Two-stage vacuum former plumbing alternatives (in 3 parts)

Results 1 to 3 of 3

Thread: Two-stage vacuum former plumbing alternatives (in 3 parts)

  1. #1
    Registered drcrash's Avatar
    Join Date
    Oct 2006
    Location
    USA
    Posts
    125
    Downloads
    0
    Uploads
    0

    Default Two-stage vacuum former plumbing alternatives (in 3 parts)

    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.)

    Similar Threads:
    Last edited by drcrash; 06-04-2007 at 01:52 PM.


  2. #2
    Registered drcrash's Avatar
    Join Date
    Oct 2006
    Location
    USA
    Posts
    125
    Downloads
    0
    Uploads
    0

    Default Two-pump two-stage plumbing (part 2 of 3)

    You can make a two-stage vacuum system with a high-volume, low-vacuum pump for the initial pulldown, and a low-volume high-vacuum pump for the long hard pull.

    The obvious way to do that is to use a three-ported valve to switch between two vacuum systems on the fly, after the initial pulldown.

    Supposing we had a fast enough high vacuum pump to keep up with leaks around the platen edge during the long hard pull, we could use something as simple as this:

    Code:
    + - - - - - - - - - - +       
    |        platen       |        
    +--------+  +---------+
             |  |
             |  |                        +------------+
             |  |           +------------+ 1st stage  +---
             |  |           |  +---------+    pump    +-- exhaust
             |  |           |  |         |(high vol.) |
             |  |        +--+  +-+       +------------+
             |  +--------+ 3-port |           
             +-----------+ valve  |
                         +--+  +--+      +------------+
                            |  |         | 2nd stage  |
                            |  +---------+   pump     +-- 
                            +------------+(high vac.) +-- exhaust
                                         +------------+
    Operation of this scheme is very simple. The pumps can both be running shortly before the plastic is brought down over the platen, with the valve set to allow the high-volume, low vacuum pump to draw from the platen. (Typically that's a vacuum cleaner.)

    After about a second, when that pump has removed most of the air and the plastic has formed around the mold, the valve is switched to allow the high vacuum pump to suck most of the remaining air out, to get good detail, and to keep sucking it hard until it has cooled below its thermoforming range.
    Then both pumps can be switched off.

    For simple manual operation, this works well---there is only one valve to be operated, and split-second timing is not required.

    The downsides of this simple scheme are:

    1. Large three-ported valves are considerably more expensive than simple 2-ported ball valves of the same size. (The valve must be large enough that the initial pulldown can be fast.)

    2. It requires a fast enough pump to keep up with leaks around the platen edge and maintain a high level of vacuum.

    Doug Walsh solves these problems with two refinements:

    1. Using two common cheap valves instead of one uncommon and expensive one---an inexpensive 2-port ball valve to open the path from the platen to the high-vacuum source, and an inexpensive check valve to automatically close the path to the low-vacuum pump. (A "check valve" is simply a one-way valve, allowing flow in one direction between its two ports, but not the other.)

    2. Adding a tank on the high-vacuum side, so that the tank, rather than the pump, performs the long hard pull. The tank is evacuated over a minute or more, allowing the use of a much slower and cheaper pump.

    Code:
    + - - - - - - - - - - +       
    |        platen       |        
    +-------+     +-------+       
            |     |                
            |     |                             +-------------+
            |     |      +-------+              |             |
            |     +------+  ball +--------------+   vacuum    |
            |     +------+ valve +---+ +--------+    tank     |
            |     |      +-------+   | |        |             |
          +-+     +-+                | |        |             |
          |  check  |                | |        |             |
          |  valve  |                | |        |             |
          +-+     +-+                | |        |             |
            |     |                  | |        |             |
            |     |               +--+ +--+     |             |
        +---+     +---+           |  high |     |             | 
        |     low     |           |  vac  |     |             |
        |     vac     |           | pump  |     |             |
        |     pump    |           +--+ +--+     +-------------+
        +---+     +---+              | |
            |     |                exhaust
            |     |               
            exhaust
    Here I've drawn the pipes and valve for the high-volume pump larger than than those for the high-vacuum pump and tank. One of the advantages of this scheme is that the large flow of air during the initial pull down only goes through the large check valve and the high-volume/low-vacuum pump. The plumbing for the high-vacuum side, including the main ball valve, can be comparatively small, and inexpensive.

    The check valve that Walsh uses is cheap, despite its size---it is a 1 1/4" sump pump check valve. (I got one for $7 at Lowes.) That's not really the "right" valve for the task, but it's cheap, it's easily modified to work better, and it's large enough to support a large flow of air for the initial pull down of a good-sized vacuum former.

    The significant modification is the addition of a rubber band to hold the flapper of the check valve lightly closed by default, rather than hanging open. This makes the valve close faster when the high-vacuum system is turned on.

    Here's how it works:

    Before forming, the ball valve is closed, isolating the high-vacuum system, and the high-vacuum pump is engaged to evacuate the tank.

    When it's almost time to form the plastic, the high-volume pump is engaged. It will suck air through the check valve, the "right way", and therefore from the platen.

