Foam Material Softness Evaluation: Testing Standards, Methods, and Applications

Foam material softness evaluation is a critical quality control process that measures the hardness, compression behavior, and resilience of cellular polymeric materials used in seating, bedding, packaging, and sealing applications. By applying internationally recognized standards such as ASTM D3574, ISO 2439, ISO 3386, and GB/T 10807, manufacturers can accurately quantify foam softness, ensure batch-to-batch consistency, and match material performance to end-use requirements. This article provides a comprehensive technical overview of foam softness testing samples, testing items, evaluation methods, and typical applications.

Technical Overview

Foam materials, also known as cellular polymeric materials, are produced by introducing gas bubbles into a polymer matrix, resulting in a structure composed of open or closed cells. The softness of a foam is one of its most important performance attributes, directly influencing comfort, cushioning, sealing force, energy absorption, and user perception of quality. Softness is governed by multiple factors, including the base polymer type, cell morphology (open-cell versus closed-cell), cell size and uniformity, material density, formulation additives, and post-processing such as reticulation or thermal treatment.

Because "softness" cannot be captured by a single parameter, professional evaluation typically combines several measurable properties: surface hardness measured with a durometer, bulk load-bearing capacity measured through indentation or compression force deflection, recovery behavior measured through compression set testing, and elasticity measured through ball rebound resilience. Accredited third-party laboratories commonly perform these tests in accordance with standards published by ISO, ASTM, EN, JIS, and GB organizations, enabling objective comparison between materials and suppliers across global markets.

Testing Samples

Flexible Polyurethane Foams

Conventional slabstock polyether foams, high-resilience (HR) foams, viscoelastic (memory) foams, and molded urethane foams are the most frequently evaluated soft foam categories. These materials dominate mattress, furniture, and automotive seating applications, and their softness profiles are typically characterized by indentation force deflection and compression set behavior in accordance with ASTM D3574 and ISO 2439.

Polyolefin Foams

Closed-cell materials such as expanded polyethylene (EPE), polypropylene (PP) foam, and ethylene-vinyl acetate (EVA) foam are widely used in packaging, footwear midsoles, and sports protective equipment. Their softness and compression behavior are commonly evaluated using ISO 7214 and related polyolefin foam test methods.

PVC, NBR, and Sponge Rubber Foams

Flexible PVC foams, nitrile-butadiene rubber (NBR) foams, EPDM sponge, and neoprene (CR) foams are used for gaskets, weatherstripping, thermal insulation, and vibration damping. Because these products are often thin and moderately firm, Asker C or Shore OO hardness measurements combined with compression deflection testing are the preferred evaluation approaches.

Silicone and Specialty Foams

Silicone foams and other specialty elastomeric foams are used in medical cushions, wound-contact layers, electronics cushioning, and flame-resistant sealing applications. Their softness evaluation frequently requires low-force indentation methods due to their very low hardness values.

Sample Preparation and Conditioning

Prior to testing, specimens must be cut to the dimensions specified by the relevant standard, with cut surfaces parallel and free of skin removal unless the test intentionally includes the skin layer. Standard conditioning is typically performed at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours in accordance with ISO 291 or ASTM D618. Foams with pronounced time-dependent recovery behavior, such as viscoelastic grades, require additional rest periods between mechanical tests to ensure repeatable results.

Testing Items

A complete softness evaluation program typically includes the following measurable parameters:

• Surface hardness: Shore A, Shore OO, or Asker C durometer values, measured per ASTM D2240, ISO 868, or equivalent JIS and GB methods.
• Indentation Force Deflection (IFD), also known as Indentation Load Deflection (ILD): the force required to indent a foam specimen by 25%, 40%, or 65% of its thickness using a circular indenter.
• Compression Force Deflection (CFD): the force required to compress the entire specimen cross-section by 25% or 50%.
• Compression Load Deflection (CLD) stress-strain curve: characteristic values such as the 40% CLD stress per ISO 3386-1.
• Support factor or sag factor: the ratio of 65% IFD to 25% IFD, indicating how quickly a foam stiffens under increasing load.
• Compression set: permanent thickness loss after sustained compression at 50% or 90%, reflecting long-term softness retention.
• Ball rebound resilience: the percentage of drop energy returned by the foam, per ASTM D3574 Test H or ISO 8307.
• Apparent density: mass per unit volume per ISO 845, ASTM D3574 Test A, or GB/T 6343, a fundamental reference for interpreting softness data.
• Hysteresis loss and energy absorption characteristics derived from loading-unloading cycles.

