Originally Posted by
drcrash
hmm... let me see if I understand this... You're saying that you want to build a heater and a platen that are each made out of 50cm (just under 20 inches) square modules, so that you can combine them into various sizes and aspect ratios to make, say, a 2 module x 3 module grid that's 100 cm by 150 cm (a bit shy of 40 x 60 inches), etc? Interesting.
What is the maximum size of thing you will be making, and what's it shaped like? In particular, how deep is the draw?
The volume of air you'll need to suck out (within a very few seconds) will depend on the volume of air under the plastic. If you'll be using negative (cavity) molds, that'll basically be the volume of the molds. If you'll be using positive (male) molds, you should think about the "tent" of plastic that you get when you stretch the plastic over the molds. Tall, peaky molds will make a bigger tent than short or gently convex molds, so there will be more air to suck out.
It's common for 2 foot x 2 foot (607 mm x 607 mm) formers to use about 3/4" inside diameter pipe (which is really about 20 mm inside). That lets a high-vacuum system suck about a cubic foot of air out from under the plastic in a second or so.
To scale that up, you want to increase the pipe's cross sectional area (and flow capacity) to match the increased volume of air under the plastic. If you increase all three dimensions of your molds by the same scale factor, you increase the volume of air under the plastic by the cube of that factor.
For example you double the platen and molds sizes in each of 3 dimensions, you increase the volume by a factor of 8. To increase the pipes' flow capacity by the same factor, you need to increase the diameter by the square root of 8, or a factor of about 2.83.
If your molds are a friendly shape, this may not be necessary, and you may only need to scale the pipe diameter up by the same factor as the platen, so that the cross-sectional area of the pipe increases with the area of the platen. (For example, if you were just using a big former to do a bunch of small parts at a whack, that would be true.)
What sizes are your tanks?
If you use a one-stage system, with your tanks ganged together in parallel to act like one big tank, you need tank volume that's several times the amount of air you'll be pulling out from under the plastic. For example, if you have a cubic foot of air to evacuate, and two cubic feet of vacuum (tanks), sucking the air out will pollute the tanks so that they have 1/2 atmosphere of air in them---not a very strong vacuum. But if you have 10 cubic feet of vacuum, you won't pollute the tanks nearly as much---they'll only have about 1/10 an atmosphere of air in them, and pull almost as hard as if you had an infinitely large tank.
You may want to make a two-stage system, where you use one tank to basically pull the plastic down, then close it off and open another valve to the other tanks. That way, one tank will suck up most of the air without polluting the other tanks and weakening their vacuum. Once most of the air has been sucked into the first-stage tank, you can use the others to pull the remaining air out without polluting them much, and pull the plastic down really hard to get good detail.
Another version of that is to use vacuum cleaner pump/motor assemblies to do the initial pull-down, then use vacuum tanks to do the final hard pull. (That's what I generally do.)
A big advantage of that kind of two-stage arrangement is that you don't need nearly as much tank volume, or as much pump capacity to evacuate the tank(s) in a reasonable period of time. You can use cheap high-volume low-vacuum pumps to do the initial pull-down, and cheap low-volume high-vacuum pumps/tanks to do the final hard pull.
By the way, what kind(s) of tanks do you have? Some tanks built for several atmospheres of pressure will implode under less than one atmosphere of vacuum. (Ask me how I know.)