Each of these is a method of testing the hardness of a rubber sample by pressing an indentor (called a frustrum) into a rubber sample and measuring the force and / or amount of penetration. Each of these methods uses a slightly different indentor, method of controlling the force, and / or method of measuring the penetration. As a result, each of these hardness measurement devices will generate a different number on the same sample, and there is no direct correlation from one “scale” to another.
In the US, Shore A is the most common method of measuring rubber hardness. Shore D is typically used for very hard rubber compounds and plastics (50 Shore D is approximately 90 Shore A). Shore M is specialized for use on O-rings. IRHD is a fully automated measuring procedure preferred in Europe. At Parker, we control each batch of rubber by measuring the Shore A hardness.
All of the test methods can provide repeatable results (all the numbers will be about the same for a given sample and test method), but the number provided by one method may be very different from that of another. For this reason, it is not valid to compare the hardness results obtained by one test method with the pass / fail limits written for another.