Molding a canopy.

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Roger Miller, Mar 22, 2007.

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  1. Mar 22, 2007 #1

    Roger Miller

    Roger Miller

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    Does anyone here have any expirence free blowing Acrylic canopies? I am going to attempt to form my own canopy. I have a heat box built that is
    4' x 4' x 10' long and I have found some 220 volt ceramic heating elements
    to heat the box with. Any suggestions or comments are welcome.

    Thanks,
    Roger
     
  2. Mar 22, 2007 #2

    Othman

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    Free blowing canopies... does that mean without a mold?

    I've seen bubble style canopies formed by laying the sheet of plastic on a sheet of plywood that has an elliptical hold cut in it (the hole is the die that will form the shape of the canopy base) and clamping the two together. Then the whole thing is heated uniformly by using large heating elements. As the plastic warms up, gravity begins to pull it through the hole in the plywood forming a bubble on the other side.

    I don't know how much control you have over the final shape. You really have to be careful about how much you heat the stuff or it will get away on you.

    Also the overall shape and the amount of heat used with determin the effect on the optics of the final product.

    I've also see the plastic heated and then draped over a male mold of a windscreen. You have to have a very nice mold if you do it this way.
     
  3. Mar 22, 2007 #3

    orion

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    One suggestion I might offer is that you might be better off going to someone that already does this and has the system bugs all worked out. One such supplier is Todd's Canopies - they free-form and mold the units, to just about any shape you want. Given all the potential sources of error in this process, it is possible that by the time you make an actual serviceable unit you will spend as much, if not more, than that charged by someone with experience.

    Regarding molding over a plug, it is important to use a soft, temperature resistant material like felt between the hot plastic and the cooler tool. Yes, it can be beneficial to heat up the tool before draping the plastic over but even so, without a soft skin between the tool and the plastic, it is very easy to get "cold" spots or areas with differential stretch. These significantly distort the view through the glass.

    Personally, I like the plastic over the tool approach better since you will be guaranteed to get the shape you want (assuming your tool is accurate) and you don't have to heat up the plastic as much as when you do free forming, thus reducing the chance of uneven or excessive stretch.
     
  4. Mar 23, 2007 #4

    plncraze

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    There is an article aboutr making a canopy on the internet. Google it and you should run into it. The article is written by Bob Walters who designed the Dragonfly. Note that when he needed a canopy for the Dragonfly he did not do it himself according to an Air Progress story. The canopy he molded was for a Duster sailplane. This story is also in "The Collected Works of Stan Hall."
     
  5. Mar 26, 2007 #5

    Roger Miller

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    Thanks for the comments guys.

    I am going to try my luck with one attempt and see how it goes.
    I bought an old used house oven this weekend and removed the thermostat,oven light and window to use on my box. I am going to assemble the heat box and see if it will heat up to the 400 degrees I need to form a canopy. If it makes it to 400 degrees I will invest the $110 for a sheet of 3/16 acrylic and see what happens. I will have less than $200 in the whole project and
    Todd's canopies quoted me $900 for a canopy and delievery.

    Roger
     
  6. Mar 27, 2007 #6

    Peter V

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    So... that means you get 4 goes until you have to kick yourself. I'd play those odds! :gig:
     
  7. Apr 5, 2007 #7

    Radioactiveboyscout

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    Roger,

    I have a little experience in this area as I molded my own windscreen for my motorcycle a couple of years ago. I used Lexan (polycarbonate) rather than acrylic but I think that what I learned is probably equally important.

    * Heat the acrylic slowly over a few hours and do not exceed the boiling point of water 212 F before two hours - this is a must for Lexan and I believe it probably applies to acrylic too. Lexan absorbs water and so if you heat it quickly the water molecules do not have enough time to escape the glass and will cause hazing as a result of vaporizing in the glass. Basically by heating it slowly you are drying it out. I did not heed this suggestion as I sometimes act like a pilot - you know you are smarter than everyone else - right?

