Q: We are having a few open joints when we veneer maple to a particleboard core. But when we glue pine, we do not have any problems. I think that perhaps the maple is too wet?
A: The problem is, as you suspect, the effect of having too high MC in the maple. The heat of the press dries the veneer slightly, creating some shrinking right away. Your press variables -- time, temperature, and pressure -- are good.
Maple shrinks twice as much as pine when the MC drops. As a result, maple is much less forgiving, so if the MC is wrong, it will show up first in maple, as well as oak and a few other high shrinking species. The fact that you have the problem only once in a while leads me to suspect that you have a few wetter pieces of maple veneer. The average MC can be correct; the problem is that there is too wide a spread of MC around the average.
You can determine the spread by using the standard deviation calculation. Measure about 25 pieces of veneer for MC, choosing pieces from throughout the shipment, not just top pieces. Then, enter the values into an inexpensive calculator that has a standard deviation option. Certainly, an SD greater than 0.5 would have me worried. In truth, it would not be uncommon to have MCs that varied from piece to piece and even within a piece from some veneer producers who are not very careful. Also, I should mention that a maple joint is much more subject to aging. The veneer needs to be freshly sanded or machined before gluing. Even a very light sanding is adequate.
I suggest that you store the veneer when you receive it in a low humidity condition. This means either a dehumidified room with an electric dehumidifier, or a room heated about 25 degrees F (15 degrees C) above outdoors to achieve 35 percent relative humidity. (Often, heating is a cheaper way to control RH than using dehumidification equipment.) If you store the maple in this small heated room, I think that many of your open joints will go away - you can try a few pieces as a test.
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