How is hardness defined?
Tests have been developed that measure the resistance of plastics including the Shore® (Durometer) test and the Rockwell hardness test. Rubbers are frequently defined on the IRHD scale.
Both methods measure the resistance of plastics toward indentation and provide an empirical hardness value that doesn't necessarily correlate well to other properties or fundamental characteristics.
Shore Hardness is the preferred scale for elastomers
Shore Hardness, using either the Shore A or Shore D scale, is the preferred method for 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 referred to as 'Durometer hardness'). The hardness value is determined by the penetration of the Durometer indenter foot into the sample. Because of the resilience of plastics, the indentation reading may change over time – so the indentation time is sometimes reported along with the hardness number. Standards include: ASTM D2240 00 ; ISO 7619 and ISO 868; DIN 53505; and JIS K 6253.
The durometer measures resistance to indentation
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.
Shore hardness is often used as a proxy for flexibility (flexural modulus) in specifying elastomers. The correlation between Shore hardness and flexibility holds for similar materials, especially within a series of grades from the same product line, but this is an empirical and not a fundamental relationship.
(Information courtesy of www.matweb.com)
