What is meant by hardness?

The hardness of a material is often one of the first criteria considered when choosing thermoplastic elastomers (TPEs). Typically, there are three types of durometer used to express the hardness of TPEs, which are type A, D, and OO. Phon Tech produces standard products ranging in hardness from 10 Shore OO (softest) to 65 Shore D (hardest). In between, Phon Tech produces TPEs hardness range from 5 to 95 Shore A. Hardness is defined as a material's resistance to indentation when a static load is applied. The indentation hardness is inversely related to the penetration and is dependent on the elastic modulus and viscoelastic behavior of the material. We follow the test method ASTM D2240 to measure the hardness of TPEs. This test method permits hardness measurements based on either initial indentation or indentation after a specified period or both. Phon Tech reports hardness values using instant reading at initial indentation.

The most common scale for TPEs is the Shore A scale. The Shore A durometer consists of a blunt indenter with a moderate spring force. Shore A instruments are less accurate when readings are above 90. The Shore D durometer is more appropriate when hardness levels are above 90 Shore A. This instrument uses a sharper indenter and higher force. Softer TPEs use the Shore OO scale for the measurement of materials below 5 Shore A.