The choice of mold making material should take into consideration what is being molded. The more complex the shape, the more complex the mold making procedure.

Items that are relatively flat can be cast in a press mold. Those are made by simply pressing the master copy into a substance that holds the shape when the master is pulled back out.

For items that only have slight to moderate undercuts you can use something called a glove mold. The classic example for this is recasting a figure's head. You can use silicone or latex to coat the head, then peel it off by pulling the "mask" up and over the chin, nose, ears, etcetera.

For items with significant undercuts, you'll need a two part mold. Those are made by a process that is roughly equivalent to making two press molds that mate together to form a complete three dimensional mold.

The glove molds are typically only good for a few copies, due to the need to stretch the mold to remove the cast items. They're simple to make, and good for making limited copies of a head for custom work. I've made several of these for copying heads so that they can be mounted on other body types.

I use press molds made from plasticene clay for making latex face masks. Think along the lines of Mission Impossible style face masks. They can be used to put one figure's face on another figure's head.

I've never done a two part mold, so any information I have about those is 2nd hand knowledge.

There are lots of web references for the terms
press mold
glove mold
two part mold
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ScottE