Molding a canopy.

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Roger Miller

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Mar 22, 2007
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I tried an experiment last night to see if my problem was a cool air problem. I made an air deflector and installed it into the box where my air is inserted into the heat box. The deflector pushed the air outward to the ends of the box. When I pulled the sample the center looked great and the areas where the air was deflected onto looked like they were colder. I guess that answers my question I need to pre heat the air before it is inserted into the heat box....

Roger
 

Peter V

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Is there a cheap way to heat compressed air? Hot air guns cost a heap. :ermm:
 

RonL

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Peter, depending on pressure and air flow volume, you might find a add on transmission oil cooler, if you move air thru it while it is submerged in boiling water, at 212 F degrees, the air will take on heat, if it needs to increase in temperature, then put it in olive oil, which smokes at 420 degrees F, or avocado oil which smokes at 520 F degrees, using a simple cooking meter you can keep the oils just below smoke temperature.

RonL
 

Roger Miller

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Mar 22, 2007
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now, doesn't vacuum forming sound a lot simpler... ?

LOL... Yes but my heat box is far from air tight.

I have been running tests moving the heaters and using fans to move the air around inside the box. This made the temperature more even inside the box
but I think I will still have problems with hot spots with only two heaters in the ends of the box. I am going to try to use four oven heating elements spaced evenly across the bottom of the box, I think this will distibute heat alot more evenly.

Roger
 

Peter V

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Also, a great way to dissipate heat is aluminum fly mesh. Cheap at any hardware store. Both diffuses air flow and evens out hot spots. :)
 

Bart

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FORGET ABOUT BLOWING AIR INTO THE BOX OR ONTO THE WORK as noted previously. All that does is cool the plastic area where the air flows, causing its shape at that point to solidify even as everywhere else continues to droop. Besides, the compressed air comes out of the nozzle a lot cooler than it was in its compression tank, per Boyle's Law or whatever, making matters even worse.

FORGET ABOUT ANY IDEA OF BLOWING THE WORK UPWARD: Gravity is your friend or enemy, and is constant, so draw the bubble DOWN into the box, aiding gravity when needed, watching from a side window with a line drawn on the opposite side of the box as an index. As it hangs drooping after heating, surface tension makes for an optically perfect bubble, just like you want. Fine tune the droop with vacuum from below.

When the work begins to droop as it heats and becomes more elastic, you can adjust its downward movement and shape with a vacuum applied under the work. Use slowly applied vacuum to suck the hot air from beneath the work once it gets to the proper shape. Actually, using the wall line index, draw the work an inch or two down past the line, as its surface tension will pull it back to the line. Fine tune the drooping bubble with the vacuum, gently applied. A water manometer of clear plastic tubing will help monitor vacuum pressure under the work. Vacuum applies exactly the same to all areas of the work, so you don't get bumps.

The template at the top will need to be secured by bolts every 1.5 to 2 inches around the circumference, and sealed to the work. Duct tape might be a good sealant. Forget about self-tapping screws, which are wonderful for drilling ragged holes that propagate cracks. Use a new and proper drill bit for Plexiglass, and take care with drilling to make nice clean holes.

For heat, why not use propane stove guts from your barbeque? You probably already own all that stuff, but just need to have aluminum foil and/or window screen over the burners to prevent hot spots and disperse the heat. In places of greater curvature, such as the leading or trailing ends of a teardrop bubble canopy, you need a bit more heat, so adjust your alu. foil heat dispersers accordingly so they feed more heat to the ends.

The box could be built with drywall sheet and 2x2 lumber, basically in coffee table planform, with plywood template for the bubble. Cheap insulation would help to achieve working temp. without delay. You probably already have the propane barbeque guts, a shop vac, and clear tubing for the manometer is ~15 cents/ft. Water and food coloring complete the manometer.

When the bubble is formed, anneal it at lower temperature for at least a few hours or more. One of those bathroom fan heaters placed under the work should do well.

Get a proper cutting disk for this type of plastic. Dremel probably makes good ones. Ask your local plastics people. In fact, most cities have thermoforming companies that make signs, etc. and you can get free advice on this stuff from them. With luck they have an oven and could do the work for you, for not too much money.

Kitchen thermometers of the sort you hang in your oven are cheap and reliable.
 

Roger Miller

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Mar 22, 2007
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Update:

I made a new heat box out of particle board and 2x2 lumber, glued and screwed together. The new "airtight box" is lined with foam insulation. I wired in my heating elements, light, and two fans to circulate the air for a more even heating. This box is airtight so I can use a vacuum to form the part.

I have made two sample parts so far they look promissing. I have a form I used to mold the samples and I am amazed at the difference in shapes you get between blowing and vacuuming a part on the same form.

Roger
 

bityman

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Aug 1, 2007
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My 2 cents about the peanut shape…

Polymers are really quirky, and they are very sensitive to temperature and strain rate (percent change in length per time). If you think of the canopy in three sections, you end up with a quarter sphere attached to a half cylinder attached to a quarter sphere. I believe the areas that make up the quarter spheres are subjected to more stress and strain than the cylindrical shape in the middle for the same given pressure. The result is they “give” earlier, and therefore they thin a little more than the rest, which induces more stress, more strain, etc.

On a material side, if you don’t already have a modulus/temp curve for the acrylic, it could be helpful. It will tell you what temps to use to get a desired formability. You shouldn’t have to go much above glass transition to get the material to move into the visco-elastic region, which should be the temp range for easy formability. Polymers can change behavior very quickly with very moderate temperature differences.
 

