Fracture strength of type IV and type V die stone as a function of time.

Statement Of Problem: During the removal of casts from the impression there is the risk of stone fracture.

Purpose: This study evaluated the fracture resistance of four die stone materials at different time intervals.

Material And Methods: Additional silicone impressions were made of a maxillary master cast. Two Type V, one Type IV die stones and one Type IV resin reinforced stone were tested. A total of 80 casts were prepared, separated, and tested on the Instron Universal Testing Machine at 1/2, 1, 12, and 24 hours to measure resistance to fracture.

Results: Significant differences to fracture resistance of the different die stone materials were observed at all time intervals except at 24 hours. The data were analyzed with ANOVA at alpha = 0.005 and n = 5.

Conclusion: It is recommended to wait at least 12 to 24 hours when separating casts from impressions to avoid fracturing casts. Residual moisture content in a stone cast may contribute to fracture.