Compression set testing quantifies the permanent deformation that a sealing ring retains after prolonged compression at a controlled temperature, making it one of the most decisive indicators of long-term sealing performance and elastomer durability. This article explains the test principle, applicable international standards such as ISO 815-1 and ASTM D395, specimen preparation, step-by-step procedures, calculation formulas, and typical acceptance criteria for sealing rings evaluated by accredited third-party laboratories.
Technical Overview
Compression set, also referred to as compression permanent deformation, is the residual dimensional loss that an elastomeric sealing ring exhibits after it has been compressed between two surfaces for a defined period at a specified temperature and then released. Because a seal maintains leak-tightness only while it continues to exert contact force against its mating surfaces, a high compression set indicates that the material has lost a significant portion of its elastic recovery and can no longer compensate for thermal cycling, vibration, pressure pulses, or flange micromovement. For this reason, compression set is one of the primary acceptance criteria in material specifications for O-rings, gaskets, and custom-molded seals across the automotive, aerospace, hydraulic, and general industrial sectors.
The result is expressed as a percentage: the lower the value, the better the seal retains its original cross-section and its residual sealing force over time. Testing is performed either as a material characterization on standard molded buttons or as a component verification on the finished sealing ring itself.
Principle of the Test
A specimen is compressed between two parallel steel plates, either to a predetermined deflection (constant-strain method) or under a predetermined force (constant-load method). It is held in this state inside an air oven or a low-temperature chamber for a specified exposure time, then released and allowed to recover. After a defined recovery period, the specimen thickness is remeasured and compared with both its original dimension and the compressed height imposed by the fixture spacers.
Governing Standards
- ISO 815-1: Rubber, vulcanized or thermoplastic — Determination of compression set — Part 1: At ambient or elevated temperatures
- ISO 815-2: Rubber, vulcanized or thermoplastic — Determination of compression set — Part 2: At low temperatures
- ASTM D395: Standard test methods for rubber properties — Compression set, covering Method A (constant load) and Method B (constant deflection)
- ASTM D1414: Standard test methods for rubber O-rings, including compression set determination on finished rings
- GB/T 7759.1 and GB/T 7759.2: Chinese national standards technically aligned with ISO 815-1 and ISO 815-2
- JIS K 6262: Japanese industrial standard covering compression set at ambient, elevated, and low temperatures
Testing Samples
Standard Laboratory Specimens
For material-grade characterization, cylindrical buttons are compression-molded or prepared from cured sheets. The most widely used geometries are Type A buttons of 13 mm diameter and 6.3 mm height, and Type B buttons of 29 mm diameter and 12.5 mm height. A minimum of three specimens per test condition is customary, and specimens must be free of porosity, flash, surface defects, and machining damage that could bias the thickness measurement.
Finished Sealing Rings
Component-level testing is performed on actual O-rings, X-rings, square-section rings, and custom-molded profiles in the as-delivered condition. For O-rings, the characteristic dimension is the cross-sectional diameter, measured at several evenly distributed positions before testing. Finished-part testing is preferred whenever the molding process, grinding, or post-curing specific to production parts may influence field behavior.
Material Families Commonly Evaluated
- NBR and HNBR for hydraulic, fuel, and general oil-resistant sealing
- FKM and FFKM for high-temperature and aggressive chemical service
- VMQ and FVMQ silicone for wide-temperature-range and low-temperature applications
- EPDM for coolant, steam, brake fluid, and outdoor weathering resistance
- Specialty composites and thermoplastic elastomers for customer-defined service conditions
Unless otherwise specified, specimens are conditioned after vulcanization and before testing in a standard laboratory atmosphere, typically 23 °C ± 2 °C and 50 % ± 5 % relative humidity for at least 16 hours, to stabilize their dimensions and internal stress state.
Testing Items
Typical test programs requested by manufacturers and accredited third-party laboratories include the following items, each combining a specific exposure condition with the standard compression set procedure:
- Compression set at ambient temperature (23 °C)
- Compression set at elevated temperatures, commonly 70 °C, 100 °C, 125 °C, 150 °C, 200 °C, or 250 °C depending on the polymer class
- Compression set at low temperatures, typically between −10 °C and −70 °C, following ISO 815-2
- Compression set after immersion in oils, fuels, coolants, brake fluids, or process chemicals to evaluate combined thermal and fluid aging
- Constant-deflection compression set (Method B) versus constant-load compression set (Method A)
- Long-duration aging studies at 168 h, 336 h, or 1000 h to support service-life prediction
Related but distinct measurements, such as compression stress relaxation in accordance with ISO 3384, are frequently requested alongside compression set to characterize the decay of sealing force rather than the decay of geometry.
Testing Methods
Specimen Preparation and Measurement
The original thickness or cross-section of each specimen is measured with a calibrated gauge having flat, parallel anvils and a resolution of 0.01 mm, applying a low and repeatable contact force to avoid additional deformation. Specimens are then placed between clean, polished steel plates with rigid spacers that limit the compressed height to the target deflection.
