One of the key factors to consider when selecting a thermoplastik elastomer (TPE) is the material's relative softness or hardness. Hardness is also related to other important design characteristics such as tensile and flexural modulus. However, due to the various different measurement scales and relationships to other material properties, confusion can arise when discussing hardness.
The most common instrument used for measuring rubber hardness is the Shore (or Shore A) durometer. This instrument uses a spring-loaded metal presser foot to penetrate the surface of the material and measure how deeply it can go. The scale ranges from zero (maximum penetration depth) to 100 (no penetration).
There are various different Shore hardness scales available, with the Shore A scale being the most commonly used. It employs a blunt presser foot and a medium-loaded spring. However, this type of durometer can become less accurate when readings exceed 90. For harder materials, a durometer with a sharp presser foot and stronger spring, such as a Shore D durometer, is used to measure greater depths.
For measuring even harder plastics, a durometer with a sharper presser foot and stronger spring, such as a Rockwell hardness tester, is used. On the other end of the scale, a Shore 00 durometer is used to measure soft gels and foam rubber.
Most materials can withstand the initial force, but over time, they may yield due to creep and relaxation. Readings can be taken instantly or after a specific delay, usually between 5 and 10 seconds. Instant readings will always show higher (or harder) readings than delayed readings. Delayed readings are more representative of both the material's hardness and its resilience. Weaker, less elastic materials are more susceptible to creep than stronger, more elastic materials.
To ensure data validity, precise testing procedures are required. In order to obtain accurate readings, the test piece must have a smooth, thick enough surface to avoid interference from the supporting surface. The required thickness is typically 0.200 inches, but harder materials with less deformation can be accurately tested with thinner samples.
