Effect of Molding Temperature on Crystal Formation and Interfacial Strength of Continuous Fiber Reinforced Thermoplastic Composites

ABSTRACT Maximizing the performance of continuous fiber‐reinforced thermoplastics (c‐FRTP) requires improving interfacial properties, in addition to suppressing void formation and fiber breakage. Although molding conditions are known to alter interfacial properties, the resin's initial crystalline state remains largely unexplored. This study investigated the effect of molding temperature on interfacial properties using four types of fibrous intermediate materials composed of carbon or glass fibers and polyxylylene sebacamide resin with differing initial crystalline states. Unidirectional specimens were fabricated via hot compression molding. Interfacial strength was evaluated using three‐point bending tests performed perpendicular to the fiber direction, while crystalline states were analyzed via DSC and polarized microscopy. Results indicated that interfacial strength increased with molding temperature regardless of the material, showing an improvement of approximately 20 MPa at the highest temperature. Furthermore, the initial crystalline state caused strength variations of up to 10 MPa. Analyses revealed that higher temperatures reduced the melt memory effect, shifting the dominant mechanism from homogeneous to heterogeneous nucleation. We propose that the magnitude of fiber–resin interaction and residual stress differences caused by crystal nuclei position govern interfacial strength. Consequently, controlling molding temperature based on the intermediate material's initial crystalline state is essential for maximizing c‐FRTP performance.