Combined Effects of Print Orientation and Post-Curing Duration on the Mechanical and Surface Properties of a 3D-Printable Resin Intended for Lingual Retainers: An In Vitro Study

This in vitro study evaluated the effects of printing orientation and post-curing duration on the flexural strength, Vickers microhardness, and surface roughness of standardized specimens fabricated from a nanoceramic hybrid resin intended for fixed lingual retainer applications. A total of 144 specimens were fabricated using a SprintRay Pro S DLP printer and SprintRay OnX Tough 2 resin and allocated to nine experimental groups according to printing orientation (0°, 45°, and 90°) and post-curing duration (1, 10, and 20 min). Of the 16 specimens fabricated per group, eight were allocated to flexural strength testing, while the remaining eight were used for both Vickers microhardness and surface roughness measurements. Flexural strength was assessed via three-point bending, microhardness via the Vickers test, and surface roughness via profilometry. Normally distributed outcomes were analyzed using a two-factor factorial model, whereas non-normally distributed outcomes were analyzed using robust ANOVA, with statistical significance set at p < 0.05. Both printing orientation and post-curing duration significantly affected flexural strength (p < 0.001 and p = 0.026, respectively), with a significant interaction (p < 0.001); the highest mean flexural strength was recorded at 45° with 20 min of post-curing (144.24 MPa), closely followed by 90° with 10 min (144.12 MPa). Microhardness was significantly affected by both factors (p < 0.001), with no statistically significant interaction (p = 0.245); the highest mean microhardness was observed at 0° with 20 min of post-curing. Surface roughness was significantly influenced by printing orientation only (p = 0.030), being highest at 90°. The effects of printing orientation and post-curing duration varied across the investigated properties, with a significant interaction observed for flexural strength. These findings provide material-level evidence regarding the influence of manufacturing parameters on OnX Tough 2 resin but do not establish optimal processing conditions for the clinical performance of fixed lingual retainers.