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The purpose of this study was to determine changes in the thickness of mouthguard sheets under different heating conditions during fabrication. Mouthguards were fabricated with polyolefin–polystyrene co-polymer (OS) and olefin co-polymer (OL) sheets (4.0-mm thick) utilizing a vacuum-forming machine under the following three conditions: (A) the sheet was moulded when it sagged 15 mm below the sheet frame (i.e. the normally used position); (B) the sheet frame was lowered to and heated at 30 mm below the top of the post and moulded when it sagged by 15 mm; and (C) the sheet frame was lowered to and heated at 50 mm below the top of the post and moulded when it sagged by 15 mm. The working model was trimmed to a height of 20 mm at the incisor and 15 mm at the first molar. Post-moulding thickness was determined for the incisal portion (incisal edge and labial surface) and molar portion (cusp, central groove and buccal surface). Dimensions were measured, and differences in the change in thickness due to heating condition were analysed using the Kruskal–Wallis test. Under condition C, OS and OL decreased in thickness from 0.36–0.54 mm to 0.26–0.30 mm, respectively, at the incisal portion and from 0.34–0.66 mm to 0.17–0.47 mm, respectively, at the molar portion. It may be clinically useful when moulding a mouthguard to maintain the thickness of the incisal and molar portions by adjusting the height of the sheet frame.