I witnessed 3D printing for the first time yesterday. A friend of mine is an old school inventor. (You know, makes things that you can touch.)<p>He has something called an Airwolf (not the helicopter) which seems to be a 3D printer capable of printing its own spare parts.<p>It's pretty cool to watch a prototype being made. There's a lot of considerations in the design, however. You have to think about cavities and bridges, since there's a critical angle that the goop will not be able to settle at. You also have material saving considerations such as how to fill an internal cavity. There's also some tricks about how to make the 3D printed thing look a bit nicer, since there's a limited resolution.