Mold failure help

Discussion in 'Fiberglass and Composite Boat Building' started by Scuff, Dec 5, 2020.

  1. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    I will prepare some illustration for you.
     
  2. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    Thank you Rx.

    I called the gelcoat mfg and had them review my procedure. Their saying I should be applying the first layer of reinforcement within 15m of last layer of gelcoat. I waited 45m this last time. Could this be the issue? I thought you wanted to wait about an hour to make sure the solvents had flashed off. Their instructions indicated between 1 and 5 hours was acceptable.
     
  3. ondarvr
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    ondarvr Senior Member

    No, 15 m is an unlikely short period of time, 45 would be more like it.

    Although the longer it sits after its ready for laminating, the more likely it is to pre-release.

    You probably didn't talk with the manufacturer, who did you buy it from?
     
  4. rxcomposite
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    rxcomposite Senior Member

    I think you are laying your gelcoat too thin. A thin gel will cause the volatiles to flash off easily because of relatively large surface area to thickness. For molds we use medium to heavy coat about 700-850 gr/m2. If thin, we bump up the MEKP as the rapid evaporation cause the volatiles to flash off and the gel coat takes longer to dry or not cure properly.

    "Tacky dry" means if you touch the gelcoat, the color will not transfer to your fingertip. It is not cured yet and will enable chemical bonding. Too cured and and the solvent in the resin will soften it causing lifting. Premature lifting of gelcoat can also be caused by too slippery surface or overwaxed. Stick a piece of masking tape on the surface and if it falls off, it is too slippery or the wax you are using is too glossy.

    Try also to reinforce your flange during laminating so it does not move. Brace it with popsicle stick and hot glue.
     
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  5. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    It is possible I've over waxed the plug. I'm using pva over the wax. I'll increase the amount of gelcoat applied per coat and just do two heavy coats instead of 3.

    Ondarvr, I got this gelcoat from fibreglast. I'm going to pick up some from eastern Burlap tomorrow as I used what I had.
     
  6. Chris Rogers
    Joined: Apr 2020
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    Location: Boston, MA

    Chris Rogers Junior Member

    I too am confused about what is happening here - but it doesn't look terrible from the pictures. Were you using something thin and floppy to mold the flange? This would be a great place for a big sheet of something thick and stiff (3/4" melamine, etc) with a "hole" cut to the outline of the board and fit tight with wax or clay to form a very beefy flange mold that won't stray from the actual middle of the trailing edge.

    Are you using two plies of 3/4oz mat as a skincoat? One will be fine - two may put too much pressure on the gelcoat as it cures - and worse if you have laps. Be careful of gelcoat thickness and maybe butt the mat edge from the flange into the corner on the skincoat so it doesn't have to go up and over that tiny detail - or use putty like other people have suggested to support the corner.

    That is a hard thing to tool into a mold like this so you're not alone in struggling to get trailing edge features right!


    Here's an unfinished, excessively long and poorly edited article from my side-project site - it may be helpful about the mold-building process:

    Building Open-Molded Production Tooling | Explore Composites! https://explorecomposites.com/articles/tooling/building-open-molded-production-tooling/


    And when it comes to building the board itself, if you tell us some details of the engineering and layup, I'm sure people here can help.
     
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  7. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    Chris, thank you for the link that was a good read. The flange is 4" wide polyethylene sheet glued to 3/4" mdf. There are end plates on the foil .. those are glued to the table. The flange is butted to the plug and glued. I applied clay to the flange/plug seam. There is no movement in the flange that I can find. I'm considering not using the pva .. any thoughts on this? I'm also going to buff the plug again to be sure no excess wax. Thank you all for helping me out. This site is a wealth of information!
     
  8. rxcomposite
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    rxcomposite Senior Member

    Attached my method of making molds and parts for airfoil shaped parts. The illustration is self explanatory but here is some tips that may be new to you.

    1. When trial assembling the parts after the bulkhead are attached, Line the other half with masking tape and packing tape so that the fill up bog will not stick to the upper part. The bog is to ensure a proper mating and fit and provide a wider area for strength. The masking tape gives thickness which should approximate the minimum thickness of the glue recommended.
    2. Do not cut the flanges all the way. Leave some tabs to grip on so you can align perfectly the edges.
    3. The joggle or recess is to allow you to finally seal and reinforce the mated parts. Without this you will have a build up of laminate and your profile will not be true to intended shape.
    4. Note that in airfoil shape, the trailing edge is not razor thin but rather with 3mm of edge. The leading edge is radiused.
     

    Attached Files:

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  9. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    Thank you very much. The illustrations make it easy to understand the procedure.
     

  10. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    Success. I used Advaco tooling resin this time. It sprayed much better than the other. It laid down nice. I did re-buff the mold and apply pva. Thanks to all!
     

    Attached Files:

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