Exploring PLA/TPU blends in pellet-based printing for multifunctional applications: Blending and interfacial properties

Multi-material pellet 3D printing using fused granulated fabrication (FGF) is gaining attention for its ability to create multifunctional models. This is driven by the extensive range of commercial polymer pellets and additives available, enabling prints with tuneable mechanical properties and functionality beyond aesthetics. Despite its potential, limited research exists on interphase properties, such as the influence of polymer ratios on interphase strength and bonding. In this study, we used multi-material FGF to 3D print polymers with varying hardness and stiffness in a single print. By blending polylactic acid (PLA) and thermoplastic polyurethane (TPU), we explored in-situ polymer blending to achieve multifunctionality. Five PLA/TPU ratios were investigated, with optimized 3D printing parameters. The mechanical and thermal properties of the resultant blends were analyzed. Notably, toughness peaked at an infill density of 80 % and a blend composition of 75 % PLA/25 % TPU. Shear strength at the interface improved by ∼320 % with a gradual transition between PLA and TPU (5.76 MPa) compared to a discrete interface (1.79 MPa). PLA-dominant blends exhibited superior compressive strength due to higher rigidity. Adding carbon black to TPU (cTPU) enhanced its electrical properties, enabling heating functionality as confirmed by thermal imaging. We also demonstrated the stimuli-responsive effect of PLA/TPU blends, showing that the hardness of an insole could be controlled through Joule heating. To illustrate practical applications, we designed a multi-functional insole integrating the optimal PLA/TPU blend with a cTPU heating layer. These findings highlight the potential of FGF for creating multi-material objects with tailored properties, paving the way for advancements in multifunctional additive manufacturing.