Study on Photocuring Printing Technology of Polyurethane Acrylate-Based Transparent Braces

A polyurethane acrylate-based photosensitive resin system was developed for 405 nm DLP printing. The formulations were prepared by blending commercial urethane acrylate RT75A with TPO-L, BBOT, and one of four reactive diluents (ACMO, IBOMA, TPGDA, or CTFA). Double-bond conversion was determined by ATR-FTIR, whereas viscosity, UV–Vis transmittance, working-curve parameters (critical exposure and penetration depth), volumetric shrinkage, and mechanical properties were evaluated. The reactive diluent markedly affected both the liquid-state behavior of the resin formulations and the properties of the cured networks. RT75A-TP showed the highest double-bond conversion (97.9%) and volumetric shrinkage (4.65%). RT75A-CT exhibited the highest viscosity at 25 °C and the largest elongation at break (119 ± 9.5%). RT75A-IB exhibited the highest tensile strength (43.3 ± 3.1 MPa) and flexural strength (57.8 ± 1.1 MPa). All formulations maintained transmittance above 80% from 450 to 800 nm. A transparent aligner with a nominal thickness of 0.55 mm was directly printed from RT75A-IB, and its dimensional accuracy was evaluated by 3D scanning. Overall, the results show that reactive-diluent selection enables the coordinated tuning of DLP processability, double-bond conversion, optical performance, and mechanical behavior.