Static and Dynamic Mechanical Properties of 3D-Printable Aligner Resins: An In Vitro Study with FTIR Chemical Characterization

Background: Directly printed aligners are advancing rapidly, but the mechanical behavior of the resins behind them is still only partly understood. This in vitro study compared the static flexural behavior, short-term stress relaxation, and FTIR profiles of five Class IIa-certified 3D-printable resins for direct orthodontic aligners. Methods: The five resins, TC-85, TA-28, DCA, Clear-A V2, and Ortho Flex, were printed as standardized rectangular bars and tested at 37 °C. Three-point bending to 1 mm deflection yielded the maximum flexural stress and the flexural modulus, while a 30 min hold at fixed deflection captured stress relaxation. FTIR added a qualitative chemical characterization. Results: Differences between resins were substantial. DCA led on every static measure, pairing the highest flexural stress and modulus with the highest final relaxation modulus and the best stiffness retention. Clear-A V2 was also statically stiff but retained force only intermediately, whereas TC-85 combined high stiffness with pronounced relaxation. Ortho Flex performed modestly under static loading yet held on to a moderate fraction of its stiffness, and TA-28 relaxed the most. Conclusions: Directly printed aligner resins are mechanically heterogeneous, and static bending alone did not predict short-term force stability. Relaxation metrics should therefore accompany static testing whenever a resin is selected for a specific clinical purpose.