Process–Structure–Property Relationships of CF/PPS Composites Formed by Hot Pressing: Influence of Tool Temperature and Cooling Rate

Carbon fibre-reinforced polyphenylene sulfide (CF/PPS) thermoplastic composites are increasingly adopted in aerospace applications owing to their high specific strength, chemical resistance, and reprocessability. However, the coupled influence of hot-press tool temperature (Tt) on microstructural evolution, interfacial properties, and mechanical performance remains poorly characterised, limiting the ability to define predictive process– structure–property relationships for this material system. This study systematically investigates the effect of tool temperature (150–195 °C) on the cooling rate, degree of crystallinity (DoC), void content (Vc), surface energy and tensile strength of CF/PPS 5-harness satin laminates formed by hot pressing. Increasing Tt from 150 °C to 195 °C reduced the laminate cooling rate from 21.09 °C/min to 14.64 °C/min, representing a 31% reduction. Composite performance responded non-monotonically, with an optimum condition identified at 180 °C. At this temperature, the lowest cooling rate that preceded the performance peak (15.93 °C/min) yielded the highest DoC (22.82%), lowest Vc (0.21%), highest surface energy (35.13 mJ/m²), and maximum tensile strength (794 ± 19.7 MPa). Exceeding this threshold at 195 °C degraded all four properties simultaneously, attributed to excessive molten dwell time promoting void reformation and reducing fibre-matrix interfacial continuity. The results demonstrate that Vc influences fibre– matrix surface energy through contact continuity, which in turn determines tensile performance through load transfer efficiency. This cascade relationship establishes a process–structure–property relationship that identifies 180 °C as the optimal Tt and provides a quantitative basis for process optimisation of hot-pressed CF/PPS composites.