Compression set testing measures the ability of packaging materials—such as flexible foams, elastomeric cushions, and sealing gaskets—to recover their original thickness after prolonged compressive loading. This article explains the testing principle, applicable international standards, specimen preparation, test procedures, calculation methods, and practical applications of compression set determination for packaging materials, providing engineers and quality professionals with a complete technical reference.
Technical Overview
Compression set refers to the permanent deformation that remains in a cushioning or sealing material after it has been compressed under a defined deflection or load for a specified period and then allowed to recover. For packaging materials, this property is critical: foams, rubber pads, and gasket materials used in protective packaging must maintain their thickness and resilience throughout storage and transportation. A material with poor compression set resistance will gradually lose its cushioning thickness, seal contact pressure, and energy-absorbing capability, leading to product damage, seal failure, or degraded vibration protection.
Compression set determination quantifies this behavior as a percentage—the lower the value, the better the material retains its original thickness and cushioning performance. Testing is typically performed in accredited third-party laboratories or in-house quality laboratories following internationally recognized standards, including ASTM D395 (rubber vulcanizates and thermoplastic elastomers), ISO 815-1 and ISO 815-2 (rubber compression set at ambient, elevated, or low temperatures), ASTM D3574 Test I (flexible polyurethane foams), ISO 1856 (flexible cellular polymeric materials), and the corresponding Chinese national standards GB/T 7759.1 and GB/T 6669.
Two fundamental test approaches are used: the constant-deflection method, in which the specimen is compressed to a fixed percentage of its original thickness (commonly 25%, 50%, or 75%) between rigid plates with spacers, and the constant-load method, in which a fixed force is applied to the specimen. The constant-deflection method is the most widely used for packaging material qualification because it directly simulates the fixed-geometry compression that occurs when foam inserts are held inside a fitted package.
Testing Samples
Flexible Cellular Foams
Flexible polyurethane foam, polyethylene foam (EPE), polypropylene foam (EPP), expanded polystyrene (EPS), EVA foam, and other cellular cushioning materials are the most common test subjects. Typical specimen geometry is a rectangular block of 50 mm × 50 mm × 25 mm, or thicknesses representative of the actual packaging application. Specimens must be free of skin defects, cells crushed during die-cutting, and visible density gradients.
Elastomeric Cushioning and Sealing Materials
Rubber pads, silicone gaskets, sponge rubber profiles, thermoplastic elastomer (TPE) cushions, and closure seals used in transit packaging and container sealing are tested as molded or cut buttons and slabs. Standard rubber specimens are typically cylindrical buttons of 13 mm thickness and 29 mm diameter, or slabs laminated to approximately 6 mm total thickness.
Composite and Laminated Packaging Structures
Fabricated packaging components such as foam laminates, corrugated-foam composites, corner protectors, and die-cut cavity inserts can also be evaluated. For these, the test deflection and specimen geometry are often defined by the product specification, and results are interpreted relative to the functional thickness requirement of the finished package.
Specimen Conditioning
- Condition specimens for at least 24 hours at 23 ± 2 °C and 50 ± 5 % relative humidity unless otherwise specified by the governing standard.
- Measure the original thickness at defined measurement points using a gauge with a flat pressure foot and low contact force, consistent with ISO 1856 or ASTM D3574.
- Prepare a minimum of three specimens per condition to allow statistical evaluation of variability.
- Machine-cut specimens at least 24 hours after fabrication to allow relaxation of cutting stresses.
Testing Items
- Compression set at constant deflection under ambient conditions (23 °C) at 25 %, 50 %, or 75 % compression.
- Compression set at constant deflection under elevated temperature (typically 40 °C, 70 °C, or 80 °C) to simulate warehousing or transport in hot climates.
- Compression set at constant deflection under low temperature (for example −25 °C or −40 °C) for cold-chain packaging applications.
- Compression set at constant load, in which a specified force is maintained rather than a fixed deflection.
- Compression set after prolonged aging (168 hours or longer) to assess long-term storage performance.
- Thickness recovery percentage after defined recovery intervals (30 minutes and 24 hours) to characterize elastic and viscoelastic recovery behavior.
Testing Methods
Constant-Deflection Method (Primary Method)
The specimen is placed between two parallel rigid plates of a compression fixture. Spacers or stops of the appropriate thickness limit the plate separation to the target compressed thickness. For a 50 % deflection test on a 25 mm specimen, spacers of 12.5 mm are used. The assembled fixture is then placed in a temperature-controlled oven or environmental chamber for the specified duration, commonly 22 hours, 24 hours, 70 hours, or 168 hours.
