Compression set is the permanent deformation left in a material after it has been squeezed for a while at temperature. In a seal, it shows up as a flattened cross-section that no longer pushes back against the mating face. The elastomer is not worn out in any visible way; it has simply stopped recovering.

The four variables

Set is not a single material property. It is the result of four things acting together:

  • Strain. How far the section is compressed. Squeeze a seal past roughly a quarter of its section and recovery suffers.
  • Temperature. Heat accelerates everything, including the chemistry that makes a compressed section stay compressed.
  • Time. A part that sits under load for months behaves differently from one cycled daily.
  • Cure. An under-cured part will take a set earlier, whatever the datasheet says.

Compression set is normally reported as a percentage: the thickness lost after a defined time at a defined temperature, measured a set period after release. Test conditions decide the number, so two suppliers quoting "low compression set" are not necessarily quoting the same test.

What you can change without changing compound

Groove fill

Under-filled grooves leak; over-filled grooves extrude and take a permanent set at the edges. The fill percentage is a drawing decision and it costs nothing to get right at DFM.

Section shape

A round cord and a rectangular profile at the same hardness behave differently under the same load. Where a seal sits in a shallow groove, a profile with a defined contact band often outperforms a larger round section that has nowhere to go.

Hardness

Harder compounds resist extrusion better and hold shape; softer compounds conform to a imperfect face. The trade is real, and it is the reason hardness should be chosen after the groove, not before it.

Compression ratio

For a masking cap that is stretched over a stud and left through a cure cycle, the strain is set by the interference fit. Too much interference and the cap splits at the rim before it ever seals; too little and it falls off in the oven.

If a part has to hold a seal for years at temperature, design the groove so the material works less. That usually beats specifying a more expensive compound.

Reading a failure

A seal that comes back flattened usually carries the evidence: a sharp edge where it extruded, a glossy band where it touched, a crack that runs from the parting line. Those marks tell you whether the groove was over-filled, whether the part was under-cured, or whether the assembly simply compressed it too far. Sending the failed part with the drawing is worth more than a page of description.

Where this lands in a quote

When a drawing shows a seal in a deep groove with a heavy interference, the fit review flags it before tooling. A small change to the section is cheap at that stage and expensive once the cavity is cut. Our durometer note covers the other half of the same decision.

Send the failed part and the drawing and we will read both.