Seals, gaskets and grommets do their job by pushing back. When a rubber part is squeezed and stays squeezed for months, it can lose some of that push. Compression set is the measurement that tells you how much.
What is compression set?
Compression set is the permanent deformation left in a rubber sample after it has been held under compression and then released. It is reported as a percentage. A result of 0% means the rubber came all the way back to its original thickness. A result of 100% means it stayed completely flat and did not recover at all.
Lower numbers are better. A seal with a low compression set keeps pressing against its mating surfaces. A seal with a high compression set goes flat, and a gap opens up.
How the test works (ASTM D395)
ASTM D395 is the standard test method for compression set. In the common constant-deflection method, a small rubber disc is squeezed to a fixed percentage of its original thickness between two plates. The squeezed sample is held at a set temperature for a set time, often 22 hours at an elevated temperature such as 70 °C or 100 °C. The right conditions depend on the material and on the customer specification.
The sample is then released and allowed to rest. Its thickness is measured again. Compression set is the share of the original deflection that did not come back:
Compression set (%) = (original thickness – final thickness) / (original thickness – squeezed thickness) x 100
Why seals fail from high compression set
- Heat. High temperature speeds up the chemical changes in rubber that cause permanent set.
- Time under load. A part that is squeezed for years can set even at moderate temperature.
- The wrong compound. Some rubbers resist set much better than others at a given temperature. Compound choice matters as much as part design.
- Too much squeeze. Over-compressing a seal in its groove raises stress and speeds up set.
How to get lower compression set
- Choose a compound rated for the real operating temperature and fluids, not just the nominal ones.
- Ask for compression set data at your temperature and time, not only a room-temperature figure.
- Design the groove so the seal is compressed within the recommended range.
- Ask the supplier to test production lots against the same method, so the number on the data sheet matches what you receive.
What to put on your drawing
If a part has to seal, put a compression set requirement on the drawing: the test method (ASTM D395), the temperature, the time, and the maximum percentage. That gives your supplier and your inspectors one clear target.
See our page on the ASTM D395 compression test, and browse the other rubber testing videos in our Video Library.
Have a part that needs to hold its shape or pass a spec? Send the drawing or sample requirements to estimator@coirubber.com and our engineering team will review it. Coi Rubber has been in business since 2003, is US owned and operated, and our Wuhu, China manufacturing facility is IATF 16949 certified.