Physical form of the reinforcement

The advantages of fabrics are in the handling of the material when fabricating a composite so that fibre alignment is maintained and fibres are draped to follow the shape of a mould or tool, improved off-axis properties (essentially isotropic for a mat) and possibly better throughthickness properties. 

Another type of product is a preform in which the fibres, whether in the form of a mat, fabric or some other construction, are initially formed to a, possibly, complex shape and stabilized with a binder. The preform is placed in a mould and a thermosetting resin injected, sometimes with vacuum assistance, to produce a component (section 11.3). The advantage of this approach is the ability to maintain the fibres in position during a complex moulding operation, increased product quality and improved productivity. 

An alternative approach to building up components in which the fibres are aligned in various directions, which is often favoured with carbon fibres, is to produce a pre-impregnated material, known as a prepreg. Unidirectional fibres are spread out thinly and impregnated with a thermosetting resin which is partially cured or B-staged. The sheet or roll material (which is a fraction of a millimetre in thickness) 

The fibre volume loading, Vf, is usually between 55 and 65v/o for an aligned fibre composite or laminate. The value may be lower for moulding compounds and GRP and in these cases it is often quoted as a weight percentage. 

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Attempts to reduce or eliminate the tendency to bum can involve careful choice of resin (see later) and of the nature and physical form of the reinforcement, but the main emphasis is usually on adding special additives to the resin composition [57]. 

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