DUROMETER encyclopedia topics

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 ASTM D2240 type A and type 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. ASTM D2240 durometers allows for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time and then read. The material under test should be a minimum of 6.4 mm (.25 inch) thick.

Test setup for type A & D
Durometer	Indenting foot	Applied mass [kg]	Resulting force [N]
Type A	Hardened steel rod 1.1 mm - 1.4 mm diameter, with a truncated 35º cone, 0.79 mm diameter	0.822	8.064
Type D	Hardened steel rod 1.1 mm - 1.4 mm diameter, with a 30º conical point, 0.1 mm radius tip	4.550	44.64

The final value of the hardness depends on the depth of the indenter. If the indenter penetrates 2.5 mm 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.

Durometers of various common materials
Material	Durometer	Scale
Bicycle gel seat	15-30	OO
Chewing gum	20	OO
Sorbothane	40	OO
Sorbothane	0	A
Rubber band	25	A
Door seal	55	A
Automotive tire tread	70	A
Soft skateboard wheel	75	A
Hydraulic O-Rings	70-90	A
Hard skateboard wheel	98	A
Ebonite Rubber	100	A
Solid truck tires	50	D
Hard hat	75	D