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Method for quickly measuring foam hardness

Writer:Anonymous   2020-02-26 11:57:19


Measuring the "hardness" of a soft sponge-like elastomer (foam) is an inexact science. Data provided by equipment such as a hardness tester, a tool for measuring hardness, is often unexpectedly widely distributed. There are many more variables that can make the readings more dispersed. These variables include operator technique, specimen geometry, and even ambient temperature and humidity.

To accommodate these variables, material specifications typically allow a tolerance of ± 5 points on the scale of the measuring device. But sometimes, even a 10-point difference is not enough to provide a satisfactory comparable reading of the same sample measured by technicians in different laboratories.

The most famous device for measuring the hardness of elastomers and rubbers is the Shore A durometer. It is intended for materials that are as soft as bottle nipples or as hard as shoe heels. It uses an indenter probe and works well on solid elastomers, but only for elastomers with Shore A hardness in the range of about 20 to 90.

To measure foam, the Shore A system should never be used. The high load of the indenter and equipment of the frustum may even damage or destroy the specimen due to penetration. Also, if the indenter probe happens to stop on a foam surface (common when measuring machine parts), the readings are further distorted.

To avoid these problems, engineers should use a Shore O hardness tester when measuring foam hardness. Its spherical indenter is more suitable for foam. The thickness of the foam should be at least 0.25 inches. If the designer needs to correlate the Shore hardness with the Shore O value, the table below uses data from the Shore Instrument Company. Therefore, converting Shore A to 30 ± 5 gives a Shore O value of 42 ± 56. A ShoreO value of 21 translates to Shore A 15-a value that is difficult to measure directly with the A system.