Shore Durometer Firmness Testing: Understanding and Applying Foam Firmness Values

Measuring the firmness of a material is one of the most important testing areas for a product expected to perform in many different applications. Given the different varieties of foam and the many uses for each, knowing a material’s firmness is effective in identifying which foam types succeed in a few areas and which do not. This evaluation is performed for foam and foam rubber Durometer products by a Shore Durometer, which measures and assigns a value for the firmness of a tested material.

Invented by Albert F. Shore in the 1920s, the durometer is a dedicated instrument that measures the firmness (or Shore hardness) of a material based on its indentation strength. This is different than a material’s scratch resistance which is gathered by comparing two materials, one of that might or may not be controlled or have precise values. A durometer may vary in size, but they are generally hand-held accessories. They include a precision dial or digital display that shows parts and a small shaft that provides down to a flat bottom. Some newer durometers have minimized the presence of a shaft or eliminated it completely. Protruding from the bottom is a 2. 54 millimeter (. 100 inch) spring-loaded pin, with either a circular or blunted cone-shaped point that records the measurements. Once rested against an object to be measured, the durometer is, in general terms, pushed downward so that the flat bottom of the shaft meets the surface of the material. Depending on the material’s firmness, the pin may be forced back into the shaft, as happens with hard materials, or indent the material, as occurs with softer products.

There are numerous skin scales on which values are measured because of the wide range of materials that can be looked at with a durometer, such as hard pouches, rubbers and foam. The three most utilized skin scales are “A, inches which ranges the widest array of materials but neither the softest nor hardest, “D, inches on which the hardest materials like pouches are recorded and “OO, inches for the softest materials like foams and rubbers. Regardless of the scale type used, statistic values are put on a mathematical scale from 0 to 100, with 100 being the hardest, and 0 being the softest.

The majority of foam products that acquire durometer rating are ranked on the OO scale, as foam is one of the softer materials that can be tested with this process. This scale includes but is not limited to varieties like Minicel, Volara and neoprene. The ratings produced on the OO scale go from 0 to 100, as with all the others. But because the skin scales measure different products, an OO value cannot be consistently compared to a material measured on the A or D scale. The skin scales do loosely overlap, so while it will be possible that a high credit scoring OO material is harder when compared to a medium or low credit scoring A or D material, the only way for a true comparison is to test the different materials on the same durometer scale. Different durometers are used for different skin scales as opposed to a single adjustable durometer because of the differences in pin types and spring pressure.

The two real-life values that can be most directly looked at from this test are pressure resistance and material flexibility. However, when measuring foam and other soft products, it must be understood that a material’s thickness and the weight being supported have a large have an effect on its pressure resistance. For this reason, a durometer value should be seen as a starting point for expectations on how a product will perform, with an knowing that externalities impact a specific material’s performance in a given situation. For example, a higher valued material will be harder and better at resisting pressure when compared to a lower value when both are measured similarly, but results may be different when using a low rated material at ten times the thickness of a higher rated material in the same application. And the harder a material is, the more unlikely it is to be flexible without breaking. Some examples of where it is important to know a foam’s firmness include custom packaging and package inserts, components like industrial gaskets, construction materials like expansion joint filler and particular sports equipment such as shoulder or knee pads.

When considering materials for any project, it is important to consider the full array of product characteristics to make sure you choose the perfect material for your job. Shore firmness is one very useful statistic and it can be used to help you create an even more informed purchasing decision.

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