When a silicone foam gasket is compressed, it changes shape. That part is expected. The more important question is what happens after the pressure is removed. Does the foam return close to its original thickness? Or does it remain partially compressed? That permanent loss of thickness is commonly described as compression set. In simple terms:
Compression set measures how much deformation remains after a material has been compressed under defined conditions and then allowed to recover.
For a gasket, this matters because sealing depends on maintaining contact between the foam and the mating surfaces. A gasket may work perfectly when it is new. The real test is whether it continues to work after thousands of hours under compression, temperature changes, vibration, and environmental exposure.
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Imagine a silicone foam gasket installed between two panels. At installation, the foam is compressed and pushes against both surfaces. That contact pressure helps create the seal. Now leave the assembly in service for a long period. If the foam permanently loses too much thickness, the contact pressure may decrease. The result can be:
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This is why compression set is particularly important in applications where the gasket is continuously compressed. Examples include:
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These terms are related, but they are not exactly the same.
Compression set focuses on the permanent deformation remaining after compression. A lower value generally means less permanent deformation.
Compression recovery focuses on how well the material returns toward its original shape after the compressive load is removed. In practical gasket design, both concepts matter. A material that recovers well can continue to maintain useful contact pressure after repeated or prolonged compression. However, actual performance depends on the testing conditions. A foam may recover well at room temperature but behave differently after prolonged exposure to elevated temperature.
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A typical compression-set test involves compressing a material specimen to a defined deformation, maintaining that compression for a specified period, and then allowing the sample to recover. The remaining deformation is then measured. A simplified concept is: Original thickness → Compressed thickness → Recovery → Final thickness |
The exact test conditions matter enormously. A test specification may define:
Therefore, two compression-set numbers should not automatically be compared unless the test conditions are comparable. This is particularly important when evaluating materials from different suppliers. |
Suppose a silicone foam sample starts at: 10 mm It is compressed to: 7.5 mm and held under the specified test conditions. After recovery, the material measures: 8.5 mm The foam has not returned completely to its original thickness. That remaining deformation is the basis for calculating compression set according to the applicable test method. The exact formula and test procedure depend on the standard being used. For purchasing purposes, the important point is this: Always evaluate the number together with the test method and conditions.
Compression set is not controlled by one factor. Several variables interact.
| 1. Material Formulation |
The silicone formulation has a major influence on how the foam behaves after prolonged compression. Polymer characteristics, curing chemistry, additives, and processing conditions can all affect recovery. This is one reason two products that look almost identical can show different long-term performance. |
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| 2. Foam Cell Structure |
Cell structure also matters. Closed-cell and open-cell silicone foams have different internal architectures. The internal cells respond to compression and recovery differently. For environmental sealing, closed-cell silicone foam is often considered because it combines the sealing characteristics of the cellular structure with silicone's resistance to heat, UV, ozone, and aging. Open-cell foam may be preferred for applications where softness, airflow, or cushioning is more important.   |
| 3. Density |
Density can influence the mechanical response of silicone foam. As discussed in our previous Silicone Foam Density Guide, density should not be interpreted as a direct measure of quality. A higher-density foam may provide greater mechanical support, while a lower-density foam may compress more easily. But density alone cannot predict long-term compression set. For a gasket application, actual compression and recovery data are more useful. |
| 4. Hardness |
Hardness can also affect how the foam responds to compression. A firmer material may resist deformation differently from a very soft foam. However: Hardness ≠ compression set. A particular Shore value does not tell you exactly how the material will behave after thousands of hours under compression. This is why hardness and compression-set data should be considered separately. |
| 5. Temperature |
Temperature is one of the most important environmental factors. Silicone is well known for its broad temperature capability, but that does not mean compression behavior remains identical at every temperature. Long-term exposure to elevated temperatures can affect:
For applications involving heat, compression testing should ideally reflect the actual service environment. |
| 6. Compression Level |
The amount of compression matters. A foam compressed slightly and a foam compressed aggressively are not experiencing the same mechanical conditions. If the gasket is designed for excessive compression, the material may experience greater deformation and potentially greater permanent set. This is why gasket design should establish a reasonable working compression range before selecting the material. |
| 7. Time |
A gasket installed for one day and a gasket installed for ten years face very different requirements. Long-term applications should consider:
For this reason, accelerated testing can be useful during material development, but the test conditions need to represent the expected application as closely as practical. |
This is where the laboratory specification becomes an engineering issue. Consider an enclosure gasket. At the beginning: Foam compressed → Contact pressure created → Seal established After prolonged use: Foam recovers sufficiently → Contact pressure maintained → Seal continues But if permanent deformation becomes excessive: Foam loses thickness → Contact pressure decreases → Sealing margin becomes smaller The exact relationship is more complicated than this simplified diagram, but the principle is important. A gasket is not successful simply because it seals on day one. It needs enough sealing margin to remain effective throughout its intended service life.
