Testing standard
ISO 2439
Flexible cellular polymeric materials — Determination of hardness (indentation technique)
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 2439 measures the hardness of flexible foam by indentation. A 200 mm circular foot is pressed into a large specimen and the force is recorded at specified percentages of the original thickness. The block is preflexed first, then allowed to recover, because a filled or freshly made foam softens over its first compressions and only stabilises afterwards.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ISO
- Edition
- ISO 2439:2008
- Material
- Rubber, elastomers & foams
- Runs on
- Series 7200 and Series 9000
What the test does
A specimen of flexible foam, at least 380 mm square, rests on a perforated support plate. A circular indentor foot 200 mm in diameter, with radiused edges, is brought down onto its centre. A small contact load establishes a repeatable datum and the original thickness is recorded. The foam is then preflexed with two full compressions and allowed to recover. After recovery the foot indents the specimen to each specified percentage of its original thickness in turn — commonly 25 %, 40 % and 65 % — and the force is recorded after a fixed dwell at each step.
What it measures, and why it matters
The output is a set of forces at fixed indentation depths, and, where required, a support factor formed by dividing the force at 65 % by the force at 25 %. These are the numbers on which cushioning foam is actually bought and sold. The 25 % figure corresponds roughly to how firm a seat feels on first contact; the 65 % figure to how it behaves under a load pressing well into it. The ratio between them is what separates a foam that supports from one that feels comfortable for a moment and then bottoms out. Because the measurement is sensitive to cell structure and cure, it also serves as a production and incoming-inspection control.
Specimen and indentor
This is a test on a piece of foam large enough to behave like foam. Almost every requirement here exists to keep the edges and the platen out of the measurement.
- Minimum plan size
- 380 × 380 mmThe indentor is 200 mm across; anything smaller lets the free edges relieve the stress and the foam reads soft.
- Thickness
- 50 ± 2 mm, or as specified
- Indentor
- Smooth circular foot, 200 to 203 mm diameter, edge radius 1 mmThe 1 mm edge radius stops the rim cutting into the surface and turning a hardness test into a local shear failure.
- Support
- Perforated flat plate — 6 mm holes at about 20 mm spacingOpen-cell foam expels air as it is compressed. On a solid plate that air has nowhere to go and adds a rate-dependent stiffness that is nothing to do with the polymer.
- Preflex
- Two full compressions before measurementRemoves the first-cycle softening so the reading describes the stabilised material.
- Recovery after preflex
- As specified, before readings start
- Let bunned or rolled stock recover fully
- Before cutting to sizeDakFoam stored compressed reads soft for far longer than it looks compressed, and this is the commonest reason a batch fails incoming inspection and passes on retest.
A specimen cut smaller than the minimum plan size is not a conservative simplification — it reads systematically soft, because the free edges relieve stress the standard block would carry.
Speed and sequence
- Indentation rate
- 100 ± 20 mm/min
- Preflex
- Two compressions to a high indentation, then recover
- Hold before reading
- A fixed dwell at each indentationFoam relaxes under a held indentation, so the force falls while the foot is stationary. Reading at a fixed dwell is what makes two laboratories agree.
- Contact load
- A small defined preload sets the zeroThe surface of a foam block is not a plane; a small preload finds a repeatable datum for the thickness measurement.
Calculations
Force recorded at each specified percentage of original thickness
The result is a set of forces, not a derived quantity. Which percentages are reported depends on the method letter and the specification.
SF = F₆₅ / F₂₅
- F₆₅
- force at 65 % indentation, N
- F₂₅
- force at 25 % indentation, N
A ratio, so it is dimensionless and independent of overall hardness. It describes how much the foam stiffens as it is compressed — the property that separates a cushion that supports from one that bottoms out.
How the test runs
- 01Let the foam recover fully from any storage compression before cutting.
- 02Cut a specimen at least 380 × 380 mm at the specified thickness.
- 03Condition at least 16 h at 23 ± 2 °C and 50 ± 5 % RH.
- 04Place it on the perforated support plate, centred under the indentor.
- 05Apply the small contact load and record the original thickness.
- 06Preflex with two full compressions at 100 mm/min.
- 07Allow the specified recovery period.
- 08Re-establish the contact datum.
- 09Indent to each specified percentage of original thickness in turn.
- 10Hold for the fixed dwell at each step and record the force.
- 11Calculate the support factor where required.
