Your first thought when you see this is that the plastic mold would melt or catch fire! You are right! The way it is done is as follows...

This is going to be a story about a foundry that cast propellers for boats. That is where I learned how plastic molds are used to cast metal.

I once rented space in a foundry that cast props. It was a dark and dusty place. Also very hot some days! Like when the furnace was heating metal. It was a little noisy then to. There are big fans running to feed air into the furnace.

Here is how they would use the plastic molds.

First they would cast in plaster, using the plastic mold. Next the plaster casting would be used to make a new mold using sand. This mold is called a