Forming Plexiglas

I am interested in learning the ins and outs of forming Plexiglas. I want to put a skylight in my airplane. I want the skylight to have a curve outward of about 2 to 3 inches. I have looked at the You Tube videos, but they are mostly about forming thin polycarbonate such as for landing light lens etc.

Does anyone here have first hand experience about the best way to heat up the plexiglas? Also what is the best material to use as a form? How smooth does the form need to be? Is there something that I should use over the form before I stretch the plexiglas over it, such as a sheet of Tyvec? I am using 3/16" thick material. It will end up being about 34" square with about a 2 to 3 inch bulge, so I am not looking for a whole lot of curve. In an effort to keep the experimentation down to a minimum, I would really prefer to hear from those who have first hand experience verses theorists.

Hello,

You can build a form out of wood and cover it with flannel. The form can be made of 1/2" thick MDF ribs with thin hardboard glued across the ribs to form the curve.

Read the materials properties for the plastic that you are forming. You want to heat it up to the exact temp that it starts to soften. If you get the plastic too hot so that it is really floppy, the weave of the flannel will transfer to the face of the plastic. For a skylight you only need to form a relatively small curve.

The challenge is finding an oven large enough for your form and the plastic sheet. You want an oven that has a temperature control. Preferably an electric oven.

The plastic sheets that I have formed have fit in the kitchen oven. Not sure what the dimensions of your part are.

Best of luck,

Wes
N78PS

Yes, finding a way to heat a large piece is the challenge. I have heard that windshield makers place the plastic in a vat of boiling oil. But I don't know what kind of oil they use, and don't know how I would heat up a vat large enough to hold a piece the size I need. The skylight is about 35" square, so I need to heat up a piece a little larger than that. The flannel is a good idea. I will remember that, if I ever figure out a solution to the other obstacles.

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Originally Posted by WLIU

You want an oven that has a temperature control. Preferably an electric oven.

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I would do some tests on a small piece first, most household electric ovens do no not hold a steady temperature, they actually turn the element on & off so there is a bit of a temperature delta. I have seen videos of plexiglass being formed, and they use specially built ovens that will hold an exact temperature. It also takes several people to get it out of the oven & on the form quickly enough to form it.

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Originally Posted by Pearson

I have heard that windshield makers place the plastic in a vat of boiling oil. But I don't know what kind of oil they use, and don't know how I would heat up a vat large enough to hold a piece the size I need.

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If you go this route, see if you can come up with a used fuel oil tank like what people with oil fired boilers use in the basement, and cut the top off. I'd use a vegetable oil with a smoke point above whatever the temp is you need to heat the plexiglass to. Unfortunately the safest way I can think of to heat the oil would be a steam jacket on the tank, and I really doubt if you have access to a boiler that would get hot enough. Any kind of burner under the tank would be rather unsafe. :)

There is a good U-tube video on this by an EAA chapter. Try Google for it.

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Originally Posted by flyingriki

There is a good U-tube video on this by an EAA chapter. Try Google for it.

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I spent the better part of a day viewing videos on You Tube. I did see one made at an EAA Chapter, but it droned on for three different videos before I realized they were just making nav light covers for wing tips. The challenge with what I want to do is the size of the piece of plexiglass. Thanks for the ideas though. Keep them comming. Eventually someone who has done this before will notice this thread and chime in here.

It might help if you posted how large the part that you have to form is.

It is not that hard really. Plan on throwing away your first try, but you will likely get an acceptable part on the second try.

I would not resort to hot oil. Hot air us good enough. I did the skylights for a Globe Swift in the kitchen oven. You might consider building an insulated box on the front of your kitchen oven to extend its size. Use the oven temp control to creep up on the right temp. Patience is important.

Best of luck,

Wes

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Originally Posted by WLIU

I would not resort to hot oil. Hot air us good enough.

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I tend to agree - I'm having visions of a gigantic turkey fryer fire. :eek:

A fellow in Kitplanes, a few months ago, made bowed side windows for his plane in his wifes' oven. He built an extension to the oven box with simple materials and was pleased with the results. Probably had the temp info you require. Sounds similar to the suggestion above for Swift Skylights. Good luck!
Link attached.

Thank you for the article "Kitchen Window". It gave me a couple of ideas. I have a small forced air propane heater I use to heat my garage during the winter. I think if I made some sort of box to hold the hot air I could use it as an oven. Also I like the idea of using air to "blow" the plexiglass. That would save a lot of labor associated with making a male mold out of wood. I will have to fabricate female curves to match the curve of the top of the wing and the front and rear cross pieces, but I think that will be less work than a male mold. Blowing it also eliminate my concern for protecting the plexiglass from scratches when using the male mold. Thanks for the ideas.

I made windshields for my Biplane using an oven that is designed for baking silk screened immages in "T" shirt shops. The process was outlined in the Acro Sport Newsletter #62. These newsletters can be accessed from the homepage of <BiplaneForum.com>
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I am going to try to use what I have on hand to heat the plexiglass. If my little propane heater doesn't work, I will investigate the link to the ovens.

Get hold of Light Airplane Construction by Leo Pazmany. He gives a good run down on making a canopy for the PL1 and PL2.
He recommends air heating in an oven with the sheet hanging from pressure clamps. Felt or flanel moulds should be covered in grease.
If you can't get a copy easily. email me 355yen@tpg.com.au and I will see if I can photopy and email the relevant info to you.
I must stress that I have never tried this process myself, at least on anything over the size of a light lens.

