Investigation of the effects of infill process parameters on the tensile and flexural properties of fused filament fabricated thermoplastic polyurethane composite

Electrically conductive carbon particle-reinforced thermoplastic polyurethane (TPU) composites are increasingly utilized in wearable electronics, flexible circuits, soft robotics, and bioelectronic interfaces. This study investigates the effects of infill density (ID: 30%, 60%, 90%) and infill pattern (IP: gyroid, cubic, cubic subdivision, octet, cross 3D, tri-hexagonal, grid) on the mechanical properties of conductive TPU samples fabricated via fused filament fabrication (FFF). A full factorial design of experiments was used, with samples examined for tensile and flexural properties, followed by microscopic examination and ANOVA. This study showed that ID and IP affect mechanical performance considerably (p < 0.05), and that the parameter of ID is predominant. Among all the combinations, the optimal combination of 90% ID with Gyroid infill pattern exhibited the best overall properties, such as elastic modulus = 74.76 MPa, ultimate tensile strength = 5.54 MPa, yield strength = 3.27 MPa, toughness = 8.412 MJ/m³, elongation at break = 228.2%, flexural modulus = 25.70 MPa, flexural stress at 10% strain = 1.779 MPa. This configuration (desirability = 0.9104) was confirmed through the method of multi-response optimization performed on a composite desirability function, which was verified with low error of measured experimental values. Microscopic examination revealed that a low ID level (30%) induced defects (voids >500 μm, poor adhesion, cracks), whereas a high ID level (90%) minimized the defects. The optimized conductive TPU composite is also a potential material for use in soft robotics or flexible electronics, where a comprehensive characterization of its electrical properties is recommended for future studies.