Compression Set: Why is it Significant for Rubber Parts?




You might already be familiar with the various characteristics of rubber materials and some of the testing methods.

For example, there is hardness, which is the resistance of a rubber to the penetration of another harder substance. There is also tear strength. It describes the force opposing the further tearing of a rubber piece that has already been torn. Of course, there is also compression set, which is another valuable characteristic of rubber parts.

But why is compression set so important? Is it only critical for specific items like seals and gaskets, or is it relevant for all materials? When evaluating the rubber parts that you need, a compression set is an essential characteristic to consider. Here’s why.

What Does Compression Set Mean?

Compression set testing records the ability for rubber parts to return to their original thickness after being prolonged to pressure stresses at certain deflections and temperatures. As materials are compressed over time, they will lose their ability to return to their original state.

The part’s loss of resiliency can reduce the capability to perform over long periods of time. This is particularly relevant for parts like O-rings, seals and elastomeric gaskets. It can result in leaks or damage to units if the item is unable to resist permanent deformation in specific settings.

How Is Compression Set Typically Measured?

The results from a compression set test are displayed as a percentage. The general rule is that the lower the rate, the more likely the material can resist deformation under specific temperature ranges and deflections.

There are multiple test methods that exist for determining the compression set of rubber parts. There are standards in place for the standardized testing process for compression set allowing the comparison of different raw rubbers and elastomer compounds. These uniform test methods are detailed, for example, in ISO 815, the older DIN 53517, ASTM D 395 and ASTM D-1414. Two of the most common include ASTM D-395 and ASTM D-1414. Both of these tests use the same equipment but assess different materials. The procedure is as follows:

1. The rubber part is measured to determine their height.

2. Spacers that are calculated to 75% of the height of the rubber part in question are needed.

3. The rubber parts is placed between metal plates with the spacers. The metal plates are tightened until they meet the spacers.

4. All of the items are placed into a chamber for a certain period of time at a specific temperature. The prevailing temperature and duration of the test may differ from test to test. So, for instance, the test could take place at:


100 °C/24 h

  70 °C/24 h

175 °C/22 h

175 °C/24 h

200 °C/70 h


5. After the rubber part has been loaded for a certain period of time (see point 4.), the metal plates are removed. The rubber part should then rest at room temperature for 30 minutes.  

6. The height is measured to record the result. The formula for calculating the compression set is:


Formula for Calculating Compression Set

Within ASTM D-395 and ASTM D-1414 are multiple methods and options, which are specific to the rubber parts being tested. For example, ASTM D-385 Method B/Option 2/Type 2 refers to three rubber parts that are 0.24 inches thick and 0.51 inches in diameter. Meanwhile, ASTM D-1414 Method B is specifically for testing O-ring or O-ring cross-sections.

It is important to keep in mind that test conditions are always specified in the material’s technical information. Comparisons can only be made for the results of rubber compounds from tests conducted under the same conditions.

It’s not unusual to be confused by the variations of testing. However, it’s one of the reasons why you should consult with experts such as HEPAKO, who are knowledgeable in the test methods and results of all rubber parts.

Why Is the Compression Set Result Important for Rubber Parts?

It’s critical to understand how your rubber parts will behave under specific conditions. There are many different rubbers and many more different rubber compounds, so it is important to narrow down the choices by at least knowing the properties of each rubber.

For example, if you’re choosing rubber for sealing or a gasket, you might want to choose the thinnest material that performs the way you need it to. Thicker items can introduce problems that impact the ability of the seal or gasket to perform at peak performance because they might be too stiff and inflexible. They also increase the chances of leaks occurring. But, if you thin out the material too much, then it won’t provide enough energy to seal or resist forces, which is why you need to understand the compression set measurement that is best for your situation.

What Rubber Parts is Compression Set Most Important For?

The compression set test is particularly important for gasket materials and O-rings. The assessment measures how much the rubber part will resist, and so a material that has a high compression set will also relax quicker. The resistance is critical for long-term sealing. The higher the back pressure of the rubber part, the greater the ability of the seal to keep dust, liquids or other foreign particles out of the application.

The type of rubber material is also significant. For example, silicone rubber materials are more resistant to heat compared to other elastomers.

What Rubber Parts Have a Low Compression Set?

Not all rubber parts have low (i.e. good), compression sets. There are products that require constant use and therefore require a lower measurement. It allows the item to return to its original shape and density after being compressed, which means it will have a longer lifespan and reduce the need for frequent replacement.

Both solid silicone rubber and liquid silicone rubber (LSR) are considered to be the best materials when a low compression set is needed. In addition, silicone rubber has a distinct advantage over other types of rubber because its outstanding compression set properties remain relatively stable over a wide temperature range whereas the same properties in many other materials deteriorate significantly in cold or heat.

Two other very important rubber materials featuring low compression sets are FKM and EPDM. FKM is highly resistant to swelling when exposed to specific elements, making it suited for a wide range of applications, like complex vacuum services, diaphragm pumps or self-extinguishing devices. The compression set properties of FKM are very good at room temperature or hot conditions. Meanwhile, EPDM is ideal for sealing brake systems, hot water, and steam. It is also widely used as a material for diaphragms or gaskets. If the EPDM material is peroxide cured, it is even suitable for higher temperature exposure and features significantly improved compression set performance.

Who Can Help You Choose the Correct Rubber Parts?

The compression set test can be confusing for many people. As there are many variables that make up the assessment, it can be difficult to know the correct settings and the best measurement for the rubber part. However, it is critical to understand this characteristic so you know if the item will suit your needs.

You don’t need to store all of this knowledge in your head or have a thorough understanding of all of the individual properties. That’s where we can help. HEPAKO manufactures rubber materials in accordance with your specific requirements and provides the results of all the tests it conducts. For many businesses around the world, HEPAKO is the first contact when precise rubber articles are required. If you need rubber parts for your organization, get in touch with HEPAKO today to discuss how we can help you.

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