Vacuum infusion molding principle

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Vacuum bag infusion – step by step

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Vacuum bag infusion

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Vacuum infusion with semi-rigid shell

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Careful resin flow rate regulation to avoid air entrapment

RESIN FRONT

VOIDS

RESI

N F

LOW

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Resin infusion possibilities

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From a centre point towards the periphery

SLOWEST!

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Resin infusion possibilities

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From the edge

MEDIUM FAST!

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Resin infusion possibilities

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Infusion from the pheriphery

FASTEST!

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Flexible, semiflexible or rigid mould?

• Vacuum bag infusion (flexible bag): suitable for small production volumes, large size products and lower tolerance demands

• Vacuum infusion with semi-stiff shell: suitable for medium production volumes, medium product size and medium tolerance demands

• Vacuum infusion/RTM with stiff (solid) moulds: suitable for large production volumes, small size products and high tolerance demands

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Blades for wind mills

• Length 30 - 70 m• 20 years life length • Lay up of two separate

halves which are glued together

• Filament winding• Unsaturated polyester, vinyl

ester, epoxy resin• Glass fibre, carbon fibre• Stiffness and fatigue

properties are important• Denmark major producer

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AmbulancePolytec, Sweden

Modular construction design possible

• Parts are manufactured separately, and joined by adhesives

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Compression molding

• A premade compound is formed by pressure in a closed mold

• Crosslinking is initiated by heating• Cost effective method for long and very

long series• SMC: sheet molding compounds• BMC: bulk molding compounds• Automotive and electrical industry most

important application areas

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SMC manufacture

Shelf life: 3 - 4 monthsMSK 20120213

SMC prepreg manufacture – step by step

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Application of resin onto plastic support film

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Addition of cut fibres

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Ready SMC is covered by second support film

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Schematic of compression molding

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SMC press

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Compression molding - process conditions

• Pressure: 20-50 kg/cm2

• Temperature:145 - 160 ºC• Time: 1 - 5 minutes• Molds: steel, chrome-

plated

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Volvo V70 TailgateBenefits with composite compared to steel:→ Reduced tooling need→ Styling freedom→ Integration capability→ Weight reduction

compared to steel→ Technology step

Comparison composite/metal series length

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V70 Tailgate

Steel plate

Theft/heat protection

Reinforcement

Directional fibres

BMC

t=3.5, 20% glass

SMC

t=2.5, 25% glass

SMC

t=2.5 (gen. Surfaces) 2.5-

4(stressed areas) , 25% glass

Glass fiber carpet

M =10,3 kg (structure only)

Production volumes – manufacturing process

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Reinforcements

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Fibre types• Glass fibre: relatively good strength,

medium stiffness (E= 70 GPa), transparent, cheap

• Carbon fibres: very good strength, high stiffness (E=200-300 GPa), black, very expensive, electrically conducting

• Natural fibres: flax, hemp, sisal, wood• Aramid fibres (Kevlar): very good tensile

strength, yellow, hard to process, expensive

• Special fibres: polyethylene fibres, boron, ceramics, basalt

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Fibres, yarns and rowings• An assembly of collimated glass

fibres is called a yarn, (tow, strand), and a group of yarns is called a rowing

• The yarns and rowings are twisted, which simplifies handling, but makes resin impregnation more difficult

• The fibre thickness varies typically between 3-25 µm (commonly 10-20 µm)

• Linear densities are given by the TEX number

• A rowing has a TEX of minimum 300

TEX 4 103d2N

densityN number of fibersd fiber thickness

TEX g km

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Characteristics for glass fibres• Based on SiO2 with added oxides of calcium, boron,

sodium, iron or aluminium• Depending on composition different glass types are

defined:

– A-glass (Alkali glass)– E- glass (Electrical glass)– C-glass (Chemically resistant glass)– S-glass (High strength glass)

• Characteristic properties are high strength, good tolerances for high temperatures and corrosive environments

• Transparency and no colour are advantages compared to other fibres

• Disadvantages are low stiffness, moisture sensitivity and abrasiveness

• Low cost has been the most critical factor when promoting their use

Composition and properties for glass fibres

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A glass C glass E glass S glass

SiO2 weight-% 72 64,5 55 65

Al2O3 + Fe2O3 weight-% 2 4 4,5 25

CaO weight-% 10 13,5 21,5 -

MgO weight-%

2 3 0,5 10

Na2O + K2O weight-% 14,5 10 < 1 -

B2O3 weight-% - 5 7,5

Tensile strength GPa 3,1 3,3 3,6 4,6

Modulus GPa 72 70 75 80

Softening point ºC 700 690 850 990

Density g/cm3 2,45 2,45 2,54 2,48

Manufacturing process for carbon fibres

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Polyacrylonitrile (PAN) is the most common precursor for carbon fibresThe strength of the fibres are due to orientation and stretching of the C-C bondsStrength can be increased by graphitisation at 1500 ºC

Carbon fibre production

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Textile reinforcements

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Classification of reinforcements

