Vacuum Pump Rating

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

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

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

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

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: 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.