Adjustable forming of thermoplastic composites for orthopaedic applications

The present study was focused on the development of a special thermoforming technique for manufacturing of continuous fibre reinforced thermoplastic composite parts with complex surface contours. In particular, a stamp forming process was modified to investigate the potential manufacturing advantages of thermoplastic composites in orthopaedic applications. An apparatus was designed which allowed the thermoforming procedure to be fully automatic, i.e. a cold pre-consolidated laminate panel, as the feed material, was heated up in an infrared heating zone and then transferred into a cold mould system, where it was stamp formed. Both halves of the mould were made of many tiny round metal sticks in a metal frame. This needle-bed mould allowed one to copy any contour by pushing it slightly on spring fixed sticks. The desired position of these sticks could then be adjusted by forcing the side plates of the metal frame together. To prevent any press mark of stick-tops on the composite, i.e. to achieve smooth surfaces of the themoformed composite parts, flexible rubber pads were needed to cover the mould surfaces. Experimental results showed that the surface profile of CF/PP and GF/PP composites formed by the needle-bed mould reproduced fairly well the contour of a saddle shaped, complex model sample. Unique properties of this needle-bed mould are that it can be repeatedly used, and that it can copy any complex surface contours, for example a bone surface, by simply adjusting the stick positions according to the special surface requirements.