Thermoforming process for fabricating oral appliances: influence of heating and pressure application timing on formability.
OBJECTIVE
This study was designed to examine the influence of heating and pressure application timing for thermoplastic soft materials on formability during the thermoforming process.


METHODS
Ethylene vinyl acetate (EVA) and a high shock-absorbing material (Hybrar) were used. Five specimens (20 x 10 x 4 mm) were heated to temperatures of 60, 80, 100, 120, 140, 160, 180, and 200 degrees C and then placed under a 4 N static weight with an indentation tip. The forming capability index (FI) was evaluated by rating the shape, size, and surface texture changes of the indentation tip reproduction in specimens using specially developed scales. The suitable temperature range for forming (STF) was determined by FI. Heat-holding capability of the two materials was also evaluated by the temperature changes in the cooling process using a digital thermometer. Timing of air pressure application was examined with the time-dependent change in negative pressure among three types of forming machines.


RESULTS
STF of the EVA (80-120 degrees C) was lower than that for Hybrar (140-160 degrees C). The time required to reach the lower limit of the STF was statistically different between the two materials (EVA: 41 seconds, Hybrar: 13 seconds) (p < 0.05). The maximum negative pressure (MNP) of the three forming machines ranged from -12 to -60 cmHg and time to reach the temperature, 5-60 seconds.


CONCLUSIONS
The results suggest that heating conditions for each type of sheet material should be predetermined by the STF. Forming process should be performed with the high MNP before reaching the lower limit of the STF.
Osaka University School of Dentistry - Division of Interdisciplinary Dentistry, Osaka, Japan.
Junko
J
Yamada
Yoshinobu
Y
Maeda
D003080:Cold Temperature; D003764:Dental Materials / Q000737:chemistry*; D017267:Dental Prosthesis Design / Q000379:methods*; D004548:Elasticity; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D010969:Plastics / Q000737:chemistry*; D011145:Polyvinyls / Q000737:chemistry; D011312:Pressure; D013314:Stress, Mechanical; D013816:Thermodynamics; D013997:Time Factors; D014618:Vacuum
D003764:Dental Materials; D010969:Plastics; D011145:Polyvinyls; C016438:ethylenevinylacetate copolymer