Q. We are having problems with our door panels cupping and the glue joints showing. Any insight?
A. Wood moves, including cupping, and glue lines are seen due to excess pressure when the wood warps, only when the wood's moisture changes, so the wood is too wet for your plant's environment. You should check the incoming lumber's MC everyday...several times a day if a lot of lumber is being processed...in most cases, nothing wetter than 7.8 percent MC is a good target. Chances are that your shop and your customer, during the heating season have a humidity that averages around 35 percent RH which means the wood will get to 7.0 percent MC. Your shop should be no drier or wetter than this...within 3 percent RH. These conditions are typical for much of North America, put a minor amount of fine tuning may be needed in a few cases.
Many people check the MC of incoming lumber with a pin meter using insulated needles...a good choice. Drive the needles half the lumber's thickness to get the core MC. But, it probably would be best to drive them 1/4 of the thickness, which will give you the average MC for the piece. This is a critical and useful number.
In most cases, the core MC is the moisture level that the lumber achieved in the kiln. Poor shipping or poor storage may change the outside MC, but it takes a big moisture difference between the air and the lumber and many months to change the core MC. True unless the lumber is exposed to liquid water.
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.
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