managing two part urethane foam

Discussion in 'Materials' started by Anatol, Apr 22, 2017.

  1. Anatol
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    Anatol Senior Member

    Hi
    I'd like to experiment with filling alu tube with two part urethane foam to increase stiffness - for possible spar application. I believe I read somewhere that you can't reliably fill a tube (say 4"x20', for arguments sake) with liquid because it expands too fast and blocks the tube, potentially making voids. Is there a 'slow' two part rigid urethane foam? Or can one add a retarder? Any advice gratefully received.
    thanks!
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Interesting question. I often wonder about the implications of using these foams in situations where the expansion front is limited, as regards build-up of excess pressures especially. I have used the material several times without harm, but not in a situation where the "open" surface was very limited. I assume you would be thinking too of a denser, more robust foam than the 1kg/cu ft commonly employed. The differences in expansion characteristics, for the different densities, other than the final volume being smaller from the same amount of liquid, for the denser material, would need to be factored in. You could experiment with a pvc rain-water pipe I guess, then cut it at intervals to look for voids, (if it did not bust). You might talk to people in the industry who use expansion foam on a daily basis.
     
  3. ondarvr
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    ondarvr Senior Member

    I've done 10' sections of 2.5" tubing, but not 20'. You may be able to do a pour from each end.

    In an aluminum tube the expansion pressure isn't a problem, what you end up with is a denser foam than the 2 lb product you start with. Using anything heavier than the lightest possible foam would make for a very heavy part.

    Slower foams are available, but may be hard to get in small quantities.
     
  4. Anatol
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    Anatol Senior Member

    thankyou for responses.
    I suppose if I stood the tube vertically, and hung out a 3rd floor window - or whatever - I could add the foam in very small batches, and let each batch expand fully before adding the next.
    Some tests are definitely in order, in PVC or whatever, to determine mix quantity to get maximum expansion before the top surface hardened and compressed the material. Probably temp dependent too. I might have to talk to the manufacturers to get some numbers.

    ondarvr - interesting. 2.5" is narrow. did you pour in one go and just let the bubbles do their work?
    Yes I'd thought 2lb or 3 lb. But 2 lb/cu ft, I'm guessing, would add about 2lb for every 10' - by some otoh estimate. So I don't see the weight problem. Mind you, I sail a boat with a spruce mast :) Damn thing weighs 600 lbs. but its pretty.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Can't see going for the lightest foam makes much sense, the overall volume is not great and you want more substance than the 2lbs, it won't add anything noticeable by way of strength or stiffening. Minimum 6lbs imo.
     
  6. Barry
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    Barry Senior Member

    How are you thinking that the foam will help the rigidity of your spar?

    The modulus of elasticity of foam would be much lower than the tubing that it is in so it would be doubtful that there is any load that the foam would carry. IE in order for the tubing to deflect less, then foam must carry some of the load.

    A spar, if it is not stayed, is mainly subject to bending stresses ( of course there are other stresses prevalent)
    Compression on one side and tension on the other side. If the tubing wall is very thin, the tubing cross section, if subjected to bending, will deform. The foam would help this a bit. But hardly enough to provide much rigidity.

    What size of spar were you thinking of building, ie outside diameter and wall thickness?

    It would be much easier to increase the thickness of the spar/tubing to get the rigidity that you need.
     
  7. Anatol
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    Anatol Senior Member

    Barry,
    my analogy - may be faulty - is the way a soda can buckles. It seems to me that foam fill will delay the onset of that buckling. This is intuition, I don't have any numbers.
    "What size of spar ..." I'm kicking around some ideas for a ~24' mast, maybe of 4" dia round tube, I was wondering if I might go with 1/16th wall and foam fill, instead of 1/8 wall.
    I'm just kicking around some ideas about composite material construction. I'm toying with the idea of doing a helical wrap of fiberglass on the outside. I know, however, that some composite combinations are not effective. ie carbon fiber over glass...
     
  8. ondarvr
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    ondarvr Senior Member

    2 lb will end up being much denser inside of a closed chamber, the 2 lb figure is for free expansion at the perfect mix ratio, humidity and temperature. Once you enclose the foam the expansion is reduced a great deal.

    I have no idea if the foam is better or worse for your application than a thicker wall tubing.
     
