Loctite and acrrylic (bad)

Discussion in 'Workshop Tips and Secrets / Tools' started by FritzW, Nov 4, 2018.

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  1. Nov 4, 2018 #21

    Hot Wings

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  2. Nov 4, 2018 #22

    Hot Wings

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    One only is used on an old Ford engine? :gig:
     
  3. Nov 4, 2018 #23

    wsimpso1

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    Which materials you are dealing with dictate which other materials you can use.

    LocTite (cyanoacrilate) and many other solvents do terrible things to polycarbonate (Lexan is polycarbonate). I suspect that your cracked part is polycarbonate based upon the looks of the cracks with many branched cracks. I also suspect that the part that did not crack is PMMA. Check out the materials carefully before applying chemicals.

    Silicone sealers work great on PMMA if it is unstressed, and is standard process in certified airplane windows and windshields. If PMMA is highly stressed, silicone sealers can cause cracking during cure.

    Prevailing torque fasteners are your friend...

    Bill
     
  4. Nov 4, 2018 #24

    wanttobuild

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    This post contains inappropriate language and is a example that the OP vowed to clean up.
    This is also an example of biased moderator behavior here on HBA.
     
  5. Nov 4, 2018 #25

    mcrae0104

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    Assuming these aren't supplied by a third party on this particular plane...

    The canopy that cracked is acrylic: https://www.sonexaircraft.com/eshop/cart.php?target=product&product_id=16457&category_id=297

    The windshield is polycarbonate: https://www.sonexaircraft.com/eshop/cart.php?target=product&product_id=16458&category_id=297
     
  6. Nov 4, 2018 #26

    Angusnofangus

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    Don't beat yourself up to bad about this. At the company I recently retired from, an OEM aircraft manufacturer, I was given a modification to do that involved Locktite, acrylic, and screw holes. I performed the mod as per the drawings by the company engineers, and guess what? The Locktite caused the acrylic to crack around the screw holes. We live and learn.
     
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  7. Nov 4, 2018 #27

    rv6ejguy

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    There are 2 types of silicones- acedic and "neutral". The latter emits alcohol or ketones when curing. Different possible reactions with certain metals and other materials. I'd also mention that Pro-seal should not be used on acrylics but I saw the Raptor guys gluing their transparencies on with it- on a pressurized aircraft no less.

    BTW, in the old days, flat head or countersunk screws were used on thousands of RV canopies and work just fine if you follow the factory recommendations. 15 years later, mine is still mint. We use nylocs to keep the screws from falling out.
     
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  8. Nov 4, 2018 #28

    FritzW

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    The windshield and canopy were both supplied by Sonex, so will the new canopy. ...the more I think about it, the more I think I remember a warning about using Loctite. Oh well, it's too late now (as I pound my head against the wall)

    20181103_105015_resized_1.jpg 20181103_105112_resized.jpg I've seen the 'feather' looking fractures before but I've never seen them turn black.
     
  9. Nov 4, 2018 #29

    Hot Wings

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    After a bit of looking it appears that Loctight also sells what they cal Dri-loc. It's the pre-applied thread locker that is common on the tips of OEM replacement bolts. I can find MSDS and product sheets, but no price listed. Is this something that HBA&DIY can do/afford?
     
  10. Nov 4, 2018 #30

    BBerson

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    It is probably a combination of chemical and strain . A drop of chemical might dry with no effect if not under pressure.
     
  11. Nov 4, 2018 #31

    Daleandee

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    As understand it ... you are correct.

    Dale
    N319WF
     
  12. Nov 4, 2018 #32

    Dan Thomas

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    The Loctite might have caused some of it, but I agree that overtightening countersunk hardware is going to crack acrylic. It will even crack Lexan; I learned that the hard way. When you use too much pressure, or countersunk stuff that won't allow any movement at all, you're going to get cracking due to thermal stresses. When I replaced my Jodel's Lexan windshield I drilled the holes versize and used nylon washers under the screw heads so that the thing could expand and contract at will. It didn't crack any more, but then I made a new canopy cover out of nice new vinyl, and new vinyl still has solvents in it that attack Lexan and cause crazing. The crazing (very fine cracks) occurred along the bend in the windshield where the stresses put tensile loading on the outside. This was in a hot climate, and that probably figured into the outgassing issue as well. The old cover had been made of a silvered nylon that kept the sun and moisture off but still breathed. Couldn't find any more of that stuff. Vinyl, being impermeable, will trap the gases against the windshield.
     
  13. Nov 4, 2018 #33

    Dan Thomas

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    I have used both acidic silicone and PRC on acrylic, with no adverse reactions. The aluminum structure around a windshield doesn't care for the acid, though. It can suffer corrosion pitting. Avionics guys often won't use it to seal antennae bases for that reason.
     
  14. Nov 4, 2018 #34

    FritzW

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    The screws were tightened correctly. Just enough so the washer wouldn't easily spin by hand. ie. just enough to keep the canopy from rattling on the screw.

    The screws weren't over tightened.

    The screws were tightened correctly.

    The screws were torqued correctly.

    The screws weren't over torqued.

    The amount of tightness on the screws was appropriate for the application and materials.

    (just getting a few responses typed out so I can save time with a 'cut and paste' every time someone says the screws were over tightened)

    ...did I mention that the screws weren't over tightened.
     
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  15. Nov 4, 2018 #35

    pictsidhe

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    Fritz, you didn't by any chance, overtighten the screws, did you? ;)


    Edit, Sorry, just realised that 3 pages into this thread, I have only waffled about solvent stress cracking and that cyano is known for it with acrylic I have missed the obvious, unlike most others...
     
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  16. Nov 4, 2018 #36

    narfi

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    I wish I could say that everytime I screwed something up.....
     
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  17. Nov 5, 2018 #37

    Himat

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    Quite probably yes.
    I can’t find the reference but the police in at least one European country found out this the hard way. Their riot control shields were made from some clear plastic, acrylic I think. Excellent strong and impact resistant shields until the rioters did throw butyric acid or something on the police. At next impact the shields shattered!
     
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  18. Nov 5, 2018 #38

    Hot Wings

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    This thread got me searching for some Loctite Dri-loc. The idea of self applied 'dry' type thread lock seemed like an idea that needed some followup. Seems Loctite isn't interested in sales below the tanker level of this stuff. At least the impression the few sites I found left. I don't like the price on request method of doing business..........

    I did find a similar product that one can actually buy: Vibra-tite VC-3

    https://www.google.com/url?sa=t&rct...ll_sheet.pdf&usg=AOvVaw3UhaKkfSFryLK7VdGze7JV

    https://www.google.com/url?sa=t&rct.../213_tds.pdf&usg=AOvVaw10eW9cIcUArwIWpSe4wo4y

    Never used this but it looks promising for some applications. Anyone have any real world experience they care to share?
     
  19. Nov 5, 2018 #39

    FritzW

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    ...well ...this is embarrassing ...right on the canopy page, in bold letters

    20181105_112505_resized.jpg I probably read that note a dozen times :emb:
     
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  20. Nov 5, 2018 #40

    lr27

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    I think I've used that stuff on one or two jobs. Can't remember any problems, but then I don't know if we would have run into problems anyway. It's been a long time, so the memories are vague. Since then I've also used nylon insert locking stuff and also square cone washers. All seemed to work, but I still don't know if proper torquing wouldn't have been enough in the first place. I suspect that's not the case with the acrylic canopy if subjected to large temperature range, but I don't know.
     

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