A hot wire knife is used for cutting foam with smooth, clean lines and to avoid damaging the foam with tears. While expensive hot wire knives can be found at most hardware stores, you can save yourself a bunch of money by making your own with a few modifications. By using a low powered soldering iron you can harness just enough heat to cut through the foam and by using a very thin, fine blade you can get a cut just like you would with a wire.
Place the soldering pencil into a vice with the tip pointing upward and toward you. Make sure the pencil is completely cool before starting to work with it or you could hurt yourself.
Twist off the rod tip from the soldering pencil until you can remove it completely from the tool. This will leave a threaded hole in the tip of the tool.
Apply lubricating oil and drill into the end of the soldering pencil to widen the hole. You need to make the hole wide enough to accommodate the shaft of the blade.
Clean the drilled out hole with a brush and reapply the oil. You will have to monitor the lubrication constantly and add more to avoid damaging the pencil.
Drive the tap into the hole to create the new threads. Make sure you remove the tap after every new half turn.
Clean the threaded hole and twist the shaft of the blade in. Make sure you screw the blade in properly or it will fall out.
Plug the hot knife into a power source and let it heat up completely. Leave the iron at full heat until the light smoking stops. The smoking is the lubricating oil burning off.
Go to thrift or second hand stores to find cheap, old soldering pencils.
Do not leave the hot knife unattended when it is plugged into the power as someone could seriously hurt themselves on it.