Shape Control for Thermoplastic Composites Made by Automated Fiber Placement Using Annealing

Abstract

The goal of this research is to undo the visible warping and relieve the residual stresses of thermoplastic composite parts manufactured using automated fiber placement (AFP) technology by designing an annealing cycle. AFP is an automated process that allows for the manufacturing of flat and round parts from various materials, including thermoplastic composites. The samples were made from carbon fiber reinforced polyether ether ketone (PEEK) tape and were manufactured on a 6- axis AFP system with a hot gas torch. The residual stresses arise due to the large temperature gradients in manufacturing and the subsequent uneven cooling. The warping is problematic because finished parts vary significantly from their intended geometry. The curvature of samples with 4, 12, and 15 layers were measured using a laser measuring system that is comprised of a platen that allows users to pinpoint x-y coordinates and a laser to provide the elevation. MATLAB was used to compute the second order polynomial that best represents this surface. The curvature at any point on this surface can be calculated using partial derivatives. Annealing in an oven and using a heat blanket were explored as out-of-autoclave options. A reduction in the mean curvature of 86%-90% was observed in unidirectional samples with an annealing cycle comprised of a three-hour hold at 200°C and vacuum pressure of 15 torr. The sample with a symmetric unbalanced layup sequence only had a 17%-19% improvement to its mean curvature.