Effect of Extruder Type, Shore Hardness, and Infill Ratio on the Mechanical Properties of FDM Manufactured TPU Parts

This study investigates the effects of Shore A hardness, extruder type, and infill ratio on ultimate tensile strength (UTS), elastic modulus (E), and elongation at break (%) in FDM-printed thermoplastic polyurethane (TPU) specimens using a multifactor experimental design, ANOVA, and regression models. Unlike previous studies that often focus on printing parameters or material hardness individually, this work presents a holistic evaluation of how Direct Drive and Bowden Tube extruder systems modify the tensile response across different hardness levels and structural densities. ASTM D638 Type-IV tensile tests indicate that Shore hardness is the most dominant factor governing mechanical performance. Considering extruder configuration, the Direct Drive system outperforms the Bowden setup because its shorter filament path and more stable material flow improve interlayer fusion. Numerically, the strongest mechanical resistance was achieved using the Direct Drive system with 95A hardness and 100% infill, yielding the highest UTS (41.2 MPa) and elastic modulus (43.8 MPa). Conversely, the weakest results occurred with the Bowden Tube system at 70A hardness and 50% infill, resulting in the lowest UTS (22.4 MPa) and elastic modulus (18.7 MPa). Maximum ductility was observed at 70A hardness and 100% infill with the Direct Drive system, reaching 703% elongation at break, while the lowest elongation (285%) was recorded using the Bowden Tube at 95A and 50% infill. Overall, the findings recommend 70A + high infill + Direct Drive for applications requiring flexibility, and 95A + 100% infill for applications demanding high strength.