When designing seals and gasket components, understanding the relationship between compression set, recovery, and stress relaxation is crucial for performance. These properties determine whether an elastomeric material will maintain a consistent seal, absorb energy properly, and perform reliably over time.
Because these characteristics are closely related, selecting materials with an ideal balance of compression properties can be difficult. In this article, we’ll break down what each of these terms means, how they are related, and what engineers need to know when selecting foam for gaskets, seals, padding, and vibration absorption.
Need help designing, testing, or prototyping materials? American Flexible Products has more than 30 years of experience helping engineers choose the right materials and ensuring they meet specifications for any design. Contact us today for expert guidance.
What is Compression Set?
Compression set (c-set) is a property that describes the extent to which a material recovers after being compressed. It represents the permanent loss of thickness after the compressing force is removed.
Compression set is one of the key factors that determines how effective a seal will be over time. If the c-set is not adequate for the application, contaminants such as dust, air, or moisture can pass through the seal, leading to performance degradation and product failures.
How is C-Set Measured?
Compression set testing measures compression and assigns a value representing the percentage of thickness permanently lost. Each material has a specific value, but materials can generally be described as having a high or low compression set.
- Low compression set: The lower the number, the less likely the material is to lose permanent thickness. For example, a material with a 5% c-set would rebound to 95% of its original thickness. Slow recovery foam (memory foam) is a common example.
- High compression set: The higher the number, the more likely the material is to become permanently flattened or deformed. For example, a material with a 40% c-set would only rebound to 60% of its original thickness, meaning it loses much of its size after compression.
It’s easy to assume that materials with a low c-set are better, but that’s not necessarily true. Some applications benefit from materials with a high compression set. For example, permanently sealed components may not require gaskets to bounce back to their original shape. Materials with a high c-set also tend to be more cost-effective. For low-stakes applications, a high compression set may be ideal in order to keep costs down.
What is Recovery?
When evaluating material performance, it helps to understand the difference between compression set and recovery. While closely linked, these concepts are used to assess different elastomeric properties.
Compression set evaluates the result of long-term compression, while recovery examines the process of how the material returns to its shape. Recovery is measured by observing how the material behaves rather than with a standardized number like c-set.
While c-set is usually the most essential factor to consider, recovery matters when materials are subject to repeated movement (such as opening a door frequently) or to vibration, impact, or shock.
What is Stress Relaxation?
When evaluating a material’s compressibility and recovery, it’s equally important to look at how the material responds to force after repeated use. Whereas c-set measures thickness loss, stress relaxation measures force loss.
Over time, a repeatedly compressed material will gradually lose its ability to exert adequate force. This determines whether a material will maintain a proper seal in the long term, making it just as crucial to evaluate as c-set.
Like c-set, stress relaxation is expressed as a percentage to illustrate the amount of force lost during compression. For most applications, a lower number is preferable, as it indicates a lower loss of force. General applications can tolerate a 25 to 50% loss of force, but for high-stakes applications, you will usually want to look for a stress relaxation value under 25% for optimal performance.
If you need help determining optimal c-set and stress-relaxation values for your application, American Flexible Products can help. With high-performance materials like BISCO® and PORON® and custom converting capabilities, we can help you select materials that deliver consistent seals even under demanding conditions.
Factors Affecting Compression
Understanding c-set, recovery, and stress relaxation is important, but these are not the only factors that determine how a gasket or seal will perform. Even the best material will have performance issues if it’s overcompressed or used in extreme conditions it wasn’t designed for. Factors that can affect performance include:
- Thickness of the gasket or seal.
- Compression ratio.
- Humidity and moisture exposure.
- Oil, solvent, and chemical exposure.
- Extreme temperatures.
- UV exposure.
Trust American Flexible Products for Expert Guidance
If your application depends on a consistent seal and lasting performance, don’t leave material selection to chance. AFP works with engineers to identify high-performance materials that meet demanding requirements for compression, recovery, stress relaxation, and durability.
With deep application expertise, we help you find the right solution for unmatched performance and reliability. Contact our team to explore your options.
