Lexan window cracks - help needed

Discussion in 'Materials' started by Wynand N, Apr 8, 2009.

  1. Wynand N
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    Wynand N Retired Steelboatbuilder

    I experienced some mayor cracking installing windows on a steel boat under construction and attached photos to show the damages.

    But let me start at the beginning. We needed a smoked colour glass to blend in with the colour of all hatches, portlights etc fitted, Since it is side cabin windows and may take a large wave or more when sailing, it was decided to use a Poly Carbonate plastic such as Lexan. Problem being, Lexan only available in 4mm thickness in smoke tinted colour in South Africa - our national dealer and importer in Maiseys plastics, largest in country - and this is to thin for the application and would fit "badly" in its allotted frames.

    It was then decided to bond the 4mm Lexan to 8mm clear Perspex to get the desired thickness and tint. The Lexan was placed on the outside of the boat. This was actually a good idea then, the bond between the Perspex and Lexan can act as a trim to hide the actual bond of the window to the frame from the outside - much like the black trim around an automobile windscreen. Secondly, the bond will also keep water finding its way between the two laminates. See first two pictures.

    To make this bond I contacted my supplier and he got SikaFlex's technical department phoned me and I explained what we want and he made his recommendations of what to use and we did exactly that. The edges to be bonded has to be treated with a Sika Primer 209N primer and then thin layer of SikaFlex 295-UV applied. This we done straight out of the book.
    All holes were drilled 2mm bigger than the bolt diameter and countersunk with countersunk tool for perfect match with bolt c/s head.

    Before fitting the window to the frame, a general layer of SikaFlex 295 was applied on the frame and primer again to the Perspex to be mounted against it. Bolts were inserted and only lightly tightened until the window made solid contact with Sikaflex and left overnight for the SikaFlex to cure and form a gasket before tightening things up.
    At this stage nothing was amiss and I have to mention that the max curve on these windows are about 6mm over a length of an average 700mm. Some are actually flat. We only fitted the starboard side windows.

    Yesterday morning hell came down on me - climbing into the boat to inspect our work, all the windows are cracked badly and I thought it was the Perspex. On closer inspection I found it was the virtually indestructible Lexan that cracked!!.
    I immediately went to the plastic supplier and they came in force with their technical guy to check things out. They found the bolts still loose and can be easily turned with allen -key (nylock nuts). The slight curvature they found not to be an issue and suspect that the holes were to small. When informed and shown on windows still to be installed the clearance is about 2.5mm per hole that was eliminated. The suspected the primer and came back later with 18 pages of specs from SikaFlex head office and again it was found we done everything out of the book and used the right stuff. They were flabbergasted just as we were.

    Then someone in the know told me of similar problems they had experiences and said these plastics has to be drilled with a blunt drill bit, actually be burned through. We took the smallest window to be still fitted on the port side and slightly enlarged the holes further with a blunt bit (burned through) and we fitted that. And just to make sure, although the windows fit loosely into its bracket, we took off another 2mm off the perimeter of the window. Guess what we saw this morning....same thing happened again.

    Can anyone help me or give some advice? This is a serious and expensive disaster and do not want this to repeat itself. Funny, in the past we fitted Perspex only windows to hulls with a lot of curvature and never had anything remotely as this happens.

    Finally, we bonded pieces of Lexan and Perspex offcuts and drilled exactly as per windows and put the bolts in and tighten with such force that we strip the allen-key holding the bolt without any crack to the Lexan....

    Attached Files:

  2. waikikin
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    waikikin Senior Member

    Countersunk bolts, I think maybe that perfect match to the countersink may be to blame plus the thermal coefficient of expansion steel to plastic? maybe use a screw cap- like a cradle washer for the countersunk head or try some round head bolts with flat washers. Regards & best of luck with it from Jeff
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly...why bolt the windows if you're gluing them as well?..one or the other, not both.

    If you have any of the lexan left, take a bit - a half decent size, uncut. Place it on a table, support it at the ends, with small supports, like a simply supported beam.
    Then over night, lay a weight, on top of cloth, on to the lexan. Say 2kg..nothing seriously heavy, but enough to slightly stress it.

    Look at the result next morning..if all ok, fine

    Next repeat, but this time, scratch the surface with an X mark, roughly in the centre and leave over night...if all ok, fine.

    Finally, where you have placed the X mark...drill, as you have before, and repeat.

    The purpose of this is to ascertain if the lexan has been made prestressed. Just like an ally plate that has locked in stresses when welded, if high restrained.

    It should fail under any of the above, if the manufacturing is suspect.

    If it does not fail...hmmmmm..interesting!
  4. Manie B
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    Manie B Senior Member

    Wynand i know this is going against "normal" thinking

    but we have had so much **** with lexan at the dam - also mostly flat, that simply never again - if its not the one thing - its flippen glue or silicon migration and and and - the list is endless - you farkin didn't do this right - then you didn't do that - so fu3k that **** NEVER AGAIN
    and after doing it to the book, 3 years later and its got to be replaced because guess what - you flippen didn't do it right - the **** doesn't last either.

