By Kate Gluck on 1/15/2021

Preventing Gasket Failure Part 1: Compression Set

Die cut foam gaskets are small but mighty. They reduce noise, vibration, and harshness. They cushion against shock. They seal out elements. They insulate against heat and sound. In short, they play a critical role in the overall performance of the part to which they adhere – whether that be an automotive headlight seal, a sound reduction panel in a dishwasher, or a compressor or blower in an HVAC unit. And yet, with improper design or incorrect material specifications, gaskets can fail… rendering the more expensive equipment and parts that they support ineffective.

There are several things that can impact the longevity of a gasket that can be addressed with proper material selection. Among those are, in no particular order:

Environment (temperature, moisture, UV, chemical exposure, etc.)
Time
Pressure (intermittent or constant)
Adhesive
Durometer
Compression Set

This article will focus on compression set.

What is compression set?

In the simplest terms, compression set is the measurement of a foam or rubber material’s ability to return to its original thickness after being subjected to a compressive force or load over a given period of time. Said differently, it is the amount of permanent deformation the material sustains after being subjected to constant or intermittent force. Foams and elastomers with “good” or “low” compression set resistance spring back once the load is released, whereas materials with a high compression set will not return to their original thickness.

Low numbers indicate high resilience. High numbers indicate that the material is more likely to remain compressed. As an example, a compression set of 15% indicates that the material regained 85% of its initial thickness.

Why is compression set resistance important?

While it is only one of several factors to consider when selecting gasketing material, compression set is essential to understanding how foams and elastomeric materials will perform. When a die-cut foam or elastomer is used as a seal, gasket, or cushion, the loss of resiliency associated with a high compression set can reduce the long-term performance of the part by producing gaps, leaks, or application failures. It is more important to consider in applications where there will be periodic stress on the gasket than in applications where the force will be constant. When there is a constant force, there is less need for material recovery properties.

What factors can impact compression set requirements?

It is important to note that the “right” compression set is the compression set that fits the intended application. Some applications need a lower compression set than others.

Temperature is the single biggest factor that can impact the compression set of an elastomeric material, particularly when it comes to closed-cell foams. As an example, when closed-cell foams are compressed over a period of time, the gasses in the bubbles that give the foam its elasticity begin to permeate out. When you add excessive heat or fluctuations in thermal cycles it will accelerate the release of those gasses, thus increasing the speed at which the material loses its elasticity. Due to the way it is constructed, open-cell material reacts differently to heat. Its ability to maintain its elasticity has more to do with the material makeup and the material’s ability to resist heat than it does the air structure. Regardless of material makeup, any gasket material that takes a compression set at elevated temperature will quite often stop functioning as a gasket over time.

Pressure and material compatibility are also factors when selecting gasket material. As an example, if you are trying to seal out the air in a low-pressure application like an HVAC seal that offers just enough air pressure to allow the air to flow through the ductwork, you need the gasket to be able to withstand at least as much pressure as the amount of pressure that is being applied to the gasket. As you compress the gasket, it will push back against the two surfaces on either side. The more pressure applied, the harder the gasket will push back. As the material loses its elasticity, it creates a mechanical failure of the gasketing material that reduces its ability to push back against the squeeze causing the gasket to fail.

How is material tested for compression set?

There are varying ASTM standards for testing compression set — each having its own specific parameters. Among these are ASTM D395, ASTM D1056, ASTM D-1414, and ASTM D3574.

One example is the ASTM D395 Method B test, in which a sample is compressed 25% and held in place for a set amount of time at an increased temperature. Once the material is cooled to room temperature and the force released, the sample's thickness is measured. "Compression set" is the percent of compression that has been permanently lost. Learn more here.

When looking at material data sheets to compare compression set from one material to the next, it is important to make sure that they are being measured against the same standards. Each element of the test including how much the sample is compressed, the temperature at which it was tested, how long it is kept at an elevated temperature, and recovery time before the material is remeasured will impact the end result. When you are looking at test results based on different standards, the numbers you are looking at will not provide an “apples to apples” comparison. A key misunderstanding of published compression set data is that published results are based on standard test sample thicknesses. A Type 1 test uses a 12.5mm thick sample, a Type 2 test uses a 6.0mm sample. Quite often the gasket thickness used is not either one of these thicknesses and the compression set experienced in actual use may vary widely from published data. This is why it is important to conduct real-world testing to determine a material’s suitability for the intended use.

Which has a better compression set: open or closed-cell foam?

As a general rule, open-cell materials are more inclined to spring back after being compressed than their sturdier closed-cell counterparts. Their lower (better) compression set makes them ideal for sealing out air and dust in products that see heavy usage. This is not to say that closed-cell foam cannot be effective, however. Learn more about the differences between open and closed-cell foams in our blog: Open Cell & Closed Cell Foams: Keys to Making the Right Selection and in Machine Design’s article: Proper Gasket Design Can Simplify Manufacturing and Extend Product Life.

Examples of materials with low compression set

In addition to silicones, neoprenes, and solid rubber, cellular (poly)urethane such as the Rogers Poron® family of products is an example of open-cell foam with excellent compression set resistance. A few examples are shown here. For more material datasheets, please refer to the resources section of our site.

Foam material selection aides

At JBC we have over 30 years of experience with converting flexible materials into custom parts. If you need assistance selecting the right material for your application, our experts are here to help -- just contact us using this easy form, we’ll get back to you as soon as possible. In the meantime, please feel free to peruse the following tools, CONTACT US using this easy form or give us a call at 440-327-4522.

Rubberlite interactive tool:
Rogers Corporation: Technical Sealing Guide
Worldwide Foam: Product Search
JBC’s Foam & Sponge Rubber Material Data Sheets

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