Compression Set in Rubber Seals: Definition, Test & How to Minimize It

When a rubber seal is compressed in service, it should spring back when the load is removed. The extent to which it doesn’t is called compression set — and it is one of the most important, and most misunderstood, properties in seal selection. This guide explains what compression set is, how it is measured, what causes it, and how to keep it low in real applications.

What Is Compression Set?

Compression set is the permanent deformation a rubber part retains after being compressed for a defined time and temperature, then released and allowed to recover. It is expressed as a percentage of the original thickness (or deflection):

Compression set (%) = (t₀ − t₁) / t₀ × 100

where t₀ is the original thickness and t₁ is the thickness after recovery. A lower number means the material returned closer to its original shape — which is exactly what a long-term seal needs.

How Compression Set Is Tested (ASTM D395)

The standard test is ASTM D395 (equivalent to ISO 815). Two methods are used, and the method must be stated when comparing values:

• Method A — Constant Strain: the specimen is compressed to a fixed deflection (usually 25%) between plates and held at temperature, then measured after recovery.
• Method B — Constant Stress: a fixed load is applied; the resulting and recovered deflection is measured. Useful when the application is load-controlled rather than gap-controlled.

Both report a percentage, but the numbers are not interchangeable. Always compare like-for-like and at the same temperature and duration.

What Causes High Compression Set?

Compression set rises with the things that break down the polymer network over time:

• Heat – the dominant driver; set accelerates sharply above a compound’s rated continuous temperature.
• Chemical exposure – swelling, extraction, and oxidative attack soften the network.
• Prolonged compression – the longer the dwell, the more permanent the set.
• Wrong hardness or compound – under-rated materials simply flow out of the joint.

Typical Compression Set by Compound

Approximate 22 h / 100 °C Method A values give a sense of ranking (always confirm against the specific grade and spec):

How to Minimize Compression Set in Your Design

• Stay inside the temperature limit of the compound — see the FKM temperature range guide for fluoroelastomers.
• Design for the right squeeze — over-compression drives set; follow the recommended compression for the cross-section. For O-rings, match the groove to the size in the AS568 O-ring size chart.
• Choose a low-set compound for static, high-temperature, or long-life joints — silicone and FKM lead.
• Control hardness — very soft compounds can flow; the Shore A hardness guide explains the trade-offs.

Specifying Compression Set for a Quote

Anlintech molds seals in FKM, silicone, EPDM, NBR, HNBR, and FFKM and reports compression-set values from qualified compounds to ASTM D395. Tell us the service temperature, dwell time, and allowable set, and we will recommend a compound and hardness — with material certification on shipment. For longer life in aggressive service, review the trade-offs in our material specification library.

Related Resources

• AS568 O-Ring Size Chart — dash numbers and groove-matching dimensions.
• Shore A Hardness Guide — how durometer affects compression and sealing.
• Material Specification Library — temperature and compression-set data per compound.