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General: what is the pressure generated by pu squirty foam

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    I am about to fix a new internal window sill with pu foam. What is the pressure generated by pu squirty foam during expanding and curing?

    Real question I suppose - how much weight should I put on top to stop it lifting? (in weight unit per square something, I don't mind imperial or metric!)
  2.  
    Enough to bend 12mm plasterboard with 400 centers!! Better to let it expand first then fit.
    • CommentAuthorCWatters
    • CommentTimeJan 17th 2017
     
    +1

    Enough to push plasterboard off studwork making the wall look pregnant. Don't ask how I know.
  4.  
    Difficult to let it expand first as the foam is supposed to hold the sill in place. There are some quite big gaps to fill (stone wall) so would it be better to fill the gaps first and then fit the sill and foam in a thin (1" - 2") fixing squirt
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      CommentAuthorSteamyTea
    • CommentTimeJan 17th 2017
     
    Generally, PU foams expand up and get warm at the same time. As they cool, they contract a bit, leaving a gap.
    This often caused problems when moulding with structural and flexible foams.
    We used to work on 3% contraction as a general rule.
    So it is the volumetric expansion (about 20 to 40 times depending on mix), minus 3%.
    To give you an idea of the pressures involved, an 20mm thick aluminium mould could easily 'expand' by 5mm.
    A 12mm composite mould would require a very sturdy 50mm by 3mm box frame around it for clamping and hinging.

    If you can, make sure that the foam can expand out the sides, that way it should expand between the bits you want to seal and the contraction and be taken from the overfill.

    I have just looked at my can of Soudal gap filling expanding foam, and it says it expands by 30 times.
    This would imply that it can exert up to 30 bar of pressure or 450 lb per square inch.
    It will actually be less than that as some of the gas will escape right away and the matrix will collapse.
    But even at 100 lb per square inch, it is going to be hard to keep something flat, so I suspect that the pressure is quite low.
  6.  
    Why not use the lower expansion foam designed for fixing in window frames?

    Something like this: http://www.touch-n-foam.com/window-and-door-foam.html

    Paul in Montreal
  7.  
    I used Soudal Flexifoam behind the plasterboard on my window reveals because it has low expansion. It didn't cause any problems.

    http://www.soudal.com/soudalweb/productDetail.aspx?p=498&ID=2715
  8.  
    ''Why not use the lower expansion foam designed for fixing in window frames?''

    Or 'dub out' with ordinary foam, cut back with a bread knife (clean knife before preparing toast) and stick cill to that 'splodge' with adhesive-grade foam.
    • CommentAuthorowlman
    • CommentTimeJan 18th 2017
     
    Peter, do you have to fully foam it all in one go because of access problems??
    If not, you could just dot foam in a few places, to do an initial "location" fix. Let it go fully off, then weight it if necessary, before repeating to fully fill the gap, - use a long flexi nozzle, and leave an escape route for excess, don't overfill.
    You should also vac off the stone to remove and loose dust. Most PU is moisture curing so if it's very dry, lightly misting the stone may get a better cure.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 18th 2017
     
    This is a real question also in filling between the edges of EWI blocks.
    Whether to do it 'as you go' by applying a bead to mating edges immediately before placing each block so it foams up immediately on placing;
    or whether to fill the joints afterwards by pushing a long thin nozzle into the hopefully tight joint at intervals.

    The former has the danger of displacing the block sideways by the edge-joint foam's expansion pressure, because the back-cement is still soft.
    In the latter the block is already firmly back-cemented in place before the foam tries to expand in the edge-joints.

    The other thing is, in the former (as-you-go bead), doesn't the foam collapse if it's touched/messed with during expansion? leaving just a thin sticky smear?
    • CommentAuthorblacksmith
    • CommentTimeJan 18th 2017
     
    Probably no use whatsoever but here goes - I embedded a couple of wood battens in the wall when I rebuilt mine so I could screw/plug into them from above afterwards. If the holes you have are big enough could you foam a couple of pieces of timber in to do similar?


    Failing that I use a concrete block to hold things down like you're describing,finished heights being acceptable and all that.


    No matter how I try I can never quite get the correct amount of foam from my gun if trying to be 'really' neat as it behaves differently throughout the year - expands like crazy when warm, not so much when cold.


    If you don't want to screw into the timber then it's not much use - sorry.


    Many years ago my first job after leaving school was installing cavity wall foam - it was not a pleasant experience and I did not do it for long but when installing it one of us drilled the joints in the wall and the other 'foamed', we had to be especially careful around windows/door frames so as not to pop them. The method used there was to use a small bamboo cane in the hole along from where the foam was currently going in and look for it moving. More of an art than exacting science.
    • CommentAuthorwookey
    • CommentTimeJan 22nd 2017
     
    It depends a lot if there is space for the foam to expand into, what type you use, the thickness of foam, and how fast you squirt it in. The stuff sold as 'glue' expands much less than the stuff sold as 'standard foam'. It expands _much_ less if applied very slowly. For fitting glazed windows I've had no trouble at all with them moving, using standard foam, but with an open side so the expanding foam had somewhere to go if overfilled. I'd be more paranoid if fitting empty plastic frames, for example.

    As others have observed it can unhelpfully lift plasterboard. Put some between a couple of bits of wood under a scale propped in place (or using weights) to work out what the force in Kg/m2 is. it would be interesting to know.
    • CommentAuthorUserError
    • CommentTimeJan 30th 2017
     
    The question's been answered, but this makes an entertaining point:
    http://www.songofthepaddle.co.uk/forum/showthread.php/2879-Funny-expanding-foam-story
    • CommentAuthorowlman
    • CommentTimeJan 30th 2017
     
    Obviously in this instance degrees/"ologies" and expanding-foam didn't mix. Very funny though.:bigsmile:

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