Measuring the hardness of a material is one of the most important testing areas for a product expected to perform in a variety of applications. Given the different varieties of foam and the many uses for each, knowing a material’s hardness is helpful in identifying which foam types perform well in certain areas and Durometer which do not. This evaluation is performed for foam and foam rubber products by a Shore Durometer, which measures and assigns a value for the hardness of a tested material.
Invented by Albert F. Shore in the 1920s, the durometer is a dedicated instrument that measures the hardness (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 which may or may not be controlled or have precise values. A durometer may vary in size, but most are generally hand-held mechanisms. They feature a precision dial or digital display that shows readings and a small shaft that extends 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 spherical 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 until the flat bottom of the shaft meets the surface of the material. Depending on the material’s hardness, 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 scales on which values are measured because of the wide range of materials that can be assessed with a durometer, such as hard plastics, rubbers and foam. The three most utilized scales are “A, ” which spans the widest range of materials but neither the softest nor hardest, “D, ” on which the hardest materials like plastics are recorded and “OO, ” for the softest materials like foams and rubbers. Regardless of the scale type used, measurement values are placed on a numerical scale from 0 to 100, with 100 being the hardest, and 0 being the softest.
The majority of foam products that receive a 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 scales measure different products, an OO value cannot be uniformly compared to a material measured on the A or D scale. The scales do loosely overlap, so while it is possible that a high scoring OO material is harder than a medium or low 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 scales as opposed to a single adjustable durometer because of the variances in pin types and spring pressure.
The two real-life values that can be most directly assessed 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 impact 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 understanding 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 than 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 less likely it is to be flexible without breaking. Some examples of where it is important to know a foam’s hardness include custom packaging and package inserts, components like industrial gaskets, construction materials like expansion joint filler and athletic equipment such as shoulder or elbow pads.
When considering materials for any project, it is important to take into account the full range of product characteristics to make sure you select the perfect material for your job. Shore hardness is one very useful measurement and it can be used to help you make an even more informed purchasing decision.