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A question about ILD, compression modulus measurement 03 Feb 2014 13:00 #1

Is it true that the thickness of the test foam block is different (4in. block vs. 6in. block) for conventional foam and latex when measuring ILD?

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A question about ILD, compression modulus measurement 03 Feb 2014 13:27 #2

Hi RidgeWhitlock,

That is indeed true. IFD for polyfoam is usually measured using a 4" thick sample, and ILD for latex foam is usually measured using a 6" sample, and each is usually compressed to 25%.

The impact is that the poly foam is compressed less than latex foam in testing, and if poly foam were tested with a 6" sample it would have a higher IFD than it does when tested with a 4" sample.

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A question about ILD, compression modulus measurement 03 Feb 2014 14:20 #3

Hi RidgeWhitlock,

There's also a little more about how ILD can vary between materials in post #6 here .

Phoenix
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A question about ILD, compression modulus measurement 04 Feb 2014 11:11 #4

I don't suppose there's a way to convert those measurements between the two types of foam, is there? I'm only looking for an approximation.

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A question about ILD, compression modulus measurement 04 Feb 2014 11:28 #5

Hi RidgeWhitlock,

There isn't one that I've found useful. Ild/ifd is only one measurement too, and therefore horrible for comparing dissimilar materials.

Using ild works not bad for comparing identical materials assuming it's measured accurately (e.g. comparing talalay of differing firmnesses) because with an identical material 'all else is equal' and you're only varying the firmness.

There's too much variability between materials for it to have helped me any, and certainly it hasn't helped me predict how a material would feel or how that feeling would change with various layers/materials above or below.

You'd need far more info than just ild. The other specifications I've seen are support factor / compression modulus, and just recently I saw a full report which was force to compress (in newtons) on x-axis vs compression on y-axis vs repeated compress/decompress cycles (represented as multiple curves) - which took into account durability/longevity also. It happened to be for coco latex dunlop. And even that wouldn't help compare materials really well.

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Last edit: by dn.

A question about ILD, compression modulus measurement 04 Feb 2014 12:55 #6

Hi RidgeWhitlock,

I would add a "ditto" to dn's comments.

Besides the different methods used to calculate it, ILD can also be misleading because it's only one of several factors that can affect the softness or "feel" of a fast response material (generally latex and polyfoam) including compression modulus, layer thickness, point elasticity, and the other layers or components above and below the layer. Foam materials also have a non linear response curve (unlike many springs) so they can also vary between each other depending on how much they are compressed and the specific response curve of the material (the response curve is typically shaped like a banana with a steeper curve at the beginning followed by a more linear firmness progression and ending with a steeper curve as you get closer to bottoming out).

If you are making a very general comparison between polyfoam that has been tested for IFD on a 4" layer and comparing it to latex that was tested on a 6" layer (and this isn't always the case by any means) and both used ILD ratings at 25% compression (which also isn't always the case) then it would probably be reasonable to use something in the range of a 20% difference but I would be very aware of how misleading this can be because the difference will also depend on the firmness of the layer and the specifics of each material and on how all the other specs and the response curve compares. Section 4.3 here on the PFA site talks about the effect of the thickness of the testing layer but even when you are comparing the same materials they are very clear that there is no general "rule of thumb" that will apply between the different testing thicknesses even when the materials are exactly the same much less when they are different ...

IFD values in the above table were obtained from testing actual foam samples. These values should not be used as anything but a guide. The actual magnitude of the IFD versus thickness change must be determined for each foam type. A simple "rule of thumb" on the degree of change is rarely accurate.

The IFD increases with cushion thickness as you read from left to right. For example, if the IFD of a 2 inch thick cushion is 6.7 lbs/50 in sq., one could expect the IFD of that IDENTICAL foam, if it were 8 inches thick, to be approximately 16 lbs/50 in sq.. However, the rate of IFD increase with increasing cushion thickness also varies with increasing IFD. Note in Figure 1 how the slope of the IFD-thickness line increases as the IFD increases, which shows that the foam thickness effect is essentially "compounded" at higher IFD levels. Please keep in mind that the values represented on this graph are approximate and are displayed here only for visualization of the concept.


Phoenix
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Last edit: by Phoenix.

A question about ILD, compression modulus measurement 07 Feb 2014 08:41 #7

Thanks so much to you both. I realize there are a lot of variables to consider but the 20% approximation is enough for me to work with and does seem to match my experience laying on both types of material, at least with respect to the foam densities I'm considering.

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A question about ILD, compression modulus measurement 07 Feb 2014 08:57 #8

One additional observation, though: given the differences in measurement, wouldn't the support factor of Talalay (~2.7-3, from reading) be roughly equivalent to HR foam (which I've seen in the 2.5 range)?

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A question about ILD, compression modulus measurement 07 Feb 2014 11:44 #9

Hi RidgeWhitlock,

One additional observation, though: given the differences in measurement, wouldn't the support factor of Talalay (~2.7-3, from reading) be roughly equivalent to HR foam (which I've seen in the 2.5 range)?


Some of the better HR polyfoams have a compression modulus/support factor that approaches Talalay yes.

Phoenix
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