I get confused when reading blurbs regarding the relationship for the ILD (indentation load deflection) of a mattress to its firmness. I understand the basic point that a higher ILD corresponds to greater firmness. My confusion (or disagreement depending on one’s perspective) arises from the relationship of ILD as an indication of firmness as a function of mattress thickness.
[Warning: the following makes reference to high school physics.]
The text I’ve seen online contends that the firmness experienced by a user for a constand ILD increases with increasing thickness of the mattress. This conflicts with my understanding of the effect of “spring constants” I recall from high school. In the spring constant world, for a given material firmness (i.e. spring constant “K”), the longer the spring having a spring-constant K, the lower the effective stiffness of the spring as a whole is. Thus, I’m surprised that various blurbs online on this topic contend that the opposite is true.
If ILD is measured not as the force per distance, but instead the force needed to compress the mattress by a fixed percentage (about 25% as I recall), then the mattress web site blurbs make a little more sense. However, even in the case of a constant-percentage-compression measurement for ILD, it seems to be that the effective firmness experienced by a user would be the same regardless of the mattress thickness.
The motive for answering the above, is that I want to purchase a portable, foldable latex foam layer for travel purposes and want its ILD to match that of the mattress I have at home. Getting to actual numbers, my home mattress (firm side of a Planck) has an ILD of 36 and a seven-inch thick foam core. Can I expect a three-inch thick latex layer to provide the same thickness as the mattress I have at home?
[Side note: I wanted to ask this on the ask-an-expert portion of the site, but did not find a list to select an expert from, that would hopefully have identified the area of expertise of each listed expert].
It’s great that you’re trying to apply physics to these materials but there’s a few things I think it might be helpful to point out. First I would caution using coil springs as a model for how foam works. Other than the fact they both push back as you apply more force the comparison is really apples and oranges. Coil springs use torsion thus the more windings/coils you have the softer it becomes. Also coil springs are also typically linear in force/deflection while most foams are not. This is why foams can’t have a K value (since they aren’t linear, ILD is not at all the same as the K value or spring rate).
As a result, ILD was created as a standardized test to help compare foams. Basically ILD just indicates how much force it takes to deflect an 8" circle into the foam approximately 40% (I know the tests say 25% percentages but my empirical testing shows 40% gives accurate results). Again ILD is just a snapshot because each foam has its own curve. For example you can also see a difference between talalay and dunlop latex foam even though they both might have the exact same ILD rating (deflection curves are different). So needless to say it will require some testing on your part if you want to match something you have already have. You’ll need to match both the deflection curve (remember ILD is just a snapshot) as well as the thickness. If you get both those right then you can be sure it will perform just like what you had already. Hope this makes sense.
As a followup to the above post from Mattrebuild, I noticed that Sleep On Latex is now providing their ILD ratings at both 25% and 40%. Soft: 20 ILD (25%) / 46 ILD (40%) Medium: 34 ILD (25%) / 76 ILD (40%) Firm: 46 ILD (25%) / 100 ILD (40%) with a note: *ILD Ratings measured using ASTM D3574-17 Test B1 / 4" .
[quote=“Sweet Dreams post=88991”]As a followup to the above post from Mattrebuild, I noticed that Sleep On Latex is now providing their ILD ratings at both 25% and 40%. Soft: 20 ILD (25%) / 46 ILD (40%) Medium: 34 ILD (25%) / 76 ILD (40%) Firm: 46 ILD (25%) / 100 ILD (40%) with a note: *ILD Ratings measured using ASTM D3574-17 Test B1 / 4" .
Bill
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Wow, that’s really interesting. I wonder why they picked 40%? Normally the ASTM standard says 25% and 65% (see below). It would be nice if all foam manufacturers started including the deflection curves on their spec sheets so it would make it alot easier to compare different materials when designing a mattress.
One difference I did note between the ASTM test and how I did mine is that I did not preflex the foam before testing so not sure how much of a difference that would make or not (it is possible that’s why I saw the rated 25% ILD’s at 40% deflection instead). Not surprisingly I don’t have an Instron machine at home so it’s not something I could really verify either.