Cheapest way to build a mold

Discussion in 'Boatbuilding' started by mosquitolaGOON, Apr 16, 2008.

  1. mosquitolaGOON
    Joined: Apr 2008
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    mosquitolaGOON Junior Member

    Well i'd like to make a long narrow real shallow running skiff like the Egret and Hells Bay or ECC. It'd be my first go at a full boat, I have some fiberglass experience but nothing as far as building a mold goes. My question is, can i just get styrofoam and carve it and shape it to the shape I want, and the lay mat and resin onto that and pop the foam off and have my hull? After that would i just reinforce it with layers and layers to strengthen it? I could do the same for the top deck as well, it could just be one big piece with walk around gunnells and a side console, maybe something like a pathfinder 15t.
  2. richard165
    Joined: Oct 2005
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    richard165 Junior Member

    Hi there

    This is a cheep way of building a mould and works well if done right.

    After shaping the styrofoam needs to be covered with a layer of masking tape then a layer of plastic packaging tape,make sure to cover all the foam.

    When doing your fibreglass work make sure you use EPOXY resin or you face the chance of your foam being melted by other resins.
  3. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    I found that shaping styrofoam was a pretty innexact science. Its not very solid. It might be better to put a layer of plaster on a roughed out shape to get a precision shape.
    Luckily, plaster sticks to styrofoam really well.
  4. kengrome
    Joined: Jul 2006
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    kengrome Senior Member

    It sounds like you want to build one boat, not a bunch of them, which usually makes a mold a waste of time. It also sounds like you want a lightweight boat, which makes fiberglass far less appealing than plywood.

    Mat is used with polyester or vinylester, not epoxy, and it is very heavy when laid up by hand. These 'ester resins will dissolve the foam you're talking about using, too, so you'll have to cover all the foam first.

    Foam is too soft and flexible for accurate molds in my opinion. You're better off building a more solid mold if you really want a homemade plastic boat.

    You cannot just strengthen the hull by adding layers and layers to it, you actually need structural components in most hull designs.

    Just a few things to think about ...
  5. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Kengrome makes some good points. Making a mould can be time consuming, and normally you wouldnt want to do it. With a project of this size, there may be some benefits.
    You could do a basic mould and then do the fibreglass "layer upon layer", and sand it to shape. The trouble is, you are sanding to shape very expensive and itchy stuff - and as Ken said, heavy.
    I have just done a strip plank canoe, and this may be a method for a lightweight but long lasting build for your type of boat. The mould isnt solid, just enough to give an accurate shape. You can sand the wood to shape before the epoxy and fg.
    I found the most difficult bit sanding the insides. I think it would be worth the effort to put packing tape on the inside of each plank before gluing them together so that on turnover, you can just pull the tape off and not spend hours sanding glue runs. If you dont use epoxy as the plank glue (use an expanding foam glue), a lot of work is avoided.
    It would be like building a strip plank kayak .
    You get the weight saving of using light timber with the long lasting 'plastic' surface.
  6. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    If its a chine boat your after, molding flat panels off melamine sheet or building a mold from similar material will be the cheapest & give you a fairly acceptable contact molded surface. Regards from Jeff.
  7. RealityBoatCo
    Joined: May 2004
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    RealityBoatCo Junior Member

    I use particle board, its cheap fast and easy. You don't have to glass over it, primer will fill everything in and it is very flat to start with.:D

  8. Herman
    Joined: Oct 2004
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    Herman Senior Member

    Is there a picture of a boat like the one you want to build?

    For one-off construction, there are several options, all with their advantages, and disadvantages. I have no idea of the boat, the scope in which it is placed (racing, experimental, cruising?) of your knowledge or possibilities, but here are some options:

    -Build a plug out of polystyrene foam, and use Ce-Sense "EPS Coat" over it. it is a polyester based resin, which does NOT attack EPS foam (polystyrene).
    Either use this model as a plug for a female mould, or use it as a male mould.

    -Carve a negative mould from high density EPS foam (CNC router helps a lot...). Vacuum a flexible film in the mould, and use that as a negative mould.

    -Set up frames, and use thin or thicker sheets of constructional foam (PVC, SAN) to form the core, laminate over that, turn around, and laminate the inside. Round built boats can use bead and cove foam (narrow strips) and chined boats can use sheets.

    -use MDF or any other sheet material to construct a mould (negative or positive) and work from there.
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