Foam material volume shrinkage rate testing is a critical dimensional stability evaluation that measures how much polymeric foams contract after exposure to elevated temperatures, humidity, or cold environments. This article provides a comprehensive technical overview of the test principle, applicable international standards, sample requirements, testing procedures, and typical industrial applications, helping manufacturers ensure product quality and long-term performance reliability.
Technical Overview
Foam materials, including polyurethane (PU), polyethylene (PE), ethylene-vinyl acetate (EVA), polystyrene (EPS/XPS), and rubber-based foams, are cellular structures produced through physical or chemical blowing processes. After demolding or fabrication, these materials may undergo post-expansion stress relaxation, cell gas diffusion, or secondary crosslinking, all of which can cause dimensional changes over time. Volume shrinkage rate testing quantifies this dimensional instability by comparing the initial volume of a specimen with its volume after controlled environmental conditioning.
The volume shrinkage rate is typically expressed as a percentage and calculated using the formula:
Volume Shrinkage Rate (%) = (V0 − V1) / V0 × 100, where V0 is the initial specimen volume and V1 is the volume measured after conditioning. Since direct volumetric measurement is impractical for regular rectangular specimens, the volume is derived from the product of measured length, width, and thickness. Linear dimensional changes along each axis are also reported to identify anisotropic behavior caused by foam rise direction or extrusion orientation.
Testing is generally performed in accordance with recognized standards such as ISO 2796 and ASTM D2126 for rigid cellular plastics, GB/T 8811 for dimensional stability of rigid foams, and ISO 1923 / GB/T 6342 for the measurement of specimen dimensions. Accredited third-party laboratories routinely conduct these tests for quality control, material qualification, and compliance verification.
Testing Samples
Rigid Foams
Rigid foam samples include polyurethane rigid foam (PU/PIR), extruded polystyrene (XPS), expanded polystyrene (EPS), phenolic foam, and rigid PVC foam. These materials are commonly used in thermal insulation panels, structural cores, and load-bearing applications where dimensional stability directly affects assembly tolerance and insulation performance. Standard specimens are typically cut to 100 mm × 100 mm × 25 mm or 100 mm × 100 mm × 50 mm blocks with flat, parallel faces and clean edges free of crushing or melt damage.
Flexible Foams
Flexible foam samples cover soft polyurethane foam (slabstock and molded), expanded polyethylene (EPE), crosslinked PE foam, and EVA foam. These materials are widely applied in footwear midsoles, cushioning packaging, upholstery, and sealing products. Specimens are usually prepared as rectangular blocks or die-cut components, and the sampling position relative to the foam rise direction should be recorded because flexible foams often exhibit direction-dependent shrinkage.
Elastomeric and Specialty Foams
Elastomeric foam samples include EPDM foam, chloroprene rubber (CR) foam, nitrile rubber (NBR) foam, silicone foam, and EVA/rubber blends. These closed-cell materials serve in automotive sealing, HVAC insulation, gaskets, and vibration damping products. Specimens must be fully cured or post-treated according to the material specification before testing to avoid confounding effects from incomplete crosslinking.
In general, a minimum of three to five replicate specimens per condition is recommended, and all samples should be conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours (or as specified) prior to initial measurement.
Testing Items
The core testing portfolio for foam volume shrinkage evaluation includes the following items:
- Linear dimensional change in length, width, and thickness after thermal aging
- Volume shrinkage rate after exposure to elevated temperature
- Volume shrinkage rate after low-temperature (sub-zero) exposure
- Dimensional stability under combined heat and humidity conditions
- Irreversible volume change after cyclic temperature exposure (optional)
- Mass loss percentage accompanying dimensional change (optional, supporting post-foaming and volatile content analysis)
Depending on the product specification, results may be reported as absolute dimensional change in millimeters, percentage linear change per axis, or overall percentage volume shrinkage. Testing can be customized to simulate actual service conditions such as hot automotive interiors, tropical humidity, cold-chain logistics, or accelerated storage aging.
Testing Methods
Specimen Conditioning and Initial Measurement
After standard laboratory conditioning, specimens are marked with reference points on each measurement face. Initial dimensions (length, width, thickness) are measured using calibrated calipers or a dimension-measuring device with a precision of 0.05 mm to 0.1 mm, following ISO 1923 / GB/T 6342. Each dimension is measured at multiple positions, and the arithmetic mean is recorded. The initial volume V0 is calculated as the product of the mean length, width, and thickness.
