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Isotropic rubber moulding

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ISOTROPIC RUBBER MOULDING

by

Timothy M. D. Buffham B. Eng. M. Sc.

A Doctoral thesis submitted in partial fulfilment of the requirements for the award of

the degree of Doctor of Philosophy

of the

Loughborough University

August, 1999

-,

Supervisor:j philip K: Freakley; Ph', D., FPRI . ..

Institute of Polymer Technology and Materials Engineering

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For my parents, Bryan and Dolly. Thank you.

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Acknowledgements

I received much help, support and encouragement during the course of this work. I

. would like to acknowledge the input from, and thank, Roger W Collins with whom I

had many lengthy conversations regarding the moulding system and its operation.

I would like to thank all the members of IPTME that have helped me during the

course of this work especially the members of RuPEC, my supervisor Philip

Freakley, Jane Clarke and Oavid Southwart whose suggestions, criticism and

knowledge of rubber and the rubber industry have been invaluable.

Other staff who deserve my thanks and without whose help things would have been

much more difficult are: Barry Clarke who helped so much in the RuPEC lab. and

IPTME pilot plant, R P Owens for his help with the physical testing equipment and

demonstrating its use, T Atkinson for his help with the photography for the project

and this thesis.

The engineering skills and knowledge of C Lines, K Ellison and M Hallam in IPTME

workshop and A. Trotter in the pilot plant were indispensable for the project, the

commissioning and modification of the prototype and helping to keep it in operation.

I would like to thank James Walker & Co. Ltd. For supplying compounds and O-rings

for the trials and comparison.

I would also like to acknowledge the contributions of the project partners Iddon Bros.

Ltd. and Euro-projects (L TTC) Ltd. and the OTI and EPSRC for funding the under

the EEM Link scheme.

Finally, I would like to thank my family for all their help and support over the last 32

years.

Abstract

The current work was initiated to develop, understand and optimise a novel

computer controlled, automated, flexible compression moulding system primarily for

the production of fluid seals. A prototype moulding system was designed and built

for the study. It was used to process a range of rubber compounds for process

evaluation.

The rheological properties of the compounds were determined with a Negretti TMS

biconical-rotor rheometer and the cure characteristics were obtained with a Wallace

Shawbury precision cure analyser. The data was used in fluid flow module of a

finite element analysis package (EMRC NISAII) to simulate the flow of uncured

elastomer in the system enabling the prediction of temperature rise and pressure

drop to aid the design and development. Temperature predictions currently

over-estimate the measured values by 10 - 20%.

The prototype moulding system, consisting of a preforming dispenser, press and

computer control system, was used to produce mouldings with the range of

selected compounds. The dispensed preforms and mouldings were studied for

signs of anisotropy. These preforms and mouldings were, however, still some way

from ideal, the preforms exhibiting an elongated lozenge shaped cross-section with

a typical shape factor of 3 and a large degree of post preforming shrinkage due to

the largely circumferential molecular orientation. The dispenser was redesigned

giving further consideration to managing the flow history and therefore molecular

orientation, relaxation and recovery of the material. The second-generation

dispenser produced considerably lower post preform shrinkage and a more

desirable cross section with a shape factor much closer to 1 (1.25).

A comparison between the conventionally moulded parts and those produced with

the prototype system was undertaken. The measurement of mechanical properties

! .of vulcanisates by tensile testing and solvent swelling methods, and mould

•. shrinkage were used to determine the extent of the anisotropy caused by molecular

" orientation.

:

Due to the elimination of in-mould flow isotropic O-rings were produced showing

uniform properties under testing and substantially better properties under testing

and substantially better properties than conventionally moulded O-rings. Sheet

samples produced by both systems were however, similar in all respects because

the elimination of flow in a flat mould during closure and compression was not

achieved.

The secondary objectives of automating the compression mould in process and the

production of flash free components, utilising the inherent viscoelastic properties of

elastomers to advantage, were also met.

Table of Contents

1. Introduction ......................................................................................................... 1 1.1 Why is Reduction of Anisotropy Important? .................................................... 1 1.2 Objectives ........................................................................................................ 2

2. Literature Review ................................................................................................ 3 2.1 Introduction ...................................................................................................... 3 2.2 Types of Polymer ............................................................................................ 4

2.2.1 Rubbers (Elastomers) .............................................................................. 4 2.2.2 The Structure and Morphology of Rubber Molecules ............................... 5 2.2.3 The Structure and Properties of Rubber (and Rubberlike MateriaL) ........ 7

2.2.3.1 Mechanical Properties .......................................................................... 7 2.2.3.1.1 Elasticity ...................................................................................... 10 2.2.3.1.2Viscoelasticity and Stress Relaxation .......................................... 11

2.2.3.2Thermal Properties ............................................................................. 14 2.2.3.2.1 Thermal Conductivity(A. or !<c) and Thermal Expansivity(ae) ........ 14

2.2.3.3Electrical Properties ........................................................................... 15 2.3 Reinforcement (and State of Mix) .................................................................. 16

2.3.1 Reinforcing Materials (and Particle Size) ............................................... 17 2.3.2 Reinforcing Mechanisms ........................................................................ 19

2.3.2.1 Bound and Occluded Rubber ............................................................. 19 2.3.2 .2Hydrodynamic Theories ...................................................................... 26 2.3.2.3Rubber Elasticity and Strain Amplification .......................................... 28 2.3.2.4lnterparticle Chain Breakage and Chain Slippage Mechanisms ......... 30

2.4 Cross-linking (Vulcanisation and Cu re) ......................................................... 32 2.4.1 Vulcanisation Mechanisms, Rates and Times ........................................ 34

2.5 Production and Manufacturing Processes ..................................................... 37 2.5.1 Pre-moulding Processes - Mixing and Milling ......................................... 38

2.5.1 .1 The Mixing Process ............................................................................ 38 2.5.1.2Mixing Equipment ............................................................................... 39

2.5.1.2.1 The Internal Mixer ........................................................................ 39 2.5.1.2.2 The External Mixer ...................................................................... 40 2.5.1 .2.3 The Continuous Mixer ................................................................ .40

2.5.1.3Milling and Calendering ...................................................................... 41 2.5.1.4Anisotropy and Pre-moulding Processes .......................................... .41

2.5.2 Moulding Processes ............................................................................... 41 2.5.2.1 Moulding Methods ............................................................. : ................ 42

2.5.2.1 .1 Compression Moulding ................................................................ 42 2.5.2.1 .2Transfer Moulding ........................................................................ 43 2.5.2.1 .3lnjection Moulding ........................................................................ 45

2.5.2.2Anisotropy in Moulding Processes .................................................... .46 2.5.2.2.1 Anisotropy in Compression Moulding ......................................... .47 2.5.2.2.2Anisotropy in Transfer and Injection Moulding ............................ .48

2.5.3 An isotropy in Special Cases ................................................................... 51 2.6 Summary and Comments on Anisotropy and Orientation in Rubber ............. 52

2.6.1 Causes of Anisotropy in Products and Mouldings .................................. 53 2.7 References .................................................................................................... 54

3. The FORM System ........................................................................................... 60 3.1 FORM System Concept - Overview ............................................................... 60

3.1 .1 The Dispenser Concept ......................................................................... 61

3.1.2 The Press Concept ................................................................................ 62 3.1.3 The Mould Concept.. .............................................................................. 63 3.1.4 The Computer Control System ............................................................... 64 3.1.5 Sequence of Operation .......................................................................... 64 3.1.6 Reduction in the Number of Processing Operations Compared with Conventional Compression Moulding ............................................................... 65 3.1.7 System Configurations ........................................................................... 65

3.1.7.1 Stand-Alone Configuration (Single-Station) ........................................ 66 3.1 .7 .2M ulti-Station Configu ration ................................................................. 67

3.2 Prototype FORM System Description ............................................................ 68 3.2.1 Description of the Dispenser .................................................................. 68 3:2.2 Description of the Press ......................................................................... 71 3.2.3 Description of the Control System .......................................................... 71

3.2.3.1Temperature Control and Set Points .................................................. 72 3.2.3.2Hydraulic Pressure Control ................................................................. 75 3.2.3.3Position Control .................................................................................. 75

3.3 System Operation .......................................................................................... 76 3.3.1 Programming the Dispenser and Press Operation Cycles ..................... 76

3.3.1.1Temperature and Pressure Setting .................................................... 76 3.3.1.2Dispenser Cycle ................................................................................. 77 3.3.1.3Press Cycle ........................................................................................ 78

3.4 References .................................................................................................... 79 4. Experimental ..................................................................................................... 80

4.1 Rubber Compounds ...................................................................................... 80 4.1.1 The Raw Polymers ................................................................................. 81 4.1.2 The Filler - Carbon Black ........................................................................ 82 4.1.3 The Additives ................................. : ................. ; ..................................... 82

4.1.3.1 Activator and Processing Aids ............................................................ 82 4.1.3.2Curatives ............................................................................................ 83 4.1.3.3Antidegradants ................................................................................... 84 4.1.3.4Plasticiser ........................................................................................... 84

4.1.4 Material Preparation ............................................................................... 84 4.1.4.1 Mixing Equipment ............................................................................... 84 4.1.4.2Mixing Procedure and Conditions ....................................................... 85

4.2 Material Properties ........................................................................................ 89 4.2.1 Determination of Rheological Properties ................................................ 89

4.2.1.1 Negretti TMS Biconical Rheometer ................................................... 90 4.2.1.2Measuring Viscous Flow in the TMS - Test Procedure ....................... 91

4.2.2 Determination of Scorch Safety and Cure Times ................................... 92 4.2.2.1 Equipment - The Wallace-Shawbury Precision Cure Analyser (PCA) 93 4.2.2.2Method - Determination of Scorch and Cure Times. with the PCA .... 94

4.2.3 Determination of Specific Heat Capacity at Constant Pressure (Cp) ...... 94 4.2.3.1 Equipment - Differential Scanning Calorimeter (DSC)' ....................... 94 4.2.3.2Method - Determination of Specific Heat Capacity (Cp) ...................... 95

4.2.4 Measurement of Density ........................................................................ 96 4.3 The FORM System Trials .............................................................................. 97

4.3.1 Methods and Procedures for the Operation of the Form Machine (Optimum Operation Procedures) ..................................................................... 97

4.3.1 .1 Dispenser Operation - Procedure for Preforming ............................... 97 4.3.1.1.1 Filling The Meter Cavity ............................................................... 97

ii

4.3.1.1.2 Preform Dispensing ..................................................................... 98 4.3.1.1.3Preform Size Range (Weight) ...................................................... 99 4.3.1.1.4Preform Consistency (Accuracy of Shot Weight) ........................ 99 4.3.1.1.5 Dispensed Preform Temperature ................................................ 99

4.3.1.2Press Operation and Moulding ........................................................... 99 4.3.1.2.1 Press Moulding/Forming Procedure ............................................ 99 4.3.1.2.2 Flash-Free Moulding .................................................................. 100

4.3.2 Test Specimen Production ................................................................... 101 4.3.2.1 Moulding Temperature Offsets ......................................................... 101 4.3.2.2Moulding O-ring Specimens ............................................................. 1 02 4.3.2.3Moulding Sheet Specimens .............................................................. 102

4.4· Physical Testing and Observations ............................................................. 103 4.4.1 General Observations - Preforms and Preforming ............................... 103

4.4.1.1 Preform Shape and Size .................................................................. 103 4.4.2 Mouldings (Product) ............................................................................. 104

4.4.2.1 Product Examination/Inspection ....................................................... 1 04 4.4.2.1.1 Rings ......................................................................................... 104 4.4.2.1.2 Sheet ......................................................................................... 105

4.4.2.2Mould Shrinkage .............................................................................. 105 4.4.2.2.1 Rings ......................................................................................... 105 4.4.2.2.2Sheet ......................................................................................... 106

4.4.2.3Swelling in Good Solvent... ............................................................... 106 4.4.2.3.1 Ring Shape ................................................................................ 107 4.4.2.3.2Volume ...................................................................................... 107

4.4.2.3.2.1 Sheet Samples .................................................................... 107 4.4.2.3.2.20-ring Samples ................................................................... 108

4.4.2.4Compression Set ....... , ...................................................................... 109 4.4.2.5Tensile Testing of Dumbbells Cut from Sheet .................................. 109

4.5 References .................................................................................................. 110 5. Finite Element Modelling (FEA) ...................................................................... 113

5.1 Model Construction ..................................................................................... 113 5.1.1 Geometric Modelling ............................................................................ 114 5.1.2 Meshing (Finite Element Modelling) ..................................................... 115 5.1.3 Boundary and Initial Conditions ............................................................ 118 5.1.4 Units ..................................................................................................... 119 5.1.5 Post-Processing ................................................................................... 119

5.2 Static Heat Transfer .................................................................................... 119 5.2.1 Conditions and Assumptions ................................................................ 119

5.3 Heat Transfer with Incremental Flow (Pseudo-flow) .................................... 120 5.4 Flow Modelling with NISAlFLUID ................................................................. 121

5.4.1 Dispenser Fill Modelling ....................................................................... 121 5.4.1.1 Ring Dispenser Geometry ................................................................ 122

5.4.1 .1 .1 Ring Dispenser Geometry I ....................................................... 122 5.4.1.1.2 Ring Dispenser Geometry II ...................................................... 123

5.4.1.2Sheet Dispenser Geometry .............................................................. 123 5.4.2 Dispense Modelling .............................................................................. 125

5.4.2.1 Ring Dispense Modelling .................................................................. 130 5.4.2.1.1 Ring Dispenser Altemative Geometries ..................................... 130

5.4.2.2Sheet Dispense Modelling ................................................................ 131 5.5 FLUID - STATIC Interface ........................................................................... 132

iii

5.6 References .................................................................................................. 133 6. Results and Discussion ................................................................................... 133

6.1 Mixing and Material Characterisation .......................................................... 133 6.1.1 The Factors Affecting Processibility ..................................................... 133 6.1.2 Mixing ................................................................................................... 134 6.1.3 Compound Rheology (Negretti TMS Biconical-Rotor Rheometer) ....... 135 6.1.4 Physical Constants ............................................................................... 139

6.1 .4.1 Specific Heat Capacity ..................................................................... 139 6.1 .4.2Density Measurement ...................................................................... 140

6.1.5 Scorch and Cure (Vulcanisation) Time ................................................. 140 6.2 Finite Element Modelling ............................................................................. 141

6.2.1 Heat Transfer Modelling ....................................................................... 142 6.2.1.1 Reservoir Geometry ......................................................................... 143 6.2.1.2Static Heat Transfer (Pseudo-Flow Simulation) ............................... 144

6.2.2 Fluid Flow Modelling ............................................................................. 146 6.2.2.1 Dispenser Flow (To Fill the Meter Cavity) ......................................... 146

6.2.2.1.1 Initial Design (Flow Geometry I of Chapter 5) ............................ 146 6.2.2.1.2 Modified Dispenser (Flow Geometry 11 of Chapter 5) ................ 150

6.2.2.2Dispense Flow (Metering and Preforming) ....................................... 156 6.2.2.2.1 What if? Modelling of Other Possible Meter Cavity Configurations ............................................................................ 162

6.2.3 Prediction of Preform Shape Change Using Fluid Flow and Static Finite Element Modelling in Combination ................................................................. 164

6.3 Experimental Work with the FORM System ................................................ 165 6.3.1 Preforming with the FORM Dispenser .................................................. 165

6.3.1.1 Filling the Meter Cavity and Preforming ........................................... 165 6.3.1.2Preform Consistency (Shot-to-Shot Repeatability) ........................... 166 6.3.1.3Preform Size Range (Weight) .......................................................... 168 6.3.1.4Dispensed Preform Temperature ..................................................... 168 6.3.1.5Preforming - Observations ................................................................ 172

6.3.1.5.1 Curtaining .................................................................................. 172 6.3.1.5.2Preform Shape .......................................................................... 172 6.3.1.5.3Lobing ........................................................................................ 173

6.3.1.5.3.1 Lobing and Molecular Orientation in O-ring Preforms ......... 176 6.3.1.5.4Preform Shrinkage ..................................................................... 179

6.3.2 Elimination of Lobes and Preform Shrinkage with a Modified Dispenser ............................................................................................. 180

6.3.2.1 Preforming with the Modified Dispenser ........................................... 182 6.3.2.1.1 Preform Shape .......................................................................... 182 6.3.2.1.2Preform Shrinkage ..................................................................... 184

Moulding - Observations, Problems and Defects ............................................ 185 6.3.3.1 Flash Free Moulding ......................................................................... 186

6.3.3.1.1 Phased Closure .................................................... , .................... 187 6.3.4 Product Testing .................................................................................... 189

6.3.4.1 Examination of Moulded Product ..................................................... 189 6.3.4.2Physical Testing of O-Rings ............................................................. 190

6.3.4.2.1 Swelling in solvent ..................................................................... 190 6.3.4.2.2 Compression Set ....................................................................... 194 6.3.4.2.3Mould shrinkage ........................................................................ 194

6.3.4.3Physical Testing of Sheet ................................................................. 195

iv

6.3.4.3.1 Mould shrinkage ........................................................................ 195 6.3.4.3.2Tensile Testing ................................................. : ........................ 196

6.3.4.4Summary of Preform Production and Physical Testing .................... 199 6.4 References .. ................... , ..................................................... ....................... 199

7. Conclusions .................................................................................................... 203 7.1 Suggestions for Further Work ..................................................................... 204

v

Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Appendix H Appendix I

Chapter One

1. Introduction

Anisotropy (or 'grain') in moulded parts has long been known about. It was studied

in the 1940's and B.S. 902 Part A2 recommends that tensile test dumbbells be cut in

the direction of the grain. The term 'grain' seems to encompass orientation, residual

stresses, the anisotropy of shrinkage during cooling and other mechanical effects

that vary depending on the direction in which the test is carried out. This anisotropy

is induced into the rubber compound in the various processing, shaping and cross­

linking stages of the rubber production process.

The control of polymer processing is a notoriously difficult problem. Rubber product

manufacture is typically low technology batch production and suffers from all the

common faults associated with this type of industry. The type of problem these

processes suffer from are: (i) batch-to-batch variation, (ii) poor process monitoring, it

being difficult if not impossible to cure 'on-line' non destructively, and (iii) the non­

linear behaviour of the process.

In this project an attempt has been made to relate anisotropy of moulded rubber

parts to their processing history and to develop a rubber moulding process for

producing products with little or no anisotropy. A novel compression moulding

system was designed, manufactured and evaluated during the project.

1

1.1 Why is Reduction of Anisotropy Important?

Anisotropy is particularly relevant to the production of moulded precision parts

where the requirement for dimensional accuracy calls for careful control of moulding

'shrinkage'. Such critical components are used in the oil industry where physical

properties and dimensional accuracy requirements are strict. Another product area

of particular interest to this project is the production of large diameter O-rings which

are notoriously difficult to manufacture to high tolerances.

Mould design, and manufacturing cost and time, are affected by the anisotropic

behaviour of rubber. Differential directional shrinkage in mouldings can lead to

moulds being 'jiggled', modified or re-manufactured so that the parts produced have

the desired finished shape (Le. an accurate circular component may have been

produced in a mould cavity that is non-circular).

An isotropic molecular structure will provide a basis for consistent physical

properties and dimensions of finished mouldings.

1.2 Objectives.

The main purposes of this research are:

[1] to devise models which relate the anisotropy of rubber mouldings to the

process strain and heat history;

[2] to evaluate a novel compression moulding system that has been designed to

minimise anisotropy in moulded rubber products.

[3] to automate the compression moulding process into a highly automated, high

volume manufacturing process to compete with injection moulding.

[4] to develop a proposed method of producing flash-free mouldings which will

eliminate the need for post-demoulding finishing operations.

2

Chapter Two

2. Literature Review

2.1 Introduction

In this chapter it is proposed to review both the factors which determine the intensity

and amount of orientation and to explore the properties which are affected by

orientation. Consideration is given to the molecular structure and morphology and

properties of elastomers, highlighting some specific properties of rubber and

rubberlike materials, their elasticity, viscoelasticity and the phenomenon of stress

relaxation. It is this type of behaviour which sets elastomers apart from other

polymers and engineering materials and makes them unique, useful, unusual and

interesting. This is followed by a detailed look at the controversial, yet highly

important, topic of particulate 'reinforcement' which is one of the factors that

contribute to the physical properties, the in-service life and behaviour characteristics

of both un-cured and vulcanised elastomers. Cross-linking and cross-link structure is

also considered briefly. A section discussing the manufacturing processes of rubber

products is also included. Such operations as mixing and milling (and calendering)

are considered as it is from these premoulding operations that a portion of the

anisotropy in components stems. Moulding, in its various different forms, is

considered because flow in processing especially 'in-mOUld flow' is deemed by

many to be a major contributor to anisotropy in rubber (and other polymer) products.

Anisotropy and molecular orientation in elastomers is a theme that runs throughout

this review with a view to determining causes, effects and, hence, possible

remedies.

3

2.2 Types of Polymer

Thermoplastics are materials which soften and flow (melt) upon the application of

heat and pressure and can be processed and formed by a number of extrusion and

moulding techniques. The thermoplastic melt is then cooled and the desired shape

frozen in. Most thermoplastic materials can be remoulded many times. Thermosets

and rubbers have to undergo an irreversible chemical process, cross-linking, usually

initiated by the application of heat, forming an interconnected network of polymer

molecules, to give them their ultimate properties. In thermoplastics molecular

orientation is frozen in by cooling whereas in rubber and thermosets orientation is

fixed in by cross-linking in a heating process which increases molecular activity and

therefore aids the recovery of orientation but the low modulus of rubber means that

the effects of orientation will be much more apparent than in thermoplastics or

thermosets.

2.2.1 Rubbers (Elastomers)

Elastomers are perhaps best known for their capacity for very large and rapid

recovery of deformation, a phenomenon that is familiar to almost everybody and the

term 'rubber', broadly, covers any of the materials that stretch considerably and

recover more or less completely. The major polymers that occupy this group are:

natural rubber (NR) and its synthetic counterpart stereoregular polyisoprene (IR or

cis-1,4-polyisoprene), styrene-butadiene rubber (SBR), butyl rubber (IIR),

polychloroprene (CR or neoprene), acrylonitrile rubber (NBR or nitrile). Elastic

behaviour can also be seen in lightly cross-linked amorphous polymers, such as

poly(methyl methacrylate) (PMMA) and polystyrene (PS) above their Tg, albeit only

in short term experiments. Elastomers are network polymers and require curing (or

cross-linking) to form an entire network that could be considered a single, large

network molecule (or macromolecule). Cross-links are chemical bonds; long polymer

chains are connected together by small molecules. In the case of sulphur

vulcanisation, links between the polymer chains consist of one or more (up to about

eight) sulphur atoms. It is the C=C double-bond unsaturation in linear chain

polydienes and random copolymers that allows the irreversible cross-linking of the

polymer chains through liberation of one of the valences.

4

2.2.2 The Structure and Morphology of Rubber Molecules

The simplest and most basic form a polymer takes and therefore the easiest to

visualise is the linear polymer which, as the name suggests, is a long continuous

chain of the repeated unit monomer (formed by linking carbon atoms together or by

linking carbon and other atoms together). Many thousands of the monomer units

may be connected together in such a way that they are analogous to a piece of

string or cooked spaghetti (Figure 2-1 (a)). To give an idea of the aspect ratio, the

strand of spaghetti would have to be two to three metres long in order to represent a

polymer molecule. It is this shape, with a large aspect ratio in the order of 103, which

makes rubber susceptible to anisotropy and orientation. The chains may be

branched (Figure 2-1 (b)) or interconnected to form three dimensional tree or

network structures by cross-linking.

Covalent bonds are formed when two atoms share one or more pairs of valence

electrons and therefore they are associated with high energies and very small

Figure 2-1 Schematic of (a) linear, and (b), branched molecular chains

interatomic distances. Covalent bonds between the atoms in a monomer unit and

between the atoms in two adjacent monomer units hold the polymer molecule

together in its configuration. Each carbon atom has four valences or bonds

connecting it to its neighbouring atoms which are angled symmetrically towards the

comers of a tetrahedron at 109.5°. Although the interatomic distance (bond length)

and the bond angles are fixed the chain is flexible because of rotational motion

about the carbon-to-carbon bonds (and each carbon-to-hydrogen bond where

5

present). This flexibility can be used to help account for polymer behaviour such as

creep, melt flow, elasticity and orientation.

In solution and in melt the molecule possesses kinetic energy which is exhibited as

a combination of the following kinds of motion: translation of the molecule as a

whole (macro-Brownian motion), rotation of parts of the molecule (crankshaft motion

or reptation) and vibration of individual bonds which causes the conformation to

change continually. The molecule can, subject to the bond angle and length

restriction and any extemal force, adopt a random arrangement in space.

Branched molecules are more complex than linear molecules and have one or more

side chains (Figure 2-1 (b)). These side chains or branches interfere with the

ordering of the molecules so that crystallinity and orientation will tend to be

decreased. The processability of the branched polymers can also be slightly more

difficult'. Polymer networks (Figure 2-2) can be formed from both linear and

branched molecules cross-linked to other molecules (or sections of the same

molecule). Natural rubber, for example, is predominantly a linear polymer that can

form a polymer network with the addition and reaction with sulphur.

Rubber molecules generally have a carbon 'backbone', they are built up of

monomers which contain carbon-to-carbon bonds and hydrogen atoms. There are

other rubbers which retain the carbon-to-carbon 'backbone' but substitute other

elements, chlorine (Cl), fluorine (F), nitrogen (N) or other groups for hydrogen (H)

atoms.

The separate molecules, or even segments of the same molecule, are attracted to

each other and held in the mass by secondary van der Waals (intermolecular)

forces not primary chemical bonds, metallic, co-ordinate or ionic as are in metals or

ionic solids nor the covalent bonds which are predominant in individual polymer

molecules. The forces causing these secondary bonds are orders of magnitude

smaller than those of covalent bonds and this permits relative molecular motion.

They are much more difficult to define as they are not constant. These force

6

increase in the presence of polar groups and decrease with an increase in the

distance between the molecules.

If, as in the melt, there is a large number of molecules, the spaghetti analogy can be

extended to a saucepan of spaghetti in boiling water. Each piece of spaghetti is

entwined with others and in continuous motion but there are no actual physical

connections between the individual strands. In the case of the polymer molecules

they are entangled but not actually joined to each other and are free to move.

/ i

(a)

I ,

Figure 2-2 Network polymers: (a) loose cross-link network, and (b) tight cross-link network

2.2.3 The Structure and Properties of Rubber (and Rubberlike Material.)

2.2.3.1 Mechanical Properties

The variables stress, strain, time and temperature and their interrelationship are

important in describing the mechanical behaviour of elastomers. Below the glass

transition temperature (Tg) elastomers cannot support tensile strains of more than a

few percent. The microstructure does not allow the molecules to rearrange

internally, consequently failure is clean and sudden and there is no gross prefracture

yield.

7

At any strain below that of failure the deformation is largely reversible and subject to

spontaneous recovery on removal of the applied stress. Hooke's law is obeyed

approximately and this behaviour is described as elastic and the energy expended

to produce the original deformation is totally recoverable. This characteristic is much

the same as that observed in many metals, glasses and ceramics. This relationship

is often written,

cr = Ea (2-1)

where cr is stress, E is Young's modulus (or elastic modulus) and a is strain.

Figure 2-3 (and Figure 2-5) shows typical behaviour that follows the relationship

given in Equation (2-1). Strain, a, is small; stress, cr, however, is large and therefore

Young's modulus, E, is large and indicates that the forces needed to move atoms

60.---------------~--, Failure

50

or 40 E z ::E - 30 t:> .. (f)~ 20

10

O+----+----+----+--~

o 0.01 0.02 0.03 0.04

Strain a

Figure 2-3 Typical stress-strain curve for a polymer such as PM MA at a low temperature (Le. well below its T g.)

from their equilibrium positions by bond stretching, bond compression or by altering

the distance that separates non-bonded, adjacent atoms must also be large.

8

A typical elastomer, in its glassy state, below Tg. has a Young's modulus in the

order of 103 MNm·2 whereas other typical engineering materials (e.g. metals and

carbon fibre) have a Young's modulus that is in the region of 2-3 orders higher at

105_106 MNm·2 .

3-,--------------.

.:-2 'E z ~ t:l In

~ -III 1

O+--~-_+-_+--r_--4

o 1 2 3 Strain ex

4 5

Figure 2-4 Typical stress-strain curve for an unfilled rubber

Considerably different behaviour can be observed in a lightly cross-linked (above Tg)

rubber or a non-elastomer in the rubbery state, a phenomenon that many polymers

exhibit in a region just above their Tg, where chain entanglements restrict the

movement of chains and act, for a short time, as effective cross-links.

If a similar tensile test is carried out above Tg (as above) the stress-strain behaviour

is very different from that below (Figure 2-4); it is still described as elastic because it

will return to its original state when the load is removed. Hooke's law is NOT

obeyed. The extension is gigantic and Young's modulus is orders lower in

comparison to the solid state (below Tg) value.

9

2.2.3.1.1 Elasticity

The topic of rubber (or rubberlike) elasticity has been a subject for many

researchers and many investigations over many years and it is covered adequately,

from both theoretical and empirical points of view, in the literature. Treloar's classic

book2 and numerous reviews concerned with both cured3,4 and uncureds elastomers

have been published.

The property of high elasticity is the result of a particular molecular structure and is

not a special characteristic of a particular substance or chemical type. For a material

to exhibit these elastic properties it is necessary for6.7

.s: (a) the molecules to be long

and flexible with rapid free rotation about many of the covalent bonds between

adjacent atoms in the molecular chain as a result of thermal agitation; (b) there must

cross-links between the long chain molecules, due to either chemical bonds which

are permanent or physical molecular entanglements; (c) the cohesive forces

between the individual chain molecules must be low permitting a high degree of

molecular movement excepting, of course, where the molecules are cross-linked as

in (b) above. However, a similar arrangement exists in unvulcanised rubber gum and

rubber compounds, which also show elastic properties, where no permanent

chemical cross-links exist, due to purely molecular entanglements and molecular

entanglements and physical adsorption of molecules to the surface of filler particles

for gum and filled compound respectively creating 'effective' cross-links.

10

If a piece of rubber is stretched the molecules will become aligned, to a greater or

lesser extent, in the direction of stretching thus becoming more ordered and moving

away from the random tangled configuration. Thermodynamically the random state

is more likely (or probable) than the deformed state and therefore work has to be

done to create this departure from the disorderly state and the rubber will resist

being stretched. On release the rubber will revert to its more natural state of disorder

and therefore it will retract, returning to a state that is indistinguishable from its

original. In the undeformed state the rubber has its maximum entropy and it will

always return to or tend towards this state if there are no external constraints. For

visualisation the simple analogy is that of a spring. The molecules uncoil and

Input Stress

Time

c ;; ;;

Output Strain

Figure 2-5 Ideal elastic stress-strain response

elongate then, in recovery, 'snap' back and regain their original form.

2.2.3.1.2 Viscoelasticity and Stress Relaxation

Time

Elastomers exhibit both elastic solid and viscous liquid behaviour in both the cross­

linked and uncross-linked states. Uncross-linked polymers behave largely as non­

Newtonian liquids of high viscosity. They can be deformed irreversibly (Le. their

response to force is flow) and their flow can exhibit a considerable amount of elastic

behaviour. The flow behaviour is most important and desirable in the processing and

forming of the component and elastic behaviour is the most important characteristic

in the vulcanisate where the plastic behaviour becomes an undesirable quantity.

11

An ideal elastic solid will obey a time-independent stress-strain relationship such as

Hooke's law (Equation (2-1)). An instantaneous constant stress imposed will give an

immediate strain response in the form of a constant deformation. If the stress is then

Input Stress

Output Strain

Time Time

Figure 2-6 Viscoelastic stress strain response

returned to zero in a similar fashion, the strain deformation will also follow (Figure 2-

5). In a polymer the response to the same instantaneous stress is very different. At

first there is an instantaneous strain deformation response after which the material

continues to deform under constant load. This time-dependant or viscoelastic

behaviour is known as 'creep'. Removal of the stress shows an immediate elastic

recovery followed by a less rapid reduction in strain which is not wholly recovered

indicating viscous or irreversible deformation (Figure 2-6).

On a molecular level this response can be accounted for by the individual elastic

and viscous components. The elastic response (the vertical portions of the strain

curve (Figure 2-6) is driven first by the elongation and alignment of the chains and

then by the recovery of the strained long chain molecules as they return to the

preferable unstrained state. The time-dependent viscous component reflects the

difficulties these long chain molecules have in moving past one another due to

intermolecular friction and entanglements which progressively 'release' under stress.

Similarly when the stress is released there are frictional losses and 'new'

entanglements to overcome.

12

In the above the time-dependent strain response of polymer to an imposed constant

stress has been discussed. Stress relaxation on the other hand is the time­

dependent stress response to an imposed constant strain (Figure 2-7). At moderate

Input Strain Output Stress

Time Time

Figure 2-7 Stress relaxation: Time dependent stress response to imposed strain

strains and brief hold times, a proportion of the higher molecular weight chains will

not slip at entanglement junctions9• When a sample is kept under constant strain,

the stress (or retraction force) decreases and eventually, when the strain is

removed, the sample will not return to its original state. The mechanism responsible

for this is similar to that described above. The molecules uncoil and elongate on

stretching and slip past each other. As entanglements are progressively released, a

new equilibrium state of random coiling is created as molecules take up new

positions and new entanglements are formed.

The time-dependent effects are, in certain circumstances, related as was shown by

Gent 10 who found a relationship between creep, stress relaxation and recovery (the

return to unstressed dimensions after the removal of a load) indicating that the

same, similar or linked processes are responsible for the behaviour. The stress­

strain behaviour described above is affected considerably by chain length 11 (as

viscosity is known to be). The longer the chains in uncross-linked elastomer or the

distance between links in cross-linked elastomer the more difficult it is to release

entanglements and the more difficult it is for the molecules to work their way past

each other and thus reach their desired ideal state. This leads to greater elasticity

and longer recovery.

13

2.2.3.2 Thermal Properties

The transfer of heat is an important consideration in both the in-service (use) and

processing of polymers. For example, thermal insulation (the control of heat flow) is

a primary function of some products and in others, such as tyres, the rubber will

become hot, due to hysteresis effects (viscoelastic loss), which is an incidental

effect.

Somewhat less well known is the fact that orientation can have an effect on thermal

properties of polymers, Plomanteer, Thorne and Helmer12 and Wetton 13 have

reported that Tg can shift due to molecular orientation, reducing with increasing

orientation. Different values of Tg can be obtained depending on the direction of

measurement, being up to 15°C lower in the direction parallel with the orientation

than in the direction normal to orientation.

2.2.3.2.1 Thermal Conductivity(J.. or kc) and Thermal Expansivity(ae)

Heat is conducted through materials via atomic vibrations. The application of heat to

a cold polymer will have the effect of increasing the amplitude of the thermal

vibrations in the microstructure. This will be conducted to neighbouring atoms at a

rate which is dependent on the strength of the bonding between the adjacent atoms.

Crystalline solids and covalently bonded materials are good thermal conductors

because they have strong bonds between the atoms and molecules. Heat is

conducted poorly in amorphous solids in which secondary forces bind the

molecules. The weaker bonding does not transfer the thermal vibrations as

effectively. The thermal conductivity of polymers is in the order of 0·22 Wm·1K"1. For

comparison typical values of thermal conductivity, for a range of materials, are given

in Table 2-1. Polymers show anisotropic thermal conductivity if the molecules are

aligned; heat flow parallel with orientation is greater than heat flow normal to

orientation, this effect was clearly demonstrated by Hands 14 for stretched polymer

samples. Increases in chain length will tend to give an increase in thermal

conductivity because thermal energy flows more freely along polymer chains than

between them.

14

The coefficient of linear thermal expansion is higher for polymers than it is for metals

or ceramics and is not generally a truly linear function of temperature thus

complicating the design of moulds for precision parts and the design of metal inserts

in polymer parts. Thermal expansion is not necessarily isotropic, Muller'5.,e.'7

showed considerable anisotropy in hydrocarbons (CnHn+2 for n=5 to -50) with

negligible expansion in the direction parallel with orientation and up to 6·8% normal

to the direction of orientation. Later, Polmanteer et al.'2 showed similar results with

correspondingly higher values for thermal expansion coefficients of elastomers with

up to 86% difference between directions parallel and normal to the direction of

orientation. Changes in the composition can produce significant changes in thermal

expansion, for example, the replacement of polymer by fillers that, usually, have

lower expansivity will reduce the resultant expansion.

Thermal Linear Thermal Specific Heat Conductivity (A. or Expansivity( a,,) Capacity(cr) !<c)Wm"K" K" kJ kg" K'

Steel -63 -1·5 x 10'0 -0·42 Aluminium -201 -2·3 x 10'0 -0·9

Water (liquid) -0·6 -7 -4·19 (ice) -2 -5 -2·1

Solid Polymers 0·1 - 0·45 4 - 20 x 10'0 1 - 2

Table 2-1 Thermal properties of polymers compared to other (engineering) materials'··1 •. 2 ••

NB: Published thermal property data shows inconsistency. The table above is a composite of data from several sources and should be considered as an illustrative guide rather than firm or exact.

2.2.3.3 Electrical Properties

Most electrical properties of polymers are determined by primary chemical structure,

in a similar way to thermal expansivity, and are generally less varied than the

mechanical behaviour. Polymers are dielectric (non-conductors of direct electric

current) with very high electrical resistivity (1010 to 10'8 Qcm). Some carbon blacks

and metal powders used as fillers can impart conductivity to a polymer due to the

formation of a network of pathways of conductive filler particles2'.22.23 that are either

contacting or less than a 1 nm apart, although alternative suggestions have been

15

made on the theme of quantum mechanical electron tunnelling by Meyer4 and

others.

The measures of polymer electrical properties are: volume and surface resistivity,

dielectric strength, arc resistance, dielectric constant (relative permittivity) and

dissipation and loss factors. In this work reference is made only to resistivit!5 and

permittivit!6 as their measurement gives some clues to the structure of filled rubber

compounds21 .22,27.28 and yield anisotropic effects29.3o.31 with differences in

conductivity varying by a factor of 30: 1 parallel with and normal to the direction of

orientation.

2.3 Reinforcement (and State of Mix)

Reinforcement is a phenomenon that occurs when a finely divided filler is mixed with

rubber, a process which is as old as the industry itself. Fillers were originally used to

dilute the rubber and lower the cost and for increased rigidity and hardness.

However, it was soon noticed that the use of certain fillers gave extra strength and

toughness thereby reinforcing the properties of the rubber vulcanisate. The term

'reinforcement' refers to the striking changes in stress-strain properties that are

noticeable in rubber vulcanisates. These changes can be characterised as: (1) an

increase in tensile strength; (2) an increase in the modulus; and (3) an increase in

the elongation at break. Much of the work done in the study of reinforcement has

been carried out using amorphous materials, such as SBR, as they do not crystallise

or undergo strain crystallisation, an effect which may hide the reinforcement. There

is evidence, reported by Wheelans32, that the presence of carbon black enhances

anisotropy in moulded rubber products.

The increase in modulus is expected because the carbon particles are rigid but

reasons for the ability of the carbon black to permit an increased elongation at break

and the ultimate tensile strength of the composite are not clear. There is a very large

body of literature on the subject of rubber reinforcement and reinforcing fillers but

there seems to be little agreement on the fundamental mechanism of reinforcement,

apart from the logical conclusion that the bond between the particle surface and the

16

rubber phase must be responsible for the reinforcing effect. In a practical sense

reinforcement may be considered as the increased stiffness, modulus, rupture

energy, tear strength, tensile strength, cracking resistance, fatigue resistance or

abrasion resistance, any of which would tend to increase the service life of a

product. It is probable that the particulate reinforcement of rubbers is not the result

of anyone phenomenon, molecular process or mechanism33.