    * Try to set up your heat box so that you have to handle the acrylic as little as possible. I remember that the Lexan I used stayed very riggid right up to the point that it hit its melting point and then it had the consistency of a huge piece of Kraft american cheese. This surprised me as I thought it would slowly melt as if it were plastic. I don't know if this applies to acrylic but a huge piece of droopy cheese is not easy to handle.

    * Keep the mold very clean. When Lexan is soft it will easily imprint anything hard it contacts. This includes oven racks. But now I have a really cool stripped windscreen on my bike. If you don't heed this suggestion be prepared with a lie like mine, yes those stripes are my artistic impression - cool aren't they?

    * Have the mold the exact size that you need. Again this applies to Lexan but I don't know about acrylic. Because Lexan stays riggid right up until its melting point you can't expect to bend it into shape at elevated temperatures. It is either riggid or soft and not in between.

    Hope this helps.

    Will
     
  8. Apr 5, 2007 #8

    orion

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    :D It's always important to stay "creative". "Yes, I meant to do that" is one of the more important answers to remember. But just as important is the answer you come up with to the inescapable question "Why?"
     
  9. Apr 5, 2007 #9

    plncraze

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    In addition to the Bob Walters article that I mentioned in a previous post there are also some pictures and descriptions in Ladislao Pazmany's "Light Airplane Construction." He had a group of builders with thick gloves ready to help push the plexiglass over the mold soon as it came out of the oven, which was a borrowed from a molding company. the molders brought the mold to a plastics company and used their oven.
     
  10. Apr 5, 2007 #10

    gahan

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    Am getting ready to do my canopy do you remove the paper cover before heating? I'm forming creases and gradual bends with a min. of elongation.
     
  11. Apr 5, 2007 #11

    Radioactiveboyscout

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    Gahan,

    Definitely remove the paper before heating. Otherwise the paper will burn and leave the tacky adhesive on the acrylic. I made the mistake and had a real tough time getting the adhesive off. Even after using the super solvent Goof Off.

    Will
     
  12. Apr 8, 2007 #12

    Roger Miller

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    Thanks Will,
    I had someone else tell me to heat it slow at first to remove any mosture but he wasn't sure how hot or how long. This helps alot. I wired my heat box up this weekend and it seemed to work ok. I need to get a thermometer and see if the two heaters I am using will get the box up to 400 F.

    There will not be any mold, it will hang upside down and when it get up to 400 F I will slowly blow air into the Acrylic and form a bubble.
     
    Last edited: Apr 8, 2007
  13. Apr 14, 2007 #13

    Bart

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    I've not done this myself, but have read extensively about it over the years. Somewhere, I have a copy of a magazine article on how the Zuni sailplane canopies are free-blown, with excellent pictures and explanation.

    Based on my recollection of that, here are a few thoughts you may wish to consider:

    1. Cover the heating elements with aluminum foil sheet and/or aluminum window screen to disperse hot spots. Otherwise, the heat will rise in a column (like cigarette smoke or smoke from a fire) and make hot spots on your acrylic sheet, causing differential heating and therefore optical distortion. The aluminum foil, etc. will more uniformly distribute the heat over the entire plastic surface. Actually, you want the more curved/pointed ends to get a bit more heat, as they must finish as tighter radiused curves.

    2. 400 degrees may be too hot. Check with the manufacturer for proper heat range, which may be more like 320 degrees, or whatever.

    3. Slow heating of the work is better, as you've noted.

    4. Having the plastic droop by gravity from an overhead template is vastly better than trying to blow it up. Gravity helps you here, rather than fights with you, and the heated plastic is held by surface tension, making for excellent optics.

    5. Don't blow the plastic down: The blowing air, being cooler than the plastic, will make cold spots and distort the optical clarity and/or shape of the hanging plastic. Rather, SUCK the plastic down into the hot box. The Zuni guy had a window on the side to look in, and a curved line on the opposite side of the box as a visual guide. As I recall, he'd suck it down ~1-2" past the line, then ease off the vacuum, and the plastic would pull back to the line before it "set" in that permanent position. He used a clear plastic tube with water and food coloring in it as a manometer to discern and set the vacuum pressure in the oven box. The box beneath the drooping plastic was sealed airtight with duct tape, or whatever. A battery powered air pump (like you plug into a car cigarette lighter to pump your tires) set in the bottom of the box with the nozzle through the box wall would do nicely, methinks, to evacuate the air beneath the drooping plastic canopy.

