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A modified vacuum infusion (VI) process, which we refer to as the progressive compression method (PCM), has been proposed to reduce the filling process. In PCM, the bag is divided into several segments. During infusion, all segmented bags are pulled upward by the vacuum. Resin is easily infused into the loose preform. Once enough volume of resin is infused, the vacuum on the segmented bags is released in a step-wise manner. The atmospheric pressure is progressively applied on the segmented bags that are inflated to squeeze the surplus resin from the wetted preform through the dry preform. The flow enhancement is achieved since the dry preform remains loose during the filling process. Once the preform is fully saturated, the post-filling stage is performed for achieving the uniform thickness of the part. In the present study, seven cases is utilized to investigate the effect of three key process variables, including the number of the compression segment, initiating timing of the next segmented compression and volume of infused resin, on the complete filling process. Numerical results show that the multi-segment compression and appropriately premature initiation of the next segmented compression can reduce the filling process; whereas, the excess infused resin can significantly increase the total filling time due to the very slow post-filling stage. Compared with the traditional VI process, PCM at the optimal case reduces the filling time by 10.15% with the same resin consumption as VI. [ABSTRACT FROM AUTHOR]

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.Materials and 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.Conclusion: 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. [ABSTRACT FROM AUTHOR]

Aim: The purpose of this study was to examine the differences of the vacuum-formed mouthguard thickness by the timing of vacuum application.Materials and Methods: The material used in this study was a mouthguard sheet of 3.8-mm ethylene vinyl acetate. Three conditions of the timing of vacuum application were examined: the vacuum was applied immediately, 5 s after, and 10 s after the sheet holder was lowered over the vacuum-forming stand. We measured mouthguard thickness at the labial surface of the central incisor, buccal surface of the first molar, and occlusal surface of the first molar. Differences in thickness in different regions of mouthguards formed under different timing of vacuum application were analyzed by two-way analysis of variance and Bonferroni method.Results: We found that mouthguard thickness differed in different regions of the central incisors and the first molars (P < 0.01). The mouthguard thickness at the labial surface of the central incisor and buccal surface of the first molar became thinner when the vacuum was applied immediately after the sheet holder was lowered over the forming stand. The thickness at the occlusal surface of the first molar did not vary with the timing of vacuum application.Conclusions: Our results suggest that the thicknesses of the labial surface of the central incisor and buccal surface of the first molar became larger when the vacuum was applied several seconds after the sheet holder was lowered over the forming stand. This finding is necessary knowledge when forming a mouthguard sheet. [ABSTRACT FROM AUTHOR]

Background: The goal of the present study was to examine the thickness of mouthguards molded under a variety of heating conditions to clarify suitable conditions during vacuum-pressure forming of ethylene vinyl acetate sheets.Materials and Methods: Mouthguards were fabricated using ethylene vinyl acetate (EVA) sheets (thickness: 4.0 mm) using a vacuum-pressure forming machine. The sheet was pressed against the working model, followed by vacuum forming for 10 s and compression molding for 2 min. Three heating conditions were investigated in which the sheet was molded when the center of the softened sheet sagged 10 mm, 15 mm, or 20 mm below the clamp (H-10, H-15, or H-20 respectively). The temperature of the sheet surface was measured using a radiation thermometer under each heating condition. The thickness of the mouthguard sheets after fabrication was determined for the incisal portion (incisal edge and labial surface) and molar portion (cusp and buccal surface), and dimensional measurements were obtained using a measuring device. Differences in thickness due to the heating condition of the sheets were analyzed by one-way analysis of variance and Bonferroni's multiple comparison tests.Results: The temperature difference between the heated and non-heated surfaces was lowest under H-15. The thickness differences at incisal edge, labial surface, and cusp were determined. The thicknesses for H-10 and H-15 were greater than that for H-20, and the thicknesses for H-10 and H-15 were equivalent at all measurement points. No differences in thickness at the buccal surface were observed for the various heating conditions.Conclusion: The present study demonstrated that a sagging distance of 15 mm provided the most suitable forming process. The results of the present study provide a standard heating condition for EVA sheet forming. [ABSTRACT FROM AUTHOR]