This works best for foam that’s at least 2 in (5. 1 cm) thick. Ideally, the blade of the knife should be longer than the thickness of the foam. That will make it much easier to slice all the way through. You may have better control over a long knife than some of the other options, so this might be a good choice if you need to make detailed cuts.
When you’re using an electric knife, it’s a good idea to line up the foam so the line you’re cutting is hanging off of your work table. That way, the edge of the knife won’t make contact with the surface while you’re cutting. If your foam is less than about 2 in (5. 1 cm), it’s probably easier to use a rotary cutter or craft knife. You can purchase electric kitchen knives at most department stores. They’re usually pretty inexpensive, so it might be worth picking one up if you have a big project to work on. [3] X Research source
Rotary cutters can be useful for cutting curved lines, but they might not be ideal for intricate work. You could also use a box cutter for thinner foam, as long as the blade is about the width of the pad. Sharp scissors may work for this, as well. However, these aren’t ideal for cutting thicker pads.
In addition, if you move too slowly or you pause while you’re cutting, you might melt the foam, causing it to harden and possibly become misshapen. Hot wire cutters are only safe to use on foam made of polystyrene and polyethylene. Do not use them to cut foam made of polyurethane—the fumes produced would be very toxic. [6] X Research source Be very careful not to touch the wire—it will burn you.
Foam cutters are expensive, so if you just need something that will work for one project, it’s probably not worth the cost.
Make sure you choose a surface that won’t be damaged by a sharp blade or heat if you’re using a hot wire cutter, like a wooden or metal table. If you don’t have a table large enough to work from, place a sheet of plywood or a large sheet of flattened cardboard on the floor. This will protect the floor while catching any foam that falls. If the foam is hanging off of your work surface, the weight might cause the piece to shift while you’re cutting. [9] X Research source To protect your hands during this project, consider wearing cut-resistant gloves if you have them.
If you’ll be working with a thick piece of foam, you may want to mark your cuts on the top, sides, and bottom. That way, it will be easy to make sure your cuts are straight all the way through the pad. Remember, it’s better to err on the side of cutting the pieces a little larger, rather than having them be too small. The foam will compress into place if it’s a little too big, or you can cut off more if you need to. If you’re cutting 2 shapes out of the same piece of foam, avoid having them share a side, just in case you make a mistake. For instance, if you’re cutting 2 cushions from the same foam pad, you’d want to put a gap of at least 1–2 in (2. 5–5. 1 cm) between the squares.
If you’re using a serrated knife, you may need to make a gentle sawing motion with the knife blade as you cut through the foam. As long as the blade stays straight up-and-down, you should still get an even edge. If you’re using a hot wire cutter, carefully push the wire through the foam in a steady, even motion. Don’t stop moving the wire once you start cutting—otherwise, you could melt the foam. If you’re making curved cuts, it might help to hold the knife up-and-down and cut from the side. [12] X Research source
If you need to, make several passes until the knife slices through the bottom of the foam. Do not press down on the foam while you’re cutting. That will change the shape of the foam, making your cuts uneven. [14] X Research source If you’re using a long enough knife blade, you should be able to easily cut through the bottom of the foam pad without having to compress the foam. Make gentle sawing motions if you’re using a serrated knife, but don’t press down hard.
If you’re planning to cover the foam with fabric, a little bit of unevenness is probably fine.
