Compression set test

compression set test

The compression set test evaluates the ability of an elastomer to return to its original shape after a standard test specimen of the elastomer has been compressed by a certain amount for a certain period of time. The test is often carried out at elevated temperatures. This is intended to simulate the recovery behavior of seals during use.

What does a low or high compression set mean?

The compression set test can be carried out using different test methods and test specimen dimensions. For example, the compression set test can be carried out using an adjustable constant force (ASTM D-395, Method A) or by compressing the standard test specimen to a specified constant compression ( ASTM D-395, Method B). In this test method, the test specimen is compressed by 25% of its original thickness. However, the result of the compression set test to be evaluated, the so-called compression set, is always given as a percentage. The smaller the percentage of compression set, the better the recovery of the material under pressure and heat and thus the sealing ability of the elastomer used. A compression set of 50% would mean that, based on the 25% compression of the test specimen, half of the deformed height remains permanently deformed after the end of the test. With a compression set of 100%, no recovery of the material would be noticeable.

Does the sealing material always have to have a low compression set?

For static applications where the seals (e.g. flat gaskets , O-rings , molded parts ) are pressed together between flange surfaces, the loss of sealing ability can lead to a leak. Therefore, a low percentage of compression set is desirable for such applications.

Although low compression set is desirable, a static seal can still fulfill its sealing function even when the material has almost completely lost its resilience and has a compression set of 90%. However, this is assuming that the system pressure acting on the seal is constant and that no other forces act on the seal that could damage the seal. Material or volume swelling due to contact with liquid media can also compensate for high compression set. On the other hand, situations where contact with a liquid causes the material to shrink will most likely lead to seal failure.