Can you use vacuum bagging with PVA to create "glulam" type panels?

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  • #555744
    Laci Nagy
    Participant

    Hi Folks,

    I am looking to build a couple of projects, a bed frame for my son and blanket box for my wife and I. I’d like to use some stock that is 38mm – 50mm (1.5″ – 2″) thick however it’s quite expensive here, particularly in wider boards.

    What I’ve done in the past is laminate 42mm x 19mm thick sticks to get 42mm x 38mm sticks and usually the glue line disappears in the planing and finishing. However I’ve only done this so far for small pieces eg table leg for side table eg 38mm x 38mm x 500mm and have had sufficient clamps to do this.

    When it comes to laminating panels that will be around 140mm x 38mm x 2030mm for the bed rails, I won’t have anywhere near enough clamps!

    However, I do have vacuum bagging equipment that I use to make model glider wings. I have bags that are long enough and wide enough to bag two rails 140 x 39 x 2000 at the one time without problems and a bench long enough and flat enough for them to lay on and stay flat.

    I usually use Epoxy (or maybe polyurethane) for this as I’m normally laminating fibreglass or timber veneer to foam cores and need it to be strong and stiff. I don’t want to use epoxy for this as I think it will make the glue line stand out, so I was going to use my usual PVA and just replace the clamps with the vacuum bag.

    Before I set this up, I was hoping to get confirmation that I can still use PVA (Titebond III) to do this and that it will still set within the vacuum bag. Has anyone had any experience in doing this ie vacuum bagging 2 boards 19mm thick to create a single board 38mm thick? If so, does it work and are there any traps for young players that I should be aware of?

    Any help or suggestions gratefully accepted.

    Regards,

    Laci

    #555750
    Sven-Olof Jansson
    Participant

    Very interesting question, Laci!

    Assuming the area of the head of an aluminium sash bar is 12.5 cm2, and the stronger assumption of the manufacturers’ claims on 250 Kg as maximal pressure, clamping pressure would be 20 Kgcm-1. Vacuum bagging pressure would – depending on barometric pressure – be around 1 Kgcm-1, evenly distributed, which I suspect would call for very flat and parallel edge surfaces for stable joints to be formed.

    Vacuum extracts moisture, so PVA glue should set quickly. PU-glue, I think work by having water added or extracted from the air, which seems to make it less useful. I’ve no experience from vacuum bagging, though.

    Attached is a simple drawing of a device I use for edge jointing thin pieces. Basically, it’s just two pieces of wood separated two other ones that are a tad taller than the boards to be jointed. I use wooden wedges to establish pressure on the edges, and also to force the boards flat. Five of them will do the job.

    /Sven-Olof

    Sven-Olof Jansson
    London, UK; Boston, MA

    #555757
    Laci Nagy
    Participant

    Thanks Sven-Olof,

    That’s a great idea. I actually want to face joint them rather than edge joint them. But it looks like I could achieve the same effect if I made one of the longer sides of the device a wedge instead so it provided pressure to the face.

    Inspired by your calculations, I did a little more math and worked out on a 203cm board 14cm wide, the vacuum bag was the equivalent of evenly placing 287kg of weight on the boards. That’s probably enough pressure to force a laminate onto a board, but maybe not enough to press two 19mm thick boards together.

    I’ll have a think about how to adapt your device for face gluing or see if I can find some way to use the dog holes in my bench with some form of holdfast to provide pressure on one edge against the bench while I use the clamps on the other edge.

    Thanks again for your response and ideas.

    Regards,

    Laci

    #555760
    Edmund
    Participant

    I’ve done some veneering, sometimes with Titebond, and it works great. If you have a thin veneer which happens to be from a porous wood, it’s not a good idea as the vacuum will sometimes lift the white glue up through the pores and to the surface of the wood — not an attractive result. Found out about that the hard way. However with non-porous woods or thicker pieces I’ve never heard of it to be a problem nor experienced a problem.

    I’ve also done vacuum bagging for making surfboards, kayaks & paddles with epoxy, polyester or urethane resins. I do almost all my veneering in the bag except for really small projects, where I’ll just use a hammer veneering technique because I’m too lazy to break out the bag and set everything up.

    As has already been alluded to above, at greater thicknesses atmospheric pressure is insufficient to correct warped boards. This is where mechanical presses become necessary — closing reluctant gaps in wood surfaces that aren’t sufficiently flat. You can watch this video for 20 seconds to see one in action: https://youtu.be/B5VGsZyXy60?t=417

    So in my limited experience the answer is: it depends. If your boards lie flat against each other, or have gaps which will close with just light pressure, then your bag will get the job done. If the gaps need considerable force to close, then it won’t. If you don’t have sufficient clamps, maybe look into building custom cauls for the project. I have a very small clamp collection compared to most woodworkers and often build clamping cauls instead of using a blizzard of clamps.

    Polyurethane glue is preferred by some of the most high-end furniture makers in the world for veneering. Craig Thibodeau uses it for veneering and sometimes marquetry on pieces which sell for 6 figures, such as https://www.youtube.com/watch?v=Ymr3EAJ7Mmo
    Craig recommends it unreservedly over any other option for veneering, and he vacuum “bags” (actually he doesn’t have a bag, he has a vacuum table, but there’s no real difference for this point) all his veneering work.

    I’ve used epoxy in woodworking where glue lines needed to be invisible and had the same success as PVA glue, although now that I think about it they were always done in dark woods — walnut, wenge or katalox, so maybe a lighter wood might show a glue line from epoxy. But if you don’t need the increased open time, gap filling or joint rigidity, there’s no advantage to epoxy, and generally PVA glues are cheaper, so might as well save the money and measuring / mixing time.

    #555761
    Laci Nagy
    Participant

    Thanks Edmund,

    It looks like i’ll be making some custom cauls for this project. thanks for all the information, the videos were great too!

    Regards,

    Laci

    #555795
    deanbecker
    Participant

    This is interesting a question please when you remove the air how does the glue dry ? It seems it would be like storeing it in the bottle

    #555798
    Benoît Van Noten
    Participant

    If one face will not be visible, you can screw the two boards together (as P.S. does for laminating the plywood workbench-top – plywood workbench video episode 1 around 13″). You can remove the screws when the glue has set.

    #555810
    Laci Nagy
    Participant

    The glue doesn’t need air to dry as such, it just needs to have the moisture removed as it evaporates. The vacuum will do that. Keeping it in a sealed bottle only allows a certain amount of moisture to evaporate into the bottle, once the air becomes saturated with the moisture no more can evaporate and the glue in the bottle stops “drying”.

    #555811
    Laci Nagy
    Participant

    Thanks Benoit,

    I hadn’t thought of using screws. It’s for a bed frame, so the inside face will always be hidden.

    Regards,

    Laci

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