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Thread: epoxy joint question

  1. #1

    epoxy joint question

    There are numerous resources on constructing epoxy resin river tables. However, virtually all of these designs have a strip of resin running parallel with the grain of the slabs on either side. This reduces or eliminates issues with wood movement since the wood/resin bond is perpendicular to the direction the wood expands.

    Has anyone seen or used resin bonded in a parallel direction with wood movement? Imagine bonding resin to the endgrain of a table top, like a breadboard. Would the movement of the wood across the top destroy the wood/resin bond? If the bond is strong enough to keep the ends from moving, would you get strange cupping or other stresses as the middle or other end of the wood pieces try to move? Does using a dovetail or box joint improve the bond strength?

    My application is attached. This would be the front of an entertainment console table; the resin center would actually be the front of a built-in subwoofer enclosure. Not pictured in the attachment are a top and bottom of the table to help support the joints.

    In all my searching I haven't seen anyone try something like this. Unfortunately it's not a simple exercise to just test the joint since it's hard to artificially induce seasonal wood movement. Appreciate any thoughts.

    IMG_1668.jpg

  2. #2
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    The epoxy panel will act independently of the timber as it is essentially a piece of hard plastic. This means the bond will break with seasonal timber movement. You will need to joint the two materials. Do a dovetail tenon on the end of the timber and then pour the epoxy. Alternatively, get some polycarbonate sheet the thickness and colour you want and work with that. Cheers

  3. #3
    Both options are prone to failure but the shear in the endgrain option would likely fail very quickly if the part is wide enough. The epoxy/wood/river thing is a fad that will pass but my prediction is there will be a lot of very expensive tables that will fail structurally over time. It will be interesting to see how many fail in a time period that customers who paid several thousand, or 10, 15, 20k for a top that failed want their money back or a re-make.

  4. Here are a few thoughts:

    1. Selection of Resin product is important. To achieve a stronger joint, choose a product that has a low viscosity and long pot life. Something in the 100-500 cps viscosity range. The two products that come to mind are Smooth On's Expoxacast 690 and Ecopoxy's Liquid Plastic. Expoxacast 690 has a 5 hour pot life (72 hr cure time). With the Ecopoxy product, you'll want to go against the manufacturers recommendation and mix with either a 13:8 ratio of resin:hardnener. These two properties will allow a deeper penetration of the resin in the endgrain. I would personally go with Ecopoxy because of the low exothermic catalyzation of the polymers. With the Epoxacast, there's a good chance you'll get off-gassing. Both have excellent UV resistance.

    2a. The greater the surface area, the better the bond. If this is a table top, use a dado blade to cut a channel (rabbet?) on the underside of each board for the epoxy to fill. I would then use a chisel or dremel to cut a christmas-tree type pattern on the channel. Every few quarter inches, I would also drill a bunch of small holes. That way the epoxy will be supporting the underside and have a larger mechanical grip on the pieces.

    2b. If you drill the holes on the underside, pour the resin with the bottom side face up first. I refer to this is a "preparation cast." Cast a small amount so that the holes get filled up and a little gets into each of the channels. Wait until the epoxy has a gel-like consistency (do not let it fully cure). Then flip the pieces back over, top side face-up, put them in the mold, then do your regular 'deep cast.' This will prevent air pockets from getting trapped in the channel/holes, which can be the weak points in the joint.

    3. In response to Wayne Lomman's comment: Some epoxies have a greater degree of flex than others. Certain products allow for the user to change the mix ratios that will affect this. While I agree that a majority of epoxy products will confirm his statement, I do believe that other products with greater degree's of flexural strength will reduce the joint failure rate.

    4. Using epoxy as a top coat for your project (or at least just this component) will add the strength of the joint. Especially if you pour the top coat on the piece while it is still in the mold and the deep cast is in the gel stage. Since Ecopoxy's Liquid Plastic has a super long gel stage, this product seems to be the best choice for the job.

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