With the Melt 'N Mold Factory from Crayola, you can use Crayola crayons to create custom swirled crayons and molded toys. This kit includes everything you need to melt, mold, and create: 24 full-size Crayola crayons in four different colors, two molds, and a mold holder.
Alpha Foundry Provides Quality, Shell Molded Aluminum and Zinc castings, Foundry Core Production & Machining Services. Our unique process allows us to produce sand castings with appearance and quality that rivals investment and permanent mold castings at significantly lower tooling and production costs.
I don't know specifically what's wrong with that, but if it was that easy, green sand casting wouldn't need to have been invented. I'm guessing the main problems are things like thermal shock breaking the clay, and removing the casting from the mold without breaking it.
Fiberglass molds are commonly used to make multiple copies of a part that may have a complex shape. Some of the advantages of using a fiberglass mold are: the materials are inexpensive, they are easy to make, last many years and can be used to produce hundreds of parts.
The limitations of three-axis machining may seem severe, but seldom truly are. Every section of an industrial injection mold or a metal forming die typically needs to be a depth projection anyway, so that the processed material could be pulled out of it easily.