1. Short2. Unidirectional3. 2D

weaves/Planar interlaced

4. 3D/Fully integrated

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Different reinforcement types

• Chopped strand mat• Continuous strand

mat• Woven fabrics,

diaxial• Woven fabrics,

multiaxial• Stitched fabrics• Braided fabrics• Knitted fabrics• Combinations

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Chopped strand mats andcontinuous strand mats

• Non-woven structures• Surface weights 150 - 900 g/m2

• Made from chopped or continuous yarns, bound together chemically, mechanically or by heating

• Emulsion binders and polyester powder binders are most common

• Good drapability• Surface veils (surface eights 10-50 g/m2) are used

to get a wanted surface finish• Mats made from other fibres are commonly

named non-wovens

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Woven fabrics = interlacing of 2 or more yarn systems

• Characterised by the crimp

• Lower crimp improves formability and resin permeability

• Crimp also reduces stiffness

plain basket twill satin

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Benefits with woven fabrics

• Good drapability• Low manufacturing costs due to

combination of two layers• Good impact resistance• Lower stiffness due to crimp• Better compression strength

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The mechanical properties for weaves depend on:

• Type of fibre• Weave structure• Stacking and orientation of fibres• Yarn twist

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Braided fabrics• Circular braiding is used for tubes or ropes

• Biaxial

• Triaxial

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Braided reinforcements

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Knitted fabrics

• Made by knitting• Loose and flexible

weaves are produced

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Stitched fabrics (noncrimp)

• Fibre layers are stiched together into one structure

• The stiching is done by sewing

• Noncrimp fabrics offer a rapid and precise lay-up of multilayered reinforcement

• Different fibre types can be combined, sunh as comingled fabrics

Spread tow fabrics by Oxeon, Sweden

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Non-crimp fabric

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Combinations

• Combination of different mats stitched together

• Ex: Combiflow mat:

• Porous flow layer for better mould filling, used in resin injection

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Parabeam – 3 D fabric

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The interphase/interface in composites

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Long term durability of composites

• Depends on the state of the resin, which may undergo:– Physical ageing– Environmental degradation– Changes in fibre-matrix interaction– Matrix stress state, due to processing, thermal and fatigue cycling,

mechanical loads• Microcracking is the first sign of damage, which can initiate:

– Fiber fracture– Interface debonding– Delamination

• The microcrack can be a pathway for moisture, chemicals, microorganisms, soil which then can lead to degradation

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Fibre-matrix interphase

• The three-dimensional boundry between the fiber and matrix

• It is critical for the control of composite properties, as fibre-matrix interaction occurs through the interface

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Interaction at fiber-matrix interface

a) Micromechanical interlocking

b) Electrostatic (dipole) interaction

c) Chemical bondingd) Chain entanglinge) Transcrystallisation

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Interphasial region in composites = the region of the matrix which is

influenced by the fibre

matrix

fibreinterphase

Fibre diameter

interface

Interphase in composites

• The interphase = a three dimensional region near the fiber with properties different compared to the fiber and the matrix

c) composite 4a d) composite 8a

a) composite 3ab) composite 7a

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Transversal fracture in composites

Transversal fracture at low elongation (< 0.2%) due to poor adhesion between the fibre and

the resin

Transversal fracture at high elongation (> 0.6%) due to strong adhesion between the fibre and

the resin

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Fibre surface treatments

• Surface oxidation; electrolytical, gases or liquid chemicals

• Surface coating by organic/inorganic chemicals (sizing agents)

• Polymer grafting onto fibre surface

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Surface treatment of glass fibres

• Surface treatment by sizeing

• Treatment composition:– Film forming

polymer (PVA)– Lubricant– Coupling agent

Some effects due to surface treatment

• Fibre protection during shipment, handling and processing

• Binding of indivcidual filaments together to ensure easier handling

• Lubrication during processing• Reduce static electricity• Improve chemical bonding to the matrix

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Remark

• If the surface treatment is not properly done, it can be detrimental to the bulk mechanical properties, and the interface properties can vary

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Non-destructive testing (NDT)

• For identification of defects without destroying the object

• Used for quality control and for in-service inspection• Delaminations, failed adhesive bonds, voids,

incorrect reinforcement orientation, variations in fiber content

• Based on differences in physical or mechanical properties, due to the defect

• Comparative methods -> qualitative information

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Ultrasonic inspection

• Transmission of sound waves through the specimen

• 0.5 - 75 MHz sound• Pulse-echo or through

transmission• Coupling mediums

(water, oil, gels) for efficient transfer of sound wave into the component

NDT methods

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Acoustic emission

• Detection of microscopic failures by recording the sound of the event

• Fiber fractures and matrix microcracks• In combination with mechanical loading• Semi-NDT• Careful interpretation of data necessary

NDT methods

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Acoustic emission

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Other methods

• Radiography (X-ray, -ray)• Computer aided tomography: defects can be

located• Thermographic inspection: based on

differences in thermal diffusivity• Vibrational inspection: ¨coin tapping¨

NDT methods

End of part 2

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