Q: We are getting a lot of warp in the panels that we make. Sometimes it happens right away when we try and plane the panel; other times it takes a few days. It is mostly cup type warp. I checked the MC and it is always 7 to 8 percent MC. What is the problem?
A: You are reporting two different problems here.
First, you have a problem with lumber cupping right after you plane it. This is the result of casehardening (which is really a word that means drying stresses). The lumber has not been conditioned (also called stress relieved). I would consider the wood to be improperly dried if it has substantial drying stress. Drying stress always results in immediate warp (unless there is a severe moisture gradient). Stress tests can be cut at any time after the lumber leaves the kiln; in the hot kiln, there is a special procedure using a microwave oven to get the correct cold lumber reading.
Second, you are reporting some cupping sometime after the pieces are manufactured. This is because the wood has changed MC and therefore natural shrinkage/swelling will occur. Such natural shrinkage means that the bark side of the lumber will shrink more than the heart side. If you check the MC after the MC change has occurred, you will always see the proper MCs. You need to check the MC before the MC has a chance to change, not after it has changed.
The presence of excessive stress is certainly a reason to go back to the supplier, but I would also cut the normal stress tests (both transverse and longitudinal) before I send the lumber back to confirm the extent of the problem.
The incorrect MC for you and your customers may not be something you can complain about because:
1. You may not have specified the MC that you want. The term "kiln dried" means nothing with regard to MC, for hardwoods, even if the NHLA rules are used. (Actually, for red oak, 6.0 to 7.0 percent MC is better, with any pieces over 7.0 percent MC being returned immediately.)
2. If you did not measure the incoming MC, it could be that the MC was OK when shipped, but changed in transit or in storage. I urge you to check the MC of incoming lumber as it is an important property.
3. The MC could be OK, but your customers have an unusual condition of RH, meaning that the typical 7 percent to 8 percent MC is too high.
4. It could be that most of the lumber is OK and just a few pieces are too wet. You may need to check a large percentage of the pieces. Sure, this is expensive to do, but the cost of a reject is even more expensive.
I know of one door company that had over 100 moisture calls every winter. After they installed an inline MC meter, checked every piece, and eliminated all that were wetter than acceptable, they had only three calls. This quickly paid for the moisture meter. Maybe you are too small for an expensive inline meter, but a handheld will do the same job, with a little more labor cost.
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