Q: We manufacture inexpensive kitchen cabinets with miter-jointed, flat-panel doors, and we have recently had a serious problem with warping hard maple doors. We purchase the mouldings in random length, and check moisture with a pin-type moisture meter. The pieces generally indicate a moisture content of 8 to 10 percent. When we build, finish and ship doors to low-humidity environments, many of the doors warp over a period of a few weeks. What causes the doors to warp? What can we do to minimize or eliminate this problem? What moisture content should we specify for our incoming mouldings?
A: In most of North America, kitchen cabinet doors will have a moisture of 6 to 7 percent MC in use. In a low RH environment, perhaps the moisture will be 5 percent MC. When wood goes from 10 percent MC to 6 percent MC, it will shrink approximately 1 percent in size. If the grain is not perfectly straight (and straight grain is rare), then the pieces will warp also. You need to have the lumber dried to the correct MC, 6.0 to 7.5 percent MC. You need to maintain the RH no higher than 38 percent to avoid moisture gain in both storage and manufacturing. Also, avoid exposure to moisture after manufacturing and before installation. Temperature is not a factor; only the RH matters.
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