Simultaneous enhancement of interlayer/inter-line bonding in 3D-printed continuous fiber thermoset composites through core-shell-structured filament printing

Continuous fiber 3D printing enables complex thermoset composite structure fabrication by monofilament deposition and layer-by-layer stacking. However, this process produces weak interfaces between layers and filaments, increasing materials' anisotropy. This study proposed an interlayer and inter-line simultaneous enhancing method through printing a core-shell structured filament. This filament was prepared using a shell consisting of polyetherketone-cardo (PEK[sbnd]C) particle-toughened matrix and a core containing dissolved PEK-C toughened matrix with continuous fibers. Microstructural observations demonstrated that the toughened particles were regionally distributed in the interlayer and inter-line, simultaneously strengthening dual interfaces. Experimental results show that the interlaminar and the inter-line shear strengths were increased by 21.44 % (i.e., 87.63 MPa) and 58.56 % (i.e., 65.61 MPa), respectively. The difference between the interlayer and inter-line shear strength was reduced by 28.46 %, indicating an improvement in anisotropy. This simultaneous improvement further enhanced the impact resistance of the 3D-printed composites, reducing the damaged area by 25.2 %. Fracture studies showed that the regionally distributed microparticles and the nanoscale thermoplastic phases formed by dissolved PEK-C form a multiscale toughening system, enhancing the interlayer and inter-line interfaces. This study's core-shell filament printing strategy and the regionalized enhancement method offer a pragmatic solution to the multi-interface problem of 3D-printed composites.