2.3.1 Reinforcing Materials (and Particle Size)

The list of particulate materials that have been added as fillers to rubber is long and

diverse and includes such materials as carbon blacks, aluminium hydroxide, calcium

carbonates and oxides (limestone and chalk etc.), calcium sulphate (gypsum),

silicas, zinc oxide, wood flour etc. They are added to reduce cost, improve

processing, to improve appearance and colour and to reinforce the polymer by

enhancing the mechanical properties in some way (e.g. hardness, tensile strength

etc.). Horn34 gives detailed information on carbon black fillers and Simmons35 has

produced an exhaustive list of non-black fillers including information on the

reinforcing ability and the properties imparted to the compound.

Among these fillers several have been shown to produce a significant reinforcing

effect and they include carbon blacks and graphitised carbon blacks36.37

,

precipitated silicas and silicates38, anhydrous silicas39

, esterified silicas4o, the

organic polymer Iignin41 and formaldehyde resins42 in particulate form.

The chemical nature of the surfaces of these particles varies widely and therefore

the only common feature between them is that they all have an extremely small

particle size. It is difficult then, for anyone studying the subject, not to conclude that

any finely divided solid material will reinforce rubber, as did Schmidt43 when he

wrote "small particle size of the pigment is of prime importance in elastomer

reinforcement, whereas the chemical nature of the pigment appears to be of

secondary importance" after having reported the strong reinforcing effects of stannic

oxide, silica, Prussian blue, polystyrene and casein.

17

In Figure 2-8 the results of Boonstra44 have been plotted. This clearly shows the

effect of carbon black particle size on reinforcement in SBA. It is generally accepted

in the literature that particle size has an effect, with the amount of reinforcement

increasing with decreasing particle size for a given volume fraction of filler particles.

The size ranges are broadly divided, for carbon blacks, as follows: at 300nm

diameter and above are thermal blacks which show little reinforcing effect (these

might be called 'dilutent'); 'semi-reinforcing' furnace grade blacks 100nm to 200nm

25~~------------------------~

20

.. Do

!. ~ c: 15 e -11)

.!!

.iij c: ~

10

5+-------~------~------~ o 100 200 300

Average Particle Size (nm)

Figure 2-8 Effect of particle size of carbon black at 50 phr. on tensile strength of SBR. Graph plotted from Boonstra data".

in diameter; 'high abrasion' fumace grade blacks at about 40nm diameter; and

'intermediate super abrasion' and 'super abrasion' grades with particle sizes below

35nm. For non-black fillers similar groupings can be identified45: soft clays etc. with

particle sizes in the range of 1000nm to 8000nm diameter are 'dilutent' fillers; hard

clays, zinc or titanium oxides and precipitated calcium carbonates with sizes ranging

from 100nm to 1 OOOnm diameter are 'semi-reinforcing' fillers; and 'reinforcing' fillers,

18

consisting of precipitated calcium carbonates, silicas, calcium silicates and

anhydrous silicas etc., at 10nm to 100nm diameter. The greatest reinforcement is

noticed in the range 10nm diameter to 100nm diameter (Figure 2-8). Very little work

on fillers with particles below 10nm has been reported.

Particle size is inversely proportional to the surface area of a particulate filler. The

effect of smaller particles might actually show the extent to which the filler can

interface with the polymer phase.

The carbon blacks and silicas are the most important reinforcing fillers for use in

industry. This is born out in the literature by the fact that polymer textbooks and

reviews, when discussing reinforcement, often concentrate on carbon blacks and/or

silicas and only mention other fillers in passing. Carbon black is the most common

and, perhaps, the most important because of its heavy use in the tyre industry.

Many other areas of the rubber industry also utilise carbon black, and its reinforcing

effect, in the manufacture of their products.

2.3.2 Reinforcing Mechanisms

This reinforcing phenomenon is the subject of several theories and explanations

although there is not really any single method which explains fully the mechanism of

reinforcement. The following sections highlight some of the more popular concepts

of particulate reinforcement in elastomers and rubber.

2.3.2.1 Bound and Occluded Rubber

On mixing rubber with a reinforcing filler, particularly fine carbon black, a change

comes about in the mixture and an increase in viscosity can be noted. When

exposed to a good solvent only a proportion of the rubber can be extracted from the

(unvulcanised) mixture. The portion that cannot be drawn into solution is known as

bound rubber and exists in the form of a rubber-filler gel that is swollen without loss

of shape. Twiss46 was the first to report (1925) observations that a mixture of natural

rubber and carbon black shows a resistance to solvent that is related to an

improvement in mechanical properties. A test was developed some twelve years

19

later by Fielding47• After the observations of Twiss, Stamberger48 noted fresh "killed"

rubber (after recent mastication) has no strength but after a time of standing gains

such strength that it can no longer be pulled apart by hand and freshly made rubber

dissolves readily but the stiff material, often standing only a few days, just swells.

Blow49 reported similarly the phenomenon that a batch of a fresh mixture of natural

rubber and channel black dissolves readily in a good solvent but if a batch is allowed

to stand before exposure to the solvent it will dissolve less readily. If the period of

standing is prolonged, solution of the rubber will be impossible and it will swell to a

gel retaining its shape, behaving as if it were vulcanised. Such storage maturation

effects have been shown by BlowsO and Leblanc and HardyS1 to evolve over a period

in the order of about 30 days. Blow49 also noted a structure viscosity and thixotropy

of the solutions.

It is clear that rubber-filler systems exhibit time-dependent characteristics. Fresh

rubber (or that recently freshened by mastication) is more easily processed and has

a lower viscosity than a compound that has been rested. It is believed that this is

due to the formation of bound or immobilised rubber over time as the material is

rested in storage. This structure can be broken down by work and it will reform if the

material is then allowed to rest again. Similarly if a rubber solution is allowed to

stand and is then worked the apparent viscosity will decrease with time due to a

'structural' (intermolecular attractions and entanglements) breakdown, structural

reformation occurs concurrently and an equilibrium point is reached. If the solution is

allowed to stand, then a structural reformation to the original state will occur due to

Brownian motion. McBain52 put forward the suggestion that this structure was due to

an orientation of the rubber "particles" on the carbon-black particles. Cotton53 also

makes mention of rubber molecules being oriented between filler particles, however,

no mention of the immobilisation of rubber is made. This orientation, if it exists,

could be a cause of anisotropy seen in products.

Occluded rubber is that part of the rubber phase that takes up the internal volume of

a filler aggregate. In doing so it becomes shielded from deformation of the

composite mixture as a whole and, therefore, many of the mechanical properties are

20

influenced by the amount of rubber which is shielded by such occlusion because

this has the effect of increasing the volume fraction of filler and decreasing the

volume of the rubber. This effect was initially pointed out by Medalia54 and

Kraus55.56.57.

The concept of a three-dimensional, two-phase lattice structure in which the longer

molecules were adsorbed, at different points along their length, onto the surface of

several different filler particles, was suggested in early studies by Stickney and

Falb58, Gessler59 (for carbon black systems) and by Southwart60 (for silica systems).

The two phases consisted of mobile rubber and semi-mobile or immobile rubber

bound to the surface of one or more filler particles, the latter creating a three­

dimensional lattice and giving a reinforcing effect. It was also found that the amount

of bound rubber could be related to the observed mechanical properties of the

composite and that bound rubber was not only affected by particle size and

structure but also by the surface activity of the particle. Baker and Walker61 , in

reporting an insoluble bound rubber effect occurring in SBR-carbon black mixes,

also reported that the amount of bound rubber increases with increasing molecular

weight. The concept of "bound polymer" has also been used by Akal2 in the study

of highly filled thermoplastics.

A mathematical model for the description of filled elastomers, the "double network

model", has been developed with a similar, two phase, structure in mind by Reichert,

G6ritz and Duschl63• They describe a superposition of two independent networks.

The first network is the rubber matrix, the second network contains the filler and

connecting strands in a "supernetwork" these are analogous to the bound and

immobile phases described above. The model gives the stress atotal as the sum of

the partial stresses 0"1 and 0"2 which represent the stress in the rubber matrix and the

stress in the filler-supemetwork respectively,

O"total = (1 - <ll) 0"1 + <ll 0"2, (2-2)

21

where the partial stresses are,

2 1 0", = G, ("" -1n, )1 () ,

- '1', "', (2-3)

for the rubber matrix and,

(2-4)

for the supernetwork. <I> is the volume fraction of the supernetwork, G, and G2 are

the Gaussian moduli of the networks, '1', and '1'2 represent the proportion of fully

extended chains and appear in the empirical factor 1 ( ) in equations (2-3) and 1-'1' A

(2-4) (when all the chains in both networks are fully extended a limit is reached and

there can be no further extension), "', and "'2 are the macroscopic strains of the

networks calculated as a probability function of the microscopic strain 11. The

predictions of the double network model of Reichert et al.63 were compared to the

stress-strain results of Mark64 (for Si02 filled Polydimethylsiloxane (PDMS)

networks) giving a good correlation. Later Frey, Goritz and Freund65 showed a good

correlation between the model and carbon black - SBR systems with various carbon

blacks and loadings. The stress/strain predictions of this theoretical model show

good agreement with the stress/strain data obtained by experimental measurement

and can be viewed as good supporting evidence that a two-phase

(polymer/supernetwork) type structure exists.

Other evidence to support a two phase structure of a polymer filler composite exists

in the literature that covers electrically conductive rubber. Changes in the

conductivity (or its reciprocal resistivity) of carbon-black-filled rubber vulcanisates

with stressing such as flexing, elongation or compression etc. have been described

by Lane and Gardener66 and by Bulgin22 and Dannenberg67 among others. The work

by shows an initial rapid rise in resistivity to high values and a slow decrease as

soon as a stationary condition is established. This return to equilibrium is faster

22

when the temperature is higher which tends to support the idea of a structure

breakage and reformation mechanism. Wack, Anthony and Guth27 and Boonstra

and Dannenberg21 show a rapid increase in resistivity to a maximum (up to 50%

elongation) then a less rapid decrease (from approx. 50%-150% elongation) for

carbon black filled rubber compounds during stretching of tensile specimens. This is

explained by assuming the breakdown of a carbon structure at lower elongation and

a realignment at the higher elongation. Their apparatus was only capable of

elongation of up to 200% but they speculated that this decrease in resistivity would

not continue until breakage occurs, but would reach a minimum and then would rise

rapidly up to rupture. Later Voet and Morawski68 conducted experiments to higher

elongation. That there is a large rise in conductivity (fall in resistivity) above 200%

elongation is clear, reaching a maximum with a very rapid decrease thereafter as

predicted by Boonstra and Dannenberg.

These results support the two-phase, bound rubber structure proposed above, the

rubber-carbon black phase (or 'super-network') being the pathway supporting

conduction by containment and alignment of the conductive filler particles, (Le.

holding the conductive particle chain together in a distinct rubber-filler phase). The

changes in conductivity (resistivity) concur with the formation and destruction of

such a network. The exact mechanism for conduction, which could be via either the

direct contact or very close proximity (less than 1 nm) of filler particles or the later

"electron tunnelling" ideas69 (a few nano-metres) between filler particles is

unimportant because the distances proposed are the same order of magnitude and

each would be sustained by this three-dimensional bound rubber phase.

O'Brien et 81.70 suggested that the degree of mobility of the bound rubber was a

proportional function of its distance from the surface of the filler particle, the greater

the distance the greater the mobility. Kaufman, Slichter and Davis7\ Ban, Hess and

Papazian72 and later Leblanc73 point to the bound rubber network consisting of three

parts: tightly bound rubber at the filler surface which will not deform, a more loosely

bound rubber layer that surrounds the filler particles and forms connections in the

form of filaments to other particles in the compound and totally unbound rubber (Le.

freely mobile rubber). This latter structure, if it exists as proposed by these authors,

is similar enough to be reasonably approximated by the two-phase model used by

23

G6ritz and co-workers63,65 and explains why they found good correlation between

model and experiment. Interestingly, Ban et al. suggested that there may be a level

of molecular ordering such as an epitaxial type orientation of the polymer molecules

on the carbon black surface and alignment of the molecules in the network (similar

to the ideas of McBain51 and Cotton53 30 years previously). This is likely to be close

range ordering and the extent to which this could affect the more longer range order

is not clear. They also thought it likely that this orientation would persist in the final

vulcanisate. This could, possibly, have a contribution to the anisotropic effects

exhibited by moulded parts.

!:!. ~ ;;

~ ii ~

RII ... .fiIl ... !rd RII ... · F'd}rrer irter.DiCfl

, , [S/\ 0.5 (.0.25) ,

~~------------------~~-------------~ H,<tcxt,<mic elfeds ci.e 10 RIIer

Figure 2-9 Based on Payne75 plotlPayne effect dynamic test. Filler-filler and polymer filler intereraction is almost completely broken down by a dynamic strain amplitude of ±O·25.

There seems to be general agreement that bound rubber is an important factor in

reinforcement. Bound rubber could also be a factor in anisotropy, if it is accepted

that the three dimensional (2 or 3 phase) rubber-filler network structure exists and

persists from the uncured stock through into the vulcanisate. Leblanc clearly thinks

that it is possible that this structure could persist in the vulcanisate when saying ''this

portion of the elastomer can undergo very large deformation during flow whilst

remaining essentially attached to the filler particles". It would be possible for this

three-dimensional filamentous structure to contribute a component to anisotropy of

the product if it were cured with this phase in a state of strain. However Leblanc

24

does go on to describe a breakdown of the three dimensional structure at high

stresses by ruptures of the connective filaments or decohesion between rubber

chains and filler particles, this happening at a point where the stress is high enough

to cause melt fracture. The question must therefore arise: Does the structure persist

in the vulcanisate? Ban et al.72 and others73.74 think that the persistence of such a

structure is likely, although its existence would be difficult to prove, being largely

hidden by the cross-links that are formed during vulcanisation.

The writer believes that a three-dimensional rubber-filler network does exist in the

vulcanisate as a distinct phase, but that this is not necessarily the persistence of a

network that was formed prior to the last working and shaping of the uncured stock

but one which starts forming after shaping is complete. The work on dynamic

properties of rubber by Payne7S.76 shows a drop in shear modulus as dynamic strain

amplitude (DSA) is increased; the effects were shown as additive. A strain amplitude

independent residual value of shear modulus was shown due to pure gum modulus,

to hydrodynamic effects due to the filler and to filler-rubber linkages (additional

cross-linkages). A strong strain amplitude dependence was shown to be due to filler­

filler interaction. In this classical view of the "Payne effect" it has widely been

accepted that the shear modulus loss is mainly, if not only, related to the filler

network formed in the rubber matrix. Figure 2-9 shows a schematic idealisation of

the effect of strain amplitude on shear modulus similar to that of Payne. Here the

strain amplitude independence is considered to be due only to pure gum modulus

and hydrodynamic effects. The strain amplitude dependence is a combination of

filler-polymer linkages and filler-filler interaction. This view is supported by Wang77

who states ''the Payne effect can serve as a measure of filler-filler interaction as well

as polymer-filler interaction". The size of the contribution of each of the interaction

components, filler-filler and filler-polymer, to the fall in shear modulus is not known.

When applied to flow, the Payne effect indicates that the three-dimensional

structure (rubber-filler network) is broken down relatively easily when it is worked

(i.e. freshened) or in a state of flow (i.e. processing). The reformation of the network

is time dependent and occurs when flow ceases and takes about 30 days, as

indicated by the storage maturation effects shown by BlowsO and Leblanc and

HardySl.

25

The breakdown and reformation of the filler network allows a mechanism for the

formation of anisotropy to be proposed. It is well known that polymer molecules can

be oriented by deformation, such as by flow. The macromolecules (long polymer

chains) tend. to be extended (un-coiled) and aligned when stressed in such a

manner. When the stress is removed the molecules recover and return to their

relaxed (coiled) state. Short molecules recover quickly but the medium-to-Iong

molecules have a recovery time that is in the order of, or longer than, the rubber­

filler network reformation time. The rubber-filler network would therefore be

reformed before the recovery of the medium to long chain molecules is complete,

preventing them from recovering fully. This process creates anisotropy in the sample

due to the orientation of, and the stress in, the molecules that are not fully

recovered.

2.3.2.2 Hydrodynamic Theories

Einstein78 calculated the viscosity of a liquid with uniformly dispersed rigid spherical

particles,

11 = 110 (1 + 2·5c), (2-5)

where "0 is the viscosity of the liquid, " is the viscosity of the mixture and c is the

volume fraction of the particles. The particles should be incompressible, should not

interact, should be completely wettable and uniformly sized and distributed.

The relationship was shown to also apply to elastic modulus (Young's modulus) of

vulcanisates with large filler particles and low filler concentrations by Smallwood79,

E = Eo (1 +2·5c), (2-6)

26

where E and Eo are the Young's moduli of the mixture and the unfilled gum

respectively.

The assumptions that are made in the derivation of equation (2-5) do not apply to

most filler particles and Guth and Gold8o added a term to account for particle

interaction to give,

11 = 110 (1 + 2·5c + 14.1c2), (2-7)

which agrees well with experiment for larger particles of about 500nm and with a

volume fraction of 0·3 or less but not for fine reinforcing fillers. The reason was

thought to be asymmetry of the filler aggregates and Guth81 proposed the addition of

a shape factor, f, being the ratio of the length to the width of the aggregate, in a

modified equation,

11 = 110 (1 + 0·67 fc + 1·62 f2c\ (2-8)

For reinforcing blacks a good fit is obtained with a shape factor, f, of around 6 for a

high-abrasion furnace black. However it should be noted that shape factors for

carbon black aggregates are much closer to a value of 2 when measured using

electron rnicroscopl2. Guth81 and others, also assumed, like Srnallwood79, that the

change in the elastic constant of the rubber caused by the inclusion of spheres

would be entirely analogous to the theory of viscosity therefore the equations would

apply to the elastic modulus, E, giving,

E = Eo (1 + 2·5c + 14.1c2), (2-9)

for spherical particles and,

(2-10)

27

for non-spherical particles.

The Guth equation is useful for expressing the mechanical properties at low

extensions. Better correlations for some situations can be obtained by the use of an

"effective filler volume fraction" (EFVF), that is, by including the volume of

immobilised rubber "as filler" within the term c (similar to <1>, the volume fraction of

the supernetwork in Equation (2-2) from the Reichert et al. double network modeI63).

The EFVF term could then incorporate the volume of bound and/or the volume of

occluded rubber which does not have any interaction with the matrix and therefore

cannot be deformed as established by Medalia83 and KrausS5 (Also see Medalia54

and Kraus56•S7

).

2.3.2.3 Rubber Elasticity and Strain Amplification

A number of authors have developed methods based on the statistical theory of

rubber elasticity for unfilled gum vulcanisates. This relates the Young's modulus of

an ideal elastomer network directly to absolute temperature. Stress <J and strain

ratio ex are related by the equation,

<J = vkT(ex - 11 ex\ (2-11 )

where v is the number of elasticity effective chains per unit volume, k is Boltzmann's

constant and T is absolute temperature. As the predictions did not agree well with

experimental evidence, a later empirically modified, and much used, expression was

suggested by Mooney84 and subsequently modified by Rivlin and Saunders85 to

(2-12)

where Cl and C2 are constants characteristic of the rubber. C2 is the term that

makes Equation 2-12 differ from the classical ideal network behaviour. If, as in the

28

case of highly swollen gum vulcanisates, the C2 term is reduced so that it is close to

zero then 2C1 is equivalent to vkT.

The concept of strain amplification has also been proposed to describe the

behaviour of filler-elastomer composites at high strains. It is assumed that filler

particles are rigid and do not deform and therefore that the actual strain in the

rubber phase of the composite is not equal to the overall strain. In effect the particle

phase is thought to increase the strain in the rubber phase.

Several methods have been proposed to determine a value of effective strain, a', in

the rubber phase. This too, is a controversial area in terms of reinforcement. A. M.

BuecheB6 suggested the expression,

a' = 1 + (a-1).,Jc, (2-13)

where a' is the effective strain ratio in the rubber itself, a is the overall strain ratio

and c is the volume fraction of filler. A modified expression was later proposed by F.

BuecheB7:

a' = 1 + (a - Vc)/(1- Vc). (2-14)

Mullins and TobinBB suggested that the volume concentration factor from the Guth -

Gold and Guth equations could be used, as this would account for the disturbed

strain distribution and the lack of deformation in the fraction of the composite which

is filler:

a' = a(1 + 2·5c + 14.1c2). (2-15)

29

This, however, only agrees with experimental evidence for large particle sizes and

moderate extensions, so for finer more reinforcing particles and higher extension the

following should be used:

(2-16)

However, for this to fit experimental results, unusually large shape factors have to

be assumed. Here too, as with the modification of the Guth equation, the use of the

EFVF that includes occluded rubber can help bring the theoretical prediction more

into line with the experimental evidence.

2.3.2.4 Interparticle Chain Breakage and Chain Slippage Mechanisms

Another explanation of particulate reinforcement was proposed by Blanchard and

Parkinson89,who suggested that there are two types of bond, strong and weak, that

form between the rubber molecules and the carbon-black filler particles. This is

thought to be assisted "by the presence of a coherent chain structure of the filler

particles themselves". The strong type is due to chemisorptive attachment and the

weak due to a physical, van der Waals, type bonds. There are thought to be

relatively few of the chemisorptive bonds and many of the physical type. The

phenomenon of stress softening can be explained by the progressive breakage of

the weaker links. As the stress is increased, the strong bonds remain intact. The

reinforcement and ultimate strength are provided by this small number of the

stronger bonds which persist until the point of rupture. This explanation has been

extended by Blanchard9o•91 who added strain hardening at large deformations due

to: (1) tightening of short molecular chains between close particles; (2) strain

alignment of molecular segments between particles and crystallisation in

crystallisable rubbers; and, (3) slip rearrangements giving greatly increased

alignment and stress sharing by highly stretched chains. Blanchard himself points

out that there are similarities with F. Bueche's interparticle chain breakage

mechanism, and with Dannenberg's molecular slippage mechanism.

30

The Dannenberg92 molecular slippage mechanism uses a combination of molecular

slippage and bond rupture and reformation to explain how reinforcement is achieved

in a rubber-filler system. The assumption is made that it is possible for rubber

molecules attached to filler particles to undergo a surface slippage or other type of

rearrangement when a stress is applied. The shortest chains are extended to a point

where it is not possible for them to extend any further without scission or without the

bond at the surface of the filler desorbing or the chain slipping across the surface of

the filler particle. It is suggested that the most probable of these alternatives is

molecular surface slippage. The process continues in this manner when the

extension limit of the next shortest chain is reached and so on. Eventually chain

lengths equalise and the stress is redistributed between the chains that are now

oriented, aligned and equal in length, giving greater strength due to this stress

sharing and increased intermolecular association. On relaxation of the stress, the

chains of equal length will, initially, for a subsequent extension exhibit a lower

modulus (Le. stress softening). A random distribution, such as that which existed

before the rubber was stretched, is recovered on standing for some time as the

normal kinetic motion of the molecules is allowed to resume.

The interparticle chain breakage mechanism proposed by F. Bueche93 states that as

the interparticie chains in a rubber filler network have various chain lengths, the

shorter chains will break first at relatively small deformations and that, since the

stress on the chain immediately before breakage is large, it will contribute greatly to

the stiffness or modulus. Strength is also influenced by the filler particles acting as a

vehicle for distributing the tensile load, more equally, over a wide number of chains

(Le. stress sharing). Chains, once broken, will not be able to affect stiffness on a

second extension hence, a softening effect will be observed. Although a particular

mechanism is not explained the replacement of the broken chains with other similar

chains is postulated for stress recovery. A process is also proposed for stress

relaxation where the filler particle is able to move through the rubber; even a very

small motion would be sufficient to release most of the tension in extended chains.

As can be seen, except for the concentration on slippage rather than breakage of

chains and the inclusion of chain alignment effects, Dannenberg's and Bueche's

explanations are similar.

31

2.4 Cross-linking (Vulcanisation and Cure)

Vulcanisation (or curing) is the process in which a molecular network is created in an

elastomer by the formation of cross-links between its polymer chains. Before 1840

the use of rubber had been confined to those applications in which it could be

supported by a substrate (e.g. cloth). This was necessary because raw natural

rubber (NR) has poor mechanical properties existing as a tacky, weak thermoplastic

mass highly subject to the problem of heat softening. The properties are

transformed, on vulcanisation, to a useful level as NR is converted into a non-tacky,

strong, highly elastic and tough material suitable for a wide range of engineering

purposes. Vulcanisation (or vulcanising or cross-linking or curing), generally, is such

an important subject in the process of rubber manufacture and production that it is

covered in numerous texts by Hofmann94•95

, Hills96, Coran97

, Morrell98, and Alliger

and Sjothun99, to name but a few among many.

Hancock's and Goodyear's method of vulcanisation requires the addition of a small

amount of sulphur to the rubber and the resulting compound to be heated to bring

about the striking property changes highlighted in Table 2-2. Cross-linking by this

method was also found to be useful for the synthetic elastomers, developed later,

such as SBR, IIR, NBR etc. but the synthetic rubbers required different proportions

of the vulcanising additives (Le. sulphur and accelerator, see below), as synthetic

elastomers are slower curing than NR therefore greater amounts of the accelerators

needed to be used.

It was later found that ingredients other than elemental sulphur cause cross-linking.

Various sulphur compounds (sulphur donors) can also be used, their sulphur being

liberated at vulcanisation temperatures. Further investigations showed that, although

rubber could be cross-linked solely with sulphur and heat, the addition of certain

other compounds would increase the rate of cross-linking which is, on its own, slow.

These other compounds, 'accelerators', can be added to the polymer-sulphur

(polymer-sulphur-filler) mix to greatly enhance the rate at which cross-linking takes

place, enabling a reduction of cure time, hence, increasing production output and

improving vulcanisate properties. Presumably this improvement was due to shorter

exposure to heating causing less degradation. Zinc oxide was also found to have an

32

effect, enhancing the action of the accelerator and it became known as an

'activator'. Later a combination of zinc oxide and stearic acid became commonplace.

Property RawNR Vulcanised NR Non-reinforced Reinforced

Tensile Strength 2.1 20.1 31 (MPa) Elongation at 1200 BOO 600 Break(%) Modulus (MPa) - 2.B 17.3 Permanent Set High Low Snap Hioh Very Hioh Water absorption High Low Solvent Resistance Soluble Swells only (partially soluble on extended (hvdrocarbons) immersion)

Table 2-2 Typical properties of raw and vulcanised natural rubber (NR)

Conversely, vulcanisation inhibitors (retarders) are used to prevent scorch, or

premature cure. These additives delay the onset of cure and slow the rate of

vulcanisation thereby increasing scorch safety (scorch time) and the time to reach

final cure. This is a consideration that is particularly important for large, thick parts

where extended heating times are necessary to obtain cure throughout. The extent

of cure in parts of widely varying thickness could be uneven because a thin section

will warm more quickly and vulcanisation will occur more rapidly. The longer

effective time at the vulcanisation temperature compared to a thicker part in the

moulding may result in non-uniformity in the state of cure in the respective portions

of the moulding. Hofmann 100 has produced an extensive work that covers additives

for rubber vulcanisation and processing in great detail.

It was also discovered that cross-linking may be achieved without heat or elemental

sulphur. Other compounds exist that can be used to cross-link rubber components:

Oxidising agents can be used instead of sulphur but vulcanisates are weaker than

those from sulphur and price and toxicity tend to rule them out as an alternative or

replacement. Free radical generators like organic peroxides, azo compounds and

33

many accelerators and phenolic resins can also be used. A cold process using

sulphur chloride has been developed to cross-link rubber. It is therefore obvious that

there are several mechanisms that form cross-links.

LS...J

~!

~ ~

il (or lee)

rS-s-, (a) (b) (c) (d) (e) (f) (g)

Figure 2-10 Different Cross-link Formations: (a) monosulphidie cross-links, (b) disulphidie cross-links, (c) polysulphidic cross-links, (d) cyclic monosulphidic and disulphidic "cross-links", (e) vicinal cross-links, (f) sulphidic chains (pendants) and (g) C-C cross-links (i.e. peroxide cure systems).

2.4.1 Vulcanisation Mechanisms, Rates and Times

Dibb0101 describes a mechanism for vulcanisation and suggests the intermediate

steps in the reaction that occur in the formation of cross-links in sulphur-accelerator

and sulphur-accelerator-zinc oxide-stearic acid systems. Vulcanisation is described

as a continuous process that begins as soon as the ingredients are mixed and the

temperature elevated. It continues long after the heating has finished and persists

into the ageing phase of a rubber component's life. Dibbo also points out that a

number of types or structures of cross-link are formed as illustrated in Figure 2-10

(a) - (e), mono- (a), di- (b) and polysulphidic (c) cross-links, cyclic links (d) and

vicinal cross-links, which are so close in proximity to each other that they act,

effectively, as a single cross-link. It can also be seen that some of the sulphur does

not contribute to actual cross-links between different molecules but links to the same

molecule or to neither in the case of a pendant sulphur chain (Figure 2-10 (d) and (f)

respectively). Pendant chains can be formed with accelerator residues at the ends.

These links are not particularly harmful, as they are low in concentration compared

to cross-links10

2, although it is conceivable that they may hinder local molecular or

segmental motion by proximity. On the other hand, these sites, especially the

pendant sulphur chains, have the potential to be formed into cross-links on

34

continued heating. It is the formation of some or all of these types of cross-link that

will, progressively, prevent the material from flowing.

Before the onset of vulcanisation the polymer molecules can move relatively freely.

There may hindrance to movement from intermolecular entanglements but this is

slight, especially at elevated temperatures, where molecular energy permits free

molecular bond rotation allowing the molecules to slip past one another easily

(macro-Brownian motion). The formation of the desired type of cross-link can be

controlled by the choice of cure system and additives for a given polymer. Figure 2-

11 shows a schematic representation of a curemeter trace for a number of cure

schemes, these can apply to both sulphur and non-sulphur vulcanisation systems: A

shows a rapid onset of cure where cross-links start to form instantly on the

application of heat. The change in the properties can be seen immediately (typical of

peroxide cure systems). In most cases this is undesirable as it will interfere with the

safe processing of the compound. If cross-linking occurs in parts of the material

Onset of Cure (vt*:aniSabon) Under-cure • i •

OpomumCUre ., . ~ : ::::: ::::: :::::: ::: ::::: ::: :::::::: ::::: f: :::: :::::: ::::::: ::::: .. .1 .......... . o

I

E

Vulcanisation Tlm, a' b·

Figure 2-11 Stages of cure (vulcanisation); Modulus vs. Time

before it has been formed into the desired shape the molecules in that section of the

35

component could suffer from residual vulcanisation stresses and/or flow-induced

molecular orientation that is "frozen-in" by curing in a similar way to that described

by White and co_workersl03.104 and Isayev105 in a birefringence study of amorphous

polymer mouldings: B shows a more desirable curve indicating that there is a delay

between the application of heat and the start of cross-linking which allows time for

the material time to flow and take up its shape, say, in moulding operations. This

delay, known as the "scorch time" is constituent dependent. Its magnitude can be

controlled by the selection of cure system and additives. The bottom of curve B is

very important: it allows time for the material to flow and take up the shape of the

former (be it a mould cavity or extrusion die). If all flow forming the component

shape can be contained within the scorch time (i.e. before the onset of cross-linking)

the component is less likely to suffer from the residual stress and molecular

orientation effects that occur in A. The other end of the cure trace shows the region

where the vast majority or all of the possible cross-links form. C shows a gradually

increasing modulus value typical of the so-called 'marching cure' that could be

obtained with SBR. 0 is plateau cure: the modulus reaches a maximum level and,

once there, it remains constant. Other properties (e.g. elongation at break) might not

follow this trend. E shows decreasing torque with increasing time, after a maximum

at b - b', and represents 'reversion'; this is the breakdown of the cross-links (and/or

polymer degradation) caused by an extended heating time. Between the sections

already discussed there exists a region where the cross-linking reaction rate is

slowed dramatically but the point of optimum cure has not been reached. The lines

a - a' and b - b' show the x and y intercepts at 90% cure and 100% cure,

respectively. That is a and b are at 90% and 100% of the maximum torque value

attained by curves 0 and E (curve C never reaches a maximum torque value) and

a'(or t90) and b'(or hoo) are the times at which 90% and 100% cure is reached. A

value of t95 is often used for the determination of best 'technical cure' time. It is

worth noting, however, that if a curemeter measures only very small strains the trace

obtained for a given sample of compound might show up the carbon-black network

effect that would be lost or hidden if larger strains were used. A strain is usually

selected to eliminate most of the structure effect.

The number and type of cross-links will dictate the properties of the

vulcanisatel06.107, and this in turn will be dictated by cure system, the amounts of the

36

additives (vulcanising agent, accelerators, inhibitors etc.) and their activity. For

example, hard rubber ('Ebonite') is cured with 30 - 50phr of sulphur and forms a tight

network of many close cross-links producing a vulcanisate that is rigid and deforms

little, if at all, when stressed. It could easily be described as hard and strong. A small

amount of sulphur, say, 2 - 4phr will yield a lower cross-link density, a much smaller

number of cross-links, and the resulting vulcanisate (soft rubber) will be elastic and

deform comparatively easily when stressed and return to its original state when the

stress is removed.

G6ritz, Sommer and Duschl10B suggested that cross-linking is not random but that a

polymer network with 'islands' of concentrated cross-links connected by molecular

chains that are lightly or not cross-linked is formed; due to the chemical bonding

process being exothermic and the mobility of the chains near a recently formed

cross-link being impeded, thus increasing the probability of a second (and third and

so on) cross-link forming in close proximity. This structure could explain some of the

anisotropic effects that are noticed in vulcanised rubber products, for example,

anisotropic voids formed during drawing caused by an inhomogenious cross-link

distribution. A similar result could be obtained if the curative/cross-linking agent

were not properly mixed into the polymer. This would seem an unlikely, though, as

mixing and distribution of the curatives in the rubber compound is not as difficult as,

say, mixing carbon-black or other parliculate filler because only dispersion

(dispersive mixing) is required and the constituents tend to melt as temperature

rises thus making dispersion easier.

2.5 Production and Manufacturing Processes

It is believed that anisotropy is induced in the manufacture of most, if not all, rubber

products. The production sequence in their manufacture can be clearly divided into

three stages: mixing, forming and curing. The major components of the

manufacturing process are reviewed with respect to how and where anisotropy is

induced in rubber products.

37

- ---------

2.5.1 Pre-moulding Processes - Mixing and Milling

The object of the mixing process is to produce a compound that has its ingredients

sufficiently thoroughly incorporated and dispersed so the later processing (Le.

forming and curing) will ensure that the product has the desired properties for its end

use. In this section the process and purpose of mixing and mixing equipment are

briefly covered.

2.5.1.1 The Mixing Process.

The mixing process consists of three simultaneous processes (i) simple mixing, (ii)

laminar mixing and (iii) dispersive mixing. There is no single formula which

determines the importance of these different types of mixing. Anyone, depending

on the particular compound, may be the critical (or efficiency determining) process.

Simple mixing (i) is the process of moving particles around in the mix whereas

laminar mixing (ii) and dispersive mixing (iii) are much more drastic: the material will

flow in laminar mixing and particles will fracture in dispersive mixing if the stresses

are sufficiently large.

During mixing several physical changes take place:

• Incorporation - The incorporation stage is where the ingredients of the compound

are 'joined together' from being initially separate. The rubber is forced between

pairs of rotors and the mixing chamber wall or rolls with the affect of destroying

the original form of the rubber. As the material is deformed under shear, fresh

new surface is created allowing it to accept, surround and encapsulate the filler

forming agglomerates.

• Dispersion - Agglomerates are broken down and distributed through the rubber

(simple mixing). Then they are dispersed to give the required fine level of mixing

when the particles have been broken down to their ultimate size (the aggregate).

• Distribution - takes place throughout the mixing cycle to increase the

homogeneity of the compound.

38

• Plasticisation - The rheological properties of the compound are determined for

further processing operations. This process is ongoing from the very early stages

of polymer mastication.

Figure 2-12 Different internal mixer configurations: (a), non-intermeshing (tangential) and, (b), intermeshing.

2.5.1.2 Mixing Equipment

Mixing machines and equipment are covered comprehensively in the literature by

White109.11o, who includes reference to a large number of patents, and others111.

There are basically three broad types of mixing machine, the internal mixer, the

external (open roll mill) mixer and the continuous internal mixer.

2.5.1.2.1 The Internal Mixer

Internal mixers can either be of the intermeshing or non-intermeshing type, both

types rely on high local shear stresses and a lower shear-rate stirring action. The

basic configurations of internal mixers are shown in Figure 2-12. The rotors travel in

opposite directions. The non-intermeshing type relies upon a shearing action

between the rotor and the mixing chamber wall; however there is a certain amount

of shear between the rotors. The wings of an inter-meshing internal mixer are

synchronised so that they come close to those on the other rotor but do not collide.

This action causes the necessary shear rates and stresses to facilitate mixing. The

majority of the mixing action occurs between the rotors rather than between the rotor

and the wall. It is difficult to conceive that anisotropy or molecular orientation could

be caused on anything other than a localised micro scale.

39

2.6.1.1.2 The External Mixer Extemal (or open roll) mixers generally consist of two rolls that are parallel and

horizontal. These too rotate in opposite directions causing the material to be pulled

through the clearance (nip or bite) between the rolls. The rolls are rotated at

different speeds to create a shearing action. The surface of the roll is used to

transport the material, through banding, back to the clearance where more mixing is

undertaken. The action of the rolls is essentially to cause uniaxial shearing of the

material and stretching of the rubber molecules in one direction only. It is easy to

visualise how this process would leave the compounded rubber in a state where

there is significant molecular orientation. This anisotropy is referred to as 'grain'.

2.6.1.1.3 The Continuous Mixer

Continuous mixers generally rely on the use a screw or screws to mix and transport

material in a similar way to that employed in conventional extruders. In some cases

these machines are a hybrid between the previously described intemal mixer and an

injection or extruder mechanism. In some machines the majority of the mixing is

carried out by an intemal mixer mechanism which dumps out directly into a screw

mechanism. Others are just screw mechanisms. The screws are often staged with

first part having coarse 'blade-like' flights resembling the rotors of an intemal batch

mixer before the geometry alters to resemble the more familiar screw. This then can

be used to feed a machine that converts the material into a useable form such as

sheet, strip or pellet. In some cases, forming through a die is the final shaping

operation. It is the flow near the exit that seems to be the cause of anisotropy that is

similar to that produced by injection moulding. The exit of the mixing chamber is

usually some type of former or preformer. The molecules get orientated in the

direction of flow through this forming process.

2.6.1.2 Milling and Calendering

A rubber calender is a machine that is used to produce rubber sheet, in various

profiles, widths and thicknesses. In some cases a calender can be used to

incorporate reinforcing materials such as fabric or wire. A calender resembles a mill,

but will be much stiffer to be able to obtain and control the required thickness of

40

material. It will consist of two or more rolls, each of the adjacent rolls rotating in

opposite directions. Rubber is passed between the rolls where the nip (clearance) is

set to squeeze the material into sheets of the desired thickness. The anisotropy

induced by milling and calendering is similar to that of open-roll mixing.

