The effect of thermoforming foils’ material and thickness on the physical properties of orthodontic clear aligners: an in vitro study

Selection of an appropriate thermoforming foils is a crucial step for fabrication of in-office aligners. The aim of this study was to evaluate the effect of different thermoforming material and thickness on the color and weight stability and elastic modulus of clear aligners. Thermoforming foils of five types and two thickness (0.7 and 1.0 mm) were selected. For each group, 12 specimens were prepared to be a standard clear aligner. Weight change, color stability and elastic modulus was measured before and after thermocycling. Data were analyzed using Kruskal-Wallis and Mann-Whitney tests. PETG aligners tended toward weight reduction, while TPU and multi-layer aligners tended toward weight gain. PETG (1.0 mm) specimens weight reduction was significant ( P = 0.041). TPU (0.7 mm) specimens weight gain was significant ( P = 0.002). PETG aligners had the highest color change (ΔE), while multi-layer aligners showed the least. The thicker foils (1.0 mm) had better color consistency than thinner foils (0.7 mm). PETG (1.0 mm) aligners had the highest elastic modulus while multi-layer aligners had the lowest. Thermocycling caused elastic modulus reduction in PETG and increase in TPU and multi-layer aligners. It was concluded that both the type of thermoforming material and its thickness affect weight change, color stability, and elastic modulus of clear aligners.