Heads will rotomold!

Here is how I will be making the children's heads in the masses. To make the ~25 heads, I carved a hard material into 5 types of faces. I split them in half so I have front and back. The next step is to vacuum form them so I have a plastic mold. Think of hollow chocolate bunnies, chocolate is poured into one half of the shell, the mold is closed, and then rotated to cover the inside to create a hollow shell. When dried it becomes that solidified hollow, lightweight version of the form. I will do this same process with liquid plastic so that I can get multiple heads out of one mold, all light weight (eliminating top heaviness) and I can't think of an easier faster method.