I'm not understanding this Skin-foam-Skin roto process

Discussion in 'Boat Design' started by Squidly-Diddly, Oct 1, 2011.

  1. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    http://www.hogislandboatworks.com/roto-molded-HDPE-drift-boats.html

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    Roto-molding our boats involves a two part process that uses two types of high density linear polyethylene resin. We first load our boat mold with the polyethylene resin that makes our boat’s outer surface.

    Our hull is formed all at once in this two part mold
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    We call this our skin. We then load our mold into an oven nicknamed the Terminator, and form our hull skin by rotating the mold while heating the oven to roughly 500 degrees. Once the outer surface has formed we stop the rotation of our mold, and drop a second resin into the mold. This second resin is then heated and rotated like the first resin, and in turn forms an extremely rigid closed cell foam that chemically bonds to the first skin. The combo material is referred to by thermo-plastic engineers as skin foam and skin foam skin. At Hog Island we call it SSB: Seamless Smooth Butter.

    DID THEY LEAVE OUT THE PART WHERE THEY ADD MORE OF THE 1ST "skin" RESIN INTO THE MOLD SO IT BONDS TO THE JUST CREATED "FOAM" LAYER?

    i SEE HOW THAT WOULD WORK FOR A HOLLOW BOAT LIKE A SOT KAYAK, OR EVEN A REGULAR KAYAK SINCE THE COCKPIT OPENING IS SMALL AND THE BOAT IS MOSTLY HOLLOW. BUT HOW IS THIS WORKING TO MAKE A DRIFT STYLE BOAT????



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    The result is a drift boat building material that it is awesome. Our material has so many things going for it on the water, in the river, and at your house. It is very tough and very quiet and naturally buoyant.

    I CAN'T HELP BUT NOTICE THEY SAY BUOYANT IS GOOD, SOMEONE ELSE SAID DRIFT BOATS ARE SUPPOSED TO SINK SO THEY STAY PUT, WAS HE JOKING???

    Our hull easily slides over rocks, gravels bars, logs, and sand bars. In addition the color of our hull doesn’t scratch, it is easy to patch, and it is very easy to clean.
     
  2. Mild Bill
    Joined: Nov 2007
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    Mild Bill Well, not entirely mild.

    Let me take a guess.

    Even though the polyethylene resin is high density and high viscosity, when it's heated up to 500 deg to promote the curing the viscosity goes down, so they have to rotate the mold to keep the resin from flowing down due to gravity.

    The mold is actually in two parts. When they say "first load our boat mold with the polyethylene resin" the singular "mold" actually means both parts of the mold - one side forming the outer surface that makes contact with the water and the other side forming the inner surface that makes up the floor/interior. When the mold pieces are brought together, the skin edges are bonded together so it all becomes one surface with an air gap in between the inner and outer skins.

    After the skins have cured to a certain extent, the high density foam is injected into the space in between the skins, then the whole shebang is kept heated and rotating to prevent the skins and/or the foam filler from flowing due to gravity.

    As for the desirability of sinking rather than merely anchoring a drift boat - ¿Quién sabe?
     
  3. FAST FRED
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    FAST FRED Senior Member

    I have been to Maine and seen Old Town canoes made with this system.

    Works great ,

    but the best part of the tour was when the tour director , who happened to be a financial guy, stated the produce hit the container with UNDER 3 man hours of labor , Total!
     
  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    So Fred, is is like Bill said, where the skin is topographically

    speaking, made as a one piece hollow(like a SOT where the deck is 3/4" from the hull) and then foam in injected into that?
     
  5. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I think the rotation has a washing machine effect. The cure is managed by the heat transfering from the outside inwards in a predictable manner, producing an even thickness, sorta.
     
  6. Juansanjuan
    Joined: Oct 2011
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    Juansanjuan New Member

    Hello,
    I am from Hog Island Boats. I saw this thread, and am here to help anyone better understand how our Hog Island boats are made. Mild Bill has it right for the most part. We mold our skin, and then drop a second resin into our mold, and form the foam core that bonds to our skin. PhilSweet is correct as well in that it is like a washing maching. The rotation is to distrubute resins. Good stuff.
     
  7. Frog4
    Joined: Oct 2011
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    Frog4 Proletariat

    same process used to make foam cored plastic tables ... should see the giant (2 story) robotic molds we've built ... spins like a Multi-axis trainer, with multi-part mold in the center and then heated with a huge microwave coil ...
     
  8. spiritgide
    Joined: Oct 2011
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    Location: Kanasa

    spiritgide Junior Member

    Rotational casting

    Roto molding is used for many products, more recently expanded to include boat production. Great method for making hollow items. Resins are generally granular, not liquid. The temp melts them to a flowable stage inside the closed mold, the rotation distributes the fluid plastic over all areas of the mold inside. Rotation is multiple axis, it tumbles so that every surface is evenly exposed to the plastic pool. When the heat cycle is shut off and cooling turned on, the rotation continues keeping the plastic distributed until it's hardened. For a foam core, a liquid urethane mix can then be added or injected that will self-expand and fill all the available space. This provides support and stiffness as well as hull perforation protection, there's nowhere for water to go. Can't sink it. How tough the boat is and how heavy it is depends on plastic thickness and formula as well as foam density.
     

  9. Frog4
    Joined: Oct 2011
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    Frog4 Proletariat

    to add to your great explanation: fire-retardants are also added to the mix ... we designed a mold system for a skyrise builder, needed light, fire-resistant ducts, we passed the UL testing and received approval ... ducts just melted, no fire or hazardous fumes ...
     
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