Gluing it the right way: Proper pressure

Q:We are gluing some maple veneers together and are using 300 psi. Does this seem high to you? We are getting some delamination; it looks like starved joints.

A:Yes indeed, 300 psi seems quite high for most situations; nearly double. But are you certain that you have 300 psi? Oftentimes, the measuring instrument measures the pressure of the ram in the press.

The area of the ram needs to be compared to the area of the wood being glued, adjusting the pressure proportionately. Even if your pressure gauge has been calibrated for the press area, if you do not fill the press, edge to edge and end to end, the wood pressure will be higher than indicated. (Example: If the ram is 60 square inches and the work being glued is 120 square inches, the pressure on the wood is half of the ram pressure. On the other hand, if you only have a small piece in the press, say 30 square inches, then the pressure on the wood is double the ram pressure! Of course, if the pressure has been adjusted already for the press size, but you only have half the press filled, then the pressure on the wood would be doubled.

Starved joints, incidentally, can result from insufficient glue being spread on the veneer, as well as from too much pressure. If there is excessive pressure, then I would expect to see excessive squeeze out. This in turn can cause the gluing people to reduce the glue spread in order to reduce the squeeze out; this will lead to trouble for sure.

A starved joint appearance can also result when the veneer is not active for gluing, due to aging or contamination, for example. Most glue companies can be of assistance in identifying the type of glue failure. I suggest sending samples to several different reputable firms for analysis.


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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.