Determination of Softness in Foam Materials: Standards, Test Methods, and Applications

Softness determination is a fundamental quality control and R&D test for flexible foam materials, quantifying how easily a cellular polymer yields under indentation and compression. Governed by recognized standards such as ISO 2439, ASTM D3574, and GB/T 10807, the measurement supports product grading, comfort engineering, and regulatory compliance across bedding, automotive, footwear, packaging, and medical industries. This article explains the applicable test samples, testing items, measurement methods, and typical applications of foam material softness determination performed by accredited third-party laboratories.

Technical Overview

Softness is one of the most important handling and comfort attributes of flexible cellular polymeric materials. For foams, softness is not an intrinsic scalar constant but a mechanical response that is quantified through indentation hardness, compression stress-strain behavior, resilience, and recovery characteristics. In practice, softness is expressed inversely through indentation force measurements: the lower the force required to indent a foam to a specified deflection, the softer the material is considered to be.

Softness determination is typically performed using a universal testing machine fitted with a defined indenter or flat compression platens, together with a conditioned test environment. Measurements are standardized internationally so that results are comparable between manufacturers, suppliers, and accredited third-party laboratories. The principal standards include:

• ISO 2439 – Flexible cellular polymeric materials, determination of hardness (indentation technique)
• ISO 3386-1 and ISO 3386-2 – Determination of stress-strain characteristics in compression
• ISO 8307 – Determination of resilience by ball rebound
• ISO 1856 – Determination of compression set
• ASTM D3574 – Test methods for flexible cellular materials (slab, bonded, and molded polyurethane foams)
• ASTM D2240 – Durometer hardness (Shore OO scale for very soft foams)
• GB/T 10807, GB/T 6669, and GB/T 6670 – Chinese national standards for indentation hardness, compression set, and ball rebound of soft foams
• JIS K 6400 series – Japanese standards for flexible polyurethane foam testing

Because foam mechanical response is sensitive to density, cell structure, ambient humidity, and mechanical history, controlled specimen preparation, pre-conditioning, and standardized test sequencing are essential for reproducible softness data.

Testing Samples

Flexible Polyurethane Foams (Slabstock and Molded)

Conventional slabstock polyurethane foams and molded PU foams used in mattresses, upholstered furniture, and vehicle seating are the most frequently tested materials. Their softness is characterized by indentation force deflection at multiple deflection levels, and the resulting values directly determine comfort grading and specification acceptance.

High-Resilience and Viscoelastic (Memory) Foams

High-resilience (HR) grades and slow-recovery viscoelastic foams require softness testing supplemented by resilience and recovery measurements. Memory foams exhibit strong strain-rate and temperature dependence, so indentation testing must be conducted under strictly controlled laboratory conditions after thermal equilibration.

Polyolefin Foams (PE, EPE, EVA, EPP)

Crosslinked and non-crosslinked polyethylene, EVA, and polypropylene foams used in footwear midsoles, protective packaging, and sports equipment are tested for compression softness, with the appropriate test method selected according to density, thickness, and cell structure.

Latex and Elastomeric Foams

Natural and synthetic latex foams, as well as PVC/NBR and other elastomeric cellular sheets, are evaluated for indentation softness and compression set to verify consistent feel and durability in bedding and sealing applications.

Specimen Preparation and Conditioning

For indentation testing, the preferred specimen is 380 mm x 380 mm with a 50 mm thickness, cut so that the faces are parallel and free of skin defects. Specimens are normally conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours, and polyurethane foams should generally be tested no earlier than 72 hours after production to allow post-cure stabilization. Dimensions are determined according to ISO 1923 and apparent density according to ISO 845, since density is routinely reported alongside softness data. A minimum of three specimens per material is recommended.

Testing Items

Depending on the material and end use, softness determination typically comprises the following measurable items:

• Indentation force deflection (IFD, also known historically as ILD) at 25 %, 40 %, and 65 % deflection
• Compression force deflection (CFD) and compression stress at 40 % strain between flat plates
• Sag factor (support factor), calculated as the ratio of indentation force at 65 % to that at 25 % deflection
• Ball rebound resilience, expressing the elastic energy return of the foam
• Compression set, quantifying the permanent thickness loss after sustained compression
• Shore OO durometer hardness for rapid surface-level quality checks
• Apparent density as a supporting classification parameter
• Flexural rigidity or bending softness for thin foam sheets and laminates

Test reports normally present the measured forces in newtons (or pounds-force for ASTM-based specifications), the calculated sag factor, resilience percentage, and full details of specimen geometry, conditioning, and any deviations from the standard procedure.

