ISO 48 specifies four methods for determining the hardness of vulcanized or thermoplastic rubbers on flat surfaces (standard hardness methods) and four methods for determining the apparent hardness of curved surfaces (apparent hardness methods). Hardness is expressed in international rubber hardness grades (IRHD). The methods cover the hardness range of 10 IRHD to 100 IRHD.
These methods differ mainly in the diameter of the scoring ball and the magnitude of the scoring force; they are chosen to suit the particular application.
This International Standard specifies the following four methods for determining standard hardness:
In the range of 85 IRHD to 95 IRHD and 30 IRHD to 35 IRHD, the hardness value obtained by the N method may not exactly match that obtained using the H method or the L method, respectively. The difference is normally not significant for technical purposes. Due to various surface effects in rubber and the possibility of slight surface roughness (for example, produced by polishing), microtesting may not always yield results that are in agreement with those obtained by normal experimentation.
This International Standard also specifies four methods for determining the apparent hardness of curved surfaces, CN, CH, CL and CM. These methods are modifications of the N, H, L, and M methods, respectively, and are used when the rubber surface being tested is curved.
In this case, there are two possibilities:
Apparent hardness can also be measured on non-standard flat test pieces using the N, H, L, and M methods.
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