Composite body closures have many attractive attributes that appeal to automotive OEMs that produce premium vehicles. Composites, particularly long fibre-reinforced thermosets, offer the potential for reduced part masses and design freedom for stylists. They allow complex geometries that are otherwise unachievable by traditional technologies, and the marketing potential of a high technology material. There are a great number of factors that limit the widespread use of composite closures in large production volumes. One such factor is the associated high part cost that results from labour- intensive processing, expensive and fluctuating raw material costs combined with short- lived composite tooling and batch curing processes. The specific definitions of Class- A may vary between different vehicle manufacturers, however the final overriding criteria for supplier and OEM alike, is to cost- effectively produce a composite panel with the requisite surface finish.
Production process Upon entering the automotive body panel market, the Gurit ethos was to develop and engineer a material system that would address the costs associated with raw material, processing and finishing ( to Class- A). As a formulator of hot melt epoxy resin systems, Gurit ( UK) has a range of in- house facilities at its disposal including a full suite of analytical and mechanical test equipment to aid in the development of new formulations and material mechanical characterisation. The result of Gurit's development was SPRINTTM CBS, a development of the patented SPRINT philosophy, which combines two dry fibre plies either side of a resin film. The principal behind this Resin Film Infusion ( RFI) variant is that the dry fibre acts as an evacuation path during cure and allows for very low void content laminates ( equivalent to high pressure autoclave cured pre-preg laminates) by out- of- autoclave processing. This material format had been successful previously on structural automotive components such as the Mercedes- Benz SLR McLaren chassis. The first aspect of the material to be addressed was the exterior surface of the part, and therefore, the first layer of material into the mould. Being the critical visual surface, the material was required to have a range of functional features. The solution ( Figure 2) is a SPRINT surface film of three discreet layers. The first is a fine glass scrim, which acts as both a carrier film and a print barrier. The central layer is a highly filled epoxy resin film that gives the resin rich surface the sanding qualities of a high build primer system. The outer scrim contacts the mould and the dry fibres in this SPRINT material evacuate the air trapped between the laminate and the mould surface. The result is a hole- free component A- surface, which saves process time and rework during painting. Optimised tack levels improve handling properties and help to prevent bridging and associated scrap - likely with conventional pre- preg systems and areas of complex geometry. The surface film has an added benefit of maintaining a class- A surface quality at joints and overlaps. This level of quality allows ply templates to be cut into the most convenient forms for ease of lay up, irrespective of position on the component.