Fiberglass mold thickness for kayak plug

Discussion in 'Fiberglass and Composite Boat Building' started by Canunut, Dec 7, 2016.

  1. Canunut
    Joined: Dec 2016
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    Location: Mt Holly, NC

    Canunut New Member

    I am making a carbon replica of my 18 foot cedar strip kayak, using the kayak for the plug. Presently I am refinishing the hull and deck to excellent condition to be the plug. This boat has provided 13 years of fitness and racing fun, and competes successfully with the latest skinny 18 foot kayaks. It fits me, and provides the stability I need but is becoming too heavy for me at 58 pounds compared to the latest 21 pound carbon kayaks at $4,000.

    The plan is in 2 parts; lots of waxing and mold release will be used:
    Deck: Lay the carbon, foam core, and carbon on the deck and vacuum bag it there. Bagging boundary will be below the hatch and along the deck to hull line so the hull nor deck is crushed by the pressure differential.

    Hull: Make a mold from the hull plug using fiberglass 2 ounce cloth on the inner layer, additional glass layers, and some leftover glass matting until it is thick enough. Vacuum bag it to the hull. remove when hardened, and then for the new boat, lay up the carbon, foam core, and carbon inside the mold and vacuum bag it. I don't want distortion of the hull mold by being too light or twisting during vacuum bagging with the final carbon-foam carbon hull

    The question: How thick should the fiberglass hull mold be? It will be used once or twice maximum. The boat is 18 feet long and 23 inches wide. I plan to use horizontal spacers to ensure the hull mold retains the right dimensions at the deck/hull joint. I'm guessing one layer of smooth 2 oz glass cloth, 2 layers of 8-10 ounce glass cloth, and use up my stash of glass matting. Epoxy resins and vacuum bagged.

    Thanks, all info appreciated. Steve.
     
    Last edited: Dec 7, 2016
  2. ondarvr
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    ondarvr Senior Member

    Most plugs are made from polyester, it's cheaper, easier to work with, and they're not vacuum bagged, bagging can distort the kayak and/or plug. If you have a bunch of epoxy it will work though.

    Thickness depends on the shape, most of the time a frame is used to keep the plug or mold straight. If you bag the mold the plug will need to be stronger to hold its shape. for one off plug shapes 3/16 will work with a good frame, thicker if not good.

    You need a gel coat type coating on the plug and mold, there are products that will work with both polyetser and epoxy.
     
  3. waikikin
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    waikikin Senior Member

    Hi Canunut,
    I'd think you'd do well to fit a flange to your kayak at the sheer/greatest beam, this will give some longti stiffness to set the shape once built into the hull & deck molds, with some care you can add some false work so a shoe box fit can be achieved at bow and stern- these areas can be hard to tape when joining- the rest can usually be reached via cockpit & deck spinners. Some alignment nodes are handy to be molded into the matching flanges.
    Jeff.
     
  4. trekker
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    trekker Junior Member

    If building a plug, can the Duratec products be the final layer before the tooling gel is sprayed ? Thanks !
     
  5. fallguy
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    fallguy Senior Member

    Neat project.

    Is the carbon look desired?

    What is the final finish?

    I would want 50 minimum to 75 ounces glass for the mould.

    You definitely need to mould a flange. Put the yak in a cradle so it can't move. Then, if possible, build a flat spot at the place on the hull where it is wide and has no reverse curves. For example, say it is 4,5" above the cradle. Put a 2x4 all around the hull screwed to the cradle and then top it with 1x4 pieces that touch the hull. Fill gaps with duct seal.

    Then wax it all and pva it all.

    Use 50-100 ounces of cloth. 1708 is good and glass the plug. Then stiffen the mould before release. Install some boards deeper than the mould and more and more boards until the mould is stiff enough to not flex.

    An alternative would be to use the male kayak as the mould. Much, much cheaper and much faster, but it would be a bit heavier because you can't really bag it.

    If a kayak 18' long is about 5 yards of glass, a 6 ounce cloth is say 2 pounds. The savings to vac bag are like one pound.

    You ought to experiment with using a shop vac for lower pull and just the yak as a male mould. It might work really well.

    By the way, ondarvr suggested 3/16 for the mould thickness and he is the resident expert. 3/16 is about 100 ounces of glass or 4 layers of 1708.
     
  6. Blueknarr
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    Blueknarr Senior Member

    It would result in a slightly larger cayac. Why not skip the female mould and build directly from existing cayac as a male plug?
     
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  7. ondarvr
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    ondarvr Senior Member

    Yes, there are several in the Duratec product line that could be used.
     
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  8. rxcomposite
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    rxcomposite Senior Member

    In general, the mold should be slightly thicker than the product, say 1.25 to 1.5%.

    Second, it must be stiff. Support the mold by egg crating with the use of steel tubings, plywood, or cardboard tubes cut in half and laminated.

    Low cost mold can be fabricated with gel coat first layer, lots of CSM and regular (semi flex) polyester. Just make sure it cured properly before removal. For higher stability and lower shrinkage, use tooling resin which is a little bit more expensive but has a higher tolerance for heat. This makes the mold tolerant of repetitive heat during production. WR is also added on the last layer to make it stable.
     
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  9. fallguy
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    fallguy Senior Member

    Okay, I am a bit confused. How is an ultrathin mould going to work with vacuum and not distort? The thickness of the mould must have a minimum starting point. No? A layer of carbon is pretty thin..
     
  10. rxcomposite
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    rxcomposite Senior Member

    I said MOLD with lots of CSM and poly, not carbon. The part is to be vacuum bagged? The mold has to be 1/4" thick with flanges and firmly supported.
    Do not confuse mold and part to be made. Hand layup, the mold just need only slightly thicker than the part. To be used for vacuum bagging, the mold has to be robust. Start with 1/4" thickness with support every 2'.
    Making mold out of Carbon is reserved only for aerospace application and autoclaved.
     
  11. ondarvr
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    ondarvr Senior Member

    This thread was started three years ago.
     
  12. fallguy
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    fallguy Senior Member

    Thanks. I sure wish we would all get a notice of an old thread. Once one person updates it; the thread pops as a 'recent' post. And trekker hit it and then I didn't check it.

    Thanks RX; as always.
     
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  13. rxcomposite
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    rxcomposite Senior Member

    You also replied yesterday.:)
     
  14. ondarvr
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    ondarvr Senior Member

    To the person asking the new question, not the original one.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Is this a new thread? From what I can see the topic is still about mold.
     
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