Vacuum Pump Rating

Returning to a thread from November, at last I have some vacform success using an old propane tank between the lab pump and the platen/table. My teeny pump evacuates the 44 litres tank to 18 inches mercury in about two minutes and my plastic of 16 inches by 10 inches is showing pretty good detail. One more tweak to follow all the good advice I got so far: I know I can improve the seal between the frame the plastic is clamped in and the table. From what Ive read so far totally airtight is not necessary esp when the tank swooshes the air in so quick. Does anyone use rubber oven seal? I assume thats heatproof enough. The frame is going to contact the seal as it comes off the oven. My book and web sources seem so vague in saying "airtight seal" but not How! Metal to metal or wood to metal is hardly airproof............. Evacuated

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Silicone gasket material in a tube will work just fine. Apply a thin layer of Vasaline to the surface you don't want the silicone to stick to and a bead of silicone to the other surface. Press the two togeather and wait for the silicone to set up.You can get the gasket material at any auto parts store. HTH Pete

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Thanks Pete
Ive used that on cars, will need a bigger tube though for my frame I think! The bathroom type of silicon seal is the one Ive used liberally on my contraption, but will that be heatproof I wonder?
Evacuated

You are sort of headed in the right direction. I manage the engineering department at a commerical "vacuformer" (we use a more generic term "thermoforming") and can give you a little advice.
You've got MORE than enough vacuum, in terms of static pressure. Where you are lacking is the RATE at which you can remove the air between the plastic and the mold. Yes, a good seal will help, and it is easy to do**, but it will only it be a marginal improvement. Most homemade vacuform machines employ vacuum cleaners, not vacuum pumps. Only a modest vacuum is needed. But, you need to create it between the plastic and the mold nearly instantaneously. Your surge tank is an excellent idea, (we use surge tanks on our machines as well) but I suspect that your vacuum lines are pitifully small (1/4"?). This creates a lot of resistance to flow that slows the process down. The amount of air moved by your pump during the critical second or two that the plastic is forming is negligible. Focus of the surge tank. It's doing all the work. I'd recommend moving it as close as possible to the mold. Increase that vacuum line to 1 to 1.5" diameter (PVC pipe). If you are using a portable propane tank with a tiny valve opening, look at removing the valve so you can get some real flow. Consider a bigger tank.
And just for grins, you might try the vacuum cleaner...
** Don't try to seal to the frame. Too many leaks. The inside dimensions of your frame should be slightly bigger (1/8 to 1/4" per side depending on the thickness of your plastic sheet) than the mold box or baseplate. Their edges in contact with the hot plastic create a seal very nicely. You can use a thin layer of hardware store 100% silicone if you wish. It won't hurt. We don't bother.
Greg Reynolds, IPMS
On Thu, 20 Jan 2005 21:21:42 +0000, snipped-for-privacy@aol.com wrote:
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Thanks Greg I >suspect that your vacuum lines are pitifully small (1/4"?).
Im using 15mm copper pipe but yes the pump inlet is about qtr inch. Tank evacuates now to 22 inches in 2-3 minutes. The pipe from the tank to the mold is about two feet cannot rearrange that til I am working in more spacious surroundings. When I got the tank I had two 22mm pipe connectors welded in the side, the gas valve on the top us closed off. I do not rely on the pump to draw down the plastic, I gave up that route! I only use the release valve between the tank and the mold. Is your silicon from a car shop or a bathrrom shop? Does it matter re heat? Ive stacks of cheap bathroom sealant
Evacuated

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As long as the label reads 100% silicone, you're good. There are a lot of cheap sealants with acrylic in them (some are sold as "paintable"). They won't last nearly as long. (Neither on your vac-u-form machine nor your house!)
Greg

Anybody in the Sonoma/Marin/SF/East Bay area in need of a pressure chamber.
Specfics on request.
Tom

Thanks everyone. I got a bit sidetracked in thinking the seal between plastic and table was vital. In fact the soft plastic makes its own seal and the results are great. I may widen the piping between tank and table to an inch sometime... Evacuated

This thread is nice and I have a couple related questions:
1) When looking for a vacuum pump I learned that thermoforming requires 25-30 inches of mercury in vacuum -- but what about the time that it takes to reach that? One book mentioned a auto intake vacuum but I did find these on the web: http://www.blowerwheel.com/vacuum-pumps-gast.htm They have the "Hg but not enough speed? My plastic will be quite thin.
2) How does having cavities in your mold effect your vacuum requirements, if at all? I am going to try molding a part with a series of 1/2" deep cavities.
Thanks.