durometer

The hardness of polyurethane and rubber is most commonly measured by the Shore® (Durometer) test. This method measures the resistance of the material toward indentation and provides an empirical hardness value that doesn’t necessarily correlate well to other properties or fundamental characteristics.

Shore Hardness, using either the Shore A or Shore D scale, is the preferred method for rubbers/elastomers. The Shore A scale is used for ‘softer’ rubbers while the Shore D scale is used for ‘harder’ ones.

The Shore hardness is measured with an apparatus known as a Durometer and consequently is also known as ‘Durometer Hardness’. The hardness value is determined by the penetration of the Durometer indenter foot into the sample. Because of the resilience of rubbers and plastics, the indentation reading may change over time - so the indentation time is sometimes reported along with the hardness number.

The final value of the hardness depends on the depth of the indenter’s penetration. If the indenter penetrates 2.5mm or more into the material, the durometer is 0 for that scale. If it does not penetrate at all, then the durometer is 100 for that scale. It is for this reason that multiple scales exist. Durometer is a dimensionless quantity, and there is no simple relationship between a material’s durometer in one scale, and its durometer in any other scale, or by any other hardness test.

The results obtained from this test are a useful measure of relative resistance to indentation of various grades of polymers. However, the Shore Durometer hardness test does not serve well as a predictor of other properties such as strength or resistance to scratches, abrasion, or wear, and should not be used alone for product design specifications.