Gaskets and Rubber Parts: The Importance of Durometer

The term “durometer” refers to the hardness of a material, usually polymers, elastomers and rubbers. The hardness of an elastomer or rubber is the ability of the material to resist localized deformation when a force is applied. Hardness, or durometer, is an important factor to consider when specifying a material for gaskets and rubber parts to service a given application.

“Durometer” indicates the hardness of a material, as well as the instrument used to measure it. The Shore durometer, invented in the 1920’s by Ferdinand Shore, is the instrument commonly used for testing and measuring the hardness of elastomers and rubbers.

The Shore durometer works by applying a standardized force or pressure through a steel probe, or indenter, to penetrate into the material being tested. The hardness reading, which reflects the degree to which the material is penetrated by the indenter, is then read from the Shore scale gauge on the durometer, either immediately or following a required penetration time. Higher numbers in a Shore scale indicate a greater resistance to indentation, and thus greater hardness, while lower numbers in the sale indicate a softer material. A durometer can be used to measure the hardness of not only solid rubber, but also foams, sponges, plastics, and other elastomeric materials.

There are multiple scales used for measuring the hardness of elastomers and rubbers. The ASTM D2240 standard recognizes twelve different durometer scales, using durometers with different combinations of spring forces and indenter geometries. The two most commonly used scales are ASTM D2240 type A and type D (often referred to as Shore A and Shore D respectively). The Shore A scale is usually employed when measuring softer materials while the Shore D scale is used with harder materials.

How is hardness testing carried out?

Durometer testing is carried out by using a Shore durometer to measure the depth of indentation in a material after a force has been applied. The indentation is created by applying a given force of the durometer’s indenter onto the test material which rests on the instrument’s presser foot. The depth of the resulting indentation depends upon the hardness of the material being tested, which is a function of its ability to resist deformation, and the duration of the test.

ASTM D2240 testing with a Shore durometer allows measurement of the initial hardness of a material, or by measuring the hardness after the test force has been applied for a given period of time. In carrying out the ASTM D2240 test, a consistent force is applied to the material being tested and the depth of the resulting indentation measured with a durometer. If a timed hardness is desired, the force is applied for the required time and the depth of the indentation read from the durometer. Material being tested should not be less than 6.4 mm (0.25 inches) in thickness.

Durometer readings vary according to the hardness of the material being tested - harder materials will resist penetration by the durometer's indenter. If the indenter of the durometer penetrates 2.54 mm (0.100 inch) or more into the material, the durometer is 0 for that scale. If the indenter fails to penetrate the material at all, then the durometer is 100 for that scale. Because the durometer of a material is measured by the depth of indentation during testing, the different Shore scales were created for different types of materials to be measured.

Why is durometer important?

The hardness, or durometer, of a material is a key factor in the specification of a material for producing gaskets or rubber parts. Durometer is a key property that determines how a material will behave or react in an intended application. For gaskets and rubber parts, this has important implications for the performance, longevity, and installation ease of the final part.

For gaskets, durometer is a key factor in determining sealing performance. Gaskets made from materials of higher durometer will resist extrusion in joints where higher bolting forces are required or where there is a large gap between the sealing faces. Alternatively, joints having sealing faces with rough or irregular surfaces may need gaskets made from a lower durometer as the material will be more conformable to these irregularities. Compression set is also a function of durometer and so due consideration should be give to this when selecting materials for parts, especially for applications which involve the repetitive movement of a part.

While one will always choose a compound or material for producing gaskets or rubber parts from a consideration of factors such as service temperatures and pressures, chemical resistance, etc., the final material specification should also include the durometer. Because durometer is one variable that is present in all different forms of a gasket or rubber part, buyers should specify the correct durometer to ensure the needed density, flexibility and pliability in the finished part.

CRG’s experienced sales team can help buyers of gaskets and rubber parts to select the appropriate material and durometer for their application. We carry an extensive stock of elastomeric and other materials in a wide range of durometers that can service a variety of applications. All parts we produce from these materials are made using proven processes in our ISO 9001:2015 certified facility.

To find out more about how we can assist you with gaskets and rubber parts, please contact our sales team at