Flow and heat transfer during compression resin transfer moulding of highly reactive epoxies

Abstract

Fast-cure epoxy systems are used for the mass production of composite parts with cycle times in the minute range. One of the major difficulties observed when processing fast-cure resins is their strong exothermic reaction during cure. This may result in a significant temperature overshoot and large temperature gradients over the thickness, and therefore gradients, in glass transition temperature, shrinkage and residual stress. Hence, the cure reaction during the injection, impregnation, compaction and curing stages need to be understood and optimised in order to reduce cycle time without sacrificing mould filling and part quality. A possibility to process such highly reactive epoxies is the compression resin transfer moulding process (CRTM), where the preform is impregnated in through-thickness direction, leading to reduced cycle times. The aim of this work is to identify processing constraints that the different CRTM variations present when using fast-curing resins. We have developed a multi-physics model to compare three variations of the CRTM process: gap injection, direct dosing and wet pressing, by studying their fluid flow and the exothermic reaction. The models show the importance of the injection strategy to avoid temperature overshoot, which can occur before the part is fully impregnated. Our results show that when impregnation time is a limiting factor, wet pressing appears to be a favourable approach for fast composite processing, doubling the available impregnation time before gel.