Effect of printing layer thickness on the mechanical properties of a direct-printed orthodontic aligner resin: An <i>in vitro</i> study

Objectives: The Objective of the study is to examine how print layer settings—50 and 100 µm affect the mechanical behavior of a clinically used direct-printed aligner resin. Material and Methods: This in vitro study investigated how printing layer thickness influences the mechanical properties of a directly printed aligner resin (Graphy Tera Harz TA-28). A total of 60 specimens were fabricated using a Uniz Nbee printer and divided into two groups based on layer thickness (50 µm and 100 µm; n = 30 per group). Standardized specimens for tensile, flexural, and compressive testing (n = 10 per test per group) were designed from STL templates and printed under identical conditions to ensure consistency. All samples underwent uniform post-processing, including centrifugation, ultrasonic cleaning, and nitrogen-assisted UV curing. Mechanical testing was carried out using a universal testing machine in accordance with ISO standards: ISO 527-2 for tensile testing, ISO 178 for flexural testing, and ISO 604 for compressive testing. The primary outcomes were strength and modulus values for each test. Data were analyzed using independent t-tests or Mann–Whitney U tests depending on data normality, with statistical significance set at α = 0.05. Results: Compressive strength was significantly higher for the 50 µm group (2.42 ± 0.61 MPa) compared with the 100 µm group (1.84 ± 0.37 MPa; p = 0.034). Conversely, the 100 µm specimens demonstrated a higher tensile modulus (114.12 ± 44.01 MPa) than the 50 µm specimens (72.14 ± 25.8 MPa; p = 0.01). Conclusion: Layer thickness influences the mechanical behavior of the Graphy TA-28 resin. Thinner layers improved compressive performance, while thicker layers increased tensile stiffness. These findings suggest that print settings can be tailored to achieve desired aligner strength and flexibility.