**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.

Method | Name | Test Description | Calculation | Diagram |
---|---|---|---|---|

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. | C_{A} = [(t_{o} – t_{i}) / t_{o}] * 100where t _{o} is the original specimen thickness and t_{i} 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. | C_{B} = [(t_{o} – t_{i}) / (t_{o} – t_{n})] * 100where t _{o} is the original specimen thickness, t_{i} is the specimen thickness after testing, and t_{n} 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

• Shrinkage

• Too much squeeze to achieve the seal

• Incomplete curing (vulcanization) of O-ring material during production