A process for prestaging thermosetting towpreg

A method for prestaging thermosetting towpreg consisting of passing the material through a tunnel oven was developed to address the difficulties inherent in making thick‐walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay‐up; and residual stresses, warping, thermal degradation, resin gradients, and excessive void nucleation during the final cure. AS‐4 carbon fiber in a 3501‐6 epoxy matrix was used to test the continuous prestaging process. With prestaging, significant reductions in towpreg bulk, voids, and total enthalpy were achieved. Cure shrinkage also was achieved. The final towpreg product had good drape and greatly reduced tack. These changes addressed many of the aforementioned difficulties. A relation between residence time at 200°C (the process temperature) versus percent degree of cure (%DOC) was developed. Manufacturing characteristics such as drape, tack, and bulk were related to %DOC. A processing window of 15 to 22 %DOC yielded towpreg with optimal characteristics. Prestaged towpreg would be well suited to manufacturing with advanced fiber placement (AFP) type equipment. Initial trials in industry have been promising.