Testing Methods

Durometer Hardness Testing

For very soft foams, Shore OO type instruments apply the lowest measuring forces and are suitable for materials below the range of Shore A. Asker C durometers, widely referenced in Japanese and Asian supply chains, are specifically designed for cellular rubbers and low-hardness foams. Specimens should be stacked or thick enough to prevent the measuring foot from sensing the underlying test plate, and multiple readings are averaged per ASTM D2240 or ISO 868.

Indentation Force Deflection Testing

In the IFD test, a flat circular indenter of 200 mm diameter (ISO 2439) or approximately 323 cm² area (ASTM D3574 Test B1) presses into a foam block supported on a perforated or ventilated base. The force is recorded at 25%, 40%, and 65% indentation after a defined pre-flexing cycle. Lower IFD values indicate softer foam, while the 65/25 ratio, known as the support factor, describes load-bearing progression. GB/T 10807 specifies a corresponding indentation hardness method for flexible cellular polymeric materials.

Compression Force Deflection Testing

The CFD method (ASTM D3574 Test C) compresses the full cross-section of a specimen, typically 50 mm thick and 100 mm square, to 50% of its original thickness, and records the stabilised force. ISO 3386-1 similarly defines the compression stress-strain characteristics for flexible polymeric foams, reporting the 40% compression stress value. CFD data are particularly valuable for gasket design, packaging cushion design, and comparing foams independent of part geometry.

Compression Set Testing

Compression set testing (ASTM D3574 Test D, ISO 1856, or GB/T 6669) compresses specimens by 50% or 90% for 22 hours at elevated temperature, then measures residual thickness loss after recovery. A foam with poor compression set will feel progressively softer and thinner in service, making this test essential for validating long-term softness durability.

Ball Rebound Resilience Testing

A steel ball is dropped from a fixed height onto the foam surface, and the maximum rebound height is expressed as a percentage. High-resilience foams typically return values above 55%, while viscoelastic foams may fall below 20%. This simple, repeatable method (ASTM D3574 Test H, ISO 8307) complements force-based softness measurements by characterizing elastic response.

Data Interpretation and Correlation

No single value defines foam softness. A rigorous evaluation correlates hardness, IFD/CFD, density, and resilience data, and where relevant, compares results against tactile panel assessments or end-product comfort trials. Test reports from accredited third-party laboratories should state the exact standard, specimen dimensions, conditioning history, and test direction (vertical versus horizontal foam rise direction), since foam anisotropy can shift results significantly.

Applications

Softness evaluation data support material selection and quality control across a broad range of industries:

• Bedding and furniture: matching mattress comfort layers and upholstered seat cushions to target IFD grades and support factor specifications.
• Automotive: validating seat foams, headrests, armrests, and door trim cushioning for comfort, durability, and fatigue resistance.
• Footwear: characterizing EVA midsole and insole foams for cushioning feel, energy return, and compression set resistance.
• Packaging: designing cushioning for fragile electronics and precision instruments based on compression stress-strain curves.
• Sealing and gasketing: selecting closed-cell PVC, EPDM, and silicone foams with the correct compression deflection behavior for consistent sealing force.
• Medical and healthcare: verifying softness and pressure-distribution consistency for wheelchair cushions, positioning pads, and orthopedic supports.
• Consumer electronics: qualifying low-hardness foam pads for gap filling, vibration isolation, and display cushioning.
• Sports and protective equipment: ensuring shock-absorbing foams in helmets, pads, and mats deliver the intended firmness and energy management.

FAQ

What is the difference between IFD and CFD testing?

IFD testing indents the foam with a circular indenter while the material is supported on a ventilated base, simulating localized body loading, and is mainly used for comfort grading. CFD testing compresses the entire specimen cross-section between flat plates and is better suited to gasket load and material-to-material comparison.

Which hardness scale should be used for very soft foams?

Shore A is generally too stiff a scale for most flexible foams. Shore OO or Asker C durometers are recommended for low-density, low-hardness materials, and the selected scale must always be reported alongside the measured value.

Does lower density always mean a softer foam?

No. Density and softness are related but independent. A high-density viscoelastic foam can feel softer and slower to recover than a low-density firm polyurethane foam, which is why density data should always be interpreted together with IFD, CFD, and resilience results.

How should viscoelastic (memory) foam be tested?

Memory foams are temperature- and time-sensitive, so tests should be conducted at precisely controlled laboratory temperature, with extended conditioning and recovery intervals. IFD values at multiple indentation levels, recovery time, and compression set provide the most meaningful softness characterization.

Are ASTM and ISO softness results directly interchangeable?

Not always. Although test principles are similar, differences in indenter geometry, pre-flexing procedures, and specimen size can produce systematic deviations. For contractual purposes, the specific standard version should be agreed upon by all parties before testing begins.