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Durometer is one of several ways to measure the hardness of a material, defined as the material's resistance to permanent indentation. The term "durometer" can refer to both the measurement, and the instrument used to generate the measurement. Durometer is typically used as a measure of hardness in polymers, elastomers and rubbers.
Durometer Scales
There are several scales of durometer, used for materials with different properties. The two most common scales, using slightly different measurement systems, are the A and D scales. The A scale is for softer plastics, while the D scale is for harder ones. However, the ASTM D2240-00 testing standard calls for a total of 12 scales, depending on the intended use; types A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Each scale results in a value between 0 and 100, with higher values indicating a harder material.
Method of measurement
Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. Durometer allows for a measurement of the initial indentation, or the indentation after a given period of time. The basic test requires applying the force in a consistent manner, without shock, for 15 seconds, and checking the depth. If instantaneous depth is desired, force is applied for only 1 second. The material under test should be approximately 6.4mm (1/4 inch) thick.
| Durometer | Indenter | Applied force (g) |
|---|---|---|
| Type A | Hardened steel rod 1.1 mm - 1.4 mm dia, with a truncated 35º cone, 0.79 mm dia. | 822 grams |
| Type D | Hardened steel rod 1.1 mm - 1.4 mm dia, with a 30º conical point, 0.1 mm radius tip | 4550 grams |
The final value of the hardness depends on the depth of the indenter's penetration. If the indenter penetrates 2.5mm or more into the material, the durometer is 0 for that scale. If it does not penetrate at all, then the durometer is 100 for that scale. It is for this reason that multiple scales exist. Durometer is a dimensionless quantity, and there is no simple relationship between a material's durometer in one scale, and its durometer in any other scale, or by any other hardness test.
Durometer of various common materials
| Material | Durometer | Scale |
|---|---|---|
| Hard Hat | 75 | D |
| Non-spark hammer | 60 | D |
| Hard skateboard wheel | 100 | A |
| Solid truck tires | 50 | D |
| Soft skateboard wheel | 70 | A |
| Auto tire tread | 70 | A |
| Door seal | 55 | A |
| Rubber band | 25 | A |
| Sorbothane | 40 | OO |
| Sorbothane | 0 | A |
| Chewing Gum | 20 | OO |
See also
Dimensionless numbers | Materials science
References
Rex Gauge product from the manufacturers (Rex Gauge Company) site *.
"Shore durometer"