Influence of Printing Orientation and Ageing on Mechanical Properties of 3D-Printed Resins for Occlusal Splints

The aim of this study was to assess the effect of printing orientation and water ageing on the flexural strength and flexural modulus of 3D printed resins for occlusal splints. Bar-shaped specimens were designed with dimensions of 64 × 10 × 3.3 mm according to ISO 20795-2:2013. Specimens were 3D printed with the Form 3B printer (Formlabs), using Dental LT Clear Resin (CL) or Comfort Resin (CO) (Formlabs), and 3 different printing orientations: as per manufacturer's recommendation (40° N = 20), parallel (0° N = 20), or perpendicular to the build platform (90° N = 20). To simulate intraoral ageing, half of the specimens per material and printing orientation (N = 10) were stored in distilled water at 37 °C for 30 days prior to testing. Specimens were tested in a three-point bending apparatus using a universal testing machine equipped with a 50 N load cell moving at a crosshead speed of 5 mm/min. Flexural strength (MPa) and flexural modulus (GPa) data were collected and statistically processed with separate analyses for unaged and aged specimens (Two-Way or One-Way ANOVA; Tukey test; <i>p</i> < 0.05). As for unaged specimens, both resin materials exhibited the highest flexural strength and modulus in the 90° orientation and the lowest values in the 40° orientation group. After water aging, all groups showed reduced flexural strength and modulus, with CO displaying up to 52% loss in flexural strength and values falling below ISO thresholds. CO consistently exhibited significantly lower flexural strength and modulus than CL, irrespective of aging.