Material of mould for FRP boat

Discussion in 'Materials' started by naserrishehri, Jun 25, 2024.

  1. naserrishehri
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    naserrishehri Senior Member

    DEAR FRIENDS
    good day
    what material do you suggest to build a mould for manufacturing a 12M FRP boat?
    i used 3MM plywood.
     

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  2. TANSL
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    TANSL Senior Member

    Just an opinion (without scientific basis) : For the internal structure you should go to 15 or 18 mm. The outer surface could be 1.5 or 2 mm to provide it with more flexibility.
     
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  3. naserrishehri
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    naserrishehri Senior Member

    I used 12mm MDF as internal frames which i gave them cut by CNC.
    I m going to reduce cost for next project so i need others experience.
     
  4. TANSL
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    TANSL Senior Member

    12 mm are possible, of course. It would be necessary to see what the interior structure is like and the weight it must support or if it has to be turned over or any other circumstance.
     
  5. Rumars
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    Rumars Senior Member

    It's a very difficult question because it's so intimately tied to the local economic conditions. Often cheaper materials mean more manual work, so it depends how much your workers cost. Everything that I write after this has to be considered from this point of view because I have no ideea how much individual materials or labor costs you. I know for sure that some methods are not economically viable in high labor cost countries.

    1. Real wood. The usual method is to strip plank the frames dry (no adhesive between the strips) then sand fair, apply fairing compound and finish sanding. Sometimes a layer of CSM with polyester is used over the wood before the fairing compound to stabilize the mold surface.
    2. Recycle the existing 3mm plywood. Scrape some of the fairing off, scarf it back together into sheets and reuse.
    3. Put widely spaced battens on the mold, staple hessian cloth (old jute/hemp sacks) over them (stretched tight), plaster with a sand-cement mix. After it's dry apply fairing compound. Or you can use some plate material to plank the battens that's cheaper then plywood, like 3mm HDF (the stuff you see in the back of a closet).
    4. Fill the space between the molds with something cheap and stable (straw bales, old bricks, rammed earth, stabilized sand, etc.) to an approximate shape, plaster over it with clay/straw or sand/cement mix faired to the frames, apply fairing compound. Another variation is to spray expanding foam, sand it back fair to the frames, put a layer of CSM/polyester over it, then final fairing.

    As you see there are many ways to do this. You could make the entire mold out of rammed earth and clay or stabilized sand without any internal frames at all, using only bent rebar as female templates to control the shape. The materials can cost nothig, it's a question of skills and how much the workers time costs.
     
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  6. wet feet
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    wet feet Senior Member

    I would be reluctant to go below 12mm for the outer skin.It will still require longitudinals between the frames and to support the chine.A small but significant point is that the initial post shows what I would take to be a plug.The actual boat would normally be moulded inside a FRP mould taken from the plug.
     
  7. TANSL
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    TANSL Senior Member

    What I interpret as the photo in the initial post corresponds to a "male" mold that has an internal structure that must support the weight of the hull.
     
  8. wet feet
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    wet feet Senior Member

    That could be a correct assumption,perhaps the OP will tell us more.I would still think 12mm would be about as thin as I would go as I would anticipate somebody kneeling or walking on the surface while applying a finish and later some wax.This applies whether it is a plug or a mould.
     
  9. TANSL
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    TANSL Senior Member

  10. wet feet
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    wet feet Senior Member

    2X 6mm or 3X 4mm will work for most of those areas and will mean that the same general skin thickness can be allowed for framing.there may also be some small areas where a piece of solid alder,gelutong,lime or basswood might be appropriate.
     
  11. TANSL
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    TANSL Senior Member

    Sorry, I may not understand you correctly but I see no reason to put down several layers of playwood. In my opinion, it is not feasible, it is very expensive and it is not operational.
    Sorry if I'm interpreting it wrong.
     
  12. wet feet
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    wet feet Senior Member

    Perhaps I should have added more detail.It is much more convenient to plan the structure beneath the skin for a particular thickness and to use multiple skins of thinner ply that add up to that total in those areas of tight curvature.The end result needs to be strong enough for workers to walk on when applying a finish or laminating.A skin which is thin enough for them to destroy by standing there would cause delays while repairs are carried out.One would only use multiple layers in those areas where the generally applied skin cannot conform to the curvature.
     
  13. TANSL
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    TANSL Senior Member

  14. Rumars
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    Rumars Senior Member

    The approach I have seen was always to install more battens on the mold, not to increase the skin thickness. The battens aren't let into the frames, just screwed on top there it's not much additional work installing them. Fairing them takes longer but it's not onerous. The battens are common industry stuff, as used on roofs, 1x2", 30x40mm, 40x60mm depending on boat size. The ply is there just to create a solid surface so you can fair the plug.
    On a male mold if the battens are close enough you can just cover them with a plastic sheet and start laminating. If you have to walk on the mold, use more skrinkwrap and watch where you're going.
     
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  15. TANSL
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    TANSL Senior Member

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