Cutting Foam at Home
Cutting polyurethane foams is easy, If you know the tricks!
What to Use
The best way to cut foam at home is by using a serrated blade. A bread knife or an electric carving knife work best. Using serrated blades will give you a much smoother edge than a box cutter or other flat blades, which can leave the foam with a ragged and tattered edge.
Another reason to use serrated blades, as opposed to flat blades, is that foam will dull flat blades very quickly. The actual foam polymers are very tough. So even though you may not be using a lot of force overall, the force exerted on the blade where it is contact with foam polymers is very high because it is very localized. Serrated blades focus more of the force you exert into the tips of the serrations; therefore they counteract the physical properties of foam. This makes cutting the foam with a serrated blade much easier and you will end up with a much straighter, smoother edge.
How to Cut the Foam
Now that you have the right tool for the job, its time to get busy. The first thing you need to do is figure out your measurements. If you are cutting a curve or design, we recommend making a template of the cut you want to do. Parchment paper or scrap cardboard make great templates but you can also use the object you are trying to cover (i.e. kitchen chair base, backrest, etc.).
Once you have your template ready, secure it on the foam with pins or weights of some kind. Draw your pattern onto the foam with a permanent marker (a fine tipped marker like a sharpie is the best choice). If you are going to reupholster the foam by pulling fabric around it and stapling it to a hard backing, you should leave a 1/4" of extra room around your template so that when you pull the fabric down, the foam has a nice rounded look and the fabric gets protected from the backing along the edges. Once you have your foam marked, just follow your lines.
Photo credit: Linda with craftersanonymous.net
If you are doing a straight cut, its way easier. All you have to do is take your measurements, mark the foam, and then cut your line. You can use a drywall T-square, when you are drawing your lines to keep them straight and to ensure accurate measurements. When it comes to cutting the foam, you can line up your foam with the edge of a table to get a straighter cut.
Extra Tips and Tricks
If you don't have an electric carving knife, you can find them at most second hand stores. You can also usually find new ones at specialty kitchen stores.
Thinner foams (~1.5" and less) can be cut with heavy duty scissors.
Closed cell EVA type foams can be cut with straight blades because they have a different cell structure.
When cutting memory foam, go slow. The foam can bind to your blade resulting in and uneven cut.
If you are using a bread knife to cut foam, use long even strokes for best results. short strokes will leave the foam with a more jagged edge
If you need to use an adhesive, make sure it is foam-safe. Many adhesives will actually melt foam. So, if you are unsure whether or not your adhesive is foam-safe, test it on some scrap foam first.