The application of microwave preheating in resin transfer moulding
Fibre reinforced plastics (FRP) are of considerable interest to the automotive industry. Intelligent application of these materials could reduce vehicle weight for higher operating efficiency, at a reduced manufacturing cost. The principal use of FRP in high volume (greater than 100,000 parts per annum) has been restricted to non-structural body panels made from short fibre reinforcement. Long fibre reinforced composites are ideal for load bearing structures resulting from a high specific strength. However, a high volume technique to produce these components at a moderate cost has not been realised. One long fibre reinforcement process with the potential to meet the high volume demands of the automotive industry is resin transfer moulding (RTM). Prolonged cycle times are an obstacle to high volume RTM. Cycle time is dictated by thermal quench near the injection gate from cold resin entering the hot mould. Heat recovery by the mould, and coincident heating of the resin to initiate cure is necessary to complete the cycle. Microwave preheating of the resin before injection reduces thermal quench. Since microwave heating is volumetric, low conductivity resins can be heated uniformly and efficiently. In-line resin preheating has been developed for its compatibility with high volume RTM. The use of an in-line microwave resin preheating system to reduce cycle time was investigated. This system was incorporated into the automatic RTM cycle. The resin temperature could be held constant or profiled during injection using a proportional-integral-derivative (PID) power controller. Both injection techniques reduced cycle time, although temperature profiling enabled coincident resin cure across the mould for a maximum cycle time reduction. Resin preheating had no adverse affects on the RTM process or laminates. This suggested that the microwave resin preheating system could be retrofitted within an existing RTM facility to reduce the component cycle time without damaging the mould or degrading component quality.