Cheap, Simple, Portable Vacu-forming Rig

INTRO

Some years ago, a friend of mine was helping a buddy remodel a Chinese restaurant. The owner asked that a long countertop be shortened. The head worker looked it over and thought doing so would be unsafe, so he told the owner it couldn’t be done. The owner couldn’t see why not: it seemed simple enough to him. He replied, “Take saw; cut here”.

I think that if you’re determined to give something a good solid try, the battle is already half won. Giving up before you’ve even started, is the only sure way to lose.

I say that as a guy who was “chicken of” vacu-forming my own parts, for far too many years. Several things conspired to keep me from even trying. One was the machinery I’d have to build: it always sounded like too much could go wrong. If it wasn’t that then it was, “They must be leaving a lot out: it can’t really be that simple”. Also, I wasn’t in any hurry to waste boatloads of sheet plastic, while learning. And no one likes feeling dumb, after failing many times.

Sound familiar? Hopefully, not for much longer.

THE LEARNING PROCESS

To misquote the restaurant owner, the main rules of the vacu-forming process are as simple as this: “Take plastic; heat it; turn vacuum machine on; put plastic on machine”.

It boils down to just five variables -- a heat source; a vacuum source; plastic sheets (of a type made for this sort of work – but the ones sold in hobby shops will be: so no worries there); and a form or shape you wish to duplicate.

The fifth major variable is practice. With that you can gain both practical knowledge and increasing confidence. And early “mold form testing” can be done for free, with a plastic bag: instead of wasting expensive sheet plastic.

Each of those four main steps has several sub-steps or things you should know something about -- but none of them are all that difficult. The laws of physics aren’t going to arbitrarily change, just because you’re vacu-forming.

I’m convinced average folks could build the very simple vacuum machine I’ll describe, in one evening, using common hand tools. After that, if a person were to make one fair attempt per evening, at making one “real” part; and then thought about what did or did not work out very well, on that particular attempt, over the next day ... and then repeated those experimental steps the next day ... I think that person would have very pleasing results in one week, and they’d no longer be intimidated by the process.

I took that a step farther: I tried to “re-work” each of the parts I made, which weren’t quite perfect. I messed the already-bad parts up, even worse, in the process ... but it didn’t matter. I learned a lot from trying to fix what had gone wrong. Each new thing I learned was applied to my next attempt to vacu-form real parts. And my results improved.

HEAT SOURCES

I’ll mostly talk about the vacuum machine I made, in this article. Suffice it to say I’m experimenting with open flames - partly because I like the idea of “portable” scale modeling. (But “Safety First,” folks! Use common sense!)

Many of you will no doubt want to stick to trying what’s been proven to work, time and again: a kitchen oven. Be sure to check out the nice, brief tutorial on the WarmPlastic.com web site, to get an idea of how “normal people” do things.

VACUUM SOURCE CONSIDERATIONS

Let’s discuss the vacuum-related machinery. What is it really doing? What basic purpose do those tools serve?

What’s really going on is that you’re creating a small difference in air pressure, above and below a warm plastic sheet. It isn’t that you’re suctioning air out of under your plastic sheet, so much as you’re allowing the weight of the air above that sheet, to push it onto a form.

Even a difference of one PSI will potentially allow work. Any air that remains below the sheet tends to create a balance. We want an imbalance. Put another way -- any air that remains below the sheet will fight the air above it.

So the more air you can get rid of, the better things work. Speed is a factor, too: your plastic begins to cool as soon as you remove it from your source of heat. When you place it in contact with a mold form, some of the heat is absorbed into that form: rapidly cooling the sheet.

AN UNUSUAL VACUUM SOURCE

Most articles suggest a wall-socket-powered, household vacuum cleaner as your system’s source of vacuum. I suggest something quite different: a small, inexpensive, sometimes even just-battery-powered device. It’s stated purpose? To rapidly inflate or deflate things like air mattresses.

