Many people probably don’t think of hardness as a quality that can be measured. The truth is, the hardness or softness of a material can be very important depending on its function. In the world of metrology, the term durometer has multiple meanings: a durometer is a tool that is used to measure hardness, the standardized method of determining the hardness of materials like rubber and plastic, and a dimensionless measurement of hardness.
Hardness scales
The technical name for the durometer instrument used with the Shore scale is a Shore durometer. There are many different durometer scales used for testing rubbers, elastomers (materials with elastic or rubber-like characteristics) and plastics, but the most common are Shore OO, Shore A and Shore D. Shore OO is used for very soft rubbers and gels; Shore A measures many materials from flexible rubbers to semi-rigid plastics; and Shore D measures harder materials, from semi-rigid to hard plastics. Each scale includes a range from 0-100 to indicate its determination: the higher the number, the harder the material.
Shore (durometer) isn’t the only scale used for measuring hardness — there are many similar scales and hardness tests, and the second most recognized is Rockwell. Rockwell scales are used to measure metals and harder plastics. It’s important to understand that across the board, hardness scales are general comparisons of hardness between materials, as opposed to exact measurements. That’s why a durometer is not considered a unit of measurement, but a dimensionless reference to a material’s hardness level.
Behind the names:
Why Shore and Rockwell? The Shore durometer testing method and scale were developed by Albert Ferdinand Shore in 1920. The Rockwell tester was actually patented a year prior by Hugh Rockwell and Stanely Rockwell, who were unrelated.
How does a durometer work?
In addition to being a numerical measurement, testing method and scale for hardness comparison, a durometer is a precision measurement tool used for this purpose. Durometer measuring gages include a pressure foot with a point, which is shaped differently depending on the scale being used. The tip is duller for softer materials (for example, in Shore OO) and sharper for harder materials (with Shore D being the hardest).
Essentially, a durometer measures hardness by making an indentation in the material with the pressure foot, and the depth of that indentation determines hardness. This is why it’s necessary to have a general idea of the hardness of your material before using a durometer: you must choose a scale based on your understanding of the material’s hardness, with the appropriate instrument tip to match. For example, if you use a Shore D test for a very soft material, the material may be damaged by the sharp point. To understand why this test works, consider the definition of hardness (according to materials science) — the ability to resist deformation, which is determined by a test that measures a surface’s resistance to indentation.
Glossary
Shore — the name of the durometer testing method and scale developed by Albert Ferdinand Shore in 1920, which are used for measuring the hardness of rubber and plastic materials.
Pressure foot — the part of a durometer used to apply force to a material when measuring hardness. The pressure foot has a tip with a conical shape, which is pointed to varying degrees depending on the hardness of the material being tested.
Hardness — a material’s ability to resist deformation, which is determined by a test that measures a surface’s resistance to indentation.
Frequently Asked Questions
Why do durometers matter?
Durometers enable us to gain a general understanding of a material’s hardness —and as a result, gauge its durability and suitability for a particular function. High precision is essential in product design, and durometers are one more measuring tool that helps you achieve that.
What types of materials does the Shore durometer test?
The Shore durometer is used to test the hardness of rubbers, elastomers and plastics. The most common durometer scales are Shore OO, Shore A and Shore D, with Shore OO measuring the softest materials and Shore D used for the hardest.
Why is a durometer not considered a unit of measurement?
Hardness scales like Shore and Rockwell are generally used to provide rough comparisons between materials, not exact measurements. This is why durometers are considered dimensionless measurements of hardness, as opposed to units of measurement.