National Physical Laboratory

Rubber Hardness

The standard test methods used for measuring the hardness of rubber and plastic are either Shore or IRHD - these are specified in BS EN ISO 868 [4] and BS 903-A26 (ISO 48) [3] respectively.

Shore hardness

Shore hardness specifies methods for determining the hardness of materials by means of durometers of two types: type A for softer materials and type D for harder materials [4], although the ASTM standard [1] covers twelve durometer scales. The method permits measurement of the initial indentation, the indentation after a specified period of time, or both. The indentation hardness is inversely related to the penetration and is dependent on the modulus of elasticity and the viscoelastic properties of the material. The shape of the indenter, the force applied, and the test duration influence the results obtained. The Shore durometer consists of a reference presser foot, an indenter, an indicating device, and a calibrated spring that applies the force to the indenter. The difference between the type A and type D durometer is in the shape of the indenter and the calibrated spring (Table 1).

Table 1:  Shore hardness scales

Shore durometer Indenter Applied force, F / mN
Type A Hardened steel rod 1,1 mm - 1,4 mm dia, with a truncated 35º cone, 0,79 mm dia. F = 550 + 75 HA
Type D Hardened steel rod 1,1 mm - 1,4 mm dia, with a 30º conical point, 0.1 mm radius tip F = 445 HD

Note: It is recommended that measurements be made with the type D durometer when the value
is above 90 HA, and with a type A durometer when the value is less than 20 HD.

The units of hardness range from 0 for the full protrusion of 2,50 mm to 100 for no protrusion. The force is applied as rapidly as possible, without shock, and the hardness reading made after a duration of 15 s ± 1 s. If an instantaneous reading is specified, the scale is read within 1 s of force application. There is no simple relationship between results obtained with one type of durometer and those obtained with either another type of durometer or another instrument for measuring hardness.

IRHD hardness

IRHD (International Rubber Hardness Degrees) provides four methods for the determination of the hardness of vulcanised or thermoplastic rubber surfaces (N, H, L, and M) and four methods for the determination of the apparent hardness of curved surfaces (CN, CH, CL, and CM). The test consists of measuring the difference between the depths of indentation of a ball into a rubber surface under a small initial contact force followed by a large total test force. The methods differ primarily in the diameter of the indenting ball and the magnitude of the indenting force selected according to the hardness of the rubber (Table 2).

Table 2:  IRHD hardness scales

Test method Indenter diameter
/ mm
Contact force
/ N
Additional force
/ N
Total force
/ N
(normal test)
2,50 ± 0,01 0,30 ± 0,02 5,40 ± 0,01 5,70 ± 0,03 Thickness >= 4 mm
Range: 35 IRHD - 85 IRHD
or 30 IRHD - 95 IRHD
(high hardness)
1,00 ± 0,01 0,30 ± 0,02 5,40 ± 0,01 5,70 ± 0,03 Thickness >= 4 mm
Range: 85 IRHD - 100 IRHD
(low hardness)
5,00 ± 0,01 0,30 ± 0,02 5,40 ± 0,01 5,70 ± 0,03 Thickness >= 6 mm
Range: 10 IRHD - 35 IRHD
0,395 ± 0,005 0,008 3 ± 0,000 5 0,145 ± 0,000 5 0,153 3 ± 0,001 Thickness < 4 mm
Range: 35 IRHD - 85 IRHD
or 30 IRHD - 95 IRHD

Note: Hardness values obtained for method N within the ranges 85 IRHD - 95 IRHD and 30 IRHD - 35 IRHD may
not agree with the values obtained for method H or method L, respectively.

From the difference in indentation depth D and using the relevant 'look-up' curve, the IRHD value can be read. These hardness scales are chosen such that a 0 IRHD value represents the hardness of a material with a zero Young's modulus and 100 IRHD represents the hardness of a material with an infinite Young's modulus.

Last Updated: 25 Mar 2010
Created: 18 Jun 2007