Direct female molding

Discussion in 'Fiberglass and Composite Boat Building' started by groper, Feb 3, 2012.

  1. groper
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    groper Senior Member

    I have started getting things ready to build my 35ft powercat, the first frames are being CNC cut as i type, and these are for building the direct female mold of the hull "shoes". I designed a symetrical hull below the waterline so that i can pull both shoes from the same mold, after this the mold will be dismantled.

    The plan is;
    to build a frame and batten mold from the CNC cut transverse frames on a strongback and pine timber battens longitudinally between frames. Once the battens are in, im going to transversely strip formica/melamine laminate (0.8mm) onto the battens and glue it down with contact cement. I was thinking of sealing the melamine joins with bog or silicone so that it may be used for an infusion layup, both sides in 1 shot. Before the layup begins, i was thinking of spraying the surface with PVA just to make sure i dont have any release problems.

    Before i begin, i just wanted to ask anyone here thats done something similar if there is any tips or points worth thinking about, material choices or anything else that may cause problems... If there is any online resources showing this process, can you please link me to it.

    Cheers!
     
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  2. Herman
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    Herman Senior Member

    If you consider infusion, make sure you test the mould integrity before laying up anything. I would close the seams with a MS Polymer probably, for its good adhesion, and flexibility.
     
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  3. gonzo
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    gonzo Senior Member

    That may be a hard way to fair a mold. The battens will create compound curves and the formica won't. The formica between the battens will probably buckle and create hard spots. That is not a good way to make a mold. Another thing you need to consider is the weight of the laminate which the mold must hold without distorting. Also, the laminate is exothermic when it cures, and the temperature will induce even more distortion on the fomica.
     
  4. waikikin
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    waikikin Senior Member

    Groper, you might like to line your mold with various appropriate width strips of "polyester board" you can get it from "Mister Ply & Wood" about 3mm thick with satin or gloss coating in white, apparently for infusion the mold must be air tight, so if you lay a skin coat first that should take care of that. If you want to use the mold twice, consider being able to split it on the center line to assist release with the hull shoe hanging from over head gantry or whatever. Definitely use wax & PVA. Jeff. also check out Jon Sayers Wasabi construction pics to see nicely executed temporary tooling http://www.sayerdesign.com/
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    If there are compound curves in this thing, it sounds like a nightmare job. And if something lets go, all the resin gets squeezed through the breach ? Nervous breakdown material ! But then again, Groper probably has more patience and application to the task than me !
     
  6. groper
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    groper Senior Member

    Hmmm.... all good points...

    Yes i based the method off the "wasabi" build i saw on sayers website... although i planned it slightly different due to the shape of this hull compared to his...

    I wanted to form the compound curvature by using strips of "material" over the battens... i dont know exactly which material im going to use just yet, but it has to be flexible enough to allow a bend radius of about 100mm if i run the strips transversely... failing that, it may just be simpler to longditudinally plank across the frames with 12-16mm MDF ripped into strips... which would be the same as the "wasabi" build...
    [​IMG]
     
  7. waikikin
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    waikikin Senior Member

    Groper, 100mm radius............. thats pretty tight man, longtitudinal sounds like the go, if you use mdf seal the back 'cos it moves/grows/shrinks with moisture - like paper.
     
  8. groper
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    groper Senior Member

    What im gonna try first, is to stand up the frames and batten it out with pine. Then my plan is to bond 0.8mm melamine laminate across these battens in wide transverse strips which will take the tight radius - ive already checked this. I might try to bed this thin laminate to the battens using gyprock stud adhesive or cornice cement etc which will fill the gaps underneath the laminate and fillet/bed itself to the pine battens underneath. Once its all cured it should be a pretty solid mold, and ill just run some silicone or other flexible sealant along the edges where the melamine sheets join.

    Both of these adhesives are very cheap, buy 20kg buckets of the stuff from bunnings for under $50. The idea is to use cheap materials to get things fair, rather than expensive materials on the hull not to mention the hard yakka fairing the hull...
     
  9. AndrewK
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    AndrewK Senior Member

    The mold in the picture is made from 16mm MDF, pine battens are dearer and not as straight. It will take more effort to fair a pine mold than MDF one. MDF is very consistent material.
    Also you can adjust the width of your MDF planks easily and use wideones in the flat areas.
     
  10. tunnels

    tunnels Previous Member

    Something no one takes any notice of is the effects that humidity has on wood . it affects the final shape of the timber you are using and the longer before the build is done the more things can go out of shape ! If its frames or planking what if its from wood or wood product it could buckle and go out of shape .
    We always coat both sides of wood with old resin just laid out on the floor and roll on with a paint roller .:D
     
  11. gonzo
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    gonzo Senior Member

    Stabilizing the mold is always a good idea.
     
  12. tunnels

    tunnels Previous Member

    Its also a time when you re-find all those filled screw holes as the screw and the filler reapear as if by magic and after all that filling ,painting ,sanding and polishing and just takes one wet day at 100% humidity and oooh dear !!:confused:
    :p:D:p:)
     
  13. groper
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    groper Senior Member

    indeed, after much consideration, ive abandoned the direct female mold for infusion for the hull shoes- not because it cant be done, but because by the time ive added up all the materials and time to build this, including all the setup time for the infusion and getting the core thermoformed and fitting properly - i would have already built the hull shoes and faired them the old fashioned way :)

    it was really dependant on the tight radius i need in the bilge and contour cut foam wont do it...and im not game to try corecork either :) i still plan on using the direct female molding for the deck however... save this for later in the build...
     
  14. tunnels

    tunnels Previous Member

    it was really dependant on the tight radius i need in the bilge and contour cut foam wont do it...

    Its possible to make your own sheets of foam so they will curve the radius ! Whats the thickness of the foam and the density !
    Theres no heat needed just a little foresight and understanding !!Interested ??:confused:
    Its really easy to do once you get started !!:)
     

  15. groper
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    groper Senior Member

    YES... i needed 15mm 80-100kg/m3 PVC foam to bend around a 100mm radius... what tricks do you have up your sleeve tunnels?
     
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