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There is no single compression percentage that works for every silicone foam product. The appropriate working range depends on:
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For example, a 5 mm foam gasket used in an electronics enclosure may require a completely different compression design from a 15 mm foam seal used in industrial equipment. The supplier's technical recommendation should therefore be considered alongside the actual assembly design. |
Thickness deserves special attention. A thicker foam layer can accommodate larger dimensional variations and may provide greater compliance. But that does not mean simply increasing thickness will solve a sealing problem. If the gasket is too thick for the available assembly space, it may be excessively compressed. If it is too thin, it may not compensate for the gap. A good gasket design balances: Thickness + Compression + Gap + Clamping Force rather than optimizing any one value independently.
These three properties are often discussed together. They should not be confused.
| Property | What It Describes | Why It Matters |
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| Density | Mass per unit volume | Influences mechanical structure and weight |
| Hardness | Resistance to indentation | Helps describe material firmness |
| Compression Set | Permanent deformation after compression | Indicates long-term recovery behavior |
A material can have:
Another material can have:
That is perfectly possible. Material selection therefore requires more than one number.
If your application requires continuous sealing, consider the following sequence.
| Step 1: Define the Gap |
Measure the real minimum, nominal, and maximum gap. Do not design around the nominal value alone. |
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| Step 2: Define the Compression Range |
Determine how much compression the gasket will experience during assembly and throughout dimensional tolerances. |
| Step 3: Determine the Available Force |
Make sure the assembly can generate enough force to compress the selected foam. |
| Step 4: Identify the Environment |
Consider:
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| Step 5: Review Compression Data |
Ask the supplier for relevant compression-set or recovery information. Do not rely only on hardness or density. |
| Step 6: Test the Actual Assembly |
This is the final step that is often overlooked. A laboratory specimen is not the same thing as a finished gasket installed in an enclosure. The actual assembly should be evaluated whenever the application is critical. |
Battery systems may experience:
A sealing material must continue to perform after prolonged exposure to these conditions.
Outdoor equipment may remain sealed for years. The gasket can be exposed to:
A silicone foam with suitable recovery characteristics can help maintain the sealing interface over time.
Machines often introduce another variable: vibration. Repeated movement can place additional demands on the gasket. Compression recovery, environmental resistance, and mechanical stability therefore need to be considered together.
Electronic equipment may require a softer interface to protect components while still providing environmental protection. In these applications, the designer needs to balance: Softness + Compression + Recovery + Protection
| Mistake 1: Looking at the Number Without the Test Conditions |
A compression-set value without the corresponding test conditions tells you very little. Always ask: At what temperature?For how long?At what compression?According to which test method? |
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| Mistake 2: Assuming Low Density Means High Compression Set |
Density alone does not determine long-term recovery. Material formulation and cell structure also matter. |
| Mistake 3: Assuming Harder Foam Has Better Recovery |
Hardness does not directly tell you how much permanent deformation a material will retain. |
| Mistake 4: Testing Only at Room Temperature |
A gasket intended for elevated-temperature service should be evaluated under representative conditions. |
| Mistake 5: Ignoring the Actual Assembly |
A material may perform well in a standardized laboratory test but still be unsuitable because the gasket geometry, gap, or clamping force is wrong. |
For a serious engineering project, a useful material inquiry should include:
| Material |
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| Mechanical Properties |
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| Dimensions |
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| Environment |
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| Processing |
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This gives the manufacturer enough information to recommend a suitable material rather than simply quoting the nearest standard grade.
Before approving a material, ask: ☐ What is the minimum and maximum gap?☐ How much compression is available?☐ What is the expected clamping force?☐ Will the gasket remain compressed continuously?☐ What temperatures will it experience?☐ Will it be exposed to water, UV, ozone, or chemicals?☐ Is the material open-cell or closed-cell?☐ What are its density and hardness?☐ What compression-set data are available?☐ Has the actual gasket geometry been tested? If these questions have clear answers, material selection becomes much more straightforward.
Compression set is the permanent deformation that remains after silicone foam has been compressed under defined conditions and allowed to recover.
Generally, lower compression set indicates better dimensional recovery after compression. However, the number must be evaluated together with the test conditions and the requirements of the application.
Silicone foam can provide good compression recovery, but actual performance varies with formulation, density, cell structure, hardness, compression level, temperature, and aging conditions.
Density can influence compression behavior, but it does not directly determine compression set. Material formulation and structure are also important.
Yes. It is particularly important when the gasket remains compressed for long periods and must continue maintaining contact pressure.
It can be. The suitability depends on the specific silicone foam grade, cell structure, compression design, environmental conditions, and required service life.
Start with the correct material grade, appropriate thickness, controlled compression, suitable hardness and density, and representative environmental testing. Good gasket geometry is just as important as the material itself.
Compression set is one of those properties that can seem like a laboratory specification until a gasket fails after years of service. For a short-term cushioning application, it may not be the first number you look at. For a gasket compressed inside an EV battery enclosure or outdoor electrical cabinet, it can become a major part of the material-selection decision. The important point is that compression set should never be evaluated alone. Density tells you about the material's mass and cellular structure.Hardness tells you about indentation resistance.Compression set tells you how much deformation remains after prolonged compression. Together, these properties give engineers a much clearer picture of how silicone foam may behave in a real application. And ultimately, the best material is not the one with the most impressive datasheet. It is the one that continues to perform after it has been installed, compressed, heated, cooled, exposed, and put to work.
