Two flexible foam pads can come off the same line with identical density and hardness, yet one feels dead and the other breathes. The difference is rarely chemistry; it is cell structure. Airflow is the property that exposes it.
Why Airflow Deserves a Defined Test in Flexible Foam
Density tells you how much polymer is present, and hardness tells you how the foam resists an applied load. Neither reveals whether the cell walls have opened enough for air to move through the part. Airflow closes that gap because it responds directly to the number, size, and continuity of flow paths through the cell network.
That makes airflow a sensitive process-control signal. A batch can hold its density and IFD within specification while airflow moves outside the expected band, indicating that cells are beginning to close or that window formation is shifting later in rise. Catching that drift early is cheaper than discovering it later through customer complaints about heat retention, poor recovery, or a boardy feel.
What the Airflow Test Actually Measures
The test is a steady-state permeability measurement. A specimen cut from slabstock or molded foam is held at a defined pressure differential, and the resulting air volume is reported as a flow rate, commonly in cubic feet per minute or liters per minute. ISO 7231:2010 and ASTM D3574 Test G both describe this approach for flexible cellular materials [1][2].
The number reflects the continuity of the cell network after foaming, not the porosity of the raw material. When cell windows are open, air moves readily through the small pathways connecting adjacent cells. When walls remain intact or membranes are thick, resistance rises and the measured flow falls.
Several practical variables can change the result without any real change in the foam:
- Specimen thickness and exposed area. A thicker specimen adds resistance, so comparisons require a fixed sample geometry.
- Skin and cut surfaces. A molded skin or a crushed surface layer can reduce flow even when the core structure is open.
- Orientation. Flexible foam is anisotropic; flow parallel to rise can differ from flow measured across rise.
- Conditioning. Temperature, moisture, and time between cutting and testing shift the result enough to matter.
- Instrument setup. Pressure drop, seal quality, and flow-sensor calibration must be controlled for shift-to-shift comparisons.
Without that discipline, airflow values from two shifts or two suppliers cannot be compared, even when both claim the same standard.
Process and Formulation Drivers of Airflow
Airflow is largely established during rise. The blowing reaction creates cells, while gel strength determines whether windows remain open. If gelation advances too early, windows trap before they can open. If cell walls remain intact or partially closed, flow resistance stays high. Surfactant level, catalyst balance, water content, isocyanate index, and mixing quality all shift this balance in different directions [3].
Molding conditions add another layer. When material is overpacked into a limited-volume mold or crushed too aggressively after demold, some flow paths are permanently collapsed. The result may still pass density and hardness checks because those properties do not isolate cell openness the way airflow does.
Because cell walls open at a defined stage of rise, airflow is sensitive to whether that opening happens early, late, or unevenly. <The Role Of Rise Time In Polyurethane Foaming> covers the timing window that shapes the open-cell network.
Relating Airflow to Production Performance
Higher airflow generally points to a more open, breathable network, but it does not by itself measure support, durability, or recovery. A foam with high airflow can still be too soft if the solid polymer phase is weak. A foam with low airflow may feel firm but hold heat and recover slowly. This is why airflow belongs beside hardness, density, and resilience in a cell-structure evaluation rather than as a replacement for any of them.
Airflow describes how easily air moves through the cell network, while resilience describes how quickly that network returns to shape after loading. <Measuring Polyurethane Foam Resilience A Practical Test Guide> covers the recovery side of the same cell-structure picture.
Need to set airflow limits for a new foam grade? Send your target density, IFD, current airflow readings, and line output to Info@chinahaifeng.com. That information is enough to relate your cell-structure target to metering accuracy, mixing-head speed, and pour pattern before you change a formulation.
A Practical Evaluation Sequence for Incoming and In-Line Foam
Use airflow as part of a small, repeatable method instead of treating it as a certificate value:
- Fix the standard, sample geometry, and conditioning procedure.
- Test multiple specimens in the same orientation for every lot or production campaign.
- Record airflow together with density, IFD, and the date, line, and material lot.
- Set upper and lower control limits from your own stable production data, not from a generic table.
- Investigate when airflow shifts even if the other measured properties remain inside specification.
When airflow drifts, shrinkage and dimensional change often follow the same process upset. <Causes Of Shrinkage In Polyurethane Foam How To Control It> covers the cure and formulation variables that can move both properties off target.
Bring Airflow Into Your Production Baseline
Airflow is most useful when it is anchored to a defined test method, a stable sample procedure, and a narrow process window. If you are setting those limits for flexible slabstock or molded foam, tell us:
- Foam type, target density, and target IFD
- Which test method your customer or QA uses
- The airflow values you are seeing and their variation
- Line output and whether you are running slabstock or molded parts
We can help connect cell-opening behavior to metering precision, mixing-head parameters, and pour pattern so airflow stops being the last property you check.
Info@chinahaifeng.com
WhatsApp: 86 13566296633
FAQ
What is the difference between airflow and density?
Density is mass per unit volume; airflow is the rate at which air passes through the foam at a fixed pressure drop. Two foams with the same density can have very different airflow values if their cell windows or flow paths differ.
What airflow value should a flexible foam have?
There is no universal pass/fail number. The acceptable range depends on the foam grade, application, customer specification, and test method. The most reliable approach is to build a control band from qualified production history using the same test procedure.
Does higher airflow always mean better foam?
No. Higher airflow can improve breathability, but it does not guarantee better support, resilience, or durability. A foam must still be evaluated for hardness, recovery, density, and the specific performance requirements of the intended part.
How often should airflow be tested?
Test every incoming lot or every production campaign at minimum, plus after any formulation or process change. For continuous lines, sample at fixed intervals and after startup, mix-head maintenance, or material lot changes so that drift is detected close to its source.
References
- ISO 7231:2010, Polymeric materials, cellular flexible — Determination of air flow value at constant pressure-drop. International Organization for Standardization, Geneva, 2010.
- ASTM D3574-17, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test G — Air Flow. ASTM International, West Conshohocken, PA, 2017.
- Szycher, M., Szycher’s Handbook of Polyurethanes, 2nd ed., CRC Press, Boca Raton, FL, 2013.
If you’re interested, check out these related articles:
The Role Of Rise Time In Polyurethane Foaming
Causes Of Shrinkage In Polyurethane Foam How To Control It
Measuring Polyurethane Foam Resilience A Practical Test Guide
SRIM Explained Structural Reaction Injection Molding Basics
RRIM Vs RIM Which Process Suits Your Structural Part