The packaging will say these are low-pressure devices. Measurements with a vacuum tune-up gauge told me my pair have one third of the negative pressure you’d get out of a common vacuum cleaner. That doesn’t sound very impressive. What is impressive, however, is seeing how quickly one of these little deflators can suck all the air out of a large, inflatable air mattress. Just minutes, to empty out many cubic feet of air. It’s low pressure ... but high volume.

Pressure isn’t a measurement of work being done; it’s only a measure of potential work. Push as hard as you can against a brick wall: lots of pressure, but no work being done. Volume indicates that work is actually being done.

Initial suction tests proved to me that pressure alone isn’t everything. A thin shopping bag pulled down not only very quickly and tightly, but with an audible snap. Even a thicker bag (gallon Zip-Lock) snugged down firmly.

SIDE BENEFITS

One nifty thing about using a deflator is that you don’t have to worry about building a fancy vacuum box; or trying to get correctly-sized hose adapters fitted and leak-free.

Cost and space savings are other benefits. These nifty little devices are available at the usual discount stores (Kmart, etc.) for as little as $10 USD, on up to about $22. Years ago, I paid $20-ish for one powered by rechargeable internal batteries. It came with an AC adapter (house “wall wart”) and a car’s adapter. I recently bought a second (wall-socket only) deflator for $4 USD at Kmart, brand new! Why so low? Winter clearance sales, on summer-only items.

GENERAL DESIGN CONSIDERATIONS

A vacuum source, by itself, is not enough. You need some sort of tool to control that pressure differential, and to put it to work. Let’s break it down, functionally. What do we really need this tool to do? We need three main things:

(1) Some sort of a flat working surface for our master part (positive) or our “mold” (negative) to sit on. It has to be solid enough to hold our molds firmly in place, while we drape warm plastic sheets over them. However, air needs to be able to get through it: to remove air from the space between the sheet of plastic, and what we might think of as a “floor”. It also has to be able to withstand the heat that our sheet of warm plastic will try to transfer to it, when the sheet touches the floor. And the edges of the sheet must touch the working surface’s floor, or an airtight seal will not be achieved, around the mold (which sits in the center of that floor). If we don’t achieve an airtight seal, no work can be done.

One common solution for this part of the tool is a flat piece of wood, with many holes drilled in it.

(2) We need some way of connecting our vacuum source to that working surface or “floor”. This is usually not an easy thing to do, because the working surface tends to be a very different shape and size than the connection from our vacuum source. We’re presumably going to desire our custom device to sit, upright and unclamped, on a standard horizontal workbench. That eliminates a bottom-inlet connector; which as we’ll see in a moment, would maximize efficiency.

There is no easy solution to this, hence needing:

(3) Some sort of right-angle-adapter, for our vacuum source’s adapter. One common solution to this problem is to introduce a relatively large, box-like enclosure. The sides of this box will extend down, from the working surface (the top); and must be as tall as needed, to be able to plug in our vacuum source’s air intake -- through an adapter placed on one side (not the bottom) of the boxy enclosure.

If you want to see two examples of the item described above, then check out Michael Benolkin’s reviews of the “Kingston Micro” or their “Canopy Master” wooden boxes.

Granted, their solutions are very pleasing, cosmetically. I think my solution is quite elegant, functionally ... and if I wasn’t freezing / rushed when I made my first test unit, and hadn’t used scrap plywood, it could also be a looker.

SUPER-EASY ASSEMBLY

What I did was to simply take three pieces of 3/8” inch thick plywood, and assign one of those functions to each of the three “slices”. The upper surface has holes drilled into it. I also rounded the outer edges, a bit -- to make it easier to self-align holding frames, as I lower them.

The slice on the bottom just has one round hole in it: that’s my dirt-simple adapter, for my vacuum source. The deflator’s air intake simply sticks up through the hole. The rest of that slice rests on top of the deflator’s flat upper side. One board; one hole; couldn’t be much simpler!

