Compression set is one of the most important properties to consider when selecting a rubber material for sealing or cushioning applications. Despite this, it is often overlooked. In materials science, compression set describes an elastomeric material’s ability to recover after being subjected to prolonged compression. Understanding this behaviour is essential for achieving reliable long term sealing performance. It also helps ensure rubber components continue to function as intended throughout their service life.
Components such as rubber seals, gaskets, O-rings, and vibration-damping products are frequently exposed to continuous pressure. To perform effectively, they must be able to recover their original shape after compression. Understanding compression set helps engineers and designers choose the most suitable material for their application. It can also help prevent premature component failure.
What Is Compression Set?
Compression set refers to the permanent deformation that occurs in a rubber component. This happens when the component is compressed for an extended period and then released.
Ideally, rubber should return to its original thickness and shape once the compressive force is removed. However, over time, some elastomers lose part of their elasticity and fail to recover fully. The amount of permanent deformation that remains is known as the permanent set.
In simple terms, the lower the compression set value, the better the material’s ability to recover after periods of compression.
Compression set is closely related to another phenomenon known as stress relaxation. While compression set measures permanent deformation, stress relaxation describes the reduction in force exerted by a compressed rubber component over time. Both properties can affect sealing performance and long-term reliability.
Why Compression Set Matters
Compression set is particularly important in applications where a rubber component must maintain a seal. It is also important where consistent cushioning is required over a long period.
If a material develops high compression set, it may no longer exert enough sealing force against mating surfaces. As a result, it may be unable to maintain an effective seal. This can lead to leaks, reduced performance, or complete component failure.
Compression set is particularly important in seals designed to retain liquids or gases over extended periods.
Common applications where compression set is critical include:
- O-rings
- Gaskets
- Static seals
- Weather seals
- Anti-vibration components
- Industrial sealing systems
Selecting a rubber compound with suitable compression set characteristics is essential for these applications. It helps maintain a constant force over time and supports reliable long term sealing performance.
How Compression Set Occurs
When rubber is compressed, the polymer chains within the material are temporarily deformed. Under normal conditions, these chains return to their original position when the load is removed.
However, prolonged compression, elevated temperatures, and environmental exposure can cause permanent changes within the material structure. Over time, the rubber loses some of its ability to recover, resulting in permanent deformation.
The material remains in a partially compressed state. This reduces its ability to provide the same level of performance as when it was new.
The severity of compression set is influenced by several factors, including:
- Rubber compound formulation
- Operating temperature
- Amount of compression
- Duration of compression
- Environmental conditions
- Ageing and material degradation
- Levels of compressive stress
How Compression Set Is Measured
Compression set is typically measured using standardised test methods such as ASTM D395 or ISO 815.
A standard compression set test places a rubber specimen under a specified compressive load and constant deflection for a defined period. The sample is then allowed to recover before measurements are taken.
The difference between the original thickness and the final thickness is then measured. This determines the amount of permanent deformation that remains after the load has been removed.
The result is expressed as a percentage.
A lower percentage indicates better recovery and improved long-term sealing performance. A higher percentage indicates greater permanent deformation.
For example:
- 10% compression set = excellent recovery
- 25% compression set = good recovery
- 50% compression set = significant permanent deformation
Actual acceptable values depend on the application and should always be assessed using relevant technical data.
The Effect of Temperature on Compression Set
Temperature has a major influence on compression set performance.
As temperatures increase, rubber materials tend to lose elasticity more quickly. Prolonged exposure to heat accelerates ageing and can significantly increase compression set.
This is one reason why material selection is so important. A compound that performs well at room temperature may struggle when exposed to elevated temperatures for extended periods.
Materials such as silicone rubber are often selected for applications involving continuous heat exposure. They can maintain sealing performance across a wider temperature range. Silicone-based products offer similar advantages.
Low temperatures can also affect performance. Compression set is typically associated with heat. However, some materials may become stiff at low temperatures. This can reduce their ability to recover effectively after compression.
Compression Set and Material Selection
Different rubber compounds exhibit different compression set characteristics and material properties. Understanding these differences is essential when selecting the most suitable material for an application.
Natural Rubber
Natural rubber offers excellent elasticity and resilience. However, it can be more prone to compression set when exposed to elevated temperatures for long periods.
EPDM Rubber
EPDM rubber generally provides good compression set resistance. It is widely used in sealing applications exposed to weather, water, and temperature fluctuations.
Nitrile Rubber (NBR)
Nitrile rubber offers good compression set performance while also providing excellent resistance to oils and fuels. This makes it a popular choice for automotive and industrial sealing applications.
Silicone Rubber
Silicone rubber is often selected for applications that require both excellent temperature resistance and low compression set. It is commonly used in medical, food-grade, and high-temperature sealing applications.
Neoprene Rubber
Neoprene provides balanced compression set performance. It also offers good resistance to weather, oils, and chemicals. This makes it suitable for a wide range of industrial applications.
As with many materials, there is no single rubber compound that is best for every application. The correct choice depends on operating conditions and performance requirements.
Does Compression Set Affect the Moulding Process?
Compression set is primarily a material property, but the rubber moulding process can also influence the final performance of a component.
Factors such as curing temperature, curing time, and mould design can affect how well a rubber compound develops its final mechanical properties. Improper curing may result in reduced elasticity, poor compression recovery, and increased compression set.
For this reason, material selection and manufacturing processes must work together to achieve the desired results.
Compression moulding, transfer moulding, and injection moulding are all capable of producing components with excellent compression set performance. Achieving this depends on selecting the correct material and processing conditions.
Designing for Compression Set
When designing rubber components, it is important to consider the amount of compression that will be applied. Designers should also consider how long the component is expected to remain under load.
Over-compressing a seal can increase stress within the material and accelerate permanent deformation. Similarly, selecting the wrong material for the operating environment can reduce component lifespan.
Designers should evaluate:
- Compression levels
- Operating temperatures
- Environmental exposure
- Expected service life
- Material compatibility
- The anticipated compressive load
Taking these factors into account helps ensure reliable long-term performance.
How Kea-Flex Can Help
At Kea-Flex, material selection is an important part of the design and manufacturing process. By understanding the specific requirements of each application, the team can recommend rubber compounds with suitable compression set characteristics. This helps ensure components deliver reliable long-term performance under their intended operating conditions.
With in-house tooling and expertise in compression, injection, and transfer moulding, we can support projects from material selection through to production. This integrated approach helps deliver high-quality, reliable rubber components that continue to perform under demanding operating conditions.
