The Shore Durometer Test and Its Values

Invented in the 1920s by Albert F. Shore, a durometer is a specialized instrument that measures the hardness of a material based on its ability to resist indentation. Durometers vary in size, but the majority are hand-held testing instruments. Pressed against the material being evaluated, such as automotive foam or a hard plastic, a durometer is forced down until its spring-loaded tip is either pushed into the shaft, or the material yields to the pin and the base of the shaft meets the surface of the test material. Harder materials force the pin back into the durometer, as happens with hard products like rubber, while softer materials like a squishy foam pad get indented by the pin.

There are various scales that measure these values, due to the vast range of materials Durometers test, from hard plastics, to soft foams. The three most used Durometer scales are “A,” a medium scale that covers the broadest range of materials, “D,” for the hardest plastics, and “OO,” for soft materials like cushions of foam firm enough for packing. On every type, measurement values are rated on a scale from zero to 100, with 100 being the hardest value, and zero being the softest.

Real-life values can be better understood with a Shore Durometer rating, in particular pressure resistance and flexibility. It’s important to note that with certain materials, the thickness will impact resistance to pressure and flexibility in different way than a smaller sample tested under a spring-loaded pin. For that reason, a durometer rating should really only be used as a starting point in understanding the performance material, but still remains a useful factor to have in your knowledge set.