Compression Set is the amount, expressed as a percentage of deflection, by which a rubber specimen does not return to its original thickness following the release of a compressive load. Compression set is the end result of a progressive stress relaxation, which is the steady decline in the sealing force that results when an elastomer is compressed over a period of time. In terms of seal life, a compression set of 15%-20% is like the seal dying, where as a compression set of 100% would be like death. Though it is still possible for a seal to perform after death (100% compression set), the parameters would have to remain unchanged and the application would need to be static. Swelling can also have a positive effect in relation to compression set.
For the sake of sealing applications, it is better to choose a material with a low compression set.
Compression set tests can be done through two different methods and are both recognized by multiple standards organizations. The two methods are; Method A (constant force) or the percentage of the original deflection for Method B (constant deflection). Method B is what is typically used for sealing materials in sealing applications. Below is a breakdown of each of these methods, including the calculations necessary to determine compression set.
|A||Compression Set Under Constant Force in Air||A constant compressive force of 1.8 kN (400 lbf) is applied to the specimen for a specified time at a specified temperature. Compression set is taken as the percentage of the original specimen thickness after the material is allowed to recover at standard conditions for 30 minutes.||CA = [(to – ti) / to] * 100|
where to is the original specimen thickness and ti is the specimen thickness after testing.
|B||Compression Set Under Constant Deflection in Air||Specimen is compressed 25% for a specified time at a specified temperature. Compression set is taken as the percentage of the original deflection after the material is allowed to recover at standard conditions for 30 minutes.||CB = [(to – ti) / (to – tn)] * 100|
where to is the original specimen thickness, ti is the specimen thickness after testing, and tn is the spacer thickness.
The following are examples of what can cause compression set failure in a sealing application
• High compression set material
• Low heat resistance of the material
• Too much squeeze to achieve the seal
• Incomplete curing (vulcanization) of O-ring material during production