This is an update of sorts to an older thread (http://www.stopmotionanimation.com/forum/topics/replacement-tree-mouth) as I've gotten some more time to work on things.
What I'd like to be able to do is replicate these faces fairly accurately so that even in the low light I'm going to be shooting them in they look good. Using milliput it's proved quite tricky to get the faces looking the same at the end. Will mould and casting in foam work for something with this shape or should I consider a different method?
Have you considered casting in Fastcast resin? This is a hard resin that you pour into a mould. It sets very quickly and can then be painted. For replacement mouths you don't really need foam.
If I were doing a cast of the mouth, I would create the mouth shapes and the teeth separately, then glue the teeth in place.
For ways to create a 2 part mould, have a look at the many tutorials on YouTube. You will need to make a silicone mould - a flexible mould to cast rigid pieces. Alternatively have a look at Blue Stuff, available through eBay, which is a reusable flexible mould material. The disadvantage of Blue Stuff is that you can only make one mould at a time. With silicone you would make one mould for each mouth shape. Teeth would only need one mould as they don't change shape.
Haven't heard of fastcast or bluestuff before, I'll have a look. I've used silicone, foam and alginate for moulding in the past. I'd got it into my head that I needed something special because I hadn't seen anything like this moulded before, just lots of armatures but I suppose that yes, the process isn't too different. Guess it's time to make some moulds.
I hadn't thought of doing the teeth separately, despite making them separately and sculpting them on before the miliput dries. Now that I think about it sculpting cavities to glue them into would probably be a great way to make glueing them in easy.
I'm not sure you should be sculpting in milliput, which is quite tricky to manipulate. Super Sculpey is a polymer clay that sculpts really well, so you could sculpt the faces directly in that, then bake them to harden.
I would still make the teeth separately, as they need to look the same from one frame to the next. So they would ideally be moulded. Then you can glue them in place into the hardened Super Sculpey. This way you make one mould for the teeth, and sculpt the finished mouths directly. Might be a cheaper alternative to making multiple moulds.
Don't forget to add some recesses for the tiny magnets to hold the faces in position.
Just read through the original thread. Nick suggested creating the back shape in epoxy putty, then sculpting on top with Super sculpey, to avoid the distortion that is like with just Sculpey.
I think this might be your best method, because the surface will have good detail and the back will be accurate. Have you tried it? (Still recommend doing the teeth separately.)
If you want practice at mouldmaking, you could take a cast of the back shape to speed up the epoxy putty stage. This also preserves your main puppet.