The vacuum system of a vacuum former really does two jobs, which require different things of a vacuum source:

1) Pulling a significant volume air out from under the plastic in a hurry, during the "initial pull-down," i.e., the first second of forming.

2) pulling the plastic down really hard, by rarefying the remaining air as much as possible, and maintaining a high level of vacuum until the plastic cools enough to harden in the new shape.

These are two very different tasks. During the initial pulldown, the volume of the space between the plastic and the mold is actually being reduced, and the vacuum level there typically does not approach the vacuum level in the tank. (Unless the plastic is very thick and/or difficult to form at all.)

After the initial pull-down, when most of the "easy" air has been removed and the space to be evacuated has shrunk considerably, harder vacuum is needed to stretch the plastic into smaller concavities or bend it around sharper convexities.

To do both of these things well by brute force, with just a single pump, would require a pump that can pump large volumes of air quickly (against little resistance) and which can also pump small volumes of air very hard. It should have both high CFM's (air flow) and high "inches of Mercury" (vacuum hardness).

Pumps like that are pretty expensive. There is a torque-like issue here.

It's easy to make a modest-sized, inexpensive pump that has little "leverage" but a lot of "throw," and can displace a lot of air quickly as long as the resistance is small. Typically such pumps are centrifugal pumps, like vacuum cleaners use; a good vacuum cleaner can pump dozens of cubic feet per minute in free air, but can't pump any air at all against a resistance of 2 or 3 PSI.

It's also easy to make an inexpensive pump that doesn't move much air per unit time, but can pump a little air against very high resistance. (It's set up with much more "leverage" against a smaller amount of air per pumping cycle.) As an extreme example, a little $20 tire inflator air compressor generate 200 PSI or so of pressure, and can be adapted and used as a vacuum pump, pulling over 24 inches of mercury, i.e., 5/6 of the way to a perfect vacuum. But it takes a long time to evacuate a sizable tank, and will wear out if you use it that way a lot. (It will probably overheat if you try to evacuate a sizable tank in one go.)

The usual solution is to use a single pump that can pump moderate volumes of air very hard, and evacuate a sizable vacuum tank in a few minutes. The tank is then used for both the "initial pull-down" and the "long hard pull."

The vacuum plumbing schematic for that looks something like this:

Code:
+ - - - - - - - - - - +       
|        platen       |        
+--------+  +---------+       
         |  |                
         |  |                                       +-------------+
         |  |        +-------+                      |             |
         |  +--------+  ball +----------------------+   vacuum    |
         +-----------+ valve +---+ +----------------+    tank     |
                     +-------+   | |                |             |
                                 | |                |             |
                                 | |                |             |
                                 | |                |             |
                                 | |                |             |
                                 | |                |             |
                              +--+ +--+             |             |
                              |  high |             |             | 
                              |  vac  |             |             |
                              | pump  |             |             |
                              +--+ +--+             +-------------+
                                 | |
                               exhaust
The ball valve is closed to evacuate the tank... the pump will then pull from the tank and not the platen. For forming, the ball valve is opened, and air rushes from the platen into the evacuated tank.

Given a big enough pipe & valve between the platen and the tank, a tank can pull a lot of air in a hurry, during the first second or so of forming. An evacuated tank can also pull very hard---as hard as the pump that evacuated it, minus small losses.

There's a problem here, though. If the tank is used for the initial pull-down, the air pulled out from under the plastic goes into the tank, and weakens the vacuum. For example, if there's one cubic foot of air under the plastic that must be pulled out, and the tank holds two cubic feet of hard vacuum, sucking the plastic down will weaken the vacuum in the tank by half. Only half of the pump's potential will be available for the long hard pull.

The usual (partial) solution to that is to use a large tank. The tank usually has at least four times the maximum volume of air to be pulled from under the plastic, so that the initial pull-down only weakens the vacuum by at a quarter at most. Three fourths or more of the pump's vacuum strength is then available for forming.

The problems with a large tank are that

(1) it typically costs more,
(2) it takes up more shop space, and especially
(3) it requires a higher-volume (read: more expensive) pump to evacuate.

Even if you can accept the longer cycle time, evacuating a larger tank will wear your pump out that much faster, and increase pump costs in the longer term.

Doug Walsh's solution to this (in his book "Do It Yourself Vacuum Forming for the Hobbyist") is to make a "two-stage" vacuum system, which uses a cheap high-volume (but low vacuum) pump for the initial pulldown, and a cheap high-vacuum (but low volume) pump to evacuate a modest-sized tank for the long hard pull.

Before showing Walsh's solution (and mine, which uses one pump and two tanks, rather than two pumps), I'll show a small modification to the basic one-pump, one-tank scheme that can allow two-stage operation, given a fast enough pump.

This works if your pump is too slow to do the initial pull-down by brute force (hence the tank) but is fast enough to keep up with leaks around the platen edge and maintain a high level of vacuum for the long hard pull.

The tank is only used for the initial pull-down, then the connection to the tank is closed with a second ball valve:


Code:
+ - - - - - - - - - - +       
|        platen       |        
+--------+  +---------+       
         |  |                
         |  |                                       +-------------+
         |  |        +-------+       +-------+      |             |
         |  +--------+  ball +-------+  ball +------+   vacuum    |
         +-----------+ valve +--+ +--+ valve +------+    tank     |
                     +-------+  | |  +-------+      |             |
                                | |                 |             |
                                | |                 |             |
                                | |                 |             |
                                | |                 |             |
                                | |                 |             |
                             +--+ +--+              |             |
                             |  high |              |             | 
                             |  vac  |              |             |
                             | pump  |              |             |
                             +--+ +--+              +-------------+
                                | |
                              exhaust
With the second ball valve closed, the pump pulls air from the platen only. Without it, it would also pull air from the tank---the air that was just sucked into it by the initial pulldown---and wouldn't achieve a higher level of vacuum than the tank.

This is a simple "two-stage" system, where the tank is used first, to perform the initial pulldown, and the vacuum pump alone is used for the long hard pull.

Unfortunately, it requires a pump that's fast enough to quickly remove most of the air that is left under the plastic after the initial pull-down quickly, and hold the plastic down hard in the face of leaks around the platen edge. That's not nearly as expensive a pump as one that could do the initial pull-down quickly without a tank; with a good enough platen edge seal and a moderately fast pump, it should work well.

It does not require a large tank, or incur the pump wear costs of evacuating one. The tank only needs to be modestly larger than the maximum amount of air to be pulled out from under the plastic, becase the pollution of the tank doesn't affect the strength of the long hard pull.

For example, if there's up to a cubic foot of air under the plastic, and two cubic feet of vacuum in the tank, the pulldown will pollute the tank by 50 percent at most, but it will still pull at least two or three times more strongly than a vacuum cleaner. (That could be useful for forming thick or difficult-to-form plastic, where even the intial pulldown requires significant force.)

Other schemes allow the use of a cheaper pump, which can't even keep up with leaks around the platen edge by itself. (Much less do the initial pull-down.) For that, we need a tank for the second stage. (The long hard pull.)