Vacuum Forming

[RedHeadKevin]

So I'm trying to vacuum form a new canopy. I've got a vacuum box, which is good and tight. The master canopy is filled with plaster (to provide some more structural rigidity) and everything seems to be working without air leaks anywhere.

I'm using K&S brand clear PETG sheets, in .030 inch and .015 inch (I'll elaborate on this later) and my household vacuum cleaner, which pulls plenty of vacuum. I'm using the burner from my electric stove as a heat source, holding the PETG in a frame, a few inches above the burner. The process works very well, with two main problems.

First, I'd like to use the thicker plastic to make the canopy. But when I go to heat it, tiny bubbles form in the plastic, like gas that's trapped (chemically) is starting to bubble out. Am I heating it too fast? Too slow? Too much? (I'm heating it until it sags, but the bubbles form before this.) Am I using the wrong kind of plastic? Is it just too thick? Could it just be a bad batch of plastic?

Second, with the thinner plastic, there aren't bubbles, and it forms pretty well, but when I remove it from the mold, the plastic has a slightly grainy or bumpy texture. It's not a smooth, crystal clear transparency like I was expecting. Is this normal, and does it polish out?

TIA

[Joseph Osborn]

FWIW, I've always understood that clear Butyrate is the best plastic for vacu-forming canopies. I suppose you could polish the PETG and dip it in Future, but that sounds like an awful lot of extra work.

[PetarB]

The issue with the bubbles for the thicker one, I'm not sure what's going on there. I think perhaps you might want to slow down the heating. I have not experienced the issue as I only use the thinner sheets for canopies.

As for the bumpy texture it's possible that this is from the buck. It's not clear whether you're using an existing plastic canopy, filled with plaster? I'm guessing the plastic on plastic contact between the PETG heated sheet and the plastic of the canopy is causing the issue. Have you tried another object, like the plaster, or some other inert non-plastic substance? I've had the best experiences either with metal or 2-part resins.

I've had very good experience with PETG, so I'm not sure that's the issue, but it may be.

Edited December 5, 2011 by PetarB

[Dndieje]

First, I'd like to use the thicker plastic to make the canopy. But when I go to heat it, tiny bubbles form in the plastic, like gas that's trapped (chemically) is starting to bubble out.

I have done some experimenting with vacforming canopies and small items like landing-light covers myself.

Like yourself I had trouble with bubbles but that was (in my case) always due to heating the plastic too quickly.

Maybe the reason you don't get the problem with thinner plastic is because you have to be gentler heating it.

The structure of the plastic does get a bit "pebbly" but probably that is because you're doing things to the plastic it does not like.

Once shaped it doesn't like to be altered and you will never be able to re-shape it keeping the plastic at a constant temperature.

As soon as you remove the heatsource it will start cooling and that probably starts the imperfections.

To be honest, in a lot of kits with vacformed clear parts you see exactly the same structure so it's generic to the material not the person doing it.

What you need is some form of Thermoplastic which is designed to be reshaped when heated.

No idea if this is an option for modellers though.

By the way, the plastic I use is from left over cat food containers (clear plastic boxes holding two little tins) heated over a candle.

The positive mold I make out of Milliput.

Hardly Leonardo da Vinci at work but is does the trick :D

You could always contact some firm that does vacform clear parts and ask them how they do it, it's hardly industrial espionage is it ?

PJ Productions in Belgium do fantastic vacform canopies.

Don't ask Broplan though, their clear stuff isn't even worth holding catfood.

[josh1813]

I can't say to much about vacu forming, but luckily I have spent the last 5 years of life at a company that makes PETG, so I can elaborate a little there.

The bubbles are being caused because you are heating the plastic too hot, and causing it to degrade. When the polymer chains began to degrade they break up and produce carbon dioxide which becomes trapped in the plastic. The object of thermoforming is that you want to get the plastic over it's glass transition temperature, or its Tg, this is the point where it appears to melt, but it actually isn't melting, just flowing. Isn't polymer science fun? PETG doesn't technically have a melting point by the way. Most PETG's have a Tg at ~ 80° C (176° F). Typically you want to exceed this by about 50-100° F, to get it to form well. As you can see it doesn't take a lot of heat to start forming PETG, in theory you could do it with boiling water, but that doesn't leave you with much of a window to form it. Long story short is that you could probably use a hair dryer or maybe something like a small space heater to heat the plastic. Most commecialy thermoforming equipment uses a series of heat lamps for instance. I have heated PET and PETG along with acrylic in your standard oven to make it workable, just set it on about 250°F if your able and it works pretty good. I just suggest you lay it on a large sheet pan or something of the like, as the racks will cause localized heating where they touch the plastic and that will distort the plastic a little bit. It also protects you from making a potential mess or starting a fire, just in case you let it flow a little bit too much and it gets to one of the oven's eyes.

Also in theory PETG, acrylic (PMMA), or the butyrl plastics are all thermoplastics, and in theory you should be able to keep heating them until they straighten back out and start over. It's just that usually they don't flow back probably and "bunch" up on theirselves. Also if you heat them to close to Tg it traps stress in the plastic which will cause it to be more brittle, or it can cause the plastic to shrink if it gets warm again.

Sorry for the ramble, but it's not often I get to talk plastic, but long of the story is you might just want to use a more controllable and even heat source instead of a flame. These plastic are acutally a lot tougher than they look, but they don't take flame too well.

Hope this helped,

Josh

[Don Wheeler]

I can't say to much about vacu forming, but luckily I have spent the last 5 years of life at a company that makes PETG, so I can elaborate a little there.

Josh

That's the best explanation of thermo-forming I've ever seen.

Thank you.

Don

[RedHeadKevin]

What he said... that was awesome. I figured the bubbles were something coming out, chemically, but I didn't realize it was because I was heating it too much. I was under the impression more heat = softer plastic = easier to form = less suction needed. Guess not. And I'm not using flame. I'm holding it over the electric coil of an electric stove. I'm using small enough pieces of plastic that the whole thing is being heated evenly across the plastic.

[josh1813]

Thanks for the feedback guys...sometimes when I start in on plastics, especially PETG, it's hard to stop.

And sorry about saying you were using flame, just reread back over your post and the brain must have got crossed up when you said you were putting the plastic in the frame. Hey I was just one letter off. Good luck with your future attempts, and if I can help anymore, just holler.

Josh

[PetarB]

A flame is okay... but you have to be very gentle. Here's a documentation of some of my canopy molding work with a candle.

http://www.macrossworld.com/mwf/index.php?showtopic=28650&st=0

I've done this a lot of times since these pics were taken, but these days I don't bother photographing the process, I have a nice dental vacformer which takes place of the candle and my toasted fingers!

[Rob de Bie]

First, I'd like to use the thicker plastic to make the canopy. But when I go to heat it, tiny bubbles form in the plastic, like gas that's trapped (chemically) is starting to bubble out.

I had exactly the same problem, and there is a solution.

The bubbles that you see are steam bubbles! Every plastic absorbs a bit of water (moisture) from the air, and when you heat the plastic above 100 degC, it will boil. So you have to get rid of the moisture in the plastic.

I know of two solutions. The one I used, since I had a vacuum pump and chamber, is to put the sheets in a vacuum for a while (15 minutes in my case), to 'suck' the moisture out. The other is to store the sheets in a warm place, I would guess between 60 and 80 deC, to drive the moisture out of the plastic.

Good luck, and let us know whether it works for you.

Rob