What temp do you need? Companies that do powder coating have large ovens that heat to 400 degrees. Possibly you could strike a deal with someone that would let you put your skylight in their oven?

Hey, that is an excellent idea. I need a temp of around 300F. I will ask a couple of friends that do powder coating.

According to Pazmany temps vary dependant upon ambient temp. He gives a working time for 1/8" plexiglass of 1.5 min at 75degF and heated to 320 Deg F Going to 2.8 mins at ambient of 120 deg F. He suggests 280 to 360 deg for heating, and thinner materials need greater heat due to their quick cooling, so your project should be OK I would think at 300 deg.

I have seen a glass shop mobil service truck work with plexiglass. You've seen these trucks haul large sheets of glass, and windows on angled racks, basically
the truck had a six foot length of schedule 40 black iron pipe approximately 2 1/2" in diameter, heated internally with a propane burner, and attached to the big rack on the truck with a couple clamps, The pipe stood vertically and was not capped. The service person fired the "bbq" burner and went about other tasks, the pipe section had an oven temp gauge, nothing too fancy,
There was no contact between the pipe and the sheet plexi, he worked it back and forth across the heated pipe until the area he wanted the bend was "at the temperture" all this was done without jigs or molds. He had done this before, can't tell you the thickness, or temps. At the time I just found the process interesting with little thought of using this method until a shielding guard for a process that used water was damaged. Myself and a coworker used this method with the pipe in a vise with a heatgun blowing through, took a little longer but we made three bends to form a box to shield the escape of water.
Sounds like you are just rolling one edge? to create the leading edge of your skylight? Check with a local glass shop, they may use this method and or have scrap that you can do the tests first.

one other note, you can us clean window glass as a work surface

Another approach is to see if the nearest A&P school has an oven for heating plex, and if they'd let you use it.

I have found a powder coating guy that said he would let me try my plexiglass project in his oven. So that may be the best way for me to go. I am doing more than just bending the forward edge of the plexiglass. I am trying to put a bulge in the center of the entire piece, which will be about 35" square. The two side edges will have a curve to match the upper surface of the wing. The front and rear edges will have a very slight curve to match the piece of wood that goes horizontally across the top of the cross tubes of the fuselage. Then, aside from those curves along the edges, I want to blow a bulge across the entire middle area that extends upward between 2 & 3 inches. It sounds like some very interesting info in the book that was mentioned. I will have to investigate the various temperatures mentioned. Anything I can do that reduces the chance of ruining a piece of this plexiglass will be well worth the time and effort. Thanks for all the great ideas.

I formed the windshield for a Merlin GT over a round piece of wood.
I used a Heatgun ( One like to strip paint with ) to heat the plexiglas.
Just heat the area and slowly bend the plexiglas a little at a time.
Make sure not to overheat and don't try to bend it to the final shape
at once.
Worked great for me !

My uncle, see ( Bally 1/3 B-17 G ) "blew" the nose for his project. We had done some trials with scrap plexi and found that it did not need to be very hot to be quite flexable, but it cools to quickly to work a big enough piece. As the plexi absorbs moisture, if it gets too hot small steam bubbles will appear. So he made a female mold with pin holes for a vacuum. The oven was coriggated sheet metal insulated on the exterior with fiberglass, with an oven element to povide the heat and a pyrex baking dish as a window. He clamped the plexi to the mold and placed it on the oven. When the plexi was heated till it was sagging he applied the vacuum and sucked it up into the mold. A couple of trials and errors-salad bowls he has a nice looking nose.

Do you recall how thick the plexiglass used was? Did you take any temperature readings to tell how hot it was before the plexi started flexing?

I will suggest that the manufacturer of the "plexi" will have that data online somewhere. I quickly googles "plexiglass properties" and came up with http://www.plexiglas.com/acrylicshee...ata/properties

If you know the specific manufacturer of the "plexi" that you are using, look up their web site for the exact data that you need. I will note that "plexiglass" is the common name for clear acrylic plastic. Search using that name also.

Best of luck,

Wes

Was out of town over the weekend but stopped by Jack's last night. It was almost two years ago that he did the nose but he recalls the thickness was 3/ 32". He basically made a giant heat gun that blew the heated air on to the acrylic for more even heating. He used a laser thermometer and recalls the temp was about 275 although the readings where very inconsistant for some reason. He orignally heated the mold but found better results by not heating the mold. His grandson has set up a website http://www.theballybomber.com with pictures. This project has been the most educational he has ever done, the basis of EAA. Good Luck Dave

David,

I checked out the website. Looks like an interesting project. I think I am going to build a jig with an air line hooked up to it, and heat the whole thing in a friends powder coating oven. I am trying to minimize the number of wasted sheets of plexiglas. I have already checked with the manufacturer and they say I need 300 to 325 degrees to blow it. I am only trying to put a nice gentle 2.5 to 3 inch bulge in a 3 foot square piece of glass. I too am out of town, so it will be awhile before I can do anything. Thanks for all the great ideas everyone.

Quote:

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Originally Posted by flyingriki

A fellow in Kitplanes, a few months ago, made bowed side windows for his plane in his wifes' oven. He built an extension to the oven box with simple materials and was pleased with the results. Probably had the temp info you require. Sounds similar to the suggestion above for Swift Skylights. Good luck! Link attached.

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