    The plastic is lowered to the platen, and the high-volume pump performs the basic pulldown.

    After that's done, the ball valve is opened, and the tank begins sucking from the platen as well. It also sucks air backwards through the check valve, from the low-vacuum pump, momentarily, until the check valve sucks shut.

    That's why the check valve is needed, and why it must close quickly... without it, the high-vacuum system would suck air backwards through the low vacuum pump rather than pulling the plastic down really hard. That is also why the check valve must close quickly---if it doesn't, a significant volume of air will be sucked the wrong way through the check valve, polluting the tank and weakening the vacuum.

    This system is just as easy to operate manually, in real time, as the simpler one with the large, expensive three-ported valve. All the operator needs to do is open the ball valve after the initial pull-down. Again, split-second timing is not necessary, because the check valve closes off the low-vacuum side automatically once the high-vacuum valve is opened.

    Using a cheap high-volume, low-vacuum pump for the initial pulldown has one major advantage for a simple manually-operated vacuum former, which may not be obvious.

    Since the high-volume pump can already be running at the moment the plastic is brought down, it will "grab" the plastic and pull it down, maintaining a seal while the operator reaches for the ball valve and opens it. The operator can bring the plastic down manually, and release it to reach for the valve, and the timing comes out about right. The first stage pulldown happens without any further intervention, freeing both hands and allowing time for a reach and a handle-turn.

    For a very simple vacuum former where the operator uses both hands to move the plastic, this is very, very convenient---no hands are needed to hold the plastic down while the high-vacuum system is engaged.

    For some thick, hard-to-form plastics, this might not work; the operator may need to apply force to keep the plastic down and sealed until the high-vacuum system is engaged. (The stretched plastic may contract, overcome the force of the low vacuum pump, and break the seal.)

    In that case, some mechanism may be needed to hold the plastic down, such as a flip frame or a raise/lower lever system (as on Walsh's Proto Form machines). Still, the low-vacuum system may comes in handy. With either a flip frame or a Walsh-style lift bar, the operator may be able to hold the plastic down with one hand, and reach for the ball valve with the other. In the meantime, the first-stage pulldown happens automatically during the reach, because the low-vacuum pump is already running.

    For more detail on a Walsh-style two-stage system, see this thread on www.tk560.com's vacuum forming discussion forum:

    http://www.tk560.com/phpBB2/viewtopic.php?t=424

    That thread has pictures of an inexpensive 2-stage plumbing system, with a few additional features. (Vacuum gauge, disconnects for the tank, etc.) There's also a diagram of Walsh's modified sump-pump valve.

    Last edited by drcrash; 06-04-2007 at 01:51 PM. Reason: replaced placeholder post with actual content


  3. #3
    Registered drcrash's Avatar
    Join Date
    Oct 2006
    Location
    USA
    Posts
    125
    Downloads
    0
    Uploads
    0

    Default Two-tank two-stage plumbing (part 3 of 3)

    Rather than using two pumps for a two-stage system, we can use one high-vacuum pump and two small(ish) tanks. The first stage will use a small tank, only modestly larger than the maximum amount of air pulled from under the plastic during the initial pulldown. Then we'll switch to the second (unpolluted) tank for the long, hard pull to get good detail.

    As before, I'll show an idealized, simple version using a 3-ported valve. In this case, it must be a non-mixing valve; that is, it should connect one port to either of its output ports, or neither, but not both. Then I'll show how to relax that criterion, so we can use a cheap 2-port ball valve instead.


    Code:
    + - - - - - - - - - - +                              +------------+
    |        platen       |                              |   first-   |
    +--------+  +---------+                  +-----------+   stage    |
             |  |                            | +---------+ (initial   |
             |  |                            | |         |  pulldown) |
             |  |        +-------+       +---+ +--+      |  vac tank  |
             |  +--------+  ball +-------+ 3-port |      +------------+
             +-----------+ valve +--+ +--+ valve  | 
                         +-------+  | |  +---+ +--+     +-------------+
                                    | |      | |        |   second-   |
                                    | |      | +--------+    stage    |
                                    | |      +----------+ (hard pull) |
                                    | |                 |  vac tank   |
                                    | |                 |             |
                                 +--+ +--+              |             |
                                 |  high |              |             |
                                 |  vac  |              |             |
                                 | pump  |              |             |
                                 +--+ +--+              |             |
                                    | |                 |             |
                                  exhaust               +-------------+
    To evacuate the tanks, we close the ball valve to isolate the vacuum system from the platen, engage the pump, and switch the 3-port valve to pull from one tank, then the other. When both tanks are evacuated, we set the valve to leave the path to the first-stage tank open.

    When we are ready to form, we lower the plastic and open the ball valve. This allows air to rush into the first-stage tank, performing the pulldown, but polluting that small tank fairly seriously and weakening its vacuum.

    Then we switch the 3-port valve to close off that tank and open up the second-stage tank. Since it's been sealed off, it's unpolluted, and can perform the long hard pull at full strength. (Initially, anyway... it will of course fill up with air leaked around the edge of the platen, slowly losing strength. But at least it'll start out with a good hard full-strength pull.)

