Additive Manufacturing of Bio-Based PA11 Composites with Recycled Short Carbon Fibers: Stiffness–Strength Characterization

Short carbon fiber-reinforced bio-based polyamide 11 (PA11) composites were developed in filament form for Additive Fusion Technology (AFT) 3D printing and benchmarked against injection-molded samples. Composites containing 15 and 25 weight percent (wt%) recycled carbon fibers (rCFs) were successfully extruded into 1.75 mm diameter filaments, whereas higher fiber contents (35 wt%) led to brittle filament failure. AFT printing with subsequent consolidation produced short fiber composites with highly aligned fibers, while injection molding generated more randomly oriented microstructures. Mechanical testing revealed that AFT-printed composites in the fiber direction achieved significantly higher stiffness and comparable tensile strength to injection-molded counterparts. At 25 wt% fiber content, AFT 0° specimens reached an axial tensile modulus of 14.5 GPa, about 32% higher than injection-molded samples (11.0 GPa), with similar axial tensile strength (~123 vs. 126 MPa). However, AFT specimens displayed pronounced anisotropy: transverse (90°) properties dropped to ~2.3 GPa for transverse modulus and ~46-50 MPa transverse tensile strength, near matrix-dominated levels. Impact testing showed orientation-dependent toughness, with AFT 90° samples at 15% fiber content achieving the highest impact energy (76 kJ·m^-2), while AFT 0° samples were ~30% lower than injection-molded equivalents. Dynamic mechanical analysis confirmed that AFT 0° composites maintained higher stiffness up to ~80 °C. Overall, these results demonstrate that aligned short fiber filaments enable high stiffness and strength performance comparable to injection molding, with the trade-off of anisotropy that must be carefully considered in design. This study is the first to demonstrate the feasibility of combining bio-based PA11 with recycled short carbon fibers in AFT printing, thereby extending additive manufacturing to sustainable and high-stiffness short fiber composites.