  9. Barry
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    Barry Senior Member

    A 24 inch aluminum tube 1/16 inch thick weighs 22 pounds
    A 24 inch aluminum tube 1/8th inch thick weighs 44 pounds.

    A 24 foot 1/16 aluminum tube filled with 2 pounds/ft>3 would weigh 26 pounds

    so you weight savings is 44- 26 pounds or 18 pounds.

    If you used 4 pound per cubic foot foam density, then your 24 foot 1/16 tube would weigh 30 pounds and give you a 14 pound weight saving.

    I doubt that you will begin to get close to the strength of the 1/8 thick tube utilizing foam and a 1/16 wall tubing.

    Without knowing the modulus of elasticity, the tensile strength of the foam, it would be difficult to determine this accurately.

    And of course this exercise only deals with theoretical calculations. Some how you would have to install the 1/16 wall tubing in such a way that there is not any stress concentrations on the 1/16 inch wall. Ie how would you fasten it. Again, I am assuming an unstayed mast. And why would you want that?
     
  10. Anatol
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    Anatol Senior Member

    Barry
    >A 24 foot 1/16 aluminum tube filled with 2 pounds/ft>3 would weigh 26 pounds

    well that's a significant weight saving- thanks for the numbers :)

    > I doubt that you will begin to get close to the strength of the 1/8 thick tube utilizing foam and a 1/16 wall tubing.

    I guess that's the question, isn't it? While I am willing to respect your educated guess, (absolutely no offense meant), I assume a controlled test would be the way to find out. I don't know that I'm in the materials testing lab business. Maybe others will chime in with relevant responses.

    But, in principle, isn't this the idea of any foam and skin composite sheet material?

    >Some how you would have to install the 1/16 wall tubing in such a way that there is not any stress concentrations on the 1/16 inch wall.

    unless of course you had some local reinforcement at the stress points. I try to avoid point loads.

    >Again, I am assuming an unstayed mast. And why would you want that?

    right. That's a whole 'nother conversation. You might want to fly a Wharram pocket wingsail...
    People seem to like unstayed rigs for drag reduction reasons afaik. Some say they create a less stressed hull. Me, I like the separation of compression and tension. Call me old fashioned. :)
     
  11. Anatol
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    Anatol Senior Member

    ondarvr, interesting. I was not aware of this. So in a tight space, 2lb foam may become 4 or 6 lb?

    What was you motivation for doing the 10', 2.5" items? What was your experience?
     
  12. ondarvr
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    ondarvr Senior Member

    Yes, 2 lb foam will increase in density a great deal, the exact amount is hard to determine before hand. In cooler temps it won't expand as quickly or as much, so the density will be even higher.

    I was making oars with thin wall tubing, the foam added a bit of strength, plus they wouldn't sink, it also made the oars feel different. I had no problem filling a 10 foot length, I already had the foam sitting around. This type of boat uses rather long oars.
     
  13. upchurchmr
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    upchurchmr Senior Member

    The foam will add almost nothing in bending strength.
    This is not the same as a foam sandwich hull, because the sides of the tube are aluminum (compared to the bend direction). The aluminum is so much stiffer than the foam that the bending stresses will never show up in the foam, it will just flex.

    A sandwich hulled boat does not have anything like the aluminum sides taking up the shear loading. All the shear stress is carried by the foam, so it has to do something.

    Filling a pipe to increase stiffness can work, but you need something close to the stiffness of the pipe. One application is filling an iron pipe with concrete, for posts at the edge of a parking area. That works great, but the weight increase might not be acceptable.

    If you were considering changing from 1/16 to 1/8 thick wall you must not really know what the needs of your boat are.
     
  14. Barry
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    Barry Senior Member


    Imbedded notes above

    The other issue to consider if you decide to do the tests is that initially, the foam might adhere to the inside of the tubing. But over time with straining/flexing and aging, this joint may erode or separate. So initially, whatever minimal strength that the foam might add, could be lost and your test figures impacted over time.
     

  15. Anatol
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    Anatol Senior Member

    upchurchmr
    thankyou for explanations - very helpful.

    lol!

    And you're right, I do not know what my loads will be. I'm still designing the sails, an the whole thing is fairly left field.
     
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