    One advantage of this scheme is that if the first tank is appreciably larger than the volume of air to be pulled out from under the plastic, it will pull pretty hard, even if it doesn't pull as hard as the second tank. For example, if it's 2x the maximum volume to be evacuated, and our vacuum is fairly hard, it will pull at least two or three times as hard as a (vacuum cleaner-type) high-volume centrifugal pump.

    A disadvantage for a simple, manually operated system is that it requires a hand on the valve at the moment forming begins. Ideally the main ball valve would open just as soon as the plastic is down and can make a seal at the platen.

    For a very simple system, where two hands are used to lower the plastic and hold it down, this may require a 2nd person to operate the valve. It's therefore more suited to a system where the plastic can be lowered and held down with one hand while the main valve is opened with the other.

    There's also a subtle gotcha with this scheme, in that using a non-mixing 3-port valve may cause a loss of seal in the moment of switching tanks, when neither tank is connected, unless the plastic is held firmly down mechanically. If the 3-port valve is the mixing type, with both ports slightly open when the handle is in the in-between position, there's another problem---for that brief period, the fresh second-stage tank will suck air from the from the polluted first-stage tank, polluting itself somewhat.

    That drawback, at least, can be easily fixed, and in fact the fix makes the plumbing a little cheaper. We can simply use a restricted-flow connection between the 2nd stage tank and the 3-port valve. This could be an "orifice" fitting (a pipe or tubing connector with a constriction to limit flow), or we can simply use a small hose to connect the tank.

    The orifice or hose size should be large enough not to impede evacuation of the tank, and to allow it to comfortably keep up with leaks during the long hard pull, but too small to allow much air to be pulled from the first-stage tank during the time the valve handle is in the intermediate position.

    That idea will also let us use a second (common and cheap) 2-port ball valve instead of a rarer and more expensive 3-port mixing valve, or a still harder to find 3-ported non-mixing valve. (Sure, you can find them, but finding large and cheap ones isn't so easy.)

    The flow requirements are so different during the initial pulldown and the long hard pull that we can simply leave the paths to both tanks open during the initial pull down, and the second tank will not be terribly polluted as long as the flow is very restricted during that time (roughly one second).

    This allows this schematic, using an orifice

    Code:
    + - - - - - - - - - - +                              +------------+
    |        platen       |                   +-------+  |   first-   |
    +--------+  +---------+              +----+  ball +--+   stage    |
             |  |                        | +--+ valve +--+ (initial   |
             |  |                        | |  +-------+  |  pulldown) |
             |  |        +-------+       | |             |  vac tank  |
             |  +--------+  ball +-------+ |             +------------+
             +-----------+ valve +--+ +--+ | 
                         +-------+  | |  | |            +-------------+
                                    | |  | |  +------+  |   second-   |
                                    | |  | +--+ ori- +--+    stage    |
                                    | |  +----+ fice +--+ (hard pull) |
                                    | |       +------+  |  vac tank   |
                                    | |                 |             |
                                 +--+ +--+              |             |
                                 |  high |              |             |
                                 |  vac  |              |             |
                                 | pump  |              |             |
                                 +--+ +--+              |             |
                                    | |                 |             |
                                  exhaust               +-------------+
    or this equivalent one using small, cheap tubing to connect the second-stage tank

    Code:
    + - - - - - - - - - - +                              +------------+
    |        platen       |                   +-------+  |   first-   |
    +--------+  +---------+              +----+  ball +--+   stage    |
             |  |                        | +--+ valve +--+ (initial   |
             |  |                        | |  +-------+  |  pulldown) |
             |  |        +-------+       | |             |  vac tank  |
             |  +--------+  ball +-------+ |             +------------+
             +-----------+ valve +--+ +--+ | 
                         +-------+  | |  | |            +-------------+
                                    | |  | |   small    |   second-   |
                                    | |  | ==============    stage    |
                                    | |  +-+   tubing   | (hard pull) |
                                    | |                 |  vac tank   |
                                    | |                 |             |
                                 +--+ +--+              |             |
                                 |  high |              |             |
                                 |  vac  |              |             |
                                 | pump  |              |             |
                                 +--+ +--+              |             |
                                    | |                 |             |
                                  exhaust               +-------------+
    To evacuate the tanks, we close the first (main) ball valve, and open the second one, and engage the vacuum pump.

    During forming, we open the main ball valve, and after about a second, we close the second one.

    One nice feature of this scheme is that evacuating the tanks leaves the the valves in the right positions for forming, by default. (Main valve closed, first stage valve open.) It's a little more complicated to operate than a one-valve system, but not much. As long as we don't need both hands to hold the plastic down at the beginning, it's pretty easy to use.

    We don't need to hold the plastic down when switching from first-to-second stage operation, because the path to the 2nd stage is already open, albeit small, and suction is maintained. We will not lose vacuum (or our platen edge seal) during the switch.

    Last edited by drcrash; 06-04-2007 at 03:03 PM. Reason: replaced placeholder posting with actual content


Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Two-stage vacuum former plumbing alternatives (in 3 parts)
Two-stage vacuum former plumbing alternatives (in 3 parts)