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Vacuum Forming, Movie Prop, Sci-fi and GIjOE Forum
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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 Posted: Wed Oct 25, 2006 7:37 pm Post subject: |
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| Quote: | | So why not just use a super powerful pump in the first place? |
Because you'd want a 60 CFM, 28 inch pump. You want 60 CFM so that you could pull about cubic foot of air out from under the plastic in about one second. After that, you don't need high volume, you only need high vacuum at moderate volume---enough to stay ahead of the leaks. And you only need that for about 15 to 30 seconds, but you typically have several minutes until the next pull.
A pump that is so brutally powerful as to suck a cubic foot a second and suck 28 inches of mercury indefinitely would be very, very expensive. Two much cheaper pumps (or one pump and two tanks) can do the job, though, by specialization of labor. You only need to pull at a high flow rate for about a second, and only need to pull hard at a medium flow rate for about 20 seconds.
Even a vacuum pump that can just keep up with leaks and pull hard (after the plastic is pulled down) is fairly expensive. It's cheaper to use a tank that can store enough vacuum to pull comparably hard at that flow rate for 30 seconds or so, with a cheap pump that can evacuate that tank in a few minutes. (I.e., a pump several times too slow to keep up with leaks, which is several times slower, in turn, than a pump that can pull the first cubic foot in first second.)
With a two-stage system and a tank, you can do just fine with vacuum pump that's dozens of times slower than you'd need to do it all by brute force.
| Quote: | | I understand the torque analogy, thought it would seem more like one pump is merely exhausted and another need take over hence---do you need a second tank on account of a second pump? |
No. Pumps don't get exhausted; they keep on chugging along, and you can use one pump to evacuate two tanks. Having two small tanks instead of one big one is so that you can use the tank volume more efficiently. One pulls the plastic down, and gets very polluted in the process, because it sucks in a lot of air.
Then you close it off and open only the other tank, which doesn't have nearly as much air to suck and can suck it much harder. (Because the volume under the plastic has gotten much smaller after pulling the plastic reasonably close around the mold, and that tank has not gotten polluted with that air; it still holds a hard vacuum.) |
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plasticfan
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Joined: 10 Aug 2006
Posts: 43
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| Posted: Fri Oct 27, 2006 7:45 pm Post subject: |
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Drcrash
Thanks for your answers. This is very interesting.
Now why couldnt a tank be quickly released to blow out the air, for another quick go round. Surely I would not stand behind this opened valve for good reason, yet it seems like a good idea to merely blow out the air via a back valve and, seal it up and continue pumping.
My only concern would be the pressure emitted. Bottle rocket heaven I guess |
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Sat Oct 28, 2006 10:43 am Post subject: |
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| Quote: | | Now why couldnt a tank be quickly released to blow out the air, for another quick go round. |
Remember that these are vacuum tanks, not positive pressure tanks. If you just open a valve into the tank, it doesn't blow out air---it sucks air in.
(Unless you used up all of your vacuum; in that case it doesn't do much of anything, because the pressures inside and outside the tank are the same.) |
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plasticfan
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Joined: 10 Aug 2006
Posts: 43
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| Posted: Sun Oct 29, 2006 11:09 pm Post subject: |
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| ah, I see. Makes sense. |
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