Rubber Compression Set: How to Review Test Reports

2What the Result Can Tell a Buyer

ISO 815-1 addresses compression set of vulcanized and thermoplastic rubber at ambient or elevated temperatures. Its public scope explains that the result depends on the compression and recovery conditions. This is a property of the tested rubber material under a stated procedure, rather than a universal service-life rating.

For procurement, the practical question is whether a report describes the material and conditions relevant to the proposed part. A result from an unidentified reference compound cannot automatically support a different production recipe. Ask engineering to identify the approval basis before adding a limit to a purchase order.

Treat the report as one piece of an evidence file. A seal drawing, compound specification, production records and functional test each contribute information that the laboratory percentage does not contain. Keeping these records connected helps the buyer understand what was approved and what must be rechecked after a change.

3Why Hardness Is a Separate Requirement

Shore hardness measures indentation response. ISO 48-4 describes durometer methods for vulcanized or thermoplastic rubber. That measurement does not replace compression set testing. Specify each property for the question it is intended to answer, and identify the applicable method.

A purchasing team might receive two proposals carrying the same hardness target. That similarity is insufficient to approve a substitution. Request the complete compound identification and the other critical requirements on the drawing, then review how the proposals were tested. Do not infer comparable recovery from a shared hardness label.

Keep incoming inspection and qualification separate. A quick hardness check can be part of a defined release procedure, but it should not silently become the only evidence for a component whose performance approval included several properties.

4A Report Comparison Matrix

This matrix is a buyer review aid, not a replacement for a laboratory procedure. Use the full applicable standard and your agreed specification to decide the required test details.

5How to Handle Reports That Do Not Match

First mark the differences rather than choosing a winner. If one report uses another exposure duration or recovery procedure, label the comparison incomplete. Ask whether the supplier can provide results on the agreed basis, or arrange a controlled evaluation with the laboratory.

Separate missing information from an actual out-of-specification result. A document gap means the evidence cannot yet support a decision. It does not establish that the material failed. Equally, an attractive percentage should not receive approval until its identity and conditions are resolved.

Record the decision and its owner. Engineering should determine technical comparability, quality should define release evidence, and procurement should obtain the required records. A clear action list prevents an unresolved technical question becoming an undocumented commercial acceptance.

6Connect Polymer Sourcing to Compound Approval

SBR or NBR supplied as raw rubber is an input to a compound. A supplier’s grade description should not be represented as the compression-set guarantee for a finished seal made with your own recipe. State whether you are buying polymer, uncured compound or finished components before requesting evidence.

For a raw-polymer enquiry, identify the reference grade, producer, critical processing requirements and trial plan. For a compound enquiry, identify the approved compound revision and the agreed cured-property specification. For a finished-part order, include the drawing revision and functional acceptance requirements.

Use Linwell’s grade selection guide to organize a material trial, and the technical document checklist to keep TDS, COA and safety information in their proper roles.

7Build a Practical Qualification File

1. Identify the component, drawing and operating conditions.
2. Write the test basis and acceptance criteria before comparing alternatives.
3. Obtain a report linked to the actual compound and specimen batch.
4. Resolve different methods or missing conditions with the responsible laboratory.
5. Keep a controlled reference evaluation for the existing approved material.
6. Evaluate the proposed material under the same agreed comparison basis.
7. Complete the application-specific or component trial required by engineering.
8. Document approval, restrictions and which changes trigger a new review.

A useful file should explain the decision to a colleague who was not involved in the trial. Include the reason for the replacement, deviations from the initial plan and any conditions attached to approval. This makes future repeat orders easier to assess.

8Questions for a Supplier RFQ

• Which compound, revision and specimen batch does this report describe?
• Which test standard and selected procedure were used?
• Are the published values typical data or agreed acceptance limits?
• Can the result be supplied on our specified comparison basis?
• What evidence connects the trial material to future supply?
• Which formulation, process or production-site changes require notification?

Also provide quantity, delivery destination and the approval stage. A request for a laboratory sample is different from a released production order. Make the distinction explicit so commercial documents do not imply a technical approval that has not happened.

For repeat supply, retain the approved evidence and identify which routine batch documents are required. Do not request a new full qualification report on every shipment unless your agreement requires it; instead, establish a proportionate release and change-control procedure.

9FAQ

Does lower compression set always mean a better seal?

It can be a useful result within a comparable test, but a seal must meet the complete application requirements. Do not select a component from this percentage alone.

Can results from different temperatures be compared directly?

They should not be ranked as an equivalent test. Obtain the full conditions and agree a common comparison basis before making a purchasing decision.

Can a raw NBR COA prove the compression set of my seal?

A raw-material COA establishes only the results within its stated batch scope. Your actual compound and component need the relevant qualification evidence.

Should every order specify the same maximum percentage?

No universal maximum is proposed here. Engineering should set a justified limit together with the method and operating requirements.

What should I do with an incomplete report?

Keep approval pending, list the missing information and request clarification. Do not fill missing conditions with assumptions.

10Conclusion

Use compression set reports to support a controlled material decision. Identify the compound, align the procedure, review the complete conditions and connect the laboratory result to the actual component trial. A traceable approval basis is more useful than a low percentage with no supporting context.

11Sources and Review Basis

• ISO 815-1:2019: compression set scope
• ISO 48-4:2018: Shore hardness scope

Reviewed October 14, 2026. The standards’ public scopes support the definitions above; the purchasing workflow and checklists are practical recommendations. They do not reproduce the full test procedures. Google Trends data could not be retrieved during this review; the topic reflects technical buyer questions, not a measured popularity claim. Cover image is generated illustration and does not document a Linwell-owned laboratory.