Moulded Silicone Gaskets: Compression Set Measured Too Soon

A recovery figure taken straight out of the jig understates long-term sealing behaviour and hides an under-cured network.

What the failure looks like on the line

Moulded gaskets are judged by sealing force over time, not by the hardness printed on the drawing.

Gaskets pass an incoming compression check yet lose height in service; the same lot gives different recovery figures depending on how long it was left to recover.

Compression set combines viscous flow and incomplete elastic recovery, so the reading depends strongly on recovery time and temperature after release. An under-crosslinked network keeps creeping; reporting only an immediate reading removes the very signal that would flag it.

Buyers and suppliers usually agree on the drawing and disagree on the conditions under which the drawing was checked. On a buyer's specification this distinction matters: a part can measure inside tolerance and still fail in use, because the drawing records geometry while the complaint records behaviour. Compression ratio is usually the first value both sides compare, and it is also the one most often quoted without the conditions attached to it.

The process window that decides it

Four variables move this result more than the rest. The ranges below are illustrative engineering guidance for a first discussion; confirm them on your own tool, material grade and part geometry before they become contract terms.

Table 1. Starting window for a joint process trial. Values are engineering guidance, not a measured survey.

Compression ratio is typically held at 25% of free height. Higher strain inflates set and hides material differences Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Test temperature is typically held at 70-175 C. Above the window thermal ageing dominates the reading When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Recovery time after release is typically held at 30 min standard. Short recovery understates permanent set A change here is cheap during a trial and expensive after tooling release, so treat it as a controlled variable with a documented re-approval.

Conditioning before test is typically held at 16-24 h. Unconditioned parts give drifting hardness and set Put it on the change-control list, otherwise an unannounced material or setting substitution will be mistaken for ordinary process noise.

Reading the response curve

The left panel shows recovered height (%) as recovery time after release (min) increases: the response rises from 58 to 92 on the illustrative 0–100 scale. The dashed line at 85 marks a proposed acceptance limit, not a universal standard. Move that line only after the test method and the sample plan have been agreed, otherwise two suppliers can both be compliant and still deliver different parts.

Read the shape rather than a single point. A curve that changes quickly near the limit is more sensitive to ordinary process noise than one that approaches it gradually, and that difference should be reflected in how tightly the setting is controlled and how often it is checked.

Where the causes actually sit

The right panel of Figure 1 splits audited failures of this type into five groups. The percentages are illustrative engineering guidance for prioritising an investigation, not a measured survey and not a prediction for your line.

Table 2. Cause split and the corresponding first check. Shares are indicative and sum to 100.

Low crosslink density accounts for roughly 28% of the illustrative split. State the recovery time and temperature on every compression-set report. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Recovery read too early accounts for roughly 24% of the illustrative split. Measure recovered height at fixed intervals rather than one instant. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Excessive test strain accounts for roughly 18% of the illustrative split. Verify specimen thickness and stack against the agreed method. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Run the same sequence on a retained good sample and on the disputed batch so the comparison means something.

1. State the recovery time and temperature on every compression-set report.
2. Measure recovered height at fixed intervals rather than one instant.
3. Verify specimen thickness and stack against the agreed method.
4. Cross-check set against a rheometer cure curve for the lot.
5. Keep a reference coupon to compare across laboratories.

Keep a retained reference sample beside every result. When a batch is disputed, the comparison that settles it is almost always between the retained part and the current one under the same method, not between either part and a number on a drawing. Record the instrument, the operator, the conditioning time and the position on the part or sheet for each reading.

Writing it into the specification and the evidence pack

Require compression set to be reported with recovery time, temperature and specimen thickness stated, and compare against an agreed reference coupon before lot release.

Write the requirement as a behaviour at a stated condition, not as a brand of material or a machine setting. State the test method, the conditioning, the sample plan and the rule for deciding a pass. Where a document is needed, ask the supplier for documentation that covers the actual construction and check its scope against your own market before approval; a document for the base material alone does not describe the finished part you are buying.

At first-article approval, freeze the drawing revision, the material and colour identification, the process window, the tool and cavity identification, and the inspection method. Any later change to grade, site, tool, or window should trigger a new first article rather than a paper waiver. Keep retained samples from the approved lot and from each production lot so that a future comparison is possible.

Table 3. Minimum evidence pack for a first-article decision and a later batch dispute.

All ranges in this article are illustrative engineering guidance rather than measured data. Treat them as a starting agenda for a process trial, not as a certificate for a batch that has not been run.