Plastics processing Manual (CIPET JAIPUR)

2014-2017

Naresh

Dhaker

CIPET JAIPUR

2014-2017

Plastic processing tech.

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INJECTION MOULDING

Historical Background

A single-action hydraulic injection machine was designed in the U.S.A.

in 1870 by Hyatt

Heating-cylinder design was first recognised in a patent issued to Adam

Gastron in 1932.

Large-scale development of injection moulding machinery design

towards the machines we know today did not occur until the 1950's in

Germany

InjectionMouldingProcess

– Over View

Solid Wide neck, Flat Product is made like bucket, cabinets, Automobile &

Industrial parts etc…. by injecting molten thermoplastic material in to a

closed mould which is relatively cool.

Type of Injection Moulding Machine

Hand Injection Moulding M/C

Plunger type Injection Moulding M/C

Reciprocating Screw Type Injection Moulding M/C

Hand Injection Moulding Machine

vertical machine consists of Barrel, Plunger, Band Heaters along with energy

regulator, Rack & Pinion system for Injecting the material by the plunger, a

torpedo and nozzle.

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Plunger Type Injection

Moulding Machine

Vertical & Horizontal Plunger Type Injection Moulding Machine

The Reciprocating Screw

The feeding zone

The compressing (or transition) zone

The metering zone

Machine components

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The Injection Process

Plasticises the material by reciprocating Screw.

Injects the molten material to a closed mould

o via a channel system of gates and runners.

Cools the Mould.

Refills the material for the next cycle.

Ejects the Product.

Closes the Mould for further cycle.

Injection Moulded Items

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Advantages of Injection Moulding Process

Parts can be produced at high production rates.

Large volume production is possible.

Relatively low labour cost per unit is obtainable.

Process is highly susceptible to automation.

Parts require little or no finishing.

Many different surfaces, colours, and finishes are available.

Good decoration is possible.

For many shapes this process is the most economical way to fabricate.

Process permits the manufacture of very small parts which are almost

impossible to fabricate in quantities by other methods.

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Minimal scrap loss result as runners, gates, and rejects can be reground

and reused.

Same items can be moulded in different materials, without changing the

machine or mould in some cases.

Close dimensional tolerances can be maintained.

Parts can be moulded with metallic and non-metallic inserts.

Parts can be moulded in a combination of plastic and such fillers as glass,

asbestos, talc and carbon.

The inherent properties of the material give many advantages such as

high strength-weight rates, corrosion resistance, strength and clarity.

Limitations of Injection Moulding

Intense industry competition often results in low profit margins.

Mould costs are high.

Moulding machinery and auxiliary equipment costs are high.

Lack of knowledge about the fundamentals of the process causes

problems.

Lack of knowledge about the long term properties of the materials

may result in long-term failures.

Machine operation sequence

The mould closes and the screw begins moving forward for injection.

The cavity fills as the reciprocating screw moves forward, as a plunger.

The cavity is packed as the screw continuously moves forward.

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The cavity cools as the gate freezes off and the screw begins to retract to

plasticize material for the next shot.

The mould opens for part ejection

The mould closes and the next cycle begins

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Mould system

A typical (three-plate) moulding system

A two-plate mould. A three-plate mould.

The moulded system includes a delivery system and moulded parts.

Screw Used in Injection Moulding Machines

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The screw has three zones with a ring-plunger assembly. The Feed Zone,

where the plastic first enters the screw and is conveyed along a constant root

diameter; the Transition Zone, where the plastic is conveyed, compressed

and melted along a root diameter that increases with a constant taper; and the

Metering Zone, where the melting of the plastic is completed and the melt is

conveyed forward along a constant root diameter reaching a temperature and

viscosity to form parts.

L/D RATIO

The L/D ratio is the ratio of the flighted length (Effective Length) of the

screw to its outside diameter.

Most injection screws use a 20:1 L/D ratio. But it may range from 18:1 to

24:1

In the case of Thermoset it may range from 12:1 to 16:1.

High L/D Ratio results the following ….

More shear heat can be uniformly generated in the plastic without

degradation;

Greater the opportunity for mixing, resulting in a better homogeneity

of the melt.

Greater the residence time of the plastic in the barrel possibly

permitting faster cycles of larger shots.

COMPRESSION RATIO (CR)

The ratio of the first flight depth of feed zone to the last flight depth of

meter zone ,

Or,

First Channel Volume of feed zone to last channel volume of metering

zone,

Typically ranges from 1.5:1 to 4.5:1 for most thermoplastic materials.

Most injection screws classified as general purpose have a compression

ratio of 2.5:1 to 3.0:1.

Thermo set screws have a 1:1 ratio.

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Higher the CR results the following ….

Greater shear heat imparted to the resin

Greater heat uniformity of the melt

High Potential for creating stresses in some resins

High energy consumption

Back Pressure (Kg/Cm2 or bar)

Back pressure is the amount of pressure exerted by the material ahead of the

screw, as the screw is pushed back in preparation for the next shot.

Effect of Back Pressure

More Homogeneous Mix

Proper Melting

More compact

Sometime leads degradation

Injection Speed (cm/Sec)

The injection speed is the forward speed of the screw during its injection

operation per unit time.

Effect of Injection Speed

Easy Injection of Material

Avoid Short-Shot

Some times leads more orientation & burn marks

Screw Rotation Speed

The screw rotation speed (RPM) is the rate at which the plasticizing screw

rotates.

The faster the screw rotation result the following ..

Faster the material is compressed by the screw flights

Increasing the amount of shear heating

Low residence time, some less melting

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Cushion

The cushion is the difference in the final forward position of the screw and its

maximum allowable forward position.

More Cushion results more residence time, some time degrades.

If the screw were allowed to travel its full stroke and stop mechanically

against the nozzle, the cushion would be zero.

With zero Cushion no hold on works.

Typically a cushion of 3 to 6 mm is used.

Materials for Injection Moulding

Acrylonitrile butadiene styrene (ABS)

Acetal

Acrylic

Polycarbonate (PC)

Polyester

Polyethylene

Fluoroplastic

Polyimide

Nylon

Polyphenylene oxide

Polypropylene (PP) **

Polystyrene (PS)

Polysulphone

Polyvinyl chloride (PVC

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TOGGLE TYPE CLAMPING

A toggle is mechanically device to amplify force.

In a moulding machine, which consists of two bars joined, together end to

end with a pivot .

The end of one bar is attached to a stationary platen, and the other end of

a second bar is attached to the movable platen.

When the mould is open, the toggle is in the shape of a V.

When pressure is applied to the pivot, the two bars form a straight line.



ADVANTAGE

Low cost and lower horsepower needed to run.

Positive clamp of the mould

DISADVANTAGE

Do not read the clamp force.

Clamping is more difficult.

Higher maintenance as lubricant is provided.

HYDRAULIC CLAMPING

A clamping unit actuated by hydraulic cylinder, which is directly

connected to the moving, closed the mould. In this case ram of hydraulic

system is attached to moving platen. There are two halves in hydraulic

cylinder, which is actually inlet and outlet of oil.

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When oil goes to the cylinder with pressure oil pushes the ram to forward

direction by which moving platen moves and mould closed and when oil

comes from the cylinder the ram come back and mould is open.

HYDRAULIC CLAMPING

ADVANTAGE

Clamp speed easily controlled and stopped at any point.

Direct a read out of clamp force.

Easy adjustment of clamped force and easy mould set up.

Low maintenance as part is self lubricated.

DISADVANTAGE

It is higher cost and more expensive than toggle system.

None positive clamp.

TIE-BAR LESS CLAMPING

Tie-Bar less clamping system is basically Hydraulic clamping system

without any tie bar.

The platen is moved on a rail system.

The main advantage of this system there is no limitation of mould platen

size.

As there is no tie bar so the mould dimension is not so important.

Also mounting of the mould is easy and it is very useful when products

eject from the mould is manual.

Much larger mould mounting area.

Larger stroke compared to the toggle type machines.

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Full machine capacity can be utilised.

Smaller machines can mould larger components.

Saves floor space.

Saves electrical energy because of reduction in the size of machine.

Has the capacity to reduce weight of the moulded component because tie-

bar stretching is not there.

Machine becomes very flexible for future modification.

Easy access to mould cavity's because of the absence of the tie bars.

Robotic arm movement becomes easy.

Fewer moving parts so lesser wear and tear so longer life for machines.

Lower lubrication required.

Removal of mould plates much simple.

Greater stability.

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BLOW MOULDING PROCESS INTRODUCTION

Blow Moulding process is widely used for producing bottles or other

hollow objects, due to its least expensive & simplest process to

manufacture.

It offers a number of processing advantages,such as moulding of

irregular curves,low stresses,variable wall thickness,the use of polymers

with high molecular weight & favourable moulding cost.

Blow moulding is operated using low moulding pressure & hence result

in Low Internal Stresses.

Commonly used materials are:PE,PVC,PET,PC, PA, POM.

It is principally a mass production method.

Blow Moulding is an alternatives process to other process like

Injection Moulding.

Rotational moulding

Thermoforming

Since the mould used for the process consists of female cavity, it is easy

to vary wall thickness & weight of the part.

This is done either by changing machine parts or melt conditions.

PRINCIPLE

Material is fed into a heated barrel of Extruder.

With the help of screw rotation & heaters the plastic is melted and

homogenised.

Melted material is forced through a set of die to form a tube or parison

(Hot Plastic tube)

parison is introduced into a mould,The mould closes & pinches off.

Blow pin is inserted through the open end of the parison to form a

neck.

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Finally air is introduced through the blow pin to inflate the parison

inside the mould.

By this, the molten Polymer copies the details of the Mould.

Lastly the moulded product is cooled & ejected.

In the finishing stage, the part undergoes, trimming, finishing,

Printing-Labeling & decorating.

BASIC REQUIREMENTS

Homogeneous melt of plastic material.

Formation of the molten resin into a hollow tube or parison.

Sealing the ends of the parison in the closing mould,except the area

through which blowing air can be supplied.

Inserting the blow pin or mandrel through the open end of the parison.

Blowing or inflating the parison inside the mould.

Cooling the blow moulded part.

Ejecting the part & trimming flash if needed.

Finishing & decorations on the product.

BLOW MOULDING METHODS

Based on the method used to create the parison or preform, two types

of blow moulding are recognised.

Extrusion Blow Moulding that uses an extruded tube.

Injection Blow Moulding that uses an Injection Moulded

preform.

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Blow moulding methods commonly used in

Industries are :-

Extrusion Blow moulding

Injection Blow moulding

Stretch Blow moulding.

Press Blow moulding & Dip Blow moulding for squeeze.

Multilayer Co extrusion Blow moulding.

BLOW MOULDING MACHINES

& THEIR CONSTRUCTION.

Blow moulding machine consists of two parts:

1. The parison Forming Unit(Extruder).

2. The Blow moulding unit.

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BLOW MOULDING UNIT

Made off two female cavities that close around a parison.

Pinch-off at one or both ends.

One entrance for the blowing air.

A cutout section.

PLASTIC MATERIALS

Those Materials which show high melt strength & good stretch properties

at the extrusion temperature are suitable for formation of parison &

blowing.

