Displacement Rheometer: a method of measuring working time and setting time of elastic impression materials.

A new method of measuring both working time and setting time of elastomeric impression materials has been developed. A Displacement Rheometer was used to monitor the development of elasticity in setting elastomers. The results obtained with the Displacement Rheometer were compared to the results of the ISO test as well as those of the Controlled Stress Rheometer. Working time using the ISO method was calculated according to the standard method described in the specification, while for both the Controlled Stress Rheometer and the Displacement Rheometer working times were the times when initial elastic recovery was observed at 23 degrees C. With the Displacement Rheometer the setting time is the time when the mixed material reaches optimal elastic recovery at 37 degrees C. Statistical analysis with ANOVA showed that for five of the test materials there were no differences between the three methods. For three of the materials the working time determined by the ISO method was significantly longer than that determined by the other two methods. The advantages and disadvantages of each method are discussed and the Displacement Rheometer is recommended as a supplement to the standard specification method.