If you want to do metal casting you have to do sandcasting or possibly, investments casting. In that case you would need to make a positive of what you where going to mold and the use that as a master for sandcasting. you can do burnout of pla or you can use special filament which do ash free burnout. Even for very low temperature metals like pewter, a typical fdm printed mold would warp. Maybe it would be possible with some of the resinprinters using high temperaturetolerance resin. Using sandcasting would be a much more realistic solution at this point. I have written a few posts about that if you are interested. you might also be interested in @fathin-shihab . She does chocolate molds in silicone. alot of the designs decision are similar.