Epoxy over XPS method

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

  1. groper
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    groper Senior Member

    Who said anything about building a hull with it???
    This is what has been happening all these years when people talk about using XPS in a yacht- they immediatly think about a hull... Well NO- i wouldnt use it in a hull either! Its not suitable for this application!
    Now- open you minds a little and think about how many other panels there are in a boat - besides the outer hull shell, and how they are loaded :)
     
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  2. fallguy
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    fallguy Senior Member

    Right. My bunks are made with 12mm core, but an xps version 2" thick might be stiffer....the tradeoff is tuff to be fair. The bunk is fairly narrow say 36". So you'd add weight, lower cost, dent the product during the build easily, but have a stiffer bunk at the end...

    To be fair Groper, the questions the OP asked were ambiguous. He might have meant hull.
     
  3. groper
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    groper Senior Member

    Well i also would not limit it to very simple things like that either- i reckon - at first glance- i could do alot more with it- actually pretty much everything save the outer hull shell!

    Im talking transverse bulkheads, floors, roof\ceiling and decks etc.

    Some applications will require a composite coring and or higher density outer core but there are alot of innovative solutions which have been borne from other industries which make all of this possible.

    Next time i build a yacht- its going to open some eyes let me tell you :)
     
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  4. fallguy
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    fallguy Senior Member


    Just a follow up question.

    Let's say, for semantics, you want a 3" core. And you build it from a 2" xps core and two 12mm cores of 5# marine foam.

    Do you think there is any chance the xps could shear away in the stack like from a heavy point load or someone jumping onto the deck?

    I still need to do the abrasion pull test I promised, but had my hip replaced Feb 4.
     
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  5. groper
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    groper Senior Member

    i wouldnt go that way... id use the full thickness of XPS and then use thin ply of say 4mm either side of the core. So for semantics - 67mm XPS and 8mm ply gives you 75mm total core thickness.
    If you try to sandwich foam between foam you use up alot of resin in the glue line as the surface of the foam is very porous and so it gets heavy/expensive quickly... the resin glue line will be stronger than the foam itself - so no i dont think it will shear along the glue line. H80 PVC foam soaks up approx 300 grams / square meter of resin in the surface porosity of the foam...

    But the proper answer in engineering terms can be found by doing the calculations which ive shown here before. Rather than rehash all that, all you need to know and learn is contained in the pdf document here Technology Manuals | Hexcel https://www.hexcel.com/Resources/Technology-Manuals
    Read and understand the one titled "honeycomb sandwich panel design technology" - nevermind the honeycomb - the theory is the same for all sandwich cores and if you input the formulae into excel spreadsheet you can calculate things very quickly and easily.

    If your not into that kind of theory and prefer a seat of the pants practical answer - they make truck bodies, caravans, trailers and large span roof panels in buildings to name but a few from XPS cored panels. It can do the job in many applications. What you need to understand is that it will be fine to use in boats- or in any application - provided you dont exceed the design limitations of the material. The main issue with XPS and boats is that its "soft" - so it doesnt handle impact loads without a suitably tough laminate protecting it. This is also why you cant use it to directly core a large hull and also why you cant even use h80 PVC foam on the bottom of a high speed planing hull - wave slamming impact loads start to crush the core and then it starts to delaminate and loose all structural integrity of the sandwich. With proper PVC foam like divinycell or airex etc- boat builders and designers increase the foam density on bottom panels of planing hulls to 120kg/m3 or even higher for larger /faster boats. XPS typically around 40kg/m3 is much softer than H80 obviously- so any hull shell on large boats is basically impossible with XPS or it dents/crushes too easy not just with wave loads, but also fenders and docking in marinas, getting blown against a wharf in a storm, intoxicated tender drivers bashing against you (dont ask), general rough and tumble of boating :)

    Back to XPS On a roof span panel for example - people walk and jump on these - however the laminate is thin corrugated steel so its tough and people wont put a dent in it easily. in terms of global strength of the same roof panels - its plenty enough to span huge distances like +8 meters and support considerable storm wind pressure loads and of course live loads - ie - the "softness" of the XPS doesnt impose overwhelming limits on the sandwich structure for this type of loading - the main property your interested in now is shear strength and the thicker the panel is - the less shear strength you need from your core material for the same design load. If you play around with the spreadsheet as i said eariler - all these things become obvious as it shows up in the numbers.

    All this boils down to why i would use plywood either side of the XPS - it will glue to the XPS with less resin/glue, and provide a tough/hard outer layer to protect it from crushing or impact type damage. Plywood is also cheap and reasonably light if you keep it thin at 3-4mm... the plywood also provides a great substrate that bonds very well to the subsequent fiberglass layers to encapsulate it and retain the tension/compression skins around it. I have built a large table using this method - it was extremely tough and very stiff - so much so, that it sparked my interest in other things i could use it for next time i build a boat :)
    said table shown below :)
     

    Attached Files:

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  6. fallguy
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    fallguy Senior Member

    How did you edge finish the table? Is that ply glued to the foam on the edge? Sure would be tricky on the radius-maybe shrink wrap?

