Technically Speaking: There are different Shore Hardness scales for measuring the hardness of different materials. These scales were invented so that people can discuss these materials and have a common point of reference.

The Shore A00 Scale measures rubbers and gels that are very soft.

The Shore A Hardness Scale measures the hardness of flexible mold rubbers that range in hardness from very soft and flexible, to medium and somewhat flexible, to hard with almost no flexibility at all. Semi-rigid plastics can also be measured on the high end of the Shore A Scale.

As you can see from the chart, there is overlap on the different scales. For example a material with a Shore hardness of 95A is also a Shore 45D.

Shore D Hardness Scale measures the hardness of hard rubbers, semi-rigid plastics and hard plastics.

How Do You Physically Measure A Material's Hardness?

The different Shore Hardness scales measure the resistance of a material to indentation. A ‘Shore Hardness' gauge (looks like a round tire pressure gauge) has a needle on a spring protruding from one end. The needle is placed against the rubber or plastic and pressure is applied.

Once the gauge is pressed firmly against the material and the needle has penetrated as far as it can go, the measurement needle will indicate the corresponding hardness measurement. 

2016-11-16