Effects of Printing Layer Thickness and Thermocycling on the Mechanical, Morphological, Optical, and Dimensional Properties of Directly Printed Clear Aligners.

Directly printed clear aligners may streamline orthodontic workflows but must withstand intraoral aging. This in vitro study evaluated the effects of layer thickness (50, 100, and 150 µm) and thermocycling on mechanical, morphological, optical, and dimensional properties of Graphy Tera Harz TA-28 material. Specimens underwent standardized centrifugation and nitrogen post-curing, followed by tensile and flexural testing, Vickers microhardness, normalized light transmittance, dimensional analysis, and field-emission scanning electron microscopy (FESEM). Thermocycling comprised 250 cycles between 5 and 55 °C. Layer thickness significantly affected strain at break, flexural force, bending strength, bending modulus, microhardness, and normalized light transmittance. The 50 µm group showed the highest strain at break and greater flexural performance than the 100 and 150 µm groups. Thermocycling reduced Young's modulus and flexural outcomes. Significant layer thickness × thermocycling interactions occurred for stress at peak, deflection at peak, microhardness, normalized light transmittance, trueness (p = 0.008), and precision (p = 0.034). FESEM showed visible layer boundaries and surface heterogeneity with thicker layers, particularly in thermocycled 150 µm specimens. Overall, 50 µm provided favorable mechanical and dimensional performance after thermocycling, but produced the greatest reduction in light transmittance. Selection requires balancing mechanical and dimensional performance against optical stability after aging.