Fabrics and fibres are pre-impregnated by the materials manufacturer, under heat and pressure or with solvent, with a pre-catalysed resin. The catalyst is largely latent at ambient temperatures giving the materials several weeks, or sometimes months, of useful life when defrosted. However to prolong storage life the materials are stored frozen. The resin is usually a near-solid at ambient temperatures, and so the pre-impregnated materials (prepregs) have a light sticky feel to them, such as that of adhesive tape. Unidirectional materials take fibre direct from a creel, and are held together by the resin alone. The prepregs are laid up by hand or machine onto a mould surface, vacuum bagged and then heated to typically 120-180°C. This allows the resin to initially reflow and eventually to cure. Additional pressure for the moulding is usually provided by an autoclave (effectively a pressurised oven) which can apply up to 5 atmospheres to the laminate.
Resins: Generally epoxy, polyester, phenolic and high temperature resins such as polyimides, cyanate esters and bismaleimides.
Fibres: Any. Used either direct from a creel or as any type of fabric.
Cores: Any, although special types of foam need to be used due to the elevated temperatures involved in the process.
i) Resin/catalyst levels and the resin content in the fibre are accurately set by the materials manufacturer. High fibre contents can be safely achieved.
ii) The materials have excellent health and safety characteristics and are clean to work with.
iii) Fibre cost is minimised in unidirectional tapes since there is no secondary process to convert fibre into fabric prior to use.
iv) Resin chemistry can be optimised for mechanical and thermal performance, with the high viscosity resins being impregnable due to the manufacturing process.
v) The extended working times (of up to several months at room temperatures) means that structurally optimised, complex lay-ups can be readily achieved.
vi) Potential for automation and labour saving.
i) Materials cost is higher for preimpregnated fabrics.
ii) Autoclaves are usually required to cure the component. These are expensive, slow to operate and limited in size.
iii) Tooling needs to be able to withstand the process temperatures involved
iv) Core materials need to be able to withstand the process temperatures and pressures.
Aircraft structural components (e.g. wings and tail sections), F1 racing cars, sporting goods such as tennis racquets and skis.
Published courtesy of David Cripps, Gurit