these rapid prototyping printers can make things in a variety of ways; they can extrude different plastics (Fused Deposition Modeling shown above) or they can even use a laser to fuse real metal particles together or sinter acetate sheeting. it's the most fun technology I have worked with because it allows the creation of shapes that could not be cast due to geometric law or any number of the laws of physics. they do take a long time to print (I printed (FDM) a hitch receiver plug that took 12 hrs,) but, if you were to machine a similar part, it may take weeks.