Following Polymers are more often blow moulded:

PE(LDPE,HDPE,HMHDPE)

PP,PVC,PA,PS,PC,PET,EVA,SAN,TPE

LDPE us used for more flexible items.

HDPE is used for Rigid Bottles, Chemical Drums, gasoline tanks.

PP is used because of high stiffness, good chemical resistance, clarity and

good glass and good resistance to high temperature.

PVC & PS for general purpose articles requiring transparency at modest

cost.

RPVC – Can be bi axially oriented in extrusion stretch blow moulding.

PC is used for containers which show good transparency, excellent

impact strength, good heat resistance, good printability.

PET is used in basically oriented stretch blow moulding & commonly

used for carbonated beverages, packaging of drinks pharmacy products &

water bottles.

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EVAL (Ethylene Vinyl Alcohol) is used as barrier layer in Multilayer

containers.

PROCESSING TEMP.OF PLASTICS.

Polymer Proc. Temp. Drying Temp. Cavity Temp.

LDPE 160-170 50-70 5-30

LLDPE 160-180 50-70 5-30

HMHDPE 160-210 50-75 10-30

PP 170-220 50-80 20-50

PVC 170-200 50-70 15-35

PS 250-280 50-70 20-40

PC 280-300 70-100 50-70

PA 190-240 70-100 20-40

PET 240-255 120-140 10-20

……………………………END……………………………………

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EXTRUSION EXTRUSION:

Continuous Process

In principle, the plastic raw material is plasticated by means of a screw

plastication unit and the molten material is continuously pumped out

through a standard orifice (die) in order to take the shape and then the

shape is set by cooling/sizing system.

Example: Film,Pipe,Tube, Profile, Monofilament, Box Strapping etc.

CLASSIFICATION OF EXTRUDERS

1 Batch – Type

1.1 Ram Extruders

1.2 Reciprocating screw extruders

g

2 Continuous –Type

2.1 Screwless Extruders

2.1.1 Disk Extruders

2.1.2 Drum Extruders

2.1.3 Other Extruders

2.2 Screw Extruders

2.2.1 Single-Screw Extruders (SSE)

2.2.2 Twin-Srew Extruders (TSE)

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2.2.3 Multi-Screw Extruders

SINGLE SCREW EXTRUDER

Single Screw Extruder

Parts & its functions

Screw Pump : Combination of Screw & Barrel

Hopper : Funnel like device, mounted on Hopper throat. Holds a

constant reserve of material.

Barrel : Cylindrical housing in which the screw rotates.

Hopper Throat : Circular opening at the feed end through which the

material enters the screw pump.

Drive System : AC/DC drives

Speed reduction gear box

Transmission system

The Single-Screw Extruder consist of a screw rotating in heated barrel or

cylinder to which the material is fed.

Feed hopper

Extruder Screw and Barrel

Drive system (motor, gear box, transmission)

Thrust Bearing

Heating and Cooling Elements

Screen Pack and Breaker plate

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Die

Temperature and pressure controls.

Definition of terms

Compression Ratio - is the ratio between the channel depth is the feed zone

to that of the metering zone.

- Usually from 1.5 to 4:1

L/D ratio - Length to nominal dia of screw

- usually 20 to 22:1

Important Specification

Nominal dia of screw

Output (kg/hr.)

Zones of Extruder & its Functions:

Feed Zone - Transport the material from hopper to compression

zone.

- Compacts, eliminates air gap

Compression Zone - Transport the material from compression to

metering zone.

- Softens the material

Metering zone - Melts, Mixes, the material pressurizes and pumps

the melt.

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Screw Nomenclature

P is the screw pitch, distance between the centre of a two adjacent

flights.

W is the channel width

L is the land width

q is the helix angle, defined as an angle between the flight to the

transverse plane of the screw axis.

D is the screw diameter, developed by rotating the flight about the screw

axis.

R.D is the root diameter

Flight is the helical metal thread of the screw.

C is the channel depth o radial distance form the bore of the barrel to the

root

SCREW TYPES

Extruder Screws

General purpose screw

PVC screw

Nylon screw

Two stage screw/vented screw

Segmented screws is also available for special purpose

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General purpose screw

PVC screw

HEATING AND COOLING ELEMENTS

There are three methods of heating extruders:

• Electric

• Fluid

• Steam Heating

Electric Heating

Induction Heaters

Cast-in Heaters

Band Heaters

• Mica Insulated

• Ceramic Insulated

The electric heating is most commonly used due to :

1 Accuracy

2 Reliability

3 Easy to hook up.

INDUCTION HEATERS

AC Current passes through coil thus setting up a magnetic flux. Heat is

generated from the resistance offered to the eddy current set up by the

flux.

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The barrel is heated directly by its resistance to the induced current

Schematic Arrangement Showing an Induction Heater in Section

Advantages :

Accurate Control of Temperature.

Good provision for cooling the barrel

No possibility for hot or cool spots.

Disadvantages :

Relatively high cost.

BAND HEATERS

They consist of Ni-chrome or other resistance wires mica or ceramic insulated,

then encased in steel cover.

MICA INSULATED CERAMIC INSULATED

Flexible, supplied as a single piece. Rigid, supplied in 2 halves

Can withstand a load of 23-31 KW/m2 Can withstand higher heating load

Shorter service life Better services life

Less expensive More costly

FLUID HEATING SYSTEM

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The heating fluid, that is most commonly used for extruders is oil. It may be

heated by any suitable means (mainly electrical). The heating system consists of

a heater a circulating pump, a surge tank, and a heat transfer channel in the

extruder barrel.

STEAM HEATING

The high specific heat and latent heat of vapourisation of water makes steam an

excellent heat transfer medium. However, this system is not frequently used

because of low maximum temperature that can be achieved, a need of working

with high pressure piping, frequent leaks of steam that require shutting down of

heating for repairs, and corrosion effects.

BARREL COOLING

Barrel Cooling is needed to prevent overheating that may cause

degradation.

For small extruders fans that blow air over or around the barrel are used

Other cooling system used include:

Cooling channels inside the barrel wall

Fins on the barrel or on the heaters to speedup heat transfer

A water-fog spray over barrel.

Continuous, controlled vaporization of liquid (Water)

Copper tubing carrying cold water is sometimes used.

HOPPER COOLING

Water-cooling is used to cool the hopper throat to prevent bridging and to

protect the rubber parts present in the screw support assembly.

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SCREW COOLING

The cooling may freeze a layer of plastic on the screw root, reducing the

channel depth thus producing more shear at a cost of throughout.

This may also reverse the required relationship between the friction

coefficient (low friction coefficient on the screw, high on the barrel),

further reducing the drag flow.

Furthermore, there is a danger that the material staying a long time near

the screw root will degrade, contaminating the product.

It is important to remember that the conveying ability of the screw is

controlled by the friction coefficient ratio : f(barrel) / f(screw) ó it is

important to maximize this ratio.

Under normal circumstances the minimum value of the ratio that

guarantees conveying is 1.4.

BREAKER PLATE AND SCREEN PACK

BREAKER PLATE

Perforated circular metallic disc of about 4-5 mm thick.

Functions

- Support for Screen pack

- Converts the Spiral flow of melt in to stream lined laminar flow

- Holds back contamination and unmelted particles.

SCREEN PACK

Wire mesh 40,60,80

Arrests the unmelted particles and contamination

Helps in developing back pressure

DIE DESIGN

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The role of a die is to form the melt into a desired linear product: fibres, films,

sheets, profiles, rods,etc.

The die is a channel, whose profile changes from that of the extruder bore to an

orifice, which produces the required form.

The dies can be classified using different criteria. For example, considering

cross section of the extrudate one may recognize dies to produce:

Solid Cross-Sections

Hollow Cross –Sections

Another classifications scheme is based on the die attachment to the extruder

barrel:

Straight –through dies

Cross –heat dies

Offset dies

SOLID CROSS – SECTIONS

A Typical Die Design for extruding a solid rod is shown in fig.

In the figure, DD is the diameter of die orifice, DB is the diameter of bore

of extruder barrel, a is the lead-in angle, and P is the die land.

Because of the screen pack and breaker plate assembly, the pressure in

the extruder (PE) is reduced by the pressure loss across the assembly

(PL).

Since the die outlet is at atmospheric pressure, the working pressure is

the die pressure (PD) given by the difference: PD = PE – PL.

HOLLOW SECTIONS

Hollow products like pipe or tubes are produced using the die design shown

in Fig.

The outer diameter of tube is determined by the diameter of the outer die

ring orifice.

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The inner diameter is determined by the mandrel diameter

To make the mandrel and outer die ring orifice concentric, centring

screws are provided.

The mandrel is held in position by a spider. In the centre of the spider a

hole is drilled to supply air down the mandrel.

To provide a smooth glossy extrudate, the die head is heated. A cold die

may cause blockage of the die.

STRAIGHT – THROUGH DIES

Those dies whose axes are arranged to be in line with the direction of

supply of melt.

Spider, Mandrel is needed for tubes

Used for the extrusion of pipe, rod, profiles and sheet

CROSSHEAD DIES

Arranged with their axes at an angle of 908 (458 and 308 are also used) to

the melt feed.

No need for spider assembly.

Used for the production of insulated wires, cables

OFFSET DIES

Combination of both straight – through die and off-set die.

Used for the production of pipe.

EXTRUDATE SWELL

Extrudate is contraction in the direction of extrusion and expansion in the

cross-section while emerging from the die is called Extrudate Swell.

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The phenomenon (previously called die swell) is illustrated in fig.

Numerically, the extrudate swell is defined as the ratio of the outer

extrudate diameter (DE) to the other diameter of the die exit (DD), i.e., B

= DE / DD

When the melt emerges out of the die lips, there will be expansion in the

direction perpendicular to flow and contraction in the direction parallel

to flow.

Constrained molecules tends to relax at the die outlet. This leads to die

swell.

This is nullified by higher take off speed.

Extrudate Swell may be reduced by :

Decreasing the extrusion rate

Increasing the melt temperature

Increasing the die land

Increasing the draw-down ratio.

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COMPRESSION MOULDING INTRODUCTION

Compression Moulding is a technique principally for thermoset plastic

moulding in which the moulding compound (generally preheated) is placed in

the heated open mould cavity, mould is closed under pressure, causing the

material to flow and completely fill the cavity, pressure being held until the

thermoset material has cured.

In this process a measured quantity of a plastic material is shaped or formed by

heat and pressure. A known weight or volume of the moulding powder is kept

in an open mould cavity. The mould consists of two halves, a male and a female

part, and is normally preheated. Mould halves are attached to the upper or lower

platens of a press. These mould halves are called the core or plunger and the

cavity. Heat is applied either directly to the mould or indirectly through the

platens. A systematic diagram of Compression Moulding is given in Fig.1.

The moulding compound, generally in powder form, is placed in the mould

cavity after which the mould is closed. Heat and pressure is applied causing the

plastic material to soften, flow and fill the cavity completely. Depending on the

characteristics of the plastic material and the design of the mould, the

temperature ranges from 140°C to 250°C & pressure range from 2000 to 10,000

psi. The mould remains heated and closed until the plastic material cures and

sets.

