Hardness is defined as a material’s resistance to indentation when a static load is applied. The most common instrument used for measuring hardness is Shore Durometer. It measures the depth of penetration of an indenter on a scale from zero to a tenth of an inch (0.1 in.). A zero reading indicates that penetration depth was at its maximum; a reading of 100 specifies there was no penetration.

The Shore A scale is the most common scale for TPEs, and the Shore A Durometer consists of a blunt indenter with moderate spring force. Shore A instruments are less accurate when readings are above 90. Shore D Durometers are more appropriate when hardnesses exceed 90 Shore A. This instrument uses a sharper indenter and higher force. Softer TPEs (below 5 Shore A) use a Shore 00 scale. Most soft gels and foam rubbers are measured using this scale.

Most materials resist initial indenture but eventually yield over time due to creep or relaxation, so Durometer readings can either be taken instantaneously or after a specified delay – typically on the order of 5 to 10 sec. Instantaneous readings typically give higher (or harder) results than delayed readings. But delayed readings are more representative of hardness and resiliency. Weak, less-elastomeric materials creep more than higher strength, more resilient materials.

Hopefully, this information on the topic of shore hardness has created a clearer understanding of this important subject. While it is, admittedly, a complex topic, it is a subject with which one must be familiar in order to effectively select the proper project material.

Material Durometer Scale
Bicycle gel seat 15-30 OO
Chewing gum 20 OO
Sorbothane 40 OO
Rubber band 25 A
Door seal 55 A
Automotive tire tread 70 A
Soft skateboard wheel 75 A
Hydraulic O-Rings 70-90 A
Ebonite Rubber 100 A
Solid truck tires 50 D
Hard hat 75 D