Importance of Pressure in UF Gluing

[Flint Miller]

To test a construction technique, I recently laminated a 16"x8" length of commercial veneer to a similar size I'd sanded down (rather unevenly) to 2-ply from a 1/8" veneered piece, using DAP Plastic Resin glue. The two pieces were curved longwise on about a 12" radius, and supported from behind only along the edges of the 16" dimension. They were clamped together using cauls running across the 8" dimension, with pressure applied only at the ends of the cauls--which if anything bowed out when clamped. In short, it seems unlikely that even 50 psi pressure was applied to the inner area of the piece, never mind anything like the recommended 175 psi for softwood.

I used plenty of glue, and excess was extruded all around. To check the effectiveness of the glue job, I cut the laminated piece into strips. Everywhere, the glue line was nearly undetectable, and there were no voids or pockets of glue.

So I'm wondering, if lower pressure is adequate to achieve this result, do I lose anything by not using higher pressure? Greater strength? Greater longevity? And why do UF glues generally stipulate higher clamping pressures than other glues?

L.F.Miller

[Chris Padilla]

Nothing beats experience and testing, Flint! If you're happy with the end result, then no worries, right? :)

In my general reading of topics like this, I've come away from the idea that manufacturers are "covering their butts" when suggesting clamping pressures in these ranges. They are also thinking more that if the joints being glued were not prepared very well (i.e. they don't fit nicely...large gaps, what-have-you), that you'd need to deform the wood to close up said gaps/poor fitment and it would take ~200 psi to do the job.

Heck, I know people who clamp things together with blue painter's tape! A lot of turners gluing blanks together or preparing intricate patterns rarely use anything more complicated than tape. Then they chuck these blanks up on a lathe and cut away.

In short, I think you are fine but you may wish to cut some curved cauls such that you get contact all along the caul when you clamp it straight.

[Chris Friesen]

In my general reading of topics like this, I've come away from the idea that manufacturers are "covering their butts" when suggesting clamping pressures in these ranges.

I suspect that the manufacturers are simply specifying the optimum clamping pressure for ultimate strength.

Less pressure may provide perfectly satisfactory joints, just not as strong as the glue is capable of giving.

[Flint Miller]

Thanks for the comments. I suspected the official numbers were for the extreme case; and I couldn't really see why a crushed UF molecule would be any stickier than a normal one. Especially since the trial strips are uniformly and inseparably bonded, I'll stay with the method, though maybe using pre-stressed cauls next time.

L.F.Miller