Q: What is meant by “vacuum casting”? Does it happen in a vacuum?
A: It is much the same as lost wax centrifugal casting. The burned-out flask, at the correct temperature for casting, is placed not in a vacuum, but on a vacuum. Thus, the vacuum is pulling against the bottom of the flask, or if it’s a chamber type, a perforated flask sits into the evacuated chamber, with a flange on its upper part sealing against a hole in the chamber, and the vacuum surrounding the lower part of the flask. Either way, the open sprue end is exposed to the air (or in higher-end machines, covered by inert gas shielding). You’re pulling air or inert gas through the investment from the mold cavity.
When metal enters the mold, it allows the vacuum to then remove the remaining air in the mold, trapped by the entering metal, and in so doing, creates a suction pull on the metal as well. To do this, you need enough vacuum, both in inches of mercury and in CFM capacity, to pull through the fairly porous investment and create enough of a drop in pressure on the outside or bottom of the flask to generate suction inside the flask when the liquid metal is introduced. The function of the vacuum is both to remove the air in the mold cavity that would prevent the metal from filling it, and at the same time to allow atmospheric pressure to help push the metal into the mold more effectively than gravity alone can do. If you didn’t use centrifugal or vacuum pressure, the considerable surface tension exerted by a small amount of melted metal would keep it sitting on the top of the mold, and it would not fill the mold cavity.