Method B — Constant Deflection (Most Widely Used)
This method reproduces the condition of a seal confined in a fixed gland and is the default procedure in most specifications. The sequence is as follows:
- Compress each specimen to the specified deflection, typically 25 % for standard buttons and 15 % to 20 % for many O-ring cross-sections, depending on hardness and size
- Assemble the fixture with spacers ensuring the plates remain parallel and the specimen is not subjected to torsion or bending
- Place the assembled fixture in a circulating-air oven, or a low-temperature chamber for cold testing, and maintain the specified temperature for the selected duration, commonly 22 h, 24 h, 70 h, or 168 h
- Remove the fixture and release the specimens; for elevated-temperature tests, allow recovery at ambient conditions for 30 min ± 3 min before remeasuring
- Remeasure the recovered thickness at the same positions and with the same gauge used for the original measurement
Method A — Constant Load
In this variant, the specimen is compressed under a constant force applied through a calibrated spring or mass, and the resulting compression set is calculated relative to the original thickness only. Constant-load testing better simulates applications where the sealing load cannot build up as the material softens, such as lightly loaded gasket joints, and is selected when the product specification calls for it.
Low-Temperature Procedure
For ISO 815-2 low-temperature testing, the fixture is conditioned in a chilled chamber at the test temperature, and the recovered thickness is measured while the specimen is still at the low temperature, because elastomers recover far more slowly as they approach their glass transition. This procedure reveals cold-flexibility deficiencies that ambient testing cannot detect.
Calculation of Results
For the constant-deflection method, the compression set CB is calculated as:
CB = (t0 − t1) / (t0 − ts) × 100 %
where t0 is the original specimen thickness, t1 is the thickness after recovery, and ts is the spacer thickness that defined the compressed height. For the constant-load method, the compression set CA is calculated as (t0 − t1) / t0 × 100 %. Values approaching or exceeding 100 % indicate almost complete loss of elastic recovery, a condition that inevitably leads to leakage.
Typical Test Conditions and Evaluation
- General-purpose NBR seals: 24 h at 100 °C, 25 % deflection, typical limit below 25 %
- Automotive EPDM cooling-system seals: 22 h or 70 h at 125 °C to 150 °C
- FKM fuel-system seals: 70 h at 150 °C to 200 °C, with specification limits frequently in the 10 % to 20 % range
- Silicone seals for sterilization service: repeated cycles at 170 °C to 200 °C to assess cumulative set
- Outdoor and refrigeration seals: low-temperature set at −40 °C or lower after 24 h or 168 h
The test report should identify the material and specimen type, the method and deflection used, the exposure temperature and duration, the recovery conditions, individual results together with the median, and the calibration status of the equipment, consistent with the quality framework of ISO/IEC 17025.
Applications
Compression set data are used throughout the product life cycle of sealing systems, from material screening to field failure analysis:
- Automotive powertrain, chassis, fuel, and brake systems, where long-term elasticity directly affects warranty performance
- Aerospace hydraulic, fuel, and environmental control systems requiring verified long-term seal resilience
- Industrial hydraulic and pneumatic cylinders, valves, and fittings
- Oil and gas valves, actuators, and wellhead equipment exposed to heat and pressure cycling
- HVAC and refrigeration compressors, including refrigerant-resistant elastomer grades
- Medical devices and pharmaceutical equipment using sterilizable silicone seals
- Electronics enclosures and battery packs, where compression set governs the durability of IP-rated sealing
- Incoming inspection, supplier qualification, comparative material studies, and root-cause analysis of leaking joints
FAQ
What is compression set in a sealing ring?
Compression set is the percentage of deformation that a sealing ring fails to recover after being compressed for a specified time at a specified temperature. A seal with 0 % set would fully regain its original cross-section, while higher values indicate progressive loss of elastic recovery and, ultimately, loss of sealing force.
What is an acceptable compression set value?
Acceptance limits depend on the application, polymer, and test severity. Most industrial and automotive specifications require values below 20 % to 30 % after a standard exposure such as 22 h or 70 h at the maximum service temperature, while demanding fuel-system and aerospace applications often impose limits of 15 % or lower. As a rule, leakage risk rises sharply once the set consumes a large fraction of the designed squeeze.
Which method should be chosen: constant deflection or constant load?
Constant deflection (Method B per ASTM D395, or the corresponding ISO 815-1 procedure) is the industry default because it reproduces a seal confined in a rigid gland. Constant load (Method A) is reserved for applications where the sealing force is essentially fixed, such as spring-loaded or lightly bolted joints.
Can finished O-rings be tested instead of molded buttons?
Yes. ASTM D1414 and ISO 815 allow testing of the finished ring in suitable fixtures, with the cross-sectional diameter measured before and after the test. Component testing is recommended for production release because it captures the effects of the actual molding and post-curing processes.
How are the test temperature and duration selected?
The conditions should mirror real service exposure. For qualification testing, engineers often select a temperature at or above the maximum service temperature and durations of 22 h, 24 h, 70 h, or 168 h; extended tests of 1000 h or longer support service-life modeling. Low-temperature set testing is added whenever the seal operates in cold or outdoor environments.
How does compression set differ from compression stress relaxation?
Compression set measures the geometric recovery of the seal after release, whereas stress relaxation, measured under ISO 3384, tracks the decay of counterforce while the seal remains compressed. The two properties are complementary, and many seal qualification programs require both.
Why does compression set increase so strongly with temperature?
Elevated temperature accelerates chemical aging mechanisms such as chain scission, additional crosslinking, and oxidation, all of which reduce the elasticity of the rubber network. The same exposure that produces only minor set at 100 °C may cause severe set at 200 °C, which is why the reported test temperature is essential for interpreting any result.