2.5.1.4 Anisotropy and Pre-moulding Processes

It is very difficult to see how grain or anisotropy could be induced into the rubber

structure in an intemal intermeshing type mixer112 because of the dynamics of the

mixing process. However, other types of mixing (open roll mixing because of its

similarity to milling and calendering and continuous mixing, because of this similarity

to the secondary mix-transport phase to injection moulding transports) could be a

source of anisotropy if the action of the mixing is to orient the polymer molecules in

a particular direction. Milling and calendering have long been known to cause

anisotropy. This can be kept to a minimum by using as high a temperature as the

material will stand without the onset of cross-linking ("setting up") 113.

2.5.2 Moulding Processes

Moulding has been defined as the act or process of shaping in or on a mould, or

anything cast in a mould. These moulded parts are successfully used in a wide

variety of consumer, industrial and engineering applications. Moulding is covered

reasonably in most polymer and rubber textbooks114.11s.116. Sommer117.118 and

White119 pay particular attention to rubber moulding and Wheelans12o.121 to injection

moulding of rubber. Moulding processes are covered here to the extent necessary to

highlight orientation and isotropic effects.

2.5.2.1 Moulding Methods

In this work only compression, transfer and injection moulding will be considered.

Casting, reaction injection moulding (RIM), blow moulding and bladder moulding are

not considered at the present time, although it is should be noted that these forms of

moulding are likely to suffer from anisotropic effects similar to those to be described.

41

2.5.2.1.1 Compression Moulding

Compression moulding is the simplest form of moulding and is what most people

think of when 'moulding' is mentioned. A charge of un-cured rubber is placed in a

mould cavity prior to closure of the mould. Usually a slight excess volume of material

is used to ensure that the mould cavity is completly full on mould closure. Figure 2-

13 shows a compression mould before and after closure. The rubber preform is

squeezed between the top and bottom parts of the mould. Excess rubber is

squeezed between the split line and is captured in the spew (or flash) groove. This

obviously means that there will be flow in the mould even if the billet is a preform

that is near in shape to that of the finished article. For many years moulders have

controlled the flow in moulds to some extent by intuitively shaping charges. Silva­

Neto, Fisher and Birley'22 used a finite difference approach to calculate pressure

and velocity fields and hence to determine optimum charge shapes for filling sheet

moulds.

r::"'lr-l'0~"""~"""::+,:,,,,,,"'1':"<:v' Top P I at e Slot

. ","VII v Plate

Figure 2-13 Schematic of typical compression moulding System

Compression moulds vary widely in terms of size, shape and number of cavities.

Obviously this depends on the size, shape of the product. Construction of the

moulds also varies. Two-plate moulds are common for thin and uncomplicated

geometry. Three or more plates are used for thicker and more complicated

geometry. The more complicated the geometry the more 'in-mould' flow will be

42

required and, generally, a greater excess volume of material will be required for

complete cavity filling (up to 50% excess can be needed for some complex parts).

Mould closure, generally, is attained with a large rigid press. It takes considerable

force to close a mould and force the material to the extremities of a cavity,

especially if it contains a high viscosity rubber. The most common types are the four

(tie-) bar (or four post) press and the side-frame (or side-plate) press.

2.5.2.1.2 Transfer Moulding

Transfer moulding is essentially injection moulding in a compression press. A typical

transfer moulding mechanism is shown by the schematic in Figure 2-14. The rubber

preform is heated by contact with the pot and the plunger. If sufficient force is

applied to the plunger in a press, as in compression moulding, the rubber is forced

to flow through the sprue into the mould cavity. A gap is deliberately left between the

transfer pot and the plunger, the size of which should be large enough to prevent

binding between the plunger and the pot and small enough to minimise the backflow

of rubber between the plunger and the side wall of the pot. The sprue is wider at the

transfer pot than at the mould cavity. Thus, when the flash pad is removed after

cure, the sprue breaks near the moulding and the majority of the sprue remains with

the flash pad. The diagram (Figure 2-14) shows a single cavity mould. However

multi-cavity mould configurations are also common.

Considerable flow is necessary to fill the mould and for the rubber is required to

pass through a relatively small orifice. Therefore there is a substantial increase in

the potential to induce molecular orientation in comparison with compression

moulding.

43

Plunger

Rubber Preform

.b,;>;"'-- Transfer Pot

~~~~~~~t-- S~rue ,,"-_ Cavity Plate

~~~~~~~-- Mould Cavity

""",....,--"<:""<"""""""""''''~_~ Piu n g er

.!t;.:>:*" __ Transfer Pot

Flash Pad

~~L_ Moulded Part

Figure 2-14 Schematic of transfer moulding set

2.5.2.1.3 Injection Moulding

Injection moulding is commonly used to obtain high production rates. The major

difference between injection moulding and compression or transfer moulding is that

compression and transfer moulding require a billet or preform to be placed directly

into a compression mould cavity or a transfer pot. An injection moulding machine

can be continually fed with granules or rubber strip. Injection moulding machines are

much more complex in terms of both geometry and machine control. An injection

moulding machine may have several temperature and pressure controls whereas

compression or transfer moulding systems normally do not. However, many

moulded parts are too complex to be made by compression moulding123• The clamp

(or press) is an integral part of an injection moulding system and must be capable of

providing high pressure clamping. The moulds need to be able to withstand high

temperatures, pressures and forces with very low levels of deformation during many

high speed closures. Figure 2-15 shows a simple injection moulding system. This

example has a screw-type injector but a ram or separated screw and ram systems

may be used.

44

Throat Nozzle Barrel Screw

Mould

Injection Chamber

Heating Fluid

Figure 2-15 Typical injection moulding system

The rubber is heated to its working temperature during its travel to the nozzle, by

heat transfer from the heated body of the ram and by work done on the rubber. The

rubber is forced through the nozzle and sprue into the mould cavity. The rubber will

become hotter as it passes through the nozzle and sprue due to the high shear

rates in the material. Hendrick and Fraser124 maintain that the physical properties of

injection moulded parts are better than those of compression moulded parts and

that, because the rubber enters a closed mould in a turbulent manner, strain lines

and stresses caused by flash or overflow are absent from injection moulded

products. As with transfer moulding, there will be considerable material flow during

mould filling. This has been investigated (for thermoplastic melts) by Spenser and

Gilmore125, White and Oee126 and Oda et al. 127, among others, but there are few

studies of rubber compound mould filling of injection machines save those of Isayev

and co-workers128•129

. There seem to be only two distinguishable mechanisms for

mould filling. In the first the melt emerges from the gate and fills the mould

progressively and evenly and in the second a 'jet' of material is projected from the

gate in filling the mould. Spenser and Gilmore, and White and Oee, observed mould

filling with special glass-windowed moulds and observed that small-diameter gates

and high injection rates could be associated with the phenomenon of jetting. A

similar jetting phenomenon was also reported by Lee, Griffith and Sommer130 for

polymer entering the cavity in transfer moulding. Oda et al. found that jetting was

related to the size of the extrudate melt entering the mould. If the extrudate swells to

less than the thickness of the mould after its exit from the gate (Le. it does not

45

contact the top and bottom of the mould) it would jet and this could also occur at low

injection rates. Oda et al. also found that by altering the geometry of the mould

entrance by placing barriers near the gate jetting could be completely eliminated.

With reference specifically to the injection moulding of rubber Deng and Isayev129

produced quantitative models for the simulation of flow dynamics, thermal history

and cure of rubber compounds in the mould. Their subsequent experimental work

for verification of the models revealed considerable anisotropy of the tensile

modulus of samples taken parallel with and normal to the direction of flow in the

mould.

2.5.2.2 Anisotropy in Moulding Processes

The is a reasonable body literature dealing with anisotropy in rubber moulding,

although it is not extensive when compared to the large body of literature which

exists relating to anisotropy in thermoplastics.

Gurney and Gough13\ Blow, Demirili and Southwart132, Chang, Yang and

Salovey133, Dinzburg and Bond134 and Hamed135 have studied anisotropy in

compression mOUlding. Lee, Griffith and Sommer136 and Dinzburg and Bond134 have

discussed transfer moulding and Lavebratt and Stenberg137,138,139,140,14\ Nakajima,

Fukata and Mineki142, Deng and Isayev129, Wheelans143 and Tsai144 injection

mOUlding. These works, and some other special cases, are reviewed in the following

sections.

2,5.2.2.1 Anisotropy in Compression Moulding

Anisotropy in rubber parts can be caused both in manufacture (of components) and

in the laboratory. This is highlighted by the tear-down test which is used to assess

the adhesion of a tyre tread to its base by measuring the force required to tear away

a strip of rubber tread from the tyre carcase. It was noticed that the force required

was different in opposite directions 131. Investigations using specially designed

compression moulds131 ,132,135, which force material in the mould to flow in a known

46

direction have shown anisotropy in swelling131.132.133, shrinkage132 and tensile

tests 132.134.135.

The experiments that were carried out in these studies were quite diverse. The

geometry of the mould cavities was generally simple and designed to produce sheet

from which test specimens could be cut. An exception was the study by Chang et al.

who used a more complex mould to investigate flow. Blow et al. experimented with

different billet placements in a circular mould to create different flow conditions (Le.

convergent, divergent and no flow) in a circular sheet mould. Anisotropy was noticed

in all of their flow conditions. The convergent flow condition showed significantly

more anisotropy than the other conditions indicating that there is probably greater

molecular alignment as the molecules converge in the centre of the mould. As

expected circumferential swelling is greater than radial swelling because the

molecules tend to be extended radially.

Hamed135 noted that secondary crack propagation occurred in different directions for

specimens cut parallel with the grain, indicating a directional failure mode. Gurney

and Gough showed that anisotropy was only evident in their samples when some

cross-linking had taken place and this, allied to the conclusion of Blow et al. that

peroxide cured samples show more anisotropy, is consistent with the idea that

anisotropy is due to residual stresses and molecular orientation that are 'fixed-in' by

curing. If it is not, the molecules can recover to a random isotropic configuration.

Chang et al. discovered that, depending on the geometry of the mould, flow does

not necessarily occur in straight lines. This means that at corners, or pins, where the

flow is curved, the local properties may have different directions from that of the

main body of the material in the moulding.

The experimental results generally concur with each other and with the in­

manufacture results; anisotropy occurs in the direction of flow.

47

2.5.2.2.2 Anisotropy in Transfer and Injection Moulding

Injection moulding of rubber products has been growing in popularity because of the

level of automation that it allows for high volume production runs 142.143. Transfer

moulding has been commonplace in the production of rubber products for much

longer but it requires a similar level of labour intensity to compression moulding and

the moulds are unwieldy. Transfer moulding is included in this section because of its

similarity to injection moulding with regard to flow, from a point just before the

material enters the mould cavity through to the completion of mould filling. It is worth

mentioning at this point that the transport and warm-up system on most injection

moulding machines resembles the screw pump type mechanism used on many

extrusion machines. There have been several investigations covering the extrusion

of elastomer or rubber compounds; these are reviewed by White145. Brzoskowski et

al. 146 used two different coloured rubbers to investigate the flow of rubber in a screw

pump and produced good evidence of a re-circulating flow in between the flights. An

important point is that the rubber remembers the orientation induced in the screw

channel. This orientation could persist into the mould cavity and affect the overall

anisotropy of the product, especially if the runner and gate system is short in length

and large in bore. The longer and narrower this screw-to-cavity feed system is, the

less likely the circulating orientation is to persist.

The majority of studies of injection moulding have used a centre-gated mould to

produce disks138.139.14o.141 or sheets142.143 from which test specimens could be cut.

Deng and Isayev129 used an end-gated dumbbell and side-gate disk and Nakajima

et al. 142 used a long, narrow, centre-gated 'snake-like' cavity as well as a sheet

cavity. Again, a number of different methods were used to highlight the anisotropic

effects, tensile properties, shrinkage, etc. All the parts in the injection and transfer

moulding studies showed considerable anisotropy. Injection speed and mould

temperature were also considered as significant factors, the rate of flow being a

variable that is easier to control with an injection machine than it is, say, with a

compression moulding press. Deng's and Isayev's rubber mOUldings were found to

be highly anisotropic and the anisotropy could be reduced significantly by using a

higher injection speed. They explained this by saying that higher stress relaxation

speed would be the cause. The current author is of the opinion that there may also

48

be an explanation of this phenomenon in 'jetting' which has been shown to occur at

high injection speeds in the moulding of thermoplastics125.126 and this is possibly why

Hendrick and Fraser124 reported better properties in rubber injection mouldings than

compression mouldings in the mid 1940's. Figure 2-16 (a) shows a mechanism of

regular ordered flow filling of the mould cavity with smooth flow fronts and (b) show

jetting which is random.

11 11 II

(a)

11 11 11

(b)

Figure 2-16 Mould filling mechanisms, (a) regular ordered flow, and (b) random irregular flow or 'jetting'

Lavebratt et 81.'s studies showed anisotropy in swelling and mechanical properties.

They also observed that the presence of carbon black increased the anisotropy. The

anisotropy was thought to be due to residual molecular orientation from the mould­

filling process. This filling however may not be simple filling flow of their disk cavity.

By delaminating the disks with a water jet-cutting technique, it was possible to carry

out further tests on the different layers. They concluded that the surfaces were

formed of radially oriented molecules produced by shear and fountain flow and that

the core orientation was the result of expanding flow147 giving tangential molecular

orientation.

49

Nakajima et al. showed that the shrinkage effects exhibited by the product from

their snake-like mould showed anisotropy that varied with distance from the gate,

reaching a peak and then tailing off gradually. This effect can easily be explained by

orientation. As the long mould was not completely filled before a no-flow situation

(flow stopped) was reached, the molecules elongate as they flow along the mould

away from the gate. In the mould, when the flow becomes static, the molecules at

the flow front (farthest from the gate) will start to recover. The molecules closest to

the gate will not have had time to extend and orient greatly in the direction of the

mould. The farther from the gate the greater the orientation of molecules at the peak

shrinkage point will be fully extended. Molecular recovery between this point and the

gate is prevented by molecular packing and, ultimately, cross-linking. The molecules

farthest from the gate will also have had the greatest recovery time.

Another problem caused by localised anisotropy, although it is not a direct measure,

in mouldings is backrinding or cut back148. This ragged indentation at the spilt line of

a mould is most likely caused by large escape flows at the mould split line caused

by mould closure pressure and thermal expansion orienting the molecules in the

direction of the escape flow. On release of mould pressure and cooling there is

greater retraction (shrinkage of these extended molecules) causing the material to

tear at that point. Control of this phenomenon is achieved by reducing the billet

volume or increasing the resistance to the escape flow in the design of the

mould149.15o, for example by using a plunger type mould.

2.5.3 Anisotropy in Special Cases

Anisotropy can be found in polymer networks produced by methods other than flow,

Hamed and Song151 have studied anisotropy induced by straining uncross-linked

rubber samples. The anisotropy is thought to be due to the rearrangement of the

molecules in the direction of strain and set due to slip rearrangement of molecular

entanglements. Theoretical models predicting the changes in properties of

elastomers cross-linked in states of flow (strain) have been produced by Berry,

Scanlan and Watson 152 and Flor/53 and these support the 'two network' scheme

proposed by Andrews, Tobolsky and Hanson154 in which there are two

interpenetrating networks, one formed in the unstrained state before flow and the

50

other in the strained state, during or after flow. Kramer, Carpenter, Ty and Ferry155

cross-linked strained samples near Tg (00) with y-radiation to minimise the

entanglement slippage in their investigation of entanglement networks. The samples

were mechanically anisotropic, but contrary to expectations the modulus was lower

parallel with the direction of stretch than normal to it. Similarly, linear swelling was

greater parallel with the direction of stretch than normal to it. The reason for this is

unclear, but could possibly be due to the difference between elastically effective

entanglements and chemical cross-links.

Alternatively Rigbi and Mark156 have shown that the anisotropy revealed by swelling

and stress-strain properties can be induced into rubber by incorporating a magnetic

filler and vulcanising in a magnetic field. The samples show slight increases of linear

elongation in the direction parallel to the magnetic field and marked differences in

stress in directions parallel and normal to the direction of the magnetic field. The

orientation of the filler in the magnetic field provided greater reinforcement in the

direction of the field, even though the particles were basically spherical.

Columnar discotic liquid crystals 157 are a special case because they are naturally

anisotropic. These materials are very highly oriented because of their molecular

shape. Results of swelling experiments on such a network are reported by Disch et

al. 158. These results are interesting because they mirror those of normal elastomers

in that there is less swelling in the direction of 'grain' (Le. the direction of the

director). Nematic polymers are also intrinsically anisotropic159.16o in character.

2.6 Summary and Comments on Anisotropy and Orientation in Rubber

Anisotropy (or 'grain') in rubber parts has been known about and its effects have

been worked around in the rubber industry for many years. However, suggestions

for control of anisotropy are vague. The term 'grain' seems to encompass

orientation, residual stresses, the anisotropy of shrinkage during cooling and other

mechanical effects. These effects vary directionally depending on the direction in

which the test is carried out, or show a significant directional component when

testing is multi-axial (e.g. immersion in solvent). It is significant to note at this point

that SS 903 Part A2: 1989161 (ASTM D_412162is a similar standard), says "Cut dumb-

51

bells, wherever possible parallel to the grain of the material unless grain effects are

to be studied, in which case cut a set of dumb-bells perpendicular to the grain", and

that the following standards all make mention of anisotropy or 'grain': BS 903 Part

A3 (ASTM 0-624), BS 903 Part A16 (ASTM 0-1460), BS 903 Part A51, ASTM 0-

378, ASTM 0-454, ASTM 0-518 and ASTM 0-797.

As described in the previous sections, anisotropy in moulded rubber can be

observed and measured in most aspects of its mechanical behaviour. Tensile

stress-stain behaviour, mould shrinkage and swelling due to the action of good

solvents seem to be the most popular methods. Tear strength and permanent set, in

tension or compression, also show differences due to grain or molecular orientation.

Lavebratt et al. 137 have shown that it is possible to use x-ray scattering to indirectly

assess the orientation of the rubber molecules in a test piece by studying the

orientation of zinc oxide crystallites which are often used in sulphur curing systems.

Tg and linear thermal expansion12.13 and electrical properties can also be used as a

measure of anisotropic behaviour9.3o.31.

Birefringence can be used to study the molecular orientation of polymer and

elastomer during forming and straining although this is of limited application to filled,

especially black filled polymers.

2.6.1 Causes of Anisotropy in Products and Mouldings

From this literature survey a few main points can be highlighted as the causes of

anisotropy in moulded rubber parts.

• Molecular orientation caused by flow in the mould seems to be the greatest

contributing factor to anisotropy131.132.133.134.135.138.140.

• Vulcanisation during flow (scorch) is an important factor giving anisotropy in

moulded parts 131.132.142.

• Anisotropic behaviour is due to the orientation rather than a difference in the

state of cure in various directions 144.

52

• The rate of vulcanisation is an important factor which determines the degree

of shrinkage anisotropy142.

• The presence of carbon black enhances anisotropy138,139.14o.141.143.

• Higher anisotropy observed in injection and transfer moulding than in

compression moulding134.142,143.

• Increased injection speed reduces anisotrop/29.

The dominating factor giving rise to anisotropy is molecular orientation occuring

during in-mould flow or mould filling; and the distribution of orientation direction is

due to a combination of different types of flow in thicker and (geometrically) more

complicated parts.

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112p. Whitaker, J. IRI, 4,153 (1970)

113M. M. Heywood, The Applied Science of Rubber Ch 5 Pt2 p342, W. J. S. Nauton Ed.(Edward Arnold, PLACE, 1961)

114B. G. Crowther, Rubber Technology and Manufacture 2nd ed. Ch8 pp340-346, C. M. Blow and C. Hepburn Eds., (Butterworth Scientific, London, 1982)

115J. G. Sommer, Basic Compounding and Processing of Rubber, H. Long Ed.Ch6 (Rubber Division, American Chemical Soc., Akron, 1985)

11BF. Rodriguez Principles of Polymer Systems: International Student Edition 2nd ed., Ch12 pp348-362, (McGraw-HiII, Tokyo, Japan 1983)

117J. G. Sommer, Rubber Chem. Technol., 51, 738 (1978)

11BJ. G. Sommer, Rubber Chem. Technol., 58, 662 (1985)

57

119J. L. White, Rubber Processing: Technology - Materials - Principles Ch20 pp519-535 (Hanser, Munich, 1995)

12°M. A. Wheelans, Injection Moulding of Rubber (Butterworth, London, 1974)

121 M. A. Wheelans, J. IRI, 4,160 (1970)

12'R. J. Silva-Neto, B. C. Fisher and A. W. Birley, Polym. Comp., 1,14 (1980)

123G. C. Hessney, Rubber Age, 70, 825 (1958)

124J. V. Hendrick and D. F. Fraser, Rubber Age, 56, 277, (1944)

125R. S. Spenser and G. D. Gilmore, J. Coil. Sci., 6, 118, (1951)

12BJ. L. White and H. B. Dee, Polym. Eng. Sci., 14, 212, (1974)

127K. Oda, J. L. White and E. S. Clark, Polym. Eng. Sci., 16,585, (1976)

12BM. Sobhanie, J. S. Deng and A. I. Isayev, Appl. Polym. Symp., 44, 115, (1989)

129J. S. Deng and A. I. Isayev, Rubber Chem. Technol., 64, 296 (1991)

130L. J. Lee, R. M. Griffith and J. G. Sommer, Polym. Eng. Sci., 24, 403, (1984)

131W . A. Gurney and V. E. Gough, Trans. IRI. 22, 132 (1946): Rubber Chem. Technol. 20, 863 (1947)

13'C. M. Blow, H. B. Demirili and D. W. Southwart, J. IRI. 8, 244 (1974): Rubber Chem. Technol. 48, 236 (1975)

133W. V. Chang, P. H. Yang and R. Salovey, Rubber Chem. Technol. 54, 449 (1981)

134B. N. Dinzburg and R. Bond, 1nl. Polym. Proc., 6, 3 (1991)

135G. R. Hamed, J. Appl. Polym. Sci., 27, 4081, (1982)

13BL. J. Lee, R. M. Griffith and J. G. Sommer, Polym. Eng. Sci., 24, 403 (1984)

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138H. Lavebratt and B. Stenberg, Plasl. Rubb. Comp. Proc. Appl., 20, 3 (1993)

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14'H. Lavebratt and B. Stenberg, Polym. Eng. Sci., 34, 913, (1994)

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143M. A. Wheelans, Rubber Chem. Technol. 51, 1023 (1978)

144B. C. Tsai, Rubber Chem. Technol. 51, 26 (1978)

145J. L. White, Rubber Processing: Technology - Materials - Principles Ch12 pp350-374 (Hanser, Munich, 1995)

14BR. Brzoskowski, J. L. White, F. C. Weissert, N. Nakajima and K. Min, Rubber Chem. Technol. 59, 634 (1986)

58

147W. Woebeken, Mod. Plas!., 40, 146 (1962)

146 J. G. Sommer in Basic Compounding and Processing of Rubber, H. Long Ed.Ch6 p 151 (Rubber Division, American Chemical Soc., Akron, 1985)

14"N. L. Catton, The Neoprenes: principles of compounding and processing Appendix 8 p199 (E. I Dupont de Nemours, Wilmington, 1953)

150E. L. Stangor, Rubber Age, 60, 439 (1947)

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160G. C. Verwey and M. Warner, Macromolecules, 30, 4196 (1997)

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59

Chapter Three

3. The FORM System

This chapter introduces the isotropic moulding system concept. It describes the

novel computer-controlled, flexible, automated compression moulding system that

was designed and built for the project; the individual components that make up the

system and their interaction. The control system, the sequence of operation of the

system and the overall system operational methodology are discussed after a brief

mention of conventional compression moulding.

Compression moulding of rubber parts is a highly labour intensive process, in

common with much of the rubber industry and has very little automation. This is

especially true for the manufacture of high precision parts such as seals for the

aerospace and oilfield sectors. Generally, for seals, a rubber chord is extruded, cut

to length and placed into the hot mould by hand. For less critical and generally

smaller seals, injection or injection-compression moulding is used to attain the high

volumes required but some of the precision is lost. These products often exhibit a

significant degree of anisotropy due to molecular orientation, mould design can be

difficult due to directional post demoulding shrinkage effects. In the service

environment seals are often exposed to solvents and components can exhibit

anisotropic swelling which may lead to a premature failure.

3.1 FORM System Concept - Overview

The isotropic moulding system (Appendix A), or FORM system, as it has come to be

known, consists of a preforming dispenser with interchangeable metering inserts, an

60

----------------

up-stroking compression press and a computer control system. These three items

coupled with a set of plunge moulds provide a flexible modular system that can be

used to produce isotropic or near isotropic mouldings from a wide range of

elastomers and, possibly, other polymeric materials.

As described previously in Chapter 2 the common phenomenon of anisotropy in

rubber mouldings is largely attributed to molecular orientation that is induced into the

material during the various processing stages and 'fixed-in' when the part is cross­

linked (cured). The combination of the dispenser, press and moulds is used to

minimise molecular orientation by reducing in-mould flow and allowing sufficient time

for molecular recovery before the onset of cross-linking. thus producing a random

molecular structure that will be substantially isotropic in nature and have consistent

properties. Additionally, automation of the compression moulding process

(eliminating labour and production processing steps), repeatability and the ability to

produce flash-free products are worthwhile but secondary advantages of the

process. The individual elements of the system and their function are described in

the following sections.

3.1.1 The Dispenser Concept

The objectives of the dispenser, a positive displacement valve, are several. The

primary function of the dispenser is as a preformer, repeatably producing a precise

and uniform preformed blank that is near both the shape and volume of the mould

cavity. The secondary functions are: to pre-work the material and break down any

residual molecular orientation ('grain' or primary orientation) from previous

production processes such as milling, calendering or extrusion, etc.; to deliver the

preform directly in to the open mould (Le. a material transport system); and, to pre­

warm the material.

Producing a blank that is already close to the required size and shape of the

finished product is considered the most important function of the dispenser because

the primary cause of anisotropy in moulded parts is trapped in molecular orientation

61

due to flow in the mould during forming1,2,3,4,5,6,7, a blank of the correct shape, size

and weight will reduce to a minimum the need for such in-mould flow.

Molecular orientation will initially be partially randomised on a macro scale by the act

of division that is required in feeding the bulk stock into the machine and, perhaps

more importantly, pre-working the material (Le. making it flow) will further break

down8,9,1o,11 any molecular orientation or intermolecular/interparticulate structure that

is present in the uncured stock from previous preparation processes.

Pre-warming the material will occur due to direct heat transfer from the body of the

dispenser, which is heated, and due to heat being generated by viscous flow

(intermolecular friction). The resulting temperature rise in the material will allow it to

be more easily processed and formed by reducing viscosity. It will also bring the

material closer to moulding temperature and reduce the 'in-mould' time.

(Conventional compression moulding is often carried out starting at ambient

temperature and the time to warm the material through can be significant). The

tendency of the molecules to recover due to increased macro-Brownian motion 12, 13

will increase.

The dispenser unit is mobile and can be moved into the press daylight to a position

directly above the mould cavity where the preforming operation (dispense cycle) is

initiated and a preform dispensed directly on to the lower half of the mould cavity.

The dispensing operation forms the blank by 'extruding' material into free space and

parts it off, thus removing the material from any constraint of the material-flow path

and allowing molecular recovery in the short time it takes the preform to fall onto the

hot mould, and before complete mould closure. Cross-linking should not take place

until the part has been completely formed.

3.1.2 The Press Concept

The press is used for the final shaping operation, closing the moulds which form the

part into its ultimate configuration. A rigid press is required to ensure that the platens

and the mating halves of the mould are parallel and do not flex. The platens need to

be capable of bringing the mould to up to the temperature that is required for curing

62

the elastomer compound. The press closure needs to be precisely controllable in

terms of position and the initial closure speed should be rapid to minimise the length

of time that the preform is resting on the bottom half of the hot mould and therefore

susceptible to scorch at the contact points. If cross-linking starts before shaping has

been accomplished, anisotropy will be increased14.15.16. The press will be required to

hold the mould closed under high pressure until the desired cure time is complete.

Certain defects (e.g. backrinding) can be prevented to some degree if the press has

a high clamp force and the mould and platens are stiff and do not move or deform

with the thermal expansion of the rubber17•18. This helps to minimise escape flows.

3.1.3 The Mould Concept

Plunge type moulds were selected for use as an integral part of the FORM system

because they offer considerable advantages over conventional two-plate moulds in

terms of both material handling and the production of isotropic and flash-free parts.

A recessed cavity in the lower half of the mould pre-Iocates the preform, effectively

increasing the target area into which the preform can be dropped from the

dispenser. A step around the cavity will, when the two halves are mated, help to

control the escape flow of excess material from the mould cavity by providing an

impeded flow path. This design also makes it possible to have a large contact

support area that does not get fouled by the excess material escaping and affecting

the dimensional integrity of the product.

It is also possible to produce flash-free components that require very little or no post

demoulding. This is not a new concept as methods have been proposed for flash­

free injection 19.20 and transfe~1 moulding. However, elimination of post-mould

trimming seems to have been accomplished in compression moulding by moving the

mould split line to 45° from horizontaI22.23. This is achieved by a combination of, first,

good mould design and careful manufacture and, second, a controlled mould

closure sequence. To achieve flash-free components it is necessary to have direct

metal-to-metal contact at the split line of the mould and to have even compression

covering the whole area of the mould and as near as possible truly parallel mould

closure (Appendix 8). To make possible metal-to-metal contact at the split line, the

mould was designed with a land 1 mm wide and 0·3 - 0·4mm in height adjacent to

63

both the inner and outer diameters of the upper mould cavity. The plunge has a

clearance, to impede escape flow, of between 0·25 - 0·3mm, opening to 0·75mm

and connecting a larger spew cavity. Further metal-to-metal contact is obtained over

a large contact support area out-board of the spew cavity.

3.1.4 The Com puter Control System

A computer system is used to control the operation of both the dispenser and the

press and is essential for the automation of the system because of the need for a

high degree of accuracy for position control, speed, timing and the sequencing of

the interaction between the dispenser and the press. The quality of the moulding

and the extent to which it is isotropic and flash-free depends ultimately on a

combination of these factors. In the future it is expected that the system could be

extended to log data for compliance with quality assurance standards such as SS

5750 and ISO 9000.

3.1.5 Sequence of Operation

The dispenser and press are controlled by the computer system to give the following

sequence of operations:

(i) Feed the dispenser with fresh material, this could be strip or cut sheet.

(ii) Pre-heat the fresh material to reservoir holding temperature (60-90°C) by

heat transfer and viscous work. The reservoir should provide sufficient

residence time to heat the fresh stock to the required temperature before it is

dispensed.

(iii) Open the press, demould products, clear spew.

(iv) Traverse the dispenser into the daylight between the platens.

(v) Dispense a metered preform and part into lower cavity of the plunge

mould.

64

(vi) Traverse the dispenser from between the press platens. Operation (i)

may be repeated as soon as the dispense operation (v) is completed.

(vii) Close press and cure part. This includes the phased closure to give

isotropic and flash-free components.

(viii) Repeat operations from (iii).

3.1.6 Reduction in the Number of Processing Operations Compared with

Conventional Compression Moulding

Conventional compression moulding usually requires as many as six processing and

storage steps of work in progress to reach the finished product. The FORM system

reduces the number of operations to three (Figure 3-1). Work in progress is

minimised by eliminating storage stages and combining preforming, forming and de­

flashing into the moulding operation.

Pre-Form -+ Store f-+ Mould -+ De-flash

Blank

(a)

Mix H Store H Mould

(b)

Figure 3-1 Stages in the manufacturing process of (a), conventional compression moulding and (b), Form system moulding.

3.1.7 System Configurations

The system is versatile and flexible because its modular nature allows the creation

of large number of configurations which are only limited by such factors as the

speed of the traverse between the various moulding stations, required material

65

residence time (for warming) and rate of cure (the in mould cure time which is

dependent on component size).

3.1.7.1 Stand-Alone Configuration (Single-Station)

The single-station configuration comprises one of each of the main components, a

dispenser (with a number of interchangeable preforming heads), a press (with a

number of interchangeable moulds) and a computer control system. The dispenser

feeds only one press (Figure 3-2) and is consequently idle for most of the time it

takes to cure the component. This is not the most flexible arrangement of the

system, because only one component type can be manufactured at anyone time,

although it may be possible to produce a range of like components. Nor does it

make the most efficient use of capital, but it does give a level of automation that is

not seen in compression moulding, although an increased pre-warming residence

time for the raw stock in the dispenser unit is a consequence of this particular

configuration. More efficient use of a dispenser unit is made in multi-station

systems.

f---- Dispenser Press I--

Figure 3-2 Stand-alone (single station) system configuration. A single dispenser feeds a single press.

3.1.7.2 Multi-Station Configuration

It is envisaged that the FORM system in a production/manufacturing environment

will be a flexible, fully automated, manufacturing cell consisting of a dispenser (or

66

dispensers) and a number of presses. Such a configuration will add a great deal of

flexibility to the system. One dispenser unit could feed as many as eight presses

traversing between them in sequence dispensing preforms directly into the mould of

each (Figure 3-3(a)). If the cure time of the component is equal or less than the time

to dispense preforms in all eight presses and return to the start point of the

sequence, then dispenser and press utilisation and cell productivity will approach

100% efficiency. Figure 3-3(b) shows another variation which is yet more flexible.

Two completely different products could be manufactured in this cell configuration.

In both configurations it would also be possible to remove any single press from the

dispenser cycle for maintenance, manual operation or product/mould proving trials

etc.

a a a: a 1 Dis~ensei

Press

a:··········;

a: a IOisp:ensel

a a,··········· a

a a a ~

-r ;r:s~ -1- ~i~~~~:f :: : ·1· -a (b) : .......••. : L..::J (a)

Figure 3-3 Two different possible manufacturing cell configurations: (a), single dispenser -multi-press configuration for medium to high volume production and, (b), multi-dispenser -multi-press configuration for volume and flexibility.

3.2 Prototype FORM System Description

The prototype FORM system used in the research reported in this thesis is a single­

station machine. It was constructed as a stand-alone system to prove the concept of

isotropic moulding. Unlike a production system, the dispenser and press are

inextricably linked.

67

3.2.1 Description of the Dispenser

The dispenser (Figure 3-4) consists of a material feed mechanism, the main body

carriage and interchangeable metering and preforming inserts. The feed mechanism

is, in this case, a double-acting hydraulic ram (050·8mm bore and 025·4mm rod)

and ('stuffer') piston (030mm in 030mm bore with tight clearance fit) which is used

to prime the system with polymer through the feed pocket and advance it through

the runner system of the dispenser to the meter cavity in the insert.

Dispense Actuator Ram (2 oil)

Heating and

Dispenser - Main Body (fixed)

Cooling Water ~~'\"_

Main Body - Outer (free)

Preform - Outer (free)

Insert Lock Ring

Dispenser - Preform Insert (fixed)

Material Feed Pocket

Feed 'Stuller' Piston

Feed Runners (6 oil)

"-''''+---+~-+-

'---Meter Cavity

Figure 3-4 Schematic section of the O-ring preform dispenser. The main body is shown in dark grey and the insert or 'head' in light grey. The meter cavity can clearly be seen formed between the fixed and 'free' (movable) parts.

The main body is a cast carcass with intemal voids for heating and cooling water, it

consists of two main parts which can be moved relative to one another and held at

any given position by means of two double acting hydraulic rams (063·5mm bore

025·0mm rod) and a continuously variable servovalve. The dispenser insert or

'head' is fitted into the main dispenser body. It, also, consists of two parts which can

68

be moved relative to each other. The meter cavity is formed in a void created

between the inner (fixed) and the outer (free) parts of the dispenser head. When the

inner and outer parts of the dispenser head are in the relative position shown in

Figure 3-4 and Figure 3-6 (a) and (b) the void has its maximum volume. The inner

and outer parts of the head are fixed to the inner and outer parts of the dispenser

main body, respectively. Heating and cooling of the insert is carried out by direct

contact with the main body which in turn takes heat from water recirculating through

a Churchill water heater.

Imagine that the system is filled with material and the meter cavity is full (Figure 3-6

(b)). The outer part of the dispenser head can move relative to the inner part which

remains stationary. The outer part can move up to give the displacement required to

shut off the feed runners and force a known volume of material out of the aperture

that is simultaneously opened at the bottom of the dispenser (Figure 3-5 (a) and

Figure 3-6 (c)). The outer part is then moved down to dispense and crop the preform

away from dispenser into the mould (Figure 3-5 (b) and Figure 3-6 (d)) before

returning to the initial position (Figure 3-4 and Figure 3-6 (a)) ready for re-filling the

meter cavity. The gap clearance at the opening of the point of the meter cavity is of

the order 0.05 - O.OBmm. This is generally small enough to prevent material from

escaping during filling. The whole dispenser unit is on a carriage that can be

traversed, by a hydraulic ram, back and forth into the daylight of the press for the

purpose of dispensing a preform directly into the hot mould.

