In the rubber business, one of the most common terms you will encounter is rubber durometer. Durometer is both a measure of hardness of the material and the instrument used to measure it. A rubber durometer can be used to measure the hardness of not only solid rubber but also cellular rubber (e.g. foams, sponges), plastics, and other elastomeric materials. There are many hardness and can be categorized dependent on the type of rubber, or application in which the method of manufacturing is formed.
What is Hardness?
The NDT Resource Center defines hardness as the material’s resistance to localized deformation. When measured with a durometer, a steel indentor of standardized geometry is penetrated into the material with a standardized pressure. The hardness reading is then read from the gauge on a Shore scale either immediately or after a required time of penetration.
What is Shore Durometer?
Albert Ferdinand Shore invented the Shore hardness scale for the durometer in the 1920s. There are 12 different scales of Shore hardness, the most common of which is Shore A. Each Shore type is defined by their indentor shape and spring force.
Because a durometer is measured by the indentation depth into a material – e.g. a durometer reading of 100 is when there is no penetration at all into the material, a durometer reading of 0 is when there is 0.100″ (2.54 mm) or more of penetration depth – the different Shore types were invented for the different types of materials to be measured.
Our durometer chart gives you an idea of how the same durometer readings compare between each Shore type. A word of warning regarding durometer “conversions”: no simple relationship exists between the measurements obtained with one type of durometer and those obtained with another type of durometer, therefore the Durometer Conversion Chart is used as a guide only and not for equivalencies between different Shore measurements. The Shore instrument chosen must always be appropriate for the specimen being tested.
ASTM D2240 details the standardized method of measuring durometer: https://www.astm.org/Standards/D2240