Constant-Load Method
A calibrated mass or pneumatic loading system applies a constant force to the specimen for the test duration. This method is useful when the packaging application involves a known static load, such as stacked cartons resting on a foam layer, rather than a fixed geometric constraint.
Recovery and Measurement
At the end of the compression period, the specimen is removed from the fixture and allowed to recover freely at standard laboratory conditions. For rubber materials per ISO 815 and ASTM D395, the final thickness is measured after 30 minutes of recovery. For flexible cellular materials per ISO 1856 and ASTM D3574, the thickness is measured after 30 minutes, with an optional additional measurement after 24 hours to capture delayed viscoelastic recovery.
Calculation of Compression Set
For constant-deflection testing of rubber materials, 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 the recovery period, and ts is the spacer thickness.
For flexible cellular materials tested per ISO 1856, GB/T 6669, and ASTM D3574, the compression set is expressed relative to the original thickness:
- Cs = [(t0 − t1) / t0] × 100 %
where t0 is the original thickness and t1 is the recovered thickness. A result of 0 % indicates complete recovery, while 100 % indicates complete loss of resilience and permanent flattening.
Reportable Test Conditions
- Test temperature and tolerance (for example 70 ± 1 °C).
- Applied deflection percentage or applied load.
- Compression duration and recovery time.
- Specimen dimensions, density, material identification, and orientation.
- Individual and median compression set values for all specimens.
Key Factors Influencing Results
- Temperature: elevated temperature accelerates stress relaxation and molecular rearrangement, increasing compression set.
- Deflection level: higher compression percentages produce greater permanent deformation, particularly in low-density foams.
- Test duration: longer exposure highlights long-term resilience limitations.
- Recovery time: viscoelastic foams continue recovering after the standard 30-minute interval.
- Specimen age and prior mechanical history: materials previously compressed in service or transit show cumulative set.
Applications
- Cushion design validation for electronic products, where foam inserts must retain thickness to maintain clearance protection during drop and vibration events.
- Medical device and pharmaceutical packaging, where sealing gaskets and sterile barrier components must maintain contact pressure over the shelf life of the product.
- Automotive component packaging, evaluating dunnage and returnable container liners subjected to repeated compression cycles and high warehouse temperatures.
- Cold-chain and temperature-sensitive shipping, verifying that foams and seals retain resilience after low-temperature exposure.
- Material supplier qualification and incoming inspection, comparing compression set performance across foam densities, elastomer compounds, and manufacturing lots.
- Shelf-life and accelerated aging studies, using elevated-temperature compression set data to project long-term storage performance of protective packaging systems.
- Regulatory compliance and customer specification verification, with results reported by accredited third-party laboratories under ISO/IEC 17025 quality systems.
FAQ
What is a good compression set value for packaging foam?
Acceptance limits depend on the application, but high-quality flexible polyurethane packaging foams typically exhibit compression set values below 10 % after 22 hours at 70 °C and 50 % deflection. For critical cushioning or sealing applications, specifications of 5 % or lower may be required. Always compare results against the applicable product specification and test condition, because values from different deflections, temperatures, and durations are not interchangeable.
Which standard should be used for testing?
Use ISO 1856 or ASTM D3574 (Test I) for flexible cellular materials such as polyurethane and polyolefin foams, and ISO 815-1, ISO 815-2, or ASTM D395 for dense rubber and elastomeric sealing materials. The Chinese national standards GB/T 6669 and GB/T 7759.1 are technically aligned with the corresponding ISO methods and are commonly referenced in domestic Chinese supply chains.
How does temperature affect compression set results?
Higher test temperatures increase molecular mobility, accelerate stress relaxation, and soften polymer cell structures, all of which raise the measured compression set. A foam showing 5 % set at 23 °C may exceed 15 % at 70 °C. Selecting the test temperature to match the worst-case logistics environment—such as a hot shipping container—is essential for realistic qualification.
What is the difference between constant-deflection and constant-load methods?
The constant-deflection method holds the specimen at a fixed compressed thickness using spacers and is ideal for simulating fitted packaging inserts. The constant-load method applies a fixed force and better simulates stacked or weighted loads where the material is free to creep. The two methods produce different results and should not be compared directly.
How many specimens are required?
Most standards require a minimum of three specimens per test condition. For quality-control programs and material comparisons, five or more specimens improve confidence in the median result, particularly for anisotropic foams where thickness direction and flow direction affect performance.
Can finished packaging components be tested instead of standard plaques?
Yes. Die-cut inserts, gaskets, and laminated structures can be tested directly when the deflection, temperature, and duration are defined to reflect actual service conditions. In such cases, the functional thickness of the component replaces the standard specimen thickness in the calculation, and results should be interpreted against the minimum cushioning or sealing thickness required by the package design.