thoughts about how to create a doll for molding and casting

First of all, ask yourself: What material do I want my finished doll to be?

Two possible answers might be:
1. I want my finished doll to be made of POLYURETHANE RESIN.
2. I want my finished doll to be made of PORCELAIN.

IF your answer is polyurethane resin, then you will be working with flexible silicone rubber molds.
In order to make a resin casting hollow, your flexible molds will need to have a silicone rubber core.
The original clay can be solid, or hollow because the core is usually made after the other parts of the mold.

IF your answer is porcelain, then you will be working with rigid plaster molds.
Since plaster absorbs moisture from the porcelain slip, and the excess slip is poured out of the mold
when the slip has thickened around the walls of the mold, then the castings will be hollow when de-molded.
The original clay can be solid, or hollow because a multi-piece plaster mold is made over the original.

The only time you really must worry about making hollow doll parts is when you are making an OOAK doll.
OOAK dolls are not molded, so they must have a hollow for the elastic, when they are tensioned.

The real question is: how are you going to test-string your doll to make sure the joints are properly designed,
before you invest the time and expense of making molds? Some doll artists use a hard material, such as
Apoxie Sculpt, or a material that can be baked hard, such as Fimo, or Cernit. In that case, you would want
to make your originals hollow, so they can be test-strung when hard.
(I use carving wax as an intermediary material.)

This is a list of BJD-making tutorials which you may find interesting, or helpful?
Ball-Jointed Doll Tutorial Links

 

darkviolet: You can definitely make a mold for a hollow part from a solid part, it's just a little trickier to figure out. What you essentially end up doing is making the mold for the outside first, then pressing clay into it to the thickness you want for the final part, then making the mold for the inside. Of course, it's very important to make sure that the shape of the core mold will allow you to remove it afterwards.

 

darkviolet: Take a look at Mothi's blog (Just This And That) to get an idea of the whole process of modeling a small doll, molding it, and casting it. She explains everything.

Have Fun !!!

 

Some drill the shafts after casting, so it is possible to make a solid doll piece. Personally, I would prefer to make the torso hollow, because any change might effect the joints there (and even if it didn't, it'd be a heck of a work to hollow out the torso pieces).