What is Shore hardness in elastomers?

Shore hardness is a technical parameter used to classify the hardness of an elastomer. To determine this, the depth to which a standardised test specimen penetrates the surface of a material such as rubber or silicone under defined conditions is measured. A shallow penetration depth results in a high hardness value, whereas a great penetration depth results in a low hardness value. Depending on the scale used, the measured value ranges between 0 and 100 and has no physical unit. Therefore, the value must always be stated alongside the scale, for example as „70 Shore A“.

Shore hardness enables the comparison of elastomers under defined test conditions. However, it describes neither the strength nor the rigidity nor the elastic recovery of a material. Elastomers with identical Shore hardness can therefore behave differently under tensile, compressive or cyclic loading.

1. Measuring principle of the Shore test

A Shore durometer consists of an indenter, a pressure foot, a spring and a display. When applied, the indenter first touches the sample. As the pressure foot approaches, the indenter is pushed back into the device against the spring force. Depending on how much the sample yields, the deflection of the indenter results in a high or low hardness value.

Figure 1: Shore hardness measurement of a rubber moulded part

The method does not only react to the near-surface material zone. If the deformation field extends to the substrate, this also influences the result. A sample that is too thin on a hard substrate or as a composite component, e.g. rubber vulcanised onto a metal support, usually appears harder than would be measured with a sufficient rubber thickness. Additional geometry effects occur with small or curved components. Elastomers are viscoelastic. Therefore, the indenter continues to sink in over time under a constant load, so that the displayed value decreases. An immediately read value is therefore not readily comparable with a value that was only recorded after several seconds. If the value does not decrease, this is a sign that the rubber layer thickness is too small and that the measurement result is distorted by the substrate, such as a hard table top or a metallic support component.

2. What is the relationship between Shore A, Shore D, Shore OO and IRHD?

There are various testing methods whose application area depends not only on how hard or soft an elastomer is, but also on component thickness, measuring surface, curvature and surface finish. Shore A is suitable for many compact elastomer components and is the most widely used scale in the field of rubber and silicone moulded parts. Shore D is used for significantly harder materials and Shore OO for very soft or cellular materials. Due to the spherical indenter and various test method variants, IRHD offers additional possibilities for small or curved components. In all, there are 12 different test methods according to ASTM D2240 as well as several others according to ISO 48-2, which use different test equipment and test procedures. The four mentioned are the most widespread. Due to the different procedures, the values cannot generally be converted into one another.

Figure 2: Shore hardness scale with examples

Shore A

The Shore A test uses a truncated cone indenter with a small, flat face. Due to the flattened tip, the test force is distributed over a larger area than with Shore D. The method is suitable for compact elastomers in the standard hardness range. Shore A is used, among other things, for the following samples and components:

• Test plates made of rubber or silicone,
• larger seals and sealing profiles,
• Hoses with sufficient wall thickness,
• Rubber buffers and elastic moulded parts,
• Roller and cylinder coverings with a sufficiently large measuring surface,
• compact components made of thermoplastic elastomers.

For a standard-compliant measurement, the sample should be sufficiently thick and have a flat measuring surface. In the case of small, thin or strongly curved components, the result is influenced by the support and the component geometry. A measurement taken directly on an O-ring is therefore not directly comparable with the Shore A value of a flat standard test plate.

Shore D

The indenter of the Shore D test is designed as a narrow, pointed cone with a slightly rounded tip. At the same time, the testing device operates with a higher spring force than a Shore A durometer. This allows the indentation resistance of hard materials to be measured in a more differentiated manner. Shore D is primarily used for:

• hard thermoplastic elastomers,
• Hard rubber and ebonite-like materials,
• hard roller and cylinder coverings,
• rigid sealing and guiding components,

Shore D is unsuitable for soft elastomers because the narrow indenter would penetrate too far into the material. Conversely, Shore A quickly reaches the upper range of the scale with very hard materials and can only represent differences in hardness there to a limited extent.

Shore OO or rather „Shore 00“

Shore OO uses a rounded, approximately hemispherical indenter. The test force is significantly lower than for Shore A and Shore D. The rounded shape reduces local stress peaks and prevents the indenter from piercing very soft materials like a sharp needle. Shore OO is suitable, for example, for:

• foam rubber and soft cellular rubber,
• Elastomeric foams,
• Gel materials,
• very soft silicone compounds,
• soft cushioning and damping elements,

The significance of Shore OO decreases at both ends of the scale. In the case of very soft gels or foams, the indentor can sink in almost completely; for harder elastomers, the scale responds only to a limited extent to further differences in hardness.