Testing Methods

Indentation Hardness Method (ISO 2439 / GB/T 10807 / ASTM D3574 Test B)

The reference method for foam softness uses a flat-bottomed circular indenter with an area of 323 cm² (approximately 202 mm diameter) mounted on a universal testing machine. The specimen rests on a ventilated support plate perforated with a regular pattern of air holes so that entrapped air escapes during loading. The foam is first mechanically conditioned with pre-flexing cycles to approximately 70–80 % deflection to remove manufacturing and storage stresses, followed by a recovery pause. The indenter then descends at a rate of 100 mm/min, pausing for 60 seconds at 25 %, 40 %, and 65 % deflection while the corresponding forces are recorded. Lower indentation forces indicate a softer foam, and the sag factor derived from the 65 %/25 % force ratio describes how support builds with body sink-in.

Compression Stress-Strain Method (ISO 3386 / ASTM D3574 Test C)

In this method the specimen is compressed between two parallel flat platens at a constant crosshead speed, generating a full stress-strain curve. ISO 3386-1 applies to low-density foams up to 250 kg/m³ while ISO 3386-2 covers higher-density materials. The compression stress at 40 % strain is the key reported value, and the hysteresis between loading and unloading curves provides additional insight into energy absorption versus resilience.

Ball Rebound Resilience Method (ASTM D3574 Test H / ISO 8307 / GB/T 6670)

A 16 mm steel ball is dropped from a height of 500 mm onto a conditioned foam specimen at least 50 mm thick. Resilience is calculated as the ratio of maximum rebound height to drop height, expressed as a percentage. This quick, reproducible test complements indentation softness by characterizing the elastic versus viscous character of the foam.

Shore OO Durometer Method (ASTM D2240)

For very soft cellular materials, the Shore OO scale provides a rapid indentation measurement suitable for incoming-material screening on sheets of adequate thickness. Durometer values are useful comparative indicators but are not a substitute for full indentation force deflection testing in specifications.

Compression Set Method (ISO 1856 / ASTM D3574 Test D / GB/T 6669)

The specimen is compressed to 50 %, 75 %, or 90 % of its thickness for 22 hours at 23 °C or 70 °C, and the unrecovered thickness is measured after a defined recovery period. Compression set validates that the initial softness is retained in service and that the foam does not take a permanent impression.

Data Evaluation and Influencing Factors

Results are reported as the mean or median of replicate specimens. Interpreting softness data requires attention to density, cell size and orientation, humid aging, mechanical pre-history, and specimen thickness, all of which measurably shift indentation forces. Interlaboratory reproducibility depends on strict adherence to indenter geometry, crosshead speed, dwell times, and conditioning requirements.

Applications

Softness determination data directly influence material selection, product design, supplier qualification, and quality control in numerous sectors:

• Mattresses and bedding: comfort layer grading, firmness classification, and consistency control of PU, memory foam, and latex layers
• Upholstered furniture: cushion and armrest specification, ergonomic comfort optimization, and incoming foam inspection
• Automotive and transportation: seat pads, headrests, bolsters, and armrests evaluated against OEM material specifications for comfort and durability
• Footwear: EVA and PE midsole and insole softness for cushioning performance and walking comfort
• Protective packaging: foam softness and compression behavior correlated with cushioning performance for fragile and electronic goods
• Medical and healthcare: wheelchair cushions, orthopedic supports, positioning pads, and prosthetic liners where pressure distribution and comfort are safety-critical
• Sports and leisure: helmet comfort liners, protective padding, yoga products, and gym matting
• Apparel and consumer goods: laminated foam sheets, bag straps, toys, and childcare products requiring controlled tactile softness

Manufacturers also use softness data for reverse engineering and benchmarking competitor materials, for verifying reformulated or recycled-content foams, and for demonstrating compliance in tenders and regulatory submissions through accredited third-party laboratories.

FAQ

What is the difference between IFD and ILD?

The two terms describe essentially the same measurement: the force required to indent a foam specimen to a defined deflection. IFD (indentation force deflection) is the current preferred terminology, while ILD (indentation load deflection) is the older trade designation still widely used in bedding and furniture specifications.

Does lower density always mean a softer foam?

No. Density and softness are related but independent properties. Foam chemistry, cell structure, and mechanical history determine indentation force, so two foams of identical density can differ substantially in softness. This is why accredited laboratories report density alongside indentation measurements.

Which standard should be selected for my material?

Polyurethane slab, bonded, and molded foams are usually tested to ASTM D3574 or ISO 2439; Chinese-market specifications commonly reference GB/T 10807. ISO 3386 is preferred where full compression stress-strain data are required, and ball rebound or compression set methods are added when resilience or durability must be documented.

What specimen size is required for indentation softness testing?

The preferred specimen is 380 mm x 380 mm x 50 mm thick with parallel, defect-free faces. Thinner products may be tested by stacking or by using flat-plate compression methods, provided this is clearly stated in the report.

How long does softness determination take?

Excluding specimen supply, routine indentation softness testing including conditioning is typically completed within five to ten working days; compression set testing requires additional aging and recovery time and may extend the schedule.

Can rigid foams be tested for softness?

Softness determination applies to flexible cellular materials. Rigid foams such as EPS, XPS, and PUR insulation boards are instead characterized by compressive strength and compression behavior in accordance with ISO 844 and related national standards.