Environmental Exposure
Specimens are then placed in a ventilated aging oven, low-temperature cabinet, or humidity chamber under selected conditions, for example:
- Elevated temperature: 70 ± 2 °C for 48 hours or 168 hours (common for EVA footwear foams and PU foams)
- High temperature: 80 °C or 100 °C for rigid insulation materials per product specifications
- Low temperature: −25 °C or −40 °C for 24 to 48 hours to evaluate cold shrinkage
- Heat and humidity: 40 °C / 93% RH or 70 °C / 95% RH per ASTM D2126 humid aging protocols
Specimens must be positioned to allow free air circulation and must not be constrained, stacked, or in contact with hot metal surfaces in a way that would restrict natural shrinkage.
Reconditioning and Final Measurement
After the exposure period, specimens are removed and reconditioned in the standard laboratory atmosphere for a defined recovery period (for example, 1 to 4 hours per ASTM D2126, or 24 hours for certain specifications) before final dimensions are measured at the same marked positions. The final volume V1 is computed identically to V0.
Calculation and Reporting
Linear dimensional change for each axis is calculated as ΔL (%) = (L0 − L1) / L0 × 100, and the volume shrinkage rate is calculated as (V0 − V1) / V0 × 100. A negative value indicates expansion rather than shrinkage. The test report should include material identification, specimen dimensions and orientation, exposure conditions and duration, individual and mean results for each axis, volume shrinkage rate, and any visual observations such as warping, curling, cracking, or cell collapse.
Applications
Volume shrinkage rate testing is essential across numerous industries where foam dimensional accuracy and aging durability are critical:
- Footwear: quality control of EVA and PU midsoles, where excessive shrinkage after heat exposure causes size deviation, delamination, and outsole separation
- Construction: verification of XPS, EPS, PU, and PIR insulation boards to ensure panel joints remain tight and thermal performance is maintained over decades of service
- Automotive: qualification of seat cushions, headliners, door trim foams, and sealing foams that must withstand high in-cabin temperatures without permanent deformation
- Packaging: evaluation of EPE, EPP, and EPS cushioning inserts that must retain cavity dimensions to protect electronic products during storage and transport
- Electronics and appliances: assessment of silicone and PU gaskets, EMI shielding foam pads, and thermal insulation foams used near heat-generating components
- Sports and consumer goods: stability validation of protective padding, helmet liners, yoga mats, and floating foam products
By integrating shrinkage testing into incoming material inspection, formula development, and outgoing quality control, manufacturers can predict long-term field behavior, optimize blowing agent content and curing parameters, and reduce warranty claims caused by post-molding distortion.
FAQ
What is the volume shrinkage rate of a foam material?
The volume shrinkage rate is the percentage reduction in the volume of a foam specimen after exposure to a specified environmental condition, calculated as (V0 − V1) / V0 × 100. It reflects the material's dimensional stability and its resistance to cell gas loss, stress relaxation, and thermal contraction.
Which standards are used for this test?
Commonly referenced standards include ISO 2796 and GB/T 8811 for dimensional stability of rigid cellular plastics, ASTM D2126 for the response of rigid cellular plastics to thermal and humid aging, and ISO 1923 or GB/T 6342 for the measurement of specimen dimensions. Product-specific specifications may define additional temperature and duration requirements.
What sample size and quantity are required?
For dimensional stability testing, specimens of 100 mm × 100 mm × 25 mm (or the material's full thickness for insulation boards) are typical. A minimum of three specimens per test condition is recommended for statistical validity, and finished parts such as midsoles or gaskets can often be tested directly.
What exposure conditions are most commonly specified?
A frequently specified condition is 70 ± 2 °C for 48 hours for footwear and flexible foams, while insulation materials are often evaluated at 70 °C to 100 °C for up to 168 hours. Low-temperature exposure at −25 °C or −40 °C and humid aging at 40 °C / 93% RH are also widely used depending on the end-use environment.
What shrinkage level is considered acceptable?
Acceptance criteria depend entirely on the product specification. As a general industry practice for footwear EVA midsoles, volume shrinkage after 70 °C / 48 h exposure is often limited to 1% to 2%, while insulation boards typically require linear dimensional change below 1% to 2% depending on the applicable standard and application class.
How is volume shrinkage testing different from compression set testing?
Volume shrinkage testing measures free dimensional change caused by temperature, humidity, or aging without external loading, whereas compression set testing measures the permanent deformation remaining after a specimen is compressed under a defined deflection and temperature. Both tests evaluate long-term stability but address different failure mechanisms.
How long does a typical test take?
Including specimen preparation, initial conditioning, environmental exposure, recovery, and measurement, a standard 48-hour exposure test is usually completed within three to five working days, while extended 168-hour aging tests require approximately one to two weeks. Accredited third-party laboratories can also design accelerated aging programs tailored to specific service conditions.