title: Heating methods for reducing unevenness softening of mouthguard sheets in vacuum-pressure formation.
abstract: BACKGROUND
  Unevenness in softening of the plastic sheet leads to a decrease in the mouthguard thickness during thermoforming. In this study, we examined the heating methods for reducing unevenness when softening mouthguard sheets during vacuum-pressure formation.
  
  
  METHODS
  Ethylene vinyl acetate mouthguard sheets and olefin copolymer sheets (thickness: 4.0 mm) were used. The following three heating conditions were compared: condition A-the sheet was molded when it sagged 15 mm from the sheet frame (under normal condition); condition B-the heater was turned off when the sheet sagged by 10 mm from the frame, followed by the sheet molding when the sagging reached 15 mm below the frame; and condition C-the sheet was inverted after heating when the sheet sagged 10 mm and was molded when the sagging reached 15 mm below the frame. The sheet was heated and pressed over the model using a vacuum-pressure machine; then, 10 s of vacuum forming and 2 min of pressure molding were applied. The sheet temperatures were measured using a radiation thermometer. Thickness of the fabricated sheets was determined for the incisal and the molar portion using a measuring device. Thickness data for each condition were analyzed by one-way anova followed by Bonferroni's multiple comparison tests.
  
  
  RESULTS
  On both sheets, condition B was smallest for temperature difference between the heated and the non-heated surface, and thicknesses after molding were greatest at all measuring portions.
  
  
  CONCLUSIONS
  By comparing changes in sheet temperatures at molding and variation in thicknesses when applying the heating method using a vacuum-pressure molding machine, we found that reduced unevenness in sheet softening occurred when the heater was turned off when the sag distance of the sheet was 5 mm less than the conventional molding, and then, the sheet was pressed when the conventional sag distance was reached.
authors.affiliation: Department of Removable Prosthodontics, The Nippon Dental University School of Life Dentistry at Niigata, Niigata, Japan.|Department of Physiology, The Nippon Dental University School of Life Dentistry at Niigata, Niigata, Japan.
authors.forename: Mutsumi
authors.initials: M
authors.lastname: Takahashi
authors.affiliation: Department of Removable Prosthodontics, The Nippon Dental University School of Life Dentistry at Niigata, Niigata, Japan.
authors.forename: Kaoru
authors.initials: K
authors.lastname: Koide
authors.affiliation: Department of Physiology, The Nippon Dental University School of Life Dentistry at Niigata, Niigata, Japan.
authors.forename: Yoshihide
authors.initials: Y
authors.lastname: Satoh
authors.affiliation: Department of Physiology, The Nippon Dental University School of Life Dentistry at Niigata, Niigata, Japan.
authors.forename: Shin-Ichi
authors.initials: S
authors.lastname: Iwasaki
mesh_terms: D004867:Equipment Design; D006361:Heating; D006801:Humans; D009064:Mouth Protectors; D011312:Pressure; D014618:Vacuum
keywords: heating condition; mouthguard; thickness; vacuum-pressure formation