Shore Hardness Scales
Shore scales can be used as a method of measuring and comparing the hardness of rubbers and elastomers. They are also commonly used for softer plastics such as polyolefins, fluoropolymers, and vinyl. There are around fifteen different shore hardness scales which can be used to measure the hardness of different materials.
The Shore A scale is the most commonly used for measuring the hardness of rubbers and can be determined using a durometer gauge. The durometer consists of a metal truncated cone indenter attached to a small spring as shown in Figure 1. The indenter is extended 2.5mm into the material. The material will resist the indentation, deflecting the spring. Every 0.001 inches that the spring is deflected relates to 1 degree shore A. A harder material will resist the indentation, this will cause the deflection of the spring to be higher and will result in a higher reading. Figure 2 gives a representation of each of the shore hardness values. Shore 70 A is the standard hardness for rubber.
Different shore hardness scales exist to measure the hardness of different materials. The Shore A scale is often used for measuring softer rubbers and the Shore D scale is used for harder ones. The shore D scale was developed to measure the hardness of hard rubbers which have a value of over 90 degrees on the Shore A scale. Similarly, shore 0 was introduced to measure the hardness of soft rubbers which record less than 10 degrees on the Shore A scale and Shore 00 was introduced for very soft rubbers or gels which recorded less than 10 degrees on the shore 0 scale. Shore B and Shore C are intermediate scales intended for use on medium and moderately hard rubbers. A comparison of Shore Scales A, D, 0 and 00 is given in Table 1.
The shape of the indenter that is used on the durometer and the spring force vary by scale. For example, shore D uses a sharp pointed indenter and a spring force of 10lbs (4.5 kg) whereas the indenter for shore 00 has a blunt point and a spring force of only 113g.
Shore hardness testing will often leave a permanent indentation in the test sample. Another consideration is that the material must be at least 6mm thick and positioned on a hard surface to get an accurate reading. If the material is too thin, the durometer will measure the hardness of the surface on which the material is placed. Due to mechanical limits of the test instruments, hardness measurements in elastomers are normally expressed in 5-degree increments.
Table 1: Comparison of Shore Scale Hardness Values
|Shore 00||Shore 0||Shore A||Shore D|
|Medium and Low-Density Cellular Elastomers, Urethane Foam and Cork||Medium Density Cellular and Solid Elastomers||Solid and Cellular Elastomers||Polyurethanes, PTFE rigid, Thermoplastic and Elastomeric, very hard|