Quantifying interfacial diffusion in photopolymer-based vat additive manufacturing

The inter-layer strength of 3D-printed photopolymers is a crucial parameter for the mechanical performance of parts manufactured via vat photopolymerization (VPP). In VPP, uncured or partially cured resin remains present during layer formation, making diffusion processes a decisive factor in the development of interfacial strength and the resulting mechanical properties. Before light exposure, the resin can diffuse into the partially cured preceding layer and, depending on the resin level in the vat and the immersion depth of the component, it may also penetrate the component's exterior surfaces. In a recently introduced multi-material approach, a methacrylate formulation was combined with an allyl–thiol formulation. Reliable layer adhesion in such systems depends on chemical compatibility between the monomers and physical bonding mechanisms such as network interpenetration and chain entanglement, both of which are influenced by diffusion across the interface. This study investigates the dependence of diffusion width on immersion time and temperature. The sulfur content in one formulation enables quantification of the diffusion zone via EDX, complemented by nanoindentation. Results show that diffusion width is controlled by printing parameters and directly influences tensile properties. These findings provide valuable structure–property relationships for optimizing VPP processing conditions.