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Casting vacuum vs just pour

What is the advantage to vacuum casting versus traditional burn and pour? I get vacuum the investment to remove bubbles and such. But at time of pour the vacuum while pouring? I figured if metal hot and flows right should not be a problem. Please enlighten me

A vacuum casting rig takes up less space than a centrifugal rig, is unlikely to fling molten metal at you if there is a pouring issue, and can generally coax metal into thinner parts of a mold than can a centrifugal rig. Relying on gravity to fill your molds does not allow thin parts to be reliably filled, which is why centrifugal and vacuum casting were developed.
FYI, both centrifugal and vacuum casting were developed for the dental industry, as were many of the tools we use regularly, such as the flex-shaft machine.

Nice explanation, Elliot…but I have heard that centrifugal fills lacy details better than vaccum. Not true?

Beats me. I always send out any cast pieces. Living in NY it makes more sense for me to pay a specialist casting house than to try to do everything myself.

John, the need for either vacuum or centrifugal casting methods is necessary because when the molten metal contacts
the investment, gases are formed which will back pressure the molten stream trying to enter the cavity. Vacuum works by
slightly lowering the atmospheric pressure and allowing gravity to fill the cavity. Because centrifugal force is much greater than gravity, the molten metal forces the generated gases to be pushed ahead
through the micro permeability of the investment. I made a rough calculation once based on my flask distance from center of rotation and speed of rotation and came up with a 7X greater force than gravity. Thin items can be cast using both methods, but centrifugal allows
the metal to be at a lower temperature (because the filling takes place quicker) than vacuum and “seems” to be overall more forgiving in achieving a good casting.

Vacuuming provides a path for preventing trapped air as you are pouring molten metal. Centerfuge does this by increasing the momentum of molten metal into the mold. In either case, I think that a “path” to allow air to escape might help. In centerfuge, a surrounding wax screen, can provice multiple “vents” to provide an air pathway. In vacuum casting, you might try an escape tube formed by running a small wax sprue wire from the deepest part of the casting back around to the sprue cup. Maybe not 100%, but when you see molten metal at the escape vent, it’s likely that you have a complete fill. …And remember, there’s always Major Murphy. …