Failure due to compression set occurs in both static and dynamic rubber items. Compression set failure looks similar to abrasion failure, in which there is a flattening on one side of the product’s cross-section. With compression set, this permanent flattening of the cross-section can be seen on both sides of the item at the original points of compression. The seal usually hardens and assumes the shape of the gland. In some instances, surface cracks may also be visible.
At the most basic level, use of an elastomer with inherently poor compression set properties or a compound that has not been properly cured can doom a seal. Even if materials are initially acceptable, the aforementioned hardening (and an accompanying loss of elasticity) can be caused by temperature increases during service. These thermal increases can be frictional (i.e. as a result of the buildup of friction-generated heat), environmental (as a result of external elements), and/or systemic (as a result of system fluids).
A limited degree of fluid-induced volume swell may help offset the effects of compression set. On the other hand, some fluids may cause excessive swell, which can exacerbate compression set.