Mold support

Discussion in 'Fiberglass and Composite Boat Building' started by mrdebian, May 5, 2022.

  1. mrdebian
    Joined: Apr 2021
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    mrdebian Junior Member

    Hi all,

    I'm in the process of building a new mold for a sea kayak (around 5.5 meters length and 56cm wide) and I would really appreciate your thoughts on the following.
    I need to make the mold as light as possible but obviously to be strong enough.
    I've already made the plug and polished to A level with about 6cm flange around it.
    The produced kayak will have 4 layers of cloth in the hull and 3 in the deck (a combination of mat with wooven). I''m talking about hand laminate.
    My questions are:
    1. How many layers of chopped strant mat will be enough to be strong but light also? Would 3x300gr and 4 x 450gr mat be enough for the hull and more or less the same for deck?
    2. I've read in an older post from @rxcomposite that "Support the mold by egg crating with the use of steel tubings, plywood, or cardboard tubes cut in half and laminated.". What's your thought on this? I've seen a couple of pictures on Internet from well known kayak manufacturers that use nothing to support the mold; just the mold itself with a flange.
     
  2. wet feet
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    wet feet Senior Member

    Don't go too light on the mould layup or you will regret it.It helps enormously to add stiffening features but the strong curvature is a good starting point.I haven't gone quite as small as a kayak,but would imagine that a split tube or foam stringer perhaps 30mm square along the keel (with rounded corners) and a light glass covering would maintain the designed rocker and a couple of external frames at around a third and two thirds of the length would both help the transverse shape and stop the mould falling over while laminating is taking place.You really don't want to be holding the mould level with lengths of broom handle jammed under the flange as you work as they can be dislodged by feet and then you face having the mould lurch over and maybe tip over a cup of resin.
     
  3. ondarvr
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    ondarvr Senior Member

    Kayaks typically have a shape that doesn't need much more support, but it does need to be thick enough to withstand some abuse.

    On thinner molds the support structure can print through if it isn't done correctly, so making it thicker can be safer.

    10mm is probably the low end of thickness if you want it to last very long.
     
  4. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Why? This is not a prudent objective.
    Strong enough for what? Don't you really need it to be stiff enough?
    Polyester? Not epoxy?
    There is no place for chopped strand mat on a quality kayak.
    I believe the objective is stiffness without unnecessary weight (and cost).
    Just a couple? There is no end to any type of pictures one can find on the Internet. I don't see your point.
     
  5. mrdebian
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    mrdebian Junior Member

    @BlueBell by saying strong enough I mean stiff (sorry I'm not a native English speaker).
    I need to be light as many times it will have to be lifted from a single person.
    I'm talking about mat for the mold construction; the kayak that I will make from the mold it has only a single layer of mat and all the rest wooven to avoid print through.
     
  6. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Thank you for the clarification.
    Sorry for the confusion.
     
  7. wet feet
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    wet feet Senior Member

    I offer no comment regarding the hull layup because I don't have specific knowledge of kayaks.Moulds are something I do know a bit about and I would never in a million years imagine a 10mm thick mould for a kayak.Nobody has to walk around in the mould and it has quite a bit of curvature.The few kayak moulds I have seen were no more than 6mm thick on the flanges and may have been less elsewhere.I would advocate double gelling the plug to give enough thickness for future compounding to restore the finish if more than a few dozen boats are likely to emerge.Where external framing is required,there is no obligation to glass the framing to the mould surface and I have used a band of coremat in the zone with a light covering of glass and then bonded the frame in place with Sikaflex-thus eliminating the risk of the glass contracting and printing through.I have known a few people who developed their own kayak and the one job they all disliked was bonding the hull to the deck,particularly in the ends and the next most disliked job was making the gel on the outside of the join look tidy.
     
