Processing methods for cross-linking plastics

These methods involve the reactive processing of a prepolymer (or oligomer), or occasionally monomers, with a catalyst or a curing agent, as part of the shaping operation. 

Transfer moulding is a variant on compression moulding, in which there is a reservoir of catalysed molten resin, an aliquot of which is transferred to the mould cavity at an appropriate point in the cycle, in a fairly homogeneous state. 

In the hand lay-up process, the reinforcing mat is cut to shape and disposed in the mould manually prior to impregnation with catalysed resin, applied with brush and squeeze-roller. In the spray process, chopped glass fibres are incorporated in a spray of catalysed resin applied to the mould. Both techniques allow the manufacture of large continuous mouldings limited only by the size of the mould. 

A variant of low-pressure moulding is catalysed resin injection into matched moulds in which the reinforcement has been preplaced prior to mould closure vacuum applied before resin injection assists mould filling. All these techniques depend on the easy flow of the prepolymer and good wetting of the reinforcement, but preclude the addition of much cheap particulate filler. 

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