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Rubber pad pulled into bell jar

So I have the V.I.C12 vacuum casting machine from Rio which has a separate investing table and casting machine side by side running off one 5 CFM vacuum pump.

My issue is with the investing table with a side draw bell jar. At about 25 inches Hg vacuum and above the 1/2” rubber pad gets pulled quite significantly upwards into the jar causing whatever I have sitting on the pad (so far just a beaker of water to test the vacuum) to tilt alarmingly. So far the beaker hasn’t fallen over, but I’m worried about a tall flask tipping while vacuuming and hitting the jar and/or spilling investment everywhere.

I’ve cut a 1” x 1” X-shaped slit through the middle of the rubber pad (Rio’s recommendation to keep the pad flat when vacuuming) with no improvement. Then I tried a little bit of talc inside the slits to help keep them open, that didn’t seem to help. Any other suggestions?

I get good vacuum (about 28 inches Hg max) and can boil room temp water in about 1 minute so it’s working properly, it just seems dangerous to have everything tilting around in there under vacuum.

Thanks in advance for your help! :smile:


My thought after your photo would be that the small amount of water in your flask doesn’t equal the weight of a flask full of investment. Physics will out.
Try the actual investment. Its cheap. Be brave and bold.
good luck

Is the plate on which the pad sits smooth, or has it been scored in any way? If so, you could fill any scratches with epoxy. Does the pad have any imperfections?

Also, the bell jar bottom is very close to the left and right edges of the pad. One or the other is more likely to be where the air is getting under the pad.

Have you tried pressing down on the bell jar during the first 10-15 seconds after the pump is started? That might help.

Just questions, but I hope that they might be helpful.

put soapy water under the pad, flatten the pad and then push down on the bell when you start the vac. if that doesn’t work try Vaseline under. Better buy the vic 9 jar for any job that will fit the smaller jar. The smaller jar will get to full vac in like 1/2 the time.

The pad is coming up because air is leaking under the pad. Ask Rio to replace the now cut pad.

For the vic 12, it would be a better machine if they made a chamber extension to fit 9" flasks.

cheers M

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Well I ended up trying it with one invested 3x4” flask and the degree tilt that occurred was pretty alarming, but it did stay upright and functioned so I guess I just hope it keeps working despite the weird rubber pad issue!

Thanks for your encouragement!

The plate beneath the pad seems quite smooth, and I don’t see any obvious flaws with the rubber pad except for the1”x1” x-shaped cut in the center that it came with from Rio.
I tried moving the jar further from the edges of the pad (not a lot of room to move though with the size of the jar) and applying light pressure to the jar during vacuuming, neither made an appreciable difference.
I was able to vacuum a flask without it toppling over though which was encouraging. Might just be a quirk of the machine I have to live with!

Thanks for your suggestions!

Next time I vacuum I’ll try the soapy water under the pad and see if that makes a difference.

Having a smaller jar that pulls vacuum faster might be nice, but seeing as it’s a side-draw jar I would have to unhook/reattach the vacuum hose each time I switch the jar. And I’m loathe to spend more money at this point! But I’ll keep it in mind as an option down the road.

It’s weird that Rio recommends cutting the pad as a solution to it pulling upwards, and in fact ships the pads pre-cut. I must not understand the physics involved because I feel there wouldn’t have been any problem in the first place without the cut in the pad!

Thanks for your suggestions, hopefully one of them will fix my issue!

I went to you tube to see what they did — this is interesting —

The pad should have hole or neds to be cut through. It it is a solid piece and you pul vaccum inside the bell, there will be higher pressure under the pad and lower above it. So higher pressure air under pad will push onto lower pressure above - this is what causes pad to bulge upwards. If there is a hole or big cutout, vacuum will be pulled also under the pad so everywhere will be the same pressure and pad will stay down.

So you’re saying that the x-sharped hole that has been cut in the middle of the pad is correct, and should actually be made wider since it doesn’t seem to be allowing in enough air to balance out the pressure differential when the vacuum is pulling?

Wouldn’t pulling too much air from under the pad affect the ability to quickly pull a vacuum though?


Pretty cool! But much braver than I am. I’m a bit nervous running my vacuum table, let alone making one from scratch!