Casting plaster in 3D printed molds is tough, often it's very hard to extract the model from the mold without breaking it, even if you use generous amounts of release agent.

The demonstrated method is to 3D print a very weak 3D mold, cast the plaster in the mold, then boil the mold to make it soft and thus make it easier to release the model.

Also demonstrated is casting a mold that can be used to cast clay figures - a slip mold.

[This video was previously posted on my Grobian account that's primarily for Machine Learning related stuff, so I moved it here.]