Lexan.....The Miracle Plastic?

[Area 51%]

I bought an extra canopy (plexiglass) before starting that miserable task as I was sure I was going to fail on the first try. Undersized acrylic drill bits followed by ever-increasing diameter reamers to keep the holes as smooth as possible was my method of choice. I used a "flapper" disc on an angle grinder followed by 400 grit sandpaper to trim the edges.
So far, the only crack I have in the canopy is where I closed it on an extension cord at the right rear corner.

The same methods, tools, and care was taken during the windscreen (polycarbonate) install, however I have cracks forming at multiple hole locations around the perimeter of the "glass". I always protect the surface during refueling, so if any gas has ever been spilled on the Lexan, it's news to me--never mind the fact that most of the cracks are along the sides and across the bow--away from the fill port.
I've never been very happy with the optics of this windscreen. It has "waves" that run from one side to the other every couple of inches or so (I blame the distortion for my crappy landings), and a second windscreen I purchased at the same time was subsequently installed on a Sonex and the owner reported the same "waves".

Did I get a bad piece of Lexan, or might there be something else going on that I can't identify? Shopping for a replacement on ebay yielded what I thought was an interesting notation. It read the polycarbonate sheet was UV resistant on only one side. Might I have gotten my windscreen on upside-down, and now it's suffering from radiation poisoning? Is there even such a thing as it being resistant on only one side? I don't remember seeing anything on the print about a "jelly-side".

Anyone else getting non-fuel related cracks in their windshields?

Bird-strikes need not reply to: Area 51%

[DCASonex]

As far as I know, ordinary Lexan does not have a UV resistant side, but there are many special versions of poly carbonate (Lexan is a trade name, originally by GE) available including with hard, scratch resistant, surfacing. I suspect the cracks you are seeing on sides and bottom of front are from screw holes, and that can happen if your drill bit or fasteners were contaminated with any oil. Replacement of windshield portion is pretty simple.

David A.

[Area 51%]

That is correct, all of the cracks are emanating from the screw holes. The drill bits and reamers I used were purchased solely to drill the holes in the canopy and windscreen and had no previous experience making holes. Is it possible they had some kind of protective oily coating that transferred to the Lexan?

In my search for replacement polycarbonate, I ran across the scratch-resistant version. They warned not to bend it and use it only for flat applications.

[Rynoth]

In my search for replacement polycarbonate, I ran across the scratch-resistant version. They warned not to bend it and use it only for flat applications.

For what it's worth, I have the same spider cracks you describe, mostly around the firewall bend, but they don't appear to be progressing beyond about 1/4-1/2 inch of the holes (so far.) My plane has spent 99.9% of its life in the hangar (need to fly it more) so I wouldn't suspect UV in my own case. Something on the fasteners maybe?

As for scratch resistance... I would not consider my windscreen scratch resistant in any sense of the term, so it might not be that.

[WesRagle]

Hi Randy,

I personally never had a problem with lexan, other than scratches from careless cleaning.

However, one potential problem that hasn't been mentioned is under-sizing of the holes. Lexan has a relatively high thermal expansion coefficient. If the holes are not significantly larger than the fastener the lexan can/will be under considerable stress at temperatures different than when the holes were marked. I always make the holes and counter sinks significantly over-sized and I am careful not to over tighten the fasteners so the lexan can move as temperatures vary.

Just a thought.

Wes

[DCASonex]

Was not intending to sound like i was recommending the scratch resistant Lexan, I have had no experience with that, only mentioning it as one of several variations available.

David A.

[Area 51%]

The undersized hole theory makes more sense to me than any kind of oil contamination. That is, unless there are plastic cleaners that are not suitable for polycarbonate that may have worked it's way under the washers. I've tried many different kinds/brands and all have been touted as "all plastics" or have polycarbonate listed in their application chart. I am currently using Turtle Wax paste wax after a soap and water cleaning. This is "post" crack behavior.

Back to the undersized holes. I would have thought this problem would have reared it's ugly head last winter with the shrinking of the holes rather than deep into summer. I plan to go up another hole size on the new windscreen from what's listed on the print, given the chance the engineer that wrote that page may have had a sticky slide-rule that day. Did I mention the canopy is fine?

Not sure how long Lexan can stay out in the Sun before UV protection should be considered, but like Ryan's bird, Chett is hangared and has only been out in the daylight 59.2 hours. I think Vampires are allowed to be out longer than that.

Took the scratch resistant Lexan comment at face value. It did seem like a great idea till the "not for curved surfaces" note came up.

[WesRagle]

HI Randy,

That is, unless there are plastic cleaners that are not suitable for polycarbonate that may have worked it's way under the washers. I've tried many different kinds/brands and all have been touted as "all plastics" or have polycarbonate listed in their application chart. I am currently using Turtle Wax paste wax after a soap and water cleaning. This is "post" crack behavior.

I've been using this stuff since I first started flying:
https://washwax.com/collections/wash-wax-all/products/waterless-car-wash-wax-all-16oz

Bought something like 4 gallons 20 years ago and haven't used it up yet. For the Lexan I lightly dust the surface with a micro-fiber cloth and then mist the surface with Wash Wax and gently spread/dry with a clean micro-fiber. Works great and seems to mask some of the minor scratches.

Wes

[DCASonex]

Having used literally tons of Lexan for machine guarding, I can assure you that any contamination of drill bit or taps (Used a lot of 1/2" thick Lexan and tapped holes in it) can result in cracking. Too small holes to allow for thermal expansion is also a possibility Look up the thermal expansion rate for Lexan and Aluminum and then work out the difference over the length of each side. Think you will be surprised by the difference. Would tend to suspect cracking from tension from very cold temperatures rather than high temperatures, since in the latter case, the material can bulge between holes.

David A.

[Kai]

I am confused!

Confusion invariably sets in at the same time each year- more precisely when we head into cooler weather and winter maintenance of the Sonex looms up. Because this winter, as on a few earlier occasions, the windshield has to be replaced. In addition, my 2003 vintage canopy now has so many scars, that replacement is inevitable.

I seem to have grasped, that we are working with two different types of clear plastic sheet: acrylic and polycarbonate.

The canopy is made of acrylic sheet. One brand name is ‘Perspex’. It is somewhat delicate to install on the plane, but once there and provided you have not been granted permanent care in a mental ward, it will tolerate a lot of abuse- especially the potential revolting catastrophy of (bioethanol) mogas spill.

The windshield is made of polycarbonate sheet. One brand name is ‘Lexan’. It is fairly benign to work with taking certain elementary precautions, and replacing an existing windshield is well within the grasp of all. But its arch enemy is (bioethanol) mogas- the instant it is subjected to just a little of this type of fuel spill, it will start to show cracks.

RIGHT??
If not, please stop me in my rant here- for in the distance I can already hear the wails of the car with the nice man inside with a long white coat and a large hypodermic needle.

So would it not be a logic conclusion to introduce acrylic sheet for the windshield as well? Probably not, because contrary to polycarbonate, you can’t easily bend it without a form/plug, as well as a hefty oven where it can be heated to some 70-80 centigrades. Not for everyone, then.

Or- is it totally hopeless? Anybody in the know? Comments? Advice?

Thx
Kai