Placing vacuum caster bowl

I have a Rio Vacuum/casting machine which has one table for both
investing and vacuuming. The pad to be used for investing came with
an off- center cross shaped slit cut into it.

When de-airing the bowl of investment I was told by one person to
put the bowl in the center of the pad. As the slit is to one side of
the pad, this means that the pad and the bowl are over the hole in
the caster which is used for vacuuming.

Another person told me that this is incorrect placing of the bowl,
and that it should be placed off-center on the table, and that I
should cut an additional slit in the center of the pad so that there
will not be any build up of vacuum under the pad. In other words,
there should be two slits in the pad–one off-center, and the other
directly in the center.

My questions boils down to — What is the purpose of the slit? and
Should the bowl be placed over the slit?

Before I do any unnecessary surgery, or ruin my caster I would like
some expert advice.

Alma Rands


I don’t run one of those machines but my instinct says that the
slit(s) are just to prevent vacuuming the bowl to the rubber.
Happens to me every time and I don’t even notice anymore. Prying the
bowl off the rubber is part of my casting ritual.

Demand Designs
Analog/Digital Modelling & Goldsmithing

Happens to me every time and I don't even notice anymore. Prying
the bowl off the rubber is part of my casting ritual. 

Jeff, lay a piece of wire window screen on top of the rubber pad,
and you’ll not have your bowl stick anymore. Or cut a couple narrow
slits into the rim on the bottom edge of the rubber bowl with a mold
knife. Same result. The wire screening is better if you’re using a
very thin bowl or perhaps a metal one from a machine mixer…

Minor point, I know. But it will save you those few seconds of

Peter Rowe

Hi Alma,

I think the slit is there to prevent air from being trapped under
the pad, causing an ever increasing bulge under the bowl, eventually
making the bowl or flask fall over. I don’t think you are going to do
any harm to the machine by cutting more slits or holes. As to
positioning the bowl, try a test run with just water in the bowl and
see if vacuum builds with no bulging anywhere, and then release the
vacuum and make sure it releases correctly. It should come to a
rolling boil in 30 to 45 seconds and stop the instant you release the
vacuum. If it holds vacuum and releases it as you require, you don’t
need to worry about placement of the bowl or cutting more slits or

Sometimes people have a tendency to over-engineer solutions to
problems that don’t exist. The engineers at Rio seem to have a pretty
good handle on what they are doing and selling. Try it the way they
sent it to you, following their directions. If it works the way it
is, why mess with it?

Happy investing!
Dave Phelps

Thanks to everyone for all the good advice about how to place my
investment bowl, and the explanation as to what the slits are for.
And, special thanks to the technician at Rio, who took the time to
explain it all to me.