MC for kitchen mouldings
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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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About the author
Gene Wengert

Gene Wengert (1942-2025) was popularly known as “The Wood Doctor.” He trained thousands of people in efficient use of wood for more than 50 years and authored foundational resources on wood technology. He worked at the University of Wisconsin-Madison and Virginia Tech. His popular "Wood Doctor's Rx" column has appeared regularly in FDM and FDMC magazine since 1978. Because so much of his advice was timeless, he asked that we continue to run his columns in memoriam.