69

~~~~-------------~--------~ (a)

I

(b)

Figure 3-5 Relative Movement of Dispenser Fixed Inner and Free Outer Component: (a), Typical dispense action (outer moved up); (b), Typical material purge position (outer moved down).

(a) (b)

(c) (d)

Figure 3-6 Dispense sequence for an O-ring preform: (a) the meter cavity filling; (b) when the meter cavity is full the narrow gap at the bottom is usually enough to prevent material escaping; (c) the outer part of the meter cavity (shown here on the right) moves upwards shutting off the filling flow and expelling the preform simultaneously; (d) the outer part is then moved down to crop preform away from dispenser, the preform then fall directly into the mould.

70

3.2.2 Description of the Press

The press is a four column (or (tie-)bar) upstroking hydraulic press. Four large

columns pass through both of the platen support plates, the upper plate being fixed.

The lower plate slides along the columns to meet the upper plate and provide good

guidance and aid parallel closure. Three hydraulic rams are used for press closure.

Two high-speed double acting jack rams (025mm bore and 018mm rod), are used

to effect near closure and opening and a large single acting main ram (0152-4mm

bore) is used for final high-pressure closure (i.e. forming and curing). The maximum

delivery from the hydraulic pump to feed the rams described in this, and the

previous, section is 23·65Mpa (~3400psi).

The platens are 400mm x 400mm. The effective moulding area is limited to the

300mm x 300mm central area. Each of the platens is heated by a 3·6kW resistance

heater. The maximum daylight between the platens (i.e. without moulds) is relatively

large at 500mm and can accommodate, with good clearance, the height of the

current dispenser, its inserts and moulds. This also allows room for modifications,

applications and adaptations.

3.2.3 Description of the Control System

The machine, as a stand-alone unit, has an integrated control system the backbone

of which is a GE-Fanuc 90-30 series programmable logic controller (PLC) and its

daughter sUb-systems. The PLC holds a sequential program that executes in a

repetitive manner until stopped by a command from the user, another device or

extemal event. The PLC program constantly scans the inputs from sensors,

actuators, limit switches etc. (reading input), and calculates solutions based on input

(decision making!) data. It sends output based on program logic (output update),

and checks for user input or override and CPU and subsystem diagnostics. One

complete cycle of these operations is often called a sweep. Each sweep will take in

instructions and information, act according to the logic of the program and send

output signals to the various elements of the system.

71

A personal computer runs a dedicated machine-control interface program which has

been written as part of this project, and is used to access PLC program and change

settings (i.e. temperature set points) and to upload/download position programming

sequences for the dispenser and the press. A hierarchical structure for the control

program was devised, with two levels of access, a 'user' level and a 'supervisor'

level. Each level is password protected. The user level (Figure 3-7) has limited

access and allows the operator only to load, program and run cycles and change

processing variables such as temperature set points and cure time. The supervisor

level (Figure 3-8) allows the operator to use all the functions from the operation of a

complete dispense and mould cycle to reading and writing individual control bits to

the PLC memory registers.

3.2.3.1 Temperature Control and Set Points

Precise temperature control of both the dispenser and platens is necessary for

successful and repeatable processing and moulding with the FORM system. The

dispenser is heated by recirculating water heated by a Churchill water heater and

the each platen is heated by a resistance heater. A set point is specified for each of

the three heater units.

(i) Dispenser Temperature Control

The control of the dispenser body temperature is devolved almost completely to the

Churchill unit. The PLC sends the set point to a Eurotherm controller. Temperature

is maintained within a ±1°C range, once set the point is attained, by water heating. If

temperature reduction is necessary, fresh cooling water is used to achieve rapid

active cooling. Such rapid cooling is used for dispenser cooling on shutdown to

increase the longevity of the material resident in the dispenser.

(ii) Platen (Press) Temperature Control

The platens are heated by resistance heaters that are controlled directly by a PID

(proportional/integral/derivative) closed-loop subroutine in the PLC. The controller

72

.., cl;" I: ~

CD W , .... 0 0

" I START-UP ~ ...

2- Start-Up Routine "lI ~

List alarms 0

"" ~ AI Temperature set and 3 " " up to temp indICator.

-..j c: w Cl> m ;;1:! r ~ ~ » " " CD .. .. :z: iD" ~ AI tl ". '<

Limited Function and Access (Level - 1)

1 OPEN 1

RECIPE

r I 1

I Ram 11 Dispenser 11 Pressures 1 Construct and Construct and edit ram edit dispenser (press) cycles. cycles.

Run and step Run and step through cycle through cycle

Including cure Including time and temp. dispenser

temp . Save and Restore Save and

Restore

No control of current pressure control (i.e. Register ROO11)

List and set the various pressures for the machine operations.

OPERATION

I I

1 Auto 11 Manual I Manual Run. Ex!. Dispense

Ram cycle Auto Run. Stuffer cycle

Stuffer in Temperature Stuffer out read and abort. Enable Disp.

axis control

Temperature read and abort.

"Y1 cC' I:

01 w clo o o " [ 'lI Cl ID iil 3 en c: 'lI m

~ en o ~

i" !!.

~ n CD

= ::t iD' Ol Cl ::r '<

START-UP

Start-Up Routine Ust alarms

Temperature set and up to temp indicator.

Setting

Mould Calibration

Full Function and Access (Level - 2)

RECIPE 1------------1 OPERATION

Registers

AI-Read R - Read R - Write Q - Read Q - Write I - Read

On-line list of thecommon register variables.

Ram Dispenser

Construct and Construct and edit ram edit dispenser (press) cycles. cycles.

Run and step Run and step through cycle through cycle

Including cure Including time and temp. dispenser

temp. Save and Restore Save and

RF!~tnrp.

Current pressure control (i. e. Register Roo11) should have hot-key instant access.

Pressures Auto Manual

Ust and set the Manual Run Int. Dispense various Ext Dispense pressures for Auto Run. Move disp. in the machine Move disp out operations. Temperature Ram cycle

read and abort. Stuffer cycle Stuffer in

Hotkey access Stuffer out to pressure(R Disp. Zero 11). (SoU)

Disp Max (Top) Enable Disp. axis control

Hotkey access to pressure(R 11 ).

Temperature read and abort.

parameters are recalculated every 0·01 s. An iron-constantan thermocouple

embedded in each platen supplies the measured temperature input for each

calculation and the output power variable necessary to maintain temperature is

altered accordingly. Temperature is maintained to ± 3°C (from observation) however

further tuning could reduce this if necessary.

3.2.3.2 Hydraulic Pressure Control

Hydraulic line pressure is maintained by direct proportional control of power to the

hydraulic pump. Line pressure is maintained to a current value set in the pressure

current-value register in the PLC memory. Set points are copied by the PLC to the

current-value register during the program sweep. Hydraulic fluid is continually

displaced and excess is dumped back to the storage tank via non-return relief

valves.

3.2.3.3 Position Control

The position control of both the dispenser and the press uses comparative-relation

function loops. The PLCs axis position module (APM) carries out an initial

comparative calculation based on the current position and the required set-point to

determine the direction of travel. Then the APM simply compares the set position

value with the current position value and drives toward the set value, sampling at

0.01 s intervals until the values are equal or greater than (or less than, depending on

the direction of travel) the set-point value. Both the dispenser and the press have a

Moire-fringe linear displacement encoder attached to provide position information.

(i) Dispenser Position Control

Control of the dispenser position is accomplished with a servovalve which ensures

accurate control and maintenance of the set-point position. The encoder has 7000

divisions for a maximum stroke length of 38mm (0·0054mm per division

(calculated)). Overshoot of set position was recorded at a maximum of 5 divisions

(0·027mm).

75

(ii) Press Position Control

Position control of the press closure is carried out by the APM, as accurate

maintenance of position does not have to be attained for long time periods,

excepting during cure when position control is effected by the natural stop that

mould closure provides. The encoder on the press has 85000 divisions over a

possible stroke length of 500mm (0·0059mm per division (calculated)). Overshoot of

the set position was recorded at a maximum of 10 divisions (0·059mm).

3.3 System Operation

3.3.1 Programming the Dispenser and Press Operation Cycles

The moulding system relies on the PLC to effect the production of mouldings. The

sequential operation of the dispenser in producing and parting-off the preform and

delivering it to the mould cavity and the phased closure of the mould are essential

processes in the moulding of isotropic components as well as in the production of

flash-free mouldings that require no, or little, finishing.

The inputs required from the operator are operation pressure, temperature and

position set points for the motion control of the dispenser and press. The structure

of the control program separates these inputs from each other, each having its own

menu.

3.3.1.1 Temperature and Pressure Setting

Set points for temperature and pressure are simply entered into the control program

via a menu option for each. The three temperatures (dispenser and upper and lower

platen) are set and, usually, not changed unless or until another change is made

(e.g. material or product change).

The pressure is defined in a similar way but there are more variables. Each ram or

set of rams has to have a defined working pressure. The value is stored in memory

and then copied to the current-pressure register at the beginning of the PLC

76

subroutine function call. This can be overridden for some functions and set manually

by directly writing a value to the current-pressure register.

3.3.1.2 Dispenser Cycle

The dispenser motion-control cycle consists of seven position set points and their

corresponding dwell times. The values that must be entered for position are in the

range 0 - 7000 (0 displacement is the lowest point the dispenser can reach and

7000, corresponds to the maximum dispenser stroke (Le. maximum preform

volume)), corresponding to the resolution of the Moire-fringe type linear encoder.

The dispenser cycle 'program' is entered into the PLC via the PC interface program.

A typical set of dispenser position control values are given in Table 3-1. This

represents a single crop-dispense motion. Positions 4, 5 and 6 are redundant. The

dwell times are used sparingly in this cycle; 1s for Dwell 2 is used to ensure that

enough material is expelled and the slight delay of 0·5s (Dwell 3) at the lowest

position set point (Position 3) is to aid cropping.

Dispenser Position Value and Dwell Register (encoder divisions/

dwell Cs» Position 1 3450 Dwell 1 0 Position 2 5850 Dwell 2 1 Position 3 200 Dwell 3 0·5 Position 4 3600 Dwell 4 0 Position 5 3600 Dwell 5 0 Position 5 3600 Dwell 6 0 Position 7 3450 Dwell 7 0

Table 3-1 Typical set point values for an O-ring dispense cycle. The first and last positions (Position 1 and 7) are set at the point of maximum cavity volume (for ease of operation only). Position 2 is the main dispense ('extrusion') stroke and Position 3 is the crop. Positions 4, 5, and 6 are set at a point above flush to aid parting although this is often not necessary.

77

----------

3.3.1.3 Press Cycle

The press motion-control cycle is very similar to the dispenser control cycle. Again, it

consists of seven position set points and corresponding dwell times. However, there

is one extra dwell, cure time, that is not strictly part of the position-control sequence

but was included with the press sequence for ease of operation. The press

displacement is also set by encoder value, in this case the range is 0 - 85000. The

full press stroke can only be obtained without moulds fitted. To determine and set

the maximum allowed stroke a separate operation has to be undertaken and the

maximum encoder value automatically read and stored by the PLC. This must not

be exceeded.

A press cycle usually consists of a complete forming closure and a single' breathe'

step followed by full closure and cure. The cure time dwell does not require a

position setting as the press will close to maximum allowed displacement under the

set pressure for the specified time immediately on completion of the press cycle.

Press Position and Value Dwell Register (encoder divisions/

dwell (s» Cure Time (s) 600 Position 1 82586 Dwell 1 0 Position 2 82570 Dwell 2 6 Position 3 82586 Dwell 3 0·5 Position 4 82586 Dwell 4 0 Position 5 82586 Dwell 5 0 Position 5 82586 Dwell 6 0 Position 7 82586 Dwell 7 0

Table 3-2 Typical press cycle for an o-ring. A single 'breathe' or 'bump off step is included (Position 2) and the mould is parted for a period of 6 seconds. The following positions are redundant but could be used if necessary for further 'bumping off or in special cases the full closure step could be shifted to, say, Position 4 and the preceding steps used a careful approach to initial full closure.

78

---- -----------

3.4 References

'w. A. Gurney and V. E. Gough, Trans. IRI. 22,132 (1946): RubberChem. Technol. 20, 863 (1947)

2C. M. Blow, H. B. Demirili and D. W. Southwart, J. IRI. 8, 244 (1974): Rubber Chem. Technol. 48, 236 (1975)

3W . V. Chang, P. H. Yang and R. Salovey, Rubber Chem. Technol. 54, 449 (1981)

4B. N. Dinzburg and R. Bond, Int. Polym. Proc., 6, 3 (1991)

5G. R. Hamed, J. Appl. Polym. Sci., 27, 4081, (1982)

6H. Lavebratt and B. Stenberg, Plast. Rubb. Comp. Proc. Appl., 20, 3 (1993)

7H. Lavebratt and B. Stenberg, Polym. Eng. Sci., 34, 905, (1994)

Bp. Stamberger, Kolloid Z., 42, 295 (1928)

9A. R. Payne, 2nd Int. Rubb. Symp., London, 11·13 Qct. 1960; Rubb. Plast. Age, 42, 963 (1961)

,oA. R. Payne, J. Appl. Polym. Sci., 9, 2273, (1965); Rubber Chem. Technol., 39, 365 (1966)

"C.M. Blow, IRI Trans., 5, 417 (1929)

'"w. Holmann, Rubber Technology Handbook, Ch4 (4.2.1), p222 (Carl Hanser Verlag, Munich, 1989)

"c. Hall in Polymer Materials: An Introduction lor Technologists and Scientists 2nd ed., Ch2 (2.9) pp49·50 (MaCmillan Education Ltd., Basingstoke, 1989)

"K. Qda, J. L. White and E. S. Clarke, Polym. Eng. Sci., 18,53, (1978)

"w. Dietz, J. L. White and E. S. Clarke, Polym. Eng. Sci., 18, 273, (1978)

,sA. I. Isayev, Polym. Eng. Sci., 23, 271, (1983)

"N. L. Catton, The Neoprenes: principles 01 compounding and processing Appendix 8 p199 (E. I Dupont de Nemours, Wilmington, 1953)

'BE. L. Stangor, Rubber Age, 60, 439 (1947)

'9H. F. Jurgeleit, Rubber Age, 90,763 (1962)

2°H. F. Jurgeleit, British Patent, 1022084 (1964)

21H. G. Gilette, Rubber World, 157, 1, 67 (1967)

22T . A. Harris and J. Lucas Ltd., British Patent, 654509 (1948)

23 J. E. Collins, British Patent, 759666 (1954)

79

Chapter Four

4. Experimental

In this chapter the materials, equipment and methods used in the evaluation, the

Finite Element (FE) modelling, and the determination of operating procedures for the

FORM system prototype are described in detail.

4.1 Rubber Compounds

Seven compounds were used in the determination of the FORM machine operating

procedure, process evaluation, and optimisation trials. Four, labelled NR, SBR1,

SBR2 and NBR (or NBR ASTM) throughout, were produced in-house and three,

labelled PB80, EOl (or Elast-0-Lion/85) and FR58 (or FR58/90) were samples of

commercially produced material kindly donated by James Walker & Co. Ltd. a

manufacturer of fluid seals.

Three of the in-house compounds, NR, SBR1 and SBR2, were selected because they

would exhibit a range of processing properties (e.g. viscosity, elasticity etc.). The

fourth, NBR, was based on the sulphur cured NBR I compound in ASTM D 2934 -

891. This was chosen to behave like a material used in the production of fluid seals.

The formulation of each of the in-house compounds is given in Table 4-1.

The exact formulation of the three commercial compounds was not divulged to the

author as this information was deemed to be commercially sensitive and therefore

only the polymer types are known. PB80 and Elast-o-Lion are NBR and H-NBR

(hydrogenated nitrile) compounds respectively and the FR58/90 is FKM (fluorocarbon

elastomer).

80

Ingredient NR SBR1 SBR2 NBRASTM (phr) (g) (phr) (0) (phr) (g) (phr) (0)

NR (SMR 10) 100 2608 - - - - - -SBR (1502) - - 100 1871 100 2496 - -NBR(45·5% ACN) - - - - - - 100 1895 CB - N330 20 522 60 1122 - - - -CB - N660 - - - - 40 998 609 1137 Stearic Acid 2 52 2 37 1 25 0·5 9 Zinc Oxide (ZnO) 5 130 5 94 3 75 5 95 Sulphur (S) 1·5 39 1·8 34 2 50 0·5 9 OPG - - 0·2 4 - - - -CBS 1 26 1·2 22 1 25 1 19 TMTD - - - - - - 2 38 IPPO 2 52 2 37 - - - -Flectol - H - - - - - - 2 38 OOP - - - - - - 5 95

Table 4-1 Formulations of the four in-house compounds produced for use in evaluating the Form system with the weight of each ingredient corresponding to the mixer fill factor.

4.1.1 The Raw Polymers

All the polymers used in the trials were standard grades.

(i) Natural Rubber (NR):

SMR 10 is Standard Malaysian Rubber with 0·1 % (by wt.) maximum dirt content after

straining2•3

. The SMR 10 used in the trials was labelled as from Dynamic Plantation

BHD. Malaysia (obtained from the MRPRA).

(ii) Styrene-butadiene rubber (SBR)

SBR 1502 (INTOL 1502) is a cold-polymerised non-pigmented rUbber4 as classified

by the International Institute of Synthetic Rubber Producers (IISRP). INTOL is

manufactured by Enichem and supplied by Enichem Elastomers Ltd., Southampton.

§ ASTM D2934 - 89 calls for the use of N539 carbon black which is a fast extrusion furnace black with low structure (FEF - LS). However this was not easily available and for this work and N660 a general purpose furnace (GPF) black was substituted in its place.

81

----- ----------

(iii) Acrylonitrile Butadiene Rubber (NBR or Nitrile Rubber)

The NBR (a Nipol N) used in the trial had an acrylonitrile content of 46% and was

manufactured by Nippon Zeon and supplied by Zeon Chemicals Europe Ltd., South

Glamorgan.

4.1.2 The Filler - Carbon Black

Two carbon blacks, Vulcan 3 and Sterling V, which are N330 and N660 respectively,

as classified in ASTM D 1765·96a5, were used in the compounds for the trials, both

were manufactured by Cabot Corporation and sourced from Cabot Carbon Ltd.,

Manchester.

The indicators of reinforcing potential and the processing behaviour, particle size6,7

(Figure 2-8) and surface area, which are obviously related, and structure (or

bulkiness) are given in Table 4-2.

Carbon Slack Particle size (nm) Surface Area (m'/g) Structure (ml/100g) Designation CATS· DSPA9

N330 29 83 102 N660 50 35 90

Table 4-2 Indicators of reinforcement and processability for N330 and N660 grades of carbon black

4.1,3 The Additives

The additives used in the rubber compounds for the trials are detailed in the following

sections, they are broadly classified by function.

4,1,3,1 Activator and Processing Aids

(i) Zinc Oxide (ZnO)

A rubber industry standard zinc oxide, supplied by BD Technical Polymer Ltd" COrby,

Northamptonshire, was used as a vulcanisation activator enabling the vulcanisation

accelerator to reach its full potential. Cross-linking efficiency can be increased by

60%.

82

(ii) Stearic Acid

BD Technical Polymer Ltd., also supplied a rubber industry standard stearic acid

which was used in the compounds as: (a) processing aid facilitating the dispersion of

filler and smooth processing, and (b) as a secondary activator increasing still further

the effect of the zinc oxide.

4.1.3.2 Curatives

(i) Sulphur (8)

In the early batches a rubber industry standard ground crystalline sulphur supplied by

Anchor Chemical (UK) Ltd. was been used. Later batches used Hays - 120 mesh

sulphur with 2·5% Mg coating, supplied by Schill and Seilacher (UK) Ltd., as the

curing agent.

(ii) Accelerators

(a) DPG

DPG or Diphenyl guanidine is a vulcanisation accelerator. DPG manufactured by

Monsanto Chemicals Ltd. and in later batches, Perkacit-DPG manufactured by

Flexsys Rubber Chemicals Ltd. was used in the trial batches of SBR1.

(b) CBS

Santocure (Monsanto Chemicals Ltd.) and later, Santocure (Flexsys Rubber

Chemicals Ltd.) (N-Cyclohexyl-2-benzothiazole sulphenamide (CBS)) was used as a

vulcanisation accelerator in all the trial batches.

(c) TMTD

The accelerator (and sulphur donor) tetramethylthiuram disulphide, under the trade

names of Thiurad (Monsanto Chemicals Ltd.) and Perkacit TMTD (Flexsys Rubber

Chemicals Ltd.), was used in the trial compound NBR.

All of the accelerators ((a), (b) and (c)) were supplied by supplied by Flexsys Rubber

Chemicals Ltd. (Formerly Monsanto Chemicals Ltd.), Wrexham.

83

4.1.3.3 Antidegradants

(i) IPPD

Santoflex IP (Monsanto Chemicals Ltd.) and Santoflex IPPD (Flexsys Rubber

Chemicals Ltd.) (N-Isopropyl-N'-phenyl p-phenylene diamine) was used in NR and

SBR1 compounds as an antioxidant.

(ii) TMO

Flectol H (Monsanto Chemicals Ltd.) and Flectol TMO (Flexsys Rubber Chemicals

Ltd.) is an antioxidant, the chemical name is 2,2,4-Trimethyle-1,2-dihydroquinoline

(polymerised) .

The antidegradants were both supplied by Flexsys Rubber Chemicals Ltd.

4.1.3.4 Plasticiser

DOP

Dioctyl phthalate (DOP) under the name Jayflex DOP (from Exxon Chemicals) is a

plasticiser. The source of supply for the DOP is unknown.

4.1.4 Material Preparation

4.1.4.1 Mixing Equipment

All batches of rubber compound were mixed in an automated, computer-controlled

Francis Shaw K1 (Intermix Mk. 4) internal mixer. The mixer has a chamber consisting

of two 'Siamesed' cylinders through each of which a rotor with wings (or nogs) passes

axially. The rotors are positioned and synchronised through a gearbox in such a way

that the wings intermesh and pass close to one another but do not contact, producing

rates of high shear in the material drawn between them. The configuration of the

internal mixer is shown in Figure 4-1. The rotor speed can be varied up to a maximum

of 150rpm. The capacity of the mixing chamber is 5·5 litres. Fill factors in the order of

0·5 - 0·6 giving a batch volume of 2·75 - 3·3 litres were chosen, from experience, for

efficient mixing. The temperature of the mixer is controlled by re-circulating water from

two Conair Churchill heat exchanger units unit. During mixing the chamber is sealed

84

and the discharge door is kept closed. A pneumatic ram under a pressure of 300kPa

(3 bar) is used to maintain pressure on the material being mixed. The filler is pre­

weighed into a hopper and fed via fluidised air slide, oil can be injected directly into

the mixing chamber and the rubber and minor ingredients are added via conveyor

feed.

4.1.4.2 Mixing Procedure and Conditions

The procedure used for mixing batches of each of the various compounds was

similar, except that the NBR which was mixed in two stages. The required amount of

the polymer, filler and rubber chemicals were carefully weighed out. A warm-up batch

was mixed, dumped and discarded, if starting from cold. The polymer, cut into uniform

pieces approximately 50 x 50 x 50mm was added to the mixer with the activators,

process aids and anti-degradants and masticated for a time before the carbon black

was added automatically from a feed hopper. A further period of time was allowed for

the carbon black to be incorporated and dispersed before the addition of the

curatives. The compound was then mixed for a pre-determined time and dumped

from the mixing chamber automatically. The compound was then sheeted out on a

two-roll mill, heated to 50a C, with a nip of 3 - 5mm, and allowed to cool. A period of at

least 24hrs from mixing was allowed to elapse before the material was used for

further operations.

The NBR compound was prepared in two stages, a masterbatch containing the

polymer, filler, activators, anti-degradants and plasticiser was mixed, sheeted and

allowed to cool for 24hrs before the curatives were added in a second mixing

85

-- ----- -------------

./"--------Feed Hopper

.,-------Ram

.,-_____ Mixing Chamber

,..--:;:::::===== Intermeshing· ... Rotors

Discharge ~---"dump" Door

Figure 4-1 Schematic Cross Section of a Francis Shaw Intermix Intermeshing Rotor Internal Mixer'·

86

procedure. The compound formulations and weights used (corresponding to the

respective fill factors) for the batches are given in Table 4-2. The mixing cycles and

conditions for all of the in-house compounds are detailed below.

(i) Compound NR

The mixing cycle and mixing conditions used for the production of the compound NR

are given below in Table 4-3 and Table 4-4 respectively.

InQredient(s)/Operation Time (sec.) NR, ZnO, Stearic Acid and IPPD 0 Carbon Black (N330) 180 S, CBS and DPG 240 DischarQe 330

Table 4-3 The mixing cycle for the compound NR in the Francis Shaw K1 Intermix

Mixing Parameter Parameter value Mixer Temperature 30°C Mixer Rotor Speed 30 rpm Fill Factor 0·6

Table 4-4 The mixing conditions for compound NR in the Francis Shaw K1 Intermix

(ii) Compound SBR1

The mixing cycle and mixing conditions for compound SBR1 are given in Table 4-5

and Table 4-6 respectively.

InQredient(s)/Operation Time (sec.) SBR(1502), ZnO, Stearic Acid and IPPD 0

Carbon Black _(N330) 90 S, CBS and DPG 330 Discharge 420

Table 4-5 The mixing cycle for compound SBR1 in the Francis Shaw K1 Intermix

87

Mixing Parameter Parameter value Mixer Temperature 40°C Mixer Rotor Speed 45 rpm Fill Factor 0·5

Table 4-6 The mixing conditions for compound SBR1 in the Francis Shaw K1 Intermix

(iii) Compound SBR2

The mixing cycle and conditions for compound SBR2 are given in Table 4-7 and

Table 4-8 respectively.

Ingredient(s)IOperation Time (sec.) SBR(1502), ZnO and Stearic Acid 0 Carbon Black (N660) 90 Sand CBS 210 Discharqe 300

Table 4-7 The mixing cycle for SBR2 compound in the Francis Shaw K1 Intermix

Mixing Parameter Parameter value Mixer Temperature 30°C Mixer Rotor Speed 30 rpm Fill Factor 0·6

Table 4-8 The mixing conditions for SBR2 compound in the Francis Shaw K1 Intermix

(iv) Compound NBR

The NBR formulation, based on ASTM D 2934 - 89\ was mixed in two stages. The

mixing cycles for each of the mixing stages are given below in Table 4-9. The mixing

conditions used for both the masterbatch (stage 1) and the final mixing cycle (stage 2)

are given in Table 4-10.

88

I nqredient( s )/Operation Time (sec.) Stage 1 (Masterbatching)

NBR (NIPOL N (46%)), ZnO, 0 Stearic Acid, TMQ and DOP

Carbon Black (N660) 120 Discharge 180

Stage 2 (Final mix) Masterbatch 0 Sand CBS 60 Discharqe 90

Table 4-9 The mixing cycles (both stages) for NBR compound in the Francis Shaw K1 Intermix

Mixing Parameter Parameter value Mixer Tem~erature 30°C Mixer Rotor Speed 45 rpm Fill Factor 0·6

Table 4-10 The mixing conditions for both masterbatching and final mix of NBR compound in the Francis Shaw K1 Intermix

4.2 Material Properties

The measurement of the material properties for all the compounds used in the trials

was undertaken using a variety of equipment. Rheological measurements were made

using a Negretti TMS Biconical Rheometer. Moulding conditions, scorch and cure

times, were determined with a Wallace-Shawbury Precision Cure Analyser (PCA) and

the density of the materials was determined with a standard laboratory balance.

4.2.1 Determination of Rheological Properties

It was necessary to understand the flow behaviour of the materials in order to be able

to model the FORM system, for predictive (design) and verification purposes, using

finite element analysis (FEA). The rheological properties of the trial compounds were,

therefore, studied using a Negretti TMS Biconical Rheometer. The values produced

provided material data inputs for the FEA models.

89

- - - - -------

4.2.1.1 Negretti TMS Biconical Rheometer11,12

The Negretti TMS Biconical Rheometer has a similar configuration to that of the more

well known Mooney shearing disc or rotational viscometer. In both viscosity is

measured by applying a strain to the sample and measuring stress. However, the

TMS differs from the Mooney in the design of the rotor (disc). The TMS utilises a

biconical rotor (Figure 4-2) to give constant shear, but in a Mooney the shear rate

varies across the radius from zero at the centre to a maximum at the periphery13.

Another departure from the traditional Mooney design is that the rubber compound is

injected into a closed rotor chamber, after a suitable warm-up period, via a transfer

mechanism (Figure 4-3). This eliminates the formation of flash or spew that would,

effectively, alter the test chamber geometry, which is an important factor in constant

shear rate systems. The biconical design also means that the shear rate is equal to

the speed (rpm) because the cone angle has been selected to be 6°. The shear

stress is directly related to the applied torque and K, the constant of proportionality.

The apparent viscosity is, therefore, simply K x torque/rpm at any given speed (rpm).

22.5 mm

a.5m

6· I 2mm : m II .., r-

..... ;......0 .... - .... - ... ( r;" 'J ~m

1.5mm

Figure 4-2 TMS Rotor Dimensions"

90

- - - -- - - - ---------

Piston

Testing Cavity

Figure 4-3 Schematic of the TMS Transfer Pot, Rotor and Rotor Cavity Configuration

4.2.1.2 Measuring Viscous Flow in the TMS - Test Procedure

The test procedure used to discover the steady-state flow properties for all samples is

a standard, seven step test used in the Rubber Process and Engineering Centre

(RuPEC) Laboratory at Loughborough. The rheometer controller requires

temperatures to be set for the ram (piston), the upper die (transfer chamber) and the

lower die (rotor cavity). Also required are the time to pre-warm the sample before

injection into the cavity, the fill time, the test mode, the sampling rate and number of

steps (where each step consists of a shear rate and a duration for which for which it is

applied). A number of runs was carried out for each specimen material, using the

following test conditions (Table 4-11).

91

Parameter/set-pointlRate Value/option Temperature (upper and lower dies and ram (piston) 100°C Pre-heat Time 240 s Fill-time 120 s Pre-cure delay 10 s Shear Rates VarvinCl SamplinCl Rate 5 readinCls/s No of Steps 7

Step Number Step Duration (s) Shear Rate (s-') Mode of test 1 10 0·1 continuous 2 15 0·4 continuous 3 20 1 continuous 4 15 4 continuous 5 10 10 continuous 6 5 40 continuous 7 10 100 continuous

Table 4-11 Parameters and Settings for the TMS Rheometer Test

Once the test parameters have been entered and the machine has reached the set

operating temperature approximately 209 of compound are placed in the transfer

chamber and the machine set to run. The piston closes down to compact the sample

and ensure even heating of the transfer chamber.

The duration of each step is long enough for a steady state to be reached. This was

checked for each sample by examining the raw data on the personal computer which

acts an interface and data logger.

4.2.2 Determination of Scorch Safety and Cure Times.

A Wallace-Shawbury Precision Cure Analyser (PCA) was used to determine the

scorch safety time (or scorch time) and the cure time of all of the materials used in the

trials. The former time is the time a material can exposed to heating before the onset

of crOSS-linking, and the latter is the time taken to reach a given degree of cure (Le.

form the required number of cross-links). These data were use to determine the

moulding conditions, time and temperature, of the moulding trials.

92

~ - ---------

4.2.2.1 Equipment - The Wallace-Shawbury Precision Cure Analyser (PCA)

The Wallace-Shawbury PCA (or Curometer) is a 'rotorless' curemeter in which near­

isothermal conditions are obtained by heating a small specimen rapidly. This makes

the material heat-up time negligible in comparison to the cure time. The temperature

at which measurements can be taken can be set at any value up to a maximum of

300°C. The sample is compressed between an upper plunger (male) containing a

torque transducer and a matching lower (female) cup which oscillates at a constant

frequency of 1·7 HZ15.

~ __ Plunger Measuring Torque

~--Gap 1mm approx.

'----Oscilating Cup (1·7Hz)

Figure 44 Wallace-Shawbury peA cavity configuration 16

The system has a Zylog Z80A microprocessor at its heart, which can monitor such

parameters as displacement amplitude and frequency, time and force in addition to

torque. Data is logged continually and torque-time curves can be plotted either during

(near real time) or after the test is complete. The display on the machine can show

instantaneous real time data. The system, once a run is complete, can scale the data

and produce a plot of cure (%) verses time and calculate the time taken to reach a

number of pre-set cure values.

93

4.2.2.2 Method - Determination of Scorch and Cure Times. with the PCA

The parameters for a cure test are entered through a numeric/function keypad. These

are test temperature, required output, C1 (Chart 1 - Torque versus Time) and/or C2

(Chart 2 - Cure(%) versus Time), strain, test duration and the cure (%) points of

interest. The PCA is then left until it has reached the test temperature. 1·5 - 3g of

material are placed in the female cavity of the lower former and the test started.

Plots of both C1 and C2 were generally obtained for each test, the test set points

were standardised at 5%, 50%, 95% and 100% (or t5, t5o, t95 and t100). t95 (Le. 195 time

to form 95% of cross-links or time to reach 95% of measured torque) is often used to

determine an acceptable cure time or 'best technical cure'. Cure times at t95 were

determined from plots of at least three consecutive runs. Compounds were frequently

retested, should they have been left for more that a month without being used.

Tests were carried out at 150°C and 160°C for all of the trial compounds and also at

185°C for the three commercial compounds (PB80, E-o-L and FR58/90) from James

Walker & Co. because 185°C is the cure temperature used for these compounds in

production.

4.2.3 Determination of Specific Heat Capacity at Constant Pressure (Cp)

A TA Instruments DSC 10 Differential Scanning Calorimeter'? was used to determine

the specific heat capacities of the trial samples, as is usual in the rubber industry'8

except when the highest precision is required.

4.2.3.1 Equipment - Differential Scanning Calorimeter (DSC)19.2o

The DSC is, as the name suggests, essentially a calorimeter as the measurement is

always proportional to the amount of heat taken up or emitted by the sample. Figure

4-5 shows the configuration of the DSC cell fitted to the DSC 10 unit. Two sample

holders, which are thermally isolated from one another, contain the sample and

reference and are heated in parallel at a pre-determined rate. If the temperature of the

sample and reference differ (Le. one takes up more heat than the other) energy is

94

applied to the appropriate sample. The difference in the heat input to maintain both

samples at the same temperature is recorded.

4.2.3.2 Method - Determination of Specific Heat Capacity (Cp)

The instrument programmer (an IBM PC compatible computer) was set to hold the

sample and reference pans at the starting temperature (30°C) for five minutes and

then to heat the samples at a rate of 10°C/min from 30°C to the upper limit

temperature (170°C) and hold it constant for two minutes. The samples were weighed

accurately and placed in aluminium sample pans, 10 - 15mg of material was required

for each test, the reference sapphire weighed 60·6mg.

SAMPLE PAN

1l1'ERMAl RADtATKlN SHI£LO

FAlRING

RUBBER O-RING

~\.L-CHI'OMI!LWlRE ....... OSC CEll CROSS-SECTION

PURGE GAS Coot.ANT YM:UUM PLATE

Figure 4-5 Schematic of the DSC 10 cell

To determine the specific heat capacity of the trial samples, a baseline measurement

and reference measurement were taken using an empty sample pan and a pan

containing sample with a known heat capacity. In this case sapphire (AI20 3) was used.

The heat flow versus temperature traces of the blank (baseline) sample and the

sapphire were compared to give the calorimetric differential (difference in V-axis

displacement) at 60°C. (Any temperature between about 45°C and 170°C could have

95

been chosen). E, the calibration coefficient, was calculated from the known specific

heat of sapphire over a range of temperatures21.22.23 by using.

C = [60E.ilqS]ilY P Hr m

(4-1)

Here E is the dimensionless calibration coefficient at the temperature of interest

(60cC), ilqs is the Y axis range scaling in mW/cm, Hr is the heating rate in cC/min, ilY

is the difference in Y-axis deflection between reference (sapphire) and baseline (pan)

curves in cm, m is the mass of sample in mg and Cp is the heat capacity in J/gCC.

Runs were carried out following the same heating profile used in the above procedure

with pans loaded with samples of trial compound. E having been determined, Cp was

calculated for the samples.

4.2.4 Measurement of Density

Densities of all the trial compounds, preforms and products, (moulded conventionally

and with the FORM system) were measured by weighing accurately (± 1 mg) in air and

water (ASTM D 297 - 81 24) at 23cC (±1 CC) and calculated by means of

(4-2)

where Ps is the density of the sample, Pw is the density of water at the test

temperature (kglm\ A is the mass of the sample in air, B is the mass of the sample in

water and C is the mass of the supporting thread in water. However a sufficiently thin

thread was used that the effect of the thread was negligible and C = o.

4.3 The FORM System Trials

Initially, a limited set of basic machine control instructions were available for the

operation of the machine. These were used to determine the full requirements of the

96

PLC control system and the dedicated computer control program in operating

conditions. The structure and requirements of the final control system are detailed

Figure 3-7 and Figure 3-8.

4.3.1 Methods and Procedures for the Operation of the Form Machine (Optimum Operation Procedures)

4.3.1.1 Dispenser Operation - Procedure for Preforming

4.3.1.1.1 Filling The Meter Cavity

The automatic fill mechanism proved inoperable and a method of filling the meter

cavity and dispenser runner system had to be devised. Lack of any type of continuous

pressure control on the flow of material in the system (apart from the intermittent

pressure applied to the material by the 'stuffer' piston which was dictated by hydraulic

line pressure, a controllable variable) made the filling of the meter cavity inconsistent.

However, two methods for consistently filling the meter cavity by manually feeding

milled strip into the feed pocket were empirically determined. Both of the following

methods were applied to the ring and sheet dispenser inserts. Initially each dispenser

had to be positioned in such a way that the maximum meter cavity volume was

available to be filled. This was achieved by implementing a series of step movements

and holding the dispenser in position until it is incremented to the next position setting

by a key stroke at the computer. The dispenser is in the correct position (i.e.

maximum cavity volume) when the lowermost faces of the dispenser insert inner and

outer parts are flush (Figure 3-4 and Figure 3-6 (a) and (b)). This point could be

detected simply by touch. Once in the correct position the axis control servovalve is

activated to maintain precise position control during filling.

(i) Fill Method I

The first method of filling the meter cavity and runner system used the entrance to the

'stuffer' piston bore as a datum for filling. Material (strip) was put into the feed pocket

and nipped of by the 'stuffer' piston at low pressure (10,000 - 15,000kPa). by means

of either a direct manual push-button control or selecting the menu option from the

computer control program. The process was repeated until the material being forced

into the 'stuffer' piston bore could be seen relaxing just into view in the feed pocket on

97

retraction of the piston. The pressure was then increased to maximum or near

maximum ( 21,000 - 23,637 kPa) and the piston operated for a set number (2-7) of

cycles. The pressure setting and the number of final 'stutter' cycles required varied

depending on the individual material.

(ii) Fill Method 11

The second method requires the system to be filled in a similar manner to that

described above with the hydraulic pressure set to maximum (23,637 kPa) but the

state of cavity fill is checked periodically by operating the dispense cycle. When the

cavity is full, the material will start extruding from the meter cavity as soon as the

dispenser operation is actuated. The preform is then weighed and that amount of

material added to the dispenser prior to and each and every subsequent dispense

thus ensuring the meter cavity is filled.

4.3.1.1.2 Preform Dispensing

As mentioned in Chapter 3 the dispenser cycle is controlled by the PLC in conjunction

with a hydraulic servovalve (continuous feedback flow control). The dispenser driven

to (up to seven) predetermined set points and held at those points for a pre-set dwell

time (dwell could be zero). For convenience the first and last position set points were,

generally, set equal to the dispenser fill position in order that the dispenser was

always ready for filling before the next dispense operation. Of the remaining five

available set points the second was set to a value corresponding to the stroke

required to expel the desired volume of material. The remaining set points were

reserved for movements to crop the material from the dispenser. In most cases only

one cropping stroke (downward motion of the dispenser outer part to a point below

flush with the inner part) was required. Initially all dwell times were set to zero. The

dispense cycle was initiated and the preform produced examined and weighed. The

position value and dwell time were altered to provide more or less material if required.

98

4.3.1.1.3 Preform Size Range (Weight)

To determine the possible maximum variation in dispensed preform size an

experiment was conducted, using the compounds NR and SBR1, in which the stroke

of dispenser was increased from near zero to maximum. The starting point of the

range was defined, for the ring dispenser, as the minimum stroke required to

consistently form and crop a complete ring and, for the strip dispenser, as the

minimum stroke to produce a preform that would crop completely and fall from the

dispenser and not hang up. The former test being more discerning than the latter.

4.3.1.1.4 Preform Consistency (Accuracy of Shot Weight)

Dispense routines, consisting of fill sequence and a dispenser stroke sequence, were

set up for a given material, dispenser insert and mould combinations. A minimum of

ten preforms were produced and weighed on a laboratory balance and the weights

recorded. The utmost care was taken to repeat the fill sequence faithfully without

deviation or error.

4.3.1.1.5 Dispensed Preform Temperature

The temperatures of the dispensed preforms were measured with a thermocouple

connected to a digital meter. The measurements were made on at least five preforms

at several points (least three) on the surface of the preform as it was being dispensed.

Care was taken to avoid false readings caused by contacting the metal surface of the

dispenser, which was heated (60°-90°C). The results were averaged.

4.3.1.2 Press Operation and Moulding

4.3.1.2.1 Press Moulding/Forming Procedure

Once the production of consistent preforms had been achieved. The moulding

sequence was ascertained. As with the dispenser, seven position set points and dwell

times could be set to control the closure of the press. A separate instruction from the

PLC invokes final press closure, which was maintained, under maximum pressure, for

99

----------

a specified cure time. The press set-up routine must be run to enable accurate

position control and determine the maximum displacement encoder value for press

closure, with moulds in situ, under maximum pressure (the value differs with mould

height and very slightly over time, with increasing use, as machine, tie bars, platens,

moulds etc. relax and/or settle). The value is automatically stored in the PLC and

needs to be noted for use when creating a moulding cycle.

The press is, essentially, a standard up-stroking compression press used for carrying

and heating the forming moulds save for the fact that sophisticated position control

can be achieved. This control over the press closure was used to aid the production of

isotropic (or near isotropic) parts and to enable the production of flash-free mouldings

as well as the more standard moulding practice of 'bumping-off' to expel trapped air.

4.3.1.2.2 Flash-Free Moulding

The ability to control press closure was used, in conjunction with specially designed

moulds, to produce flash-free mouldings, one of the significant advances of the

FORM system . The preform, of approximately the mould volume was dispensed into

the plunge mould cavity and the press closed to form the part. After a very brief

duration, in some cases just enough time to complete mould closure, the mould is

'cracked' open for a 'breathe' (perhaps 0·03-0·06mm) in the order of a few seconds.

The resulting O-rings are separated from any flash and need no further cleaning or

trimming to comply easily with SS 6442:198425.

For the optimisation of flash-free moulding a number of press cycles were developed

where the 'breathe' opening distance was known (0·038, 0·05, 0·01, 0·16, 0·26 and

0-4mm nominally) having been measured with feeler gauges during dry runs and the

positional accuracy of the press was ±Q·0294mm (±5 encoder divisions, usually as

overshoot). The 'stuffer' sequence was kept constant for each material and several

mouldings were produced at each 'breathe' distance and 'breathe' time (Le. the press

open time) which was altered from 1-8s in 1 s steps. On mould opening the ring was

examined in situ and then again on removal from the mould after cooling.

100

4.3.2 Test Specimen Production

Both O-rings and sheet were produced, for testing, with the FORM system and by

conventional compression moulding. Conditions for moulding (Le. cure time and

temperature), for all compounds, were determined with a Wallace-Shawbury PCA,

however, test specimens moulded using the commercial compounds were also cured

using the vulcanisation conditions specified by the manufacturer (Table 4-12). The

conventionally manufactured O-rings were commercially produced by James Walker

& Co. Ltd. and cured to their specification.

Compound Cure Time at Post Cure 185°C (mins) Time (hrs) Temp.(OC)

PB80 4 - -EOl 4 6 150 FR58 6 12 230

Table 4-12 Vulcanisation conditions used for the commercially produced O-rings and some of the in-house moulding of PBSO, EOl and FR58.

4.3.2.1 Moulding Temperature Offsets

Before moulding samples the moulding temperature offsets were determined. The

measurement of temperature in many moulding systems is achieved by a dedicated

thermocouple mounted in the platen. This is true for both the conventional hydraulic

press and the FORM system press. There is often a difference between temperature

at the point of measurement and the surface temperature of the mould. This needs to

be taken into account when moulding.

The offsets were established by measuring the temperature of a mould surface over a

range of temperatures. Sufficient time was left after a setting change to allow the

temperature to stabilise. In order not to damage the moulds and ensure good mould

contact the thermocouple was placed in contact with the metal surface of a flash or

spew groove covered with compound and the wire positioned in a vent groove for

closure so neither the mould nor the thermocouple was damaged during mould

closure. Temperature readings were compared to those of the set points and an offset

determined. It is assumed that the difference in PCA cavity temperature and set point

are negligible. It is further assumed that the conventionally produced O-rings were

moulded at a temperature close to that specified.

101

4.3.2.2 Moulding O-ring Specimens

Conventionally manufactured O-rings were produced in three compounds PB80, EOl

and FR58. As is common in industry, the rings were manufactured from extruded

chord which was cut to length at 45° to its extrusion axis. The chord was then placed,

by hand, into a hinged two plate mould, the mould and press were closed and the ring

cured. The ring was then removed from the mould after the prescribed cure time and

post cured if necessary.

O-rings were produced with the FORM system using an automatic cycle. The

dispenser producing a metered preform and cropped it directly into the mould. The

mould closure sequence for all samples includes a single 'breathe' step. The mould

dimensions are given in Table 4-13.

Production Mould Type Inner Diameter Cross Section Diameter Method (mm) (mm)

Form System Plun~e 199·53 8·64 Conventional Two plate 198·0 8·3

Table 4-13 C-ring mould dimensions.

4.3.2.3 Moulding Sheet Specimens

Test sheets, nominally 2 mm thick, were moulded conventionally and with the FORM

system. The conventionally moulded sheets were prepared in a three-plate (picture

frame) mould. A billet of milled sheet of approximately correct weight was placed on

the bottom plate, in the centre of the frame, covered with the top plate and placed into

the upper daylight of a double daylight, up-stroking, hydraulic press powered by an

electric pump. Press closure included a single bump-off or 'breathe' to expel trapped

air and keep all moulding conditions, for both conventional and FORM processes, as

consistent as possible. Vulcanisation times and press temperatures were consistent

with those specified and/or determined in the PCA. Post curing, where appropriate,

was carried out in a standard laboratory oven.

102

Specimen sheets made with the FORM system were produced with a dual meter

cavity dispenser and two cavity mould combination. The preforms were dispensed

and cropped into the mould cavity simultaneously.

Production Mould Type Length Width Thickness(mm) Method (mm) (mm) (2mm nom.)

Form System PlunQe(2 cavity) 152·9 90·3 1·94 Conventional Three plate(frame) 122·5 120.0 1·97

Table 4-14 Sheet mould dimensions.

4.4 Physical Testing and Observations

4.4.1 General Observations - Preforms and Preforming

The preforms were observed during and after cropping from the dispenser.

4.4.1.1 Preform Shape and Size

Noticeable irregularities could be seen in the dispensed preforms after a short period

of time. The shape of the preform at the instant of cropping was regular and of even

cross section, for both the ring and the strip specimens. However, after only a few

seconds of recovery, irregularities began to appear in the form of a lobing effect on

the ring preforms and a bulging effect on the sheet samples.

(i) Filling and Packing the Meter Cavity

This phenomenon was investigated, for both dispenser insert types, with a standard

dispense motion sequence and the fill pressure was varied from low (stall at approx.

7000kPa) to maximum pressure (23637kPa). The size and shape of the preforms

were noted.

(ii) Dispensed Preform Size (Rings)

Rough measurements were made, with a 300mm rule, of the dispensed preform size

(diameter) after they had been cropped from the dispenser. The initial measurement

was made as soon as possible after crop and further measurements were made at

103

30, 60, 120, 180 and 240s (±10s) and a final size measurement was made at 24hrs

(±4hrs).

4.4.2 Mouldings (Product)

A number of tests were carried out on the mouldings that were produced by the

FORM system and by conventional moulding methods. It has already been stated that

moulding procedures and conditions were as near identical as possible to enable a

fair comparison.

4.4.2.1 Product Examination/Inspection

All of the mouldings produced (FORM and conventional), were inspected visually after

demoulding. In cases where flash was present it was, generally, trimmed unless the

ring was produced as part of a flash free moulding trial in which case it was left for

examination.

4.4.2.1.1 Rings

In order so set a stringent test for acceptance of rings for further testing, SS

6442:1984: Limits of surface imperfections and elastomeric toroidal sealing rings ('0'­

rings)25 was used as a baseline. Limits for flow marks, non-fills, foreign materials and

indentations are not tight at two in any 25mm of circumference. All instances of non­

fills and foreign materials were instantly discarded. The remaining rings were rejected,

in this study, if more that two of the other faults were visible on the whole

circumference. Limits for offset (or mismatch), backrind and combined flash (after

trimming where necessary) were in accordance with the standard.

The conventionally produced rings were examined closely for join marks and a

number of rings (conventional and FORM) were cut into eight equal sections and the

densities measured as a test of evenness.

104

4.4.2.1.2 Sheet

Sheet specimens were treated in a similar fashion to the rings previously described.

The surfaces needed to be smooth and free from all imperfections. All sheets with

non-fills, foreign materials and indentations noticeable on the major surfaces were

rejected and discarded. Flash was trimmed from both the conventionally moulded and

FORM system sheets. The FORM system sheet mould was not intended for the

production of flash free mouldings. The design of the plunge mould, however,

prevented any instance of offset.

4.4.2.2 Mould Shrinkage

Mould shrinkage is generally defined as the difference between the dimensions of the

moulding and those of the mould cavity at room temperature26•27

.

4.4.2.2.1 Rings

To determine the mould shrinkage of the a-rings, measurements were taken of the

rings across their inner diameter and across the ring on the line of the diameter to

provide ring cross section diameter measurements. Rings were also carefully cut, in

line with their diameter and the cross section measured in different directions.

Measurements were made with a travelling microscope for whole ring diameters and

some of the cross section diameters. The cross section diameters were generally

measured with a Shadomaster shadowgraph and checked with the travelling

microscope and/or a dial gauge.

In order to ensure that the true diameter of the rings was being measured a card with

concentric circles 1mm apart and diametric lines 45 0 apart was used as a guide. The

rings were easily centralised and measurements taken.

(i) FORM system

The FORM system ring diameters were also checked by carefully cutting a ring in line

with its diameter and placing the split ring into the mould ensuring the ring was tight to

the inner diameter of the cavity and measuring the gap. The ring circumference and

105

diameter were then calculated. Component measurements were compared to

measurements taken from the mould cavity.

(ii) James Walker (Estimated)

The shrinkage of the conventionally moulded rings produced by James Walker was

estimated from measurements taken of samples (as above) and comparing them with

the stated dimensions of the mould used for their manufacture.

4.4.2.2.2 Sheet

The sheet mould and sheets were measured with a Vernier calliper. Great care was

taken not to compress the sheet with the calliper during measurement, spot check

measurements were made with a travelling microscope. The sheet thickness was

measured with a dial gauge. The average was taken of at least three measurements

for each dimension. The mould cavity and sheet measurements were compared.

4.4.2.3 Swelling in Good Solvent

To investigate the integrity and highlight any molecular anisotropy present, samples of

the mouldings were placed in a good solvent for swelling. The solvents chosen for the

swelling test were all SlR grade obtained from Fisher Scientific. The swelling regimes

are shown in Table 4.15. The tests were carried out broadly in accordance with

standards2B•

Material Solvent NR (NR), SBR1 (SBR), Methanol SBR2 (SBR), NBR (NBR) Toluene PB80 (NBR) and EOl (H-NBR) FR58 (FKM) Acetone

Table 4-15 Material and Solvent Regimest

t The regimes as stated in the table were complete however some extra tests were carried out in the process, i.e. the FR58 was also immersed toluene and methanol in some the tests).

106

4.4.2.3.1 Ring Shape

(i) Cross Sectional Area

A segment of each ring was cut and marked for identification. A thin slice was then

taken from each segment and the freshly exposed surface on each side marked. It is

assumed that these two faces have the same shape. The segment was placed in

solvent and the slice placed on a shadowgraph and photographed. When the

segment was removed from the solvent, a thin slice was taken from the end that had

been marked and placed in a Petri dish with a drop of solvent to keep it saturated and

the dish placed on the Shadowgraph so it could be photographed. The area of the

cross sections of each of at least three different segments from three different rings

was then determined.

(ii) Circularity of Cross Section

The largest and smallest diameters of the cross section of each of the ring segments

was measured before and after immersion in the solvent from the shadowgraph

photographs.

4.4.2.3.2 Volume

The volume of both ring and sheet specimens was determined before and after

immersion in solvent.

4.4.2.3.2.1 Sheet Samples

The sheet specimens 38·1 x 12.7mm (±O·1mm) were prepared for immersion from

sheet nominally 2mm thick. The sheets were assigned reference directions, A and 8

(Figure 4-6). The conventionally moulded sheet reference direction 'A' was parallel

with the direction of milling and therefore any mill grain. The FORM system sheet

reference direction '8' was parallel with the direction of extrusion from the preform.

Five samples were cut from each sheet. Their dimensions were measured with a

Vernier calliper, periodic measurements were made with a travelling microscope. All

the samples were immersed in toluene except those cut from sheet moulded from

107

FR58 which was immersed in acetone. The dimensions were measured after 1,7, 14

and 21 days immersion in the solvent. Volume was calculated from the linear

dimensions.

4.4.2.3.2.2 O-ring Samples

The change in volume due to the action of solvent on sections of the moulded O-ring

was measured simply by displacement of water in a measuring cylinder. The sample

was placed in the cylinder, before and after swelling and water from a burette to a

graduation (e.g. 20ml) which was recorded after making sure all air bubbles were

eliminated. The amount of water added from the burette was also recorded the

difference between the two measurements was taken as the volume of the sample B ..

I-r- r "I

'1 DDDDD 122·5

D D D 152·

D 9

D

I~ 1200 ~I-~ \.. .,)

90·3

All DIMS IN MM

Figure 4-6 Schematic of Moulded Sheet Showing the Direction of Sample Cutting

4.4.2.4 Compression Set

Compression set tests were carried out on segments cut from O-rings produced both

conventionally and with the FORM system. The test was carried out in accordance

with SS 903: Part A629. However, the standard test pieces, could not be used.

Sections of O-ring (5 off) 25mm (±1 mm) in length were cut, and their height (vertical

diameter) measured with a dial gauge. They were then clamped in the apparatus

(Figure 4-7) with spacers limiting the compression, nominally, to 75% of their original

height. The samples were placed in an oven at 100°C (± 3°C) for 24hrs~25'

108

On removal from the oven the sample heights were measured and recorded after a

recovery of 30sec, 30min and again after a period of some months.

r-_Compressed 0-Ring Segment

Plates

Figure 4-7 Schematic of Compression Set Test Apparatus

4.4.2.5 Tensile Testing of Dumbbells Cut from Sheet

Tensile tests were carried out on SS 903:Part A2 Type 2 dumbbell30 specimens (5 off)

cut from the 2 mm thick sheet in each direction, parallel and normal to the reference

directions A and S (Figure 4-6).

A Hounsfield Test Equipment H10KM Universal Testing machine and 500l laser

Extensometer31 were used with a 1000N load cell. Samples clamped, in spring loaded

jaws, between the fixed base and driven crosshead and were extended at a rate of

500mm/min. The extension was measured by the laser from two reflective markers

attached to the sample gauge length. The width and thickness of each sample were

entered into the controller to enable output of tensile stress for predetermined

extensions (100, 200, 200, 400 and 500%) and break, the extension at break was

also recorded and mean and standard deviation calculated.

109

4.5 References

' ASTM D 2934 - 89: Standard Practice for Rubber Seals - Compatibility with Service Fluids, Annual Book of ASTM Standards,09.02, American Society for Testing and Materials (1989)

2G. F. Bloomfield (revised by G. M. Bristow) in Rubber Technology and Manufacture 2"" ed. Ch4 pp80 -81 (C. M. Blow and C. Hepburn Eds.), (Butterworth Scientific, London, 1982)

'W. Hofmann, Rubber Technology Handbook, CH2 pp16 - 18, (Carl Hanser Verlag, Munich, 1989)

'G. J. van der Bie, J. M. Rellage and C. Vervloet (revised by L. H. Krol) in Rubber Technology and Manufacture 2nd ed. Ch4 p92 (C. M. Blow and C. Hepburn Eds.), (Butterworth Scientific, London, 1982)

