I’ve done a lot of vacuum pressing using Unibond 800 but this is the first for this to happen. I needed a wide glue-up around 23 inches square. The bend was very shallow, if you laid it down on a flat surface the highest point of the curve would be 2 inches. Here are the other details –
1) 6 plies of commercial veneer – outside skins 1/16 walnut, 4 inner plies english brown walnut 1/16. I had to run all the plies in the same grain direction to get the width I needed, which might be where the pressing failed?
2) All surfaces had glue rolled on with a foam roller.
3) Glue was recently purchased.
4)Shop temperature 75 – 80 degrees.
5) Total pressing time 10 hours given that this was not a high stress bend.
Results, shortly after removing it from the bag, it returned to almost being flat and the entire assembly was very flexible. In the end I put the walnut skins over wiggle board with regular Titebond which worked fine. Any ideas on what caused this would be greatly appreciated.
Replies
You should always use an odd number of plies; otherwise the stress is on the glue line. Water-based glues do not set in a vacuum bag -- they set after being removed from the bag. I have no idea if that's an issue with Unibond 800 -- perhaps things would be different with epoxy.
Using different species of laminates in the same grain orientation is not a great idea as the expansion and contraction will be different. It would be preferable to alternate the grain orientation and make true plywood -- then the expansion/contraction is controlled by the long grain plies. I doubt that is what led to the failure though.
Mike
I do not know the properties of Unibond 800.
I always use epoxy for laminated bends and curves as it sets to a rigid glueline, which is actually holding the wood in the final position.
The fact the assembly was flexible leads me to think the glue had not set or is the wrong glue to use.
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