Cracks in Headlight Covers? What did I do wrong?

[Alphamacaroon]

Well that didn't take long for my beautiful headlight covers to turn to junk... Any ideas what I might have done wrong?



Holes were drilled with special plexiglass drill bit. They did not have any cracks when I put them on. Cracks seem to have shown up after some driving time.
Screw has a nylon washer to protect it from scratching the plexiglass.
It's not any tighter than it needs to be— just tight enough to form the glass to the contour.
Pretty much every single hole has cracks around it.


141007

Thoughts? Maybe I need to drill the holes bigger and add a a rubber grommet around the screw? Maybe I need to use a rubber washer instead of a nylon washer? Only thing I can think is that I guess I just put them on too tight.

[Gordon Levy]

I would say the holes weren't large enough and the screws were still putting stress on them and they were over tightened. You tighten them barely snug.

[Alphamacaroon]

Yeah I thought I kept them fairly loose, but apparently not enough. Well I guess that’s how you learn. 😀

[Kodiak]

Bevel the drilled hole on both sides just enough to take the sharp edges down and minimal torque on the screws.

[Jim1855]

Might want to try a softer washer, one with a bit of "squish".
Vibration, metal and hard plastic isn't a good combination.
Jim

[Tuftster]

At least you have a set of covers. Three years and still waiting on mine :(

[thriftychicken]

If you used any thread lock, it will 'craze' many plastics.

[aquillen]

I would also note that stress is one of two ways this happens. The ideas above on mounting, tension are probably your fix. From my experience you could have what I'll describe next, and stress will exacerbate the problem. Bottom line, any cleaner, solvent, etc., other than specific acrylic cleaners just might cause your problem as well. Since you have a lot of cracks I suspect chemical induced cracking is also part of the problem:

Acrylic plastics, especially any edge not well polished after any cut/machine work is subject to micro crack initiation by stress and also very much by solvents and various chemicals, followed by a slow propagation to long cracks. Offending solvents include acetone, ketones, iso and denatured alcohol, petroleum solvents, cyanoacrylic (super) glues, even some acrylic rated (! ask how I know !) adhesives and a lot more. When I was fabricating my 818's custom tail light lenses by sawing and milling used Chevy red 3rd brake light lenses, Cadillac clear backup lenses and run of the mill acrylic clear sheet, I had a devil of a time with hairline cracks that grew to be visible after gluing parts together. The cracks show up under a 10x magnifier within a few seconds to a minute or so after contact with a fluid and can be seen growing for several minutes after the surface has been wiped dry or even heated to dry faster. Usually in a day or so a few become visible without a magnifier. This is all with no other stress applied.

I researched adhesives for many hours and materials and techniques for cleaning, prep and bonding. I bought WeldOn-4 and had cracking with it. 3M and Henkle have acrylic bonding agents that are near impossible to find in less than industrial quantities for big $. Most silicone adhesives cure with out-gas materials that may also trigger this. I trusted 3M 730 on my front lenses edges, no problems as yet (since Sept 2019) Flame treating edges to get them glass smooth also turned out to be a challenge and was not a sure fire prevention. Fortunately I used tail lenses via LKQPickYourPart scrap yards nearby which were cheap enough that I could just keep testing.

I don't use any of the usual glass or paint cleaners/detergents (I know that is hard to avoid when washing the car) on my installed lenses, both in my back home made and the FFR supplied ones in the front. The only thing I trust to clean my lenses is a dedicated plastic specific cleaner, such as Plexus (Plexiclean probably equal). I was unable to find any micro cracking on edges that I soaked with Plexus. And, medical alcohol wipes in the little foil packs = micro cracks.

Once the cracks started, no solvent, adhesive, I tried would fill and bond them to hide the damage completely. Strangely though I did have a little luck with the thin (blue bottle) Bob Smith Industries Insta-cure super thin "super glue". It actually triggers some microcracking yet oddly at the same time did a fairly decent job of filling and sealing the edge overall. I ended up trusting it for the bonding work as well as trying to seal spots that showed visible cracks. The nature of the cracks on my tail lenses are hard to see so I'm lucky.

[freds]

What Art Quillen said in the previous post.
Just to add:
After getting my driver side window into the frame having lubed it as described in the instructions (and congratulating myself, as I didn't find it an easy task!) I used Brakleen to clean it. Almost immediately cracks developed around all the holes for mounting the sliding windows.

As an aside:I believe the headlight covers are Polycarbonate not Acrylic

fred

[aquillen]

Correct - polycarbonate. It will do the same sort of cracking too.

[Alphamacaroon]

If you used any thread lock, it will 'craze' many plastics.

Now this is super interesting. I did use thread lock because of the very fact that I didn't want to over tighten them. I'm starting to suspect this might be the very cause.

Any others using thread lock? If not, have you had any problems with them loosening? Or maybe I can find some plastic safe locker. Thanks— learned something new today!

[edwardb]

Now this is super interesting. I did use thread lock because of the very fact that I didn't want to over tighten them. I'm starting to suspect this might be the very cause.

Any others using thread lock? If not, have you had any problems with them loosening? Or maybe I can find some plastic safe locker. Thanks— learned something new today!

Oops. That's it. Your pictures are exactly what happens when Loctite gets near that material. I haven't used anything on mine and they haven't loosened. I've had them off/on a couple times and didn't seem to be changed. If you're determined, a little clear RTV could work.

[Nigel Allen]

Rubber washers will help hold screws in place. Could also wrap some plumbers tape around the screw thread to prevent loosening as well.

Also loved your drive video.

Cheers, Nigel

[Alphamacaroon]

Thank you everyone for the help. Seems like the mystery is solved. I had never heard of this before— always love to learn something new. Just not as fun when it involves breaking something :D

[Bob Cowan]

The rubber or soft plastic washer need to be on both side of the plexiglass. Any hard surface contact will start the cracks.

I agree with clear silicone bathtub caulk or teflon tape to act as a thread locker.