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The International Rubber Hardness Degree (IRHD) was introduced in the 1960s as a standardised method for measuring the hardness of elastomer materials, such as rubber.
The goal was to create a hardness measurement method capable of offering a more thorough and reproducible analysis with respect to Shore hardness.
Imagine the IRHD hardness tester as a graduated gauge which, when held perpendicular to the surface being tested, applies a specific force according to the chosen scale for a total of 5 seconds.
The hardness value is calculated on the basis of the initial and final indentation difference of the spherical indenter in a rubber sample.
Due to the shape of the indenter and its field of application – particularly soft elastomers – the test cycle leaves no permanent marks on the test sample.
Unlike the Shore hardness, which has as many as 13 different scales according to the ASTM D2240 and ISO 48-4 standard, the IRHD hardness only has 8.
If you use or have ever used Shore hardness, you will certainly have noticed something very interesting from the table.
The testing of non-planar surfaces – as it is the case with the widely known O-rings – is regulated by the ISO 48-2 standard.
This is a considerable advantage of the IRHD test over the Shore hardness test, which, on the contrary, has no such specific similar application, except with specially designed accessories capable of respecting the orthogonality of the indenter in relation to the surface of the sample being tested.
It is particularly important to bear this in mind, as it is directly related to the writing of the final IRHD value.
Let us take the following value as an example and analyse it: 58 IRHD SN
There are many similarities, but also many the differences, between the two.
After all, we must point out that Shore hardness is the mother of IRHD.
However, because of this relation between the two, there is a tendency to use them as if they could be directly converted into each other.
The truth, however, is that the regulations see these conversions as incorrect.
Scientific ITP-type IRHD and Shore A hardness comparison studies (inter-laboratory tests, i.e. tests carried out internally by several certified laboratories) have been carried out, which have established similarities and differences between specific scales for certain parameters such as reproducibility and precision.
During these direct comparisons, similarities emerged in terms of accuracy but not in the reproducibility of the results.
(see ISO 48-4:2018; Rubber, vulcanized or thermoplastic — Determination of hardness — Part 4: Indentation hardness by durometer method (Shore hardness) page 13 para. A.4.2)
Consequently, we can say that the two hardnesses are comparable, but not convertible.
To answer this question, we have written an e-book that aims at clarifying the measurement of the hardness of rubber and polymers in general.
It is an unedited and free e-book that makes reference to regulations and scientific studies, to give a clear picture of all that the 3 methods – Shore, IRHD and Rockwell – can offer.
We are the only company in the Italian market that personally manufactures the entire range of Shore durometers in all the scales specified by the ISO 48-4, ASTM D2240, ISO 868, and ISO 21509 standards.
