Evaluating the Effect of Infill Density of 3D-Printed PLA-TPU Structures Using Load Time Analysis

Abstract

The combination of rigid Polylactic Acid (PLA) and elastic Thermoplastic Polyurethane (TPU) in a 3D-printed component may overcome individual polymer mechanical limitations. This multi-material concept is supported by the flexibility of the fused deposition modelling (FDM) technique of 3D printing. The present study evaluated the influence of core infill density (25%, 50%, and 75%) on the characteristic load-time profiles of FDM-printed PLA-TPU structures under uniaxial tensile loading. Results indicate that higher infill directly increases mean maximum load capacity from 15.2 N at 25% infill, to 30.1 N at 50%, peaking at 52.8 N at 75%. Concurrently, higher infill successfully delayed initial structural fracture from 1 to 1.2 and 2.6 seconds, respectively. The profiles revealed a multi-stage failure characterized by stiffness mismatch. The rigid PLA layers initially carry the load, indicated by early stress whitening. Subsequently, the load redistributes to the elastic core, stabilized by geometric engagement and possibly strain-induced crystallization. However, excessive PLA intensifies the interface stiffness gradient, causing severe delamination. Practically, analyzing these load-time curves provides references for identifying failure timelines and maximizing mechanical response in multi-material 3D-printed parts.