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I have recently taken up prop making and just started my first foam-built costume from a video game. These kinds of costume armour builds are often built out of the various foam floor mats you can buy in say Home Depot or BJ's or Five Below for around $1 for 4 sq. foot tile.

The one thing that puzzles me is why foam dulls a sharp steel blade so easily and so quickly.

I have tried x-acto knives, those snap-off knives, even surgical scalpels. Regardless, after cutting several linear feet (maybe in the neighborhood of 5-10?), the knife blade begins to dull rapidly and no longer produces nice cuts, but begins to tear the foam instead. You can always sharpen your blade on a stone, but in short order it will again dull and tear.

I generally do all my cutting on one of the green, self-healing cutting mats. This could explain the tip dulling, but it is actually the whole length of the blade exposed to the foam that gets dull.

It happens across types, thicknesses and densities of foam. The floor mats are about 0.5 " and medium density. The craft foam from Michaels is 6mm, high density and rigidity. The roll I have is 0.25" low density, low rigidity

It absolutely baffles me how soft squishy foam just destroys the edge of a metal knife.

I am guessing it has something to do with the molecular organization of the foam or something, but would love to know if anyone has a good explanation for why and how this happens.

eidylon
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  • Have you seen if the blade gets hot, and how hot it gets? – pho Mar 01 '14 at 00:17
  • not that I've noticed... maybe would be slightly warm to direct touch, but not such that you can just feel heat being thrown off by it. You have me curious now though if it is warm to the touch. I'll have to see about that. – eidylon Mar 01 '14 at 00:24
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    This is strange and interesting if true, and I have no clue. – DumpsterDoofus Mar 01 '14 at 00:27
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    Do you have a microscope? I suspect fibers are getting torn out of the foam and are sticking into the micro serrations in the blade and padding it so that the metal no longer makes good contact with the foam. I have foam mats, scalpels, and a microscope. If I'll give it a try and if I can replicate it, post an answer. – Brandon Enright Mar 01 '14 at 00:40
  • Also, what are you cutting the foam against? Is the blade passing through the foam into some other backing material? – Brandon Enright Mar 01 '14 at 00:43
  • No microscope. I'll update the Q with more details regarding the cutting. – eidylon Mar 01 '14 at 00:55
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    FWIW, I have the same observation, cutting into polystyrene foam insulation sheets. I'm also curious about the explanation. I think heating is not the answer, as I've observed no noticeable heating (blade is still cooler than my fingers). A solution for the OP's dulling problem: cutting foam works really well with a hot wire cutter, which can be bought or built fairly easily. Just make sure you work in a well ventilated area, as the fumes are pretty nasty. – Kyle Oman Mar 01 '14 at 01:19
  • @PranavHosangadi - Checked this weekend, after cutting a bunch of the foam, no, the blade is not at all warm to the touch. – eidylon Mar 10 '14 at 20:17
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    Aeromodelers cutting even very lightweight 3 mm "Depron" foam, or plastic film, have observed the same for years. To avoid tearing, replace the X-Acto blade every few minutes. And buy them in packs of a hundred. – Camille Goudeseune Jul 28 '19 at 02:12
  • @KyleOman I eventually was given a hot-wire. While it works very well for the hard, crunchy types of foam, like styrofoam or insulation foam, it doesn't work very well for the softer, rubberish, EVA foams. Just FYI. – eidylon Nov 09 '21 at 18:48

4 Answers4

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"Soft and squishy" is a bulk property. That's not due to either the polyurethane or the air fraction individually, but of the combination.

However, at the scale of a knife's edge, that foam is in fact locally either air or polyurethane. Cutting the air bubbles is trivially easy, but that small fraction of polyurethane is actually quite tough. So, while you're putting only a few newtons of force on the knife, at the edge those few newton are concentrated on only a few points. Let's assume that each point where you're cutting the foam is 0.1mm square and the force locally is 0.1 N. That gives a pressure on the edge of 10 MN/m2, or 10.000 bar. Quite a lot.

MSalters
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My guess is that the foam has some type of sillica in it which may stick to the knife having a detremental effect on its cutting ability possibly cleaning the blades regularly with alcohol could help this but i'm no chemist, hopefully this is helpful

Daz Hawley
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Having previously done academic research on various aspects of knives, I agree with @MSAlters - the actual polymer you're cutting is quite tough. The practical solution is to do what professional knife-users, like butchers, carpet layers, whatever, do: run the blade through a hand-held sharpener after every few strokes. I now do this when indulging hobbies, whether cooking or carpentry. Works!

rdt2
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  • Maybe stropping the blade after every few cuts would also be effective. Sharpening removes material. Stropping does not. – Solomon Slow Aug 27 '15 at 23:39
  • Agreed. But a strop, usually made of leather and quite large, is less convenient than a small hardened-metal sharpener. – rdt2 Aug 28 '15 at 00:04
  • @rdt2 - Yeah, that's what I do now. I got this nice little Smith's pocket sharpener, and after cutting out every 2 or 3 pieces, I just sharpen the blade again. But man, it did bug the crap out of me how foam won over a steel blade! LOL . – eidylon Aug 28 '15 at 19:22
  • Strictly speaking, what butchers and chefs do every few minutes is not "sharpening", but "honing". And then they sharpen perhaps once every day. I can't tell what the Smith's Pocket Sharpener does; whether it actually sharpens by grinding or simply hones. – Mads Skjern May 19 '19 at 05:59
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Use a heat knife. Theatre folk use them religuosly to cut foam of any type. The blade wont dull, becuase a heat knife blade is already there ("butter-sharp").

user9435
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  • Aircraft homebuilders cut large foam blocks to an airfoil shape with a "hot wire" through which electricity flows at constant current, to melt instead of cut. – Camille Goudeseune Jul 28 '19 at 02:08
  • I actually got a hot-wire that someone donated to me, ... but it's mixed effectiveness. It works EXCELLENT for things like styrofoam or the pink/green insulation foam boards. But rubber foams like EVA foam, it had a really, really, really hard time cutting through. – eidylon Mar 23 '21 at 21:54