Polycarbonate (Lexan) Skylight

[Nev]

I installed my skylight yesterday and got small cracks around 3 of the holes. I see there are a number of threads discussing this already, but I’ve decided to put it here in case there’s some revolutionary advances that I’ve missed. Mine is a large skylight that I’ve modified to do in one piece. The result is that I am dealing with compound curves (I’m not as smart as I thought I was )

I would have preferred to use Acrylic and heat formed it, and it’s something I’m still considering, the downside is that it’s quite a big task. Probably I’d make a mold and take it to a commercial shop and get them to do it.

The Polycarbonate on the other hand forms over the compound curves quite well. I suspect my problem occurred when I used Loctite on the machine screws, and it’s made the poly around those holes brittle. Unfortunately I only learned that after the fact.

I also had a tip to apply some heat to relax the lay of the poly where it bends, and I’ve tried this. It definitely looked like it was removing the stress from the sheet, however when I removed the skylight it still had its original flat shape. Probably it would help though, as I only need to reduce the stress where the holes are.

My main concern is what happens when I get fuel on it as inevitably it will. I’ve been trying to source a sheet that has Noxtat treatment for chemical resistance here in NZ but so far no luck.

Originally I had planned to use an adhesive to bond the Polycarbonate to the two center stringers. I still think it would be pretty strong, and even if it isn’t, the sheet is still captured on all four sides and almost impossible to remove without removing the side strips first. I even thought about not having anything retaining the center stringers. However it does “float” up at the aft end and would probably drum.

F5192880-6578-4B0D-857A-28A2925D30A6.jpeg

[rodsmith]

Sorry to hear that. Mine is 1/8th Lexan. I didn't try to heat form it. I did drill 3/8th holes for #6 screws and used a strip of aluminum over the stringers to help spread the stress of the screws and installed them fairly loosely. I would like to know what temperature is required to heat form the lexan. Would be beneficial for installing the lexan on my gullwing doors.

[Sir Newton]

I you are capable of building a model. Vacuum forming thin plastics is very simple to do. Youtube the subject. I am of the same mindset for my doors.

[Nev]

I did drill 3/8th holes for #6 screws and used a strip of aluminum over the stringers to help spread the stress of the screws and installed them fairly loosely.

I like your idea of using an aluminium strip to spread the load - might try that.

[Bcone1381]

Here is a link to a 27 page PDF file that the Lexan Manufacturer publishes titled Lexan Sheet Processing Guide. P.8 has the procedure for Drape Forming Lexan. It seems to me to be geared to industry guide....but we are fabricators aren't we!! Knowing that they recommend around 150C is good gouge.

https://bluerhine.com/wp-content/uploads/2016/08/Sabic-Lexan-Processing-Guide1454154199.pdf

[Gerhard Rieger]

Hi Nev, the cracks around the holes originate mostly from using a sharp drill bit, You make the cutting edges blunt and round the sharp corners and then you can say, burn the holes through, you also have to make them at least 1,5mm larger than your screw. Thats info I got from a aircraft window manufacturer and it worked when I replaced all the windows of a friends airplane.

[Nev]

This is good to know thanks !

[svyolo]

I have seen pop rivets on lexan covered doors and windows, with and without metal strips over them. I don't know if the rivets were AL or steel. I might try it on some scrap first.

[marcusofcotton]

..I suspect my problem occurred when I used Loctite on the machine screws, and it’s made the poly around those holes brittle. Unfortunately I only learned that after the fact.

Loctite will definitely cause cracks in polycarbonate even when just on the threads and not on the poly. Same goes for acrylic.

[quadra]

the list of incompatible chemicals with those plastics is actually very large... petrochemicals, most solvents, and a whole bunch of chemicals that are easily available and used as bases in cleaners will cause stress cracks. These plastics are very notch sensitive and any cracks, holes or small radius corners are where you will create failure. In my workplace we have had serious failures from loctite attacking a variety of plastics, and chemicals like isocyanyurate ( superglue ) and its accelerator (liquid isobutyl compounds ) causing instant stress hazing and material failure- cracks that can propagate deeply into a section and cause it to fail with limited load.
BUT!!... that is also less common now days as suppliers and end users are following the manufacturers specs. these materials are light,strong ,durable and impact resistant but need to be treated with care.

[Battson]

Well, I would say that Acrylic appears to be less sensitive to common aviation and automotive chemicals than Lexan, because there's not much which hasn't been sprayed or spilt on our windows by this point

[quadra]

fuel, isopropyl alcohol, surface cleaners and degreasers... also, acrylic is pretty damn awesome optically, and is far MORE resistant to scratching, but does not come close to polycarbonate for impact resistance. If your skylight suddenly became a sunroof would you be as cool as a key lime pilot?

[AKKen07]

I think acrylic is more resistant to scratching than poly generally.

[way_up_north]

This is a video of a do it yourself oven forming

lexan and acrylic...quite a bit to the process...but it can be done

[Nev]

That’s an impressive video. Very good home setup.

[quadra]

fuel, isopropyl alcohol, surface cleaners and degreasers... also, acrylic is pretty damn awesome optically, and is far MORE resistant to scratching, but does not come close to polycarbonate for impact resistance. If your skylight suddenly became a sunroof would you be as cool as a key lime pilot?

sorry, that was what I did intend to say - corrected

[jim.mclaughlin924]

I have had good luck using 1/8 all alum pull rivets and with an alum strip over the lexan. I enlarge the hole in the lexan to 3/16 and bevel both sides of the hole. I did buy a couple of the special drill bits for plastic that are supposed to prevent damage when drilling.

[svyolo]

How thick did you make the strips? I made mine out of .040 yesterday but I may upsize them to .063 or .080 to spread the load a little better.

[jim.mclaughlin924]

I have used both 025 and 032, about 1/2 inch wide and 2 inch spacing

[svyolo]

My screws are 4" apart. Maybe I will stick with what I have.

[Bcone1381]

Does the four place plans call out any Material for the Lexan Cap Strip? The Patrol calls for a .500" wide strip over the lean 1/8" thick...quite a bit more robust than what is being discussed. ref Patrol Plans p.17 and p.25.

Another viewpoint on drilling Lexan/polycarbonate P. 18 of the pdf document above calls for
-standard twist drill bits and
-avoid heat build up with frequent clearing out the swarf....swarf increases friction & heat buildup....also frequently cooling the hole with compressed air.

The Lexan Sky Light from the Patrol Plans......
Screen Shot 2021-06-08 at 7.44.05 AM.png

[Glenn Patterson]

We did not do the skylight however a Bearhawk builder ions ago recommended a good idea for fastening Lexan. The method may work for a skylight and is good in particular for the windshield. The builder recommended drilling the holes oversized then using poly fuel line for bushings in the Lexan holes. The poly bushings keep the screw threads from coming in contact with the sides of the holes to soften the attach points. We used the poly fuel line bushings on the windshield and side windows. The thought is that with the temperatures ranging from -40C(-40F) to +37C(98F) that the poly bushings may be good with the temperature swings. The Lexan would pull tight in the extreme cold and expand in the summer sun to push and pull on the screws.

[rodsmith]

I really like that idea!