cutting upholstery foam

SWMBO has decided that we need to cut the cushions on our sofa down from 6" thick to 4". It's a 2ft wide cut, so electric bread knife is a no-go. Anyone got any bright ideas?

Let her do it

Does a hot wire work for cutting this kind of foam?

Edited By modeng2000 on 31/07/2020 14:20:01

Hot wire stretched between two blocks?? Might be possible if you do it outside, as I expect the fumes are not all that pleasant.

I’m sure you’ve already discounted two other possiblities - a very big bandsaw and buying new 4” foam.

Edited By Steve Pavey on 31/07/2020 14:19:45

Hot wire foam cutter . Plenty on Amazon as your starter for 10 , or model aircraft suppliers such as say Leeds Model Shop ( no connection ) .

Trevor.

Buy new foam 4" thick.

I, and folk I know, have tried a number of ways to thin down wide, thick, foam. None were objectively successful. All were stressful.

Nearest to success was a large electric carving knife on 18" wide (I think) foam which made it up to a bit below the "Not good but it will ...... have to do" level. Actually the first thing tried but not considered good enough so ....

Hot wire didn't do well at all. Hard to keep good tension and a constant cut speed in wide materials. In retrospect the fumes probably weren't good either.

Other advantage of buying new foam is no regrets when SWMBO says "I wish you hadn't cut the cushions down. They are too hard now."

Clive

Edited By Clive Foster on 31/07/2020 14:23:51

Much easier to buy new foam than build a hot wire cutter for a one off project. There are cheap ones for same on ebay but they cut less that 3-4" thickness, not 24".

Paul.

Edited By Paul Lousick on 31/07/2020 14:32:03

Cut 2" off the sofa legs.

In the flexible polyurethane foam industry large blocks are cut with big factory-sized "band knives". And....the blade lubricant used is critical.
Hot wire cutting won't work and might well give rise to toxic fumes as well as the risk of igniting the foam.
Flex PU foam dose not have a defined melting point. It is a thermoplastic and gradually softens at higher temperatures. The softening depends on the specific material, but is around 100 C.

As others above have suggested, buy new foam blocks.

Do you have an upholstery foam supplier in your area ?

They use band knives to cut the foam from the large blocks it is cast in. I had some replacement cushion foam cores made for a motorhome & the band knife the supplier used made very easy work of cutting parallel slices from a block that required a fork lift truck to place it on the machine. Taking thin slivers off large pieces of foam was easy on the machine. Far easier than struggling to do it yourself. & doing a worse job.

Nigel B.

If you're ex BT and kept your 50v iron transformer I happen to know that the old open wire electric fire elements will make a polystyrene foam cutter 3 ft long. You need to make an old style carpenters saw frame, the type that tensions the blade with two lengths of string twisted up with a stick.

I use these people**LINK**

Very good!

Jim

Excellent idea and only takes a few minutes and no need to take the covers off.

hiding now. regards

Definitely NOT hot wire! One of the reciprocating blade electric carving knives will do the job (carefully!). Ideal woukd be a long blade razor sharp sword,

rgds

Bill

My experience of the reciprocating blade carving knives is that they cut foam well if the blade is longer than what is being cut and you feed at an appropriate rate. Too slow being almost as bad as too fast.

If the cut in the foam is a little less than twice the length of the blade its possible, with some difficulty, to get an adequate cut if some variation around the overlap of the blade is acceptable and the thickness doesn't have to be accurately even.

Cut more than twice the blade length will be uneven in the junction areas. Never found a satisfactory technique for peeling it apart and making a straight cut.

The professional band knife makes the job look trivially easy.

Clive

As someone who has actually cut foam slabs on several occasions, I recommend a purchase. In my case, if the result was not acceptable, then a purchase would be made. Nothing to lose and everyone agreed in advance.

If you have no carpenter hand working skills, this will be challenging. This process employs a full size hand saw. And you should make a practice cut to see how patient, accurate and persistent you can be.

Warning, Don't expect to measure the final surface against your surface plate. However it was better than i expected. I did use a coarse old (but sharp) saw. The old rule of thumb applies here, the softer the workpiece, the coarser the blade. Also, make long and slow strokes to keep on the lines. Best that the saw starts on a corner, so you can aim the saw against two of your perimeter lines. Another piece of advice, whether drawing a straight line or using a band or power saw, keep shifting your eye focus to the distant point where you want to end the cut.

Procedure: Draw a visible cut line around the perimeter with a good marker that will dry and not bleed later. Stand the slab against a solid surface, which could be a sheet of building board or wall. Double-face removable carpet tape will help to stabilize. You need to stand with the edge of the slab central to your body, so you can maneuver the saw to make full length strokes without impediment.

Let the saw do the cutting of its own weight, don't apply pressure down.

Paul

Edited By Paul Fallert on 31/07/2020 19:23:00

From the above I get the distinct impression that new cushions are going to be bought irrespective of what happens. It just depends on the consequences of the decision either way.

Buying new ones before the old ones are wrecked does mean that they can be changed back.

I would not recommend trying hot - wire - as mentioned by others it does not work. I did try it and it just melted the foam into a sticky goo along the cut line - left a brown/yellow sticky stuff over the full cut width. Had to run the wire really hot for it to work at all and the fumes were violent. The foam was unusable afterwards.

I have in the past cut foam with a very sharp carving knife(Not electric) took it very slow and do not apply any pressure on the foam to hold it I just rested my free hand on top behind the cutting edge. It did take some starting but got there in the end. But it was not as wide as your piece.

Sounds like opening the wallet time, not as much as I'd expected if I use JimmyB's people

Thanks for the advice, saved me wasting time

Think laterally? Cut lots of 4" strips off the 6" thick slab, to make 4" X 6" strips, and glue the 4" edges together. A proper spray adhesive for foam will produce secure joints that aren't much stiffer than the bulk foam. A well-sharpened carving knife and a metal straightedge - and some patience - will work fine.