Sports mouthguards are generally fabricated by thermoforming thermoplastic elastomer sheets. Depending on the molding method, molding machines can be roughly classified as either suction molding machines, which use the negative pressure created by a vacuum generator as the molding force, and pressure molding machines, which use compressed air (positive pressure) from a compressor as the molding force [1] . Suction molding machines use the breathability of the working model to suction the sheet against the contact surface of the model. Suction molding machines can be categorized according to the vacuum generator that creates the negative pressure into vacuum blower machines, which use the suction pressure of a blower, and vacuum ejector machines, which create negative pressure with a vacuum ejector using the Venturi effect. Vacuum blower machines achieve a low pressure of −0.010 to −0.025 MPa but a high flow rate, whereas vacuum ejector machines have a high pressure of −0.075 to −0.095 MPa but a low flow rate. The suction pressure produced by the vacuum generator cannot generate a molding pressure higher than atmospheric pressure, and the molding pressure cannot be adjusted because the level of vacuum achieved is fixed. In contrast, pressure molding machines use compressed air from a compressor as the molding force to apply molding pressure from above to press the sheet against the model. Many pressure molding machines use compressed air at a pressure of around 0.4 MPa [1] .

In dental thermoforming, the differences in breathability and porosity of working model materials depend on the type of plaster [2] and the effect of surface treatment agents on the breathability of plaster models [3] . In addition, the thickness of the mouthguard is affected by the molding method [1] [4] [5] , the softening state of the sheet [6] - [8] , and the shape of the model [9] - [12] . However, it is not known to what extent differences in molding mechanisms, such as the molding method of the molding machine and molding pressure [13] , affect the molding operation or the molded product.

The purpose of this study was to clarify the effects of differences in molding mechanisms on the minimum forming temperature and molding time in dental thermoforming. The null hypothesis was that the lower molding temperature limit and molding time were not affected by the molding mechanism.

The lower molding temperature limit was about 90.0˚C for the vacuum blower machine, about 77.5˚C for the vacuum ejector machine, about 77.5˚C for the pressure molding machine at 0.090 MPa, and about 67.5˚C for the pressure molding machine at 0.45 MPa. The lower molding temperature limit was higher for lower absolute values of molding pressure.

The molding time was approximately 15.63 s for the vacuum blower machine, approximately 5.27 s for the vacuum ejector machine, approximately 1.78 s for the pressure molding machine at 0.090 MPa, and approximately 1.05 s for the pressure molding machine at 0.45 MPa. The molding time was shorter for pressure molding than for suction molding. Significant differences were observed between all conditions except between the pressure molding machine at 0.090 MPa and 0.45 MPa (P < 0.01) ( Figure 5 ).

The results of this study showed that the lower molding temperature limit and molding time depended on the forming mechanism. Therefore, the null hypothesis was rejected.

Mouthguard sheets are thermoplastic elastomers with rubber elasticity. In order for a sports mouthguard to exhibit these material properties, it is important to understand that rubber elasticity decreases when the moldable temperature is exceeded and to carry out the molding operation accordingly [1] . The ratio of the molding pressures of the vacuum blower, vacuum ejector, and pressure molding machines is 1:5:25, and thus it is assumed that there are large differences in molding capacity. However, in mouthguard thermoforming, optimal molding temperatures at different molding forces have not been determined. The moldable temperature of ethylene vinyl acetate sheet has been reported to be 80˚C - 120˚C [14] , but in the present study, the lower molding temperature limit of the pressure molding machine (0.45 MPa) was 67.5˚C, suggesting that molding at low temperatures may be possible at high molding pressures. In addition, because the lower molding temperature limit for the vacuum ejector machine (−0.090 MPa) and the pressure machine (0.090 MPa) was the same at 77.5˚C, it was inferred that the molding force depends on the absolute pressure value regardless of whether positive or negative molding pressure is used.

Because the molding time may be affected by the area of the suction and exhaust holes on the molding table, a cylindrical model was used in Experiment 2. The molding time was significantly longer for suction molding than for pressure molding. This may be due to the time required for each molding machine to exert the necessary molding pressure [14] . In a preliminary experiment, a time of 1.1 s was required to exert molding pressure with the suction molding machine and 0.1 s with the pressure molding machine. A significant difference in molding time was observed between the vacuum ejector machine and the pressure molding machine with the same absolute molding pressures, indicating that it was the difference in the molding mechanism that affected the molding time. The difference in molding time between the vacuum blower and vacuum ejector machines was due to the difference in molding pressure. In other words, a molding machine with a lower molding pressure may require longer molding time.

In order for sports mouthguards to be effective in preventing or reducing injuries, the thickness and material of the mouthguard are important [15] - [17] . If the sheet is heated for a long time, the mouthguard becomes thinner and its physical properties deteriorate [6] - [8] . In other words, mouthguard performance is affected by the thermoforming process. In order to fully utilize the material properties of a mouthguard, it is important to carry out molding operations that take into account differences in molding mechanisms. This study revealed that the formable sheet temperature and molding time differ depending on the type of molding machines. This is a matter that directly affects the thickness and material properties of the mouthguard, so it is necessary knowledge for those who perform dental thermoforming.

A comparison of the differences in lower molding temperature limit and molding time due to molding mechanisms in dental thermoforming revealed that the lower molding temperature limit depends on the molding pressure and that the molding time is longer for suction molding than for pressure molding. These results suggest that the lower molding temperature limit and molding time may affect the thickness and material properties of the mouthguard. In the future, we plan to investigate the effects of the lower molding temperature limit and molding time on the thickness of the mouthguard.

This study has two main limitations. The first is that only one type of ethylene vinyl acetate resin was investigated. The softening temperature may vary depending on the vinyl acetate content and type of resin, and further investigation is required. Second, the study was limited to specific molding equipment. If the molding pressure is known, there is a high possibility that the results of this study can be applied, but more detailed investigation will be required as it may be affected by the shape of the heater and molding table.