Lets say you have a 3D printer, and have a burning desire for a milling machine, or some other tool. Plastic is not strong enough and steel is overkill, so what do you do?
3D print the part, like say a bracket.
You then acquire silicone rubber caulk thinned down a little with paint thinner. Don't use too much.
You then use that to make a mold around the 3d printed part. Wait a day for it to cure and remove the printed part from the mold. Don't tear the mold.
You then melt tin-antimony solder or lead solder in a crucible. Pyrex lab glass works well also.
Pour that into the silicone mold. After cooling you have a die cast part. You can also use this method to replicate items like pulleys, car parts, and even make another 3d printer.