Cutting foam is difficult with traditional methods. The best way is with a hot wire. If you read Hackaday, it is a good bet you can figure out how to use electricity to make a wire hot without any help. However, there’s something clever about [MrGear’s] minimal build.
As you can see in the video below, he uses a 9V battery, a clip, some popsicle sticks, and the wire from a ballpoint pen. He also used a switch, but we couldn’t help but think that was unnecessary since you could just unclip the battery to turn the device on and off. Since he used hot glue to attach the switch to the battery, replacing the battery would be a pain.
This is a quick tool for short-term use: we imagine shorting out a 9V battery will require you to replace the battery fairly often. Of course, you could probably use a lantern battery if you wanted something that would last longer. Just don’t use a lithium ion battery since shorting them is very dangerous.
If you want something more involved, have a look at [Grant Thompson’s] build (see second video, below) that is a lot more substantial. He uses a strand of steel hanging wire for the cutting element and a spring to keep it tense. He also has a good trick for getting the waves out of the wire, that might come in handy even if you build the [MrGear] version. Looking for something between those two extremes? We’ve covered several foam cutters in the past.