PLA is the most common filament used in consumer 3D printers. It has a very low melting point so even hot water can soften it. The PLA printed mold-mold (lower right) has vertical sides and it would be pretty well impossible to remove the plaster mold from it without significant damage to the corners. So I just heated them a little and they peeled off easily. Making these two mold halves confirmed how accurate 3D printers are nowadays, the pieces mate precisely. Notice there are no notches, I feel they are not needed (lining up the outer edges perfectly positions the pieces).
Beer Bottle Master Mold Project
A project that took several years of failures and blind allies and is finally coming together - so much simpler than expected!