5ASTM D 1765 - 96a: Standard Classification System for Carbon Blacks used in Rubber Products, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

BE. Schmidt, Ind. Engng. Chem., 43, 679 (1951)

'B. 8. Boonstra, Polymer, 20, 691 (1979)

BASTM D 3765 - 96 Standard Test Method for Carbon Black- CTAB (Cetyletrimethylammonium Bromide) Surface Area, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

9ASTM D 2414 - 96a Standard Test Method for Carbon Black - n-Dibutyl Phthalate Absorption Number, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

lOp. K. Freakley, Rubber Processing and Organisation, Ch3 p51, (Plenum Press, New York, 1985)

"A. King, Plastics Rubb. 1nl. 14 (1), 23 (1989)

12S. N. Ghafouri and P. K. Freakley, Polym. Test, 11, 101 (1992)

"P. K. Freakley, Rubber Processing and Organisation, Ch2 pp22-23, (Plenum Press, New York, 1985)

"P. Khunkamchoo, Ph.D. Thesis, Loughborough University of Technology (1993)

'SWallace Test Equipment - Precision Cure Analyser Manual D14a, H. W. Wallace Ltd. (1982)

lBp. K. Freakley, Rubber Processing and Organisation, Ch2 p37, (Plenum Press, New York, 1985)

17DSC10 Differentail Scanning Calorimiter Operator's Manual, TA Instruments (1994)

lBp. K. Freakley, Rubber Processing and Organisation, Ch2 p30, (Plenum Press, New York, 1985)

19M. E. Brown, Introduction to Thermal Analysis Techniques and Application Ch4 pp25 - 38, (Chapman Hall,1988)

20G. Kampf in Characterization of Plastics by Physical Methods - Experimental Techniques and Practical Application Ch4 pp179 - 191, (Hanser Publishers, Munich, 1986)

"D. C. Ginnings and G. T. Furukawa, J. Am. Chem. Soc., 75, 522 (1953)

22D. A. Ditmars et al., J. Res. Nal. Bur. Stand. 87 (2), 159 (1982)

23H. Y. Afeeiy, J. F. Liebman and S. E. Stein in NIST Chemistry WebBook [http://webbook.riiSl.govj, NIST Standard Reference Database Number 69, (W.G. Mallard and P.J. Linstrom Eds), (National Institute of Standards and Technology, Gaithersburg, USA 1998)

110

-----------------

24ASTM D 297 - 81 Standard Test Methods for Rubber products - Chemical Analysis: Part A 15, Annual Book of ASTM Standards, 09:01 , American Society for Testing and Materials (1985)

25BS 6442: 1984 British Standard Specification for Limits of Surface Imperfections on Elastomeric Toroidal Sealing Rings ('O'-rings), British Standards Institution (1984)

26A. W. Fogiel, H. K. Frensdorff and J. D. MacLachlan, Rubber Chem. Technol., 49, 35 (1976)

27 J. G. Sommer, Rubber Chem. Technol., 51, 368 (1978)

2BBS 903:Part A16:1987, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of the Effect of liquids, British Standards Institution (1987): ASTM D 471 - 79 (reapproved 1991) Standard Test Methods for Rubber Property - Effect of liquids: Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1991)

29BS 903:Part A6:1989, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of Compression Set after Constant Strain, British Standards Institution (1989)

30BS 903:Part A2:1989, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of Tensile Stress-Strain Properties, British Standards Institution (1989)

31Houndsfield Test Equipment H10KM Operating Instructions, Rev. A - 1/11/88, Houndsfield Test Equipment Ltd., Surrey (1988): Houndsfield Test Equipment 500L Laser Extensometer Operating Instructions, Rev. A - 26\02\90, Houndsfield Test Equipment Ltd., Surrey (1990)

111

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Chapter Five

5. Finite Element Modelling (FEA)

Modelling the FORM system using FEA was undertaken in a number of stages. The

early stages were to aid in the design of the dispenser and the later stages to predict

the behaviour of the material as it passes through and out of the system. The work

was roughly divided in two areas, heat transfer and flow modelling. The latter,

comprising the greater proportion of the work, was itself sub·divided into two

discemible section, (a) filling the meter cavity, and (b) dispensing from the meter

cavity.

The Engineering Mechanics Research Corporation (EMRC) NISA (Numerical

Integrated elements for System Analysis) 11 suite of finite element programs had

been chosen because the NISA 11 standard package was already available within

the department and there was also a certain amount of user experience that could

be drawn upon. It was also desirable that the package could be run on an IBM PC

compatible platform to ensure that the modelling would be accessible to industry, off

the shelf, if one of the results of the project were to be an FEA based design tool for

the end user. Another important feature of suited FEA software such as NISA 11 is

compatibility between modules.

5.1 Model Construction

Model construction was similar for all models. Three main stages are required to

solve a problem. These are pre-processing, analysis and post-processing. NISA 11

requires a structured ASCII text file input in order to perform an analysis. If the

112

format is known to the user then any text editor is suitable. The required information

consists of such elements as: (i), the type of analysis (e.g. heat transfer, fluid, 20,

3D etc.); (ii), material properties (e.g. density, viscosity, thermal conductivity etc.);

(iii), discretisation or meshing (co-ordinates of nodes, the element boundaries and

their connectivity); (iv), the boundary conditions (e.g. applied forces and fluxes,

attachment points and constraints etc.); and, (v), the initial conditions (e.g.

temperature, velocity etc. at the start of the problem). The NISA 11 specific structure

is shown in an abridged NISA 11 input file in Appendix C.

To help the user, the process of entering and formatting the input information is a

sophisticated graphical user interface (GUI) DISPLAY Ill. The GUI has a

considerable drawing capability and the option to enter the information by means of

typed input and/or pull down menus as well as analysis interface options such as

dedicated forms and menus for the entry of NISA 11 specific data for the creation of

NISA 11 input files (<filename>.NIS files). Display III is also capable of reading,

displaying and manipulating the binary output or results files (<filename>26.DAT

[model data) and <filename>27.DAT files [results data)).

5.1.1 Geometric Modelling

For analysis the system geometry (boundary and the interior) needs to be modelled

or defined mathematically. DISPLAY III allows items to be defined in space

graphically. Complex geometry was built up from simple geometric entities, locations

in space (called Grids), straight or curved lines (called Lines), surfaces (called

Patches) and solids (called Hyperpatches). The lower order entities, such as grids,

can be used to construct higher order entities such as lines (e.g. at least two grids

can be connected to form a line) or patches (e.g. at least three grids can be

connected to form a surface). Conversely, lower order entities can be derived from

higher order entities (e.g. the surface, above, formed by three grids could be used to

'extract' the three lines that form the edges of the surface). Other functions were

also used. Circles, ellipses, etc. can be formed as parametric items and entities can

be copied, mirrored, translated, rotated, etc. This means that, in most cases, there

113

are numerous routes to defining any, even highly complex and intricate, geometric

form.

Before these tools were used to define the geometry of problems, the systems were

studied to determine if they were regular or it there were any axes of symmetry.

Many of the problems posed by the proposed FORM system geometries are simple,

regular shapes with an axis of rotation. Discoidal, cylindrical and annular shapes

constituted much of the geometry for the proposed, and ultimate, FORM system

flow paths. These were modelled as 20 problems rather than 3D problems, thus

making the geometric definition much simpler and vastly reducing the run time

required for the analyses. More complex geometries with planar, mirror symmetry

were halved along the plane of symmetry. This was done under the assumption that

the behaviour of the system would be balanced and the section modelled would be

representative of, and exhibit the same characteristics as, the section omitted.

Figure 5-1 shows how the geometry of a squat cylinder, representing a transfer pot,

can be simplified to the shaded rectangle representing half the cross section. This is

the same information required to generate a solid of revolution thus the whole is

actually defined (or definable) in space.

5.1.2 Meshing (Finite Element Modelling)

Once the geometry had been defined to give the problem some bounds in space,

the process of meshing (or discretisation) was to divide the problems into, and

define the finite elements on which, the calculation of the solution depends.

A mesh is created on the geometric entities Patch, for 20 surface and 20

axisymmetric problems, and Hyperpatch for 3D solid problems. This was achieved

using the 'Automesh' mode for heat transfer models with simple geometry and

parametric meshing, a semiautomatic mode, for fluid flow models, using the FEM

(Finite Element Meshing) menu, FEG (Finite Element Generation) option. In most

the patch or hyperpatch and the number of divisions in the x and y or x, y, and z

directions of the entity local co-ordinate system were selected, respectively.

114

100

le 1

~~",.- .•• -.- •.• I_._._.- .. -.-.--.-

Figure 5-1 Diagram Showing how model geometry can be simplified. The shaded rectangle shows is the geometry that needs to modelled for am axisymmetric solid model.

Great care needed to be taken when creating a mesh, because NISA 11 elements

have a directional component which is derived from a directional component of the

geometric entities from which they are created. For example, four grids (1, 2, 3, and

4) were used to create two lines (L 1 and L2) and a patch (P1) was created from the

two lines. In Figure 5-2 (a) the local Cartesian co-ordinate positive directions

(labelled a. and P), for a patch created from line L 1 to line L2, are shown and in

Figure 5-2 (b) the patch was created from L2 to L 1. The lines were both created in

the same direction, 1 to 2 and 3 to 4, respectively, and hence the a. directions

coincide with each other. The directional components of the elements in a mesh

need to coincide or the results and predictions may be unreliable. This is particularly

important for 3D flow modelling, where incorrect elemental directions can influence

the NISA 11 results.

115

t(: -'

(a) (b)

Figure 5-2 Two NISA 11 Patches, showing the directional component that is derived from the method and order of creation in DISPLAY Ill.

Mesh generation was not carried out with the auto-meshing routine (Automesh) in

DISPLAY III/NISA 11 for flow modelling because it had proved unreliable in the past

for NISA II/FLUID'. The 'stiffness type' of element had to be defined for both

methods of mesh generation. This consisted of assigning two variables NKTP and

NORDR, the former specifying the element type and the latter specifying the

element shape and the number and position of associated nodes (e.g. NKTP = 3

NORDR = 1 is an 'axisymmetric solid' quadrilateral element with 4 nodes).

The validity of a generated mesh was checked with routines that check the size,

shape and connectivity of the elements and nodes. Duplicated and erroneous nodes

and unconnected elements would be highlighted so that they could be altered.

From experience and information in the literature, a set of practical rules for

meshing2.3

,4 was developed early during the modelling:

(i) A finer mesh and therefore an increased number (or density) of nodes

should be located where the model is constrained, loads are concentrated

and heat fluxes applied.

116

(ii) Nodes should be located where displacements and temperatures are

constrained and/or concentrated (e.g. the narrowing of flow through a tube of

decreasing diameter).

(iii) Nodes should be located where springs and masses and their thermal

analogues are present.

(iv) Nodes should be located along lines and on surfaces where the

pressures, shear stresses, heat fluxes and surface convection are present.

(v) Nodes should be located along lines of symmetry.

(vi) Nodes should be located at interfaces between different materials.

(vii) The element aspect ratio (ratio of the largest to the smallest element) of

the entire model should be no more than five and element density should

vary gradually rather than being abrupt.

(viii) Symmetric configurations should have symmetric meshes.

(ix) Elemental and nodal density should be increased in areas of the model

where high gradients or directional changes are expected.

(x) Where possible, in areas of the model where gradients are low, meshes

should be uniform.

(xi) An element type that is suitable to a particular analysis should be chosen,

some elements and solution methods are unstable and do not converge to a

result.

117

5.1.3 Boundary and Initial Conditions

The analysis required the specification of boundary and initial conditions such as

applied temperatures and velocities and the initial temperature of the material. (e.g.

if the dispenser body were held at ao°c and the piston was pushing the material

through the system at 20mm/s and the new material entering the system were at

room temperature 20°C). These were entered through the FEM (Finite Element

Mesh) menu 'Boundary Conditions' option.

5.1.4 Units

NISA 11 is entirely independent of units of the physical quantities specified in the

input data. The only requirement is that the units be self-consistent. The SI system

of units is based on the fundamental dimensions: length, (L), measured in meters,

(m); mass, (M), measured in kilograms (kg); and, time (t) measured in seconds, (s).

On some occasions, the SI recommended decimal sub-multiples of length,

millimetres (mm) and mass, grams (g) were used.

5.1.5 Post-Processing

The results of the analysis stage were converted into a more manageable (or man­

readable) form with DISPLAY Ill. Graphs, charts and contour plots were produced

and some taken as hard copy. The output from the analysis is in the form of binary

results files «filename>26.DAT and <filename>27.DAT) that require decoding.

ASCII text data can be saved in an output file (<filename>.out) if so desired.

Typically the results calculated for each node and in, say, heat transfer analysis the

temperatures of each node at each iteration would have been recorded.

5.2 Static Heat Transfer

The very early work was carried out before the acquisition of the NISA 11/3D-FLUID

module. The NISAII/HEAT heat transfer module was used to estimate the warm-up

time, (i.e. the time required to heat the given volume of material to the proposed

working temperature of aO°C). This was modelled in order to simulate different

11a

- - -----------

geometries of the dispenser storage 'reservoir' or buffer stock of material if it had,

say, been left over night and allowed to cool. The reservoir geometries considered

are given in Appendix O.

5.2.1 Conditions and Assumptions

To enable fair comparison of each of the suggested geometries several

assumptions were made and wherever possible the settings for the FEA heat

transfer model were identical to those of all the other models. The assumptions and

standard conditions were as follows:

(i) A set of standard material values for mass density, thermal conductivity

and specific heat capacity were used in all models, these values were 0.0014

g/mm3, 0·00017 W/mm.K and 1·1643 J/g.K, respectively.

(ii) All models were treated as 20 axisymmetric (Le. volumes of revolution)

and the heat generation element type NKTP = 103 and quadrilateral elements

NOROR = 2 with a nodes per element were used.

(iii) As a termination condition all designs would be required to reach the

same temperature of ao°c to within 2°C throughout the whole volume.

(iv) The whole volume of the reservoir would be considered to be filled with

material at room temp (20°C) with no voids.

(v) A constant temperature of ao°c was applied to all external (edge) nodes.

(vi) There would be no heat flow across the axis of symmetry.

(vii) Element meshes were all created by the NISA 11 'Automesh' function.

5.3 Heat Transfer with Incremental Flow (Pseudo-flow)

The heat transfer modelling was developed to include elements of movement and

time in an attempt to simulate the flow of the material through the dispenser. The

119

flow path was considered as cylindrical. Although some of the runners in the design

are not actually cylindrical, the cross-section was approximated as a tube with the

same volume as the runner.

At the start of the flow path, the modelled cylindrical 'plug' of rubber is heated at

80°C around its circumference and at one end. This simulates heating at the wall

(boundary) of the flow path and the piston. The elements which form the other end

of the plug are restricted so that no heat flow across this face can occur. The heat is

then applied for a fixed period of time (15 seconds), representing the residence time

of the volume of rubber in that section of tube. The period of 15 seconds was

chosen as this was the design target interval between dispensing each shot.

The FE model was run for the required time step and the nodal temperature data

from the run was saved and mapped onto the next section of the flow path as the

initial starting conditions. This second, and subsequent, iterations were the same in

every respect except the piston end was not heated. This process was repeated

until the 'apparent volume' traversed by the plug of material equalled the total

volume of the length of flow under consideration.

At the point where the diameter of the flow narrows and divides, the nodal

temperatures of the previous run were transposed onto the new geometry of

reduced diameter tube in such a way that the warmer outer temperatures were

maintained at the extremities and the cooler inner temperatures at the inner part of

the new geometry. This was considered to be a reasonable approximation.

5.4 Flow Modelling with NISAlFLUID

Modelling of the flow using the NISA 11/3D-FLUID module was undertaken in two

sections, the flow through the dispenser representing the situation of flow when the

meter cavity is being filled and flow during dispense. The flow during fill is driven by

the 'stuffer' piston whereas the dispense flow is driven by the relative motion of the

120

-- - ---_._----

inner and outer parts of the dispenser insert (meter valve). These were considered

to be two distinct and separate phases, because the action of the meter valve

physically isolates the material in the meter cavity from that in the rest of the system.

5.4.1 Dispenser Fill Modelling.

A 3D model of the internal geometry of the flow path was created and meshed as

previously described. Where possible, a central core of simply shaped (rectangular)

geometric entities (and hence elements and nodes) ran the length of the flow path.

This was good modelling practice and aided the modelling of more complex

geometric features such as constrictions, expansions, changes of direction (bends),

divisions and combinations.

Two families of models were created. One was based on the dispenser geometry

required for producing O-ring preforms and the other for the two cavity, geometry

required for sheet or strip preforms. The model input requirements were very similar

for both sets of models. The most commonly used functions, commands, analysis

and input data used in the modelling are given Table 5-1.

The models created, although referred to here by the dispenser from which the

geometry was derived, only represent the dispenser in that they are actually models

of the material (virtual rubber) in the respective dispenser.

5.4.1.1 Ring Dispenser Geometry

Two geometries were modelled for the ring dispenser. These are given below. The

two designs are very similar because the type and design of the feed mechanism

and the upper part of the dispenser and dimensions of the annular meter cavity (set

out in the initial design brief) had been fixed at the time they were created.

5.4.1.1.1 Ring Dispenser Geometry I

Geometry I (Figure 5-3), the first of the considered designs, had the simplest

geometry from the point of view of both modelling and manufacturing. The 'stuffer'

piston will cause flow from A in the negative x direction (the 'entry' plane is in y-z for

121

the purposes of NISA 11 flow modelling), through a 90° bend at B in the x-y plane

(flow in the negative y direction). At C, the flow is divided into four equidistant

diverging runner channels (in the x-z plane), at right angles to the preceding flow, to

enter the annular meter cavity at D. The flow 'exit' (NISA 11) is at the bottom of the

meter cavity in the x-z plane. A shaded representation of the flow geometry is shown

in Figure 5-3 and the hyperpatch wire-frame geometry that was meshed and used in

the modelling is shown in Figure 5-6 (a).

5.4.1.1.2 Ring Dispenser Geometry 11

The flow in the second geometry considered for dispensing ring preforms (Figure 5-

4) is similar to that of geometry I from A' through to B' and differs at C' where the

flow divides in to six equidistant runners that are at 45° to the flow in the axis y.

Distance B'- C' is less than B - C as the overall height is the same. The points,

labelled D', are where the flow enters the meter cavity. The exit plane (E') is, again,

in the x-y plane the at the bottom of the meter cavity. The wire-frame representation

of the modelled half is shown in Figure 5-6 (b).

5.4.1.2 Sheet Dispenser Geometry

The upper geometry of the sheet dispenser (Figure 5-5 and Figure 5-6 (c)) before

C" is similar to that previously described. The flow division and cavity arrangement

after C" was, however, considerably different. Four runners (two on each side) feed

two separate metering cavities through the points at D" with the exit plane at E" in

the x-z plane as before.

122

NISA 1113D-FLUID Command LabelNariable Description

Executive Command Block ANALvsis FLUHT Fluid and heat transfer analvses DIMEnsion AX Axisymmetric solid problem

3D Three-dimensional Problem FILEname <file_name> Output binary data files name (6 characters of less in

DOS as designation from SAVEfile will be appended (e.a. file26.DAn

SAVEfile 26,27 Data file designations: 26 - model and analysis data, 27 - analvsis results data

INITialc (ondition) U, V, W, UVW and T Specific initial conditions for velocities U, V and W in [VALUE/S] directions x, y and z respectively and temperature T

condition reouires value STDS ON/OFF Steady state analysis. The FLCNtl analysis data card

needs to be implemented for both ON and OFF conditions for iteration control in the former and time-step control in the latter

BOUNDary ON/OFF Automatic computation of domain boundary. U=V=W=O and U=V=O for 3D and axisymmetric problems (i.e. extremities of the wireframe are assumed to be walls with no-slip boundary condition)

VDISip ON/OFF Viscous dissipation or heat generated due to viscous [Conversion factor] effects is accounted for in the analysis (conversion

factor onlv if inconsistent units are used) Model Data Block

ELTYpe NKTP [value] Element type (3 - AXIS., 4 . 3D) NORDR [value] Nodal ord~r 1 or 2 (4 or 8 nodes -AXIS. and 8 or 20

nodes - 3D NODEs Nodal values string Nodal ID and co-ordinate definitions ELEMents Element values string Element and material ID, nodal and elemental

connectivitv oarameters MATFluid DENS [value] Material property data for fluid generally values for

VISC [value] density, viscosity, thermal conductivity and specific CON~ r\valu~] heat capacity are required SPEC value

NONNewtonian POWER [value] Non-Newtonian power law fluid, the value required is the non-Newtonian or power law index n

Analysis Data Block FLCNtI NLSTP [value(,;SO)] Fluid load case control. NLSTP requires a value for

ITMAX [value] the number of time steps and ITMAX specified the maximum no. 01 iterations(in STDS) or iterations per time step if transient analvsis used.

BCDVAR U, V, Wand T [value] Variable Nodal Boundary conditions these generally take precedence over globally set boundary conditions

BCDR U, V, Wand T [node ID] Global boundary condition release (i.e. exit plane nodes need to be released when BOUNDary - ON)

ICDS U, V, Wand T [node ID Specified nodal initial condition a node ID and value] boundary condition value required

PRINTcntl U, v, W, T, P, SXX, Selects the data required to PRINT to file value SYV, SZZ, SXY, SXZ, and SYZ [value)

required 0 for all -1 for none.

Data Terminator Block ENDdata I Input Data Terminator

Table 5-1 NISAl3D-FLUID Input, Analysis and Data Commands used in the Modelling of the Form System

123

5.4.2 Dispense Modelling

Modelling of material flow through the meter cavities was carried out using a

different set of FEA models to those described above. The dispense stroke was

treated as a distinct and separate action to that of filling. The geometry also looks

somewhat different to that previously pictured this is because the dispense is a

transient situation, the inner and outer parts of the dispenser moving relative to one

another creating the narrow opening through which the material is forced.

124

~ If!f:-

r z L,

Figure 5-3 Ring Dispenser Flow Path Geometry I

125

[' ,

Figure 5-4 Ring Dispenser Flow Path Geometry 11

126

Figure 5-5 Sheet Dispenser Flow Path Geometry

127

(a)

(b)

(c)

Figure 5-6 Wire Frame Diagrams of the Actual FEA Model Geometry: (a), Ring Dispenser Geometry I; (b), Ring Dispenser Geometry 11; and, (c), Sheet Dispenser Geometry. The plane x-y is a Plane of symmetry in all cases therefore only half the actual geometry needs to be modelled for FEA.

128

5.4.2.1 Ring Dispense Modelling

The annular geometry of the meter cavity for the ring preform allowed very simple

axisymmetric modelling of the dispense flow. The lower narrow section, or tail,

represents the slit that is created when the dispense stroke is triggered .

.,,----A.rea Modelled

........ _____ Dispenser Inner

Figure 5-7 Scrap Section of Geometry Modelled in Axisymmetric Meter Cavity Dispense (the arrows show relative motion of dispenser inner and outer)

5.4.2.1.1 Ring Dispenser Alternative Geometries.

A number of alternative geometries were modelled in a "what if?" manner. These

were divided conveniently into two sets, in the same way as the dispenser design

and other geometries. The conditions for all models were standardised to enable fair

comparison. The material parameters and volume flow rate were consistent and the

nominal preform diameter was set to be consistent with that of the FORM system

design. Details of the standard material parameters and flow rate are reported in

Table 5-2.

129

Parameter Value

Flow rate 6·71 x1 0·sm3/s

Equivalent to a constant speed of 10mm/s movement in standard dispenser meter cavity

Initial Temperature(INIT T) 60°C Density (DENS) 1200 kClfm3

Specific Heat Capacity (SPEC) 1500 Thermal conductivity (COND) 0·17 W/m/K Viscosity (VISC) at 1 s·' (K) 100000 Pa Non-Newtonian Power Law Index, n

0·25 (NONN)

Table 5-2 Model Parameters for "What if?" Dispense FEA.

Alternative Cross Sections for the Annular Preform Cavity.

A couple of more radical geometries were modelled to gain an idea of the effect of

cavity shape on flow. It is assumed that it would be possible to produce the meter

cavities of the geometries devised. Examples are given in Figure 5-8.

y Figure 5-8 Example NISA 11 Meshes of Alternative Annular Dispenser Geometry

5.4.2.2 Sheet Dispense Modelling

The flow of material through the strip dispenser meter cavities could not be

modelled as simply as the annular cavity (Le. using axisymmetric elements) because

a solid of revolution cannot be formed. Only a single cavity was modelled because

of symmetry. The commands used for the model are described in Table 5-1. This

was, essentially, a 3D flow problem similar to those previously described.

130

5.5 FLUID - STATIC Interface

NISA 11 static packages were used in conjunction with the output of NISA/3D-FLUID

in an attempt to model the shape change of the preform after it has left the

dispenser. The normal stresses in the exit plane of the 3D-Fluid 20 axisymmetric

model were saved at every time step iteration, extracted from the output file with the

FLUTL (NISA/3D-FLUID Utilities) package and mapped manually onto the simple

rectangular meshes used as the starting geometry for resolution of the stresses.

Figure 5-9 shows the logic of this two stage modelling process. The stresses are

integrated over a defined 'surface' (which is an in edge in 20 axisymmetric

modelling), the 'surface' (edge) normals are computed to be positive when pointing

outwards. These values were then applied manually, as boundary conditions, to

elemental faces using loadcase type L 1. Two types of model geometries were used:

(i), normal stresses were mapped onto an axisymmetric solid; and (ii), normal

stresses were mapped onto an axisymmetric shell.

The STATIC modelling was conducted in much the same way as that previously

described for 3D-fluid. Table 5-3 gives the commonly used commands and analysis

data for STATIC modelling.

Stage 1

FLUID/STATIC Interface

Stage 2 *--------

-+--------

-+-------- ---------.

___ --3D Fluid Model (stress determination)

Flow Exit Plane

STATIC Model (stress resolution)

t Stresses mapped according to time step (i.e. most recently logged values applied

t, closer to the interface).

Figure 5-9 Logic of the FLUID to STATIC modelling. Above the FLUID/STATIC Interface Represents the NISAIU3D-Fluid model, below the stresses are mapped on to a separate model for resolution.

131

· - ------------

NISA 11I3D-FLUID LabelNariable Description Command

Executive Command Block ANALysis STATIC Static analysis BLANk common n=50000 Assign size of dynamic memory (used as swap file

durino processino) FILEname <file_name> Output binary data files name (6 characters of less in

DOS as designation from SAVEfile will be appended (e.g. file26.DAn

SAVEfile 26,27 Data file designations: 26 - model and analysis data, 27 - analvsis results data

GEOM properties ON/OFF, Computes and saves geometric properties (volume. LlST/NOLlST mass etc.)

Model Data Block ELTYpe NKTP [value] Element type (3 - AXIS., 4 - 3D)

NORDR [value] Nodal Order 1 or 2 (4 or 8 nodes -AXIS. and 8 or 20 nodes - 3D)

NODEs Nodal values strino Nodal ID and co-ordinate definitions ELEMents Element values string Element and material ID, nodal and elemental

connectivity ~arameters MATErial EX, EY, EZ [values] Elastic moduli (force/area)

NUXY, NUXZ NUYZ [values] Poisson's Ratio DENS [value] Density

Analysis Data Block LDCASE KSTR [value] Element Stress calculation L1 Data [values1 Element ID, Face and property value PRINTcntl AVNDstresses[value] Averaged nodal stresses

DISPlacements[valuel Nodal displacements Data Terminator Block

ENDdata Input Data Terminator

Table 5-3 NISAlSTATIC Input, Analysis and Data Commands used in the FLUID/STATIC Interface Models,

5.6 References

'Wilde & Partners, Stockport, Private communication (1995)

20. W. Nicholson and N. Nelson, Rubber Chem. Technol., 63, 368 (1990)

3NISA 11 User's Manual, CH3 NISA Capabilities, Engineering Mechanics Research Corporation, USA, (1994)

4NISA 3D FLUID User's Manual, Appendix 0 Modelling Hints, Engineering Mechanics Research Corporation, USA, (1994)

132

Chapter Six

6. Results and Discussion

This chapter presents and discusses the results of the experimental work described

in Chapter 4 and the modelling described in Chapter 5. The first section of this

chapter deals with factors that affect the production and processibility of the uncured

rubber compound, It reports the results of the tests that were employed to determine

the material quantities required for: (i) the finite element modelling, and, (ii) the

production of the moulded samples used for the physical testing for the evaluation of

the FORM system. These are covered in, respectively, the second and third

sections.

6.1 Mixing and Material Characterisation

6.1.1 The Factors Affecting Processibility

The main factors affecting the properties of polymer compounds are molecular

weight, or molecular weight distribution, molecular structure and the type of filler.

Molecular weight is the Single most important factor in determining the viscosity of a

polymer (and hence its processibility)l,2. Polymers of lower molecular weights have

a lower viscosity and are, therefore, processed and formed more easily. Polymers

with higher molecular weight will, all things being equal, have superior strength

properties. Unfortunately, all things are not equal and polymers with higher

molecular weights, in general, exhibit greater deviations from Newtonian flow

behaviour. This can, at least qualitatively, be explained by the argument that the

average number of entanglements per chain is greater, and the probability of there

133

being an entanglement that will break down under a lower average stress is greater.

Molecular orientation will also tend to be higher, after processing, when the

molecular weight is high. This orientation may enhance strength in the direction of

the flow (parallel to the molecular orientation) but this is likely to be at the expense

of properties in the direction normal to the flow (orientation).

The amount and type of filler significantly affects the processing behaviour of a

compound. Particle size3.4 and structure (bulkiness) have an effect on both the

processibility and the ultimate properties. Horns has described, for a range of both

processing (rheological unvulcanised behaviour) and ultimate vulcanisate (cured)

properties, the effects of decreasing particle size (CATB6) and increasing structure

(DBPA\ General trends showing the effect on properties related to these indicators

are, in general terms: (a), the smaller the particle size the more reinforcement and

the poorer the processability; and (b), the higher the structure the stiffer (harder

processing) and less 'nervy' the unvulcanised compound, for the same volume

fraction of filler. There are also other measurese•9

•1o

, all broadly based upon some

measure of particle/aggregate size. The fillers used in the trial compounds produced

in-house were a highly reinforcing N330 grade and a moderately reinforcing N660

grade (as classified in ASTM D 1765-96a11) respectively.

6.1.2 Mixing

The ASTM NBR compound12 was mixed using a two-stage method because

batches that were mixed using a single-stage mixing cycle needed an abnormally

high mixer power consumption and ran very close to the safety dump temperature,

In one case the batch dumped prematurely and showed signs of premature scorch.

This was indicated by visible lumps of cured material and a tendency for the batch

to crumble. There was also failure to completely incorporate the carbon black,

indicated by low final batch weight and a visible coating of unincorporated carbon

black. Confirmation that single-stage batches of NBR, which were not prematurely

dumped and on visual inspection of the mixed compound seemed acceptable, were

scorched was obtained from curemeter tests. All batches of the four rubber

134

compounds that were mixed in the Francis Shaw K1 Intermix using the recipes and

mixing cycles given in Chapter 4 were satisfactory.

6.1.3 Compound Rheology (Negretti TMS Biconical-Rotor Rheometer)

The Negretti TMS rheometer13,14 was used to determine the relation between the

shear stress (t) and the shear strain rate (y) (hereafter referred to simply as strain

rate or by its symbol y) and the apparent viscosity (11a) for each of the trial

compounds. It is assumed that the behaviour of each compound can be well

described by the power-law relationship,

(6-1 )

where the constants K and n are, respectively, the consistency constant (or

apparent viscosity (11a), K '" 11a at y = 1 S·1) and the non-Newtonian (or power-law)

index. Since the relationship between shear stress and strain rate is not direct

proportionality (as it is for Newtonian fluids) the concept of an 'apparent viscosity'

(11a) is often used to enable a quantitative comparison between compounds. At a

specified strain rate, 11a is defined as

11a = 't/y. (6-2)

Apparent viscosity is also temperature dependent and the relationship can be

described by,

T\ _ T\ e·b(T, • Tr.,l '18 - ,,8 ref (6-3)

(or

K - K e·b(T, • Tr • ., t- ref , (6-3a))

where 118 ral is the apparent viscosity (or Kral the consistency coefficient) at the

reference temperature, Tral, and 11a is the apparent viscosity (or Kt the consistency

coefficient) at the desired temperature Tt and b is a constant.

135

Strain Rate Apparent Viscosity (Pa.s) at 1000 e (5·') NR SBRl SBR2 NBR EOl PB80 FR58 0.1 330000 900000 520000 570000 922850 1223090 965500 0.4 172500 425000 250000 282500 361350 475640 262980 1 86000 205000 142000 158470 173960 210550 119530 4 30500 67500 49500 60590 58470 61790 37050 10 14700 32100 24700 29800 28840 30710 18750 40 4680 10100 8750 10230 9420 11220 7050 100 2010 3990 3600 3540 - - 3980

Table 6-1 Apparent viscosity for each compound for each strain rate measured

Table 6-1 shows the apparent viscosity of each of the trial compounds for each of

the measured strain rates at the reference temperature (100°C). It should be noted

that the temperature quoted is the rheometer test starting set-point temperature and

that there will be a certain amount of shear heating, caused by viscous dissipation in

the material during the test. Table 6-1 allows a direct comparison of the materials

under similar flow conditions. For example, compound SBR1 has a lower value of

apparent viscosity than compound FR58 at a strain rate of 0.1s-\ but at 0·4s-1 the

position is dramatically reversed. This indicates that flow is easier for SBR1 in the

former condition and that, therefore, it would be processed more easily than FR58

and that the converse is true in the latter condition. The trend of a reduction of

apparent viscosity with an increase in strain rate (and to an extent temperature) is

clear for all of the trial compounds. The NR compound, obviously, has the lowest

apparent viscosity in all flow conditions and the SBR1 compound has one of the

highest, as expected. No value is quoted for either EOl or PB80 at the highest

strain rate (100s·1) because the measured shear stress values were far too low,

indicating that the condition was too extreme and melt fracture (or possibly wall slip)

had occurred rendering the measurement unrealistic.

Another, perhaps more useful, method of analysis is a plot of log (shear stress) vs.

log (strain rate) (Figure 6-1 and Figure 6-2) which gives a straight line of slope n 15,

the non-Newtonian (power-law) index. In Figure 6-1 the log (shear stress) vs. log

(strain rate) curves show the aforementioned comparative trends which highlight the

differences in the behaviour of the compounds under conditions of flow. Figure 6-2

shows a log (shear stress) vs. log (strain rate) plot for a range of test starting

temperatures for the compound SBR2. This plot also shows the best-fit trendline

136

(and its equation) for the 100°C curve the gradient of which is calculated giving the

power-law index, n. Appendix E gives the log (shear stress) vs. log (strain rate) plots

for all of the trial compounds.

·1 -0.5

Log ~ vs. Log y for all Compounds at 100·C 3

0.5

o 0.5 1.5

Log y (Strain Rate) 2

~NA

--SBAl SBA2

- NBR - EO!. - PBaO ~FA58

Figure 6-1 Log (shear stress) vs. Log (stra in rate) plot of all compounds at 100·C.

A plot of In (shear stress) vs. temperature (at constant a strain rate of 1 S·l) yields the

temperature dependence of the viscosity index, b1S Figure 6-3 shows the plot of In

(shear stress) vs. temperature for each of the measured strain rates for the

compound SBR2. The best-fit trendline is shown for the curve corresponding to the

test conducted at a strain rate of 1 S·l. Its gradient is index b. Plots of In (shear

stress) vs. temperature for all of the trial compounds are given in Appendix F.

137

Log 1: vs. Log .y for SBR2

3

2.5

en .. ! y = 0.2762x + 2.0867 r8O

•

C u; ~ --- -- 90· C .. '" 1.5

1OO·C J: !!!.

--- 120·C .. Cl

.3

0.5

I

-1 ·0.5 0 0.5 1.5 2

Log y (Slraln Rale)

Figure 6-2 Example log (shear stress) vs. log (strain rate) plot of Negrettl TMS blconlcal rotor rheometer data. The data shown are for SBR2 compound.

en .. '" ~ u; ~ .. '" J: !!!. .. .5

:I 5

4

3

2

o 80

-+ 85

In 1: vs. Temperature (·C) for SBR2

: : - ... -•

Y = ·0,Q11 x + 6.0854

--~-------+-------+

90 95 100 105 110

Temperalure (·C)

! ~

-+- 0,1 • ....... 0.4

1 -- 4 ....... 10 -- 40 -+- 100

115 120

Figure 6-3 Example In (shear stress) vs. temperature plot of Negretti TMS biconlcal rotor rheometer data (for SBR2).

138

-- ---------------

The constants that were determined from the shear stress measurements made

using the TMS rheometer for use in describing the behaviour of the materials in FEA

and lumped parameter the modelling of the flow in the FORM system dispenser are

summarised in Table 6-2.

Material (consistency constant) (or 1'1. at 1s" and 1'1.,0' at 1s"

n non-Newtonian (or power-law) index

temperature dependent viscosity index

Table 6-2 Constants determined from the Negretti TMS Rheometer shear stress measurements and used to describe the behaviour of the compounds in the in the FEA and lumped parameter modelling

6.1.4 Physical Constants

6.1.4.1 Specific Heat Capacity

The differential scanning calorimeter (DSC) was used to measure the specific heat

capacity of the trial rubber compounds'6,17. The values are given in Table 6-3

Compound Specific Heat Capacity (Cp)

JlkgoC NR 1627 SBR1 1563 SBR2 1347 NBR 2017 EOl 1663 PB80 1231 FR58 1070

STD' 1500

Table 6-3 Specific heat capacities measured for the trial compounds

*The values in the tables given under the label STD are not measured values but values selected to be representative of a typical compression moulding rubber compound. These values were used as a reference throughout the modelling phases of the current work.

139

6.1.4.2 Density Measurement

The densities of each of the trial compounds were determined by carefully

measuring the weight in air and water (in accordance with ASTM D 297 - 81 18) and

are given in Table 6-4.

Compound Density (p) ko/m3

NR 1049 SBR1 1171 SBR2 1112 NBR 1218 EOl 1206 PB80 1317 FR58 1845

STD" I 1200

Table 6-4 Densities of the uncured trial compounds

6.1.5 Scorch and Cure (Vulcanisation) Time.

Samples of all of the trial compounds were tested in the Wallace-Shawbury

Precision Cure Analyser to determine their scorch and cure times at 160°C. The

results are given in Table 6-5.

Compound Scorch Time at Cure Time 160°C (secs) at 160°C(min)

NR (20 phr CB) 120 8 SBR1 (60 phr CB) 90 10 SBR2(40 phr CB) 180 15 NBR(ASTM) 30 7.5 PB80 90 8 EOl - 15 FR58/90 60 16·5

Table 6-5 Scorch and cure times for all trial compounds at 160°C

Further cure tests were carried out at 185°C on the commercially produced

compounds PB80, EOl and FRS8 (Table 6-6) to see how this compared with the

recommended processing conditions (Table 6-7). It should be remembered that the

EOl and FRS8 undergo considerable post-cure and, according to Hofmann19, it is

only in this post-cure that the cross-linking reaction is completed and the good

vulcanisate properties are obtained in the fluorocarbon rubber (FRS8).

140

Compound Scorch Time Cure Time at 185·C (secs) at 185·C(min)

PB80 30 4 EOl - 6 FR58/90 - 7

Table 6-6 Scorch and cure times for the industrially produced trial compounds at 18SoC, the temperature recommended for vulcanisation

Compound Cure Time Post Cure Min. at 185·C (min) Time (hrs)/Temp. (OC)

PB80 4 -EOl 4 6/150 FR58/90 6 121230

Table 6-7 Cure for the industrially produced trial compounds as recommended by the manufacturer

Moulding temperature offsets were applied to the PCA set temperatures when

moulding samples with both the FORM system and conventional sheet moulding.

The author has assumed that the conventionally produced O-rings were made with

a mould surface temperature close to the 185°C stated. The FORM system is

consistently about 6-8°C lower in temperature at the surface of the mould than the

temperature measured by the thermocouple in the platen. Similarly the conventional

hydraulic press used in the current work is 10°C below that of the set-point. The set­

points were, therefore, adjusted by adding the offset to the machine set-point to take

this factor into account and obtain the correct cure temperature at the mould

surface.

6.2 Finite Element Modelling

The conditioning history of the material prior to cross-linking plays a key, if not

dominant, role in determining the degree of molecular orientation in the vulcanisate.

This orientation will occur in the FORM system in the preforming dispenser unit and

during mould closure.

The flow and heat histories were investigated using finite element analysis (FEA),

which has been developed into a very powerful resource for design and diagnostics

in many engineering contexts20• Much of this modelling was carried out before the

dispenser design had been finalised, and metal cutting started. The results had a

141

significant influence on the design of the FORM system prototype produced for the

current investigation.

FEA relies on various physical phenomena for chemical, thermal, electromagnetic,

solid mechanics, fluid mechanics and dynamics being expressed in terms of partial

differential equations. These can be solved numerically and thus FEA can be used

to approximate temperatures, stresses, pressure, velocity and magnetic field etc.

The fundamental information for the application of FEA to rubber products has been

known for more than quarter of a century but, excepting the case of tyre

manufacture, has only been applied to rubber technology over the last five to ten

years. Nichelson and Nelson21 suggest the following reasons for why this might be

so.

(i) rubber is nearly incompressible, developing high stresses in regions of

confinement.

(ii) rubber undergoes large strains, indeed its compliance is one of its major

attractions.

(iii) rubber is often bonded to much stiffer materials.

(iv) rubber components are often small and thin.

(v) failure often occurs at the interface with a stiff material.

(vi) rubber material properties expressed as a strain energy function are very

difficult to characterise experimentally.

The finite element modelling was carried out using an 'off the shelf' application to

encourage its use by making it readily accessible and easily reproducible 'on the

desk-top' in an industrial environment22. The finite element analysis software that

was chosen to conduct the work was the NISA 11 family of applications from EMRC.

6.2.1 Heat Transfer Modelling

The early modelling was simply concerned with direct heat transfer. Several

concepts for material feed mechanisms were considered, and two were shortlisted

for further consideration.

142

6.2.1.1 Reservoir Geometry

The design concepts were narrowed down to two (i) the injector and (ii) a transfer

pot. The geometry is defined in Appendix O. The results of the static heat transfer

analysis are summarised in Table 6-8. The geometries with smaller volumes warm

up quickest. Results show that it can take some considerable time to heat up a

mass of rubber from room temperature to 80°C by simple conduction. Some of

these times are clearly excessive and could not be tolerated in a production

environment. A significant reduction in the warm-up time can be achieved. without

excessive loss of reservoir capacity. by increasing the contact surface area of the

rubber with the reservoir by adding a heated core.

The early work was carried out using a mesh constructed of 8-noded (2nd order) 20

axisymmetric elements (NISA element type NKTP = 103). This seemed to cause

some temperature instability at some of the corner nodes on some of the elements

(Figure 6-4 (a)). The problem only occurs when a radical temperature change is

required and can be lessened by ramping the applied temperature rise (Figure 6-4

(b)). Figure 6-5 shows a model in which 4-noded elements were used. The smooth

contours indicate the absence of instability.

On further investigation it was discovered in literature that Oamjanic and Owen23

and Barrett et 81.24 have shown. that when FEA is applied specifically to heat

transfer problems. a mesh of 4-node elements gives better results and exhibits

fewer oscillations than an identical mesh of 8-node elements. 4-node elements were

used subsequently and the work repeated. However. the re-calculated results only

showed an error in the order of a few percent by the end of each run.

Reservoir Shape and Volume (mO) Warm up time (min.) size (mm)

Pot - 0200 x 50 1·571x10·o 157 Pot- 0115 0·519x10·o 130 Pot - 0115 x 50 with 0·484x10·o 92 030 core Injector - 050 x 130 0·158x10·o 68 stroke Injector - 050 x 130 0·153x10·o 35 stroke with 010 core

Table 6-8 Warm-up times for the five proposed reservoir geometries.

143

6.2.1.2 Static Heat Transfer (Pseudo-Flow Simulation)

A draft design for the dispenser was used as the basis for heat transfer modelling.

This early design consisted of a large diameter (50mm) main bore with a flow

division into four smaller diameter bores (30mm) distributing the flow to the meter

cavity. Volume flow rate was used to determine the residence time of the material in

the system. The results of this calculation were considered to be lower than was

desirable so a similar calculation was conducted on a similar geometry (in terms of

flow path length) but representing bores that were smaller in diameter (main bore

30mm and sub-division bore 20mm).

Flow Rate Temperature QC (m3/15s) Dispenser Geometry (bore diameter mm)

50130 30/20 0·lx10·· 23 21

0·01x10" 54 43

Table 6-9 Direct heat transfer modelling of two dispenser runner configurations at two volume flow rates volume flow rates

The predicted temperature for the two schemes modelled is given in Table 6-9.

There does not appear to be significant difference in the two systems. However it is

worth noting that the residence time of the material in the system with the smaller

bores is less than half that of the system with larger bores for a similar temperature

gain. There is also a very noticeable step in the rate

narrow bores in each system. The narrower

144

of temperature rise in the

bores give a greater

""""""'" """""" .. 11 ." " ... ,. ... "." ll ." "." n.lt " ... a ... "." s,.., " ... ..... " .• ... n " ." G." 11.13 .... .. ... , ... " " , tl

"." 'It.'' .... "' ... a .14 "' ... 1',43 " ...

(a) (b)

Figure 6-4 Model of the 0S0x130mm injector (a) early in the sequence (S minutes) shows the nodal instabilities clearly (the temperature ranges from 1S.43· - 81.30· C); (b) the same model much later (nearly 70 minutes) shows a much lessened, but still present effect (the temperature ranges from 78.46· - 80.0· C)

,£W_ oo .•

" .• ".n 19." " . ., "." ?f ,1I

n ...

"It.,, ", .n

"'." ... ., ..... "'." "' .•

Figure 6-S Example of a 2D-axisymmetric model of the 0 200mm pot geometry, constructed with 4-noded elements, after a period of about 160 minutes. The smooth contour bands indicate that the instabilities (Figure 6-4) are absent (the temperature, in this case, ranges from 70.00· - 80.00· C)

145

temperature rise for a given volume of material because heat transfer takes place

through a greater surface area. The fact that the residence time of the material in

the system is much less does not seem to be too detrimental. These results only

take direct heat transfer into account. Viscous heating has not been considered.

6.2.2 Fluid Flow Modelling

6.2.2.1 Dispenser Flow (To Fill the Meter Cavity)

More sophisticated models of fluid flow through the dispenser were created using

the NISA 11/3D-FLUID module. The aim, in the first instance, was to predict the

temperature and pressure history of the material in the dispenser and later to predict

stresses and preform shape change. The methodology and data produced are

intended to help further development of the FORM system. They could possibly be

the basis of a set of design rules or be incorporated in a design package to aid the

design of dispenser inserts and moulds.

6.2.2.1.1 Initial Design (Flow Geometry I of Chapter 5)

This modelling was undertaken concurrently with the design. By the time that this

model was created, the feed mechanism (a reciprocating stutter piston) and the

meter cavity geometry had been settled. The interconnecting flow path however was

still under consideration. Previous heat transfer modelling had been used as the

basis of the design decision to change the bore diameter from the initially proposed

50mm to 30mm in order to obtain a better heating rate.

A set of material parameters, labelled STD and stated elsewhere (pp 141-142) in

this chapter, were contrived, purely for the purposes of modelling. They represent

realistic values for a typical compression moulding rubber compound. The STD

values were used in the modelling of the first generation dispenser design. Figure 6-

6 shows a plot of the predicted total pressure drop across the entire flow path and

also the terminal temperature. The initial starting temperature for the material was

20°C and the body temperature of the simulated dispenser was set at 60°C. The

volumetric flow rates used in the modelling are given in Table 6-10.

146

Volume Flow Rate (Q) Piston Speed Volume Flow Rate(Q) Piston Speed (m3/s) 10.6 (mm/s) (m3/s) 10.6 (mm/s)

141 200 42·4 60 124 175

, 38·9 55

10·6 150 35·3 50 84·8 120 31·8 45 70·7 100 28·3 40 63·6 90 21·2 30 60·1 85 14·1 20 56·5 80 7·07 10 53 75 3·53 5

49·5 70 0·71 1 45·9 65

Table 6-10 Volume flow rates and the corresponding piston speeds calculated for a 30mm diameter bore

As can be seen from Figure 6-6, for the STD input parameters temperature rises to

nearly 600 e at the lowest flow rates. At the highest flow rates a temperature of 80 0 e, a rise of some 60 0 e, is attained. The pressure drop across the system is an

indication of the work needed to force the material through the system.

Pressure drop and temperature are the quantities calculated by the finite element

model but throughout the rest of this chapter the pressure drop has been converted

to the force required to make the material flow through the system, because this is a

more useful engineering quantity from the design point of view. Full pressure drop

and temperature rise details are given in Appendix G. Figure 6-7 shows the FE

model output for the three starting temperatures, 20°, 60° and 1000 e. The

relationship,

P = F/A (6-4)

was used to calculate the force required from the pressure drop output from the

model. The bore was assumed to be 30mm in diameter and the maximum force

(47908N) was calculated from the maximum hydraulic pump delivery (23637kPa) in

a cylinder 50·8mm in diameter.

147

.." cC' c ~

CD en , en

" ~ CD 11/ 11/ C ~ CD Cl. ~

0

" .. ::J Cl. .... CD ~

3 ~

co ::J !!!. .... CD

~ 3 ~ " CD (Xl ~ ..

Cl. -.. ~

::J .. .. .. ~

Cl. .... c ~

CD

0' ~

Cl. or " CD ::J 11/ CD ~

::!! 0 ~ cc

CD 0 3 CD .... -<

120,OE+6

100,OE+6

80,OE+6

60,OE+6

40,OE+6

20,OE+6

70

60

-I CD

50 -5 CD -+-Total Pressure Ql c Drop (across

40 ;; system)

30

20

10

Temperature (temp. rise + initial temp.)

OOO,OE+O +---+---+----+----+----+----+---+----+----/-----+ 0

o 20 40 60 80 100 120 140 160 180 200

Piston Speed (mmls) Proportional to Volume Flow Rate (a)

-." 0_,

~'" VII: -i~ C~ 9O,OE+3 en 3.:.. !!l.." ~. ~ 80,OE+3 AI " -~ .,,~

AI ~ iil'" 3 !:, 70,OE+3 ~; ~ ~c.

~ iilO' ~ ~ E 60,OE+3 Max, Force :::!> AI 0 - Available :e ..

>- -+- 20· C AI .. (47908 N)

" 50,OE+3 ~ ;;:: 60· C 0 en 0 en -~ - ." ~100·C .,. :r .,

40,OE+3 CD ~ ~

c. ':; iij' C' ., ." ~

~ -::I ., 30,OE+3 .. u

~ ~

0 ~ u. C' AI 20,OE+3 .. ~ c. a ~ 10,OE+3 ::I ;:;: ~

~

iD OOO,OE+O 3 ~ 0 20 40 60 80 100 120 140 160 180 200 ::I -3 Piston Speed (mmls) 0 Proportional to Volume Flow Rate (0) c. !!.

This flow path design (flow geometry I of Chapter 5) was rejected because the force

required for flow through the system starting at 20°C, which is akin to, say, starting­

up the machine at the beginning of a shift after it has been left over night, is too

high. At 60°C, the proposed working temperature, operation would be possible at

the highest flow rates for a material with the same characteristics as STD. However,

even moderate flow rates come close to requiring the maximum possible force. This

leaves little spare capacity. A compound with the same rheological characteristics

as STD would be a typical moulding compound with a typical viscosity so that there

would be little scope to process high viscosity compounds.

6.2.2.1.2 Modified Dispenser (Flow Geometry 11 of Chapter 5)

The above FE modelling exercise was repeated on the next generation deSign, flow

geometry 11. From experience, the results of the FE modelling are of the expected

order, but to provide further verification the theory of flow of viscous polymeric fluids

through channels of simple cross-section described by Brydson25 and by

Fredrickson and Bird26 was also used to calculate the pressure drop across the

system. For the purposes of this modelling two simple cross-sections are

considered, namely, a cylindrical tube and an annulus. There are some non­

cylindrical channels in the dispenser design and these were approximated by the

equivalent cylindrical pipes for this modelling exercise. For a pipe

,Jr3n+llnr 4Qln

]

2 Ll~JlJl"R3J M = --=---:----=

R (6-5)

Here R is the radius of tube, Q is volumetric flow rate, L is the length of the pipe, K

is the consistency constant and n is the non-Newtonian or power-law index. For an

annulus

M= Qn2KL rlmJ(R,+Ro)(Ro-R,)21J1J-n (Ro - R,) L 2(2n+ I)

(6-6)

where Ri and Ra are the inner and outer wall radii, respectively.

150

-." c: _ .

3'" 'Cc: co iil Q.",

~c» Dl41 3 co coQ. CD ir ... :!: 3 g 0 .. Q. o !!.­=." cE~ .. n _co ~; .. .c a!:. ::I ' "'ll --co 0 3 ' 'C~ co 0 iil :e -.. !:; n co Cl .... So" N~ 00 _. :e "'''' oco ·0

.. 3 5.!. .... 0< 8= .0'

0'< :::!I ::I

;: co co 3 co ::I -.. ::I Q.

g E ~ >-..

;;:::

.s ~ ::I

! ., ~ o LL

70,OE+3

60,OE+3

50,OE+3

40,OE+3

30,OE+3

20,OE+3

10,OE+3

Max. Force Available (47908 N)

__ STD(20)C

--- STD(60)C STD(100)C

_ ..... ....-~ ..... _ ............ .-...--...... ---:-~==:===:===J--STD(20)M -+- STD(60)M -+- STD(l OO)M

OOO ,oE+o +I------+�------+�------~I~----_rI------~I------+1------+I------~I~-----rl ----~I o W ~ M ~ 100 lW 1~ lM 1~ 200

Piston Speed (mm/s) Proportional to Volume Flow Rate (a)

-----

Since the consistency constant K is temperature dependent, the calculation was

divided into sections each representing a portion of the flow. The parameters are

assumed to be constant for each section of flow. This technique is called 'lumped

parameter modelling'. The required force predictions based on the results of both

the finite element and the lumped parameter modelling (based on Equations 6-5 and

6-6) for the STD 'compound' are shown in Figure 6-8. The notation adopted for the

figures is as follows: CAPS (e.g. STD) indicates the set of material properties used

for the predictions, (number) (e.g. (20)) indicates the model start temperature and M

or C represent Model result (FEA) and Calculated result (lumped parameter

modelling). The results show that it would be possible to process this compound

(STD), to some extent, under all of the starting conditions. The higher flow rates

under the 20°C starting condition, however, could not be processed as the force

required exceeds the maximum that can be delivered. This was not considered a

problem because it would be very rare, if at all, that processing of rubber

compounds would take place in the FORM system at such low temperatures.

The predicted temperatures for the same flow conditions are given in Figure 6-9.

NISAl3D-FLUID outputs the result of its viscous dissipation calculation directly as

fluid temperature. To obtain the temperature from the lumped parameter modelling it

is necessary to assume that the volume average temperature rise ~T is related to

the work done by the material and is directly proportional to pressure drop ~p27.28.

This relationship can be represented as

~T = ~P/pCp, (6-7)

where p is the density of the material and Cp the specific heat. The temperature of

the material after flow through the system, then, is the initial temperature plus ~T.

The plot of terminal temperature vs. flow rate (piston speed) is given in Figure 6-9.

As expected, because viscosity decreases with increasing temperature, the

temperature rises for lower starting temperatures are greater than those for higher

temperatures.

152

0I"T1 :l _. 0.'" I: - ~

I: '" 3",

'l "0 ' ",,,, Q.~ • • • • "0'" • • • • • • • • 01 ~ • • • • • • ~ 3 • 01 _ .

3 i: "'- lOO --'" '" ~ 3 3"0 0'" Q.ji\

'" - 80 =1: -+- STO(20)C :S' CD

"'''' E --STD(60)C !O'

STD(100)C '" ~ 1! ar~ = 60 --*- STD(20)M ::1,0 i! -. ~ ..

__ STD(60)M :l 01 Q.

'" n E -~ ..

....-. STD(100)M ~ '" 0 0-

c.n 3 ., W "0 '" 40 "'= ~ 0

~~ !:;'" '" '" .. 0 o 3 20 --", t.>-~-< ~= 0"0 o~

01 ~ 0 :l _. Q.n 0 20 40 60 60 l OO 120 140 160 160 200 -~'" 00. Piston Speed (mmls) 0C' n'< Proportional to Volume Flow Rate (Q)

:!I :l ~

'" '" co 3 '" :l -

-"T1 " -, 3'" "," CD ~ c.CD

en ", , AI ... ~ 0 AI ::0 3 CD CDJ:I

-" CD _, ~ ~

3 CD c.

0 ... c.o CD ~ =n :j' ~ g ",< , !" E

", Q)

ijj' 1ii () >.

'" ::l

"' -", g CD 'C CD Q) C. ~ ... :;

~ 0 er ~ Q)

(J1 !!!. .=. .l>- Q) - ~

~ 0 ~ IL

n 0 3 ", 0

" ::l C.

"' n AI n " ii' -CD c. .,. '< "T1 m :I> AI ::l C.

70,OE+3

60,OE+3 -+-STD(60)M ___ NR(60)M

50,OE+3 (47908N) -*- SBR2(60)M ~~a:X:'F~o~r!ce~A~~va:il:a~bl:e~~::;:~;;;;;:;::,~::~~~~======~~~~~~~~~~ SBR1(60)M

.:::::;~~~=~;;;;~;;;~;~~~ -lIE-NBR(60)M

:;~~;~;;;~;~~~~~~~::::::::: -+- EOL(60)M

-!- PB80(60)M

;::~~~;:~::::::: - FR58(60)M ~::::~~====tir====r===i --STO(60)C ::: -+- NR(60)C

