Rubber seals are expected to maintain their shape, elasticity, and sealing force for years—sometimes decades. But over time, many elastomers permanently deform under constant compression or temperature, leading to leaks, pressure loss, or total seal failure.

This permanent deformation is known as compression set, and understanding it is key to designing reliable gaskets, O-rings, bushings, and other elastomeric sealing components.

In this guide, Primo Rubber Co. explains what compression set is, why it occurs, how different materials compare, and what you can do to minimize it.

What Is Compression Set?

Compression set is the amount of permanent deformation a rubber part retains after being compressed for a period of time under specific temperature conditions.

Formally, compression set is defined (ASTM D395) as:

The percentage of a rubber material’s original thickness that is not recovered after being compressed and released.

For example:

  • A seal compressed by 25% but only recovering to 15% compression afterward has a high compression set.
  • A seal that nearly returns to its original size has a low compression set.

Lower compression set = better long-term sealing performance.

Why Compression Set Matters

Rubber seals rely on elastic recovery to maintain contact pressure. If the material takes a set:

  • The seal loses force against the mating surface
  • Gaps form
  • Leakage begins
  • Pressure drops in pneumatic or hydraulic systems
  • Fasteners need retightening

In safety-critical systems (medical, automotive, aerospace), compression set is a leading cause of long-term failure.

What Causes Compression Set?

Several factors contribute to permanent deformation:

1. Heat Exposure

Heat accelerates polymer relaxation and crosslink degradation.
Higher temperatures → higher compression set.

2. Continuous Compression

Rubber under constant load slowly relaxes and loses elasticity.

3. Chemical Swelling

Exposure to incompatible oils, fuels, or solvents weakens the elastomer network.

4. Poor Material Selection

Some rubber types naturally resist compression set better than others.

5. Improper Cure

Undercured elastomers struggle to return to original shape.

6. Thin Cross Sections

Thin seals have less material to recover and may set faster.

Which Rubber Compounds Perform Best?

Here’s a general ranking from best to worst compression set performance:

Excellent (Low Compression Set)

  • Silicone (VMQ / LSR)
  • FKM (Viton®-type fluoroelastomers)
  • EPDM
  • HNBR

Good

  • NBR (Nitrile)
  • Neoprene (CR)

Poor

  • Natural Rubber (NR)
  • SBR (styrene-butadiene)
  • Butyl rubbers under high heat

Silicone is the standout performer at high temperatures, while EPDM is strong when exposed to weathering or steam environments.

How to Reduce Compression Set in Your Application

1. Choose a Better Base Polymer

Select rubber materials known for low compression set under your operating conditions.

Peroxide curing often gives lower compression set than sulfur curing in many compounds.

3. Avoid Excessive Operating Temperatures

Every 10°C temperature increase can drastically worsen compression set.

4. Increase Cross-Section Thickness

Thicker seals retain more elastic recovery.

5. Design for Partial Compression

Avoid compressing seals more than 20–30% unless the geometry specifically allows it.

6. Use Backing Rings or Gland Supports

These prevent extrusion and improve long-term shape retention.

7. Consider Dual-Durometer or Spring-Energized Designs

A soft sealing surface backed by a stiffer core or spring maintains long-term load.

Common Applications Where Compression Set Is Critical

  • O-rings in pumps and valves
  • Static gaskets in industrial machinery
  • Outdoor seals exposed to heat cycling
  • HVAC and refrigeration systems
  • Medical device valves and septums
  • High-temperature automotive components
  • Electrical enclosure gaskets

In all these cases, selecting the right elastomer is critical to long-term performance.

Summary: Designing for Low Compression Set

Need Recommended Material
High temperature Silicone, FKM
Outdoor & steam EPDM
Oil-resistance + durability HNBR
General-purpose sealing NBR, Neoprene
Lowest possible compression set Silicone

Need Help Reducing Compression Set in Your Seal Design?

Primo Rubber Co. specializes in injection molding custom rubber components. Contact Primo Rubber Co. today to discuss your project and find the ideal material and design solution for your rubber parts.