A moulding pressure of 2000 psi on the total projected area of the part is

generally recommended. To this must be added approximately 800 psi for every

inch of the vertical height of the moulded part, to arrive at total moulding

pressure required.

After the material has cured sufficiently, the mould is opened and the piece is

ejected by knock out pins. The cure time depends on the size and thickness of

the part. This may vary from 20 seconds to 10 minutes. The cure time cannot be

predicted accurately, but must be determined by experience or by trial.

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MATERIALS

Thermosetting resins: Phenol formaldehyde, Urea formaldehyde, Melamine

formaldehyde, Polyester, Epoxy.

Special Thermoplastics: Ultra high molecular weight HDPE.

VARIABLES AND FLOW PROPERTIES

VARIABLES :- The following variables to pay

attention during moulding.

1. Flow Characteristics

2. Curing Characteristics

3. Particle Characteristics

(„grinds‟)

FLOW PROPERTIES:- There are three independent flow

properties of concern thermoset

moulding.

1. The ease of flow (fluidity),

2. The total flow that occurs

before the material sets,

3. The time available for

flow

PROCESS OPERATION

1. Pelleting (an optional stage) 2. Preheating (also optional);

3. The moulding stage; 4. Finishing operations.

PROCESS OPERATION

1. PELLETING:

Pelleting permits accurate measurement of the charge to the mould reduces

contamination and facilitates preheating. It does not,however, always fit in very

well with more automated compression systems.

2. PREHEATING:

Preheating is useful in that it reduces expensive moulding time, allows rapid

heating of large pellets or masses of powder. Helps to remove moisture and

other volatiles prior to moulding and because it advances the cure has been

claimed to reduce moulding shrinkage.

3. THE MOULDING STAGE:

The powder material (specified) is placed inside the heated mould cavity and

mould is closed, predefined pressure is continuously applied on to the

moulding.

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Since the material will start to cure as soon as it comes into contact with the

heated mould. It is important that flow and shaping be completed before the gel

point is reached ( i.e. the material shows first signs of being cross linked)

The optimum cure (i.e. cross-linking) for one property is not necessarily the best

cure for another. It is therefore necessary to establish what properties are

important in the finished moulding and use cure times and temperatures that

give a good compromise to the various requirements, including that of cost. It is

particularly important that flow and cross-linking process are controlled and are

in the correct sequence. While improvements in the operation can often be made

by such refinement as breathing (venting) they will be no value if the basic

process is not correct.

4. FINISHING OPERATION:

The part finishing is done depending upon the product comes out after curing.

Sometimes it is observed that additional material is coming out from the

moulding as a flash which can be removed after curing. To make more

attractive some other finishing operations can be performed on to the finished

product.

MOULDING SEQUENCE

The moulding sequence in compression moulding is as follows

(Fig. 2) :(i) Mould open, Cavity cleaned,

(ii) Pellet loaded, Mould closing,

(iii) Mould closed - Curing stage,

(iv) Mould open, Moulding ejected.

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MOULDING CYCLE

The compression moulding cycle, in many instances the dwell time will

comprise a larger fraction of the moulding cycle than indicated in Fig. 3.

MACHINERY & EQUIPMENTS

COMPRESSION PRESS

Simplicity is the keynote of the compression moulding press. It basically

consists of two platens that close together, applying heat and pressure to mould

some material into the desired shape. Most compression presses are

hydraulically operated though some are also operated pneumatically. The

platens move up and down on four corner posts under pressures that typically

can range from 20 to 1000 tons, depending upon press size. Platen themselves

can range in size from 8 in. square to 5 ft.square.

COMPRESSION PRESS

Various degrees of automation are available in most modern hydraulic presses

to feed material and eject the part after cure. Older, simpler systems had

temperature, pressure, dwell and time controls. Today‟s modern equipment has

more sophisticated microprocessor controllers.

Presses used for moulding thermoset are available in many different shapes and

design and can be classified as either hand, mechanical or hydraulic types. Most

compression moulding presses are of the upstroke type as shown in the figure 4.

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HYDRAULIC SYSTEM

UPSTROKE TYPE:

In this type, a hydraulic ram moves the bed or bolster of the press upward to

close the mould. The strain rods or tie rods hold the upper and lower parts of the

press in accurate alignment. So that the two parts of the mould do not mismatch.

Adjusting collars on the strain rods allow the daylight opening between platens

to be adjusted to the requirements of the mould. The daylight opening is the

maximum distance between upper and lower p1aten. This opening must be large

enough so that the compression moulding can be removed from the die.

The two parts of the mould are bolted to the upper and lower platens of the

press. These platens have heating channels, which are backed with asbestos

board insulation. During setting-up, the two platens of the press must be

checked for parallelism.

UPSTROKE TYPE:

The disadvantage of the simple upstroke type compression moulding press is

that the return is slow, relying entirely on gravity, although pressure can be

applied fairly quickly. This limitation can be sorted out by incorporating a

pushback ram in the machine.

DOWNSTROKE TYPE:

A downstroke press has a fixed lower bolster and a moving upper bolster

(fig.5). The down stroke machines are better suited to the moulding of unusually

large components, which require a longer stroke. The downstroke type press,

overcomes the disadvantages of the up stroke press.

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HEATING AND COOLING OF MOULDS

Heating is generally done by cartridge type electrical heaters even though hot

oil system can also be used. The mould cooling is done by circulation of cold

water through the channels.

TYPES OF MACHINES

Presses used for moulding thermosets are available in many different shapes

and designs and can be classified as either hand, mechanical or hydraulic type.

TYPES OF MOULDS

Compression moulds are usually constructed of case-hardened tool steel,

in order to withstand the high pressures of compression moulding. There

are three basic types of Compression moulds :

1. FLASH TYPE

2. POSITIVE TYPE

3. SEMI POSITIVE – FLASH TYPE

FLASH TYPE

The flash type of mould is used to produce a shallow shape (Fig. 6). With this

type of mould. a slight excess of the moulding powder is loaded into the mould

cavity and, on closing the top and bottom force. The excess powder is forced

out and 'flash' is formed. This can be removed quite readily, though it tends to

make the process somewhat wasteful. However the moulds are relatively cheep

and are suitable for building up into tools containing multi-impressions.

Another advantage is that, while to some extent it is wasteful as far as raw

materials are concerned, very slight labour costs are necessary in weighing out

the powder.

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POSITIVE TYPE

The positive type of mould produces a vertical flash in the direction of

moulding pressure (Fig. 7). In this mould, the upper part of the mould (the

force) fits closely into the lower part (the cavity). This type of mould is also

easy to manufacture, but no allowance is made for placing excess powder in the

cavity. Should this occur in error, the mould will not close. If however, the

correct charge added, only a small amount of the flash results and it can be

easily removed. A disadvantage with such a mould is that the gas liberated

during the chemical curing action will be trapped, thus resulting in blisters. This

type of mould is suitable for moulding high bulk factor material and is used

only on a small scale for molding thermosets.

It is however, used for moulding laminated plastics and certain rubber

components. Some of the reasons for discarding this type of mould are the

necessity for weighing or measuring an exact charge, excessive wear on sliding

fit surface of the top and bottom forces and difficulty of ejecting the moulding.

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SEMI POSITIVE – FLASH TYPE

The semi-positive flash type of compression mould combines the features of

both the flash and positive type(Fig. 8). The moulding powder is forced into a

complicated mould, at the same time, making allowance for excess powder and

flash. This type of mould is expensive to manufacture and maintain and is

therefore used where long runs are required. With such a mould, it is also

possible both horizontal and vertical flash. The area of the faces of the mould

which come in contact with each other when the mould is closed, is generally

referred to as the 'land'.

ADVANTAGES

Lowest cost molds

Little "throw away" material provides advantage on expensive

compounds

Often better for large parts

Lower labor costs

Minimum amount of wasted material & Improved material efficiency

Internal stress and warping are minimized.

Dimensional accuracy & stability is excellent.

Shrinkage is minimized and closely reproducible.

Thick sections and large parts are practically possible.

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Lower molding pressures allow molding of large parts on presses of

lower tonnage

LIMITATIONS

Offers least product consistency

Not suitable for fragile mold features, or small holds

Uneven parting lines present a mold design problem

High impact composites make flash control & removal difficult.

The depth of the molded holds is limited to 2 or 3 times their diameter

Shot weight must be tightly controlled

Dimension across the parting line may be difficult to hold but good

accuracy may be obtained through tight process control.

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TRANSFER MOULDING INTRODUCTION

For some applications, it is desirable to close the mould first and then introduce

the moulding compound in its fluid state through a small opening or gate

leading to the mould cavity or cavities. This technique is called transfer or

plunger moulding.

This is variation of compression moulding and is particularly valuable when

intricate shapes have to be moulded. It is also useful when metal inserts have to

be incorporated (as in electrical component). Even with the use of pellets there

is danger of their moving during the liquefaction process and causing movement

of the inserts.

It is used frequently when the mould sections are very delicate, when the

moulded parts has thick sections 3.2 mm or more, or when an insert is retained

in the cavity for moulding in place. In such applications, closing the moulding

containing a moulding compound that is not yet fully liquid (as in the case in

transfer moulding), flow speed and pressure can be controlled to minimize the

possibility of any such damage.

PROCESS

OPERATIONS:

In transfer moulding, a known amount of moulding powder is placed in a heated

chamber (transfer pot), outside of the mould. When liquefaction is completed a

piston forces the resin through a connecting channel into the mould. As in

compression moulding the mould is kept heated and this completes the chemical

reaction which leads to hardening of the moulding. A systematic diagram of

Transfer Moulding is shown in fig.1.

TYPES OF TECHNIQUE

1. TRUE TRANSFER OR POT TYPE TRANSFER MOULDING

In true transfer or pot type transfer moulding, the mould is closed and then

placed in an open press. The charge of moulding compound is introduced into

an open pot at the top of the mould. The plunger is then placed into the pot and

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the press is closed. As the press closes, it pushes against the plunger, which in

turn exerts pressure on the moulding compound, forcing it down through a

vertical passage called a sprue and through runners and gates into the cavities.

After the curing, the mould is removed from the press, the plunger is

withdrawn, the mould is opened, and the parts are ejected. (Fig. 2)

Pot type transfer moulding may also be done with the bottom half of the mould

bolted to the lower press platen and the plunger bolted to the upper press platen.

The upper mould half, containing the pot, may then be manually placed over the

lower mould half. Or, it may be suitably supported and guided so that the

opening of the press separates the lower mould half from the upper mould half

and in the same motion, pulls the plunger out of the pot. The procedure is

reversed when the press closes.

2. PLUNGER TRANSFER MOULDING

As much material is wasted in the large pot, it is generally more economical to

use plunger transfer moulding instead. In plunger moulding, the plunger is

essentially a part of the press rather than part of the mould. It is usually driven

by a hydraulic circuit and a cylinder attached to the head of the press and can,

therefore, be considerably smaller in diameter than the pot type plunger. The

mould is held closed by the clamping action of the press, independent of the

plunger movement or force. The behavior of the moulding compound is

identical, however, channels, called sprues and runners, direct the flow of

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material from the pot to the cavities passing through a restriction, or gate, just

before entering the cavity. A single pot can feed many cavities. Air in the

cavities displaced by the incoming materials must be expelled through

strategically placed vents.

