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This study was performed to evaluate the fracture resistance of three improved die stone materials according to water/powder ratio. There are lots of handling conditions which affect the physical properties of improved dental stone, and it’s well known that the water/powder ratio significantly affect the strength of die stone. If water/powder ratio was incorrect, following disadvantages were showed : (1) susceptibility to dimensional change due to abrasion, (2) limited reproduction of fine detail, (3) lack of strength. The maxillary master casts were made of additional silicone impressions (Exaflex, GC America, Inc, USA). Three type IV die stones such as Fuji Rock (GC Europe Intreleuvenlaan, Leuven, Belgium, Velmix (Kerr, Manufacturing Company, USA), and Crystal Rock (Maruishi Gypsum Co. Ltd. Japan) were tested. A total of 160 casts were prepared, separated, and tested on the Instron Testing Machine (Model 4201, Co. USA). The obtained results of this study were as follows : 1. Fuji Rock and Velmix less 3ml than the water/power ratio of manufacturer’s instruction showed the highest resistance to fracture. According to increasing water/powder ratio, fracture resistance was significantly increased (P<0.05). Crystal Rock showed the highest fracture value when it was mixed with the water/power ratio of manufacturer’s instruction. 2. Water/powder ratio of the manufacturer’s instructions and less 3 ml than that showed lower fracture value of hand mix than that of vacuum mix. Water/powder ratio of more 3ml, 6ml than manufacturer’s instructions was not significantly different between hand mix and vacuum mix (p>0.05). 3. Velmix had the highest viscoelastic value among three die materials when it was mixed with the manufacturer’s instruction. Viscoelasticity was decreased according to increasing water/powder ratio.
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