~~~~~~~~~::::=:::-:==:=====:::-:===::===l SBRl (60)C SBR2(60)C

-*- NBR(60)C ___ ___ E-O-L(60)C

~:::~- -+- PB80(60)C

40,OE+3

30,OE+3

20,OE+3

FR58(60)C

OOO,OE+O .J----~--~---~---~--~---~---~---~--~----o ~ ~ M M 100 1~ 1~ lM lM 200

Piston Speed (mmls) Proportional to Volume Flow Rate (0)

"t1"t1 m-' >'g '" ~ 105 ::> CD c.cn , - ... : ~ 1: ... : 3-i

100 ::::===: -+-- STD(60)C "'CD CD ~ C.3 __ NR(60)C '" _. '" ::> " SBR1(60)C ~!!.

95 ~~ "'-3 CD --SBR2(60)C CD 3 ----", • __ NBR(60)C ~ CD • 3 iil 90 .-.-- -+-- E-O-L(60)C oc c.~

~ -+- PB80(60)C CD CD I-> : ="' ;e - FR58(60)C -'- 85 ::> 0 e '~ tp ~

~ = - STD(60)M E -+-- NR(60)M - 8. ~ 80 ~ E SBR1 (60)M " ./ ~ n ....

SBR2(60)M (Jl 0 (Jl 3

'" 75 --NBR(60)M 0 I: EOL(60)M ::> c.

--PB80(60)M "' 0' FR58(60)M ~

:::!O 0 :e 65

I.Q .. 0 3 CD 60 --< 0 20 40 60 80 100 120 140 160 180 200 = n Piston Speed (mmls) '" Proportional to Volume Flow Rate (0 ) c:; I: ~ -CD C. C' '<

On the basis of the results of this modelling, the design of the dispenser and primary

(a-ring) meter cavity was fixed as flow geometry 11.

Later the same models were employed to predict the behaviour of all the trial

compounds (Figures 6-10 and 6-11)). Again calculations were carried out for the

three starting temperatures but for the sake of brevity only those for the 60°C

starting point are reported now. Further plots are given in Appendix H. The

temperature rise in the material as it flows across the system is important, as one of

the purposes of the dispenser unit is to pre-warm the material to a working

temperature of at least 60°C. The previously described heat transfer modelling

shows that direct heat transfer takes a considerable time. The more rapid the rise in

temperature the higher the throughput attainable. The temperature rises indicated

by the modelling are encouraging in this respect because elevations in temperature

of 5° to over 20° are obtained at the lowest flow rate. It was noted that the finite

element predictions retum consistently higher values than those calculated with the

Equations 6-5 and 6-6.

6.2.2.2 Dispense Flow (Metering and Preforming)

Modelling of flow for the dispense cycle was treated as a topic that is distinct and

separate from flow in the feed and runner system. The FORM system dispense

motion physically shuts off the runner system from the meter cavity and at the same

time creates the opening through which the material is forced to exit the dispenser.

This creates two independent flow systems. Figures 6-12 and 6-13 show the forces

required to dispense a preform for the a-ring and the sheet dispenser units,

respectively. The sheet dispenser requires less force for its operation and all the trial

compounds could be processed although PB80 requires the maximum force at the

very highest flow rates.

156

""TT '" _. Q.'" c: 'tI' m CO ",en 0':"

200,OE+3

tIIN ':T'tI 0_ c: 0

1 BO,OE+3 --Q. O 0' .... COO' III , O'n -CO CO , -CO O..Q O'c:

160,OE+3

140,OE+3 (149713 N)

__ STD fD ~.

Q.CO -.Q. tII _

"0 CO < '" CO : iil Q.c: O'tII '«

~ -0 :r -

(J1 CO c: -..j 33

III CO n:!! :r0 :i' ~

~ ____ NR ~ 120,OE+3 Cl> SBR1 " ~ 0 __ SBR2 u.

100,OE+3 " --lIf-- NBR Cl> ~ __ EOl ·S 0' BO,OE+3 --+-PBBO Cl> a:

- FR5B 60,OE+3

CO iil -!" 40,OE+3

~ 0 0 20,OE+3 3

" 0 c:

'" OOO,OE+O +----I------1I-------+----+----+----+----+----+------j

Q. tII OOO,OE+O 30,OE-6 60,OE-6 90,OE-6 120,OE-6 150,OE-6 1BO,OE-6 210,OE-6 240,OE-6 270,OE-6 CO >< n Volume Flow Rate (m3/s) CO

" --':T CO Z m ;0

n" 0-· 3'l! -gc; co> ::I ' CI. ... ..... .. :l! ::1"0 0-C 0 0:: .,.0 ID Cl III ID .,., CD~ S' 5 . .,.a ID Cl.

~cr 0' nCl. : c;;' ",,, ID ID CI.::1 _. .. ::I ID -< ::I"c ID iil ~CD .. .. " < ~2. '" c ID 3 ~ ID

::!I

~ i 0' , III .. ::I" ID ~

" a 0' ~ ~

~ .. ~ o IL .., e -S .,. ~

160.0E+3

140.0E+3

120.0E+3

--+-STD 100.0E+3 --NR

-+-SBR1

----SBR2 80.0E+3 ---NBR

---EOl

-t-PB80 60.0E+3 -FR58

40.0E+3

20.0E+3

OOO.OE+O +----_+_---_+---~f__---+_---_t_---_+----t_---_+_---___i

OOO.OE+O 30.0E-6 600E-6 90.0E-6 1200E-6 150.0E-6 180.0E-6 210.0E-6 240.0E-6 270.0E-6

Volume Flow Rate (m'/s,

The temperatures that are achieved by the flow from the O-ring and sheet meter

cavities are plotted in figures 6-14 and 6-15. The temperature rise in the metered

dispense phase of the FORM process is much more significant than in the filling

phase and, from a production point of view, this is an asset. The material is

significantly warmed by working in the preforming stage. This brings the temperature

of the preform much closer to the vulcanisation temperature and could reduce 'in­

mould' cure time. The fact that this large step rise in temperature does not occur

until just before the material is expelled from the machine means that the chance of

the material curing in the dispenser and preventing machine operation is low.

Comparison of the two charts shows that higher temperatures are predicted for the

material in the O-ring dispenser than for that in the sheet dispenser. This result is

not unreasonable as the flow path geometry in the sheet dispenser is considerably

shorter than that in the O-ring dispenser. Hence the pressure drop, force required

and temperature, which are all related, would be lower.

159

Z"T1 -cC' Ch e >", =CO

--'" 5' , .... :;::::;:.a:a. co -t co co (;'3 3'" co co " '" -!!'. 3 e

'" 0 co c. .. ~= ::D) " -, cc" '~ G'

C' ~ '< ~ -::r " co -ca - ~

'" ., !: c. n E

~ 0 .,

Cl 3 l-Q ..,

0 e

" c. III

5' -::r co 0 , ~,

" cc c. iir .., co :s III co '" .., '" co c. 0' -co c. C' '<

115

110

105

100

95

90

85

-+- STD --- NR

SBR1

""*'" SBR2 -lI!- NBR

--EOL

-+-PB80 --FR58

80 +------+------~------+_----_+------~------~----_+------~----~

OOO,OE+O 30,OE-6 60,OE-6 90,OE-6 120,OE-6 150,OE-6 180,OE-6 210,OE-6 240,OE-6 270,OE-6

Volume Flow Rate (m'/s)

Z"T1 1ii cS' »5; =CD :::!!'" ::I':' ;:::;:UI CD-I !tCD

~.g CD CD

aD1 32' o ~ Q.CD

~~ S' 2!. IQ::I • CD

Q.

