Bubble canopy forming from ploycarbonate sheets

[ScratInTheHat]

I've looked all over the net to get information on bubble canopy forming from ploycarbonate sheets (Lexan, etc.).

I'm not talking about small model scale but usable on prototype vehicles.

Can anyone point me in the direction I need to go to get some information on this process?

 

[patprimmer]

GE

Whatever Bayer call themselves now.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

http://www.makrolon.com/BMS/DB-RSC/MakrolonCMSR6.nsf/id/home_en

Took about 3 seconds.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ScratInTheHat]

Took you three seconds to give me something I already had.

I don't need to do anymore looking through a materials site I need to have the bubble canopy manufacture process.
I already have a UV resistant polyC picked out. It's thermoformable at just about 280F. What I'm wondering is if this is a double molding or some kind of vacuum process.

Now if you are suggesting that I send them emails asking about this process I got a 'been there done that' for you.

No replies.

What I would like to have is a paper, book, article or better yet just ask someone that has done this before or knows how it is done. I don't mean the high dollor mil spec injection process either. That will do me no good.

I would like to know how they make bubble canopies for gliders and light aircraft. I know it is done from sheets of material with a molding process of some kind. I need to keep the finish and optics intact.

 

[patprimmer]

GE and Bayer have published data. I have copies in my archives somewhere. If they won't give it to you I can't say why. Maybe it is how you put the question. Maybe you looked in the wrong place, maybe they have recently withdrawn that type of support.

From memory you can do it several ways with regard to the details.

Basically you do it the same as acrylic and if you can get data from any major acrylic sheet manufacturer that will help a lot.

With PC, first you need to dry it.

At a guess, I would say dry at I think about 120 deg C overnight, best done but not absolutely necessary in a dehumidified oven.

Once dry heat to I think about 180 deg C.

Once heated to a rubber like state mould to shape.

Here is where there are options.

1) You can drape it over a male shaped mould made of whatever is convenient. Wood is popular.

2) You can use vacuum to draw it into a male shaped mould. Thes are sometimes aluminium and have a system to clamp the sheet down over the mould and seal the edges so it can hold a vacuum.

3) clamp the edges to a flat surface with flange the shape of the base of the model. Blow air under the sheet via a valve built into the flat surface. Stop blowing with air pressure once you have the right height on the bubble.

Sorry if this is brief and rough, but there is published data out there with great detail and diagrams and examples. You just need to find it. I do not have the time nor inclination to write a 4 or 5 page document.





Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ScratInTheHat]

Just knowing that they did have published data is great news. I had all that you just told me except for the drawing down into a female mold (I think that's what you meant to write). This is the process that I think would be the easiest to setup. I was looking for confirmation on that process.

I may still have to go with the male mold just for practical purposes of making the mold.

The biggest question I have now is the heating system. Most of the setups I've seen are a coil array. I’m wondering if it can be setup with hot air delivered from heat guns in a containment. Radiant transfer may be the best for uniformity.

I’ll hit them up with more requests.

You’ve been a great help!

 

[patprimmer]

Yes it was a typo. I meant draw down into a female mould. It is called vacuum forming.

When making canopies you need to be careful not to disrupt the surface and thereby lose clarity.

Regards

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[Demon3]

Vacuum sucking it down into a female mold is how you make refrigerator liners (from HIPS or ABS). That's how all fridges and freezers are made so if you can find someone with a large thermoforming machine for that then you could use it for your parts. Typically such machines can handle a sheet 1x2 metres.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[ScratInTheHat]

I just ran over a post on a homebuilt aircraft site that said these canopies are "Free Blown". This is the way they keep the optical clarity.

Now this is an idea that I had thought about but it seemed to be approaching the realm of an art. What I had envisioned was similar to the vacuum female molding method just without the mold. In my idea the peak of the bubble could be adjusted by tilting the jig so that gravity forced the bulge to the desired position. You'd have to have a way to vent the heat quickly to stop the bulge while holding vacuum I would think

This could be completely off the mark on what a "Free Blown" process is all about. Have either of you, or anyone else for that matter, even heard of "Free Blown" forming?

 

[SincoTC]

"Free Blow" forming was used extensively on mult-crew aircraft during WW2 and for many years after to produce the navigators "Astro Dome" for sextant sightings. It was the easiest way to produce a flanged hemispherical bubble without the necessity of a mould or former. The plastic sheet was just clamped over a suitable hole, softened with radiant heaters and when the material started to sag, a shot of air at a few PSI differential pressure did the trick. It is only suitable for symettrical shapes from a gentle "saucer like" bowl to a full hemisphere, or even beyond if you split the hole plate. I suspect that the plane homebuilders cut a shaped face to fit the fuselage after it's blown, so it's not too obvious that the actual canopy is spherical.

I think any "Teardrop" shape would require a mould/former; a male former is the easiest to make and is often wood as patprimmer stated above. To avoid the risk of marking the surface, it is usual practice to cover the former with green baize material. A common type of vacform machine will have the former mounted on a table that can be rapidly raised once the material at the correct heat, this pushed the former into te material and reduces the volume of air to remove when a vacuum pulls it onto the former. Heating is usually by electric radiant bars.

Hope this helps.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

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[Demon3]

I guess you may be able to roto mold the part and cut it in two. Anyone know if that's feasible?

http://www.devicelink.com/mddi/archive/96/04/013.html

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

A roto moulded PC part will have reasonably poor surface finish and about half normal impact strength.

Free blown is the 3rd method I mentioned.

You can get tear drop shape by shaping the flange. The later model Spitfires had a Perspex bubble. I don't know for sure, but I suspect it was free blown.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[berkshire]

Bubble canopies for aircraft are free blown or vacuum formed in an oval mold form then cut in half after forming.
The thing that makes polycarbonate a bear to form is the higher molding temperature and the water absorption.
High canopies are made by free forming the sheet into a U shape then clamping that into a blow mold to form the fore and aft curve.
Mecaplex Ag in Switzerland use an optically perfect mold and drape their canopies.

 

[pierdesign]

pressure vacuum forming is used for large cosmetic appearance parts, eg. instrument housings, that plain vacuum forming would have problems with material section thinning due to draw depth. Simply, it's standard vacuum forming with slight positive pressure blown on the other side.

Vacuum (pressure) forming tools for optically clear material are polished aluminum, epoxy or epoxy with a ceramic coating.

Canopies as you describe are almost always post processing heat treated (tempering almost) and/or polished as the thermal cycling, handling and tool just about always introduce imperfections. Also, canopies are typically thicker sheet material than is typical for vacuum forming, as such poor themal conduction limits their mass-manufacturing processes. Often, canopies are hand fabricated in multiple passes using combination of drape forming and local area heat forming.

Some fabricators successfully drape form thick polycarbonate sheet for armoured vehicle ballistic windows, but the curvature is slight.