When the material is introduced into the pot, it is usually a measured charge in

compacted form, preheated to a temperature approaching that of

polymerization. Only sufficient material for a single shot is loaded at one time.

The force that moves the charge of preheated material out of the pot is

transmitted through a plunger, which is closely fitted to the pot to prevent

leakage of material through clearances between the plunger and the sides of the

pot. Sealing grooves usually are cut into the plunger to further reduce leakage.

When a transfer mould reaches the end of its cure cycle, the entire shot is

ejected, including the gates, runners, sprues and the cushion of cured material

(called the cull) formed in the pot.

Preheating of material is very important in transfer moulding. Cold material

flows slowly, with the result that the first material to enter the cavity may

polymerize before reaching its final destination. If that occurs, poor quality

mouldings can then be expected, both from the standpoint of appearance and

that of their mechanical properties.

3. SEMI AUTOMATIC TRANSFER MOULDING

In semi automatic transfer moulding, the operator actuates the press each cycle

to close the mould and then manually introduced the moulding compound into

the transfer pot, which is in top half of the mould. Next, the operator actuates

appropriate controls to cause the plunger to descend into the pot. The press

controls take over to time the cure cycle, open the mould, and eject the parts

from the cavities using ejector pins in the mould. The operator lifts the parts

from the ejector pins and initiates another cycle.

4. FULLY AUTOMATIC TRANSFER MOULDING

In fully automatic transfer moulding, the press recycles automatically thereby

feeding moulding compound into the transfer pot and removing parts at the end

of each cycle, Automatic transfer presses are often configured horizontally so

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that when the moulded parts are ejected from the cavities, they will readily fall

into a container or conveyor belt below the open mould.

MOULDING SEQUENCE

The mould is closed and material is placed into the hot transfer chamber of fully

automatic transfer moulding and plastication.

The plunger descends into the pot, causing material to melt and flow through

runners into hot mould cavities.

Curing under the pressure of transfer ram.

After cure, the press opens, the plunger retracts, and parts are ejected with cull

and runners.

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Deflashing and mould cleaning

TYPES OF MOULD

1. INTEGRAL POT TRANSFER MOULD

The integral pot transfer mould was the first to be used and is so called because

the pot and plunger are built as an integral part of the mould. Round pots are the

most common, but other shapes also can be used to accommodate particular

sprue locations that might be needed with minimum material waste.

The integral pot mould frame is a three plate type with the pot contained in the

middle section. The transfer plunger is mounted in the top section of the mould

frame and the cavity in the bottom section. The area of the pot must be at least

10% larger than the total clamping area (horizontal surfaces that are in contact

with plastic material) in the cavity section. This is to prevent the unclamping

force from overcoming the clamping force and causing the mould to flash.

After the material has cured, the moulded parts are ejected by the action of the

press moving the mould ejector bar, but the cull and sprue are held to the

bottom of the plunger by one or more moulded dovetails. A lateral blow with a

wooden stick or a soft hammer releases the cull and thereby clears the mould.

2. PLUNGER TRANSFER MOULD

Plunger transfer moulds or plunger moulds, describe mould that use an auxiliary

press ram to force the plunger into the pot (or cylinder), moving the material out

of the pot and into the cavities. Transfer pressure and speed of transfer are

readily controllable, independent of clamping pressure.

Pot size (hence cull size) in a plunger mould need only be large enough and

deep enough to accommodate the full charge of material.

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Maximum pot area, on the other hand is established by determining the force in

tons that can be delivered by the auxiliary ram of the press and dividing that

figure by 3.5. This will assure that about 540 kg/cm2 force will be available as

moulding pressure, which is sufficient for most transfer grade material

formulations.

The auxiliary rams usually are atop the upper, fixed platen of the press and are

downacting. The clamping ram moves the lower platen upward to close the

mould. After the mould is clamped, the material is loaded into the pot, and the

auxiliary ram is energized. Ratio of the clamp ram to the auxiliary ram is

generally 3 to 1 or 4 to 1.

When the material is cured, the auxiliary ram retracts and the press is opened.

The moulded parts, cull and runners are ejected simultaneously by the action of

the mould ejected bar.

ADVANTAGES

Loading a preform into the pot takes less time than loading preforms into

each mold cavity.

Tool maintenance is generally low, although gates and runners are

susceptible to normal wear.

Longer core pins can be used and can be supported on both ends,

allowing smaller diameters.

Because the mold is closed before the process begins, delicate inserts and

sections can be molded.

Higher tensile and flexural strengths are easier to obtain with transfer

molding.

Automatic de-gating of the mold's tunnel gates provides cosmetic

advantages.

LIMITATIONS

Molded parts may contain knit lines in back of pins and inserts.

The cull and runner system of transfer molding leaves waste material, but

this scrap can be greatly reduced by injection molding with live sprues

and Runner less Injection Compression (RIC).

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Fiber degradation of orientation occurring in the gate and runner system

reduces the molded part's impact strength.

Compared to compression molding, high molding pressures are required

for the transfer process, so fewer cavities can be put into a press of the

same tonnage.

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ROTATIONAL MOULDING INTRODUCTION

Rotational moulding is a process of making hollow articles.

The part is formed inside a closed female mould.

In this process the mould rotates biaxially during heating and cooling

cycle.

Rotational moulded pieces are stress free because the pieces are produced

without any external pressure.

The ability to manufacture large containers of capacity 30,000 gallons as

well as small items like golf ball is responsible for the growth of this

process.

The Process requires relatively in expensive equipment and exerts on

only small pressure on the material being formed.

PRINCIPLE

The principle of the process is that finely divided plastic material

becomes molten when comes in contact with hot metal surface of the

mould and takes up the shape of that surface.

As only female mould is used, the only pressure exerted are those

induced by gravity and centrifugal force.

The polymer is then cooled while still in contact with the metal mould to

get the solid copy of the surface.

Rotational moulding permits to make a wide variety of fully and partially

closed items.

Articals of Roto moulding

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ADVANTAGES AND DISADVANTAGES

Advantages

The major advantage of rotational moulding as compared to other plastic

moulding processes is that it can make very large parts.

It requires comparatively low cost input.

The products are stress free with strong out side corners. There are no

weldlines, sprue or gate marks.

Here impact toughness is improved and failure due to brittleness is

reduced

The external dimensional details can be easily moulded with better

surface glossiness .

The colour changes in the product can be made easily. Similarly mould

changes can also be done rapidly.

Multilayer moulding is also possible for providing chemical resistance

and strength to the part.

Good control over wall thickness variation is also achievable as compared

to blow moulding or thermoforming.

Moulding can be done with metal inserts and minor undercuts.

No scrap or very little scrap is produced.

Low tooling cost.

DISADVANTAGES

The moulding cycles are longer compared to blow moulding and

thermoforming.

In case of big parts loading and unloading is very labour intensive.

The process is not suitable for parts with wall thickness less than 0.03”.

The conversion of plastic granules to powder form increases the

equipment and process cost.

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LIMITATIONS

1. It is an open moulding process and so there are no cores inside the hollow

parts

2. Surface details and dimensions can only be provided and controlled on

the side of the part.

3. The process requires heating and cooling of not only plastic material but

also the mould as well.

4. The long heating cycle increase the possibility of thermal degradation.

5. It is not suitable for materials with less heat resistant to withstand the

long heating cycle.

6. The material must be capable of being pulverised into fine powder that

flows like liquid.

7. Removal of plastic sticking onto the surface of cavity requires careful

application of mould release agent.

ROTATIONAL MOULDING PROCESS

Loading

Heating & Moulding

Cooling

Unloading

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LOADING

This step includes weighing of the charge for a particular product then

transferring it to the open cold mould.

The mould surface usually coated with a mould releasing agent.

The raw material can be in the form of powder or liquid state.

The wall thickness can be controlled by varying the amount of raw

material charged.

After the material is charged the mould is closed and clamped to the arm

of the machine.

Then the mould is moved to an oven for heating

HEATING & MOULDING

The mould fixed to the arm now moved to a closed chamber where it

undergoes intense heating.

During heating the mould rotates in two planes perpendicular to each

other.

The rotational speed varies in the range of 0-40 rpm on minor & 0-12

rpm on the major axis.

4:1 ratio is the most commonly used for symmetric article.

For moulding unsymmetrical products a wide variability of ratios is

necessary.

The revolving motion distributes the plastic material uniformly over the

inside surface of the mould.

The plastic material fuses into layers to form a hollow article.

In case of hot air oven the temperature should be between 200ºc to 500ºc.

The moulding cycle time varies from 2 to 20 minutes depending upon the

wall thickness of the article.

The wall thickness can vary from 2 to 12mm or more.

The heating chamber should be large enough to house the mould and

rotate it freely.

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COOLING

For cooling the mould is transferred to the cooling station while still

rotating.

The cooling should be made as quickly as possible to avoid the plastic

part to shrink away form the mould.

Otherwise the part will get distorted.

Cooling can be done by air or water. To provide faster cooling cold water

is sprayed over the mould.

UNLOADING

After cooling the mould is transferred to the unloading station.

In this step the mould is opened and the cooled part is taken out.

It can be done manually or with mechanical assistance.

The ejection can also be done by forced air.

The mould is cleaned and the charge is loaded for the next cycle.

HEATING SYSTEM IN ROTATIONAL MOULDING

The rotational molding process heats and cools both the mold and the

plastic material.

Cavities are build up with materials having high thermal conductivity, in

order to minimize the time required for heat to pass through the wall of

the cavity.

Rotational molds may be heated by either an open-flame method, a hot

air re-circulating oven method, or by a hot-oil jacketed mold system.

Molten Salt-it leads to corrosion.

Infrared Heater-Very efficient but costly method.

The most used system is a re-circulating hot-air oven.

RE-CIRCULATING HOT AIR OVEN METHOD

In this system a positive displacement circulating fan distributes air

through a system of ducts into the swept volume of the oven.

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The capacity of the fan (cubic meters of air per minute), will determine

the number of air changes per minute.

On contemporary machines, air should be changed in the oven

approximately 25-30 times per minute in order to provide an effective

heating for the mold.

Direction of the air in the oven is generally caused by the directional

louvers so that no “dead spots” are created.

The static pressure capability of the fan system provides force to push the

air over the mold and provide the scrubbing action of the hot air on the

mold.

On contemporary machines, air should be changed in the oven

approximately 25-30 times per minute in order to provide an effective

heating for the mold.

Direction of the air in the oven is generally caused by the directional

louvers so that no “dead spots” are created.

The static pressure capability of the fan system provides force to push the

air over the mold and provide the scrubbing action of the hot air on the

mold.

The absorption of the heat by the mold transmits through to the powder to

create the molded parts.

The medium for heating hot-air ovens may either be natural gas or oil

with a modulating burner.

In some cases, electric heaters are used to generate the hot air

environments.

The regulation of air temperature in the swept volume of the oven is

controlled by sophisticated electronic temperature control devices.

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The time that the mold remains in the oven is known as “Oven residence

time”.

The medium for heating hot-air ovens may either be natural gas or oil

with a modulating burner.

In some cases, electric heaters are used to generate the hot air

environments.

The regulation of air temperature in the swept volume of the oven is

controlled by sophisticated electronic temperature control devices.

