Which is better, low or high compression set?
Compression set results can be confusing because they are calculated in different ways under different testing methods and specimen dimensions. Tests may be run either by applying a known constant force to the test specimen (for instance ASTM D-395, method A) or by compressing the specimen to a known constant deflection (ASTM D-395, method B, which compresses the specimen by 25% of its original thickness). Although, whatever the test, results are always expressed as a percentage and the lower the percentage, the better the material’s ability to seal.
How come lower compression set is better?
Compression set measures residual deformation, so it basically measures the percentage of deflection that the specimen failed to recover (i.e. for how much the specimen failed to return to its previous state).
So if the specimen completely recovers to its original thickness, there is not residual deformation and the compression set is therefore 0%. On the other hand, if the specimen remains completely squeezed and doesn’t recover at all, it would have a 100% compression set. Thus a good compression set has a low percentage value and a poor compression set has a high percentage value.
Why is low compression set desirable in sealing applications?
For static applications where a gasket or a sealing item are compressed, the loss of ability to seal may cause leakage, hence for such applications a low compression set value is desirable. Compression set values increase over time and with temperature.
Although low compression set is desirable, it all depends on the final application. For instance, an O-ring may continue to seal even when it is almost unable to recover and has 90% compression set, assuming that system pressure remains steady and that no forces that might physically damage the seal are present (especially the case with FKM and low temperature sealing), because swell caused by contact with fluid may compensate for increased compression set. On the other hand, in a situation where fluid contact shrinks the material, high inherent compression set will most certainly lead to sealing failure.