Effect of processing parameters on interfacial adhesion in co-extruded continuous carbon fiber–PLA composites

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Co-extrusion-based continuous fiber 3D printing offers significant potential for producing high-performance composite parts; however, the interfacial bonding mechanisms between the fiber and matrix remain insufficiently understood. This study investigates the effects of the plastic flow multiplier, composite nozzle temperature, infill density, and embedded length on the interfacial adhesion of a single pre-impregnated carbon fiber tow co-extruded with PLA. Tow pull-out tests targeted the slippage failure regime, identified as critical for characterizing interfacial behavior. The peak load increased linearly with embedded length across all infill densities (R 2 ≥ 0.92), consistent with the scaling of the fiber–matrix bonding area. The apparent interfacial shear strength, which normalizes peak load by this area, also increased with infill density to a maximum of 23.55 ± 3.73 MPa at 100%. Within the tested parameter ranges, a plastic flow multiplier of 1.5 yielded the highest mean peak load with low variability, and a composite nozzle temperature of 230 °C was associated with the highest mean peak load and fewer observed fiber defects, although differences between levels were not statistically significant. Scanning electron microscopy and X-ray computed microtomography analyses of selected specimens provided microstructural evidence of the slippage mechanism and manufacturing-induced defects that may affect interfacial performance.