Compression set is one of those properties that sounds technical but has a very real impact on whether your gasket still seals a year from now. At NEDC, we cut low compression set gaskets every day, and I have seen plenty of cases where a gasket material was chosen for its initial sealing performance only to fail six months later because it took a permanent set. In this post, I will explain what compression set is, how it is tested, why it matters, and how to minimize it in your gasket design. For our full product overview, see our low compression set gaskets pillar page.
What is compression set in gasket materials?
Compression set is the permanent loss of thickness that happens when a gasket material is compressed over time. When you compress a foam or rubber gasket, it gets thinner. If the material has good compression set resistance, it recovers most of its original thickness when the pressure is removed. If it has poor compression set resistance, it stays thinner permanently. Rubber compression behavior is what determines whether a seal will hold up over months or years of service.
The compression set value is expressed as a percentage. Here is a simple example: if a gasket starts at 0.250 inches thick, and after being compressed for 22 hours at 158F it measures 0.225 inches permanently, the compression set is 10%. That means the material lost 10% of its thickness permanently. A lower percentage is better. Materials with compression set values under 10% are considered excellent, while values above 25% are generally poor for long-term sealing.
Why compression set matters for long-term sealing
Compression set matters because a gasket that has taken a set is thinner than it was when installed. If the gasket was originally compressed 30% to fill a gap, and it takes a 20% compression set, it now only fills 10% of the gap. In many cases, that is not enough to maintain a seal.
This is especially critical in three scenarios. First, in enclosure gaskets that are under sustained pressure for months or years, the material slowly takes a set and the sealing force decreases. Second, in applications where the gasket must fill a gap and compensate for surface irregularities, a material that takes a set loses its ability to conform. Third, in enclosures that are opened and closed repeatedly, a material with poor compression set recovery will not re-seal after the second or third closing. For more on how compression rates affect sealing, see our page on compression rates in rubber gaskets.
Compression set test methods explained
There are three primary test standards we reference at NEDC for compression set testing:
ASTM D395 (solid rubber): This is the standard for solid rubber materials. Method B compresses a sample to 25% of its original thickness for a set time (typically 22 or 70 hours) at a specified temperature, then releases it and measures the permanent thickness loss. The result is a percentage. For solid silicone rubber, compression set values of 15-25% at 70 hours and 350F are typical for good quality materials.
ASTM D1056 (sponge and foam): For cellular elastomers like silicone sponge gaskets, ASTM D1056 includes compression set requirements as part of the material grade specification. The test is similar in principle to ASTM D395 but adapted for the lower compression forces and cellular structure of foam materials.
ASTM D3574 (urethane foam): This standard covers flexible polyurethane foam, including PORON urethane foam. PORON 4701-40 and 4701-50 typically show compression set values under 10% at 50% compression and 158F for 22 hours.
Materials with excellent compression set resistance
Not all gasket materials are equal when it comes to compression set. Here are the materials we cut that perform best:
- Silicone foam (BF-1000, BF-2000): Compression set values under 5% at standard test conditions. These are among the best materials available for long-term sealing.
- PORON urethane foam (4701 series): Compression set values under 10%. PORON is specifically engineered for cyclic compression and maintains thickness recovery over thousands of cycles.
- Norseal silicone rubber (9030-9070 series): Compression set values in the 15-20% range. These are solid and sponge silicone materials that perform well across a wide temperature range.
For applications that also require soft compression, these same materials are among the softest we cut. See our soft gaskets page for more on ultra-soft materials that also have low compression set.
How to minimize compression set in your gasket design
Minimizing compression set comes down to four factors:
- Material selection: This is the single most important factor. Choose silicone foam or PORON urethane foam for the best compression set resistance. Avoid generic sponge materials with compression set values above 25%.
- Compression percentage: Most foam and sponge gaskets are designed to be compressed 25-50%. Compressing beyond the material’s recommended range accelerates compression set. Stay within the manufacturer’s recommended compression range.
- Temperature exposure: Elevated temperatures accelerate compression set in all elastomers. If your gasket will see high temperatures, choose a silicone-based material rather than urethane or nitrile, because silicone maintains its compression set resistance at higher temperatures. See our low temperature gaskets page for the other end of the temperature spectrum.
- Cycling behavior: If the gasket will be repeatedly compressed and released, choose PORON urethane foam. It is specifically engineered for cyclic compression and maintains recovery over thousands of cycles.
More information on low compression set gaskets
If you need help selecting a gasket material with low compression set, or if you need test data for a specific material grade, contact us at sales@nedc.com. We cut low compression set gaskets from silicone foam, PORON urethane foam, and Norseal silicone rubber every day, and we can provide compression set data, material samples, and design recommendations for your specific application.
FAQ
What is compression set in gasket materials?
Compression set is the permanent loss of thickness that occurs when a gasket material is compressed over time. It is expressed as a percentage: if a 0.250 inch gasket loses 0.025 inches permanently after compression testing, the compression set is 10%. Lower values mean better thickness recovery, which is critical for maintaining a seal over the life of the enclosure.
How is compression set tested in rubber and foam gaskets?
Compression set is tested using ASTM D395 for solid rubber, ASTM D1056 for sponge and foam elastomers, and ASTM D3574 for polyurethane foam. The test compresses a sample to a specified percentage of its original thickness for a set time at a specified temperature, then releases it and measures the permanent thickness loss. Common test conditions are 22 or 70 hours at temperatures from room temperature to 350F.
What materials have the best compression set resistance?
Silicone foam materials like BF-1000 and BF-2000 have compression set values under 5%, making them the best available. PORON urethane foam (4701 series) typically shows compression set under 10%. Norseal silicone rubber materials show compression set in the 15-20% range. Among solid rubbers, high-quality silicone compounds have compression set values of 15-25% at elevated temperatures, which is better than most other elastomers.
How do I minimize compression set in my gasket design?
Minimize compression set through material selection (choose silicone foam or PORON urethane foam), staying within the recommended compression percentage (typically 25-50%), accounting for temperature exposure (silicone performs better at high temperatures), and choosing PORON for applications with repeated compression cycling. Material selection is the single most important factor.