Epoxy over XPS method

Discussion in 'Boatbuilding' started by mvoltin, Nov 16, 2018.

  1. fallguy
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    fallguy Senior Member

    I would not bother setting up a wire cutting method to cut 2' wide for two sheets of xps for a dinghy.

    My point is run over by you. I CANNOT buy 1/2" foamular 1000! They don't make it and I don't want to fab it!

    I am building an ultralight; no reason to add 10 pounds of core at 1".
     
  2. Eric ruttan
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    Eric ruttan Senior Member

    What is 10€ each?
    I'll back up. What is your glass and epoxy cost per kg?
    The whole point of corrugated glass is to replace expensive structural foam with higher performing, cheaper, lighter core.
     
  3. fallguy
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    fallguy Senior Member

    My epoxy cost is about $100 per gallon. Or about
    what are you aiming for with the rhetoricals here??
     
  4. Dejay
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    Dejay Senior Newbie

    Approximately what you quoted in Euro, I just used rough numbers. 13€ for 1kg epoxy. Fiberglass I haven't priced yet.

    So are you saying that if you'd use 600gsm on airex or corecell and replace that with corrugations you wouldn't need to bulk up the outer laminates to get more impact strength? It would seem logical that this would be needed to some extend. Somehow with the "non existing" backing foam you need to bulk up the surface to gain some local strength and stiffness back.

    For insulated truck bodies you use something like 1.5mm laminate on XPS or about 3kg/m².

    So again very roughly you'd have roughly 1kg of laminate extra from corrugations, plus additional laminate for both surfaces to increase impact strength. That might add up to 2-4kg extra per m² in weight and cost.

    To bulk up the surface impact strength maybe you could also use lightweight fibers. Lightweight polyester or natural fibers at about 1.1g/cm³. Or aramid at 1.4 g/cm³. It's lighter than fiberglass with 2.5g/cm³. Or really thin wood veneer. But if its so thin that it becomes porous it's probably back to a density of natural fibers at 1.1g/cm³.
     
  5. Rumars
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    Rumars Senior Member

    Fiberglass is not very stiff, so the corrugation has to be pretty stout. Basicly the same thing as making top hat stringers, the sides have to resist buckling. 400gsm UD is not going to make it. You need biaxial in the sides, and you need a lot. Infusion only makes the thing worse since stiffness comes from thickness and infusion reduces it. The governing thing here is stiffness, not strength. I repeat, make a panel and test it against the laminate you want to replace. If you want to build a Harryproa your panel must be able to replace whatever laminate and core the designer specified.
     
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  6. Eric ruttan
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    Eric ruttan Senior Member

    It is replacing a foam core, right? So how stiff is the foam it is replacing compared to the corrugated glass?
    How do we get an idea how much to replace the foam core?
    How does infusion reduce the thickness?
    Yes, agreed. and I'll repeat. Which and How would you test said properties?
    Ya, Ya, but is not this statement a copout? Foam panels are well understood, and rather simple. Foam so only available is a few thicknesses. same with glass. The known properties of any given panel are well known. It should be rather simple to do a small number of simple tests to show relative performance between two panels, right?
     
  7. Eric ruttan
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    Eric ruttan Senior Member

    I think you misunderstand.
    foamular 1000 only comes in 2' widths. and 8' long. You cannot get it wider or thinner.
    So when you said you wanted to buy 50$ boat building sheets you meant that they also must be exactly the right thickness. If you do not want to cut 1 1/2" sheets into 1/2" sheets, fine. Now I know. I did not know that before.
    what is the kg/m^3 of your 1/2" ultralight core?
     
    Last edited: May 27, 2020
  8. Eric ruttan
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    Eric ruttan Senior Member

    this post really confused me.
    Epoxy over XPS method https://www.boatdesign.net/threads/epoxy-over-xps-method.61414/page-9#post-876912
    let me try and straighten it out . hope I don't screw up too bad.
    this question I asked to Dejay. but also, thank you for answering.
    this is me asking rxcomposite.

    To which fallguy replied
    Which is an excellent answer. So yes, thickness can compensate for low shear, but a natural limit is exterior skin damage.


    Let me ask a follow up before hurrying onward.

    Why would water impact damage the exterior skin in a low shear thick core?

    Continuing with trying to clarify the post that confused me.
    hey now! you credit me with rhetorical actions when they could be rooted in ignorance or stupidity.
    I guess I should thank you. Thank you sir!

    But the question was a serious one. Is it not possible to have a low shear core that also has a good skin bond?
    Continuing with trying to clarify the post that confused me.
    Much better answer than the question.
    So, we can have a low spec core, and compensate with a thicker section. but we worry about core skin attachment.

    As an aside. fallguy did the glass need to be 'thinish'? If so, why? If not, ignore this.

    now the sprint to the finish.
    Agreed. on both the XPS and the chaos of chop.
    Flatterer. Stop flirting. :D:D:D
    They were not rhetorical. They were aimed at getting rxcomposite to engage and teach me a thing. I missed and hit you instead. I am not gonna complain.

    So, looking at the Boston Whaler hull, does it not seem that a very light XPS former making a sparse glass corrugated core that has low shear and low compressive strength, but excellent skin adhesion at the glass webs, that can be cheaply made thicker and still be light, might make a decent 'high value' (cheap) core?
     