The middle slice is my “incompatible size and shape” adapter, between the other two slices. It’s just a big square hole, cut into a single flat board. That means there is no need to cut perfect corners / edges on a set of four different side pieces; nor to make sure that all four side pieces plus a top and a bottom, become totally airtight. That one board with one hole serves as all four walls. Glue it in between the top and bottom slices, and you’re done!

OTHER CONSIDERATIONS

Technically, that middle hole may be called a “plenum” area. One hidden advantage to my type of vacuum enclosure is that the plenum area is quite tiny. (It measures only 3.375 cubic inches.) A large plenum will hold a lot of air -- and all of that air must be removed, first, by our vacuum source. That’s not efficient. A small plenum can be evacuated quickly, so the vacuum source can focus on its primary job: removing air above the table, not below it.

A related bonus: no long hoses, connecting our vacuum source to our custom enclosure. That’s another big, empty area that could potentially be fighting our vacuum source.

Keep in mind that it’s inevitable that you “waste” some plastic, with each part you’ll make -- since you have to be able to hold a sheet in some sort of holding frame, and to have the edges of the sheet self-seal to the table border.

To date I haven’t used any gasket materials, where the wooden enclosure meets the deflator unit’s upper side. I’ve ignored that so far: mostly to make it easier to take one top off, and drop another one on, for design testing.

Now that the process no longer intimidates me, I can already imagine it would be nice to have a working area of twice that size or greater, for making the kinds of parts that tend to be longer than they are wide. I’m toying with the idea of a much smaller working surface, too. I’m even wondering if I can’t re-flatten and reuse scrap pieces.

Be sure to check out the download-able drawings I’ve included, if you want to duplicate my test machine. What’s there is a slight “shoulda done it this way” upgrade: the original bottom and middle slices were swapped around.

HOW WELL DOES THIS DEVICE WORK?

On only my second-ever vacu-forming attempt, I pulled a 0.040” inch thick plastic sheet down over something taller than it was wide; and got results of about 85% of the shape I had tried to obtain. (Bottom corners were soft: the mold form needed to be raised up more.) On attempt number three I got almost perfectly usable results. That despite the mold form having several sharp 90-degree-or-worse corners, and a few other things you’re supposed to try to avoid.

No wimpy super-shallow mold forms here; no sir! And no super-thin plastic, which easily forms over anything. No sissy tests for this kid! I test stuff like I mean it!

(In case you’re wondering, the project seen here is a sleigh-like custom car body, designed to fit on a Chuck Wagon kit’s chassis. It’s based on a drawing Dave’s Showrod Rally displays on holidays. Yes, I asked for permission.)

CONCLUSION

I’ve read many vacu-forming articles over the years. Most accidentally gave me the impression that messing up was the norm, and making a usable part on your first try has odds similar to winning the lottery without buying a ticket. I finally understand why; and no longer worry.

It boils down to this: every new part you’re trying to make is essentially a prototype. Even if you’ve vacu-formed parts often, that particular part “master” or mold form is a prototype: a first. There are bound to be a few bugs to work out, with anything brand new. I no longer see a few bad parts, before I get one right, as failure. (Early "mold form testing" can be done for free with a loose, unheated plastic bag. Instead of wasting expensive sheet plastic, just watch for where the bag wrinkles badly..) Shoot for a “C” letter grade, on try number one. Learn. Accept what worked well. Try again; shooting for a “B” grade. You may want to accept that “B”? If not, then shoot for an “A”.

REFERENCES

If you want more info, be sure to check out the nifty little vacu-forming book I review this month, or buy a copy of DVD #7 in the CultTVman’s Fantastic Modeling series. They’re both well worth the money; and it’s always nice to see what others say about a technique.

I know of at least six articles by Fine Scale Modeler on the subject of making vacu-formed machinery or parts. Check out their Mar/Apr 1985, Aug 1986, Oct 1986, Mar 1998, May 2002, or Mar 2005 issues for a variety of viewpoints.

Good luck! Have fun with your new toys and skills!