    Are the seams painted in?

    May I copy for my table?

    Did you do the same in the galley countertops?

    And thank you for explaining plywood. I am assuming you would glue the panels to the foam by hand and avoid the crushing problems of vac, or use vac with some means to support the panel edges? I bagged 2" foam unde 9" hg and it mashed quite a bit. I may not have had enough bag there either.

    Thanks for all the time posting. Is that your wife; she is lovely.
     
  7. groper
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    groper Senior Member

    i decored the foam edge with a router then back filled with epoxy/qcells mix, then painted it. im sure there could be other more effieint ways of sealing the edge tho...

    The teak plank timber "look" was created by masking off the plywood and painting the black lines with black paint before clear coating the whole thing. cloak and dagger stuff :)

    Yes the galley counter tops were done in the same way. Sure you can copy what ever you like on the internet :) For the galley counter tops i wrapped them in plastic masking tape and set them in position - then i back filled the edge to create a bull nose from solid epoxy and qcells mix. The counter top was removed, then the bullnose finished and faired into teh cabinets below - then painted - then the countertop glued back in position. THis way the plywood edge was not visible...

    I simply glued it by hand using some weights to press it together - no vacuum - after all were only bonding the cores here- no real benefit in tons of pressure... I make up an epoxy glue mix using silica and qcell and apply it with a notched adhesive trowel so there is lots of even thickness lines of glue all over - then press together. Takes all of 5 mins to do. If you try it im sure you will be impressed with the stiffness of the finished product. I think i used 25mm XPS foam between 4mm ply on this table btw.

    Yes- the lady is my wife and she is the best thing that ever happened to me :)
     
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  8. rxcomposite
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    rxcomposite Senior Member

    Core is good as long as its shear properties are not exceeded. As a general rule, increase thickness to reduce shear requirement. This will also reduce stress on the outer skin to a point where it is very thin and easily punctured. Covering an EPS with packing tape will enhance its overall properties but easily punctured. Increasing skin thickness will only add weight. Covering it with high modulus carbon fiber will make the surface tough and spread the load more evenly but it is counter intuitive. Expensive skin on cheap core? Not a good design. There is a limit on core design.
     
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  9. rxcomposite
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    rxcomposite Senior Member

    Good design practice. Low modulus skin on low shear property core. Cheap + cheap.
     
  10. groper
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    groper Senior Member

    Hi RX - long time no see :) ive been sailing around the world for a while so not too much bd.net from me in the last couple years... funny you just popped up - i just emailed a boat yard in the philipines about renting a shed to build another cat in Cebu :)

    What are your thoughts on the idea of using thick XPS foam core between plywood skins for a low cost sandwich panel? - Obviously not interested in using this for an outer hull shell, but i think there could be many other places it could be used such as floor panels, saloon/coach roof panels, bulkheads, many things actually... id appreciate your opinion on it?
     
  11. rxcomposite
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    rxcomposite Senior Member

    Hi Groper. I do pop up once in a while.

    Cebu is a good place. Lots of boat/shipbuilders there and skilled labor. Cost of living is cheaper than Metro Manila.

    I commented on XPS above. I built one a long time ago. I wanted a stiff bookshelves without too many division. I glued thin plywood over a cheap foam core that we were not using. Came out fine and stiff as I wanted. We also built 1/2" interior panels using honeycomb core for boats. Not exacty the cheapest built but we can groove and bend it so it is good for one off design. Very light but sound passes through. More of for privacy divisions. Most of our tables are decorative plywood with foam core and routed wood edging. Varnished, it looked awesome. When it is a door, it is foam cored with kiln dried wood for hard points and pre molded FRP skins. We bond it with microballoons/epoxy and seal the edges with a strip of laminate. We bond it while the part is still in the mold for a clean and distortion free finish.

    Most interior cabinets, shelves be wood panels and foam core finished with plywood edging. Edging can also be molded PVC which comes in 1/2" and 3/4" width. Countertops, we lined it with Corfam for its good looks and lightweight.

    Thin roof panels would also be nice as it is more stable. We used these fancy PVC sheet with embossed designs but it was not dimensionally stable at high heat. It would sag and distort but not the FRP cored variety. Bed base, Hmm... I would use honeycomb cores, the paper kind. Not good in shear but good in compression.
     
    Last edited: Mar 3, 2019
  12. Ad Hoc
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    Ad Hoc Naval Architect

    My old stomping ground :D
    Spent many years there - as we had one of our shipyards based there.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Yep. One of the best city in the Philippines and the people are friendly and trusting. Don't like Metro Manila where I grew up. Too hot + traffic + pollution + expensive.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Sadly Cebu has become a little Manila now since i first went there in the mid 90s.
    It is no longer as pleasant as it once was, and has these +++ as you note.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Yes, Cebu has grown a little like MM. But you would not believe what MM has grown to. I would not want to drive in Makati (and most of MM) anytime.
     
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