By Gene Wengert, wooddoc@uwalumni.com
Pressure drop can cause open joint.
Q. We have an r.f. panel press. We have begun to notice a fair number of small open joints. The joints are opening only a small amount. It is like we do not have enough pressure, yet I see glue squeeze out and also see the pressure guage indicating good pressure. Where do we go next?
A. Let’s assume that you have freshly prepared, flat surfaces and that adequate adhesive is applied to the wood. The pressure is applied to the edge of the panel which squeezes out any excess adhesive, as is suppose to happen. However, then the pressure drops a slight amount. This slight drop in pressure is enough to open a few of the joints a little bit. As the excess adhesive has already been squeezed out, with this slight opening, there is no longer any adhesive to fill the gap. The result is an open joint. Check the hydraulic pressure to make sure that is does not drop even a little bit, once side pressure is applied.
A second option is that after you apply the side pressure, you then apply top pressure to flatten the panel and align any individual staves that are sticking up. As the panel flattens, I believe that you can imagine how a few of the joints would open just a small amount. Again, because the side pressure got rid of the excess glue, these small gaps will remain empty and create a poor joint. In this case, check the individual staves to make sure that the sides are perfectly square with the top; with cupped lumber, sometimes the cross section shape of a stave is trapezoidal instead of a perfect rectangle. Consider applying top pressure first.
Gene Wengert, "The Wood Doctor," has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
Updated: Oct 09, 2012