Void formation in resin transfer moulding
In recent years interest has grown in the use of composite components within the automotive industry. Fibre reinforced plastic (FRP) components are of particular interest to the industry, since lower tooling costs and part consolidation can be utilised, whilst lighter, stiffer components can be produced. Several methods are available to produce FRP components at high volumes, including compression moulding (using dough and sheet moulding compounds), reinforced reaction injection moulding (RRIM) and liquid moulding processes (resin transfer moulding (RTM) and structural reaction injection moulding (SRIM)). RTM is a closed mould process, which is widely used to produce components economically in low volumes using matched moulds to produce two good surfaces. The absence of a high volume manufacturing technology, however, has impeded the acceptance and advance of RTM within the automotive industry. A research programme was established at the University of Nottingham to address the problems associated with the use of RTM for high volume manufacture. This programme has considered the topics of process technology, processing characteristics of polyester resin systems and fibre preforms, fibre wet-out and interfacial bonding, mould design, microwave pre-heating of reactive resin systems and process modelling. This thesis concerns the research which was undertaken to identify the causes of void formation during the impregnation and polymerisation stages of RTM, and methods of reducing the final void content within the component. The impregnation phase of the RTM process was identified as being the stage where the majority of voids were formed. A study of oil impregnation (having a similar viscosity to that of resin) into reinforcement was undertaken to determine the reasons for uneven flow and air entrapment. The dry reinforcements were studied to assess the microstructure of the preforms in order to determine reasons for obstruction of the resin flow. Fabric stitching, thermoplastic binder and size deposits were identified as potential causes of flow impediment. Fibre orientation and preform stacking were also assumed to assist in the development of uneven flow, leading to air entrapment. A major factor determining the formation of microvoids within fibre bundles was identified as the transverse impregnation of resin into high Tex fibre bundles. The major moulding process variables of injection pressure, vent pressure, fibre volume fraction, mould temperature and resin pre-heating have been assessed, to determine their effect on the void content within unidirectional and CFRM reinforced polyester laminates. It was observed that vacuum assistance during impregnation reduced void formation, although higher exotherm pressures and the possibility of monomer boiling arise from its use. A simple impregnation model was developed to assess the microscopic impregnation rates between fibre bundles, in the capillary between fibres and transversely into fibre bundles. The results from this model were compared with actual moulding histories. The findings of the overall work are discussed and suggestions proposed for the reduction of void content in RTM automotive components.