I created two simple molds, one for a sphere, the other for an angular spheroid approximation:

I forgot to put a releasing agent inside the mold, which bit me later on. Last time, I used grease. I then used clamps to hold the mold halves in place, and then poured the pot metal in, once it got to about 170 F.

Once the metal had cooled, separating the two mold halves proved to be difficult. I tried putting them in the freezer, but that did not improve things at all. I finally took a screw drive and a hammer and split them apart. Half of each mold was ruined.

I believe that portions of the mold were not smooth enough, causing the metal to infiltrate behind the rough spots. So, even a release agent might not have done the trick. On the other hand, there is essentially no metal remaining inside the mold, and the PLA did not melt or warp. The definition of the cast parts shows each layer clearly (which is both good and bad). The little piece at the bottom shows how some of the metal invaded the hexagonal fill space in one place.