On the interlaminar bonding adhesion of the PLA-TPU printed multimaterial

Abstract Multi-material 3D printing (MM3DP) combines materials like polylactic acid (PLA) and thermoplastic polyurethane (TPU), allowing parts to have both rigid and flexible sections. This capability is particularly beneficial for applications that require strength and flexibility, such as flexible joints. However, strong adhesion between these materials is difficult due to their different thermal and mechanical properties, often resulting in weak interfaces. This study investigates the effects of layer height (LH), line width (LW), and temperature gradients on PLA-TPU adhesion in fused filament fabrication (FFF). Lap shear tests and failure mode analyses indicate that lower LH and larger LW improve adhesion due to changes in filament contact and internal structure. Additionally, applying a temperature gradient at the PLA-TPU interface enhances bonding consistency, especially at higher LH. Optimal adhesion and lowest variability are observed at lower LH. In some cases, the TPU layer deformed before properly stressing the PLA-TPU bond, due to its low stiffness and high thickness. Moreover, the combination of sample geometry and material properties induced rotational moments during shear tests, which make it difficult to properly assess the interfacial adhesion. These findings provide practical guidelines for improving interfacial adhesion in multi-material prints and highlight the need for further methodological improvements.