What’s Shore hardness and why should you care?

You’ll have seen the ‘Shore’ number crop up once or twice, when selecting a product’s material for its unique application – but you may have found yourself wondering what it means. What is shore hardness and how is it measured?

In short, ‘Shore’ relates to the hardness of the material you’re using. It’s usually measured using the Rockwell hardness test or Durometer test – and both can work out how well plastics will fare when it comes to indentation. For engineers, at least, gleaning and understanding a material’s ‘Shore’ value is key to a successful project.

Named after its inventor, Albert Ferdinand Shore, Shore hardness offers different scales for measuring the solidity of different materials. (Shore was born in 1876 and lived until 1936, creating the ‘Shore’ scale in the 1920s.)

There are close to 15 different scales, depending on their intended use. Each scale will result in a value from 0 and 100, with the higher number relating to increased hardness.

There are differences between the Rockwell and Shore Durometer, though. While both measure the resistance of plastics towards indentation – the higher the number means a higher hardness of your material – Shore hardness uses either the Shore A or Shore D scale.

The Rockwell method, meanwhile, is used for ‘harder’ plastics:

• Polycarbonate
• Nylon
• Acetal
• Polystyrene

Rockwell vs Shore hardness

A standardised test used industry-wide, the Shore D hardness technique measures the depth of penetration of an indenter, generally using ASTM D2240 and ISO 868 test methods. With readings taken between 0 and 100, Shore hardness measures are dimensionless.

The hardness Shore D method comes into its own when a material is too soft to be measured by the Rockwell test. But what about the Shore A test? This is recommended for soft materials, like rubber or thermoplastic elastomers (TPEs).

What kinds of materials is ‘Shore’ hardness applied to?

Mostly, plastics – like elastomers and rubbers – are measured by their ‘Shore’ hardness. It’s worth knowing, though, that the value does not necessary compare with the plastic’s basic characteristics.

For elastomers and rubbers, in particular, the appropriate Shore hardness scale would be Shore A. Polyolefins, fluoropolymers and vinyl also fall under this category. The Shore D scale, meanwhile, applies to harder plastics, and Shore O and H are in existence – just not as widely in the plastics industry, in particular.

How exactly is the hardness of plastics measured?

To ascertain the hardness of plastics:

1. The indenter foot on the Durometer is inserted into a sample of plastic.
2. Next, a needle is placed against the plastic before pressure is applied to the material.

Conducted according to ASTM D2240, ISO 868 & ISO 7619-1, a Shore hardness test can be vital for engineers. During the test:

• The surface of the test material will be flat and positioned over an area with plenty of space to allow the pressure of the Durometer’s foot to easily come into contact with it.
• This will be over a radius of a minimum of 6mm from the indenter point.

Note, the flat surface is extremely important, since accurate hardness readings with a durometer can’t be made on surfaces which are rounded, rough or uneven.

1. For the minimum time of an hour, test pieces will be conditioned immediately before testing – in instances where this is possible.

2. Placing the test piece on a hard, flat or rigid surface, the pressure will be applied as quickly as possible but without shock.

3. Recorded at specific times, Durometer readings will be taken at three seconds (vulcanised rubber) and 15 seconds (thermoplastic rubber).

4. A total of five measurements will be taken at varying points on the test piece. Each will be noted, with the mean value calculated and recorded.

5. If the needle has entered the plastic as far as it can – and the gauge has been pressed firmly on the material – the needle used to measure the plastic will show a corresponding measurement relating to its hardness.

The indentation reading all comes down to how flexible the rubber or plastic is; this reading may change and it’s why it’s important to write down the time the measurement is taken, along with the number which correlates to its hardness.

What about bendable rubbers?

Of course, there isn’t one, single way to measure the hardness of a material. While they’re designed to offer a reference point for anyone using plastics, the scales are slightly different.

When it comes to bendable rubbers, for example, a Shore A hardness scale will be used. Measuring the solidity of the material – since they can vary widely – the scale offers helpful data which can be used to great effect by engineers.

When can the Shore hardness test with the Durometer not be used?

While we’ve been extolling its virtues so far, there are instances when this test cannot be used so accurately. For example: it doesn’t work so well when it comes to monitoring plastic’s resistance to abrasion or scratches. Neither will it determine its speed of wear.

In instances where an engineer is working with the same product line, there can be a link between flexibility and Shore hardness in materials which are similar.

When might you need to consider Shore hardness?

If you’re selecting an original rubber mould and need to create another one, Shore hardness can prove a vital feature to monitor. For instance, when manufacturing a dainty or flimsy child’s toy (a doll with slip legs, for example), you’d want to opt for a Shore 30A rubber. The reason for this is it would offer plenty of flexibility to allow you to easily take the model out of its mould. Without a hardness of Shore 30A, the doll may easily break.

To give you an idea of the Shore numbers applied to different materials, a measurement of 0 could be applied to chewing gum, for instance. Meanwhile, the mid-way point on the scale (50) would be assigned to a car tyre, for example. It’s worth knowing that once a material reaches a Shore A 95 range, it will resemble a plastic over a flexible material when it comes to how it feels. In this instance, the Shore A and Shore D scale will briefly overlap. The latter relates to rigid materials.

Shore hardness isn’t necessarily required for plaster casting but proves hugely useful when it comes to selecting materials for making a cast or mould. Often, engineers are faced with the conundrum of which silicone rubber would be best for such an application – and the Shore hardness durometer helps to simplify this process.

In instances where the model used for testing has severe undercuts, Shore A would be beneficial. Easier to stretch, it’s also simpler to seal correctly. Following curing, it’s much less stressful to remove the model from its rubber mould, too, meaning the project should run without a hitch.

What about rubber with a higher Shore hardness number?

Be aware of rubber with a higher Shore hardness reading, as this can prove inflexible and stiff. This may result in the aforementioned breakage of the mould or cast during de-moulding. That said, this type of rubber will be ideal for concrete castings, offering higher resistance to extrusions.

When mould-making, which durometer should you select? You’ll want to look for a firmness which is ideal for standard, everyday mould making. In toy making, for example, Shore A 10 is a good reading to look for. It’s important to always keep in mind its intended application, before selecting the right components for your project.

Let’s look at some materials and their Shore hardness

To give you an idea of different materials and their Shore hardness, we’ve put together a handy table below:

Material/component	Shore hardness
Rubber stamps	50 Shore A
Pencil erasers	55 Shore A
Car tyres	65 Shore A
Shoe heels, leather belt	80 Shore A
Windscreen wiper blades, golf balls	60 Shore A
Abrasive handling pads	75 Shore A
Mouse pads	80 Shore A
Book cover	40 Shore D
Phone cord	90 Shore A
Wooden ruler	70 Shore D

 

Portable durometers make production and quality control a breeze

Thanks to advancements in technology, portable durometers are now available. These can help engineers test Shore hardness much more easily, making the devices ideal for areas of production and quality control.

They’re also useful for technical service projects, since they’re mobile and compact in size. Some durometers of this kind can even be equipped with an LCD display, for increased ease of use. On top of that, they may also feature a convenient 360° dial and internal memory. A sleek, USB interface for data transfer adds to their appeal, as does internal rechargeable batteries.

So, why should engineers care about Shore hardness, then? In short, the data gleaned from the test can help projects run more smoothly. A useful measure of comparative resistance to indentation of different polymers grades, it is typically used as a substitute for flexibility in the production of specs for elastomers, for instance. Since studies show a link between flexibility and Shore hardness in similar materials, it’s vital the test is completed before work commences.

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