Effect of bench cooling on the dimensional accuracy of heat-cured acrylic denture base material

Objectives In order to obtain an insight on the internal stress caused by both polymerization and thermal shrinkage during the fabrication process of heat-cured denture resin, the effect of bench cooling on the dimensional accuracy of a heat-cured denture base resin was examined. Methods A dimensional change of a dumbbell-shaped specimen during the fabrication process was measured directly by using the strain gauge method. After polymerization, the specimens were treated in one of the following two processing methods: (1) rapid cooling: the specimen was removed from a stone mold within a container of boiling water at 100 °C and then left to cool in a thermo-stabilized room of 20±1 °C; (2) bench cooling: the flask was left to cool in a thermo-stabilized room of 20±1 °C for 140 min, after which, the specimen was removed from the stone mold. The strain from deflasking was derived from the difference in the strain, before and after the removal of the specimen from the stone mold. The strain differential, before and after cooling, was determined as the total strain. Results The bench cooling for the heat-cured denture base resin reduced the strain caused by thermal shrinkage during the fabrication process. The observed reduction in the strain was 26% for the CL (direction of centerʹs length), 11% for the EL (direction of left-edgeʹs length), and 12% for the EW (direction of left-edgeʹs width), when compared with the results obtained from the rapid-cooling method. Conclusions The flask should be slowly cooled to room temperature, since the internal stress developed by thermal shrinkage will be relaxed during the cooling process.