Roger Miller

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Mar 22, 2007
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Update.

I have been Vacuuming test shots and moving the heaters arround to see what effects the varying temperatures have on the shape. I think I found the "sweet spot" for the heaters to get the desired shape I'm looking for. I'm still having one problem maybe one of you can help me with. My problem is after I form the canopy and I cool the part the sides of the canopy are wavy at the base of the canopy where it is sandwiched between the two pieces of wood.
I've tried a couple of different ideas to fix this problem but no luck.....

Thanks,
Roger
 

BBerson

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Does anybody know about the different grades of Plexiglass Acrylic sheet for a bubble canopy?

I think I need to buy "cast acrylic sheet" instead of "extruded" for better optics.
But what about Mil-Spec standards?

I found aircraft quality mil-P-8184 costs about 10 times as much as normal sheet. Is there much difference and what grade acrylic is normally used?
BB
 

BBerson

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Here is a canopy mold I made from the existing old canopy shown in the background.
I hope to make a new canopy using the vacuum slip form method described in this article by Jim Miller. http://manosparnai.projektas.lt/e107_files/public/1207897539_2_FT233_canopy.pdf

Now I need to make a 4'x6' oven. I am planning to build the oven with waferboard and 2x4's and use my red dragon hot air blower for heat.

Any thoughts?
BB
 

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tyc

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Sep 16, 2008
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You might want to consult with your acrylic sheet supplier with regard to how to form it.

Sunlamps work well in this regard.

Note to, acrylic is usually carried in stock up to 4" thick; this bit of knowlege may come in handy should you elect to engage as well in making viewing ports fo PVHOs; decompression chambers, small submersibles and the like.

Hemispheres are the simplist to form and consideration should be given to the minimum acceptable guage you want to end up with, as this will determine the initial guage which you purchase; if formed properly, the guage at the top of the hemisphere will be one half that of the guage of the flat sheet you start with.

Depending on how the canopy is to be shaped, you may wish to consider making your moulds (male and female) first, then acquire the acrlyic in powder form as opposed to stock sheet form.

Consider asking GE (Lexan is their trade name for acrylic; which also has a slight purple color to it which you may or may not find to be objectionable.)

Consider asking Rhoam and Haas as well.

Hope this helps.

tyc
 

BBerson

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My acrylic supplier sent a sheet of 1/4" instead of the 1/8" I had ordered. Took another two weeks to get the right thickness. They have no experience with forming.

I tried heat lamps and could not get an even heat. The oven seems the best option.
note: GE lexan is polycarbonate.
BB
 

lr27

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Um, that's REALLY not so. Lexan (aka polycarbonate) and acrylic are VERY different. The trademark name for acrylic that I've heard most is Plexiglas. I'm not sure how polycarbonate holds up in the sun, though I think there are versions which are sold for using on greenhouses. I vaguely recall that it's a bit softer than acrylic. It definitely takes quite a bit more heat to form it. On the other hand, polycarbonate is very tough stuff. It doesn't crack like acrylic and it withstands impact quite well. To the point where they make bicycle helmets out of it. Acrylic often cracks when you try to cut or drill it. I know it's very easy to cut polycarbonate without cracking it, but acrylic cracks so easily that you can cut it by scoring with a knife and then snapping it over a straight support. I assume that polycarbonate doesn't have the same problems with drilled holes that acrylic does.
snip

Consider asking GE (Lexan is their trade name for acrylic; which also has a slight purple color to it which you may or may not find to be objectionable.)

snip
tyc
 

lr27

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P.S. I don't recall if this is a problem with acrylic, but with polycarbonate it's a good idea to use a very slow feed rate, and maybe slow blade speed, or it will melt and grab the saw blade! (It's been a while, mostly I remember the blade grabbing, not so sure if the feed or blade speed is more important.)
 

bmcj

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I'm not sure how polycarbonate holds up in the sun, though I think there are versions which are sold for using on greenhouses.
My 2004 car has (per the sales brochure) polycarbonate headlight covers. 5 years of sunlight has crazed and clouded the lens well beyond any hope of seeing through it. The transparency is gone, and the clouding is bad enough that even the translucency is beginning to be limited.

Bruce :)
 

conestogaman

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I have 2 experiences with polycarbonate to relate.

The first is my neighbor, who is meticulous with his truck's appearance. He has a product to make the crazed headlight lenses clear again. Standard Ford lenses (whatever they are made of). They still have a slight yellow tint. I don't think they are Lexan or Polycarbonate (blue tint), although they seem tougher than plexi.

Most auto manufacturers try to find the cheapest material they can to keep production costs low and share holder dividends high.

Aircraft builders try to find the lightest, strongest, most durable material they can.

Second, I installed a polycarbonate corrugated patio cover 24 feet by 24 feet that has withstood 70 mph winds. It is very impact and crack resistant. Much tougher than plexi. Plexi is much more brittle than Poly

Ok, I have 3...

Semi-tractor trailers use Lexan marker and stop/tail light lenses for the extreme durability. Trucks have severe harmonic issues to deal with, as well as bouncing along for many thousands of miles in the elements and severe altitude and temperature swings, all the while, being pummeled with general road debris and (insert your favorite meteorological event here).

I have no evidence that the trademarked Lexan is better than un-trademarked polycarbonate, only my personal observations. I was a trucker for many years...:tired:

I've never melted any of it, but I'm willing to try...
 
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