The time that the mold remains in the oven is known as “Oven residence

time”.

The oven residence time necessary to cure a part will depend upon the

wall thickness of the part, the type of plastic material being used to mold

the part, and the conductivity of the metal of the mold.

Aluminum with a higher conductivity, allows heat to transfer from the air

stream to the mold and the powder at a much faster rate than does steel.

Thinner gauge aluminum helps to increase the conductivity.

OPEN FLAME HEATING

In the case of the rock and roll machines there was no heated oven; an

open –flame method used whereby a manifold of gas jets was placed to

evenly heat the mold.

As the mold rotated about the major (rolling) axis, the heat was imparted

directly onto the mold surface, and transferred through to the plastic.

The machine was inexpensive to manufacture, but the operating cost were

significantly more than the closed oven type of heating system.

All of the thermal energy not imparted to the mold went into the

atmosphere creating increased temperature in the work environment and

the loss of energy.

Open flame machines are still used for very large tanks that are too large

to fit in conventional re-circulating ovens or where the quantity of tanks

is so small as to not justify the expense of building a large oven.

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HOT OIL HEATING METHOD

The hot oil jacketed mold system was one of the earliest systems used for

rotational molding.

In closed oven with re-circulating hot air, their will be heat losses due to

the extra volume in the oven not filled by the mold.

The jacketed mold maintains the mold temperature very close the the

temperature of the hot oil being used.

Therefore, the hot oil system generally uses a lower temperature for

molding since the oil is in direct contact with the mold and imparts the

heat energy very quickly.

The difficulty of using jacketed hot-oil molds is that the expenses of the

molds is considerably more than used in the other types of heating.

THE HEATING TIME DEPENDS UPON

THE FOLLOWING FACTORS.

Part Size

Wall thickness

Resin

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THE SPEED DEPENDS UPON

THE FOLLOWING FACTORS

Part size

Geometry

Resin

Heating rate

Thermal conductivity of mould metal

Sometimes due to over heating the air inside the mould gets expanded

and some internal pressure builds up, which may distort the mould as

well as the part.

To avoid this, mould can be vented.

To provide vent a small pipe is placed which runs from inside to outside

of the mould. To prevent entry of the vent pipe a small amount of glass

fiber can be added.

ROTATIONAL MOULDING MACHINES

Three basic types of machines are :

i. Batch type

ii. Semiautomatic type

iii. Continuous or rotary type

Batch type is used in prototype or low volume production. This method

requires less capital but most involvement of manual labour.

Continuous or rotary type method include three basic stations arranged

120º apart from arms attached to a central hub containing the drive

mechanism.

Advantage of this system is minimal labour and high production rate.

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ROTATIONAL MOULDING

BY CAROUSEL-TYPE MACHINES

The carousel type machine is a three-station rotary indexing type with a

central turret and three cantilevered mould arms.

Individual arms are involved in different operations simultaneously so

that no arms are idle at any time.

All operations are automated and at the end of each cycle the turret is

indexed 120º, thereby moving each mould arms to its next station.

Newer carousel machines being offered today have four arms.

The additional arm can be used in a second oven, cooler or load station,

depending on, which is the most time-consuming part of the over all

cycle.

The four-arm carousel machines increase the production by allowing the

indexing from station to station to occur more frequently than could be

managed on a three-arm machines.

.

MULTILAYER ROTATIONAL MOULDING

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It is used to combine two different colours of the same material or two

dissimilar material into one part.

It offers potential advantages of increased stiffness.

When solid and foam are combined, improved barrier properties and

permeation resistance by using thin inner or outer layer of low permeable

material.

It is a two staged process in which the skin of one material is combined

with an inner layer of another material.

The first shot of material moulds in the normal fashion and the material

adheres to the mould.

When adhering and curing of the layer is completed the mould is

removed from oven and second shot material is added.

While producing very thick parts care should be taken not to thermally

degrade the outer layers at the expense of optimizing properties of inner

layers.

The double process is at its best when two walls adhere to each other.

Two different colors of virgin and reprocessed combinations of the same

material would be ideal.

Dissimilar materials such as nylon and PE that don‟t bond to each other

are being used, but there are some limitations.

Two materials must be chemically compatible.

They should have similar processing temperature and similar co-efficient

of thermal expansion.

MATERIAL CONSIDERATIONS

FOR ROTATIONAL MOULDING

All thermoplastic materials can be rotationally moulded.

HDPE, LLDPE, LDPE,PVC,PC,ABS,PS,Acrylics, Nylon , TPU, SAN

Polyesters are the materials which are commonly used.

The various properties considered in selecting the proper material are

grindability, particle distribution,particle mesh size, pourability, bulk

density and fusability.

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The material should be able to ground to a fine powder and the common

size is about 300µ and maximum size is upto 400/500 µ.

To provide fine grinding the high speed impact mills are used.

The particle size distribution should also be uniform to provide uniform

conductance of heat.

The most common mesh size for rotational moulding ranges from 16 to

50.

The material should produce less volatiles during heating.

The most common mesh size for rotational moulding ranges from 16 to

50.

The material should produce less volatiles during heating.

MATERIAL PREPARATION

A process used to reduce the pallets or granules to a smaller size is called

grinding or milling.

In this process the granules fed into the centre of two plates, each with a

series of radially arranged cutting edges.

One plate is held stationary while other is rotated at high speed.

The gap between the cutting edges of the two plates is narrower at their

peripheries than the centres.

Any individual granules subjected to cutting action, generates frictional

heat and increases the temperature of metal cutting face.

Hence the temp must be controlled so that it doesn't raise beyond the

melting point of granules.

This ground particles will be passed through a series of vibrating sieves

through which the finer particles will fall and be collected for use.

The oversized particles held on the sieve are conveyed back to the mill

for further disintegration.

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MOULD MATERIALS

Moulds are not so expensive, but entirely depends upon the quality level

of the moulded parts and the method of heating to be used in the process.

Three types of mould materials in common use are

Cast aluminum

Steel sheet metal

Electroformed copper-nickel

Cast aluminium moulds are widely used for small to medium sized parts

requiring number of cavities.

Steel sheet is preferred where surface finish is not critical and for the

larger moulds of simple design.

Electroformed copper-nickel moulds are most expensive but offer a very

smooth finish.

This type of moulds are best when very intricate surface and precise

detail is required on the finished part.

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PROCESS VARIABLES

There are many potential variable in the rotational moulding process, that

can affect the size of the part being produced.

If there is any variation in the amount of plastic material charged into the

cavity, the wall thickness will change accordingly.

The shrinkage and part dimension also vary with a change in wall

thickness.

The speed and ratio of rotation determine the number of times a specific

location on the cavity passes through the puddle of plastic material and

the direction in which it enters and exits the puddle.

A change in these molding machine settings can affect the uniformity of

the wall thickness of part.

The molding machine speed, ratio of rotation, oven temperature and

other processing parameters must accommodate all the parts being

moulded.

Variation in over time, temperature and air velocity can affect final part

size.

The hotter the plastic material becomes, the more it expands, the material

will then contract or shrink more as it returns to room temperature.

The speed with which plastic material is cooled will affect shrinkage.

Cooling the material quickly will result in a low shrinkage factor.

Cooling the material slowly increase shrinkage, but the shrinkage will be

more uniform.

These Variations in shrinkage encourage warpage and make it difficult to

maintain uniform dimensions.

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Variations in the amount of mold release used can increase or decrease

the tendency of a hollow part to pull away from the cavity as the part

cools and shrinks.

The best approach is moulding parts to close tolerance to establish the

optimum moulding cycle and then maintain those conditions.

The speed of rotation of the mold must be slow enough to ensure the

gravity holds the plastic material in a puddle in the bottom of the cavity.

FAULTS AND REMEDIES IN ROTATIONAL MOULDING

PROBLEMS POSSIBLE SOLUTIONS

Production rate is too low

(or the heating cycle too

long)

Raise the oven temperature.

Use a higher-melt index

powder.

Use a lower density powder.

Powder fuses across deep

narrow draws in the mould

resulting in incomplete

mould fill

Increase the speed of rotation.

Use a lower-density powder.

Use a higher-melt index

powder.

Modify the mould, if possible.

Moulded piece contains

bubbles

Or

Piece has a rough inside

surface

Raise the oven temperature.

Increase the heating cycle.

Use a higher-melt index powder.

Use a lower-density powder.

Improve mould wall uniformity.

Decrease the wall thickness, if

possible by

decreasing the amount of

powder in every charge.

Make sure the interior surface of

the mould is dry.

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Piece is yellow, brown or

other discolouration

action.

Decrease the heating cycle.

Lower the oven temperature.

Check the mould wall for

contamination such as rust.

Piece sticks

in the mould

Clean the mould surface and apply a

suitable mould release agent.

Decrease the heating cycle or oven

temperature.

Increase the cooling cycle.

Vent the mould.

Modify the mould so that the part

tapers to a slightly larger dimension

toward the end of the mould through

which it is removed

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Piece is brittle

Use a low density powder.

Use a lower-melting index powder.

Redesign the part to eliminate sharp

corners.

Piece is warped

Decrease the cooling rate.

Lower the oven temperature.

Rotate the mould during cooling.

Vent the mould.

Improve the wall uniformity.

Allow short air cooling before water

cooling.

ROTATIONAL MOULDING PROCESS Vs BLOW MOULDING

PROCESS

Rotational moulding process have clear advantages over other process

like blow moulding and injection moulding.

The ideal shape for a part for Blow Moulding is a cylinder that is closed

on one end with a small opening at the other end. The best shape for a

rotationally moulded part is ball.

Extrusion blow moulding machine cost more than the rotational moulding

machine for a given size and capacity.

Blow moulding machines are powered by electricity which is 40 % more

costly than the natural gas that is typically used for heating in the

rotational moulding process.

The moulds for blow moulding are normally higher in cost than rotational

moulding.

Blow moulding has advantage over rotational moulding of being able to

process many thermoplastic materials including ABS and PPO.

Multilayered walled parts like fuel tanks are blow moulded but much

more costly moulding machines are required.

The blow moulding process has advantage of being able to process

materials as-received in pallet form. It eliminates the cost of pulverizing

the pallets into fine powder.

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Blow moulding process ideally suitable for capacity upto 1000 its . But

container with capacity 10,000 or 50,000 Hrs are most common in

rotational moulding process.

Blow moulding is preferred for larger volume, lighter duty barrels.

Rotational moulding dominates the market for smaller volume specially

barrels with improved toughness.

ROTATIONAL MOULDING & ITS APPLICATION

Polyethylene

Industrial products : Tanks drums, containers, nesting pallets, floor

maintenance machine components and tanks, medical carts, viedo game

housings, news paper and magazine vending machines, tool chests,

shipping cases.

Transport Products.

Consumer Products.

Recreational Products.

Agricultural products.

POLYPROPYLENE

PP is specified for many of the same applications as PE. PP‟s higher

stiffness and increased temperature resistance allow it to perform in

applications where PE isn‟t quite enough.

All these application takes advantage of PP‟s chemical resistance,

stiffness, and heat deflection temperature.

Other applications include large chemical shipping drums, radio active

material containers and high-temperature air ducts.

