You can't transmute lead into gold, but these days it is apparently possible to change a part, 3D printed in PLA plastic, into a metal part.
The method used by maker Jeshua Lacock of 3DTOPO, emulated the eternal lost-wax casting method. He printed parts in PLA on his Ultimaker 3D printer and then surrounded them with plaster and sand. By heating this to some +1200F the PLA burned away, leaving a cavity in the plaster mix corresponding exactly to the original printed part. It was then simply a matter of pouring in liquid flowing aluminum and waiting for it to cool.
This process looks reasonably straightforward, assuming you have the equipment to do the job. We're wondering whether this process could be simplified by a project that created a machine to do the casting? Software could automatically create sprue shapes to print the required shape for casting, and a high-temp furnace could burn out the PLA and melt metal scraps for the pour.