Reinforced Composite materials moulding methods
The use of fibre materials to reinforce plastics is widespread. Injection moulded parts such as water tanks, helmets, trays, ducts etc., In such application the reinforcing material is normally in the form of short rovings (Definition: rovings are long and narrow fibre), mixed with and injected with the carrier material. A considerable industry has also built up using woven materials as well as rovings, mainly with epoxy and polyester resins (bending agents), to provide large reinforced plastic parts, or to provide strength to vacuum formed parts.
Techniques for making such large parts involve the use of liquid resin systems in combination with fibrous reinforcement. A male or female single surface mould is used, giving good surface finish on one side only. The moulds are inexpensive, such materials as wood or plaster of paris or reinforced plastics being often used. The technique is to apply a gel-coat to the surface to ensure good surface finish, then to follow this up with layers of reinforcement and further resin, ensuring that the resin thoroughly impregnates the material.
Hand lay-up is the simplest and oldest process for moulding of reinforced plastics. It is a popular method for large and complex items manufacturing. It is a low volume production method. Resin and mat are placed on the mould by hand, with brushes and squeegees used to distribute the resin and eliminate air bubbles. Since parts are normally large, and the process is relatively slow, slow room temperature curing resins are normally used, with external heat, usually radiant, applied to speed up the curing as appropriate and when required.
The process is unpleasant, and many of the materials used give rise to allergic reactions. There is some concern for the health of the workers involved.
The spray-up process is essentially similar. Continuous glass fibre roving is fed through a fibre chopper and spray gun. Roving and fibre are sprayed together onto the mould surface building up the part wall thickness as desired. Since chopped roving is being used, as against woven mat or preform, the strength of the finished article is not equal to that of a good hand-laid product.
To achieve denser, higher quality products, hand or sprayed parts may, during the curing process, be subject to compacting forces by bag-moulding, i.e. by placing a flexible sheet over the lay-up and using vacuum or pressure to apply uniform force over the surface of the moulding. This may take place in an autoclave, providing heat as well as pressure.
Another technique used is filament winding. In this a continuous roving or tape is passed through a bath of resin then wound on a mandrel. When cured the resultant product is very strong, due to the high reinforcement content and its excellent disposition. The process is, of course, limited to axis-symmetric products.
Other than these methods still lot of methods are followed in reinforced fibre industries. Some of the methods currently in use are:
1. Matched die moulding
2. Compression moulding
3. Low pressure low temperature compression moulding
4. Transfer compression moulding
5. Resin transfer moulding
6. Reaction injection moulding
7. Injection moulding
8. Vacuum bag moulding
9. Vacuum infusion moulding
10. Autoclave moulding
12. Continuous laminating
Similar techniques are also utilised using preforms of reinforcing material, coating these preforms by immersion or applying resin by spray (or both) before and during positioning on the mould, to give optimum distribution of reinforcement.