PP is a relatively new as rotational molding material.

PVC

PVC material has many applications like.

Industrial Products : Flexible and rigid airducts, machine feet, air and

water filters, gaskets, tires and floor scrubber squeegees and bladders.

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Medical Products : Examination chair arms, Flexible anesthesia face

masks, blood pumps, respiration squeeze balloons and anatomical

teaching models.

Consumer products : Figurines and life-size statuary, soft cushioning

furniture, picture frames, artificial fruit and toys.

Recreational Products : Soft, cuddly, and noise-making squeeze toys; life

like doll and animal heads and body parts; toy wheels, energy-absorbing

sports helmet liners.

NYLONS

Nylon becomes the choice where PE isn‟t quite good enough.

Nylon is a choice when the application requires more temperature

resistance, tensile strength, or chemical resistance in contact with oil and

gasoline.

Typical uses for nylon include fuel storage tanks, high temperature ducts,

truck radiator surge tanks, large chemical shipping bottles, air horns,

pressurized water treatment tanks, air intake manifolds.

POLYCARBONATE

PC‟s impact strength allowed it to take this application away from glass.

This material‟s excellent out door weatherability is another important

consideration.

Other uses include pressurized beer containers and dispensers, air-cleaner

housings, heating and intake ducting, illuminated traffic signage,

transparent food and medical containers.

…………………………………………………………………………

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73 | P a g e

Thermoforming process

Introduction

Thermoforming tech.is secondary processing technique. The process involves

heating a thermoforming sheet to its softening temp. or forcing the hot and

flexible sheet the contours of mould by vacuum where it is held until cool.

Thermoforming Process

Clamping

Clamping of Sheet .

Heating

Heat the sheet by Radiant heaters

Pre-stretch

Air is introduced to blow a small "bubble"

The mould is then raised into the pre-stretched sheet.

Vacuum

A vacuum is applied on Sheet.

Plug Assist

Where a deep draw is required a top "plug" may be used.

Cooling and Release

The material is allowed to cool by air or water spray.

The moulding is released from the mould by introducing a small air

pressure.

Finishing

After moulding, any mould finishing may be performed, trimming,

cutting, drilling, polishing, decorating etc.

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Material

Acrylonitrile Butadiene Styrene ABS

Polystyrene PS

Polycarbonate PC

Polypropylene PP

Polyethylene (sheet and foamed sheet) PE

Polyvinyl Chloride PVC

Acrylic PMMA

The softening range and hot strength are important properties.

Types of mould

Plaster

Wood

Plastic mould

Aluminium

OR

Good quality wood

Aluminum

Mils Steel

Plaster of Paris

Epoxy

Plaster(Plaster of Paris)

Most commercial moulding plasters are not strong enough to be in prototyping.

Plaster are inorganic calcious materials that hydrolytically react and harden

when mixed with water, soap such as murphy‟s oil soap.

Advantage :- Cheap, quick, no venting is necessary and so intimate production

of details is possible.

Disadvantage :- A max. of only about 50 forming is possible. The surface

being very soft.

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Wood

The wood is kiln-dried before shaping to minimize stress relief during

fabrication. After through drying the surface can be sealed with temp.

Advantage :- Cheap, longer life span then plaster moulds, higher impact

strength.

Disadvantage :- Limited life say approximately 500 forming. During

repeated forming, wooden mould should not be allowed to become too

hot and its dimensions should be checked regularly.

Plastic moulds

Plastic mould are used where mould surface temp. is not exceed 600C where

drape vacuum forming used epoxy and unsaturated polyester resin together with

glass fiber are the mould materials of choice.

Advantage :- easily manufactured, low thermal conductivity ,little or no

finishing is required.

Disadvantage :- Some materials are sensitive to high forming temp.

Applications (Thermoforming)

Baths & Shower Trays Yoghurt Pots

Ski-Boxes Boat Hulls

Machinery Guards Vehicle Door Liners

Refrigerator Liners Sandwich Boxes

Parts of vehicle cabs Exterior Shop Signs

Heating systems

Convection ovens

Infrared radiant heaters

Electrically power infrared heaters

Types of forming processes

1. Straight vacuum forming

2. Pressure forming

3. Plug-Assist forming

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4. Free forming

5. Drape forming

6. Snap-back forming

7. Matched –die forming

8. Mechanical forming

Straight vacuum forming

In this process a plastic sheet is clamped in a frame and heater. When the hot

sheet becomes rubbery or elastic, it is placed on female mould cavity. The

vacuum is applied and atmospheric air is cooled the sheet.

Pressure forming

In this process a plastic sheet is formed in a mould. The air pressure on the

plastic is used of force the material against the and heated till softened. Or the

mould is airtight seal the mould and air pressure is than applied or made the

product.

Plug-assist-forming

A male plug is used in this method of vacuum forming. Plastic sheet is clamped

in the female mould and after the heat- softened sheet is sealed across the mould

cavity. A push the sheet causing to stretch. When the plug has completed its

penetration stroke and Sheet is transfer from the plug surface to the cavity

mould. This method permits deeper with more wall thickness. The formed

article in shaped. The plug are made up of metal , wood , or thermoset plastic.

These should be heated to a few degree less then the temp. of the plastic in

order prevent premature cooling.

Free forming

Some of the products need very high optical clarity.if the forming is very done

using a mould would result in undesirable changes in the surface quality of the

part. To avoid this ,the techniques known as free forming or free blowing is

used. In this technique, the part is expanded with air pressure. The size of the

bubble is often monitored by an electric eye. When the bubble reached desired

size. The air pressure is reduced to a level maintains the size of the bubble while

part cooling.

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Drape forming

In this process the plastic sheet is clamped and heated then drawn over the

mould either by pulling it over it over the mould or by forcing the mould into

the sheet. When the mould has been forced into the sheet and a seal created.

Vacuum applied beneath the mould forced the sheet over the male mould. by

draping the sheet over the mould , that part of the sheet touching the mould

remains close to the original thickness of the sheet.

Snap-Back Forming

Snap back forming is the modified form the drape forming. In snap back

forming the material is heated to the sag point and then drawn slightly into a

vacuum box below the part. This pre- stretching this centre of the part and

usually about one- half to two-third of the total draw that the part with receive.

A second step is then activated to give more draw. the male mould is pressed

against the material to draw it farther. During this drawing step , the thickness

of the centre of the material by contact the mould and thinning near the edges.

Finally the part is formed by applying a vacuum through the male mould and

causing the part to „ snap back‟ against the outside the male mould. the

advantage of this process is the uniform wall thickness. Only drawback being

that it requires longer cycle time.

Matched Die Forming

This method resembles to that of compression moulding. The material is heated

to the sag point and is trapped and formed between male and female dies that

are made up of wood, plaster, epoxy, or other material. the Clearance between

male and female mould decides the wall thickness. No vacuum or no air is

applied in this process. mould are placed until the plastic cools & cures.

Comparison of thermoforming & injection moulding

Thermoforming Injection moulding

Sheet & raw stock Granule & raw material

Die are made wood ,plaster of paris The mould cost is very high ( mild

steal)

If the no of articles to be moulded is

less then

High production

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Operation set up quickly Operation takes more tune

Sheet can be printed or decorated

before forming

Processing Before decoration not

possible

Holes and undercuts can‟t be produce

by thermoforming

Holes and undercuts be produce by

Injection moulding

Pressure required is lower then

injection moulding

Pressure required is very high

Lower cost is original cost Higher original machine cost.

THERMOFORMED DISPOSABLE CUPS

Introduction:

Wide applications such as Ice-cream cups, Tea cups, Thmblers for serving cool drinks, coffee etc.

Plant Capacity:

10,000 cups per day on single shift. Sizes 100 ml. cups and 250 ml. cups.

Raw materials:

- HIPS/PVC sheets of up to 0.2mm thickness.

Plant & Machinery:

Thermoforming machine

Thermoformed article cutting press

Compressor

Process:

Fix the sheet on the frame above the mould and clamp properly. Heat the sheet to softening point.

Remove heaters, Lift the mould up.Apply vacuum to get the desired shape.Remove the moulded item from the mould.Separate the moulded cups from the sheet by a cutting press.

Land & Building:

Total land is around 3000 sq. ft.

Total covered area is around 1500 sq. ft.

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Utilities:

Total connected load is around 15 KW.

Manpower:

Around 3 persons.

Estimated cost oc project:

Cost of Plant & Machinery is around 3 lakhs. Total capital investment is around 5 lakhs.

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CALENDERING Basic principle

It is process of squeeze a plastic melt between two or more rotating or rolls to

from continuous film and sheet. It is technique of making paper , metal and

rubber industries.

Materials

The PVC – resin which is generally processed in calendaring plant.

1. PVC

2. PS

3. PP

4. PE

5. ABS

Are also used for calendaring process.

Types of calendaring

1. L-CALENDER 2. INVERTED L- CALENDER

3. F- CALENDER 4. Z-CALENDER

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Heating system

a) High Pressure steam is used on calendaring plants. Wark pressure range

680-1380 KN/m and temperature range 120-1850C.

b) High pressure hot water (H.P.H.W.)

Advantage of calendar process

1. Delivery/output of a calendar id high.

2. Increase in maximum width of film/sheet.

3. Better control of film/sheet thickness

4. Better optical properties.

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TECHNICAL MANUAL ON

“MODEL QUESTION AND ANSWERS IN

PLASTICS PROCESSING”

INJECTION MOLDING

1 MARK QUESTIONS AND ANSWERS

1. Maximum daylight = Minimum daylight +_______________.

Ans. maximum mold opening

2. Polyacetal material requires no predrying before molding. State true or false

Ans. False

3. A higher screw rpm will cause the screw to return to its starting position

sooner. Say True / False

Ans. True

4. Increase of screw rpm will result in larger cushion say True / False

Ans. False

5. Depression on molded component is called ______________ defect.

Ans. Sink marks

6. Cushion size is the amount of plastic ahead of the screw before the injection

Stroke begins say True or False

Ans. False

7. An increase in the cushion size

a)Reduces the effective operating length of the screw.

b)Increase the effective length of the screw

c)Does not change the effective operating length of the screw

d)None

Ans. a

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1. Why hopper throat cooling is necessary for barrel?

Ans. Hopper throat cooling is necessary for barrel to avoid bridge formation. If

too cold granules also may slip each other and there by the output may be

reduced.

2. What is cavity pressure?

Ans. The pressure generated in the cavity when the molten material is forced

inside the cavity is called as the cavity pressure

3. Distinguish between speed & injection pressure

Ans. Higher injection speed (cc/sec) can push the melt to farthest part of mould

at a shorter time before the melt freezes. Injection pressure (kg/cm2) is required

to overcome the resistance to the flow of the melt in the mould. It depends on

the temperature of mould, flow ratio and melt viscosity.

3. Match the following.

1. Thin wall molding requires a) Dehumidifier

2. Molding will have burn mark b)High pressure injection machine

3. Moisture remover device c) Non return valve

4. To prevent melt leakage into barrel d) Inadequate venting

Ans.1-b,2-d,3-a,4-c


1. Draw the neat sketch of an injection-moulding machine and name its parts.

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2. State the merits of preheating

Ans. The merits of preheating are,

1) Reduce the cycle time

2) To improve the quality of product.

