foam over male mold construction

Discussion in 'Fiberglass and Composite Boat Building' started by bruceb, May 31, 2023.

  1. bruceb
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    bruceb Senior Member

    How is the foam attached to the male mold so that it is easily removed when the outside has been glassed? Also, any other quick tips? Any input is appreciated!
    B
     
  2. fallguy
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    fallguy Senior Member

    There needs to be more information about the mould.

    But let's take a simple case of a jon boat hull or dinghy hull you want to glass outside first.

    You can hotglue the panels to the mould.

    You can screw a wooden cleat to thicker foams and then screw or glue that cleat.

    If you have no access to the backside; you can also screw into the core and into the mould and work around the screws or you can use a hardened trim nail. You can also make the moulds collapsible, so a screw can be pulled thru the foam and the hole repaired.

    But I like hotglue best.. super fast.
     
  3. wet feet
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    wet feet Senior Member

    Much depends on the nature of the hull as boats with compound curvature pose more of a challenge then less complex shapes.Hot melt glue can be an aid,but there is always the risk that the foam will let go before the glue when the hull is released and a number of divots will have to be repaired.In the era when IOR hulls were being built as one offs it was quite common to use fishing line to attach the foam to the ribbands and there could easily be an amount of shaping of the foam sections to fit the hull.Edges could be aligned with temporary cleats and microballon paste used to bond the edges.When the outer skin had been laminated and faired it was a quite simple job to get inside the hull and run a knife through all the loops of fishing line.
     
  4. bruceb
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    bruceb Senior Member

    Thanks Fall, I haven't built a mold yet, I am trying to find out what I should know. I plan on building a couple of new "modern" floats for my Dragonfly 800. My current ones were poorly constructed, over weight and not shaped right. I also plan on making my boat fold so I think it is easier and better to start over and build new ones. For my project I need to build two floats from the same male mold, probably using airex foam or possibly corecell and glass and epoxy. I am a fairly experienced builder with wood and fiberglass and epoxy, and I build flat panels with glass or carbon and foam, but I have never tried to do a hull. I have decided it is time I learned how. My current larger tri- a Buc 33 was built over male a mold with foam, glass and polyester and I know they built several over the same molds. It is quite light and has lasted just fine.
    I know I can fasten the foam to a wood strip mold, but what can I use that will make it fairly easy to remove the hull from the mold? The floats will be about 25' long, 14" wide, 24"+ deep and fairly narrow forward so access to the insides is going to be limited.
    Bruce
     
  5. rwatson
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    rwatson Senior Member

    Did you not like

    You can hotglue the panels to the mould.

    You can screw a wooden cleat to thicker foams and then screw or glue that cleat.

    It doesn't get any easier than that.
     
  6. bruceb
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    bruceb Senior Member

    If I hot glue the foam to the mold, does the hot glue release easily enough to remove the mold from in the hull? I need to reuse the mold for the second hull. maybe I am using the wrong kind of hot glue but when I have tried it on foam/wood, it sticks pretty well and pulls a chunk of foam out when I try to remove it. I need the mold to drop free of the inside of the hull.
    B
     
  7. bruceb
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    bruceb Senior Member

    I want to fasten the foam over the mold, fair and glass it, and then remove the mold from inside of the hull. (lift it out the top) I assume I will have to partially compress the mold and lift it out of the top of the hull. Maybe leave the ends of the hull spread some and glass them together later. I will then glass the inside of the hull, install bulkheads and bracing and glass a deck/top on last.
    B
     
  8. fallguy
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    fallguy Senior Member

    All you do is spot the hot glue; then you can knife cut it or use an oscillating tool to cut it.

    unless we have an idea about the moulds, it is hard to present a good answer

    the fishing line idea sounds good, too
     
  9. bruceb
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    bruceb Senior Member

    I have not built any molds so any ideas there would help. I think the fishing line idea would be practical, I don't see how I could get a knife or any tool between the mold and the foam to cut glue.
    Does anyone have suggestions on how much space I can leave between my strips of wood with out getting low/high spots on the finished project? Without any experience I am planing on using 3/8" divinycell, at least for an experimental section. The 3/8 foam should be strong/stiff enough and still easy enough to bend into place for the curves. I am open to suggestions, I have not ordered any materials yet, I plan on getting just enough foam to do a test section.
    B
     
  10. fallguy
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    fallguy Senior Member

    You need an overall plan. You cannot simply put foam on stations and glass the outside and release it all. The thing will misshapen, twist, perhaps hook or rocker, etc.

    Usually, the thing is flipped, moulds in place and set into a cradle for the inside work. Then you remove the mold stations from above.

    It sounds like you plan wood ribbands over the moulds as well. All of your questions depend on whether round hulls or hard chines. For 3/8 divinycel, I'd want ribbands spaced about 6" apart. Tape the entire jig in blue tape and spot hotglue to the tape. Flip the thing into cradle after exterior laminations and then remove the parts from above. If the hull has reverse; the entire process must be carefully planned as you cannot unscrew ribbands from the stations, but you can from station cleats.
     
  11. bruceb
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    bruceb Senior Member

    I understand, I was simplifying things. I have helped pull some smaller cat hulls and kayaks from their molds and i do have a plan to partially compress the mold to remove it. I will have some plywood forms to preserve the form shape. Remember these are small and fairly light amas and as such not too hard to handle. I roll the 450 lbs floats on my 33 by myself with no issues. The separation of the foam/mold is the main question I had. The blue tape seems practical, along with the fishing line. My floats will be very simple with almost straight sides down to a round bottom for most of their length, at the stern they transition to a hard chine near the transom, I am just copying some proven successful ones. I am comfortable with the design, they will work well for my use, I just have to build them. I will use a carved block of foam at the front where the bow reverses so the mold will still be able to be lifted straight up. It also saves trying to glass inside a narrow space at the bow. If it gets too hard to do, I will just make two molds and expect to destroy them to remove them. The main shaping bulkheads will be cnc cut and I will make several sets if necessary. The mold still has to separate from the part though.
    B
     
  12. fallguy
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    fallguy Senior Member

    Well, if you part the stations upside down; you would need to remount them. So, you unscrew crossmember, flip mould, and reapply stations...basically all marked out and use same screwholes to put back together.

    But if ribbands then it would be miserable for sure.

    For reverse curves; you make a parting in the station..
     
  13. wet feet
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    wet feet Senior Member

    Without a sketch of the intended hull shapes it is a bit difficult to provide concrete suggestions.The number and spacing of ribbands is determined by the shape of the hull and the thickness of the foam.More curvature leads to closer spacing and thicker foam permits greater spacing.A bit of experimenting with foam offcuts will be beneficial.The mould may need to be built in sections if you have either a retrousse stern or tumblehome in the sections.The ribbands should stay in place on each section and the sections may need to be bolted together with care taken to place the bolts in order that they may be removed without a fight.
     
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  14. Rumars
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    Rumars Senior Member

    You could also explore other methods like using female stations. This allows you to screw or sew from the outside, instead of having to go into the float. You glass the inside first, and install the bulkheads before releasing.
    Splitting the mold on the centerline is also possible, the seam is on the keel and the deck centerline.
    To avoid having to glass in the tip, the float terminates with a small bulkhead and the tip is a separate piece made from solid foam and fiberglassed only on the outside.

    Ultimately you have to do a test to see what attachment system suits you better, screwing, sewing ot hot glue.
     

  15. wet feet
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    wet feet Senior Member

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