How interlayer interfaces affect hygro-mechanical ageing in 3D-printed PLA

3D printing, or additive manufacturing, has emerged as a versatile technology enabling the efficient fabrication of complex, customized components. However, despite its numerous advantages, the long-term durability of printed parts, particularly under humid conditions, remains a key concern, especially for polymer-based materials like polylactic acid (PLA). Previous studies have demonstrated that water diffusion into PLA is influenced by environmental parameters such as temperature, humidity, or its molecular structure. Yet, classical diffusion models, which assume homogeneous materials, fail to capture the layered and anisotropic nature of 3D-printed structures. In this study, the role of interlayer interfaces in water diffusion and mechanical degradation in PLA components fabricated by fused deposition modeling (FDM) is investigated. A combination of theoretical modeling and experimental testing is used to assess the influence of interface density on moisture uptake and adhesion strength. The results demonstrate that water diffusion and mechanical degradation are both strongly influenced by the number and properties of interlayer interfaces. The diffusion model was validated by comparing its predictions with gravimetric measurements across four layer-height configurations, showing excellent agreement. This study provides new insights into the ageing mechanisms of 3D-printed PLA parts and suggests design strategies to improve their performance in humid environments.