Shore hardness scales – rubber tactility

Trying to describe the relative tactility and hardness of an elastomer is difficult without a sample reference. The table below offers a level of guidance and comparison between scales.

Rubber hardness is measured by comparing the difference of depth of indentation between the small initial force and the much larger final force applied by a standard size and shape impacting gauge. ISO International Rubber Hardness Degrees or IRHD uses a dead load to indent a rigid ball into rubber specimens, expressing the results in the range ‘0’ (corresponding to zero elastic or Youngs’s modulus) to ‘100’ (effectively infinite elastic modulus). Shore measures the materials resistance to a spring loaded blunt point impressed on the surface.

The primary difference between the two main Shore durability scales is that Shore A uses a truncated 35degree conical probe and Shore D uses a 30degree conical needle point probe.

shores durability scales

shore durability scales
Illustration of shore A and shore D durometer indenters showing how they are used to measure the hardness of a material (usually elastomers, rubbers, and polymers). Courtesy Wikipedia.