Effect of forming temperature on the properties of polymeric diaphragm formed thermoplastic composites

An experimental study has been conducted to evaluate the effects of forming temperature on both the physical and mechanical properties of components formed from continuous carbon-fiber reinforced poly(ether ether ketone) (PEEK) using the polymeric diaphragm forming process. Non-destructive evaluation, mechanical testing and microscopy were used to examine single-curvature 90° bends formed between temperatures of 415°C and 335°C. A non-standard compression flexure test was designed to evaluate strength and stiffness properties, and the results compared to theoretical predictions. The optimum forming temperature range for the 90° single-curvature bend was found to be between 360°C and 390°C. A reduction of 13 and 8% in load to failure in a flexure test was observed at forming temperatures of 335°C and 415°C respectively. Degradation of physical properties was observed by both C-scan and microscopic evaluation, which correlates with the reduction in mechanical properties. Residual stresses due to cooling are found to cause a part closing of 2·5 degrees from the forming tool angle, regardless of forming temperature. Interply slip during forming was investigated and results show that complete relative movement of adjacent plies, necessary to accommodate part definition, occurred. This indicates the absence of fiber deformation in this single-curvature component, within the entire processing range of 335°C to 415°C.