So I would use teeth I cut out of a resin cast to make that part of the mold. I would use a plasticine "borrowed casting" of the gums (I talk more about this technique in this blog post), physically removing any bubbles before molding it. Bubbles can be removed from plasticine but not from rubber! I would make sure the teeth and the gums fit very well together before making molds of either. The big challenge would be not squishing the models and ruining the fit during this process.
Which brings us to this picture. Why, you may reasonably ask, is there a jawset model in my freezer? Very simply, freezing the clay helps keep squishing to a minimum, so I can check the teeth and gums for fit and do as little damage as possible. This kind of plasticine, Monster Clay, contains a high amount of wax, so it freezes very well.
Pictured here are another secret weapon in my arsenal, rubber tipped shaping tools. These also helped keep squishing to a minimum, as they made it possible to apply just the tiniest, most precise amount of pressure to only the areas it was needed. Unlike big fat fingers.
Right, rubber poured over the models. Coloring it like this helps to show when the two parts of the rubber are fully mixed, so that it can be stirred only as much as necessary, avoiding excess bubbles. Pouring the rubber into the molds from relatively high up, in a thin, slow stream, also helps to break bubbles that have formed while stirring. Brushing a thin coat of rubber over the model, letting it partly set, and then pouring the rest of the rubber in also helps, as bubbles trapped against the model can more easily rise to the surface and break that way.