Epoxy over XPS method

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

  1. Eric ruttan
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    Eric ruttan Junior Member

    Dejay and Eelco;

    I also looked at 700mpa XPS. I got a sample to Rob Denney who suggested it equilivent in compressive strength to H80.
    The problem is it is a very fine cell, which makes it take less glue per m², (saves weight) but has less skin bond, as the large bubbles in the pvc foam help the glass bond to the foam.

    We were chatting about this on Robs mail list.
    I wanted to share an idea. Imagine your typical 25psi compressive XPS in a vac infusion. Now hot wire the XPS into trapazoid (see pic) and weave biax through the trapazoids to make a corrugated glass core.
    As the corrugations will flow resin, you dont need to grove or hole the foam. As the core glass will bond to the top and bottom layers, you dont care about foam/glass interface.
    The foam is just there to corrugate the glass.

    The corrugated glass should be very stiff in both directions, as in one direction the section is a truss, and in the other it is a triangular beam.

    Your thoughts?
     

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  2. Eelco
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    Eelco Junior Member

    Is the skin bond weaker than the underlying foam itself? Or does it just rip at the interface when peeled off because the stress concentrated there? Not sure this is really a problem. Epoxy does not bond great to flat PS sheet, so indeed microstructure may be important; perhaps a primer coat would be ideal?

    As for you concept, yeah I like it. Ive been thinking about something similar which I think would also result in a quite practical boat-building process overall. That is, incrementally build the boat in longtitudinal sections; glue on another section, then fully glass that section, overlapping the previous one. That should give you glass shear webs throughout your hull, with a spacing of your choosing. You could also glass them one foam block at a time, rather than a section at a time, to get longtitudinal shear webs as well. That will take over a lot of the responsibility of the foam, in terms of transmitting shear from one skin to the other. The foam would just be there as formwork and to prevent indentation, in the limit where these shear webs do all the shearing.

    If you had a nice cnc wire cutting machine and some software skills to turn your hull design into blocks, this could be a very convenient method of hull construction. Or you could cut 2d templates out of some plastic sheet to form the sections of your hull, coarsely glue the XPS in place between these templates to form a section, and then wire cut them to shape in place, using the templates as a non-melting guide.

    One of those things; sounds good on paper to me; but I have not actually seen anyone do it, which may be a bad sign.
     
  3. Eelco
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    Eelco Junior Member

    Note that these boat building foams have a shear modulus in the range of 20MPa. The in plane shear modulus of glass composite is more than 40GPa; or 2000 x as much.

    So if youd have 0.3mm glass shear web every 30cm, the contribution to shear stiffness coupling between the skins from the glass webs would already be 2x that of the foam. Only in the plane of the web of course; it would do very little in the other direction; but very often the requirements for shear strength are anisotropic anyway. So if you are around that 1/1000 foam to glass ratio, the glass for sure cannot be ignored, and it gives some extra redundancy; if the foam delaminates or cracks, that section wont go completely floppy right away, and the crack wont propagate as readily.

    You could carve your blocks at an angle to get these non-orthogonal webs as you picture; in that case it would act as a shear web in-plane, but would also have an effect out of plane. Hard to compare apples to apples to having shear webs in all directions I suppose. And also hard to combine with a 2d template type of approach.
     
    Last edited: May 23, 2020 at 6:27 AM
  4. Eric ruttan
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    Eric ruttan Junior Member

    the PVC foams like H80 are groved ~2mm square to increase the strength of the PVC/glass interface.

    With corugated glass this is not needed, as the glass foam interface is irrelavant to the structures strength. The foam is just a way to make corrugated glass, which is way stronger and way cheaper than foam.

    I was thinking of the technique in the context of a vacuum infusion process. Specifically for a Harry Proa like Dejay linked above.
    Lay the bottom strips in, lay in the corugated cloth, and lay the top strips and continue as normal.
    I would not consider wet layup personally. I have had too may friends develop sensitivity to epoxy exposure doing projects. Robs vac infusion, which he calls 'Intelegent Infusion' is just way smarter in all the ways, and gets excellent ratios.
    These trapazoids fit naturally with 'II' as the corugations flow the resin, so no extra holes carved into the foam to fill with resin.

