title: Fracture strength of type IV and type V die stone as a function of time.
abstract: BACKGROUND
  During the removal of casts from the impression there is the risk of stone fracture.
  
  
  OBJECTIVE
  This study evaluated the fracture resistance of four die stone materials at different time intervals.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: University of Washington, School of Dentistry, Seattle, USA.
authors.forename: E R
authors.initials: ER
authors.lastname: Schwedhelm
authors.forename: X
authors.initials: X
authors.lastname: Lepe
mesh_terms: D000704:Analysis of Variance; D002133:Calcium Sulfate / Q000737:chemistry*; D003760:Dental Impression Materials; D003764:Dental Materials / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation; D004868:Equipment Failure; D005069:Evaluation Studies as Topic; D008422:Materials Testing; D008437:Maxilla; D003765:Models, Dental; D012307:Risk Factors; D012828:Silicones; D013314:Stress, Mechanical; D013997:Time Factors; D014867:Water / Q000737:chemistry
chemical_list: D003760:Dental Impression Materials; D003764:Dental Materials; D012828:Silicones; D014867:Water; D002133:Calcium Sulfate