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Junior vacuum cast issue

I have a vac-u-cast junior from Stuller.
Too many bubbles on the casting. When I called and asked I was informed it pulled 29 inches and that that is plenty for investing. I don’t cast with it.
My previous machine pulled 32 inches and it seems I need more vacuum than I’m getting from the Junior.
Anyone else have this issue or a suggestion to improve the performance?

For investing pulling 29 psi is great. The max is 30 Psi of the Jr. if there are bubbles on the casting that is caused by

Hello Sam_Brown, have you ever considered or tried centrifugal casting? Regards, Richard Lucas

Sam, generally this is caused by wax that isn’t cleaned before investing. Use methyl alcohol on a lint free cloth and lightly rub the wax. Make sure the cloth is wet enough to get alcohol into all the nooks and crannies. Do this right before investing but make sure all the alcohol is dry before investing.

Thanks for the replies. Occasionally I would get a bubble with my old machine which pulled more vacuum. This machine seems underpowered. Andy explained 29 inches is sufficient but I don’t think so. I don’t cast very often now days, but back in the past I would throw 3/4 times a week. Just wasn’t the issue that it is now. I’m doing everything else the same. If I had it to do over I would buy another machine

You can vibrate the flask, try touching a sander to the plate. Make sure you use soapy water around the seal of the jar. You can reduce the free space inside the bell jar, buy a smaller bell jar, or make a small jar from tubing and a thick piece of plexiglass. The altitude of your studio will change the value on the gauge. Higher altitude probably does not reduce effectiveness. You might also try more water in the investment to reduce viscosity. Read the instructions on the investment. Kerr offers several mix ratios…

cheers marty