IRHD

During the IRHD test, a spherical indenter is used instead of a conical one. First, a small initial load is applied to the sample, and then the indenter is pressed into the test piece with a defined major load. The IRHD value is calculated from the difference in the indentation depths.

Depending on the IRHD method, balls with different diameters and correspondingly adjusted test forces are used. This allows the method to be adapted to different hardness ranges and specimen dimensions. ISO 48-2 distinguishes between procedures for flat test pieces and for the determination of apparent hardness on curved surfaces. IRHD tests are used, amongst other things, for:

• standardised elastomer test plates,
• O-rings and cords,
• small seals,
• thin elastomer components,
• curved moulded parts,
• small components on which a Shore durometer cannot be safely placed.

For small cross-sections, micro-IRHD testing is available. Due to the small spherical indenter, it can be used on O-rings, thin seals and small moulded parts. However, the apparent hardness determined may deviate from the material hardness measured on a flat standard test piece.

Figure 3: Different test methods
Key: Dimensions in mm | Angle = full opening angle | Green = effective surface

3. Execution of tests and influencing factors

For each hardness measurement, the corresponding sample must have a suitable surface that enables the testing device to be placed securely and perpendicularly. Unevenness, parting lines, pores, coatings or contamination alter the contact between the contact area, indenter and test material. If an individual test piece is too thin, some methods allow several layers to be placed on top of each other under defined conditions. This procedure is not suitable for arbitrary finished parts, as air gaps, different surfaces and relative movements between the layers influence the result.

Near an edge, the material can yield sideways. Measurements must therefore be carried out at a distance from the sample edges and from previously used measurement points. In the case of small seals, O-rings or profiles, the standard-compliant geometry can frequently not be maintained. The measured value is then output as an apparent component hardness.

Other key influencing factors are:

• Temperature: The deformation resistance of many elastomers changes with temperature. Measured values at room temperature cannot be directly transferred to operating conditions in the cold or heat.
• Conditioning: Storage duration, humidity, temperature profile and previous mechanical stress can alter the result.
• Reading time: Elastomers exhibit stress relaxation and sink slightly a few seconds after indentation of the test specimen. Therefore, it must be specified when the value is read.
• Contact pressure: Tilted, excessively fast or uneven placement produces additional scatter. A test stand reduces operator influence.
• Ageing and media contact: Oxidation, post-cross-linking or plasticiser loss can lead to hardening. Swelling, hydrolysis or chain scission can reduce the measured value. The course depends on the material, medium, temperature and time.

A single measuring point is usually not sufficient for elastomers. Multiple measurements at separate locations capture local variations and enable an average value to be provided. If there is significant scatter, individual values or the range should also be documented. The specifications of the agreed test standard remain decisive.

4. Shore hardness as a characteristic value in component design

Hardness influences the contact and deformation behaviour of an elastomer component, but does not determine it alone. In the case of a seal, hardness, cross-section, installation space, compression, friction, pressure and temperature interact. With the same geometry, a harder compound can yield less and exhibit higher resistance to gap extrusion. At the same time, a higher assembly force may be required, and minor surface irregularities may not be compensated for as effectively.

For bearings and buffers, component stiffness depends heavily on the shape factor. A thin elastomer layer between rigid plates behaves significantly stiffer under compression than a freely bulging test specimen of the same compound. The Shore hardness remains the same, but the force-displacement characteristic of the component does not.

For roller coverings, hardness influences, amongst other things, the contact area and local deformation. For hoses and diaphragms, wall thickness, reinforcement layers and material modulus are frequently more informative than hardness alone.

A reliable design therefore uses the Shore hardness as one of several input parameters. Depending on the load case, calculations require, for example, stress-strain curves, dynamic moduli, compressibility, coefficients of friction or time-dependent material models.

5. Typical hardness ranges of elastomers

EPDM, NBR, FKM or silicone can be manufactured in various hardnesses through their formulation. The following ranges serve merely as a technical guide for standard compounds. Adjustment beyond these values may also be possible through the production of special compounds.

Figure 4: Bar chart of the hardness ranges of the Shore A and D scales