DIY Forming Lexan Canopies

[jyl]

Does anyone here have experience forming Lexan into windshields and other complex curved shapes? I am wondering how a home DIY'er can fabricate a shape that is curved in two axes (not curved in one axis like the side of a barrel, but in two axes like a the top of a bubble canopy). A shape like, say, a bubble canopy.

My understanding is that Lexan becomes plastic in a certain temperature range, so you could heat it up in an oven and then press it over a form. But if the piece is too large for your oven, can you place it over the form and then heat it with a heat gun set for the "plastic" temperature, forming an area at a time?

It sort of seems plausible, but I couldn't find any DIY stories about actually doing this, which makes me wonder it it is actually practical . . .

[island911]

I've some experience with it. (teach an Engineering Plastics class too)

PC (PolyCarbonate, Lexan) can be cold-formed, in many cases (bend & worked like a pice of sheet-metal) but will hold "memory" and want to "creep" back, close to it's original shape.

You can heat and form into a shape (thermoforming) thou the T_G (glass transition temp) is fairly high, for PC.
. . .maybe use a PC/ABS blend for more workable temps.

Also, localized heating can cause some problems, as plastics expand where heated.. . .then cool and can warp. (not to say it cant be done . .just that may be problematic.

Someone on the board use to form big thick windshields . . bell, I think it was.

[widebody911]

There was a guy making curved corner windows for early VW buses by laying the plastic on a mold and cycling hot water over it until it melted down to the shape he wanted.

[island911]

yeah .. .that's not going to work with lexan . . .not even close.

http://www.matweb.com/

[jyl]

Thanks for the link - I'm going to sift through there.

Searching further, I've found pages on DIY thermoforming of Lexan with a heat gun, but always little pieces or a simple local bend.

[BlueSkyJaunte]

Didn't bell make a bulletproof windshield for somebody?

[BlueSkyJaunte]

Man, is my memory good or what?

lexan targa top project........

[xlr8]

homemade bubble canopies

[bell]

nice memory blue
i used old bricks to make an oven big enough for the windshield......then used a kerosene "torpedo" type heater to heat things up, took quite a few hours but it was 1" thick.
something less than 1/4" would be alot easier to form and would need less heat soak (time), translates to an even less efficiant oven.

xlr8's link on bubble canopies is pretty damn informative

[widebody911]

Originally posted by bell
xlr8's link on bubble canopies is pretty damn informative

Yeah, makes me want to try it just for the hell of it! Looks pretty neat!

[BlueSkyJaunte]

I say we start making lightweight rear targa glass...

[cegerer]

I'm in! I've always thought it would be cool to make an R-style rear Targa glass with the louvers .....

[island911]

Hey, good idea. Use some thin, light-weight plastic . . maybe have it zip-in.

[jyl]

Russ, that is a great link. Thanks.

Hmm, a good friend is an accomplished glass blower - I wonder if he could manipulate the "blowing" to control the shape even better. Come to think of it, conceivably I could use his studio's ovens - but I'm not sure they can even go as low as 200-300F!

[island911]

jly, what type of canopy are you looking to build? . . .airplane, glider, boat, HPV?

Hey blue, did ya read to the end of my first post?

Originally posted by island911
. .. .
Someone on the board used to form big thick windshields . . bell, I think it was.

[Zeke]

I used to work in a vacuum forming shop and pattern shop. If you can make your form or mold, I'm sure you can get some oven time for cheap. Now, I don't know what kind of shape you want to make, but let's say you want to make a 911 windshield.

First, you would take a stock glass one and make a plaster mold on it with layers of burlap and plaster. If you really do this, I can steer you towards the proper plaster. Next, you would build a cradle to hold and brace your mold. Make the outside dimensions flat and square. Now, you drill holes in the thing all over about 1/8 " and on a 4" grid. You then will have to enclose the backside of the mold, usually a plywood box around the cradle will do. You can seal it with caulk. Put a hole in it for a hose; find out ahead of time what size.

You are ready for the plastic. The plastic is held in the warming oven until it begins to sag on it's own. Immediately place the plastic on the warmed mold and turn on the vacuum. It takes more than a ShopVac to do this, so a production shop is really the place to go for the final product. The sides of the mold have to be designed so that there is a seal around the perimeter. You might have a clamp piece made up to push the soft plastic down to the edges. Some 2-sided tape on the perimeter of the mold ready for the soft plastic could help, I forgot what we used. The thing is, you can't afford to lose any vacuum once you start to pull the part down. It goes into your mold in mere seconds.

You will have overhang which will have to be trimmed to fit.

BTW, a brick oven and kerosene heater is not by any means out of the question. But, you will still need powerful vacuuming force. Maybe 2-3 vacs would pull it down. Uneven heating and cooling will produce distortion. So, optically correct compound curves are quite difficult. While I was at that shop, we made a prototype fighter jet canopy, so I know what went into that. About a month's work by some very accomplished pattern makers and plastic formers. It was a many 1000's of dollars piece.

[BlueSkyJaunte]

Originally posted by island911
jly, what type of canopy are you looking to build? . . .airplane, glider, boat, HPV?

Hey blue, did ya read to the end of my first post?

LOL, nope! Just further proof I never read any of your stuff.

You started talking all MatSci geek-speak and I just skipped over the rest of your post.

[bell]

Originally posted by island911


Hey blue, did ya read to the end of my first post?

i missed that one too.......you also have a good memory island

[jyl]

Originally posted by island911
jly, what type of canopy are you looking to build? . . .airplane, glider, boat, HPV?

Err, well, I'm kind of embarrassed - well, it would be a windscreen/roof bubble for a Vespa scooter, OKAY? There, I said it. I'm tired of using the 911 for errands around Berkeley, the parking sucks and that's even if you're willing to squeeze in between two dented minivans. A cheap 'n cheerful scoot would be great, although not so very stylish. But, it also rains here, quite a lot sometimes. I remember, when I was a kid in France, there were a lot of scooters bussing around with windscreens, roofs, even fabric doors unzipped at the bottom to let the rider put his feet down. I couldn't find any sources online (not even searching in French, "scooter + toit + pluie" etc) so I thought about making one . . . then I simply got interested in how one forms plastic.

[island911]

LOL .. .you said "toit"