I made this mold in MeshMixer and plan to print with PLA, then pour molten Pewter into the mold. I simply imported the STL into MeshMixer, did a "Make Solid Offset" and then a plane cut to make the face open.
I will post pictures after printed and after casting.
Finally got the pewter and cast it (after many interruptions.... watch out for fillers inside the pewter your melting down).
After the metal had been poured, I waited a little bit and then pulled the halves apart. the metal was nicely hardened and everything looks great so far. I found there is a chemical that makes PLA rubbery, I plan to soak the cube next and pull the rest of the little bits off.
Will post more pictures after it has cooled down and gets cleaned up
If you use supports, make sure they only come up enough to touch the bottom of the mold, anymore and there will be supports inside your old.
I scaled this one up just a tad to make the cast 60mm x 60mm. Printed solid, this mold is about 6.4oz