Method for quickly measuring foam hardness

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release time : 2020-06-01 15:11:01

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Measuring the "hardness" of soft sponge-like elastomers (foams) is a science. The data usually provided by equipment such as a hardness tester (a tool for measuring hardness) is often widely distributed unexpectedly. There are many variables that can make readings more dispersed. These variables include operator skills, sample geometry, and even ambient temperature and humidity.

In order to adapt to these variables, the material specifications usually 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 and comparable reading of the same sample measured by technicians in different laboratories.

The equipment used to measure the hardness of elastomers and rubbers is the Shore A durometer. It is intended for materials that are as soft as a bottle nipple, or as hard as the heel of a shoe. It uses indenter probes and works well on solid elastomers, but only for those with Shore A hardness in the range of about 20 to 90.

In order to measure foam materials, the Shore A system should not be used at all. The indenter of the truncated cone and the high load of the equipment may even damage or destroy the specimen due to penetration. Moreover, if the indenter probe happens to stop on the foam surface (common when measuring machine parts), the reading will be further distorted.

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To avoid these problems, engineers should use the 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 Shore Instruments data for conversion. Therefore, converting Shore A to 30 ± 5 will result in a Shore O value of 42 ± 56. A ShoreO value of 21 will be converted to Shore A 15-this value is difficult to measure directly with the A system.

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