Hardness (Shore A / Shore D)

Definition and Measurement Principle of Shore Hardness

Shore hardness (Shore A / Shore D) is a standardized parameter for the indentation hardness of elastomers and plastics. What is measured here is not “hardness” in the metallurgical sense, but the resistance of a material against local indentation. This matters in sealing technology, because seals are locally deformed under contact pressure, and precisely this local deformability influences the function.

Measurement is carried out with a durometer (hardness tester). It has a spring and a standardized indenter. The indenter is pressed into the sample with a defined force. The indentation depth is converted into a value from 0 to 100: 0 stands for very soft (large indentation depth), 100 for very hard (small indentation depth).

For viscoelastic materials such as elastomers, the result also depends on the measurement time, because the material flows under load. Therefore, it is decisive whether an instantaneous value or a value after a defined time is reported.

Shore A vs. Shore D: Selection, Application Ranges, and Comparability

Which scale makes sense depends on how stiff the material is and which indenter/spring combination delivers sufficient resolution. Shore A is typically used for softer elastomers and flexible plastics. Shore D is designed for significantly harder plastics and hard elastomers.

The two scales are technically different, because indenter geometry and spring force differ. As a result, measurement values arise that do not represent the same mechanical state. A numerical comparison along the lines of “A = D” therefore does not work reliably.

Conversion tables between A and D exist, yet they remain approximations. For specifications, incoming inspection, or releases, they should only be used with great caution, because small differences in test setup and material behavior can produce large deviations.

Standards, Test Conditions, and Typical Sources of Error

For testing, ASTM D2240, ISO 868, and ISO 48-4 are mainly used (historically also ISO 7619-1). The standards specify how the durometer is to be applied and which conditions secure comparability. In practice, the test condition often decides whether a value is robust.

A central point is the specimen thickness. Measurements close to the standard usually only succeed from about 6 mm thickness, because thinner specimens are more strongly influenced by the substrate. Temperature and measurement time must also be defined, because elastomers respond sensitively to them.

Typical errors arise when component geometry and test setup do not match:

• Wall too thin or soft support: the measurement value often appears too low (apparently softer), because substrate and deflection are co-measured.
• Curved or small surfaces: the indenter does not stand stably; the indentation is distorted.
• Insufficient edge distance: the edge “yields”, and the value becomes unstable.
• Temperature deviation: material is often softer when warm and harder when cold.
• Inconsistent measurement time: instantaneous reading and timed value are not identical.

For test plans, this means: scale (A or D), measurement time, temperature, support, and specimen geometry should always be documented, so that results remain comparable between laboratory, production, and supplier.

Importance of Shore Hardness in Sealing Technology and Measurement on Finished Sealing Parts (Micro Shore A by Bareiss)

In sealing technology, Shore hardness directly influences how a seal works in the installation situation. Softer materials usually conform more easily to roughness and shape deviations of the mating surfaces. At the same time, the tendency toward extrusion into gaps frequently rises when pressure and gap dimension are unfavorable. Harder materials remain more dimensionally stable, yet they can require higher assembly forces and a better mating-surface quality, so that the necessary contact pressure for sealing is generated.

Nevertheless, hardness is only one design criterion. For a robust selection, pressure, gap dimension, temperature, medium, and motion type must also be considered, because they strongly determine the deformation and aging behavior.

A practical problem is that seals in operation are often thin-walled and therefore not available as a standard-conforming, thick specimen. Standard Shore measurements are then hard to reproduce. A complementary method tailored to real seal geometries can be sensible here.

Brief Description of Micro Shore A (Bareiss Method)

Micro Shore A is a micro-suitable indentation test that can be used specifically to determine hardness directly on the finished sealing part. This is particularly helpful when no standard specimens are available, or when the components are significantly thinner than is typical for standard-near Shore measurements. In practice, measurement is typically possible from about 1 mm wall thickness.

Clear documentation matters, because Micro Shore A is its own method: method, scale, measurement time, and support conditions must be recorded, so that measurement results can be compared cleanly internally and with suppliers.

In the end: Shore hardness values are very useful when the test conditions are unambiguous. For critical applications, a short technical alignment on the test method and the interpretation of values is often sensible.