Scrapers
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Flat seals
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Sealing sets
O-ring measuring towers
Turned parts
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Turned and milled parts
Bushes or bearings
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Large format 3D printing
Post-processing of 3D printed parts
Heat-resistant 3D printed components
Plastic Components with Flame Retardancy
Compression Set
Definition and Limit of Meaning
Compression set (German: Druckverformungsrest, DVR) describes the permanent set of an elastomer after a defined, prolonged compression. In other words, it captures how much of the original deformation remains in a rubber or TPE material after unloading, instead of recovering elastically as it should.
Compression set is measured as a percentage from 0 to 100 %: 0 % means full recovery (no permanent deformation), 100 % means no recovery at all (the compression remains fully “set”). The parameter therefore answers the question: how well does the geometry recover after a defined compression? It answers only indirectly, however, how well a seal performs over the long term, because compression set primarily captures geometric recovery rather than the actually remaining contact force in the installation situation.
What matters is the limit of meaning: a compression-set value is only meaningfully comparable when test conditions such as compression level, temperature, duration, recovery time, specimen geometry, and medium match. Even a different temperature or test time can shift the value significantly.
Distinction from Related Parameters
In sealing technology, compression set is often discussed together with other viscoelastic parameters. Each of them describes a different “response” of the material to load, time, and temperature.
| Parameter | What is held constant? | What is observed? | Typical relevance |
|---|---|---|---|
| Compression set | Deformation over time, then unloading | Permanent set after recovery | Recovery of static seals |
| Stress relaxation | Deformation constant | Force decreases over time | Decay of contact force in service |
| Creep (creep) | Force/load constant | Deformation rises over time | Setting behavior under sustained load |
| Rebound resilience | Short, dynamic impulse | “Springiness”/energy return | Dynamic applications, damping |
Compression set and stress relaxation are often connected in practice, because both result from the viscoelastic structure of the elastomer. Nevertheless, they provide different answers: compression set describes how much deformation remains; relaxation describes how much force is lost, even though deformation stays the same.
Test Principle, Calculation, and Important Test Parameters
The test principle follows a clear sequence. First, the initial thickness of the specimen is measured. Then it is compressed by a defined amount (often 25 % compression) and stored at a defined temperature for a defined time. Afterwards, it is unloaded, a defined recovery time is observed, and the thickness is measured again. The difference shows how much of the original compression did not come back.
The calculation is expressed as a percentage relative to the originally set compression. In practice, this means: the larger the remaining thickness loss after the recovery phase, the higher the compression set.
For interpretation, a few parameters dominate that should always be read together with any data-sheet value:
- Compression level (e.g., 25 %, or other values depending on standard/application)
- Temperature (room temperature or elevated)
- Test duration (hours to days)
- Recovery time after unloading (because recovery is time-dependent)
- Specimen geometry (thickness/cross-section influences recovery)
- Medium (e.g., air or oil; interaction with the material is possible)
This procedure already shows why compression set is so commonly used in sealing technology: it reflects a typical load case in which a seal stays compressed for a long time and afterwards must at least partially recover.
Influence of Temperature, Time, and Aging
Under sustained compression, both physical and — at higher temperatures, more pronounced — chemical changes occur in the elastomer. Physically, molecular chains and filler structures rearrange themselves, so that recovery decreases over time. Chemically, aging processes can alter the network structure, which further reduces recovery capability.
In many cases, a clear trend follows: as temperature and duration rise, compression set rises. For seals, this is particularly relevant when they operate for long periods near the upper service temperature or when thermal cycles occur.
Influence of Material and Hardness (e.g., IRHD)
Compression set depends strongly on the material type (e.g., different elastomer families) and on the hardness. Hardness is frequently expressed as IRHD (International Rubber Hardness Degrees), i.e., a standardized rubber hardness. Harder or differently crosslinked compounds can recover better or worse under certain conditions; without identical test conditions and formulation details, a general ranking is often not reliable.
Therefore, in practice: compression-set values are only meaningful together with the associated temperature and test time. Data sheets do not state these conditions by accident — without them, the parameter is easily misinterpreted.
Standards and Methods: ISO 815-1 vs. ASTM D395
For compression set, two standard families are particularly widespread. ISO 815-1 describes methods for determining compression set on vulcanized and thermoplastic elastomers at room or elevated temperature. The standard covers method variants, including for the type of storage and unloading.
ASTM D395 is also very common in the international context. A frequently used approach is the test case with constant deflection in air. In practice, compression-set values therefore often appear as “ASTM D395” or “ISO 815-1”, together with the supplementary method and test conditions.
For technical communication, what is decisive is: a compression-set value should always be documented together with standard, method, and test parameters. Without these, a comparison between materials or suppliers is usually only apparent.
Practical Relevance in Hydraulics and Pneumatics: Interpretation and Selection
In hydraulic and pneumatic sealing systems, the question is why compression set matters at all. A static or quasi-static seal depends on maintaining contact pressure at the sealing surface after installation and throughout operation. When the material sets permanently, the recovery reserve drops, and the seal is more likely to enter a region where leakage becomes probable — under pressure drops, vibrations, or temperature swings.
A low compression set is therefore often advantageous, because it points to better recovery. Nevertheless, the percentage value should not be read as an absolute sealing guarantee. The real sealing function additionally depends on the installation space, preload, surfaces, media compatibility, and the force progression over time.
For rough classification (only when test conditions are identical), the following orientation can help:
| Compression set (low → high) | What it means for recovery | Typical effect in a static seal* |
|---|---|---|
| approx. ≤ 10 % | Very good recovery | More reserve against set loss |
| approx. 10–30 % | Medium recovery | Often acceptable, depending on tolerances/temperature |
| approx. ≥ 40 % | Low recovery | Higher risk with low contact pressure or in cold conditions |
*The actual sealing performance results from the overall system and cannot be unambiguously calculated from compression set alone.
For selection, it is usually most effective to choose test conditions that reflect the application case: which temperature is sustained, how long does the compression act, which medium is present, and how high is the actual deformation in the installation space? Only then does compression set become a parameter that reliably helps with material comparisons in sealing technology. For complex cases, specialized material and application consultation can be sensible.
