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What is wrong with my Rubber ? Part 2 - Hardness Variations

12 Jun 2012 | Posted by Andrew Onions

This is the second in the series and here I thought we should deal with another common and perhaps slightly more technical issue when evaluating Rubber Sheet materials, and answer the question:

Why do I always get variations in the Hardness of my material?

Let's start with the basics - you need the right equipment for the job. Pushing your nail into a material or trying to compare one piece of rubber with another by pulling and twisting just isn't going to work.

There are a variety of manual and electronic Durometer devices available on the market. Manual, hand held devices tend to be most common as they are easy to use and are relatively low cost.

Hand held digital hardness meters were increasingly popular a number of years ago but most of the main manufacturers have since ceased supply due to the increased level of inaccuracies which became apparent in their use.

Electronic devices can sometimes accommodate testing for a range of products including plastics and metals.
MacLellan Rubber doesn't recommend any particular manufacturer but I've included images of typical products below.

Durometer for rubber hardness testing

Equipment should be recalibrated frequently. Most manual units are spring loaded so performance will deteriorate over time. Most equipment is now supplied with reference blocks to enable regular recalibration.

So what about the mechanics of measuring - it's simple. Isn't it? Applications



MaterialHardness Scale
Soft Rubber, Sponge Rubber Skinned Urethane, Cork Shore 0 & 00
Medium Hard Rubber, Flexible Polyurethane, Soft Polymers, FeltsShore A, IRHD
Medium Hard Rubber, Polyurethane, PlasticsShore C
Hard Rubber, Rigid Polyurethane, Thermo Plastics, Epoxy ResinShore D

Standards



BS903-A2:1995, ISO 48:1994Physical testing of rubber. Method for determination of hardness(hardness between 10 IRHD and 100 IRHD)
BS903-A57:1997, ISO 7619:1997Physical testing of rubber. Determination of indentation hardness by means of pocket hardness meters
BS EN ISO 868:1998Plastics and ebonite. Determination of indentation hardness by means of a durometer(Shore Hardness)
ASTM D1415-88(1999)Standard Test Method of Rubber Property-International Hardness
ASTM D2240-02Standard Test Method of Rubber Property-Durometer Hardness
DIN 53505Testing of Rubber, elastomers and plastics; hardness test Shore A and Shore D
DIN 53519-1Testing of elastomers; determination of the ball indentation hardness of soft rubber, International Rubber Hardness(IRHD), hardness testing of standard samples