An experimental and theoretical study of the effect of sample thickness on the Shore hardness of elastomers.

Objective: The Shore hardness test is a quick and convenient way to measure hardness of elastomers. The test specifies that the specimen should be at least 6mm thick; however, published work in the literature indicates that workers often use much thinner samples. The aim of this study was to investigate the effect of thickness on the measured hardness of a range of dental elastomers and develop a theoretical relationship to predict the effect.

Methods: 7 dental silicone elastomers were selected to give a range of hardness values. Shore A scale hardness measurements were made on standard thickness samples, and those of lower thicknesses. A theoretical analysis was derived, whereby the effect of thickness of the sample on its measured hardness can be predicted and was tested using the results obtained.

Results: The results show reasonable agreement between theoretical and experimental values. The measured Shore hardness increased inversely with thickness, the effect being most pronounced with thin samples. Results ranged from 66.3+/-1.6-80.9+/-0.4 for 1mm thick to 35.0+/-1.7-69.1+/-0.6 for the 6mm thickness. Ratio of the measured hardness of 1mm/6mm ranged from 1.97 for the softest material to 1.17 for the hardest.

Significance: Measurements obtained with a Shore A hardness tester on samples of less than the specified 6mm thickness will give erroneously high values, the error being greater as thickness decreases. The theoretical treatment proposed gives a basis on which data obtained from studies that used thinner specimens can be better analysed.