Shore hardness testing is the usual way to measure how soft or hard a rubber or plastic product is. When a silicone product is specified as "Shore A 50", for example, this is the test behind the number.

Principle

A steel indenter of defined shape is pressed perpendicularly into the specimen surface under a set force. When the presser foot sits fully flat on the surface, the tip of the indenter protrudes a certain distance beyond the foot, which is the depth to which it has penetrated. The indenter is connected to a linear measuring device, and the indentation depth is converted mechanically or electronically into a Shore hardness value. The deeper the dent, the softer the material and the lower the reading.

Governing standard

Products are tested to ASTM D2240 (Shore hardness). The standard specimen is generally 6.4 mm thick. If a single piece is too thin, several pieces can be stacked, but a single piece is preferred.

Shore A and Shore D

• Shore A: for softer materials such as silicone rubber and ordinary rubbers.
• Shore D: for harder materials such as rigid plastics.

The two durometers differ in indenter shape and applied force, so their readings cannot be converted directly. When comparing the hardness of different products, check that they were measured with the same durometer type and on the same specimen thickness.