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Shore hardness and IRHD hardness: which is better?

Shore hardness Vs IRHD hardness: the actual difference

The Shore measurement method is certainly the most widely used in product quality control. And the reason is very easy to understand if one analyses the calculation method on which it is based.

We are talking of a linear scale that measures the degree of deformability of plastics and rubber (Yung modulus). This is an essential aspect that allows Shore hardness to have a very wide range of applications.

In fact, its versatility guarantees suitability for many materials, from the softest (e.g. Shore 000-s scale, see ASTM D2240) to the hardest (e.g. Shore D scale, see ASTM D2240, ISO 48-4).

This is precisely the substantial difference from IRHD (International Rubber Hardness Degrees).

The latter is in fact based on a substantially different method of analysis, if not opposite, considering that it is not a linear scale.

And it is precisely thanks to this specific aspect that the IRHD test manages to overcome the weaknesses of the Shore test.

We are talking about the difficulties of analysis that Shore hardness has always entailed by definition, but which it would be useful, and in some cases vital, to consider.

Why is the IRHD method for measuring the hardness of rubber so little known

If IRHD hardness came about as an evolution of Shore hardness, one wonders why it is perhaps the one least used in quality control.

The reason is actually very simple.

It is indeed because this improvement has been confined to a very limited field of specialisation that IRHD hardness is only used to test the hardness of the softest and smallest rubber specimens (see ISO 48-2).

In terms of type of material analysed, all the 8 IRHD scales can be compared to only 2 shore scales: Shore A and Shore AM.

This explains why its use is so limited.

Yet, its potential lies precisely in this application specialisation, of particular interest for the medical, aerospace and automotive sectors.

But then, how does one choose between IRHD and Shore A hardness?

The answer is not easy, since there are many aspects that must be taken into consideration to make a conscious choice.

Generalizing, we can say that IRHD hardness is preferable to Shore in these circumstances:

  • when measuring small and soft-rubber materials
  • if materials have to meet strict safety and performance standards
  • if you deal with materials of minimum thickness (generally IRHD is recommended for < 6 mm)

In order to choose which hardness method best meets your business needs, contact us, we will be happy to guide you and find out the ideal solution for your quality control department.


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