Dentures aren't molded. Well, not exactly, anyway. It's odd and kinda complicated.
There are at least two ways of making molds in dentistry. Dentures are made through impressions - you mix up an impression material and make an imprint in it. That is used to make a cast, which are generally made out of something pretty close to rock. However, it's not as tough as rock (I have fond childhood memories of breaking old ones with a hammer), and so it might not be up to your needs. (this
should give you an idea of the process involved). I'm also not sure how good it is at fine details - teeth are pretty smooth, after all.
In making dentures, casts are then used as reference points for the construction of the denture - usually a wax model is made, the teeth are implanted, then the wax model is sent off to a lab to be finished (which involves a stone lost-wax mold - you can do it in-house, but dental labs can afford to use pressure to remove bubbles, which ensures a better fit).
There's also a neat way involving vacuum forming an acrylic sheet. It makes mouthguards and other things. A dental vacuum former goes for $600+, and it obviously can't do anything that's too three-dimensional (since it works from a flat sheet and you'd have to break the mold to get the result out), but it would be good for detail panels. Or you could make halves and then glue the halves together.
(EDIT: Or make two halves and then make a proper mold from them