After sculpting the teeth in sculpy and baking them (to help retain it retain it's shape when being cast), I super glued the base of the teeth to some foamex board and then glued a segment of a plastic bottle around it (to act as a wall for the mould).
I mixed some dragon skin (a type of 2 part silicone) and injected it in, making sure that the teeth were ciompletely covered.
When the dragon skin set I pulled it away from the sculp. Seeing that the mould was OK, I mixed some fast cast (a two part resin) to cast a copy of the teeth. I took the cast out to find there were a couple of air bubles in the mould. This wasn't to great a problem as the extra resin caused by the airbubble could be chipped off or sanded down.
Seeing that a duplicate of the top row of teeth didn't look that great as the bottom row, I sculpted another row and repeated the mould process. I then cast a large number of the bottom teeth to go inside the deferent replacable mouths (for diferent facial expressions and lip sync). As I was using a fixed row of teeth for the top set, I only really needed one, but I ended up casting a few spares with the excess fast cast from casting the multiples of the lower set. With the lower sets I did different depths of casts. I did this to save time on sanding back the mouth shapes with slighter exposures of the bottom teeth.