Quote Originally Posted by armatac View Post
well you can, print a part, then you can use it to make a mold using somethign like silicone RTV (room temperature vulcanizing) rubber. This done properly (vented/parting line) then allows you to pour whatever substance you want to make it out of.

Typically for steel or aluminum parts you want prototyped they print them in wax and then using the lost wax method of casting generate the part. A company I did design work for hand many samples made up like this to test them. Much cheaper than running a CNC to make them and faster.