Shore Hardness Test

Definition and Purpose of Shore Hardness

The Shore hardness test (also called durometer hardness) is a standardized indentation hardness method for plastics and elastomers. An indenter (a standardized indentation tip) presses into the material surface with a defined spring force, and the measuring device shows the resistance as a numerical value on a Shore scale. A high Shore value means that the material is harder to indent.

Measurement is performed with a durometer — a handheld or bench-mounted device that couples indentation depth and spring force in accordance with the standard. In sealing technology, Shore hardness is used primarily to compare and select sealing materials, for example for O-rings, molded seals, or wipers. In practice, the parameter quickly answers the question of how “soft” or “hard” a compound feels in an indentation test, which often correlates with installation behavior, adaptability to sealing surfaces, and extrusion tendency.

Scale Choice and Standard Reference (Shore A, D, 00; ISO/ASTM)

Shore hardness is not a single scale but a scale system adapted to different material hardnesses. For a value to remain interpretable, scale and standard must match the measured value, because a numerical value is only meaningful in the context of its scale. Therefore, a “70” on Shore A describes something different from a “70” on Shore D.

Relevant standards in practice are primarily:

• ISO 48-4 for elastomers,

• ISO 868 for plastics and hard rubber,

• ASTM D2240 as an internationally widespread standard that covers several durometer scales.

Which Scale for What?

The scale choice depends on how strongly the indenter can typically indent the material. In sealing technology, this is particularly important because many materials lie in the soft to medium range.

Values from different scales are not directly comparable, even when the number looks the same. For material comparisons, the same scale should therefore always be used.

Which Standard Applies to My Material?

For reliable data sheet or laboratory comparisons, the Shore value alone is not sufficient. Decisive is according to which standard testing was carried out, because standards define measurement conditions and device types.

In many cases:

• Elastomers (sealing compounds): orientation along ISO 48-4.

• Plastics / hard rubber: orientation along ISO 868.

• International comparability: often ASTM D2240, particularly in global supply chains.

Test reports should therefore at least state: Shore scale (A/D/00) + standard + if applicable read-out time and temperature, so that measured values between suppliers and laboratories remain genuinely comparable.

Test Procedure and Influencing Factors (Specimen, Time, Temperature)

Shore hardness appears in everyday use as a simple parameter, yet the measurement reacts sensitively to operating conditions. The specimen needs sufficient thickness and a flat surface, so that the indenter actually tests the material and not indirectly the substrate. With specimens that are too thin, the hard substrate can limit the indentation path, and the Shore value then appears artificially higher.

In addition, geometry plays a role. On curved, structured, or very small measuring surfaces, a reproducible placement is difficult, so scatter increases. Furthermore, temperature and conditioning influence the result, because elastomers typically react softer at heat and stiffer at cold. For reliable comparisons, specimens should therefore be stored and tested under comparable conditions.

Why a Time Specification Can Be Important

Elastomers are viscoelastic — they deform in a time-dependent way. Under constant load, the indenter can penetrate somewhat further because the material locally “gives way” (relaxation/flow on a small scale). As a result, an instantaneous value and a value after a defined indentation time sometimes differ noticeably.

When test reports or data sheets state Shore hardness, it is therefore often sensible to also document the read-out time. As a result, it becomes clear whether two values really reflect the same measurement regime or only look similar.

Checklist of Typical Sources of Error

• The specimen is too thin, and the substrate influences the measured value.

• The surface is not flat, or measurement is performed on curved areas.

• The wrong scale is used (A instead of D or vice versa).

• Standard, temperature, or read-out time are missing in the documentation.

• Too few repeat measurements are taken, although scatter is relevant.

Significance for Sealing Technology and Distinction from Other Parameters

In sealing technology, Shore hardness serves as a quick, established comparison value to classify materials with respect to their indentation resistance. Softer sealing materials usually adapt more easily to sealing surfaces under contact pressure, which can support initial sealing. Harder compounds frequently show more resistance to deformation in gaps, which reduces the extrusion tendency under pressure load. The suitable Shore hardness therefore often arises from the question of which sealing function dominates in operation: adaptation to the surface or shape stability under pressure and gap loading.

Typical Shore Ranges and Practical Effects (Soft vs. Hard)

Many sealing elastomers lie roughly in the range of about 40 to 90 Shore A. Polyurethane (PU) is frequently used in seals in the upper Shore A range, when higher shape stability and extrusion resistance are required. Such ranges should be understood deliberately as orientation, because recipe, fillers, and degree of cross-linking shift the behavior even when the Shore value is similar.

What Shore Hardness Does Not Say (Distinction) and When IRHD Makes Sense

Shore hardness describes the indentation resistance, yet it does not automatically follow how high tensile strength, E-modulus, wear behavior, or compression set (permanent deformation after compression) are. Particularly with seals, media resistance and temperature resistance are additionally decisive, because they often limit service life and sealing function more strongly than pure hardness.

As a complementary hardness method, IRHD (International Rubber Hardness Degree) is frequently used with rubber. IRHD is also standardized and can, depending on specimen geometry and test requirement, be a sensible alternative — for example when defined test conditions or different specimen shapes are in focus.

In the end: for a reliable material selection, Shore hardness is often sufficient as a first filter, yet for critical applications a coordinated consideration of further parameters and test conditions is sensible; if needed, specialized consultation can help.