A hot wire foam cutter is a very handy tool for constructing foam patterns. The process of using foam patterns is known in the trade as 'lost foam casting'. As described on the lost foam casting page, the molten metal is simply poured in. As the hot metal comes into contact with the foam, the foam vaporises, leaving a cavity for the metal.
The hot wire foam cutter is useful because one-off parts can be made quickly and easily.
The cutter is basically just a heated resistance wire (usually 'NiChrome', but can be as simple as a guitar string) through which a current is passed, causing the wire to heat up enough to slice through the foam. Red heat is NOT needed or desired.
A variable voltage power source is required. This is due to the fact that different lengths of wire have different resistances, and that different foams will require different temperates.
one (physical designs)
One simple design is just an automobile battery charger with a light dimmer wired in series on the 120VAC side. The charging clips go to the resistance wire.
Real fancy versions, CNC control