Rubber and flexible parts - how do I get the hardness I want?

Hardness is officially defined as “the resistance of a material to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to bending, scratching, abrasion or cutting.”

Within the field of engineering we most commonly refer to ‘indentation hardness,’ which measures the resistance of a sample to material deformation due to a constant compression load from a sharp object. The tests work on the basic premise of measuring the critical dimensions of an indentation left by a specifically dimensioned and loaded indenter.

There are many different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to identify the test method used.

Touch can be subjective

We find the problems of defining hardness occur predominantly within the realms of the more flexible materials such as rubber.

This is because our touch can discern a difference in rubber hardness, however this is subjective – with most metals we detect them all to be ‘hard’. Further factors that affect grip, such as texture or friction can also impact our perception of hardness.

Material thickness can be misleading

We also know the thickness of the material can have a big effect on how a part will feel. The thicker it is, the less flexible it becomes, and yet the hardness reading will not have changed.

When considering material selection, treatments and coatings can also be considered. For example there are readily available ‘rubberised paints’ that can easily add a further dimension to a proposed design.

In terms of how to select the right hardness, there are only two real ways forward. Option one is to use a Durometer on another product that feels right and the second is to prototype – something we would always recommend!

Prototyping options

In general todays prototyping options still struggle with elastomerics, however Polyjet allows for a range of hardness’s to be 3D Printed, simultaneously if required. Then there is vacuum casting if aesthetics are also an issue. Both permit the required geometry to be created in a range of hardness’s for evaluation prior to moving to tooling.

In both these cases the materials used are not production materials, they are acrylic or polyurethane and will approximate the hardness usually to within ±5 Shore A. Other properties such as tear resistance will also be reduced and if a full component evaluation is required, then it will be necessary to use development tooling and look at compression moulding or injection moulding to get production materials.

Common hardness scales include:

Shore

The Shore Durometer Test is the one we use most commonly due to the ability to measure the hardness of polymeric materials. There are different Shore Hardness scales for measuring the hardness of different materials:

The Shore A00 Scale measures rubbers and gels that are very soft.

The Shore A Hardness Scale measures the hardness of flexible elastomerics that range in hardness from very soft and flexible, to hard with almost no flexibility at all. Semi-rigid plastics can also be measured on the high end of the Shore A Scale.

Shore D Hardness Scale measures the hardness of hard rubbers, semi-rigid plastics and hard plastics.

The differing scales measure the resistance of a material to indentation. A ‘Shore Hardness' gauge has a needle on a spring protruding from one end. The needle is placed against the rubber or plastic and pressure is applied. Once the gauge is pressed firmly against the material and the needle has penetrated as far as it can go, the measurement needle will indicate the corresponding hardness measurement.

It is a relatively inexpensive, quick and easy method.

IRHD (International Rubber Hardness Degree)

The IRHD test method originated in Europe. It provides a measure of the indentation resistance of elastomeric or rubber materials of various shapes and sizes, based on the depth of penetration of a ball indentor. It is especially important in determining the hardness of rubber. For highly elastic materials, IRHD and Shore A are comparable. The IRHD test is usually non-destructive, and as a result, is a popular method for final product inspection; the test takes 35 seconds.

Vickers  

A Vickers Hardness Test can be used for all metals and has one of the widest scales amongst hardness tests, performing on both the micro and macro scales. It has one scale that covers its entire hardness range with load weights up to 50 kilograms.

Knoop  

This microhardness test is used on very small metal parts, brittle materials or thin sheets with material features that are unable to be tested by the other methods. It employs a test load of 1000 grams or less.

Rockwell  

The Rockwell hardness test method, is the most commonly used hardness test method. It is generally easier to perform, and more accurate than other types of hardness testing methods. The Rockwell test method is used on all metals and some plastics, except where the test metal structure or surface conditions would introduce too much variation, where the indentations would be too large for the application or where the sample size or sample shape prohibits its use.

Brinell  

The Brinell Hardness Test can be applied to almost any metallic material and is the method most commonly used to test castings and forgings that have a grain structure too coarse for other hardness testing methods.

For support and advice with your flexible project, please give us a call.