Compression Set

Significance and Test Method

Elastomers are not ideal elastic materials. If a component made of an elastomer material is compressed over a longer period of time, it does not completely return to its original shape after it has been released. The component remains partially permanently deformed. If the deformation takes place at high temperatures, this effect is intensified, even if the upper temperature limit typical for the elastomer has not yet been reached.

Significance for Application Technology

Depending on the test parameters, the compression set determined allows conclusions to be drawn about the quality and degree of cross-linking of a finished part. Thus, it is possible to evaluate and compare different elastomer formulations. If the tests are carried out at different temperatures for an appropriate length of time, we can also use the compression set values to assess the long-term behaviour of the elastomer components or provide a service life straight line.

The Compression Set Test

The compression set is usually tested according to the standards ISO 815-1, DIN ISO 815-1 (this replaces DIN 53517, which is still noted in old data sheets or specifications) or ASTM D395. The CS test procedure is carried out at either ambient or elevated temperatures, depending on the standard and requirements. Measurements at low temperatures are defined in (DIN) ISO 815-2 (see also compression set at low temperatures). The CS can be determined both with standard test specimens as well as with finished parts.

General Procedure of the CS test in accordance with relevant standards

In the standardised CS test with elevated temperature, a precisely defined elastomer specimen is

  • measured at its initial height
  • compressed in a defined manner using a device, usually by 25 % of its initial height
  • exposed to a defined temperature for a defined period of time, often 24 hours, in a laboratory oven in the compressed state
  • then released and cooled in a defined way (depending on the method, cooling takes place in the compressed or in the released state)
  • and measured at its new height after a defined time

The compression set describes the remaining part of the initially applied compression in percent. It therefore refers to the deformation trajectory and not, as often assumed, to the initial height of the specimen.

Downloads

For more information on compression set, its measurement and interpretation, see our technical reports and the comparative table of standards for CS testing.

This detailed technical report tells you more about compression set testing of elastomers, CS test devices, the purpose of the test and the interpretation of test results.

In order to be able to make supplier-independent statements on the possible service life of standard NBR O-rings, which also include typical batch and production-related variations, long-term tests on NBR O-rings were carried out and evaluated by the Richter O-ring test laboratory over a period of 2 years. Important findings were obtained regarding the influence of temperature, cord thickness and formulation.

If a seal shows very high permanent deformation well before the end of its service life, this can cause seal failure. In the technical article, you will learn more about permanent deformation as a cause of failure, the mechanisms that can lead to premature and greatly increased permanent deformation, prevention and testing options.

If a seal shows very high permanent deformation well before the end of its service life, this can cause seal failure. In the technical article, you will learn more about permanent deformation as a cause of failure, the mechanisms that can lead to premature and greatly increased permanent deformation, prevention and testing options.