    6. As I recall, the guy and his assistant then pulled the plastic in its template out of the oven and quickly removed all the retaining bolts, which were set about every inch around the perimeter of the template, while the plastic was still pretty warm. This to prevent heat-stress cracks from propagating from any bolt holes.

    7. Plywood template thickness was ~3/4" and the upper edge was radiused with a router and ~1/4" radius bit, to make an easy droop for the plastic without stress concentrations. The edge of the template had felt on it to make the plastic comfy.

    8. As I recall, the canopy was kept in a box at ~200 degrees for ~24 hours after blowing, to heat anneal the plastic.

    9. Smart builders are known to do the canopy first, then build the plane to fit it, for best mating of the two.

    10. I very much regret to say that I've heard that Glen Breitsprecher passed away. He was a great guy and maker of many fine aircraft canopies. His sad passing also means the loss of a wealth of experience that can't be replaced, esp. since he was generous with his knowledge and advice. If you can find somebody like him, your best and least expensive bet is probably to have him do the work for you. Just sayin'.
     
    Last edited: Apr 14, 2007
  14. Jun 9, 2007 #14

    Roger Miller

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    I finally got a chance to try out my heat box. It took about an hour to come up to 240 degrees and stoped. I had several air leaks that I stoped and the then the temperature went up to 260 degrees and stoped. The sides of the box felt like they were insulated good but I could feel lots of heat escaping through the top of the box. The top was covered with 5/8 particle board, I might have to cover the wood with a sheet of foam board like the box is made of....
     
  15. Jun 11, 2007 #15

    N2T18S

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  16. Jun 12, 2007 #16

    Roger Miller

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    That's a very imformative article. It looks like I'm doing everything right so far. His box is smaller than mine, I think I will cut down the height of mine and insulate the top and bottom of the box. I will try again this weekend and see if we can reach about 300 degrees.

    Roger
     
  17. Jun 12, 2007 #17

    Nev25

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  18. Jun 18, 2007 #18

    Roger Miller

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    I cut down the height of my heat box and insulated the floor this weekend. The box reached the "magic" 300 degree mark easy. I made a small sample mold and pulled a shot of .100 acrylic to test the shape of my mold. The test sample heated great and it was very easy to blow to the desired height, the only problem I had was the overall shape. The canopy I'm trying to made is for a 2 person tandem seat airplane so the shape is long and narrow. The sample I made had a slight "peanut" shape to it. The plastic didn't stretch as much in the center as it did on both ends. I think my problem is one of two things.

    1. The air I'm using to blow the plastic comes into the box in the center of the plastic and may be cooling the plastic in that area. I'm using an air muffler to diffuse the air but I may need to heat the air or something.

    2. I noticed the shapes of the templates used in the previous articles were more ellipse shaped than rectangular with radiused ends. This might also be my problem.

    Any thoughts?

    Roger
     
    Last edited: Jun 18, 2007
  19. Jun 19, 2007 #19

    Dana

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    You can control a lot with careful heating. Some years ago I worked in the thermoforming business... shape wasn't so critical since we were vacuum forming over or into a tool, but there was a good bit of work controlling the thickness of the final part (naturally, it tends to thin out where it's stretched the most). On our largest machine (which for about two weeks was the largest in the world, making parts up to 13 feet long), we had a grid of infrared heaters over the blank sheet... all were controlled individually, and by varying the heat in different areas we could control how (or more precisely, where) the sheet stretched, and reduce thin spots. If you're introducing cool air over the plastic then you'll definitely have problems.

    -Dana

    Fugitive from the law of averages!
     
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  20. Jun 19, 2007 #20

    bmcj

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    Would a small internal circulation fan take care of this problem?

    What do you do about the air being drawn in to fill the increasing volume over the plexi? Wouldn't it also act to cool the canopy unevenly (depending on where the vent opening is)?

    Bruce
     
    Last edited: Jun 19, 2007

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