Evaluation of mechanical tests conducted before and after thermoforming of aligners

Abstract

This study aimed to devise a method for determining the thickness and studying the mechanical properties of thermoplastic materials used in making clear aligners before and after thermoforming. 1） Test specimens were obtained by cutting the top and lateral surfaces of a sheet in the form of a thermoformed rectangular parallelepiped. The determination of thickness and evaluation of material properties were conducted before and after thermoforming. 2） We devised three types of holding jigs and conducted compression tests on the aligners’ shape. The apparent elastic modulus of the aligner was compared and examined through the stress–strain diagram. 1） Deformation and variation in thickness were observed following thermoforming; however, the elastic modulus remained constant. No significant differences were observed in the elastic moduli of materials with different thicknesses. 2） The results of mechanical tests on the aligners indicated that the apparent elastic modulus increased with the thickness of the material. There were no variations in the thickness and physical properties of the aligner materials before and after thermoforming; however, as dentition morphology is complex, we assumed that a greater sheet thickness obtained by thermoforming would increase the elastic modulus of the aligner. Aligner holding jigs were developed and used to study the mechanical properties of the aligners. These properties were affected by the shape of the jig’s holding and joint sites as well as the thickness of the thermoplastic material.