3) To obtain the desired property like clarity etc in the moulding.

4) Energy saving

3. Discuss in brief the process of Gas assisted injection moulding.

Ans. Gas assisted injection Moulding is used to produce hollow out injection

moldings by controlled injection of an inert gas (nitrogen) into the polymer

melt. The gas does not mix with the plastic but instead continuos channel

through the hotter, less viscous section of the stream. The gas maintains the

pressure throughout the Moulding cycle. During the cooling phase the gas

ensures the positive contact between the polymer and the surface of the mould.

Swirl marks free surface is possible in this process.


1. Compare and contras hydraulic clapping system verses Toggle clapping

systems.

Ans. HYDRAULIC SYSTEM:

• .Much higher original cost

• .Higher horse power needed. Therefore more expensive to

run.

• Unlimited stroke potential

• .Direct readout of clamp force

• .Easy adjustment of clamp force

• .Easy mold setup

• .Varies stroke to mold height

• .Clamp speed easily controlled or stopped at any point

• .Low maintenance as parts are self lubricated

TOGGLE SYSTEM:

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• .Lower original cost

• .Lower horse power needed, more economical to run

• .Limited stroke potential

• .No Direct readout of clamp force

• .More difficult

• .More involvement in mold setup

• Constant mold stroke

• Clamp speed more difficult to control and stop

2. .With a neat sketch explain the In line- reciprocating screw type injection

moulding process.

Ans. In this type of machine the plunger/spreader that is the key to the plunger

type machine is replaced by the reciprocating screw, which moves back and

forth inside the heating cylinder. as the screw rotates the flights pick up the feed

of the granular material from the hopper and force it along the hot cylinder

barrel.

As the material comes off the end of the screw the screw moves back to

permit the plastic material to accumulate. At proper time the screw is moved

forward, acting as a plunger and propelling the plasticized material through the

nozzle and sprue into the mould cavities. Measuring the back travel of the screw

regulates the size of the change.

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EXTRUSION


1. Increase in back pressure will result in ___________(increase / decrease) in

output.

Ans. Decreases

2. What is the relation between die diameter, bubble diameter and layflat width.

Ans. Blow ratio (BR) = Dbmax / Dd

or Blow up ratio = 0.637 x lay flat width

Die diameter

Lay flat width = (πDd/2) BR

Or Lay flat width = 1.57 x bubble diameter.

Where,

Dbmax = maximum diameter of bubble

Dd = die diameter.

3. Hopper cooling is done.

a) To prevent slipping of granules b) To avoid bridge formation

b) To increase output. d) All of the above

Ans. b

4. The die used for coating of wire and cable ___________

a) Cross head/offset die b) straight die c) coat hanger die d) T- Die

Ans. A


1.State the importance of screw cooling

Ans. Screw cooling is recommended to prevent the decomposition of the heat

sensitive material

2. Name the different extruder screws.

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Ans. The types of screws used are,

a) General purpose or the Polyolefin screw

b) PVC screw

c) Polyamide screw

d) Barrier screw

e) Twin screw

f) Vented screw

3. .What is guesseting device?

Ans The guesseting device is used for folding of the film so that the appearance

of the bag is small but it can take large amount of material.

4. Why Pretreatment of the substrate / web is essential before coating?

Ans. The pretreatment of the substrate / web is essential before coating so as to

increase the adhesion of the surface with the polymer.

5. State the different pipe-sizing methods

Ans. The different pipe sizing methods are,

1) Vacuum trough

2) Sizing sleeve

3) Extended mandrel

4) Sizing plates.


1. .State the functions of screen pack and breaker plate.

Ans. The functions are,

Breaker plate:

Its principle function is to provide support to screen

pack. The breaker plate helps to convert the spiral flow of the polymer melts

leaving the screw into a linear, axial one. It also supports the screen pack.

89 | P a g e

Screen pack:

Its main function is to eliminate the particulate contaminants.

2. Explain in detail the causes and remedies for “Melt fracture”.

Ans Melt fracture or rough surface finish is caused when the melt temperature

is too low. It can also happen due to narrow die gaps.

It is a die-entry effect

In any converging flow there are tensile and shear forces

If tensile stresses become large and if they exceed

the tensile strength of melt, the desirable smooth

laminar flow is lost completely.

The extrudate emerging from die exit will be of irregular

shape. This phenomenon is called “Melt fracture”.

3. Enlist the factors, which affect the output of an Extruder.

Ans. The factors that affect the output of the extruder are,

Factor Direction of change Output

Screw speed Increase Increase

Channel depth Increase Increase

Helix angle Increase Increase

( max. upto 300)

Back Pressure Decrease Increase

Length of Metering Increase Increase

Viscosity Increase Decrease

.

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4. What is bambooing ? Explain the root cause in detail.

Ans. Bambooing is the defect that is observed during the extrusion of pipe etc.

this defect arises due to improper die design, die not having proper polish ,

defective or intermittent working of melt pump which causes melt surge.

The melt as it proceeds along the die channel, has a velocity

profile with maximum at the centre and zero at the wall.

As it leaves the die lips, the material at the wall has to

accelerate to the velocity at which the extrudate is leaving

the die.

This generates tensile stress and if the stress exceeds Tensile

strength, the surface ruptures causing the visual defect -

“shark skin”.

If the conditions causing shark skin becomes more intensive,

Eg. Pressure at the extruder becomes excessive or die

temperature drops, the extrudate “snaps back” --

“Bambooing effect”.

BLOW MOLDING


1. Jerry can with handle is produced by _____________

a. Injection blow moulding

b. Extrusion blow moulding

c. Injection stretch blow moulding

d. Extrusion stretch blow moulding

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Ans.b

2. Small pharmaceutical bottles with accurate neck finish

is produced by

________ Process.

Ans. Injection blow molding

3. 200litres capacity oil barrel are economically produced by ______

Ans. Intermittent blow molding

4. The main root cause for „pitted surface‟ defect in blow

molding is____________

a) Excessive melt temperature

b) Inadequate air venting

c) High blow ratio

d) None of the above

Ans. d

5. All bottles are Bi-axially oriented. Say true or false.

Ans. False

6. Odd shape containers are made by die shaping say true or false.

Ans. True


1. Name any two merits of Injection blow moulding.

Ans. The two merits of injection blow molding are,

• Perfect neck finish.

• Scrap free product.

2. State the different types of blow moulding processes?

Ans. Injection blow moulding, Extrusion blow moulding, Injection stretch blow

molding, Dip blow molding, multilayer blow molding, special blow molding.

3. State the various moulds materials used in blow moulding.

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Ans. The various materials used in blow Molding are Al, steel, beryllium-

copper alloy, kirksite.

4. State the merits of biaxially oriented blow molding products.

Ans. It enhances the mechanical properties, optical properties, chemical

resistivity & improves barrier properties.


1. Compare & contrast extrusion blow molding and injection blow molding

Ans.

2. What do you mean by stretch blow molding? Explain the process

Ans. Stretch blow molding is the technique in which the use of mechanical

assistance is done to stretch the part in the longitudinal direction at the same

time the blowing of the part stretches it in the hoop direction or the radial

direction. This biaxial stretching occurs when the parison or the preform is

blown into the desired shape in the blow mold.

SR.

NO

EXTRUSION BLOW MOLDING INJECTION BLOW MOLDING

1 Best suited for shorter runs and quick tool

change

Best suited for longer runs

2 Machine costs are less Machine costs are more

3 Tooling costs are 50 to 75% less than

injection molding.

Tooling costs are more

4 Accuracy depends on die design Very high accuracy can be obtained

5 Seam lines or pinch marks are possible in the

product

No seam lines or pinch marks possible in

injection blow molding

6. Scrap generation is more Scrapless or negligible scrap generated.

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The longitudinal stretching of the parison is accomplished using a

telescopic mandrel or core pin that extends to push on the bottom of the preform

at the same time the air is being injected to push against the walls to stretch the

material radially. The temperature of the process is properly selected so as to

obtain proper orientation.


1. .Mention types of blow molding techniques and explain with neat sketch of

ram

type of blow molding process.

Ans. The different types of blow molding techniques are,

• Injection blow molding

• Extrusion blow molding

• Multicolor blow molding

• Dip blow molding

Ram type or the accumulator type of blow molding is the process in

which the extrudate flows from the extruder into an external chamber or

accumulator. At an appropriate moment in the cycle, a ram in the chamber

advances and injects the resin in the die that is mounted on the outlet of the

accumulator to form the parison. Very large parts can be made using the

accumulator system because the volume of the accumulator is several times

larger than the injection volume possible with the reciprocating screw machine

and the injection could be very fast using the ram type of machine. The

accumulator can be heated to maintained the proper temperature of the resin so

only resin with good heat stability should be used since the time that the resin

may be at high temperature could be quite long.

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THERMOFORMING


1.In thermoforming plastic sheet heated to the –

a) Melting point b) Sag point c) Tg –point d) none of the above

Ans. b

2. In Drape forming technique vacuum is used for forming say true or false.

Ans. True

3. Canopies for racing vehicles is produced by-

a)free forming or free blowing b) drape forming

c) Pressure forming d)Mechanical forming

Ans b

4. What is the other name for free forming

Ans free blowing.

5. Bath tubs are made out by _____________ thermoforming technique

Ans. Drape forming

6. Which of the following materials not usually thermoformed

a) HIPS & ABS b) PMMA c) PVC d) POM & Nylon

Ans. d

7. Where the production runs are very short in thermoforming _______ molds is

preferred.

Ans. wood


1. Enlist thermoforming techniques.

Ans. The different thermoforming techniques are,

1. Straight vacuum thermoforming

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2. Pressure thermoforming

3. Plug assist thermoforming

4. Reverse draw thermoforming

5. Match die thermoforming

6. Drape forming

7. Ring thermoforming

8. Free forming

9. Snap back forming

2. .State any two merits of Plug –assisted forming.

Ans. The advantage of plug assisted forming are,

• Better wall thickness uniformity

• Exact shape of required can be obtained.

• 3. State the merits of die-matched thermoforming.

• Ans. The advantages of die matched thermoforming are,

• 1.Very large parts can be manufactured

• 2. Complicated shapes can be easily manufactured

• 4. .State the various mould materials used in Thermoforming

• Ans. The materials used are,

• 1. Wood

• 2. Aluminum

• 3. Epoxy

• 4. Steel

• 5. .Match the following:

• a) Refrigerator liners 1) Extrusion

• b) Air bubbles film 2) Co-injection

• c) Two colour moulding 3) pad printing

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• d) Curved surface printing 4) Thermoforming.

• Ans. a-4, b-1, c-2,d-3


1. Classify Thermoforming processes. Explain the process of plug-assisted

forming with merits over drape forming.

Ans. The thermoforming process are classified as,

• Straight vacuum forming

• Pressure forming

• Plug assisted forming

• Reverse draw forming

• Free forming

• Drape forming

• Mechanical forming

• Snap back forming

• Matched die forming

Plug is used in the plug assisted thermoforming to push the material into the

mold. The plug can be used to carry the material towards the area where it

would be too thin if straight vacuum or pressure forming were used. Plugs can

be made of wood, metal. the plug is generally heated to few degrees less than

the temperature of the plastic.

