Hi all! I’m doing a project where I need to cut 1 1/2" memory foam into strips, and I’d like to do it square and clean. Somebody posted a comment on a post a while back describing how they did it for their job, but I can’t find it now. (Something about compressing the foam first, using a particular knife, etc.) Anybody have any wisdom for me?
I have had good results making straight cuts in most kinds of foam rubber and memory foam uncompressed using a snap-blade utility knife with a brand new, full-length blade. I extend the blade all the way and cut at an angle so as much of the blade is in the cut as possible. Brand new is important, if the blade has been used even a little it probably won’t cut well. With a very sharp blade there is no “sawing” motion required.
I’ve never cut memory foam - but we produced assemblies for aircraft customers that included various foam inserts (almost like upholstery cushions). We had a Bosch 1575A with several different lengths of knife blades for different thicknesses. It sort of works like an electric knife that uses 2 counter moving scalloped-edged blades to do the cutting. I was then a pricey tool and I’m not sure that it is still made. I see on Amazon that there are alternatives:
The Hercules one looks much like the old Bosch - and was probably sourced from Harbor Freight - but is no longer available on their site.
The current Bosch model doesn’t seem to be readily available this side of the pond - but can be ordered (very expensive for a 1 off project)
You might also experiment with a much less expensive electric knife:
Thanks Mecha and fred! I’ll try a new snap knife, and if that doesn’t work, I’ll check out an electric carving knife.
It occurs to me that the foam might drag on the blade, distorting the foam. So I’ll try the narrowest blade I can find (narrow from edge to back) and maybe put some wax on it, to reduce friction.
Lubing the blade absolutely will help if the foam tries to “grab” it. You can also use your off hand to spread open the cut as you make it.
I’m not sure exactly what you mean by a “clean cut” but if you can tolerate a little fuzz and roughness then fred is correct about power tools being useful. A serrated knife like you might use in the kitchen for bread cuts foam effectively. So do the wavy knife-edge jigsaw or recip saw blades like these:
…I use those for cutting foam, both in a jigsaw and manually with a handle. They cut very well but they don’t leave the cleanest of edges so that might not be an option if appearance is important.
Thanks again, Mecha! I’d not considered knife edge jigsaw blades. It looks like people use them for rigid foam insulation. Do they work for super-squishy foam like memory foam?
Yep, I wasn’t clear when I said, “clean cut.” I’d like the cut to be square and sharp cornered, and looking like it was made in a single cut, looking like a factory cut. A little fuzz is fine though.
I have not tried the jigsaw blades on memory foam so I can’t give any feedback on that, but I have used them many times on soft open-cell foam and they cut that just fine.
A place I worked used a bandsaw with a wavy(rather than toothed) blade for cutting dense foam. Also harbor freight sells a fairly inexpensive hot knife, not sure how the effect on memory foam would be. A hot cutter with just a wire may work better, which isnt terribly hard to make.
When EIFS was all the rage - we bought a number of hot knives from Demand Products Co. They work better (faster and less prone to breakage) than hot wire cutters on rigid foam. They work poorly IMO on foam rubber - with the melted material oozing and getting in the way for the cut. The idea of using a long serrated/scalloped edge bread knife has some appeal to me. we had some bread-knife look-a-likes that were useful for cutting insulation bats, Another thought is that you might try sandwiching the foam top and bottom clamped between boards (or plywood) swished down with just the narrowest cut line exposed. Then use a knife or knife-edge jigsaw blade to make the cut.
A knife-edge circular saw blade might also work with that clamped foam if you use a piece of rigid buiding foam underneath
I was imagining the hot knives to be messy on soft foam and you mostly confirmed that.
I’ve only tried a hot knife on soft foam once and it was messy for me as well. I have never tried it on foam rubber, but I imagine it wouldn’t work. Hot knives/wires work by melting the material. Rubber doesn’t melt, it burns, so even if it could cut the foam rubber it would necessarily leave a nasty burnt edge.
Thanks fred! I’d never heard of knife-edge circ blades.
I would give these a try.
@ChrisP, thanks for that! Yet another tool I’d never seen before.
But I think I’m going with straight blades for now. I understand serrated blades are better for tough, fibrous materials, and this foam is soft. I think serrated blades would rip at the foam rather than slicing it, though a lot depends on the particular serration pattern, of course.
I am sure those blades will cut memory foam well, but if the goal is to have a clean cut then any sort of reciprocating cutting motion is right out.
So supposedly they make a T shank jigsaw blade for cutting foam that acts like that foam cutter Fred posted about. I’ve seen a guy in the hangar use one not sure where it came from. it’s like 6 or so inches long and is wavy not serrated. as best as I remember.
Another thing - lightly compressed with the straight end device - jig saw with blade - and wizz wizz on though at full tilt it appeared. again I wasn’t doing it - but dude was making foarm blocks for tool box drawers with it. Think snap on 42 inch job - drawers were I don’t know 28 or so on one side and 3 inch thick - whatever it is. foam block was 2 piece one red the other yellow so it had a witness color layer.
Those are probably the blades I linked to on Amazon higher up in the thread. I like them a lot, but they may not leave a clean enough edge for Koko’s purposes.
yes that’s exactly it. cut very clean and was a pretty straight line.
Only thing I think would work better is a wire or a band saw with the right blade. I think you can get wires for coping saws and the like that works for this. Or it seems like that would work. depending on the thickness.
About compressing the foam to cut it, I’d worry that would make the cut not perfectly vertical. I guess that didn’t happen?
I bet it depends on how hard you conpress it and the foam’s rebound.
in the case of this tool box foam dude cut it took it well but it’s not like he compressed it 1/2 height to make the cut either.