Warping parts
November 3, 2013 | 6:00 pm CST

Q: I find your column very useful, and I hope you are well. We manufacture inexpensive kitchen cabinets with mitered 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 an expensive pin-type moisture meter. The pieces generally indicate an MC 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? Thank you for your advice.

A: The key point is that wood products (solid lumber or glued-up solid wood or composites) will not warp or change size over time unless their moisture content changes. Therefore, we know that if you are getting warp over the period of several weeks, then the MC has changed from the time you manufactured the door until you notice the warp.

In most of the United States, the in-use MC in a home is around 6 to 7 percent MC. Therefore, if your mouldings are coming in at 9 to 10 percent MC (and maybe a little wetter at times?), they are going to dry after manufacturing. Most certainly, shrinkage will occur during drying. Although this high MC might be OK for softwoods, as softwoods machine better at higher MCs oftentimes, it is totally unacceptable for hardwoods like maple.

You need to insist on lower incoming MCs. I suggest that you specify nothing higher than 7.0 percent MC (shell and core) measured with the meter. By specifying the meter to be used, you will also eliminate any confusion about MC measurement between you and your supplier. I would much rather pay a little more for lumber at the correct MC than have to pay for door replacement after the door has been manufactured and installed.

Special note: Oftentimes, you can raise the RH in your plant to avoid shrinkage while you are handling the wood. This only postpones the drying and shrinkage problems until the customer receives the doors. If there will be a problem, it seems much better to have it occur before the product leaves your plant. Therefore, do not humidify your plant much above 35 percent RH (7 percent EMC).

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About the author
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.