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 “pattern” and will be used to make the sand mold for the metal to be poured into. This is done in a Sand box!! I bet you had already guessed that? The plaster pattern is placed in the box and sand is packed around it. This is special sand that cannot be reused. It is mixed in a big machine with waterglass just before it is to be used. This is packed into place and CO2 it blown over it to harden it.
Once this is ready the metal is heated in the furnace and then poured into the mold. It is allowed to cool 12 hrs or so (this depends on the size of the casting – bigger equals longer). Once it has cooled the sand is broken away and discarded. The casting is cooled for another 12 or so hours and then ground and polished.
related catagory Injection molding
Link to this article：How to Cast Bronze and Other Metals with Plastic Molds
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