Compression Set vs Stress Relaxation: What Seal Test Reports Actually Show

Compression set and compression stress relaxation answer different questions about a rubber seal. Compression set concerns dimensional recovery after compression is released. Compression stress relaxation concerns the change in reaction force while the rubber remains compressed. A useful seal specification distinguishes the two before deciding which evidence the application needs.

For purchasing and engineering teams, the practical risk is comparing percentages that describe different measurements. A low compression-set value is useful information under its reported conditions, but it does not state the force remaining in an installed seal. Likewise, a force-retention result does not describe every aspect of recovery after disassembly.

Start with the measurement you need

A compression-set test compresses a specimen under defined conditions and evaluates its recovery after release. ISO 815-1:2019 covers testing at ambient or elevated temperatures. Its public abstract explains that the result depends on the compression exposure and the subsequent recovery conditions. Record both stages when requesting or comparing reports.

That means a result needs more context than a percentage and a rubber-family name. Ask for the compound identification, specimen description, method, imposed deformation, exposure temperature and time, and release and recovery procedure. A comparison is meaningful only to the extent that the relevant conditions have been aligned or their differences assessed.

Keep the original report with the comparison sheet. A shortened quotation may omit the release procedure or recovery period, leaving apparently similar figures based on different tests. Mark missing details as unresolved rather than selecting the lowest number automatically.

Understand force loss under held compression

ISO 3384-1:2024 addresses compression stress relaxation at constant temperature. The specimen remains at a fixed deformation while its counterforce is measured. The standard distinguishes measurements made at test temperature from those made at standard laboratory temperature; these methods do not produce equivalent results. It also limits valid specimen comparisons to similar sizes and shapes.

For a seal review, use this distinction to ask a precise question: does the report describe force during the intended hot condition, or force measured under a different temperature procedure? Identify the method explicitly instead of treating the standard number alone as a complete test description.

Ask whether the report expresses force remaining or force lost, and how the reference value is defined. The direction of a percentage matters. Keep the measured force information and reporting basis where available so that a later reviewer can understand the result without reconstructing the laboratory calculation.

Do not convert one result into the other

These tests observe different responses. A compression-set percentage should not be relabeled as a percentage loss of sealing force, and a force-retention percentage should not be relabeled as dimensional recovery. A proposed relationship between them needs evidence for the compound and conditions concerned.

As a practical review approach, start with the failure question. If the concern is recovery after the assembly opens or its interface changes, dimensional recovery may be relevant. If the concern is force retention during a sustained installed condition, stress-relaxation evidence may help. Some applications need both, along with other material and assembly evidence.

A drawing that simply requests “good compression performance” leaves these decisions open. Replace that phrase with named properties, test methods, conditions and acceptance limits selected by the responsible engineering team. Do not import a numerical limit from an unrelated application merely because it is easy to find.

Connect laboratory evidence to the installed seal

Neither report is a complete leakage qualification for a machine. Keep the material evidence connected to the actual interface, permitted tolerances, contact media, temperatures and operating sequence. Identify differences between the laboratory specimen and the supplied component, including whether the report represents the proposed compound.

Define a separate functional acceptance plan where the application requires it. State the leakage measurement method and limit, the stages of operation to be evaluated, and any restart or thermal-cycle requirements. Use safe test arrangements established by the responsible engineering team. A completed material test should not silently become a service-life guarantee.

Request a reviewable evidence package

• Identify the compound and the part or specimen represented by the report.
• Separate compression-set requirements from stress-relaxation requirements.
• Record the method, geometry, deformation, temperature and exposure period.
• Preserve recovery conditions for set tests and the force-reference basis for relaxation tests.
• List the assembly performance still to be demonstrated before approval.

When reviewing IKSONIC O Ring options, use these questions to define the evidence needed for the proposed seal. The IKSONIC contact page provides a route for discussing the application and asking which supporting documents are available. Agree the required evidence before treating a material description as an approved sealing solution.