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

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.

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.

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.

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.