What will multiple vacuum pumps do to a vacuum system, namely to PSI?

Long story short, I'm looking to make a vacuum forming table, and have most of the thing designed. However, I am looking to use two (maybe three) cheap vacuum pumps until I have the money to get a decent vacuum pump. I know that the pumps (4 CFM) if stuck in parallel will get me roughly 8 CFM, however, what will that do to the PSI? The pumps have a 90 PSI pull (if thats the right phasing to it) and I'm unsure what grade of pip/tubing to get for this. Thanks in advance!! 

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DoctorWoo (author)  rickharris4 years ago
My main thing is for a surge tank for a quicker pull. From most of my looking, a surge tank is needed for thicker pulls. I may have read some wrong info, though
All of the smaller vac formers I have used and one which was a commercial system used for forming things as large as Samsonite suitcase use a relatively low vacuum, They are easily able to form acrylic at 3 mm thick and with care I have done up to 5 mm thick.

Acrylic is one of the most difficult materials to vacuum form. Styrene is much easier.

Getting the material to the right temperature is critical so it sags and drapes over the pattern, then a moderate pull will suck it down.

No surge tank or other complicate things.

UNLESS you are intending to vacuum form something HUGE.

The biggest Vacuum form made in the UK was the Topper dingy which is around 3.4 meters long.

You can see here in this hot tub forming video how slowly the vacuum is applied - and the huge sag in the material.


shows a smaller educational sized machine.

DoctorWoo (author)  rickharris4 years ago
This is actually all really handy info, so thank you for all this! As I'm sure you know, I may have dived into this project a little sooner then I should have...thankfully, nothing is made yet.

I plan to make a 2'x2' table, mainly for pulling prop related item (visors, armor pieces, etc). Some of the higher end stuff (armor for one) would be better with a thicker, tougher material, such the reason for my thoughts of needing a surge tank. On top of that, the how-to I'm using also called for a 30 gallon tank and 10 CFM vaccuum pump. Seeing as how that pump can get pricey, it raised this question.

However, it looks as if I do not need that setup after all. This raises another question then: what would be the recommaned vacuum needed to pull sheets of plastic and get decent details (ie, the grooves of a stromtrooper helmet)?
Sorry, that's a "how long is a piece of string" question ;-)
Too many variables. The best vacuum is lots of it, in a leaky situation like this, you also potentially need a lot of flow to achieve it. Su it depends on the size and shape of the piece, and the plastic, and how much money you've got.....
DoctorWoo (author)  steveastrouk4 years ago
I was kind of afraid of that. But it didn't hurt to ask! However, if your game to answer another stupid question, I was curious if you had any other tid bits to better help with my selection of vacuum pump.
The best advice I can offer is.....suck it and see....

If I had to do it, I'd try either a big shop vac, or a rotary vane pump. I think you'll find a good shop vac is a pretty good method.

DoctorWoo (author)  steveastrouk4 years ago
Sounds like as good a plan as any!
Thankfully, I know truckloads of people that work in some sort of trade, so getting my hands on a decent shop vac won't be hard. If that doesn't do quite what I need, I'll try a pump, and possible even a tank and pump system like I've been seeing.

Although my original question was stupid, and now unneeded, I got quite a few more answers to questions I had. Thanks!
Are you building a suck and blow, not a joke?

Some forming table’s blow hot pressurize air under the plastic as it heats the plastic and then when the plastic is heated to the desired temperature they trip a valve and it sucks the plastic into the mold.

If you use vacuum motors use identical motors connected in parallel, the differential pressure between the ambient air pressure and the air pressure inside the chamber should only be 24psi, however the amount of air moved is crucial to the size of the table and the depth of the mold. So the bigger the more air to move, the more motors.

The 90 is probably 90 cubic feet per minute which is actually quite slow for a large table.
DoctorWoo (author)  Josehf Murchison4 years ago
The plan was (and I think may still be) to build just a basic vacuum former with a oven box and suction table.

As far as the second point, thats also so handy info to have. I've said it a few dozen times on this question now, but my pneumatic knowledge is virtually non-existent.

And it is 90 PSI. The CFM is 4.2. However, the pump I was looking out is cheap (not only in price, but quality, recent googling showed) and I may be abandoning it.
And the blind man picks up his hammer and saw.

I take it the vacuum pump you want to use is the same kind you would use with a bell jar.

This is the vacuum molding machine I use.

It has a heat lamp to heat the plastic and I power it with a shop vac.

Plastic is very soft when it is hot so you don’t need 90 PSI to form it, but you do need air flow to form it before the plastic cools to the point it won’t form to the mold.
No vac pump call pull 90PSI
What you'll need is generous piping that can take a lot of flow, and withstand the pump pressure. Even a decent shop vac might pull adequate vac for what you want.
DoctorWoo (author)  steveastrouk4 years ago
I just looked more into it: you (as you already know) are right about PSI. My knowledge of pneumatics systems of any kind is slim to none...if it changes anything, the air intake of the pumps is rated at 90 PSI, and there is no pascal rating to it at all.
+1 No need for massive vacuum You need flow and a shop vac is fine. Even better if you remove the filter and let the motor have better air flow.

The hardest thing in VAC forming is getting the pattern right and getting the material to the right temperature.

Get either wrong and your plastic will web and spoil the form.