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Author Topic: Pulling a canopy, what temperature?  (Read 1415 times)

Offline Jim Fruit

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Pulling a canopy, what temperature?
« on: December 01, 2012, 03:25:56 PM »
Hi Guys:

I am in the process of pulling a canopy for my scale Rearwin Speedster. I have created the plug and the plywood fixture for holding the clear plastic. I have used this method before, but honestly it was as much as 40 years ago, so I have forgotten what temperature I set the oven at. Do any of you have a suggestion of what temperature setting I should use? I am using K&S Engineering clear PETG, .030 thick. Thanks for your help.

Jim Fruit
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Offline Jim Fruit

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Re: Pulling a canopy, what temperature?
« Reply #1 on: December 01, 2012, 04:40:53 PM »
Thanks, Ty:

That gives me a good starting point. Just a little concerned about this one in that it is a very deep pull, about 4" by 4" by 4" deep. If it proves to be too deep (tearing the plastic),I may just use a heat gun. It is a little difficult to describe, but it is the same method that I used on my Super Solution. The windshield is streamlined, but consists of three flat panes bent to a streamlined shape. I could used the heat gun to bend the individual sheets separately over the three flat/curved !! panes. I would then build the framework to hold the panes out of aluminum. Sounds like fun.

JHF
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Offline Paul Smith

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Re: Pulling a canopy, what temperature?
« Reply #2 on: December 02, 2012, 10:32:15 AM »
I don't use the oven.

I use the grill on top of the Jen-Aire stove.  It's not done by temperature, but by when the plastic starts to sag.  Holding it with asbestos gloves, I turn it over and back a few times until the sag looks right.  It's an art.

PS: Sometimes, when the pull doesn't go right, I can reheat it an do-over.
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Offline Jim Fruit

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Re: Pulling a canopy, what temperature?
« Reply #3 on: December 02, 2012, 04:34:17 PM »
Paul:

I use the "sag" method too, but I could not remember what approximate temperature to start at. I pulled two windshields this morning. The first one went perfectly. On the second one, I let things get in my way like Ty mentioned. I did not get a complete plunge before it cooled, just a little short. So I tried reheating it. It softened but when I tried to plunge it again, I trapped some air and the warm air made a huge bubble on the face of the windshield - sort of gun turret like. The real issue that is developing for me however, is that the plunge is so deep as to pull the plastic too thin. I am concerned that the end result might not be strong enough because the .030 thickness gets stretched to thin. It looks like I will have to create a frame such as the real plane used and then apply the full thickness .030 over that in panes. This is exactly how it was done on the real plane. Hopefully it will appear more scale.

Jim Fruit
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Offline Tim Wescott

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Re: Pulling a canopy, what temperature?
« Reply #4 on: December 02, 2012, 04:47:08 PM »
There are a number of videos on vacuum forming on youtube.  They make it look very convincingly easy.

All of them use the "watch for sag" method of temperature control.  Many of them use astonishingly easy-to-make vacuum-form boxes (basically a box with a hole for the vacuum hose and a bit of pegboard for the top).
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Offline Paul Smith

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Re: Pulling a canopy, what temperature?
« Reply #5 on: December 02, 2012, 06:55:11 PM »
I have found that .030" is kinda thin for a lot of bigger jobs.  As you mention, it draws down quite thin.

I have had to make a pair of plywood picture frames with bolts about 2" apart to stabilize the plastic prior to forming.  A bit of work, but as I see, necessary.

Also, you need the proper vacuum vents in the right places to let the air escape.
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Paul Smith

Offline Jim Fruit

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Re: Pulling a canopy, what temperature?
« Reply #6 on: December 02, 2012, 08:00:28 PM »
Tim & Paul:

Actually I am not vacuum forming, just plunging hot PETG over a mold. So, in theory, the air escapes downward as I plunge the hot plastic downward. I just screwed up my second attempt by inadvertently trapping air in the already existing bubble from the first plunge. Funny, actually.

Paul, I also use a plunging frame. However I just cut openings 1/8" larger than the perimeter of the plug into two pieces of 1/4" plywood. I then make a sandwich of ply/PETG/ply held together with several steel C-clamps. The whole thing goes into the oven. Popping the C-clamps removes the new windshield from the sandwich after forming.

All is a moot point however. I was not satisfied with the appearance due to the thinness of the end result. I have already made an aluminum frame to receive the full .030" thickness instead. Isn't scratch building fun?

JHF
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Offline Steve Helmick

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Re: Pulling a canopy, what temperature?
« Reply #7 on: December 15, 2012, 07:52:57 PM »
I'm pretty sure that I stumbled over a canopy molding page recently, and went looking for it. Didn't find it, but did find this'n with Google, and it looks like some good info. I think a helper would be very handy, especially if equipped with a heat gun as well as the suggested wet rag.   H^^ Steve     http://www.ultralajt.webs.com/largecanopy.htm
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Offline RandySmith

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Re: Pulling a canopy, what temperature?
« Reply #8 on: December 19, 2012, 03:41:11 PM »
I would suggest using bake setting instead of broil, easier to control the heat and not melt the plastic

Randy
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Offline RandySmith

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Re: Pulling a canopy, what temperature?
« Reply #9 on: December 19, 2012, 05:50:10 PM »
or if your really quick use a heat gun  :-)

Randy
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Offline Jim Fruit

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Re: Pulling a canopy, what temperature?
« Reply #10 on: December 19, 2012, 07:17:48 PM »
Steve:

Thanks for the web site. I find two of his points very interesting. One was the thickness of his material. While I can't relate to his "4mm" thickness, I believe it is considerably thicker than the .030 thick material that I was using. I think that the failure that I experienced was because I have an approximate 4" deep plunge and the material was just stretching too thin as the end product. It made it look too uneven. The second surprise was how he used wood frames only on the two sides. I usually make a four sided frame and just set it in coffee cups in the oven. Then when the material begins to sag, it is stable enough with a frame all the way around to just grab and plunge. The material cools fast enough that I have never had to have anyone helping with a wet rag.

Randy:

I have used a heat gun before. I did it on my Laird Super Solution. There was only a single bend it a straight line with no compound curves. So, on that one I made the mold, cut a piece of plastic large enough, clamped weights on each side of the plastic and heated the ridgeline of the plastic until it sagged down over the mold. It worked perfectly.

Jim Fruit
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