    I think the glass web give excelent strength in both directions, have excellent inpact resistance, survivability, and repairability. I really posted here to expose the idea to critque. So thank you.
    this is exactly where I went. DIY cnc hot wire cutters are really easy. All the RC guys are using them. But hand cutting with hot wire is also possible. I had thought to infuse strips of glass to each side of a sheet, and cut the corrugations out of the core, so the multitude of pieces would be boumdiin thier relative positions. Making it easier to lay in to the mold/form.

    it might. But i think it is a cheap soluition. And is kind of fiddly. And the domain is boats, so no one wants cheap.

    On paper I think it could save ~75% of the cost and save weight for 12mm+ cores

    But it may be a bad idea. It is certianly way more fiddling.
     
  5. Eric ruttan
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    Eric ruttan Junior Member

    please clarify to me what is, and the desirability of a 2d type of approch.
     
  6. fallguy
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    fallguy Senior Member

    The more I think about it the less appealing it becomes.

    I considered cutting 45 degree channels in it. That would add 40% to the surface area and create infusion channels, but no matter how you slice it (haha), it suffers from compressibility problems.

    sticking with san
     
  7. Eric ruttan
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    Eric ruttan Junior Member

    Can you detail the compressibility problems?
     
  8. Dejay
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    Dejay Senior Newbie

    I think the 3D core infusion XPS works because it creates strong epoxy filled hexagons. And you only need a certain amount of sheer strength, the rest of the strength comes from fiberglass.
    I've been considering building a kind of "cheese grater" for XPS sheets with resistance wires that get heated. So you waffle your XPS sheets from one side but not all the way through, then from the other side in the perpendicular direction. That way you could get pretty cheap foam with infusion flow channels.
    But I haven't found 45kg/m³ XPS foam. Do you have a source?
    I'm mostly interested in that for building rather large cabin walls with something like 800mm insulation. The hull should be build out of more structurally proven materials.
     
  9. Eric ruttan
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    Eric ruttan Junior Member

    i agree the 3d core gets it's properties from resin, which seems expensive and heavy, on top of the foam cost.
    I agree on the sheer strength. Not much is needed. But then things like delamination from impacts and various loads like in plane loading stress the foam glass interface worried me.
    With glass corrugation you have a glass to glass bond, so those worries go away, for me.
    I do not think of the foam densities. But you can get 25psi compressive strength at most any? Building supply. Orange green or blue. I have Foamular 250, which i think is about 40kg/m³
    I can also source 700kpa at the green one as a special order. This is in the US, and pre pandemic, so it might have changed. The foam is for thermal seperation under concrete slabs.
    Did you mean 80mm? 800mm seems excessive. But me too. I was that thinking 2 or 3 inch (50-75mm). I figgure it keeps the heat out or in and the boat dryer. Why not cruise to the cold parts too! And the hull is stiff!
    I cannot see how it is not, at least, better than foam core, structuraly. But I expexct motivated reasoning on my part.
    If I can convince some knowledgeable people that this makes a resonable structure, I'll build a E25 out of it.
     
  10. Dejay
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    Dejay Senior Newbie

    Oh wow, I think this is a really interesting idea! I think even very light cloth would increase the strength dramatically. You'd have to calculate it. But I'd guess it basically becomes very similar to a honeycomb material.

    You could make a very simple jig with a resistance wire going up and down between two metal bars. Then you could simply push or drop your XPS sheet through it. Like feeding through a table saw or planer.

    But it's a lot of work weaving this together.

    And you'd probably get uneven print-through.

    It might also not be cheaper in materials than importing PVC foam from China. Except if you want 8cm foam for insulation :D

    And you could probably also make something like thin 2mm wood veneer work as a SIP.

    I think for build well insulated tiny houses or houseboats this is a very interesting idea. Long term I want to build a 15m / 50ft trimaran with a rather gigantic 12m x 3m cabin on top. I've been wondering how I could vacuum infuse a giant panel and this seems like the best way so far!
     
  11. Dejay
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    Dejay Senior Newbie

    Yes that does sound a bit excessive. There wouldn't be much boat left haha. Ideally I'd like to be able to heat and cool just using solar power.

    I've attached some data sheets and manuals for the 3D core I've saved, unfortunately in German but maybe you can translate with google chrome.
    What is interesting is that they calculate something like 75g/m²/mm for hexagon plus 200g on each side. So for 10mm you add 750g of resin. The PET foam is even more. It adds quite an excessive amount of weight and cost.
    They also say you should only vacuum down to 0.5bar with the XPS. Not sure if that is a problem for infusing large panels.
     