The advantages of the drape forming over the plug assisted forming are,

– Better wall thickness can be accurately obtained as

compared to the drape forming.

– Less space is required around the mold, so the trim

scrap is less in plug forming.

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ROTATIONAL MOLDING


1. In rotational molding process plastics material perfectly-

a) Melts b) fuses c) Both (a) & (b) d) None of the above.

Ans. B

2. Rotational molding process has four principle steps. ( 1) loading (2) ___

(3)

Cooling & (4) _____.

Ans. Heating and unloading

3. Rotational speed of major axis should be slow & it should not be more than –

a) 100 rpm b) 85 rpm c) 60 rpm d) 75 rpm

Ans. C

4. State true or false:

The amount of material controls the wall thickness of rotational molded

products.

Ans.True


1. What do mean by grindability for rotational molding?

Ans. Grindability of the material means that the resin can be ground to fine

powder. Resin grades that have very low melting may not be easy to grind in the

high speed impact mills.

2. What is Pourability in rotational molding?

Ans. To tumble properly in the mold the plastic powder must flow easily

without any external pressure. This property is called pourability. A minimum

flow rate of 185 gm/min characterizes acceptable rotational molding powders.

3. What is mixed resin in rotational molding process?

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Ans. The mixed Resin are characterized by two different melting points and

thus will have different softening points. Due to different softening points it

would fuse and stick to the walls of the mold and have two layer molding.

4. How do you achieve the openings in rotational molded tanks?

Ans. Openings can be obtained in the rotational molded tanks using metal

inserts at the position where the inserts are required with the insulated plugs.


1. What are the advantages & disadvantages of rotational molding.

Ans. Advantages of rotational molding:

• Very large parts can be made with relatively low cost

equipment.

• Low pressures are used and the temperature used are also

less

• Stress free parts can be made using the rotational molding

• If parts of equivalent sizes are made then less investment as

compared to injection or blow molding.

• Less cost of equipment and ease of making the mold.

• Multilayer and Multicolor product possible.

Disadvantages:

• simple shapes can only be manufactured

• poor dimensional tolerance control

• generally thicker overall walls

• low part mechanical properties.


1. What is rotational molding process? Explain the process with a neat

sketch.

Ans. Rotational molding uses the rotation of the mold in the heated chamber

to form the part. It is mainly suited for the formation of very large, seamless

and hollow stress free parts. The process uses no pressure and hence the

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mold can be simple and inexpensive. The temperatures used in the rotational

moldings are less as compared to other processes.

Process:

The rotational molding process has four principle steps,

these are

1. Loading

The loading steps begins with careful

weighing of the charge of the starting material.

The starting material is usually a finely ground

thermoplastic powder. The charge of the starting

material is loaded manually into an open ,

cold mold that is prepared from

inside by coating the mold release agent.

2. Heating and cooling

The heating cycle is done in an oven. The heating oven must be large

enough that the entire mold assembly can be placed inside it and rotated freely.

The mold assembly is rotated throughout the heating and cooling cycle. The

heating cycle for the mold is often quite long and so it is common to have

multiple molds that are cycled together. The mold is rotated biaxially so that is

even mixture of the molten plastic inside the mold. The rotation speed

determines the thickness of the molded product.

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3.Unloading

Unloading steps begins with the removal of the lid of mold. The cold part is

removed manually or with a mechanical assist. Mold release is sufficient to

allow removal.

.

CALENDERING


1. In L-Type calender –

a) offset roll on the top & the take off is from the bottom roll

b) offset roll is at the bottom & the take off is from the top roll

c) Both (a) & (b)

d) Neither (a) nor (b)

Ans. b

2. which of the following is not a type of Calender

a) „L‟ type b) „F‟ type c) „E‟ type d) „Z‟ type

Ans. E -Type

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3. Most common type of intensive mixer using calendering is______

a) Braw blender b) Planetory mixer c) Banbury mixer d) None of the above

Ans. c


The golden rule as such in calendering is to use temperature as high as

possible provided no sticking to rolls & any degradation is caused.

Ans. True


1. What is plate out in case of calendering?

Ans. Transfer of tacky deposit of compound to the roll is plate out defect.

2..Match the following for the calendering take off:

a)Bright film finish 1) Engraved roll

b)Mat finish 2) Shot peened roll

c) Orange peel 3) Chrome plated roll

d) Embossed 4) Sand blasted roll

Ans. a-3, b-4, c-2, d-1

COMPRESSION AND TRANSFER MOLDING



Storing under cryogenic can enhance shelf life of thermoset compounds.

Ans. True

2. .State true or false

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PTFE can be processed using conventional moulding machinery.

Ans. false

3. Down stroke press is used ____________ products (smaller/larger)

Ans. larger

4. Cull is associated with ________transfer molding (Pot type / Auxiliary ram

type)

Ans. Pot type


1. Name the various test methods for checking the flowability of thermosets.

Ans. The test methods are

1) Cup flow test

2) Spiral flow test

2. Name the various transfer moulding processes.

Ans. Various processes are

a) Pot type transfer molding

b) Auxillary ram type transfer molding

c) Screw type transfer molding

3. State the various preheating methods thermoset material.

Ans. Screw preplastifying preheating,Infrared preheating, High frequency

preheating.

4. Match the following:

a) Cull is must 1.Compression & Sintering process

b) Flash is must 2. Transfer molding

c) PTFE processing 3. Compression molding

d) Spiral flow test 4. Pot type transfer molding

Ans. a-4, b-3, c-1, d-2

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1. Explain the process of compression molding with the help of a neat diagram.

Ans. Compression molding or the matched die molding is the process used

exclusively for thermoset molding. In this process a charge of thermoset resin is

put into the cavity of a matched heated mold that is in open condition. The mold

is closed by bringing the male and female half together and pressure is applied

to squeeze the resin so that it uniformly fills the mold cavity.

While under pressure the material is heated which causes it to cross link

and to harden. When the material is hard the mold is opened and the part is

removed in hot condition.

2. What are the process variables in compression molding.

Ans. The process variables are,

1. Design the article to be produced

a) projected area and depth

b) wall thickness

c) obstruction to vertical flow pin etc.

2. Speed of press in closing

a) use of slow or fast acting self contained press.

b) For fast acting press by hydraulic line pressure

3) Plasticity of material

a) preheating

b) density

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c) position of the charge in cavity

4) Overall temperature of the mold

temperature variation within the cavity and force of the mold.

5) Surface condition of mold cavity and force

a) highly polished chrome plated surface

b) polished steel

FRP


1. FRP „I‟ beams are produced by _________ process.

Ans. Pultrusion

2.Which one of the following is a high pressure process.

a. Spray up technique b. RTM

c. Matched –die-moulding d. Centrifugal Casting.

Ans. b

3. ___________ type of Glass fiber is mostly used in FRP Fabrication.

Ans. fibrous form

4. State True or False

FRP boats are produced by matched die moulding

Ans. False

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1. Define “ Gel coat” and what is its function.

Ans. The thin unreinforced layer of the resin placed on the surface of the mold

in the hand lay up process is known as the gel coat. It forms a protective surface

layer through which fibrous reinforcement does not penetrate. It also improves

the gloss.

2. Classify FRP processes

Ans The FRP process has been classified into, Open type,contact

molding,matched die molding, RTM, continuos( Pultrusion, filament etc).


1. Write short on vacuum bagging.

Ans. In vacuum bagging a mold release film is placed over the reinforcement

or resin composition. The film edges are sealed and vacuum is drawn from

within. Atmosphere pressure works the film towards the laminate to create a

smoother surface, eliminate voids and force out entrapped air and excess resin..

It provides good surface and void removal benefits.

2. What do you mean by matched die molding? Explain in brief

Ans. Matched die molding:

Very complicated shapes can best be form if both male and female

molds or dies are used together. These molds must be mated or matched so that

they fit together without interference and leave only the space between them for

the material. In matched die molding the clamp material is heated to the sag

point and then is transferred to the molding station where the male and female

molds are immediately brought together to squeeze the material.

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RECYCLING PROCESS


1. The technique of burning the solid municipal waste is called ____

Ans. Incinerating.

2. PET can be chemically decomposed by ______

a) Hydrolysis

b) Alcoholysis

c) Glycolysis

d) All of the above

Ans. d

3. .The most important factor that influences control degradation is degradation

in presence of

___________.

a) CO b) O c) Inert gas d) none of the above

Ans. b


1. Write short not e on factory waste recycling

2.Draw flow chart for manufacturing waste woven sacks to pellets/granules

Ans.

PP/HDPE

WOVEN

SACKS

SORTING

(MANUALLY)

CLEANING &

DRYINGSHREDDING

MIXEREXTRUSIONGRINDER

EXTRUSION PELLETS

FLAKES

LUMPS DRIED SMALL

PIECES

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1.Write short note on PET Recycling.

Ans. The chemical recycling of PET is a Glycolysis process

The process consists of,

1. REACTOR (UNDER HIGH PRESSURE & TEMP):

Ethylene glycol and PET waste mixed together under high pressure and

temperature.

2. FILTER:

The product formed during the process is further filtered in this step.

3. REACTOR (UNDER VACCUUM)

In this step the excess glycol is removed and antimony oxide is added as

catalyst.

4. POLYMER STRANDS BY APPLYING N2 PRESSURE

In this step the polymer strands are separated using Nitrogen pressure.

These strands are then passed through the cooling trough.

5. GRANULATOR

The polymer strands are then passed through the granulator where they are

made into small size granules.

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SECONDARY PROCESSING TECHNIQUE


1. . Which plastic material has excellent Electroplatability.

a. ABS b. PC c. HDPE d. Nylon 6

Ans. a

2. . Which one of the welding technique is widely used in producing PVC

folders.

a. Vibration welding b. High frequency welding

c. Ultrasonic welding d. Hot air welding

Ans. b

3. Very fine reproducibility of printed patterns can be achieved through

a. Screen printing b. Flexographic printing

c. Pad printing d. Rotogravure printing.

Ans. d

4. The function of Transducer in Ultrasonic welding is _____________

a. Step up the input frequency

b. To convert Electrical energy into Mechanical energy

c. Step down the input frequency

d. None of the above

Ans. b


1. What are different methods for decorating the plastics ,

Ans.The different methods for decorating the plastics are,

a) Labeling

b) Printing

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c) Hot foil stamping

d) Transfer printing

e) Texturing

f) Embossing


1. Write various casting methods

Ans. The various methods are,

1) Surface casting or gated casting

2) Embedding and potting

3) Dip casting

4) Encapsulation

5) Fluidised bed coating

6) Slush casting

7) Static powder casting

8) Continuos casting

9) Film casting or solvent casting

Dip casting:

In the dip casting method the part is immersed in a paint filled

vessel and then withdrawn and then the excess paint is allowed to drain. The

parts are air dried and baked if necessary. The process is easily automated by

moving the parts by overhead conveyor from the dripping into the drying or the

curing areas. Filtration is essential to remove the particular contamination

introduced by the surrounding and by the plastic parts.

Plastics processing Manual (CIPET JAIPUR)

Engineering

Transcript of Plastics processing Manual (CIPET JAIPUR)