Page 319 - Plastics Engineering

302 Processing of Plastics
43.8 Reaction Injection Moulding
Although there have been for many years a number of moulding methods
(such as hand lay-up of glass fibres in polyester and compression moulding
of thermosets or rubber) in which the plastic material is manufactured at the
same time as it is being shaped into the final article, it is only recently that this
concept has been applied in an injection moulding type process. In Reaction
Injection Moulding (RIM), liquid reactants are brought together just prior to
being injected into the mould. In-mould polymerisation then takes place which
forms the plastic at the same time as the moulding is being produced. In some
cases reinforcing fillers are incorporated in one of the reactants and this is
referred to as Reinforced Reaction Injection Moulding (RRIM)
The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend
themselves to the type of fast polymerisation reaction which is required in this
process - polyesters, epoxies, nylons and vinyl monomers. However, by far
the most commonly used material is polyurethane. The components A and B
are an isocyanate and a poly01 and these are kept circulating in their separate
systems until an injection shot is required. At this point the two reactants are
brought together in the mixing head and injected into the mould.

Fig. 4.47 Schematic view of reaction injection moulding

Since the reactants have a low viscosity, the injection pressures are relatively
low in the RIM process. Thus, comparing a conventional injection moulding
machine with a RIM machine having the same clamp force, the RIM machine
could produce a moulding with a much greater projected area (typically about
10 times greater). Therefore the RIM process is particularly suitable for large