Resin-dependent mechanical anisotropy in laser vat photopolymerization correlates to the initial rate of polymerization and critical energy

• It is shown for the first time that mechanical anisotropy in objects printed with laser VAT photopolymerization is resin-dependent by comprehensively analyzing a set of resins covering 4 orders of magnitude in elastic modulus. • The link between the degree of anisotropy (DA) and the polymerization kinetics and Jacobs working curve parameters is introduced. • The degree of mechanical anisotropy is higher for resins a higher initial rate of photopolymerization and a lower critical energy. • A linear correlation between the initial rate of polymerization and the mechanical anisotropy was found. • A logarithmic correlation was found between the critical energy and the DA based on the elastic modulus. The degree of mechanical anisotropy in objects printed with laser vat photopolymerization (VPP) remains controversial. It has been stated that objects with a higher degree of mechanical isotropy are produced with VPP as compared to other polymer-based additive manufacturing techniques, such as fused filament fabrication (FFF). However, reports on the evaluation of resin-dependency of the mechanical anisotropy obtained with VPP are scarce. Furthermore, the degree of anisotropy (DA) was quantified using different procedures. Here, six commercial resins were selected to evaluate how the DA correlates to the initial rate of polymerization (R P0 ), critical energy (E C ), and penetration depth (D P ) for materials with a broader range of properties. State-of-the-art procedures to calculate the degree of mechanical anisotropy are discussed, and an ideal method is proposed, namely, the ratio of the standard deviations related to the inter- and intra-layer forces: DA=(sd inter /sd intra ). The elastic modulus ( E ) was confirmed isotropic with the three resins that were previously reported. However, objects printed with the additional resins that polymerize at higher initial rates (R P0 =72.1 mM/s) and with lower critical energies (E C = 0.36 mJ/cm 2 ) appear more anisotropic. A linear trend was obtained for the scaling of the mechanical DA with R P0 . Moreover, a logarithmic correlation between E C and the DA in E was found, which appears inappropriate for E C as a function of the DA in the maximum stress (σ Max ). This study aims to spur research on the mechanisms underlying the dependence of the mechanical DA on the resin-curing behavior for objects fabricated by VPP.