Investigation of the effects of varying the processing parameters in diaphragm forming of advanced thermoplastic composite laminates

Diaphragm forming offers several advantages over other forming techniques in the manufacture of advanced thermoplastic composites. The technique can be used in the forming of parts with complex curvature and can produce excellent surface finish. This work investigates the effect of buckling in both single- and double-curvature moulds, while forming carbon fibre-reinforced poly(ether ether ketone) (APC-2). A control system was set up to provide linear displacement of parts at rates of 1–100 mm min⁻¹. Buckling was established for both cross-ply and quasi-isotropic lay-ups in a double-curvature elliptical dish mould. Forming rate experiments were also carried out on a single-curvature 90° mould, with no buckling occurring at forming rates up to 100 mm min⁻¹ The conditions likely to cause buckling were calculated for the 90° female mould, using both the tensile properties of the diaphragm material and interply shear data for APC-2 laminates. An investigation was also made into the spring-forward effect on 90° parts formed using male and female tools for both APC-2 and carbon fibre-reinforced poly(ether imide) materials. The parts made from the male tool using these materials had a larger spring-forward effect in each case. The influence of part thickness was investigated and found to reduce the spring forward which occurred. The effect of mould radius of curvature was also investigated and found to be negligible. The effect on part quality when varying the consolidation pressure was investigated for [0°/90°]_2S and [0°/±45°/90°]_S APC-2 lay-ups in the male 90° mould. The parts were ultrasonically C-scanned to assess their quality; interlaminar shear tests were also carried out to validate the ultrasonic tests. It was found that a consolidation pressure in excess of 200 kPa was required to fully consolidate these parts.