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Thread: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

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    HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    I noticed that one of the two mounting "wings" on my Mk 15.5 center section broke yesterday on a shallow boat trip. I am not sure exactly how it could have happened, but there was obviously some stress that contributed.

    It would be great if I could just buy a replacement Mk 15.5/ Mk 16 center section, but those are as common as rocking horse sh#t even if I could afford one. Alas, I am no longer single with ample discretionary funds, so it is either try to fix it or live with it. I can do the latter, I have dived my rig with the center section unmounted before (besides the diluent & oxygen plumbing keeping it stable) and obviously hard mounting to the case is not critical to life support. However, I would be interested in any advice from composite or adhesive experts on a good mending approach. Have attached some photos to help you get an idea of the situation.

    Notice the break happened under the metal bushings/helicoils. Two photos are with the broken-off mount bracket wiggled back into place. I was thinking of damming it up with some high strength epoxy. However, thinking that the shear force on the joint would most likely defeat any epoxy mend unless somehow the bond interface was reinforced. Like some fiber or metal pins or...?

    Thanks in advance for your advice.
    Last edited by teksimple; 30th December 2012 at 19:32.

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    Some research on my own revealed that epoxy would be worthless for repairing the ABS plastic of the molded center section. I need a specific ABS repair solvent. So am thinking of thisSo am thinking of this

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    Modifications are performed on the biopak center sections by a couple different methods that could be relevant. Calling Sandy...

    If it were me I would probably grind off the damaged part and let the other fasteners and ports keep the center section in place. There is the finite possibility that you fix it and get a more serious chip/crack later. Then I could sell you my 15.5 for a million billion dollars...

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    I believe from what I read in the 15.5 manual and the biopak manual that the materials are identical. I've had good success with a few repairs.

    I had a center section that was dropped on its top and broke a 2 inch section of the lip that is on top of the lid o-ring off. I built up the area to rough shape by hot melt plastic welding it and then machined it back to factory shape. It worked well in my opinion and I never had any stress cracks or failure.

    For you area that would be more difficult. You need to have the lugs back in the mix and fixed to the body of the unit. Epoxy will not give you a chemical bond but its mechanical bond will be pretty good. I'm with Cys on grinding most of the area off to give you the most surface area (rough it up as well so your epoxy will key into the ABS) Then I would build a tidy mold on the area to dam the epoxy up and pour a mix of good epoxy (not the stuff in tubes from a hardware store - I like the West Systems epoxy) with some black pigment and short strand chopped carbon fiber and let it set. I have had good results with epoxy on the center sections when its prepped just right and done properly.

    That being said you should decide what your skills are like with the materials. Under pressure as it all starts to go exothermic and the timer is counting down the whole thing can turn into a bad hot mess.

    You'll spend 10 hours to get a 6% better result than if you just left it and let the thing hang by all the other points.

    I'm a sucker for the 6% and would do it but that's also my kryptonite. I spend more time fiddling than diving.

    I do have some scraps of the ABS if you were keen and wanted some material to experiment on that was not part of you unobtainable center section.

    Cheers,
    Sandy

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    Kent,

    Here's what I would recommend - you can buy the 6-32 Threaded Inserts from McMaster-Carr, and then drill two new holes beneath the ones that broke off. Then, place the Threaded Inserts on the tip of a soldering iron, and once they are hot, gently insert them. Use a pair of small needle-nose pliers to hold the Insert in the holes while you withdraw the soldering iron.

    One thing I do with these inserts, is actually take a small chip of the ABS, and once the Insert is hot enough, melt a bit of the ABS on the knurled end of the Insert to fill-in any gaps or irregularities - sorta like a "glue".

    From what I remember, your Center Section is of the older variety - the newer ones have that "wing" hollowed out on the bottom, so this fix might not be appropriate for those.

    Then - you can re-bend the sheet-metal bracket (or even make a new one) to match the new hole placement.

    ABS does really well being melted to itself, from my experience. I've fixed a ton of Mark 15 O2 Sensor plates this way in the past.

    Hope this helps!

    Kevin.

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    Quote Originally Posted by teksimple  View Original Post
    I am not sure exactly how it could have happened, but there was obviously some stress that contributed.
    I know how it happened. You are a monkey with your stuff! :)

    It looks like your center section is done for. I'll trade with you for $12k.

    See you next year!

    Charlie

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    Hi Kent-

    Man, I can't even imagine how P****ed you were to find this break!

    99 percent chance you already know this, but: Should you choose to go the bonding route, your first try will have the best chance of success. The freshly BROKEN rough surface is plenty rough enough, provided it is truly **CLEAN**.