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  12. Eric ruttan
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    Eric ruttan Junior Member

    I also contemplated a hot wire jig, and just pushing or letting the sheets fall through it. And perhaps that is resonable.
    But doing with a cnc hor wire, which are now rather cheap, seems a better approch, as a straight trapazoid id not really ideal. One would want radiused corners, and a curve instad of a flat trapazoid side is significantly better for strength, and minimal weight cost. Also, one can tune the glass density according to the desired properties, by adding or removing angle on the corrugations. Which is easy to change and experiment on a cnc hotwire. Harder on a jig.

    Also, i think it is key for the glass to go from one side clear to the other and get a glass to glass bond.

    I suggest 75g/mm/m² is super heavy. As is the 200g/side resin weight, which is the same as H80/H100.
    At 80mm thickness, you are saving so much weight and cost going corrugated, I don't know how to justify anything else.

    Do you want the g/mm/m³ weight for 80 i calculate for these corrugations?

    Yes it is fiddly. But 8' long planks, with a glass strip on the back, holding them in a relative place and parallel, should make it less fiddly. And with Rob's II, you have all the time to tweak and fiddle and even vac, before the epoxy flows.

    I am worried about the foam compressing and folding the glass. That would reduce the strength of the web. But I think by adding curves to the angled spans, they act as glass tensioners, taking up length as the foam deforms. But I don't know.

    I am gonna do a smallish sample with 2" foam and just a hot wire bow and templates, and see how that looks being destroyed.
     
    Last edited: May 23, 2020 at 3:29 PM
  13. Dejay
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    Dejay Senior Newbie

    Yeah I wondered about that too especially since you shouldn't crimp fiberglass and sharp corners would create larger gaps. You could try to use flat band kanthal wire but it's probably not stable enough, or too big and creates too big gaps.

    Any CNC large enough would work, you'd just need to mount a hot wire and move it laterally and up and down.

    You could also build a kind of "wire wagon" that translates the movement of its wheels into straight up down movement. Could be quite an interesting challenge haha. You'd drive it over the sheet and it plunges the wire below itself. Advantage would be that it pushes down the foam at the same time.

    Would love to see the calculations for 80mm.
     
  14. fallguy
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    fallguy Senior Member

    Gurit M80 compress 148psi
    Xps 250 25psi

    Even if you overcome the shear issues; you still have this issue.

    You can engineer it to say 37 psi, but there is no easy way to get to 148...

    My boat got hit by a golf ball today. We spent 30 minutes looking for the dent.

    Ain't one.
     
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  15. Eric ruttan
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    Eric ruttan Junior Member

    Dejay; you may want to search youtube for cnc hot wire. these RC guys are doing neat stuff, and the kits seem cheap.

    This is all a guess, but if you are talking fiberglass one would use for the hull, to also be corrugation glass, say 400gsm uni (.015"??), then the hot wire is roughly a typical 26 gauge (~.015"), which suggests the stretched hot wire CNC like the RC guys do. I don't think you can get something that thin stiff enough to not be a tensioned hot wire. I cannot see a one side cnc working.
    e.g. Hot Wire 4 Axis CNC Foam cutter https://openbuilds.com/builds/hot-wire-4-axis-cnc-foam-cutter.2361/

    XPS 25 psi is ~25kg/m³
    80mm gives 25kg/m²*0.001m/mm*80mm = 2kg/m²
    400gsm uni glass with 3mm trapazoid tops and 80mm 45 degree down *2 to go back up to where we started gives 163mm liner length, of witch 3/163 is straight, and 160/163 is going up and down. the curves i choose give an arc length of 1.6.
    This gives 400* 3/163 + 400*160/163* 1.6 = ~636gsm glass weight
    Now add a resin weight of 50% if you are infusing.
    and finally a surface wet out weight of whatever XPS needs. I think it will be much less than 200gsm per side that PVC needs, as XPS has a much finer cell, but you do have to count it twice for the slants witch is 160/163% (98%) of the length and 1.6 times as long per linear meter.

    What core would you like to compare against?

    why do you say there is a compression issue? The point is to use XPS to make a uni glass truss, which can be any arbitrary large compression strength or shear strength depending on the truss configuration, right? The XPS will never see any load after the vac infusion. If it does, you done broke something, and all bets are off.
     
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