Analysis of the Pyrolysis Behavior of Injection-Molded CFRP Plates and the Properties of the Resulting C/C Composites

Production of short-fiber-reinforced carbon/carbon composites (SF-C/C composites) is typically based on the pyrolysis of short-fiber-reinforced plastics (CFRPs) produced by compression molding. The production of C/C composites is generally a highly time- and energy-intensive process, particularly for complex geometries. Injection molding can offer an attractive alternative due to its high level of automation, cost efficiency, and ability to produce complex geometries. However, in contrast to compression molding, the shorter fiber lengths resulting from the compounding process and the parameter/geometry-dependent cavity-filling behavior in injection molding lead to a complex three-dimensional fiber orientation distribution. This can profoundly affect both the pyrolysis behavior of the CFRPs and the properties of the resulting SF-C/C composites. In this study, CFRP plates (150 × 150 × 4 mm3) were injection-molded at varying injection rates and mold temperatures. The influence of these parameters on the properties of the resulting SF-C/C composites was systematically investigated. Characterization included shrinkage and warpage behavior, porosity, microstructure via light microscopy and X-ray computed tomography, and flexural properties. The results show that the homogeneity of fiber orientation within the component is critical for controlling warpage during pyrolysis. In particular, asymmetric flow-line formation leads to non-uniform shrinkage across the thickness and promotes warpage. Therefore, achieving a homogeneous and/or symmetric distribution of the fiber orientation is essential for producing warpage-free SF-C/C composites by injection molding.