Random & short fibre composites

Random reinforcements can be applied during hand lay-up as prefabricated mat or via the spray-up process as chopped strand mat. Similarly, a continuous strand mat can be laid down in a swirl pattern to give random directionality reinforcement. Both hand lay-up and spray-up methods produce plies with nominally equal properties in all directions (except through the thickness). This method of manufacture is a very economical way to build up thickness, especially with complex moulds. Mechanical properties such as strength and modulus are less than other reinforcement methods, but are significantly higher than unreinforced resin.

Short fibre polymer matrix composites present yet another option to the design engineer. Peak strength and peak stiffness are the main drivers behind creating continuous fibre reinforcements but these are not always the primary design requirement for new components. Manufacturing with short fibre composites is less expensive than manufacturing with continuous fibre composites and some components are more naturally suited to this form of reinforcement due to their geometry. Manufacturing processes for short fibre composites are better suited to high volume production of cheap complex shape components than are continuous fibre composite manufacturing processes. Injection, compression and transfer moulding have all been adapted to produce short fibre composites. Short fibre composites are well established for both thermosetting resins and thermoplastic matrix materials.

The optimum length of the fibres in a short fibre composite depends on the composite system being considered. Optimum reinforcement lengths can be determined based upon the composite system and the manufacturing route chosen but the manufacturing route often damages the fibres, resulting in a range of possible fibre lengths contained in the final composite part. Further the orientation of the fibres in some manufacturing processes can be difficult to control and hence maximisation of the benefit of the reinforcement is hard to achieve.

Few short fibre manufacturing techniques manage to align the fibres in a single direction. It is common to have in-plane random reinforcement or indeed three dimensional random reinforcement. Components made from chopped strand mat or sheet moulding compounds have properties that can be approximated as being isotropic in the plane of the sheet with a markedly different stiffness and strength in the direction normal to the plane of the sheet. Components made from bulk moulding compounds (BMC) for compression or injection moulding can contain fibre orientations and distributions that can change throughout the moulding.