Understanding and Applying Shore Durometer Hardness Values

Understanding the hardness of a material that can be used in a variety of applications is vital for selecting its proper use. With the different varieties of foam and the multitude of uses for each type, hardness is one of the most important factors that help decide the practicality of a particular foam type in a given application. The hardness testing for foam and foam rubber products is performed by a Shore Durometer, which assigns a hardness value to a tested material.

Invented by Albert F. Shore in the 1920s, the durometer is a precision instrument that measures a material’s hardness, also known as Shore hardness, based on its indentation strength. This is different than a material’s scratch resistance which is gathered by comparing one material to another, as opposed to comparing a material against a uniform control, which allows for much greater precision. Durometers can vary in size, but are most often hand-held mechanisms. Their most prominent feature is a round dial that displays readings, often atop a cylinder that extends downward, ending with a flat bottom. Protruding from the base is a 2.54 millimeter (.100 inch) spring-loaded pin that records the measurements with either a spherical or blunted cone-shaped point. When placed on the material to be measured, the durometer is pushed downward until the flat bottom meets the surface of the material. Depending on the material’s hardness, the pin may be forced back into the shaft or indent the material, which happens with harder and softer materials, respectively.

There are many scales that evaluate material hardness because of the wide range of products that can be measured with a durometer. These range from hard plastics to rubbers and foam. The three most utilized scales are “A,” which covers the widest range of materials but neither the softest nor hardest, “D,” which tests the hardest materials like plastics, and “OO,” for the softest materials like foams and rubbers. Regardless of the scale type used, tested materials are placed on a numerical scale from 0 to 100, with 100 being the hardest and 0 being the softest.

All Foam Factory, Inc. 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. The ratings produced on the OO scale go from 0 to 100, as with all the others. But while the three most used scales overlap in general terms, there is no precise conversion from one scale to the other. To correctly compare a material in one scale to a similar material based in another, they must both be measured on the same scale. Also, different durometers are used for every scale as opposed to an adjustable one, because of the variances in pin types and the precision of the spring pressure.

The applied values that can be most directly gathered from the results of this test are pressure resistance and material flexibility. However, with foam and other soft products, it must be understood that a material’s thickness and the weight being supported have a correlation to its pressure resistance. Therefore, the durometer value should be looked at as a rough overview of how a product can be expected to perform, with an understanding that externalities impact a specific material’s performance in a given situation. For example, a high value material will be harder and better at resisting pressure than a lower value if both are used in similar ways, but results may be different when using a low value material ten times the thickness of a higher value material in the same application.

The five Foam Factory products that have received Shore Durometer values are neoprene, high quality neoprene, 2 and 4 pound polyethylene foam roll and 2 pound cross-linked polyethylene (XLPE). All five materials are ranked on the OO system.

The two neoprene foam types both received approximate durometer values of 50, placing them in the middle range of hardness for the OO scale, indicating a moderate flexibility but still landing on the soft end of the full hardness spectrum. Flexible while maintaining some rigidity, neoprene is excellent in athletic equipment like gloves, waders, wetsuits and knee and elbow pads. In an industrial capacity, its flexibility helps it function as insulation, expansion joint filler in masonry and concrete work and filler support sealant in traffic bearing joints.

2 pound polyethylene roll received a 51 rating, making it comparable to the neoprene in terms of hardness, flexibility and pressure resistance. The 4 pound polyethylene roll received a value of 64, placing it near the medium-to-soft range for uses that require a sturdier product that still provides cushioning. These values allow polyethylene roll to be used in many applications, including thermal insulation, pressure sensitive mounting tape, acoustic backing, package inserts and grounding.  It’s also used in specialty products like table pads, running shoes, prosthetic devices and EKG pads.

3 pound XLPE foam received the lowest durometer value of all Foam Factory foams at 45, though it still places near the middle of the OO spectrum for hardness. Softer than the other varieties, it has good flexibility and being able to be cut in thicknesses as thin as ¼ of an inch makes it extremely useful for insulation, construction expansion joints and as an industrial gasket. It is also used as custom packaging and in flotation equipment, with its low moisture permeability and high buoyancy.

When evaluating materials for a project, it’s important to take into account the product’s full range of characteristics to make sure you select the right material for the job. Shore Durometer is one very useful measurement and is one of many tools at your disposal for making an informed purchase decision.