Flexural Stiffness and Force Retention of Six 3D-Printed Thermoplastics for Orthodontic Appliances: An In Vitro Screening Study

Background/Objectives: Directly printed orthodontic appliances rely mostly on vat-photopolymerisation resins; pre-polymerised thermoplastics processed by fused deposition modelling (FDM) and selective laser sintering (SLS) are a less-explored, single-component route, but are poorly characterised. This study compared six thermoplastic material–process systems, supplied for biomedical or dental use, using a single intraoral-simulating protocol. Methods: Thin specimens (25 × 5 × 0.6 mm) of polyethylene terephthalate glycol (PETG), polycarbonate (PC), polyamide (PA), polyether-ether-ketone (PEEK) and a thermoplastic copolyester elastomer (TPC) printed by FDM, and of polyamide 12 (PA12) by SLS, underwent three-point bending and 10 min stress relaxation at 37 °C in artificial saliva (n = 5). Differences were assessed using the Kruskal–Wallis test with Dunn post hoc comparisons (Holm correction), and Fourier-transform infrared (FTIR) spectroscopy was used to compare virgin and printed materials for the five FDM polymers. Results: Apparent flexural moduli were 989 (PA12), 750 (PC), 687 (PEEK), 509 (PETG), 85 (PA) and 32 MPa (TPC), a very large material effect (ε2 = 0.95), with post hoc comparisons separating only the extremes of that range. Relaxation over 10 min ranged from 2.6% (PC) to 22.1% (PA12) among the five materials in which it could be quantified, with TPC falling below the load-cell resolution; expressed as force, PA12 lost the most (0.86 N) and yet still delivered 3.01 N, which was above the initial force of PEEK, PETG, PA and TPC, though not separable from that of PC, which retained 97.4% of its own. Virgin and printed spectra were concordant (Pearson r ≥ 0.986, against ≤ 0.80 between different materials). Conclusions: The PA12/SLS system was the stiffest, followed by PC and PEEK, but, as the only laser-sintered material, its polymer and process effects cannot be separated. PETG and PC combine useful stiffness with accessible FDM; PA and TPC are far more compliant. These specimen-level findings provide a screening basis for selecting material–process systems for printed orthodontic appliances, preceding device-level validation.