  8. ondarvr
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    ondarvr Senior Member

    I help companies build tooling every day, while a well cared for and pampered mold can be built thin, they survive much longer being slightly thicker. Customers come to us every week and ask how to resurrect old heavily used and abused molds, and some with almost brand new molds that are cracked and badly damaged after the first few parts.

    The easiest and lowest cost way to lengthen the useful lifespan of a mold is to built it strong enough to exceed the stresses it may experience in normal use. Flexing and impacts are the primarily forms of damage.

    If you plan to increase the gel coat thickness so it can be sanded and buffed more times, it means you need to build it even stiffer to prevent it from flexing. If it flexes the thick gel coat can easily crack, creating costly repairs. The flange needs to be large and stiff, a support structure, even just a glassed over 2×4 helps tremendously. This will stabilize the entire mold and help eliminate flexing.

    Molds that are carried and moved around by hand (light kayak molds) tend to get banged up and dropped more often.

    If the mold is used by the owner/builder, it can be made thinner because it will be handled with greater care. Hire a couple of inexperienced helpers and it can be destroyed quickly with wedges and hammers if it's not built strong enough.

    A core can help prevent damage from external sources, plus increase stiffness, but can add complexity and time to the build depending on the exact size and shape of the mold. Cores tend to print through if the laminate under it isn't thick enough.

    One of my first real jobs in the composites industry was building kayaks, I still have one of the kayaks from 50+ years ago.
     
    Last edited: May 6, 2022
  9. ondarvr
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    ondarvr Senior Member

  10. mrdebian
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    mrdebian Junior Member

    @ondarvr you forgot to paste the link to the article.

    Many thanks for the valuable information.

    A normal deck of the kayak that I pull from the mold gets about 2-2.5kg of gelcoat. In the mold what I will spray about 5-6kg while every 2kg will be different color in order to help me knowing how much I sand in future problems/repairs etc.

    How do you calculate relatively accurate how many layers let's say of mat (300-450gr) you need to use to achieve a thickness of let's say 6 or 10mm?

    Thanks
     
  11. ondarvr
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    ondarvr Senior Member

    The link should be there, I can see it.
     
  12. wet feet
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    wet feet Senior Member

  13. ondarvr
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    ondarvr Senior Member

    There is no right or wrong way to build a mold, it all depends on your needs and expectations. We supply primers that are compatible with EPS foam for one-off molds or plugs, they aren't durable, but it's the right method for the desired result.

    When building a mold it's much more time and cost effective to build it slightly stronger than needed, compared to underbuilt and in need of repairs frequently.

    If you've dialed in the strength required for molds seeing a certain amount of use and life expectancy, then your methods are fine. Building a production mold for the first time means there's no experience to draw from, so an extra layer or two of CSM is the insurance policy.
     
  14. Kayakmarathon
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    Kayakmarathon Senior Member

    I built a fiberglass kayak of very similar dimensions and purpose about 25 years ago. It weighed about 15 Kg. I did not use mat or woven roving. I used 4 oz ( 125 g / m^2) cloth for the outer layer. The outer layer was a single piece that ran the entire length of the mold. The inner layers were 9 oz (280 g/m2) TWILL cloth. Next to the 4 oz cloth I place the twill cloth diagonally to the center of the mold. Each diagonal section overlapped the previous one by 4 inches (10 cm) . The third layer is a single piece of twill that runs the entire length of the hull. The fourth layer is twill placed diagonally, but its overlaps should be 90 degrees to the first diagonal layer. The fifth layer is a single piece if twill starting 25% from the bow and ending 75% from the bow.

    The deck will have the same number of layers and weave orientation; however, use only 4 oz cloth. Don't forget to build walls along the center of the boat to support the deck, even if you plan to build bulkheads. Each center wall should be 80 to 100 cm long
     

  15. mrdebian
    Joined: Apr 2021
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    mrdebian Junior Member

    @Kayakmarathon is not clear to me what you mean by saying 'build walls along the center of the boat to support the deck'

    Do you mean to add extra tape on the hull or to the deck?
     
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