This International Standard specifies four methods for the determination of the hardness of vulcanized or thermoplastic rubbers on flat surfaces:
Method N Normal test
Method H High-hardness test
Method L Low-hardness test
Method M Microtest
and four methods for the determination of apparent hardness of curved surfaces using methods N, H, L and M, respectively:
Methods CN, CH, CL and CM.
The methods differ primarily in the diameter of the indenting ball and the magnitude of the indenting force, these being chosen to suit the particular application. The range of applicability of each is indicated in figure 1.
Method N: The normal test for hardness is the appropriate method for test pieces of thickness greater
than or equal to 4 mm and is preferably used for rubbers in the range 35 IRHD to 85 IRHD but may be used for those in the range 30 IRHD to 95 IRHD.
Method H: The appropriate method for test pieces of thickness greater than or equal to 4 mm and hardness in the range 85 IRHD to 100 IRHD.
Method L: The appropriate method for test pieces of thickness greater than or equal to 6 mm and hardness in the range 10 IRHD to 35 IRHD.
NOTE 1 The value of hardness obtained by method N within the ranges 85 IRHD to 95 IRHD and 30 IRHD to 35 IRHD may not agree precisely with that obtained using method H or method L, respectively. The difference is not normally significant for technical purposes.
Method M: The microtest for hardness is essentially a scaled-down version of the normal test method N, permitting the testing of thinner and smaller test pieces. It is the appropriate method for test pieces of thickness less than 4 mm and is preferably used for rubbers in the range 35 IRHD to 85 IRHD but may be used for those in the range 30 IRHD to 95 IRHD.
NOTE 2 Because of various surface effects in the rubber and of any slight surface roughness (produced, for example, by buffing), the microtest will not always give results agreeing with those obtained by the normal test.
Methods CN, CH, CL and CM: Apparent-hardness
tests on curved surfaces.
These methods are modifications of methods N, H, L and M for cases where the rubber surface tested is curved. Two cases exist, depending whether
a) the test piece or article tested is large enough for the hardness instrument to rest upon it;
b) it is small enough for both the test piece and the instrument to rest upon a common support.
A variant of b) would be where the test piece rests on the specimen table of the instrument.
The procedures described cannot provide for all possible shapes and dimensions of test pieces but cover some of the commonest types such as "O" rings. Determination of the apparent hardness of rubber-covered rollers is dealt with separately in ISO 7267-1:1986, Rubber-covered rollers — Determination of apparent hardness — Part 1: IRHD method, ISO 7267-2:1986, Rubber-covered rollers — Determination of apparent hardness — Part 2: Shore-type durometer method and ISO 7267-3:1988, Rubber-covered rollers — Determination of apparent hardness — Part 3: Pussy and Jones method.