    Now we are going to lamited auto glass - many specs available - and to get the "tint" on the inside we use "film" as what is used on cars for "smash and grab" or to darken the windows, once again MANY types available

    Down side is to get curves, it is very costly to make up so basically we are going flat, and glass dont scratch, it lasts forever and you can even have it so thick that it is bullet proof for them pirates :D

    And i can assure you that an extra 10 maybe 20 kgs makes no difference to a 43 Steel Dix displacing big tons

    sorry about my ranting but it drives me up the wall when the "experts" wanna ******** me about what works, when the flippen "expert" cant operate a jigsaw :mad:
  5. Landlubber
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    Landlubber Senior Member

    Could it simply be the different expansion rates of the two [lastics, lexan really does grow. It has to be remembered when fitting particularly long wondows in cats for instance. Ask the supplier about the expansion rate, it can be 10mm per 1000!
  6. daiquiri
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    daiquiri Engineering and Design

    Perspex and Lexan have both quite similar coefficients of thermal expansion, around 7*10-5 m/m/°C.
    The strange thing is that some of cracks don't seem to originate from the bores. Could be the glue shrinking/expanding and pulling plates while hardening? Just guessing here.
    In any case, maybe the best and most economical thing would be to search for a supplier of a single-layer thick lexan or perspex plate outside of South Africa.
  7. Luckless
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    Luckless Senior Member

    What was your drilling method? My first thought would be there is a difference in thermal expansion rates of the two materials that is causing breaks along stress points introduced by drilling.

    Random thought for if you're going to have to throw the cracked windows out anyway. Use a fine marker to trace the cracks on the windows as they are now, then stick one of the windows in a freezer for a few hours to see if the cracks expand any more. If they expand on the chilled one and not the warm ones then I would chalk the issue up to thermal expansion for sure, if not, then still no idea what the issue is.

    You said that Lexan only available in 4mm thickness in smoke tinted colour in South Africa, do you mean that the smoke tinted colour you want is only sold in 4mm sheet? Would it be possible to get full thickness clear sheet, and then apply a tinting film sheet to both sides of the Lexan after? This would at least give you a bit of replaceable scratch protection on your windows.
  8. Wynand N
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    Wynand N Retired Steelboatbuilder

    Thanks for you guys thus far. Let me answer some questions raised.

    Lexan is only available in 4mm sheet in my part of the woods -regardless of colour.
    The different coefficients of thermal expansion between Lexan and Perspex, if any, should not have been a problem as the windows were bonded about 2 weeks ago and no **** happened until fitted to the hull.
    Furthermore, the boat sits in a factory where the difference in temperature is minimal and temperatures are still in the mid to high twenties C.

    When installed, no visible cracks happened or any sounds heard and the bolts were not tensioned yet - just pulled the window against the SikaFlex and left to cure overnight before tightening. One window has a crack from side to side....

    The problem is how to get the windows out again with that Sika cured:?:
  9. mydauphin
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    mydauphin Senior Member

    Getting windows out is easy. I use a serrated wire in between window and frame. Cut it out. You can also use other wire, Also contact Sika they may have softerner.
  10. Wynand N
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    Wynand N Retired Steelboatbuilder

    Ad Hoc, your reply makes sense and perhaps the Lexan was not made pre-stressed.
    We are setting up the test you described and will see how it goes - the Easter weekend will be a help as we would only be able to check the bend test by Tuesday morning. I all is OK we will continue as prescribed.

    However, we are fitting a clear Perspex window - counter sunk with the bolts and primer applied, really tighten up without the Sikaflex and see how that holds up by Tuesday. If OK, we will probably go the thicker Perspex way with a plastic window tint applied - but my client will have to give the OK for that decision.

    Why glue and bolt:?: This boat will not be a marina queen and she will spend her time on the big blue. My client is a marine engineer and like things a bit over engineered just to be on the safe side of the parting line...
  11. waikikin
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    waikikin Senior Member

    Wynand, you can try bolting your windows without the counterunk bolts, the clearance you give the bolt holes is meaningless if the head of the countersunk bolt interfaces the countersink in the plastic, I was taught never to use countersunk bolts/screws with perspex or install them "loose" & as a back up only to the adhesive/sealant, also I never install bolts to the "middle" of the window corner radius but scale the fixings to each side of the pane to even as possible spacings inclusive of the radiused corners. All the best from Jeff.
  12. Landlubber
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    Landlubber Senior Member

    Jeff is very right mate, don't countersink the perspex for the very same reasons you made the holes clearence holes.....asking for trouble
  13. jehardiman
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    jehardiman Senior Member


    Just a question, is there now or recently been any painting or solvent use in the building shed?

    I also concur with not using a countersink if you are going to through bolt it, I have used cup washers in the past. If the curve is slight then you can thermoform it a little to get it to lay better.

    Here is an old thread on what I have done before.

  14. rugludallur
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    rugludallur Rugludallur

  15. Kingo in oz
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    Kingo in oz New Member


    Just another take on your problem - i actually think it is a combo of the countersunk heads, oversize holes, semi-tightening, and the curing of the Sika, but mostly the latter two.

    The Sika will cure at different rates depending on the thickness and the surface area exposed to the atmosphere. By only tightening the bolts until the plastic makes contact with the adhesive and then leaving the assembly to cure, this is allowing the varying forces of the curing Sika to push and pull the plastic as it pleases. So you may have one section of the join curing first and this pulls the plastic sheet in its direction, then as another section cures it pulls the sheet back in that direction. The effect of these forces may be exacerbated by the countersunk heads which could be forcing the sheet up and down (in and out).

    My chemistry knowledge of adhesives, sealants in general, makes me think that it would be better to use a very thin bead of Sika and tighten the bolts fully at the beginning. The Sika will take much longer to cure but i doubt this would be an issue for you.
    It probably would also help if the holes weren't countersunk and you used pan-head bolts or similar. (as others have already mentioned)

    Anyway, good luck with it!
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