    My inclination would be to take Sandy up on his offer for bits of BP 60 center section material and do some bond tests with whichever adhesive system you settle on. My one adjustment to his advice would be to not bother w/ the carbon as your thixotropic (thickening) agent. Milled or chopped fiberglass fibers are readily available at places like West Marine and should do just as well.

    If you decide to re-bond the inserts during the repair, I'd put some stainless screws into the inserts' threads w/ a bit of release agent on the screws' threads. Epoxy and similar adhesives are VERY sneaky stuff and will otherwise find their way into the insert threads and cause significant grief no matter how careful you are. Be absolutely certain none of the release agent finds its way onto the proposed bonding surface- either the center section or the outside surface of the inserts- either directly or secondarily via your fingers!

    Good luck and be sure to report your outcome!

    Ken

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    This is great. Thanks for all the feedback, with the exception of chunters According to the info from Plasti-Mend (supported by the reviews on Amazon), I am thinking there is really no reason why I cannot use the the PM-Black to make my center section (all but aesthetically) better than new.

    Has anyone else done solvent-based repair on ABS? Any specific reasons why this isn't the way to go? Seems like way less work than heat melding. Quote:


    Plasti-Mend Black is a specially formulated mix of plastic resins and solvents designed to repair ABS plastic used for RV holding tanks, Motorcycle and Auto parts and other uses. It can be used to repair holes of any size, cracks and even modify or build special tanks or parts. It works by chemically melting an application of plastic resins into the existing plastic, creating a new layer of ABS plastic bonded into the original plastic. P- M Black works only on ABS plastic or other solvent reactive plastic, such as is normally used for most RV holding tanks and sewer plumbing fittings. It will not work on polyethylene plastic. This is usually the milky-white plastic used for water tanks or the hmilky-grey plastic used for some holding tanks. These require thermal welding to properly repair. P- M Black is a permanent repair for ABS holding tanks and other items since it bonds into the plastic, making it stronger than the original. Sometimes ABS plastic cement for gluing plumbing fittings is used but it does not contain the proper solvent and resin mixture to properly bond for an effective repair. There are other repair adhesives for temporary repair but none permanently bond into the plastic for an effective repair like Plasti-Mend.

    To Re-Attach Mounting Tabs etc.
    Apply resin to break area and position broken tab. Using screwdriver, smooth resin around break area and apply extra resin if necessary. Use duct tape to hold tab in place until resin dries.
    Or;
    Apply resin to a small piece of mesh and the break area. Position the broken tab and apply the mesh for support. The tab can be covered with mesh for extra support as long as it does not interfere with mounting. Brush on extra resin taking care to keep resin from interfering with mounting. For best results, allow to cure for at least 24 hours. Grind/file off excess resin that interferes with mounting.
    But (why is there always a but?)...these guys don't ship to Hawaii or Alaska! I Googled a bunch of comparable products, and they don't ship to Hawaii or Alaska, either. OK, I reviewed the MSDS's, and am thinking the latter is the problem. Those flammable organic solvents probably can't be sent legally via air. UPS "Ground" to Hawaii and Alaska is actually space-available air cargo. That means I need to have it shipped surface cargo since I don't want to wait the 4-6 weeks for USPS parcel post even if they would accept such material. Will have to have a friend/relative drop it off at a freight terminal on the West Coast, if I can't find something like this in stock locally. Then its 5-6 days even if I plan the container ship departure schedule perfectly. Grrrh. What a hassle.

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    My suspicion is the MEK and/ or the Acetone. Certain hazardous chemicals are restricted by the feds from any air shipment . Some that are somewhat less hazardous or are in smaller quantities are classed "cargo aircraft only" meaning they cannot ship as space available cargo on an airplane w/ passengers. You are correct that USPS does not allow shipment of above solvents.

    The description does indeed look promising but I'd again urge you to do a couple of test bonds on appropriate scrap first to avoid rude surprises. Also, keep in mind that the repair is a *process*, meaning that surface prep and material application technique are every bit as important as choosing the correct adhesive.

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    Re: HELP from composite/adhesive experts to fix broken Mk 15.5 center section mount

    It sort of looks like originally the block was welded to the side of the canister - If it was mine, I would look at having some new blocks fabricated and either grinding away the existing broken block then gluing the new fabricated block on top of where the old block was.... Doing it manually, take a mold of a block that is not damaged.
    Last edited by DwayneJ; 2nd January 2013 at 20:11.

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