IT '< ... ::r CD

<;' ~ n o 3

" o C ::I Q. UI

S· ... ::r CD UI ::r CD ~ Q.

or " CD ::I ., CD ~

" <; Q.

~ Q.

IT '<

90

88

86

84

U 82 ~

~ ::I

f 80

" Q.

E ~ 78

76

74

72

70+-------+-------+-------+-------;-------;-------1-------1--------r------~

OOO.OE+O 30.0E-6 600E-6 90.0E-6 1200E-6 1500E-6 180.0E-6 210.0E-6 240.0E-6 270.0E-6

Volume Flow Rate (m3/s)

-+-STD

----NR __ SBR1

--SBR2 --NBR --EOL

-+-PB80 -FR58

6.2.2.2.1 What if? Modelling of Other Possible Meter Cavity Configurations

Two other O-ring meter cavities were modelled on a "What if?" basis. It was

assumed that it would be possible to manufacture the proposed geometry. The

same initial conditions were used for these models as were used to model the 0-

ring meter cavity. Only the STD material parameters were used.

20mm

12mm

18mm

5mm

Opening 1 mm wide

(a) (b)

Figure 6-16 Dimensions of the "What if?" O-ring meter cavity cross-sections. The centre of the 1mm opening falls on a 97mm radius.

Configurations designated 0001 and 0002 are shown in Figures 6-16 (a) and (b).

The geometries are considerably different but they have been designed as flow

channels 'funnelling' down to give a similar opening (the direction of flow is top to

bottom in the figure). 0001 has a wider flow path and narrows later whereas 0002

narrows immediately and then becomes a 1 mm wide parallel channel. It should be

remembered that the figure represents a cross-section of an annular cavity. The

characteristics of the flow in each case have considerably different pressure­

temperature-force relationships (Figure 6-17).

162

o:!! Occ OC: ... c;;

'" en :::I , 0. ...

160,OE+3 75

Maximum Force 0'" 0"" og Nn c: CD .. ~ _. CD

(149713 N) 73 140,OE+3

71 ".c ccc: - ::::; ' 120,OE+3 :rCD CD 0. 69 C/)'" -i:::l 00. -3 CD

'" 3 -.., ~ CD -.~

'" '" --.., c: '" ~ ~ CD

'" ~ ~ 3 CD

'" m CD n (,) ;-:r

~ CD 0. ...

Z 100,OE+3 67 ~ -.. 3 0001(N) u

~ ..,

0 CD __ 0002(N) u. 80,OE+3 65 iil .., - -+-0001 (OC) .. c: .: C;; ____ 0002 (0C) :::I ~

er 63,9 .. 60,OE+3 a:

61 0 ~

:::!! 40,OE+3

0 :I; 59

:::I -:r CD

20,OE+3 57

~ :r '" - OOO,OE+O +----t----f----+-----1C-----t-----t----+-----t---+ 55 ::;; ~

OOO,OE+O 30,OE-6 60,OE-6 90,OE-6 120,OE-6 150,OE·6 180,OE-6 210,OE-6 240,OE-6 270,OE-6

3 CD

Volume Flow Rate (m3/s) -CD ~

n '" < ;:;: iij' ..

6.2.3 Prediction of Preform Shape Change Using Fluid Flow and Static Finite Element Modelling in Combination.

Two methods were used to try and predict the shape change of the preform as it is

released from the constraint of the meter cavity. In the first, an axisymmeteric solid

cylinder was created onto which the normal stresses were mapped. The dimensions

of the aperture were assumed for the size of cylinder and the length calculated from

volume. The stresses were progressively released with a time step function to

simulate the rate of extrusion from the dispenser. In the second a similar method

was applied to axisymmetric shell elements.

(i) Axisymmetric Solid Model

Figure 6-18 shows the element mesh before and after the resolution of the applied

stresses. The time step function is clear from the resulting predicted deformation but

the result is quite clearly incorrect. The expected result would show a swelling and

shortening of the preform. Zero deformation was expected at the top of the model

because these nodes were fixed in space representing the point of attachment

before the preform is cropped from the dispenser.

\ \\\ \\\\

Figure 6-18 Axisymmeteric solid finite element mesh and the deformation predicted.

164

(ii) Shell elements

The results from the axisymmetric shell element modelling were more inconclusive

than those reported above

It is not known whether the models created or the method used are fundamentally

flawed or the analysis package is simply incapable of being used for modelling in

this manner. It could be that it is not currently possible to predict the deformation of

the fluid but only entities external to the fluid, other analyses using the results

obtained from the flow module in the static modelling are possible. For example, the

deformation of a pipe due to the pressure in the flow in that pipe can be resolved but

this is not usually performed on the flow rubber.

6.3 Experimental Work with the FORM System

Once the FORM system prototype had been built and was installed in the laboratory

at Loughborough work began on determining, developing and optimising the

operating procedures and evaluating the system performance. Mouldings were

produced both with the FORM system and by conventional compression moulding.

A series of tests were carried out to investigate the anisotropy, molecular orientation

and general performance of the parts and the results compared.

6.3.1 Preforming with the FORM Dispenser

The dispensing operation is a central part of the FORM concept. It is necessary to

shape the material, to meter the quantity and to manage the flow for the production

of isotropic or near-isotropic mouldings.

6.3.1.1 Filling the Meter Cavity and Preforming

An automated feed mechanism was designed to allow swift and easy charging of

the runner system and meter cavity. It consists of magazines that can be pre-Ioaded

with cut strip or milled sheet, slotted into position over the feed pocket and then

forced into the feed pocket by means of a pneumatic cylinder. However, the rubber

jammed in the mouth of the feed pocket, when the stuffer ram was actuated,

165

material failed to be forced into the runner system. The automatic feed mechanism

was abandoned and filling the meter cavity had to be achieved by manually feeding

the cut milled sheet into the feed pocket and actuating the stuffer ram under push

button-control.

This was a setback in terms of eliminating the inconsistency that manual input often

brings to any production or processing operation. The preforms that were produced

initially varied considerably both in terms of dimension and in terms of shot weight.

To investigate the cause, the length of the dispense stroke was set to a fixed value

and the displacement obseNed with the dispenser empty and in a variety of filled

conditions with a number of compounds. The dispense stroke was measured with a

rule (±O·5mm) and on the machine computer control VDU which was set up to

display the current Moire-fringe encoder reading during a dispense cycle (the

obseNed accuracy was no more 5 divisions which equates to ±O.027mm). The

dispense stroke remained consistent throughout. Therefore the only other possible

source of origin for the obseNed variation had to lie within the steps that constitute

the filling procedure. Two methods, previously described, of filling the meter cavity

consistently were determined and, with great care, repeatability was attained.

6.3.1.2 Preform Consistency (Shot-to-Shot Repeatability)

The consistency of the preforms produced by the two cavity filling methods was

remarkably high as indicated by Tables 6-11 - 6-13. Although it has to be mentioned

that achieving this level of accuracy was painstaking and laborious. It does, however

give great confidence in the capability of the system to meter highly accurate

preformed charges repeatably if a suitable automated feed mechanism, such as a

screw pump, can be found.

The O-ring dispenser provides better accuracy than the sheet dispenser, standard

deviations for both dispenser units are well below two. The sheet samples measured

166

~---------

Shot No. Recorded weight (g) 1 50·4 2 49·9 3 49·7 4 50·8 5 49·3 6 50·4 7 50·4 8 49·8 9 51·4 10 49·3

Mean SO Coel. of Maximum Minimum Range Variation Weioht weioht

50·14 0·667 1·33 51·4 49·3 2·1

Table 6-11 Shot-to-shot accuracy and repeatability for NR. The weights of 10 individual shots are given shots are given. The dispenser was setup to deliver a preform of a target weight of 50g

Material Mean SO Coel. of Maximum Weight Minimum Range Variation weight

NR 44·60 0·71 1·60 45·50 43·30 2·2 SBR1 45·98 0·90 1·96 47·60 44·60 3 SBR2 43·62 0·86 1·97 45·20 42·30 2·9 NBR 40·92 1·88 4·60 43·20 37 6·2 PB80 52·27 0·94 1·80 53·40 50·40 3 EOL 48·34 0·93 1·92 49·50 46·40 3·1 FR58 71·94 1-11 1·54 74·60 70·60 4

Table 6-12 Results of the experiment to determine the accuracy and repeatability of the O-ring dispenser.

Material Mean SO Coel. of Maximum Weight Minimum Range Variation weight

NR 36·09 1·32 3·66 38·70 34·10 4·6 SBR1 36·22 1·21 3·33 38·50 34·50 4 SBR2 34·80 1·53 4·41 37·20 32·50 4·7 NBR 28·84 1·54 5·35 30·90 26·80 4·1 PB80 39·11 1·28 3·27 41·60 37·80 3·8 EOL 34·52 1·58 4·57 36·50 32·00 4·5 FR5B 57·08 1·21 2·12 58·90 54·50 4·4

.. Table 6-13 Results of the experiment to determine the accuracy and repeatablhty of the sheet dispenser.

for this experiment were produced from two cavities. If the cavities are treated

individually then the standard deviations fall slightly. There is a slight but consistent

difference in the delivery from the two cavities which amounts to no more than about

3g, depending on the compound, for the above test.

167

The cause of this difference is not known but the explanation favoured by the author

is an imbalance in the flow in the dispenser runner system. The cavity sizes were

measured for an obvious difference in size but none was found. This could have

implications for the development of the FORM system for mUlti-cavity applications.

6.3.1.3 Preform Size Range (Weight)

The possible operating envelope was determined for two dispenser inserts. This is a

measure of minimum and maximum possible dispense; it is an indicator of the

versatility of the machine and highlights one of its limitations. The system could be

used to produce a range of products that have similar dimensions but different

volumes, for instance, a range of O-rings with the same diameter but different

gauge.

I Minimum DisDense((])i Maximum Dispense (Q) O-rirm Disoenser NR I 6·3 I 120·5 SBR I 6·8 I 135·70 Sheet Disnenser NR I 1·5 I 89·3 SBR I 2·4 I 84·2

Table 6-14 Maximum and minimum weights for dispensed preforms

6.3.1.4 Dispensed Preform Temperature

The temperature of the preforms was measured at dispense. This is the only direct

method of comparison and verification of the finite element modelling. It would have

been desirable to make a measurement of pressure drop directly from the flow of

the compound rather than inferring the pressure drop from the material and the

dispenser geometry but this was not possible with this equipment.

The predicted temperature rise has been replotted in Figures 6-19 and 6-20, as the

measured temperature of the material verses dispense time. The results for the in­

house and commercial compounds have been separated for clarity. As can be seen

the measured temperatures are all lower than those predicted by the FE modelling.

The reason for this could be simply that there are heat losses from natural

convection that occur as soon as the material of the preform is released from

168

constraint. There is a noticeable tendency for the results to converge at the lower

dispense times (higher flow rates).

169

3!! CD CD .. I: .. iil I: ~'" CD , Q. .... ~ ... ... oc ~ _. 0" ,"0 ~ CD _. :s :s ..

CD CD Q.Q.

;n' " -ga; :sO' .. ~ ~ 3 -- U

CD 0

3 -Cl) "0 ...

CD :s ~

15 .. -I: ... iil

Cl) c. .. E ~

0 -...I Cl) ... I-e -". E CD ... S· ~ , ". ... 0 Il. I: .. CD n 0 3 "0 0 I: :s Q. .. -;; m "0 ~

CD Q. ,;-c: 0 :s .. .. :s Q.

115

110

105

100

95

90

85

80+-----~------~------r_----_+------;_----~~----_+------+_----~------~

o 2 4 6 8 10 12 14 16 18 20

Preform Dispense Time (s)

--NR

--SBRl ___ SBR2

--M-- NBR

-.- NR(ACT)

-+- NBR(ACT)

....... SBR1(ACT)

--a- SBR2(ACT)

'C"T1 ~ _. (I)'" Cl. I: _. ~ g.(I)

er er» "N "0 .. C " _. CI. ..

3"5: (I) " .. .. .. (I) I: Cl. ~ (I)'C CI.~ -(I) 0'0' 03 , ... :::::!.CD " 3 "''C Cl. (I) _. ~ .. .. "ge­" iil ::: .. :"' 0 ... ...

::r (I) n o 3 3 (I) Cl !: .:c 'C Cl Cl. I: n CD Cl. n o 3 'C o I:

" Cl. .. " m

115

110

105

cr o -Cl) ... :::s l$ 100 ... Cl) Co E Cl) I-

E ... ~ ... Co

95

90

85

~==. ===-: -----------------..-

80+------;------~----~r_----_r------+_----_r------+_----_+------~----~

o 2 4 6 8 10 12 14 16 18 20

Preform Dispense Time (s)

-+-EOL

--PB80

--.- FR58

-PB80(ACT)

-FR58(ACT)

--><- EOL(ACT)

6.3.1.5 Preforming - Observations

6.3.1.5.1 Curtaining

The 0·05 - O·OBmm gap clearance between the inner and outer parts of the

dispenser at the bottom of the meter cavity proved small enough to prevent material

escaping during the process of filling the meter cavity. At the normal operating

temperature of 60°C only the NR and SBR compounds exuded material through the

gap when filling was taking place. This effect could only be observed if the stuffer

was being used at or near maximum hydraulic pressure. The curtain has a very

similar appearance to extrusions from narrow annular dies29•3o which crinkle as a

result of post-extrusion swelling and a subsequent drawing process. The curtain can

prevent the preform from falling into the mould cavity after it is cropped from the

dispenser. The problem is simply resolved by reducing the fill pressure.

6.3.1.5.2 Preform Shape

The shape of the preforms was unexpected. The author and co-workers had

assumed the preform would be reasonably regular in dimension and section. This

was not the case. The preforms from both the O-ring and sheet dispensers were

more elongated than expected in the direction of their extrusion, in some cases

extremely so. Sections of preforms were taken and examined; some examples are

shown in Figure 6-21. The extent of this elongation (Le. the shape of the preform)

seemed to be governed more by the pressure of fill than dispense rate. The shape

of the preform section could be modified by altering the fill regime without

significantly affecting the volume of the dispense. The FORM system was designed

with a certain amount of elongation in mind. The extrusion direction during dispense

and the mould closure direction are the same so that orientation in that direction will

be disrupted on mould closure. The shape of some of the preforms was

unacceptable. In some cases moulding was prevented because when the two

halves of the mould approached, the preform would splay or fold and fall outside the

cavity and produce a short-shot.

172

(a) (b) (c) (d)

Figure 6-21 Sections of C-ring preforms showing substantial elongation in the direction of dispense (top-to-bottom) and predominant tear-drop shape. (a) an extreme tear-drop shape in PBSO, (b) FR5S, (c) NR (meter cavity filled under high pressure) and (d) NR (meter cavity fill pressure modified).

6.3.1.5.3 Lobing

Immediately after cropping the preforms were regular in shape. Subsequently,

however, they were observed over a period of minutes to deform with bulges or

lobes appearing (Figures 6-22 and 6-23) , indicating considerable anisotropy in the

preforms.

50mm

Figure 6-22 The bulging or lobing effect seen in the dispensed sheet preforms. The bulges are in registration with the meter cavity feed runners indicating that there is a memory effect that is geometry related. The material pictured is PBSO.

173

100mm

Figure 6-23 Photograph of an NR O-ring preform showing the lobes that appear shortly after it is freed from the constraint of the meter cavity. The lobes occur on the top edge of the preform in registration with the six runners that feed the meter cavity.

Figure 6-24 An O-ring preform after a period of several months. Recovery is probably complete and the shape changed form a circle at the time of dispense to an almost regular hexagon with the lobes prominant on the corners.

It is suspected that this deformation is a molecular recovery effect. The long-chain

molecules have been oriented and extended in flow and they start to relax and

recover when flow stops. The recovery process continues for many days or even

weeks although visible deformation has ceased (Figure 6-24) .

174

The extent of this effect is governed by the pressure used to fill and pack the meter

cavity. Figure 6-25 shows a sheet preform that was produced using the same

conditions as those for Figure 6-22, except that the meter cavity was filled using a

lower pressure. It is clear from these photographs that the rubber has a memory of

its processing history.

50mm

Figure 6-25 A PBSO sheet preform dispensed after the meter cavity was filled under a moderate pressure. The memory effect correlating to the form of the runner system is still evident but the preform is essentially flat.

This memory effect is time-dependent31 and can be illustrated by Figure 6-26. In this

dispense, the memory of the runner system has almost disappeared. This preform

was the first dispensed after the dispenser had been left standing overnight. The

extended period of time that the material experienced in the meter cavity allowed

molecular recovery and relaxation to take place. The recovery would also be

enhanced by the elevated temperature of the dispenser increasing the ability of the

long chain molecule to move (macro-Brownian motion)32.

175

100n1ll1

Figure 6-26 A regular preform produced after the material experienced an extended period of relaxation in the meter cavity.

6.3.1.5.3.1 Lobing and Molecular Orientation in O-ring Preforms

There are some striking effects that can be seen in the lobes that occur on the 0-

ring preforms and these can be related to the conditions under which the preform

was produced. The first, and most common, condition is the complete lobe (Figure

6-27 (a)), the second has for the purposes of this work been termed 'suckback'

(Figure 6-27 (b)) which is a partial lobe and the third and final condition is the 'ideal'

or desired condition where there is no lobe (Figure 6-27 (c)).

176

- --- -- -----------

20mm

(a)

(b)

20mm

(c)

Figure 6-27 Three different effects that can be seen in the lobes that give an indication of the molecular orientation in the O-ring preform.

177

These three effects can be explained in terms of the molecular orientation that

occurs during the flow of material from the runner system into the meter cavity and

the molecular recovery that occurs when flow ceases. Long-chain molecules are

long and thin when they are extended and when they recover they reduce in length

and increase in diameter or thickness. During flow the molecules get extended

parallel with the direction of flow. The lobe occurs when meter cavity filling is carried

out at the higher pressures. The orientation of the molecules in the lobe is not in the

direction of expected flow, i.e. along the rubber and circumferentially around the

annular cavity (Figure 6-27). If they were, then the most likely result would be the

that the suckback condition would occur as molecules highly elongated parallel to

the direction of flow shorten in length as they recover. The small lumps to either side

of the 'drawn-in' portion of the ring could be caused by the recovery of the molecules

that are just turning the corner as they increase in diameter during their recovery.

The lobe, then, is formed by the recovery of molecules that are oriented in the

direction normal to the flow. This situation occurs at the junction between the runner

and the meter cavity just after the meter cavity has been filled and flow from the

runner has nowhere to go. The desired condition occurs when the meter cavity is

filled optimally and the recovery of the molecules in the direction of flow draws in just

enough material to create the near perfect preform. This perfect fill condition is rare

and the author does not know if it would be possible to devise a method of

determining when this point is reached.

178

6.3.1.5.4 Preform Shrinkage

Preform shrinkage is another effect that can be attributed to the orientation.

Circumferential orientation in the annular meter cavity induced by flow during the

course of filling the meter cavity produces dramatic preform shrinkage was noticed.

In an attempt to measure the shrinkage, it was found the there was also

considerable ('extrudate') swelling with the preforms being considerably larger,

initially, than the dispenser aperture. The swell and shrinkage were measured

roughly with a 300mm rule at a range of times from dispense to approximately 240s.

A high degree of accuracy is not claimed for the results given in Tables 6-15 and 6-

16.

Material Diameter in mm at Time (±10s 0 30 60 120 180 240

NR 225 205 195 195 190 190 SBRl 215 200 202 197 197 197 SBR2 212 202 197 195 193 192 NBRl 215 203 197 197 197 197 PB80 210 200 200 197 195 195 EOl 215 201 200 198 197 197 FR58 212 200 200 199 198 198

Table 6-15 Rough measurement of dispense diameter vs. time after dispense for maximum pressu re fill.

Material Diameter in mm at Time (±10s 0 30 60 120 180 240

NR 205 203 203 197 197 197 SBRl 200 197 195 195 192 192 SBR2 205 202 197 195 195 195 NBRl 200 200 197 197 195 195 PB80 207 202 200 200 198 197 EOl 205 203 203 200 200 197 FR58 202 200 197 197 197 196

Table 6-16 Rough measurement of dispense diameter vs. time after dispense for low pressure fill.

Although the experiment was not rigorous, there is a trend. The initial size of the

preform can be varied by some 10% depending on the material and pressure used

to fill the meter cavity.

179

6.3.2 Elimination of Lobes and Preform Shrinkage with a Modified Dispenser

The lobing (Figures 6-23 and 6-24) in the preforms produced by the system as it

was originally designed (Figure 3-4 and a three-dimensional representation of the

flow path is shown in Figure 5-4) indicated that the preforms contained significant

anisotropy immediately prior to mould closure and that the orientation would be

cured-in because, at the time of initial mould closure, the preform was still relatively

regular (Le. recovery and relaxation had not taken place to a significant extent). The

phased closure (close mould to initially form and warm part - open mould slightly to

allow recovery - close mould to form and cure part) in the direction of preforming

seems to go along way to alleviate a substantial amount of the molecular orientation

in the moulding as illustrated by results given later in this chapter. The relaxation

and recovery that caused the preformed rings to shrink from their initial diameter

and the elongated shape of the preforms (Figure 6-21) were considered to be

problems that would be significant in the production environment and for mould

design (size).

Scrap section and plan view of the flow spUtter plate: Divergent flow occurs in the direction of the arrows. The flow of material divides in the down tube and recombines after the petaloid pedestal before entering the meter cavity.

Figure 6-28 Configuration of the modified dispenser unit. The feed flow path changed considerably.

In the case of a low-stress deformation round a bend followed by the high-shear

deformation of passing through a narrow die shortly thereafter, the low-stress

deformation will be remembered at the die exit causing the extrudate to curl even

180

though the material has been through the high-shear deformation31. The time-scale

for which the material has memory is determined by its viscosity and modulus in

steady flow. Clearly if the time-scale of the material can be accounted for in the

process, then it would be possible to produce preforms that would not suffer from

these orientation effects. Increasing the residence time of the material in the meter

cavity to exceed the natural time-scale of the material would be a resolution (Figure

6-26) but this would have a drastic effect on production rate.

The dispenser was modified by redesigning the O-ring dispenser insert, the main

body (outer and inner) remained unchanged. The two-fold rationale was to manage

the flow and hence the molecular orientation in the meter cavity: (i) to increase the

residence time of the material in the system between entering the meter cavity and

the last major disturbance in the flow and, (ii), to create predominantly radial rather

than circumferential molecular orientation in the meter cavity. The six runners were

eliminated and replaced by an extended flow channel and a much shorter four-way

flow splitter which opens the flow out into a 'disk-like' flow path in order to fill the

meter cavity around the its whole circumference. The direction of material flow

would then be radially, from the centre of the 'disk' into the annular cavity rather than

circumferentially around the cavity because the material would not have to flow

around the meter cavity during filling. The direction of flow is indicated in the inset of

Figure 2-28.

Comparison of dispenser insert features: Original

• long narrow 6-way flow division material recombination in meter cavity

• Iow residence time of material in dispenser

• meter cavity filled at 6 points equi­spaced on circumference

• circumferential molecular orientation

Modified

• short, smooth and wiser 4-way flow division and material recombination before meter cavity

• increased internal volume and therefore increased material residence time in dispenser

• continuous circumferential feed to meter cavity during filling

• radial molecular orientation • 'disk-like 'flow path'

181

----_._--

6.3.2.1 Preforming with the Modified Dispenser

O-ring preforms were produced with the modified dispenser and examined in a

similar way to that described above. Essentially, the filling and metering phases of

the dispenser were not changed. Only the flow path between the main feed bore

and the meter cavity was changed. The geometry of the meter cavity itself was not

changed.

Preform consistency (shot-to-shot), preform size-range (weight) and dispense

temperature were all similar to those reported above, indicating that these measures

are governed primarily by the geometry of the meter cavity and its action rather than

the flow history of filling it. They also provide further evidence of the metering

capability of the of the dispenser.

6.3.2.1.1 Preform Shape

The preforms produced with the modified dispenser do not show the pronounced

elongation in the direction of dispense that was exhibited by the those produced with

the initial dispenser design. Figure 6-29 shows examples of preform sections. When

they are compared to the examples in Figure 6-21 it is clear that they are much

closer to the desired O-ring shape.

(a) (b) (c)

Figure 6-29 Section of preforms produced with the modified dispenser. (a) FR58 preform showing "squarish" section but this is a considerable improvement, (b), NR with an almost

182

round section and (c) PBSO showing the most marked improvement in shape; this a massive improvement on the extremely elongated tear-drop shape that was produced with this compound previously.

(a) (b)

Figure 6-30 Ring preforms produced with the dispenser of modified geometry. (a) a PBSO preform shortly after «1 hr) preforming, this preform is (b) a PBSO preform some months after preforming.

Figure 6-30 shows the shape of complete rings produced with the modified

dispenser. Figure 6-30 (a) shows a preform shortly after dispense and this could be

described as 'O-ring like'. From the point of view of limiting 'in-mould' flow this is

near-ideal as it is close to the desired dimensions before forming in the mould has

taken place. The effect of molecular orientation has not been eliminated completely,

however, Figure 6-30(b), shows a similar PB80 preform after a period of months has

elapsed. The polymer's memory of the processing history has now become evident.

A slight squaring of the preform is noticeable, which is an effect related to the

memory of the flow between the four petaloid pedestals of the 'splitter' plate in the

dispenser.

183

6.3.2.1.2 Preform Shrinkage

The reduction in diameter of the preforms produced with the modified dispenser is

considerably less marked than those made with the initial dispenser design. A

similar set of rough measurements was undertaken and the are results reported in

Material Diameter in mm at Time (± 1 Os 0 30 60 120 180 240

NR 227 225 223 223 223 220 SBRl 217 217 215 213 213 213 SBR2 225 222 218 215 215 215 NBRl 220 220 217 215 215 215 PB80 210 212 205 205 205 205 EOl 218 214 213 210 210 210 FR58 215 213 210 210 210 210

Table 6-17 Rough measurement of dispense diameter vs. time after dispense for maximum pressure fill in the modified dispenser unit.

Material Diameter in mm at Time (±10s 0 30 60 120 180 240

NR 205 203 200 200 200 200 SBRl 202 200 198 198 195 195 SBR2 207 205 205 200 200 200 NBR1 200 200 200 198 196 195 PB80 205 203 203 200 200 200 EOl 207 205 203 202 202 200 FR58 200 200 200 198 198 197

Table 6-18 Rough measurement of dispense diameter vs. time after dispense for low pressure fill in the modified dispenser unit.

Tables 6-17 and 6-18. A similar amount of swell is obtained and the preforms are

larger than the diameter of the aperture through which they are produced. The size

of the preform is also affected by the fill pressure in a similar manner, as those

produced with a high meter-cavity fill pressure are larger than those produced with

lower pressures.

The molecular orientation in the preform will be radial rather than circumferential.

Therefore the molecular relaxation and recovery will tend to pull the top and bottom

(viewed in section) of the preform towards each other, as the molecules shorten,

rather than causing the significant reduction in diameter seen previously. Orientation

effects in the preforms produced with the modified dispenser design are

184

considerably less noticeable than in the preforms produced with the original

dispenser design.

6.3.3 Moulding - Observations, Problems and Defects

Positive land ,---Mould cavity

Metal-ta-metal ,--------O=contact point

___.....:==",."'--Spew cavity

Figure 6-31 Schematic cross-section of the O-ring mould showing the O-ring cavity, the metal­to-metal contact points: (i) at the positive land adjacent to the mould cavity and (ii), the large metal contact area outboard of the spew cavity

Figure 6-31 shows a schematic cross-section of the a-ring mould. A complete ring

preform, is dispensed directly into the cavity and the part is formed on closure of the

mould. A phased closure sequence, opening and closing the mould (akin to the

industry standard practice of 'bumping-off,33) is employed to expel trapped air and

allow molecular recovery for (i) the production of near-isotropic parts and, (ii) the

production of flash free parts.

Several problems with moulding were encountered during the course of the

moulding trials. Many of these problems are encountered in conventional moulding

systems34.35.36.37. Blisters (air trapped in the compound) can be attributed directly to

the piston feed mechanism chosen for the FORM system prototype. The blisters

185

were more prevalent in the compounds of higher viscosity, the FR58 especially. It is

thought that the problem of trapping air in the rubber compound would be resolved

in a production system that employed a different feed mechanism. Short shots were

(a) (b)

Figure 6-32 The results of an elongated preform folding over in the mould. Both samples shown are FR58 (a) shows the effect in the moulded part and (b) shows the catastrophic failure that can occur after immersion in acetone.

not typical and occurred in the case of setting up the machine or determining the

programme (recipe) to be used. In rare cases the preform would hang up on the

dispenser or a preform with a pronounced curtain would stand proud of the mould

and not be pushed correctly into the mould on closure. The final problem of folding

is specifically related to the elongated preform shape produced by the dispenser.

The preform folds as the mould closes and if it is partially scorched it may not knit

together in the mould properly during cure. The effect can be seen if Figure 6-32.

6.3.3.1 Flash Free Moulding

The flash-free moulding mechanism employed in the FORM has been developed

with the goal of manufacturing parts without flash and eliminating the need to for any

subsequent processing (deflashing). It differs significantly from previously proposed

methods for injection38,39 and transfer40 moulding which even-out pressure across

186

the by mould with and in-built flexibility. In addition the mould surfaces are

manufactured with a certain roughness in order to allow air to escape at the surface

but not through the rubber. The FORM system approach relies on mould design and

phased closure. Even pressure across the entire mould surface is also an important

factor in the FORM process, but it is achieved by having a controlled compound

charge and highly parallel rigid mould. The phased closure and mould design

however, are perhaps, the most important components of this process. The mould

has raised lands adjacent to the cavity to trap escaping material in a highly strained

state on initial mould closure. The mould is then opened by a small amount for a few

seconds. The highly strained material recovers during this open period. The

molecules recover into the product and into the main body of the flash. They tear

apart and the flash separates from the component as the recovery proceeds. In

subsequent closure of the mould for final curing, metal-to-metal contact is achieved

between the lands and the opposite half of the mould in the void created where the

rubber has tom and separated.

6.3.3.1.1 Phased Closure

The phased closure used to produce flash-free components is governed by the time

of the initial mould closure and the distance and duration of the first mould opening.

The duration of the first mould closure is the least critical part of the sequence. The

only criterion is that cure should not set in to any great extent. Generally in this study

initial mould closure took no more than 1 s. The mould open distances were set at

0·038, 0·05, 0·1, 0·16, 0·26 and 0·4mm and the mould open time was set between 1

and 8s in 1s steps. The results are given in Table 6-19. The numbers quoted in the

table represent the minimum duration of mould opening to obtain a flash-less a-ring.

Where a second number is given in parentheses this indicates the duration of the

upper limit of the flash-free moulding window.

187

Mould O~ en Time (5)

Mould Open Distance 0·038 0·05 0·1 0·16 0·26 0·4 (mm)

NR - 5 3 3 3 3 SBR1 - - - 4 4(7) 4(7)-SBR2 - - 4 3 3 3 NBR - - - 6 6 5 PB80 - - 5 4 4 4 EOl - - 5 5(7) 4(8) 4(7) FR58 - - 7 5(7) 5(8) 5(8)

Table 6-19 Mould open distance and duration required to obtain components completely free of (separated from) flash. The numbers in parentheses indicate the upper limit of the flash-free moulding window.

At the smallest gap openings (0·038 and O·OSmm) between the moulds flash-free

components are not produced. With the single exception of the NR compound, this

is thought to be because the rubber is still partially confined in the mould cavity and

between the lands. Recovery of the strained material between the lands is not

possible because of this physical constraint. As the duration and the gap opening

increase flash-free components are produced by natural molecular recovery in the

highly strained rubber.

The upper limit of the flash-free moulding window, noticed in some cases, is caused

by the material in the lower half of the hot mould having time to flow under gravity

and spreading onto the land. Material is then trapped and cured between lands on

subsequent mould closure. It was possible to produce flash-free components with all

of the compounds in the trial. The more elastic, lower viscosity and more easily

processible compounds form flash-free parts at lower openings and opening

durations. The explanation of this phenomenon probably lies in the ease the

molecules in the compound have in recovery due to Brownian motion. Figure 6-33

shows a photograph of the flash separated from the component in the mould.

188

Flash separated from 0-ring

Metal surface of mould

O-ring (in cavity)

Walls of plunge cavity

Figure 6-33 Photograph of an O-ring and flash in the plunge mould immediately after opening. The flash can clearly be seen to have separated from the O-ring. The land adjacent to the cavity where metal-to-metal contact was achieved after the phased closure (breathe cycle) is visible.

6.3.4 Product Testing

To assess the capability of the FORM process in producing isotropic mouldings, and

to provide a comparison with conventional compression moulding, a number of tests

were carried out on the a-rings and sheet that were produced with the FORM

system and by conventional methods. Swelling due to the action of solvent and

mould shrinkage were measured for both sheet and ring samples. In addition a

visual examination was made. The solvent swelling tests were perhaps the most

useful in showing the anisotropy that occurs in moulded parts. The test is simple if

not necessarily quick. The area and shape (of the cross-section) of each sample

were measured and the volume calculated before and after immersion in a range of

solvents. Tensile tests were carried out on the sheet samples and compression set

tests on the a-ring samples.

6.3.4.1 Examination of Moulded Product

The first tests for both the a-rings and sheet that were moulded for the system trial

was an initial visual inspection (based on BS 6442:198441) to make sure there were

189

no obvious defects. a-rings and sheet with obvious defects were rejected

immediately. The conventionally compression moulded sheet and commercially

produced, conventionally moulded a-rings were also inspected. The conventionally

moulded a-rings were inspected for join marks and other manufacturing defects.

None were found.

6.3.4.2 Physical Testing of O-Rings

The results of the physical testing of the moulded a-ring samples, both FORM and

conventional, are given in the following sections. Although rings were moulded from

all trial compounds with the FORM system, only three compounds, PB80, EOl and

FR58, were used to mould a-rings conventionally. These results are reported now.

The results appear under the headings Conventional, FORM I and FORM 11. These

represent those conventionally moulded, FORM moulded with the original dispenser

and FORM moulded with the modified dispenser, respectively.

6.3.4.2.1 Swelling in solvent

(i) Shape change (circularity)

The measure of shape used in this test is the degree of non-circularity, which is

defined as the ratio of the difference between the maximum and minimum

measured diameters to the minimum diameter. The larger the value the greater the

degree of non-circularity (or less round) the a-ring. This acts both as a measure of

the quality of initial standard of manufacture showing the shape of the section

(roundness) before swelling and also as an indicator of shape change, hence

anisotropy and orientation, by the action of the solvent.

190

Material Shape - Non-circularity of O-ring section (% of Minimum Diameter)

Conventional FORM I I FORM 11 Prior to swellinQ

PB80 1·25 1·3 1·25 EOl 2·55 3·8 2·68 FR58 3·86 1·25 1·64

After swelling in methanol PB80 4·28 2·5 3·3 EOl 2·81 3·17 2·06 FR58 2·19 2·34 2·23

After swellinQ in toluene PB80 4·84 2·24 2·36 EOl 8·1 2·13 3·0 FR58 3·75 1·21 1·65

Table 6-20 The degree of non-circularity of the O-ring cross-section before and after immersion in toluene or methanol.

(b)

Figure 6-34 Shape change due to the action of solvent in SBR1 O-ring moulded in a conventional two plate mould. The resolution of uneven stress can be seen in (a) and distinct anisotropy caused orientation due to escape flows at the mould split line cured-in. Highly strained molecules swell less in the direction they are elongated causing the dimples in the top and bottom of the swollen sample in (b)

191

The conventionally produced rings show a greater degree of non-circularity in most

cases. This is probably because the there was a slight amount of offset (or mould

mismatch) in the conventionally moulded rings that was not present in the rings

produced by the FORM process. The mismatch was not outside the standard limits

but is bound to show up in the result. (Examples of this mismatch can be seen

clearly in the sections of rings shown in appendix I). The FORM system rings are

similar which is not unexpected as both sets were produced with the same mould

set. There is a definite trend towards increasing non-circularity for all samples with

swelling.

Figure 6-35 Example of FORM system a-ring where despite the fact that considerable swelling has taken place the over all shape has remained constant indicating that the part is almost isotropic.

Using the area of the cross-section of the swollen O-ring sample as a measure

swelling it is clear from Table 6-21 that the FORM system rings swell significantly

less than those produced conventionally. Moreover, in most cases the FORM 11

rings swell less than the FORM I rings, but the difference between these two is less

marked. Figures 6-33 and 6-34 show sections of rings before and after swelling in

solvent.

192

(ii) Change in size (area of section)

Material Area of a-ring cross-section (% size increase)

Conventional FORM I I FORM 11 After swellina in methanol

PB80 14 8 4 EOl 13 2 6 FR58 12 8 4

After swellinq in toluene PB80 90 60 32 EOl 63 60 64

FR58" - 4 6 After swelling in acetone

FR58 78 I 53 I 47

Table 6-21 Change in size of the O-ring cross-section due to the action of solvent.

(iii) Change in volume

Material Change in Volume (% of oriqinal volume)

Conventional I FORM I FORM 11 After swellinq in methanol

PB80 110 105 102 EOl 104 104 104 FR58 101 101 101

After swelling in toluene PB80 200 196 192 EOl 256 242 235 FR58 125 121 119

After swellina in acetone FR58 I 272 I 266 261

Table 6-22 Change in volume due to the action of solvent on sample of O-rings

The full picture of the amount of swell due to solvent cannot be gained without a

measure of the total volume, but the area measurements given above are useful for

highlighting anisotropy. The volume change due to swelling shows that swelling is

less in the rings produced by the FORM system. This could be due to different cure

conditions but every effort has been made to ensure that the moulding conditions for

all samples were constant. The reason for the difference preferred by the author is

that the molecular structure in the FORM rings is one of molecules that are coiled

and relaxed before cure takes place. This coiling of the molecules provides a greater

number of available cross-link sites in a given volume and a more even 'tighter'

network of cross-links is obtained.

• FR58 (a flourocarbon elastomer) has good resistance to swelling in toluene so the size changes are very small. There was no measurable increase in size for the FORM 11 samples.

193

6.3.4.2.2 Compression Set

Apart from the fact that the standard test specimen could not be used, the

compression set test was carried out in accordance with SS 903: Part A642• The

results in Table 6-23 show similar results to those of the swell tests. The more

isotropic structure of the mouldings produced by the FORM process show greater

compression set resistance for similar reasons.

Material Compression Set (%)

Conventional FORM I FORM 11 PB80 20 18 18 EOl 15 15 15 FR58 14 20 12

Table 6-23 Compression set in a-ring samples

6.3.4.2.3 Mould shrinkage

Material Mould Shrinkage (Diameter (mm)l% shrinkage)

Conventional FORM I FORM 11 Mould dia. 198mm Mould dia. 199·53mm Mould dia. 199·53mm

PB80 194·95/1·54 197·04/1·25 197·311·12 EOl 191·57/3·25 193·4213·1 195·04/2·25 FR58 190·24/3·92 191·41/4·07 191·95/3·8

Table 6-24 Mould shrinkage results for FORM and conventional a-rings manufactured from the commercial compounds.

