Mold failure help

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

  1. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    I completed the first half of a mold for my daggerboard. On the trailing edge the gelcoat and the resulting layup lifted in two areas rendering the mold unusable. I'm going to reassemble the flange for a repeat attempt. I need to understand what I did wrong so that I can mitigate it this time around. I laid 2 layers of .75/oz matt against the gelcoat. I'm using iso poly. I didn't feel these two areas when I laid those two in so I'm thinking it may have been when those initial layers were applied and cured? Pics attached. The good news is the plug easily released and the gelcoat surface was smooth.
     

    Attached Files:

  2. fallguy
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    fallguy Senior Member

    Kind of a confusing post.

    The gelcoat on the mould failed?

    But the release was fine?

    How was the release fine if the mold failed?
     
  3. Scuff
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    Scuff Senior Member

    The gelcoat lifted away along the trailing edge. Hard to see in the pictures.
     
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  4. wet feet
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    wet feet Senior Member

    Does the phrase "gelcoat repair" carry any weight?
     
  5. Scuff
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    Scuff Senior Member

    I'm an amateur at this. The whole layup is warped away from the plug in two places. I don't see a way to repair it. Any insight is appreciated. Thanks.
     
  6. fallguy
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    fallguy Senior Member

    Ondarvr is the man on this stuff.

    See if you can message him.
     
  7. ondarvr
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    ondarvr Senior Member

    I can't tell much from the pics, they were too blurry to see much detail.

    Looks like air behind the gel coat, at least from what I could see.
     
    Last edited: Dec 5, 2020
  8. Scuff
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    Scuff Senior Member

    I believe that's happened. I feel like I would have noticed it when I put the first layers of glass down. I guess it came away from the flange and unfortunately some of the plug and I didn't catch it.
     
  9. rxcomposite
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    rxcomposite Senior Member

    It is called bridging (air pockets). Whenever you have a sharp step or inside angle, fill it up with bog (paste mixture of resin and cab o sill) after the gelcoat, before you do the laminate. Glass refuses to bend into sharp corner no matter how much you saturate it with resin,
     
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  10. gonzo
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    gonzo Senior Member

    Is it warped, as in it pulled away from the mold? In that case you can simply fill the low spots.
     
  11. Scuff
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    Scuff Senior Member

    Gonzo the gelcoat appears to have lifted up away from the plug in two places. Could you recommend an approach to patching it? Rx it lifted on the trailing edge where the transition to the flange is almost flat. Thank you for the suggestion to fill the sharp edges with filler I'll definitely do that. I found a small amount of play in the flange where it meets the trailing edge right where the problem occured. I think working the resin into the matt may have moved it enough to cause the gelcoat to come up?
     
  12. rxcomposite
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    rxcomposite Senior Member

    We did a lot of molds of airfoil shaped parts with flanges as I was in the aerospace industry using vacuum bagged, prepreg process.

    You can fill up the imperfect edge with resin + cab o sill then sand. Can no longer apply gel coat so it will be ugly.

    To get the perfect fit, the finished mold is waxed, the plug is placed back in, and the second half is laminated in. Like an oyster, you split it open.

    Laminate when the gelcoat is tacky dry. Do not let it go full cure as you won't have proper adhesion with the gel coat and fiberglass.
     
  13. ondarvr
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    ondarvr Senior Member

    If you say it lifted, like pre-release, then it can be due to an uneven gel coat thickness.

    Thick gel coat will shrink more than the surrounding gel coat and pull loose from the mold.

    This may be working in combination with the sharp radius features and pre-releasing.
     
  14. Scuff
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    Scuff Senior Member

    I was planning on making two female molds around the plug. I hadn't worked out how to actually produce the foil. Ideally it would be a single lamination. Any insight on approaches to do that. Thank you.
     

  15. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    I tried again this morning with a similar result. I found the flange on that side had a little movement when pushing down and corrected it. I thought that was the problem but appears not. The trailing edge transition to the flange is nearly flat where the issue occurs. The leading edge where there is a sharp transition is fine. My gelcoat application is a 350 gram mist coat and then 3 500 gram coats all at 2% 15-20 minutes apart. I'm using a g100 gun with a fn8 4.7mm nozzle. I checked thickness with my caliper and I get a consistent .030". I waited 45m and then started to fill the sharp edges with filler and it had lifted. Shop temp 72f
     
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