Giving
38 HARDY COMPOSITES
Fabrication tcchniques
Vacuum bag moulding Pressure bag moulding Autoclave moulding
Vacuum impregnationmoulding
Resin transfcr moulding
Reinforced reaction jnjec~
lion moulding
Processing techniques of composite materials
Reinforccment
Hand layup
imum in design flexibility with the minimum in equipment in- vestment. But it does involve a lot of manual labour. A closely related fabrication method, tape laying which uses pre-impreg- nated ribbons has also become an important composite fabrica- tion method. This, unlike the hand lay up is an automated process.
GIVING SHAPE 39
Spray-up: In spray-up, bundles of filaments used as rein- forcement, and which are called roving are fed through a chopper into a resin stream and then sprayed directly into an open mould. Spraying-up equipment is usually inexpensive and port- able and roving is the least expen-
sive form of reinforcement. Spray-up
Laminate
Vacuum-bag
Vacuum-bag: Closely related to spray-up a.nd hand lay up, vacuum- bag moulding involves the placement of a flexible film over the resin and fibres in the mould. The joints around it are sealed and a vacuum is created.
This results in better adhesion be- tween layers and hence in a better composite.
Pressure-bag: This is analogous to vacuum-bag molding except that air pressure is applied directly to the rubber sheet. Pressurized steam may be used to bring about a change in the property of the resin, a process called curing. Both vacuum-bag and pressure-bag I Mould
moulding are more labour-in ten-
s-ivetRaIl hand lay-up or spray-up. Pressure-bag
Autoclave moulding: Additional heat and pressure may be applied with autoclave equipment. The equipment which is similar in principle to a domestic pressure cooker is general- ly expensive. But autoclaves do allow higher reinforcement levels resulting in faster cures and superior properties in the
40 HARDY COMPOSITES
Resin applicator
Pultrusion Filament winding
Laminat,,- Continuous strand roving
~~~
composites. Exotic, high-temperature resins can be used by this technique to make novel composites.
Reinforced reaction injection moulding ( RRIM): It invol- ves the mixing together of reactive resin components and injecting them into a mould while they are still liquid. Inside the mould they polymerize to their final shape.
Polyurethanes, high temperature polymers, polyesters, nylon and epoxy are all available as commercial RRIM sys- tems materials.
Filament winding: Providing the highest strength-to- weight ratio, filament winding consists of feeding reinforce-
ment filament or roving through a resin batch and winding it on a mandrel A. mandrel is a core around which paper, fabric or resin im pregna ted glass is wound to form pipes, tubes or vessels. Special winding machinery lays down the im- pregnated roving in predeter- mied patterns, giving maximum strength where required. After the appropriate layers are applied, the wound mandrel is cured and the molded part removed from the mandrel. Filament winding provides the greatest control over orientation and uniformity.
Pultrusion: This is a continuous method for moulding parts with a constant cross-section. It is perfect for moulding pipes, beams and fishing rods.
Pultrusion involves passing con- tinuous roving through a resin bath and then drawing the resin- im pregna ted reinforcemen t material through a steel die. The die controls the shape and is usual1y heated to initiate cure.
But the final cure is carried out in
GIVING SHAPE 41
Injection moulding
an oven or heating chamber through which the stock is drawn. The process yields continuous lengths of material with high unidirectional strength as well as high reinforce- ment to resin ratio. Pultrusion and filament winding are two of the fastest growing composite fabrication methods.
Injection moulding: It is the fastest growing method of fabricating composites. Injection moulding can produce com- ple}t, highly detaIled parts ranging in size from small precision components to automobile bumpers. In this process, pellets, compound
concentrates or resin-reinfor- cement blends are heated until they are fluid. They are then injected under high pres- sure into a cold, closed mould.
Thermoplastic matrices are perfect for injection mould- inK- Injection moulding of thermosets is also undergoing
rapid growth, for which the process usually involves injection of cool reactants into a mould that is heated to carry out curing.
Resin transfer moulding: In this process, fibre mats are placed in the desired orientation in a mould which is closed and filled with low viscosity resin. The mould is then heated to cure reactive resins. But retaining fibre orientation during resin transfer moulding is sometimes difficult and placement of fibre mats is often labour intensive.
Cold stamping: It is a high speed process, which involves preheating reinforced ther- moplastic sheet blanks and stamp- ing them in to the desired configuration on metal stamping process. The ready integration into current production lines and
'Reinforced thennoplastics sheet ~
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R
Heat source
Cold stamping
42
I l
The puppy keeps its cool
HARDY COMPOSITES
the high mechanical properties available with a rapidly grow- ing family of engineering resin matrices and continuous rein- forcements promise to make cold stamping a key process in the industrial composites area.
The thermoplastic advanced composites, promise sig- nificant advantages over thermoset systems. Their damage tolerance is 10 times as high. They have improved microcrack
GIVING SHAPE 43
resistance and negligible moisture absorption. They also have superior flame and radiation resistance. They require no refrigeration and do not undergo change in properties even after extended storage. Parts may be reheated and reformed which means that the scrap can be recycled. Furthermore, no toxic emission is produced during processing. Despite these advantages, the use of thermoplastic reinforced composites is still small and thermoset matrices command around 90 per cent of the advanced composite market. Part of this dominance is due to the early development of thermosets that could be used with the reinforcements, be fabricated into light weight complex parts and tolerate the heat as well as mechanical and fatigue loads placed upon them. Neverthe- less, a wide and growing family of new materials and their novel combinations promise more thermoplastic advanced composites in the future.
However, just making a composite is not the end of the matter for the manufacturers. The newly formed composite has to be tested for its inherent qualities. Also strict quality control measures have to be taken to ensure that there is uniformity in the quali ties of the composites produced in each batch. There are many parameters which must be assesed before a composite is released to the market. There are stand- ard methods for testing most of the properties. But certain inherent defects in composities cannot be directly assessed by these methods. Defects such as wrinkles, presence of mois- ture, unintended inclusions, damaged reinforcements, matrix cracking and reinforcement misalignment cannot be assessed by conventional methods. Specialized testing procedures have to be employed to assess the quality of the composite when these parameters have to be judged.