Design of Experiment-based process parameter optimization for enhanced tensile performance in TPU for FFF

This study examines the influence of fused filament fabrication (FFF) process parameters, nozzle temperature, bed temperature, and infill pattern, on the tensile performance of two commercial thermoplastic polyurethane (TPU) filaments: Raise3D Premium TPU 95A and FiloAlfa FiloFlex TPU 80A. A two-stage experimental design was used: a two-level factorial design to assess main effects and two-way interactions, followed by response surface methodology (RSM) based on a face-centred central composite design to model and optimise the thermal parameters after fixing the infill pattern for each material. Tensile specimens (ASTM D412-16, Die C) were tested under uniaxial tension. As ultimate tensile strength and elongation at break were unreliable, due to non-rupture or failure outside the gauge section, the response was assessed using stress at fixed strain levels (M100 to M700). For TPU 95A, nozzle temperature dominated at low strain and infill pattern at high strain. The RSM models showed non-significant lack-of-fit but limited predictive capability; the optimal region varied with strain, giving a composite optimum of 210 °C nozzle and 40 °C bed temperature. For TPU 80A, nozzle temperature and infill pattern were dominant; rectilinear infill was chosen for its superior rupture resistance. The RSM models were statistically significant (R² = 81–86%), and bed temperature, mainly through its quadratic effect, emerged as the dominant parameter, revealing nonlinear behaviour missed during factorial screening. A composite optimum of about 228 °C nozzle and 60 °C bed temperature was identified. Overall, FFF processing conditions strongly influence the tensile behaviour of TPU, with optima specific to each material. Notably, bed temperature, often treated as fixed or secondary, can play a critical role in the tensile performance of flexible TPU.