The shrinkage in the conventional and FORM I O-rings is comparable. The likely

molecular orientation in these two sets of rings is in the same direction,

circumferentially around the ring. The extrusion of the chord used for the

conventional rings will orient the molecules in the direction of flow along the chord

and the flow required to fill the meter cavity is also in the same direction. The

recovery of the molecules in these rings will have the effect to reduce the size

(diameter) of the ring. In the FORM II rings the radial molecular alignment shows

less shrinkage in the diameter. The results of the mould shrinkage measurements

are in good agreement with those of previous studies in literature43,44,45,46

194

6.3.4.3 Physical Testing of Sheet

Tests were carried out on the sheet samples manufactured conventionally and with

the FORM system. Molecular orientation as a result of processing (Le. milling 'grain')

prior to moulding is a well known phenomenon. Therefore all the tensile tests on the

sheets were conducted in two mutually perpendicular directions.

6.3.4.3.1 Mould shrinkage

The mould shrinkage of the sheet sample produced for the trial was measured. The

dimensions of the moulds were carefully measured with Vernier callipers. The

average dimensions are given in Table 6-25. The reduction in linear dimension,

Mould Length(mm) Width(mm) Thickness (mm)

Conventional 122·5 120·0 1·93 FORM 152·9 90·3 1·97

Table 6-25 Dimension of the moulds used for the production of sample sheet for the trial

expressed as a percentage change of the original mould dimension for the FORM

and conventional products, is given in Tables 6-26 and 6-27, respectively. The ratio

of the change in the two major directions is also given as a measure anisotropy of

shrinkage. All of the sheets measured in the test shrank in the two major directions

Table 6-26 Mould shrinkage and anisotropy of mould shrinkage for the sheet sample produced with the FORM process

195

Material Chanae in linear dimensianl%) lenath Width Thickness Anisatropv

NR 2.68 1.23 -1.59 2.18 SBR1 2.24 1.98 -2.97 1.13 SBR2 2.12 2.11 -0.98 1.00 NBR 2.78 1.23 -2.74 2.26 PB80 1.25 0.95 -1.27 1.32 EOl 3.92 2.75 -3.34 1.43 FR58 3.21 2.38 -1.75 1.35

Table 6-27 Mould shrinkage and anisotropy of mould shrinkage for the sheet sample produced by conventional compression moulding

(length and width) and expanded in the direction of mould closure (thickness); this is

indicated by the negative numbers in the thickness column. The shrinkage cannot

be due only to the thermal expansion of the material because, if it were, the sheet

would not get thicker in the direction of mould closure, it would shrink. This is due to

molecular orientation in the flow directions as closure of the mould forces the charge

to fill the extremities of the mould. Molecules are extended in this flow and their

recovery after demoulding is responsible for the reduction in the two major directions

and the increase in thickness.

6.3.4.3.2 Tensile Testing

The tensile tests carried out on the sheet samples moulded by both the FORM

system and conventional means demonstrate the prevalence of anisotropy in

compression moulding. Tensile tests conducted in the two mutually perpendicular

directions chosen show differences in strength and elongation at break. A small but

noticeable trend can be seen in the results given in Table 6-28 and typical examples

of the stress strain curves obtained are given in Figure 6-36.

196

26

24

22

20

18 ~

ca 16 Q.

:E 14 -(/) (/) 12 I!! -10 (J)

8

6

4

2

0 0 40 80 120 160 200 240 280 320 360 400 440 480 520 560

Strain (%)

Figure 6-36 Typical stress-strain curves for selected compounds

NA - SBA1

SBA2 - NBA - EQL

- PB80

Overall the tests , the elongation at break is lower in the direction labelled parallel (/1)

(i.e. with the milling 'grain' or in the case of FORM the parallel with the longest side

of the rectangular mould) . A closer look at some of the individual results, for

example NR, shows that there is significant difference of 10% between the

elongation at break in the two measured directions perpendicular Cl ) and parallel

(11). A value of 10% is typical of this set of data indicating that there is significant

molecular anisotropy. The range for the ratio 1. : II for this data se is from +20% to

-10%. These results indicate that the molecular orientation, due to an extension of

the polymer molecules in processing, is generally in the direction 1/. The molecules

are already partially extended in the II direction before the test and the tensile load

is applied. Therefore the amount of further extension possible, before the molecules

reach maximum extension and then rupture, is less than might be expected. This is

true for all of the conventionally moulded samples and about half of those moulded

using the form system.

Globally the strength at break data does not show any significant trend in either of

the measured directions but, as above, individual compounds show significant

197

differences in the two measured directions indicating anisotropic orientation of the

molecules in the components.

Direction of test in Modulus at 300% Strength at break Extension at Break relation to direction elongation (MPalSD) (%/SD) of moulding (MPalSD)

NR FORM I! 6·2210·08 25·7/1·2 570/11 FORM 1. 5·43/0·49 29·211·8 629/23 Conv.!1 4·3410·17 15·35/2·76 533/40 Conv.1. 4·56/0·33 18·53/2·78 570/30

SBR1 FORM I! 20·7/1·5 26·0/1·5 390/41 FORM1. 19·6710·56 25·410·8 39419 Conv.!1 19-43/0·63 25·3/0·8 389121 Conv.1. 20·010·5 26·1/1·4 399133

SBR2 FORM I! - 7·70/0·51 271132 FORM1. - 8·49/1·13 265137 Conv.!1 - 7·6110-46 212124 Conv.1. - 7·39/0·17 258116

NBR FORM I! 21·1/0·7 24·212·28 389/53 FORM1. 21·6/0·6 24·710·5 380121 Conv.!1 21·610·9 22·6/2·4 320159 Conv.1. 21·8/0·4 24·3/0·8 359126

PB80 FORM I! - 16·31/0·23 352117 FORM1. 15·05/0·36 15·40/0·44 315/19 Conv.!1 16·10/0·85 15·87/0·84 293/55 Conv.1. 15-46/0-30 15-63/0-54 330/44

EOl FORM I! - 26-8/0-9 223111 FORM1. - 25·1/0·9 236114 Conv.!1 - 26·311·2 237119 Conv.1. - 24·9/1-6 256123

FR58 FORM I! - 16·34/1·56 267/34 FORM1. - 17-21/0-95 253157 Conv.!1 - 16·85/1·15 245/65 Conv.1. - 17-36/1·32 276/54

Table 6-28 Results of tensile test carried out on dumbbells cut from moulded sheet. The directions FORM 11 and 1. represent the direction parallel to and normal to B (Figure 4-6) respectively and similarly Cony. 11 and 1. represent the directions parallel to and normal to A (Figure 4-6). A is the direction of the milling grain. Mean modulus at 300% extension and strength at break are given together with extension at break. Standard deviations are for all measurements are also given.

These results do not support the idea that the FORM sheets are isotropic but they

do highlight the fact that there is a significant amount of anisotropy in most moulded

components. The effect of mill grain and molecular orientation due to in mould flow

does seem to be evident in the moulded sheets, especially the conventionally

moulded sheet. However the effects are very small. On examination of the

processes the similarity of the results could have been expected because the two

198

sheet moulding processes are similar. A small charge of polymer is placed in the

centre of the mould cavity and, on mould closure, is forced to flow to the cavity's

extremities. It is difficult to imagine how the FORM sheet dispenser could be

modified to prevent this need for in mould flow and permit the production of isotropic

sheet.

6.3.4.4 Summary of Preform Production and Physical Testing

The tests on the O-ring samples show that the FORM system is capable of

producing rings that are substantially isotropic. The formation of a complete ring

preform that is near the size and shape of the mould cavity limits flow to the

absolute minimum and phased closure (moulding 'breathe') allows rapid recovery of

molecular orientation in a semi-constrained state. These effects and elevated

temperature all combine to reduce molecular orientation. The O-rings produced

using the FORM process compare favourably with similar rings produced

conventionally. Post-demoulding distortion is the same or less and when immersed

in solvent the FORM system rings distort less even at high swell ratios.

The results of the FORM system sheet production are, unfortunately, not so

encouraging. The behaviour of the sheet on demoulding is comparable to

conventionally produced sheet. The effects of orientation are measurable. This is

undoubtedly due to the fact that the material has to undergo considerable flow in the

mould as closure forces the material to the extremities of the cavity. The orientation

due to flow is too severe for recovery to be aided by a 'breathe' cycle and the

material has to undergo similar amounts of flow in both the conventional and FORM

sheet moulding processes.

6.4 References

'w. Hofmann in Rubber Technology Handbook, Ch3 (3.2.4) pp50-S2 (Carl Hanser Verlag, Munich, 1989)

2F. N. Cogswell, Polymer Melt Rheology: A Guide for Industrial Practice, Ch4 (4.2) pp77-81 (George Goodwin Ltd., London, 1981)

199

3E. Schmidt, Ind. Engng. Chem., 43, 679 (1951)

4B. B. Boonstra, Polymer, 20, 691 (1979)

'J. B. Horn, Rubb. Plasl. Age, 50, 457 (1969)

BASTM D 3765 - 96 Standard Test Method for Carbon Black- CTAB (Cetyletrimethylammonium Bromide) Surface Area, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

7 ASTM D 2414 - 96a Standard Test Method for Carbon Black - n-Dibutyl Phthalate Absorption Number, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

8 ASTM D 3265 - 96 Standard Test Method for Carbon Black - Tint Strength, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

9 ASTM D 3493 - 96 Standard Test Method for Carbon Black - n-Dibutyl Phthalate Absorption Number of Compressed Sample, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

10ASTM D 1510 - 96b Standard Test Method for Carbon Black - Iodine Absorption Number, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

11 ASTM D 1765 - 96a: Standard Classification System for Carbon Blacks used in Rubber Products, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1997)

12ASTM D2934 - 89: Standard Practice for Rubber Seals-Compatibility with Service Fluids, American Society for Testing and Materials, (1989)

13A. King, Plastics Rubb. 1nl. 14 (1), 23 (1989)

14S. N. Ghafouri and P. K. Freakley, Polym. Test, 11, 101 (1992)

"p. K. Freakley, Rubber Processing and Organisation, Ch2 pp17-18, (Plenum Press, New York, 1985)

1BM. E. Brown in Introduction to Thermal Analysis Techniques and Application Ch4 pp25 - 38, (Chapman Hall,1988)

17G. Kampf in Characterization of Plastics by Physical Methods - Experimental Techniques and Practical Application Ch4 pp179 - 191, (Hanser Publishers, Munich, 1986)

18ASTM D 297 - 81 Standard Test Methods for Rubber products - Chemical Analysis: Part A 15, Annual Book of ASTM Standards, 09:01, American Society for Testing and Materials (1985)

"'W. Hofmann in Rubber Technology Handbook, Ch3 (3.3.15) p123 (Carl Hanser Verlag, Munich, 1989)

'OAnon, Machine Design, 65,12,27 (1993)

21 D. W. Nicholson and N. Nelson, Rubber Chem. Technol., 63, 368 (1990)

22p Dvorak, Machine Design, 65, 5,102 (1993)

23F. Damjanic and D. R. J. Owen, Nuclear Eng. Des. 69,109 (1982)

24K. E. Barrett, D. M. Butterfield, J. H. Tabor and S. Ellis, Int J. Mech. Eng. Edu. 18,59 (1989)

200

25J. A. Brydson, Flow Properties of Polymer Melts 2nd ed., Ch2 pp 18-28 and 212-213. (George Goodwin Ltd., London 1981)

26A. G. Fredrickson and R. B. Bird, Ind. Eng. Chem. 50, 347 (1958)

27F. N. Cogswell, Polymer Melt Rheology: A Guide for Industrial Practice, p137 (George Goodwin Ltd., London, 1981)

2BA. W. Birley, B. Haworth and J. Batchelor, Physics of Plastics: Processing, Properties and Materials Engineering, Ch 3 p59 (Carl Hanser Verlag, Munich, 1991) 29F. N. Cogswell, Polymer Melt Rheology: A Guide For Industrial Practice, p105 (George Goodwin Ltd., London, 1981)

30 J. S. Schaul, M. S. Hannon and K. F. Wissburn, Trans. Soc. Rheol., 19, 351 (1975)

31F. N. Cogswell, Polymer Melt Rheology: A Guide For Industrial Practice, p49 George Goodwin Ltd., London, 1981)

32W. Hofmann, Rubber Technology Handbook, p222 (Carl Hanser Verlag, Munich, 1989)

33 J. Menough,Rubber World, 173, 1,67 (1983)

34M. A. Wheelans, Rubber Chem. Technol., 51,1023 (1978)

35J. G. Sommer, Rubber Chem. Technol., 51,738 (1978)

36J. G. Sommer, Rubber Chem. Technol., 58,662 (1985)

37M. A. Wheelans in Injection Moulding of Rubber, Ch5 pp188-195 (Butterworth & Co. Ltd., London, 1974)

38H. F. Jurgeleit, Rubber Age, 90,763 (1962)

39H. F. Jurgeleit, British Patent, 1022084 (1964)

4oH. G. Gilette, RubberWorld,157,1, 67 (1967)

41 BS 6442: 1984 British Standard Specification for Limits of Surface Imperfections on Elastomeric Toroidal Sealing Rings ('O'-rings), British Standards Institution (1984)

42BS 903:Part A6:1989, British Standard Methods of Testing Vulcanised Rubber,Part A6, Determination of Compression Set after Constant Strain, British Standards Institution (1989)

43J. R. Beatty, Rubber Chem. Technol., 51,1044 (1978)

44K. Nakashima, H. Fukuta and M. Mineki, J Appl Pol. Sci., 17, 769 (1973)

45A.w. Fogiel, H. K. Frensdorff and J. D. MacLachlan, Rubber Chem. Technol., 49, 34 (1976)

46J. D. MacLachlan and A.w. Fogiel, Rubber Chem. Technol., 49, 43 (1976)

201

Chapter Seven

7. Conclusions

From this work on the design, development, modelling, and evaluation of the novel

computer-controlled compression-moulding, or FORM, system it is clear that the

moulding of rubber components is not simple. The treatment of the material prior to

forming and curing has a significant effect on the orientation of the long-chain

molecules in the mouldings. The effects of this molecular orientation are

phenomena that designers, toolmakers and moulders have worked around and had

to cope with for many years.

Analysis of extruded chord and preforms etc. can be used as a window into the

molecular orientation induced in premoulding processes. It is well known in polymer

processing that a screw can put a curl or wave into the extrudate. In this study

particular attention was paid to the preform recovery behaviour and shape. Long­

term observation showed that recovery occurred over several months.

Preforms produced with the first generation O-ring dispenser showed considerable

anisotropy. In the case of the O-rings this anisotropy was largely undetectable in the

moulded product. O-ring preforms showed considerable shrinkage (molecular

recovery), in the order of a 10% reduction in diameter, in the first 240 seconds after

production. The second-generation dispenser gave a more reliable shrinkage of no

more than 5%.

The preforms produced with the initial dispenser were elongated, lozenge or tear­

drop in cross-section and having a shape factor of 3 typically but 7.5 in extreme

202

cases. Those produced with the modified dispenser have a shape factor of 1.25

typically, with some examples being very close to 1. In the latter dispenser the

molecular orientation is managed so that elongation during, and recovery after

dispense occur in the same direction. The fact that these preforms are very close to

the shape of the mould cavity also means that very little flow takes place in the

mould during shaping.

The preforming O-ring dispenser, as initially designed, produced preforms that

showed considerable anisotropy, due to their flow history. The internal flow path was

successfully re-designed to account for the memory the polymer has of its flow

history. The modified dispenser delivers substantially isotropic preformed rings.

The novel compression moulding system was used to produce O-rings and sheet

and comparison with conventionally moulded O-rings and sheet was undertaken. A

range of tests to determine anisotropy were carried out on both the FORM and

conventionally moulded O-rings and sheet.

In comparison FORM system O-rings show better integrity than conventionally

moulded samples. The FORM system O-rings exhibit less mould shrinkage and

compression set in standard tests. In solvent swelling tests measurements of

change in shape, cross-sectional area and volume were made. The new moulding

system shows less change in shape and lower swelling as measured by area and

volume.

Mould shrinkage and tensile testing were used as measurements of the anisotropy

of the sheet samples produced by both manufacturing methods. In both cases there

was no obvious difference between the two methods, however the results do serve

to highlight the significant level of anisotropy that can occur in moulded rubber

products with typically a 10% difference in the extension at break in the 2 mutually

perpendicular directions measured.

The O-rings produced with the novel compression moulding system show

considerably less anisotropy than the conventionally moulded O-rings. The sheet

203

samples produced with the novel moulding system are of comparable anisotropy

with the conventionally moulded sheet.

Finite element models of the flow geometry were created to predict the temperature

rise and pressure drop and therefore calculate the force required, firstly to enable

flow through the specified geometry and secondly to dispense a metered preform.

The methods and techniques developed for this modelling can be used for the

further development of the system or applied to flow in other rubber processing

applications. At the moment the models give an over-estimate of up to 20% of the

temperature rise measured for dispensed preforms.

The combination of a computer control system and the preforming dispenser has

enabled the automation of the, still relatively labour intensive, compression moulding

process and production rates for compression moulding with this system could be

brought in to line with those of injection moulding.

A method of producing flash-free compression moulded a-rings was developed. A

combination of mould design and the precise control of the closure profile to form

the part utilise the inherent viscoelastic properties of uncured rubber compounds

and enable separation of flash from the component in the mould.

7.1 Suggestions for Further Work

The FORM system, as is stands now, can produce isotropic flash-free a-rings.

However, there are areas in which it could be could be improved. The development

of a new feed mechanism to replace the current reciprocating piston would enable

better, more precise control over the, seemingly critical, fill pressure. Further system

development could be to redesign the sheet moulding dispenser, and possibly the

mould set, to produce near-isotropic sheet. The design and production of preforming

dispenser inserts and mould sets could be extended to other products or component

shapes to enhance the product range.

204

The development of the finite element modelling could include such factors as

convection to atmosphere to see if the modelling can recreate reality more faithfully.

The prediction of the change of shape of a dispensed preform should remain a goal

of the modelling associated with the FORM system. If this tool could be developed it

would be a most powerful asset and enable rapid design of dispenser and mould

sets.

205

Appendix A

Piston and Feed pocke

> <

L ______ ~L-__________ J

Press Dispenser

This schematic shows the dispenser and press configuration. The dispenser is

mobile and can traverse between the platens, produce a preform and retract before

the press/mould closure sequence is initiated.

Appendix B

r-------------------

! ______________________ J

Above a schematic of the mould used for producing flash-free O-rings is given ..

above. Below are a series of pictures, representing the section enclosed by the

dashed line, showing the different stages of flash"free moulding,

Here the mould halves are approaching initial closure. The preform which is near

shape and size, rests in the cavity.

On initial closure (above) flash is formed and molecules get extended and trapped

between the lands due to high flow rates. -' ',.'

. .,

The mould is then opened again and molecular recovery allowed. The molecules in

the flash between the lands recover and tear apart separating the flash from the

ring.

Subsequent mould closure allows metal to metal contact.between. the lands for

curing. ., ":.

Appendix C

**EXECUTIVE data deck for 3D-FLUID [EMRC NISA] ANALYSIS: FLUHT Analysis type. DIMENSION : AX Axisymmeteric FILE : sq15 SAVE:26,27,54,55 STDS : ON,0.3,0.3,0.3 NONNEWTONIAN : 0.288 VDISIP : ON,0.5E-04 *ELTYPE

I, 3, 1 *NODES

Out put data file types

Non-Newtonian index Viscous disipation factor

Nodal co-ordinates 1, , " 2, , , ,

9.89500E+01, O.OOOOOE+OO, O.OOOOOE+OO, 0 9.93667E+01, O.OOOOOE+OO, O.OOOOOE+OO, 0

3, , I , 9. 97833E+01, O. OOOOOE+OO, 0. OOOOOE+OO, 0

227, , , , 1. 02450E+02, 3.80000E+01, O.OOOOOE+OO, ° 228, I I I 1.04200E+02, 3.80000E+01, O.OOOOOE+OO, ° 229, I , I 1.05950E+02, 3.80000E+01, O.OOOOOE+OO, ° 230, , , I 1.07700E+02, 3.80000E+01, O.OOOOOE+OO, 0 231, , , , 1.09450E+02, 3.80000E+01, O.OOOOOE+OO, 0

* ELEMENTS Elemental Connectivities 1, 1, 1, 1, ° 1, 2, 9, 8, 2, 1, 1, 1, ° 2, 3, 10, 9,

216, 217, 224, 223, 175, 1, 1, 1, 0 218, 219, 226, 225, 176, 1, 1, 1, 0 219, 220, 227, 226, 177, 1, 1, 1, 0 220, 221, 228, 227, 178, 1, 1, 1, 0 221, 222, 229, 228, 179, 1, 1, 1, 0 222, 223, 230, 229, 180, 1, 1, 1, ° 223, 224, 231, 230,

*MATFLUID Material data DENS, 1,0, 1.40000E-03, VISC, 1,0, 8.53400E+01, COND, 1,0, 1. 70000E-04, SPEC, 1, 0, 1.16430E+OO, *FLCNTL, ID: 1 10,50,1,1.0,0.001,0.lE+09,1.0,1.0,0.0,250.0 * BCDVAR

** BCDVAR SET : 1 1,U 1,V 7,U 7,V 8,U

229, V

O.OOOOOE+OO, , , O.OOOOOE+OO", O.OOOOOE+OO, , , 0. OOOOOE+OO, , , O.OOOOOE+OO",

,-1.50000E+01",

0, 0, 0, 0, 0,

0,

Bountary conditions Velocities in u and v directions

o

° ° o o

o

*rCDS

229,T 230,U 230,V 230,T 23l,U 23l,V 231,T

, 8. OOOOOE+Ol, , , , O.OOOOOE+OO" , ,-1.50000E+Ol",

8. OOOOOE+Ol, , , O. OOOOOE+OO, , , O. OOOOOE+OO, , , 8. OOOOOE+Ol",

** rCDS SET = 1 1,T 8.00000E+Ol 2,T 8.00000E+Ol 3,T 8.00000E+Ol 4,T 8.00000E+Ol 5,T 8.00000E+Ol 6,T 8.00000E+Ol

2l4,T 8.00000E+Ol 2l5,T 8.00000E+Ol 2l6,T 8.00000E+Ol 2l7,T 8.00000E+Ol 2l8,T 8.00000E+Ol 2l9,T 8.00000E+Ol 220,T 8.00000E+Ol 221,T 8.00000E+Ol 222,T 8.00000E+Ol 223,T 8.00000E+Ol 224,T 8.00000E+Ol 225,T 8.00000E+Ol 226,T 8.00000E+Ol 227,T 8.00000E+Ol 228,T 8.00000E+Ol 229,T 8.00000E+Ol 230,T 8.00000E+Ol 231,T 8.00000E+Ol

*ENDDATA

0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0

Initial conditions ( temperature)

Appendix D

All dimensions in mm. A - A represents Axis used in FEA Model. Hatched area represents usable volume.

200 mm dia. Cylindrical Pot.

50 mm dia. Ram Injector.

1------'30-----... 115 mm dia. Cylindrical Pot.

;

i

, .... /' ...

. . ~ > '

sP

SO mm dia. Ram Injector with Heated Core.

: • .J • . ,

115 mm dia. Cylindrical Pot with Heated Core.·

U· - ... . , : "

"'2. 5 :::j~~s;~;;~;;~~~~=-::-=-=-=-~-i5.'.. •. r . ,.

'" I------,.l,,~-..-...ll

Appendix E

Log Shear Stress Vs. Log Strain Rate for SSRl 3

2.5

2 Y =< 0.2077x + 2.2645

1.5

... : ~ '.

1

0.5

.... :, -' .. ~ .. :.-.... -- .

-I -0.5

o 0.5 1

log Y (Strain Rate) 1.5 2

--ao·C • 90·C

-""'lOQ'C ~120·C

j

j

J

j

j

j

j

3

2.5

1.5

1

0.5

-1 -0.5 o

Log 't VS. Log yfor SBR2

y = 0.2762x + 2.0S67

. ~.'. . if ~ ~ . ,,-

.-,'

0,15 i ~ .. r. ;. . ,. _ .1 Log y (Strain Rate)

--SO°C ---90°C --100°C' --120°C

1.5 2

1.5

1

0.5

-1

-0.5

o

Log S ...... S .... Vs. Log St", .. Rate 10, .. S. a

, ..

." . ~ '., . ,.,' .

0.5

Log r (Sfrlli/l Rate) . 1

1.5 ,', - .

. '" • i

--ao·c ---90·C -'-100·e ~120·C

I

~

3

2.5

2

1.5

1

0.5

-1 -0.5

o

Log Shear Stress "S. Log Strain Rate for EOL

Y = 0.2295)( + 2.226

-.~ -.. ';:.~ .~.

0.5

log Y (Strain Rate) 1 1.5

2

--eoGe ---90Ge ........ lOOGC

-H- 120oe

2.5

1.5

1

0.5,.

-1 -0.5

o

Log Shear Stress Vs. Log Strain Rate for PBSO 3

y" 0.1982)( + 2.3107

.! . .

. -

0.5

Log y (Strain Rate) 1

1.5 2

--BO·C ---90·C ........ 100·C

~120·C

Log Shear Stress Vs. Log Strain Rate for FR5S-90 3

2.5 -. -- -= & .,.

:u

2

Cl> .c e .. 1.5 Cl> 0

..J Y = 0.2057x + 2.1134

, . -,.

. "-. .

0.5

. -1

-0.5 o _

0.5

Log Y (Strain Rate) 1

1.5

.. ... ---

--90°C ---100·C ......... 120·C

j 2 j

j j j j j

j j j j j j j j

j j j j

j j

j

j j

!

I ... !

0.5

-1

-0.5

o

1.5

log s"-"'"'SS vs. log SI""" ..... "" HR c_ ... <.5

1 .. _ .

". '.~,

0.5

LQg Y (Strain ~ate) 1

1.5

--aooc ---90·C --'-100·C ~1<OoC

2

j

J

Appendix F

6

In Shear Stress vs. Temperature ("C) for HR Compound

y = -O.0127x + 5.7352

2

1

O+----------r---------+--------~------~--~--------~--------+---~----~--------~ 90 95 100

Temperature ('C) .

105 110 115 120

80 85

__ 0.1

...-004

....-1

-M-4 ....... 10 __ 40·

-+-100

1

1

1

In Shear Stress vs. Temperature (·C) for SBR1

7

~ .- . :."... ~. 1

y = -0.0069x + 6.0193

2

.' ~- .

80 85 90 95· 100 . -- 105 110

Temperature ("C)

115 120

__ 0.1 ___ 0.4

........ 1

-*""4 ___ 10

-+-40

-1-100 -Linear (1)

.

f • (.

In 't vs. TemperatureeC) for SBR2

7 r

6~~====~~------~~==============~ ~

__ 0.1 5 . -en en

. i __ 0.4

l!! -4 I-

-: . .....-1

UJ - ---4 .. lIS Cl)

--10 .c: 3

y = -0.011x + 6~0854· UJ I-- --40

~

.. ~100

c - 2 --

1 . -' .: .. : • ' • .(>!"-'~"" ."

'<'. , " ' 0+_------~1r---~--+---~~~,--~--~r-------r-1------+-1-------r------~ '. - ' .. '- :'. -. .

80 85 90 95" . 100: , 105 . 110 115 120

Temperature ee}

In Shear Stress vs. Temperature (GC) for NBR· .- -" . . .... p..' ... -

7

__ 0.1

______ - __ . ~..,.....~....:...: ___ .:....~ ____ ~ __ =:::::::::::::::::..._ ;=:.4 ""'" __ 10

__ 40

y ~-0.0247x + 7.6269 -1-100 -Linear (1)

2

1

O+--------+--------+-----~~~~~--+--------+----~~+-------~--------~ 110 115 120

80 85 90 ... 95 . .. 100

Temperature (OC)

105

In Shear Stress vs. Temperature ("C) for EOL

7

6~====~=---~-----------

14 y = -0.0107x + 6.2878

I ~3L---------------~----~--------,· .~~ .. -,-. ~--:--------------------------+ .5

2

.. ';" .. 1

,'" H. : . .!.. _~~ ::._;:'.::. •• ._

O+---------~----~--~--~~~-+~------~~------~----------~--------~---------;· 80 85 90 95 100 105 110 115 120

. Temperature (OC)

__ 0.1

-4-0.4

-+-1 __ 4

__ 10 ___ 40

-+-100

-j .. m .If!

!!!. .. .5

In Shear Stress vs. Temperature (OC) for PBSO

6

5~=====-----------~===== 4

" , .. y = -0.0293x + 7.3n2

3

,',' ..

2 , .

1

.... '-' o+-______ ~----~--~---+------~----~+-~--~----~-+-------r------~----~ 86 87 BB 89 90

80 81 82 83 84 , 85

Temperature ('C)

__ 0.1

"""'0.4 -.-1

-*"'4 ......-10 __ 40

-+-100

In Shear Stress vs. Temperature (OC) for FR5S·90

- ;

_ y '" .().0339X +8.2811 -

... , .......

2

__ 0.1

....... 0.4

-k-1 __ 4

.....-10 __ 40

-+-100 _Linearl1)

0+-__ --------~~-----------r--------~~+-----~-----+------------~------------4 1 , . .. ~

105 .-

110 115

100 .". - .... Temperature ("Cl 95

120

90

Appendix G

TEIf>ERATURE

67.U

66.32

65.53

64.73

63.94

63.1.4

92.35

6:!..!56

60.76

59.97

TEHPERATURE

74.33

72 . 24

7j,.:1.4

70.:!.!!

59.35

Sample plots of the flow of SBRlin the dispenser meter cavity, showing temperature at increasing flow rates.

TEtf'ERATURE

90.47

89.29

88.12

86.95

85.77

74.60

65.42

62.25

61.07

59.90

TEHPERATIJRE

142.2

132.6

123.0

103.4

94.%

92.92

90.%

84.06

75.36

69.75

80 ~ LUMPED PARAMETER CALCULATION --+ STD(20)

• NR(20)

SBR1(20)

70 .. x SBR2(20) -+ +

-+ • NBR(20)

+ + • E-O-L(20) -+ + • • .. + + • ... .. ..- • .. PB80(20) 60 + + • .+ • FR58(20) • •

+ • • • - STD(80) • • --" • • NR(80) 6 +

50 • • ,. SBR1(80) - + - -.. - SBR2(60) .. ., -a: --- • NBR(60) .. - -- -~ 40 7< • E-O-L(60)

" .. - ~ 0 PB80(60) to • • ~ • • --e -.. r -:1 x • FR58(60) 0. ___ '- 1 -1 • i:J • ~ 30 • x • STD(loo)

.~ ... ..¥ -¥ III X • • • I- _a-- - --- )()( • - NR(lOO)

0 x X K • • ~

,- • • SBRl (100) • • .. 0 • • • • " °A - • • ,. • SBR2(loo)

20 • • 0 • • . " • • • • ! • NBR(loo) . fo' ..... ...-.- . • .- 0 • If t " . J .. -- • ! •... • • • t • x E-O-L(loo)

I • • • t • • • • .. .. .. ----. • -t .. PB80(lOO) •• • , , f.-::I---'--- • • .- • • .. i - d 10 "'d - o FR58(loo) ---"i .... 0 0 0 f - .:: - • 0 0 0 • 0 0

• • • • • • 0 • .... 0

0 20 40 60 80 100 120 140 160 180 200 Piston Speed (mm/s)

Proportional to Volume Flow Rate (Q)

~

ca ~ ~ ., co c: ., Co co i5 co co 0 ~

u <I: Co 0 ~

0 ., ~

:::J co co ., ~

Cl.

LUMPED PARAMETER CALCULATION

1,60E+08

1,20E+08

1,00E+08

8,00E+07

•

6,00E+07 I • • • -- • - 0 • • • . -. • • --- .. • 0-M· iI - • -11 • • J'!. .. •

It - -4,00E+07 - - -

---

• - • .. - " - x..... • x

~

2,00E+07 ----

O ,OOE+OO +----+-----~----~------~------_+------_+------_+------~------~------~

200 180 160 140 120 100 80 Piston Speed (mmls)

Proportional to Volume Flow Rate (a)

60 40 20 o

-+-- STO(20)

• NR(20) SBR1(20)

x SBR2(20)

--+- NBR(20)

---E-O-L(20) -+- PB80(20)

I- FR58(20)

STO(80)

• NR(60) SBR1 (60)

SBR2(60)

• NBR(60)

• E-O-L(60)

• PB80(60) FR58(60)

-- STO(l 00)

- NR(l 00)

-+-- SBR1(100)

--+- SBR2(100)

-..- NBR(l 00)

--- E-O-L(l 00)

---- PB80(l 00) . - FR58(100)

90 NISA" DATA STO(20) •

• NR(20)

SBR1(20) 80 .. • SBR2(20)

+- +-• NBR(20) -+ • + +- • E-O-L(20) + .. • + -+ • 70 + + +

+ • + PB80(20) + +- • -. • - - FRS8(20) • • + • • • • - STD(80) 60 • • -+ • • NR(80) 6 ..

SBRl (80) 0 +--.. er - -- SBR2(80) .. 50 a: • • NBR(80) • .. + • ---- • -. E-O-L(80) • ~

• .-:::I - s / -i • • • PB80(80) la 40 • -f • ~

I' __ ,,-f -t 0 ! . FRS8(80) ..

Cl. -1- --1--'-- .'

., E o ~1 " · STD(l00) .. .'

! . t' 1I I- 0 -t • , " - NR(l00) .. ...... • 30 0 • ~

,. SBR1 (100) ~ ~ SI I • 0 J I .. i " , • • SBR2(100) , 1 ~ .- I .. ~ • • • • NBR(l00) o , • • • • • • • • • 20 • ..... t • • • ~ . ...- t t x E-D-L(l00) .. - f-=1-I~ • -It • ..

1 • PB80(l00) • • ~-- • FR58(l00) 10 . J~ - ~

• • • • • • • • • • • - • • • • • ~ . • •

0 0 20 40 60 80 100 120 140 160 180 200

Piston Speed (mmls) Proportional 10 Volume Flow Rate (0)

1,80E+08 NISA 11 DATA

-- --- .. - - -~---.. ------ .. • •

• • - - - • --f'---1--.-. • • .. , , • • 2,OOE+07 • • . ... • -. - • ., - - • • • •

O,OOE+OO +-------+_------~------_r------_+------_+------~--------r_------+_------+_----~ 200 180 160 140 120 100

Piston Speed (mmls) NISAII DATA

80 60 40 20 o

-+- STD(20)

• NR(20) SBR1(20)

• SBR2(20) . • NBR(20)

--E·O·L(20)

-+- PBBO(20)

- - FR58(20)

STD(60)

• NR(60) SBR1(60)

SBR2(60)

• NBR(60)

E·O·L(60)

• PB60(60)

FR58(60)

- - STO(1 (0)

- NR(100)

I-+-SBR1(100)

... SBR2(1 (0)

1-'-NBR(l00)

• E·O· L( 1 (0)

• PBBO(l00)

• FR58( 1 (0)

Appendix H

120,OE+3

100,OE+3

g E 80,OE+3 ! .. ~

Cl> 40,OE+3 ~ o

u..

20,OE+3

LUMPED PARAMETER CALCULATION

, 11

__ STD(20)C

-e- NR(20)

SBR1 (20) __ SBR2(20)

-lIf- NBR(20)

-+- E-O-L(20)

-+- PB80(20)

- FR58(20)

- STD(60)C

--NR(60)C

SBR1 (60)C

SBR2(60)C __ NBR(60)C

-- E-O-L(60)C -+- PB80(60)C

FR58(60)C

- STD(100)C

- NR(100) __ SBR1 (1 00)

-e- SBR2(1 00) __ NBR(100)

-- E-O-L(1 00)

OOO,OE+O +----+------1-----+----+----+----+----+----+------,1-------1 -lIf- PB80(100) o 20 40 60 80 100 120 140 160 180 200 -+- FR58(1 00)

Piston Speed (mmls) Proportional to Volume Flow Rate (Q)

-----120,OE+3 NISA 11 DATA

-+- STD(20) __ NR(20)

SBR1(20) ___ SBR2(20)

100,OE+3 __ NBR(20)

-- E-O-L(20) -+- PB80(20)

g - FRS8(20)

~

E 80,OE+3 - STD(60)

., - -+- NR(60) .. >-.. SBR1(60)

;: SBR2(60) 0 60,OE+3 - NBR(60)

" ., E-O-L(60) ~

':; Cl'

-+- PB80(60) ., ~ ~

FRS8(60) ., 40,OE+3 - .. u .- - STD(100) ~

~ : ~ 0 .. • u. , :: • - NR(100)

~ '-'-- '-"-~~~--,. "J\,....{. - - -+- SBR1 (100) __ SBR2(100)

20,OE+3 -.- NBR(1 00)

--- E-O-L(1 00) __ PB80(1 00)

OOO,OE+O -+- FRS8(1 00)

0 SO 100 1S0 200 2S0 Piston Speed (mmls)

Proportional to Volume Flow Rate (Q)

Appendix I

The following pictures are thin sections of O-rings that have placed on a Shadograph

and then photographed. The primary reason was to investigate the shape change due

to anisotropy immersion in a good solvent.

The first two sides each show three pictures each of an O-ring section made from a

different compound. The first side shows conventionally moulded O-rings and the

second side shows sections of rings in the same three compounds moulded with the

new compression moulding system. Anisotropy can clearly be seen in the

conventionally moulded sample for FR58 (lowest on the page). The second set of

photos are all considerable more regular.

The area growth was measured by counting the graduations and the degree of

circularity was measured by taking two diameters on each picture, the longest and

shortest giving that ratio.

The following pictures are each show a before and after (Le. one not swollen and one

swollen) for the compound PS 80 in both toluene and Methanol. In this way they can

be used to show shape change. It was the shape change here that was the most

important factor under investigation. The shape change due to swelling, if any, will

highlight molecular anisotropy and orientation.

James Walker Samples. (Top Down: PBSO, Elast-o-Lion and Fr5S/90)

. L

:"

',' . , ..

..... ' .

'" ~.,.

.:. ··';1(:'·~,0·;· , .... "

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" "

:-', · •. ,f,'~·

. ' .' Form System Samples.

(Top Down: PB80, Elast-o-Lion and Fr58/90)

,t, ~.

i ....... ! . : . : !.~

'(,0: :'::'

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: ,-'

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.-:\

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, .. ~

. "

.:

James Walker PB80 - Before and After (Solvent: Methanol)

• '~'~ •• <

.', .

';- .

.. ,. . '," t',<'J.- - '. L,!

"

------------------------

::rt."-." ...

#f .• !::. :: I : . ...... .... . -...... ,. ., ..... . i,'

: ~ j; : : .. ··1", .'. .... + ••. - .. . . .. .. ~

",'

, ~: ',~ '"'i, •. '~",.",.,"

',~. ": ;".: '.' " ~ ..

"

•. ', .. j :-'.,:;- ,~":;: - ".'

,',",'.',.;.' J , •. :';' ":'~:;'f~f!cf~""#: : : !rH <, ••

James Walker PB80 - Before and After (Solvent: Toluene)

.\.\f.:.:-.i-:"i' .\

: '". f""

,"

• tr ) t:. . >~.:Jr':> c

,i ..... -, r- •• '

fI 1

".

, . . . . ~. ~i :.

".'

Form System PB80 - Before and After (Solvent: Methanol)

: <!~t~t;\;~.:t~~ ',:

,".'-

. " "

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..' ;( .. , .~ .'

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. : .;.":

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/;t~r.{~};,:, ':':

: }/...:;!}..~-5~;

-- ::

>~~~t1~v. '\.';' r : _ . , .

----------------

Form System PBBO - Before and After (Solvent: Toluene)

-'!". ;;" ':,{~~:~·~.~;;1~~~~~.: .:,~ .. ~.~.~:': .~ . • -',0