What the report has to contain
- Reference to ISO 2439 and the edition, and the method letter used
- Material identification and density
- Specimen dimensions and original thickness
- Conditioning atmosphere and duration
- Recovery time allowed after any storage compression
- Preflex procedure and recovery period
- Indentation rate and dwell time
- Force at each specified indentation percentage
- Support factor where determined
- Number of specimens and the mean
What the machine must be capable of
A frame with enough daylight and platen area to take a 380 mm square specimen and a 200 mm indentor, force measurement to ISO 7500-1 Class 1 across a range that may run from tens of newtons to over a kilonewton, and a crosshead holding 100 ± 20 mm/min. Indentation depth is taken from crosshead travel, so the datum must be established under the defined contact load rather than by eye. The support plate must be perforated: open-cell foam expels air as it compresses, and on a solid plate that trapped air adds a rate-dependent stiffness belonging to the apparatus rather than the material. The indentor edge radius matters as well, since a sharp rim cuts into the surface and turns a hardness measurement into local shear.
What goes wrong in practice
Incomplete recovery is the single largest cause of disputed results, and it usually appears as a batch that fails incoming inspection and then passes on retest a day later. Undersized specimens are the next, and they fail in one direction only — always soft — which makes the error easy to miss because it looks like a consistent material difference. Testing on a solid rather than perforated plate inflates the readings for open-cell grades. Skipping the preflex gives a first-compression figure that the material will never produce again. And reading the force at a variable moment rather than after a fixed dwell introduces scatter, because foam relaxes measurably while the foot is stationary.
ISO 2439 or ASTM D3574 Test B1
| ISO 2439 | ASTM D3574 Test B1 (IFD) | |
|---|---|---|
| Indentor | 200 mm circular foot | 203 mm (8 in.) circular foot |
| Minimum specimen | 380 × 380 mm | 380 × 380 mm (15 × 15 in.) |
| Preflex | Two compressions | Two compressions |
| Usual reported points | 25 %, 40 %, 65 % indentation | 25 % and 65 % IFD |
These two are close, deliberately so, and a foam quoted under one is often assumed to satisfy the other. The indentor diameters differ slightly and the reported points differ by specification, so the figures should be treated as comparable in ranking rather than numerically equal.
Questions we are asked about this test
What is ISO 2439?
It is the ISO method for the hardness of flexible cellular materials by indentation. A 200 mm circular foot is pressed into a large foam specimen and the force is recorded at specified percentages of the original thickness. The specimen is preflexed and allowed to recover first, so the reading describes stabilised rather than freshly made foam.
Why does the specimen have to be so large?
Because the indentor is 200 mm across and the foam around it carries part of the load. A smaller block lets its free edges relieve stress that a full-size specimen would carry, so it reads soft — systematically, not randomly. The 380 × 380 mm minimum is what keeps the edges far enough from the foot to stay out of the measurement.
What is the support factor and why does it matter?
It is the force at 65 % indentation divided by the force at 25 %, and because it is a ratio it is independent of overall hardness. It describes how much a foam stiffens as it is compressed further. A cushion with a low support factor feels fine at first touch and then bottoms out under load; a higher factor means the foam keeps developing resistance, which is what makes a seat supportive rather than merely soft.
Why is the support plate perforated?
Open-cell foam has to expel air as it is compressed. On a solid plate that air is trapped under the specimen and contributes a stiffness that depends on how fast you indent rather than on the polymer. Perforations let it out, so the measurement is of the foam and not of a partly sealed air cushion.
Why preflex before measuring?
Flexible foam softens over its first few compressions and then stabilises. A first-compression reading is therefore higher than anything the material will produce again, and is not representative of the foam in service. Preflexing with two full compressions and then allowing recovery moves the measurement onto the stable part of the material's behaviour.
My incoming foam failed hardness and passed on retest. Why?
Almost always incomplete recovery. Foam shipped in a roll, or stored under other stock, stays compressed at the cell level for much longer than it looks compressed, and it reads soft until it has fully recovered. Allowing the full recovery period before cutting specimens — not merely before testing them — removes this, and it is worth writing into the incoming procedure rather than relying on judgement.
Is indentation hardness the same as density?
No, and confusing the two is the commonest error in foam specification. Density describes how much polymer is in a given volume and largely governs durability; indentation hardness describes how firm the foam feels. A low-density foam can be made firm and a high-density one soft. A cushion specified on hardness alone can feel exactly right and wear out quickly.
Running ISO 2439 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Typically 50 N to 1 kN depending on grade and indentation depth | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 over the working range | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Gripping | Circular indentor foot of 200 mm diameter with a radiused edge, over a perforated support plate | Our compression anvils, built to the specimen |
| Environment | 23 ± 2 °C and 50 ± 5 % RH, conditioned at least 16 h after any recovery period | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.