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Matt,
It depends on the condition and the brand of the vacuum pump. I have 2 and both can pull at least 27 HG which is what you need if you are going to pump the air out of silcon mold ruber.
I would be careful using it for casting.
I use mine for a couple of seconds and don't let it go over 24 HG. The reason for that is that resin reacts quicker to the catalyst and that action appears to create gases. If you go for it and try to remove all the air, you will end up with a ruptured mold and a blob of harden bubbles like insallation foam.
What you really want to do with resin is to pressureize it.
I use a 3 gal pressure paint pot and place the mold filled with resin inside it and apply 60 psi of pressure to it.
Since a liquid cannot be compressed, and a gas can, the airbubbles are compressed to microscopic proportions. The pre vacuuming before the compression gets the big bubbles out, but not the tiny ones.
 

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Premium Member
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Matt,
You still need a vacuum chamber to make the mold. Without it, you can't compress the casting resin. If you try, the tiny airbubbles that are left in the silicon rubber will colapse and you will end up with a ghoulish looking mutation (for a figure) or a funny set of windows.
Get yourself a good HG guage. You can get some good ones for about 6 bucks (if you are lucky on ebay) You still have to pay shipping on them. Take that guage to your friends house. Attach it to the vacuum pump with a valve to close the open end of the intake.
Start the pump up and then close the valve.
If it is a good one, then you should get at least 27 HG in a second after closing the valve.
If you get 25 HG make sure that there are no leaks in your plumbing and stop any that you find and restart.
Of course, that doesn't tell you how many cubic ft of air space it removes in what time.
Both mine go to HG 27 and remove 3 cubic ft of air in a minute. Slow, but fast enough for me since a 3 gal pot is about half that.
I have a couple of Thomas vacuum pumps. One double action and the other single.
Here is a link to the double.
http://cgi.ebay.com/3-5cfm-THOMAS-V...ryZ46548QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
The single doesn't evacuate air as quickly as the double, but works great as a vacuum clamp and a back up if the other goes out.
Sears carrys a good economic pressure pot for about 99 bucks. You can go online and have it sent to your local sears instead of paying for shipping.
You will have to convert the pot top by taking out the siphon tube and stopping the air output line up. Cost is about 12 bucks.
To use the same pot as a vacuum chamber you will need a 12x12 3/4 inch thick piece of acrylic (new, not used). Carfully drill a hole an inch and a half from 1 edge. I forget the diameter of the hole, but it will need to be the size required to tap the plastic to recieve the threaded pipe you will need to fit the coupler for your vacuum line. I use an regular M type tip and coupler. Use the coupler on the vacuum side, so that air can escape when you remove the line. Other wise it will take you a good hour to get your piece out, maybe much longer if you get a good seal on your threads and all.
 
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