VACUFORMING

Last modified: Saturday, January 6, 2001 6:20 PM

Vacuforming is another useful technique to add to your bag of tricks. Essentially, a sheet of plastic is heated, then stretch-formed over the master pattern by the suction power of a vacuum. The resulting plastic shell more-or-less resembles the pattern. Although it's no substitute for molding and casting techniques in situations which require you to precisely duplicate the pattern and capture surface detail, it allows you to do some things which you can't easily do using those techniques. One of those advantages is the ability to create a very thin wall of uniform thickness. It's not easy to do this via casting: You either have to make a fairly rigid two-piece mold that's well designed to avoid trapping air bubbles during casting, or you have to slush cast it in a one-piece mold and not expect the thickness to be uniform.

On thing you should realize is that vacuforming actually creates a mold-- a negative image of the object. The surface with the greatest fidelity is the inside surface, or the side which touches the pattern. However, because of the uniformity of thickness, a fairly accurate impression is created on the outside, and that's the side that's most often considered the "working" side. But it should partially explain why vacuformed parts lack the crisp detail of cast parts: You're not really seeing the "best" side. Also, because of this, you're not actually duplicating the pattern. You're creating a skin over the pattern which will always be slightly bigger than the actual pattern.

There are other differences as well-- vacuform is cheap and fast. It doesn't require a mold, and that cuts production costs. Also, styrene is pretty cheap. The most time consuming part of production is heating the styrene-- once it's hot enough, the actual vacuforming takes about a second or two. Other differences come from the material used-- styrene is light, strong, and resilient in thin sections, and can be "welded" to itself using solvents like Tenax.

The best part is that a vacuforming "machine" is remarkably low-tech, easy to build, and relatively cheap. The most expensive part is the thing which creates the vacuum-- a household vacuum cleaner. If you're smart like my wife, you own a Hoover Mach 2.1 Windtunnel-- maaaan, that baby can suck!!! And if you're smart like my wife, you quickly realized that the hose that came with the machine was waaay too short, so you went out and bought the extra long one. And if you're smart like me, you nabbed the original hose for a project just like this!

The rest of the machine is a no-brainer. You find or make a box for the platform that your pattern rests on, connect the hose to it, seal any cracks to make it more-or-less airtight, and screw some pegboard onto the top. I used a wooden box that housed that fun game where you steer a steel ball through a maze and try to keep it from falling in the holes. Since I hadn't played it in years, and it mocked me cause I'd never beaten it, it wasn't much of a sacrifice. I used a hot glue gun to seal the seams at the edges, and around the hose orifice. In the picture, you can see that I've sealed off some of the pegboard holes with duct tape. I wanted this to be fairly adjustable so that I didn't have to use large pieces of sheet styrene every single time. By masking the holes you're not using, you create a better, more focused vacuum.

The frame which is leaning against the machine on the left is just a piece of pegboard with a hole cut out in the center. To keep it adjustable, I was planning on making several different sized frames-- pegboard is pretty cheap. Note that the hole size should be close to the size of the platform's active area-- therefore, it doesn't matter that the frame has those holes in it-- they're outside of the active area. The biggest challenge was to find a way to secure the plastic sheet to the frame. I first tried to secure only the corners of the sheet under some pegboard scraps which were screwed to the frame. I discovered that it wasn't sufficient. Air would infiltrate through the sides of the sheet, lessening the vacuum effect. My hasty solution was to use duct tape to seal those holes on the frame, and I was pretty amazed that the tape survived the oven's 550 degrees Fahrenheit that I used to heat the plastic. In the future, I may make full length strips to clamp the plastic sheet, but on the other hand, the duct tape worked pretty well. It really doesn't have to stay in the oven for very long, after all.

I can't give you any particulars about how hot and how long, since it probably depends on the plastic you're using-- never think that you can get away without experimenting! I was frustrated that this stuff required so much heat, since I've heated other plastics that went limp in two seconds under a heat gun. The most useful clue is when the plastic in your frame begins to sag under its own weight. If it starts dripping molten plastic, then you've left it in too long.

Once it's limp enough, don't lolligag around; crank up your vacuum cleaner, yank the frame out of the oven and force the frame over the pattern before the plastic has a chance to cool. You may have to help it seal to the vacuum. A little heat pain doesn't last for very long. Once the vacuum catches though, the sucker forms in a split second. It doesn't take long at all for the plastic to cool either.

Here's the result of that mighty Mach 2.1 Windtunnel vacuum cleaner. It does a pretty good job around the house too, considering the hopeless task it has. Notice that it got a really good vacuum, and tried to suck the plastic sheet through the pegboard holes on the platform. As you can see, the duct tape did survive, but it's not a pretty picture.

The lower picture shows what kind of amazing vacuum was created. Notice that not only did it get a good impression of the Joe torso, but it also got a good impression of the hole in the bottom! That projected inward about 3/4 of an inch, and needed to be cut out with an Exacto blade so that I could remove the pattern from the shell.

Also notice that there's a limit to the undercut which it can capture because of the direction the vacuum is pulling the plastic sheet. However, if more of the back surface were captured, you wouldn't be able to remove the pattern from the shell! So I think there's an art to laying out pieces that you want to vacuform, which comes from experience.

I did want to show you what a vacuforming failure looks like. The sheet on the left was not heated long enough. I reheated it and tried again. The second time, air infiltrated through the sides which weren't sealed with duct tape, so only a partial vacuum was created. I didn't try with this sheet again-- it was a hopeless failure.

So as you can see, vacuforming isn't too hard or too expensive. Chances are, you've already got everything you need to make a machine. You can find sheet styrene at hobby shops, or if you plan to do a lot of it, or really big things, buy it from a plastics distributor. I haven't mentioned much about what you'd use it for though-- I've got that problem myself! I suppose you could make little designer outfits for your pet cockroaches, but I was thinking of making brass brassieres for my S&M figures-- Wow, wouldn't that be groovy? (At least you have to respect my vain attempt to slip some sick humor and sexual innuendo into a pretty dry subject.)

--Jimbob, 03/15/99