    Last edited: May 27, 2020
  9. Eric ruttan
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    Eric ruttan Senior Member

    If you used honeycomb would you need to boost impact strength? is not every cell an unsupported span of glass?
    I am saying that it seems one can increase thickness with corrugated glass, using an XPS former, and reduce cost and get the performance of a thinner, more expensive, heavier foam, cheaper. And your hulls will be stiffer.
    what is the super highest end cores? Aramid or Aluminum honeycomb? They don't have a backer, right?
    But lets say you are right. we need to increase skin weight. Glass is 5$ a KG.
    Need is a funny word. 1.5mm skin is for toughness. Because trucks are driven by people who drive trucks. They get beat up and smashed all the time. If that's how you sail, add another layer.
    If that's how you drive your boat, sure, but you don't need it both sides. read about the Boston whaler above.
    If one drives a boat like a truckdriver. But, remember, this core is how much lighter than the core and resin of airex or corecell?
    I don't know of any design that does this. But make yourself happy.
     
  10. fallguy
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    fallguy Senior Member

    No. Because all the work and product involved in developing the core will not result in a superior product. The product you describe would be heavier than competitors, no? Why would I spend time and effort building or buying such a product for a heavier result?

    Have you calculated the weight? And isn't shear still possible, aka delamination, and you've only mitigated it? Or are you corrugating very small to the point it is more of a glass hull? Obviously the glass in a Whaler hull is 'thinnish' to reduce some weight.

    I may be out of my league on the science of what you have planned, but as a buyer of foams, so far 1/2" foamular 250 is of zero interest as a hull material.

    Hope you don't find me disrespectful. Not at all the intent.
     
  11. Dejay
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    Dejay Senior Newbie

    Yes and I assume they either use honeycomb for extreme applications like racing where damage is either acceptable or not expected. Besides the span for honeycomb is 10m or so not 160mm.

    In a cruising boat you have to expect everything and still be safe. Truck drivers don't drown when their truck box is holed. Or loose their house and all worldly possessions.
    I'm not trying to shoot holes in your idea. Ok ok, I am trying to shoot holes :D
    But that's how you test a hypothesis.

    So the relevant material property for localized impact damage would be compressive strength right? Shear strength is done by the webbing.

    XPS Jackodur 700 has a density of 38kg/m³ and a compressive strength of 0.7 MPa compared to 0.8-1.5 MPa for Airex.
    But Bulk modulus is only 10 MPa compared to 50-100 MPa for Airex. So higher density XPS would compress more easily and is more elastic in compression but wouldn't break much faster?

    Is that right? Doesn't sound so bad?

    I think what Rumars is saying here is that vacuum infusion leads to higher fiber / resin ratio so it is thinner and less stiff compared to hand laminate with higher resin / fiber ratio. So hand laminate can be (very slightly?) stiffer. Corrugated plastic typically is thicker and less dense material and closer spaced.

    So 400gsm would result in a webbing that is about 0.2mm thick and wouldn't be very stiff in compression. Of course the trapezoid form gives it strength but you'd need to run the numbers and test to see if it's enough for the loads.
     
    Last edited: May 28, 2020
  12. Dejay
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    Dejay Senior Newbie

    Some more thoughts about that sewing machine idea, stitching girders into the foam.
    I think thicker sticks forming a girder would be stiffer than thinner webbing. And could be less weight too.

    And maybe you don't need such a complicated sewing machine, just sort of nails that you shoot into the XPS foam from one side. Premade tubes with some fibers fringing out of the ends to connect to the laminate when infusing. Then you could just punch them in from one side. This would also act as a infusion channel.

    Or just have a hollow needle with fiber roving inside that you punch through the XPS sheet. Then blow pressurized air through the needle while letting out roving and retract the needle to keep the roving in place inside the sheet. Then cut the tow outside of the sheet.

    That shouldn't be too hard to build. Building a CNC is relatively straightforward and there is the example of the 5axismaker that has tilting and rotating tool head. All you'd need is to turn on and off pressurized air and a cutting tool for the roving. And maybe that needle should be heated to cut into the XPS sheet.

    With something like 50k carbon roving it would be plenty of thick and stiff. Not sure how much money and weight that costs. Attached is a sketch of how I imagine this, not sure how you would actually lay out those girders. Probably more like tetrahedrons instead of pyramids.

    XPSGirderSheet v2_2.jpg
     
  13. Rumars
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    Rumars Senior Member

    Correct, that is what I was saying.
    As to your sewing machine idea, there are knitting machines that can make what you want. So a good way of doing it is to knit the reinforcement and then pour the foam onto it. Of course this is not for the amateur anymore.
     
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  14. Dejay
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    Dejay Senior Newbie

    Right, expanding PU foam could be used as well. Or insulation spray foam. From what numbers I've found it has less compressive strength but higher bulk modulus than XPS at the same density. So it's stiffer but crumbles more easily.
    But that would close the channels to vacuum infusion so you'd rely on the polyurethane foam as a matrix for the internal struts.
     

  15. Rumars
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    Rumars Senior Member

    Foam is stiff in all directions, corrugations only in one. That's why hexagonal or triangular patterns are used in honeycomb panels. The foams own stiffness can be ignored and you can use only the stiffness gained by the separation of skins. I would calculate the panels corrugations as a series of top hat stringers.
    As to how and what to test for I already said it: strenght, stiffness, shear, slaming loads, point loads, whatever else you like. I would simply send my samples to a materials testing lab. Cobbling together testing equipement is possible, they are usually hydraulic or electric powered machines with recording equipment.

    I do believe you will get some interesting results using corrugated panels. For example high longitudinal or transverse stiffness (depending on how you arrange the corrugations) and thick skins for the other direction. Weight will go up significantly compared to a "normal" foam cored panel. Cost is relative.
     
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