The challenge of stamp forming high-strength thermoplastic composites for transportation

Thermoplastic composites have already successfully been introduced as a light weight material for mass applications in cars in the form of injection or flow molded parts. Short cycle times, weldability, high corrosion resistance and energy absorption as well as the ability to produce very complex geometries and reduce assembling costs by integral constructions were important factors to compete against conventional materials. However, the usage of discontinuous fiber reinforcement limits material strength and stiffness. For thin walled, high strength composite parts, continuous fiber reinforcement by woven fabrics is seen as a future alternative to light alight sheet metal structures. For this investigation, low cost glass fabric reinforced thermoplastic sheet, produced by double belt press technique, was deep drawn with matched metal molds in cycle times of only 10-15 s/part. The main problem far complex shaped parts is undesired fabric wrinkling. To understand the mechanisms of wrinkling, shear experiments were performed using a special picture frame. Evidence was found that fabric wrinkling was strongly influenced by fabric pre-stresses (membrane tension). A new blank holding system was invented and installed on a large scale pilot plant, capable of applying membrane stresses during the rapid deep drawing process and preventing the sheets from wrinkling even at high drawing ratios.