Q: We make dining tables (two pieces with a leaf) that we send to Arizona. We are having trouble with a gap developing in Arizona at the edges of where the leaf joins the two permanent top pieces; the center of the leaf fits tightly. These tables are tight all along the joint when they leave Minnesota. Our plant is 40 to 45 percent RH, which seems to keep the tops in good shape here. The MC of the tops here in Minnesota is 5.2 percent MC, checked with a pin-type moisture meter. We store our tops before we finish them in an unheated warehouse. Could the cold have any effect?
A: First, the rule is "Wood doesn't change size or shape unless the MC changes." So, we know that the tops are changing MC. We also know Arizona is quite dry (5 percent EMC), so the edges of the tops are shrinking.
Why is your plant so humid? You indicate that you have found out that a high RH minimizes the problems in the plant. But what it is really doing is postponing the problems until the customer gets the tables. Your plant is the equivalent of 8 percent EMC wood in your plant will be trying to achieve 8 percent MC. I think that you should consider an RH of about 33 percent RH, which is 6.5 percent EMC.
Your readings of 5.2 percent MC are incorrect. Pin meters cannot read low MCs (under about 7 percent MC) reliably. Further, if your plant is 8 percent EMC, you should be getting readings of 8 percent MC. Also, an unheated warehouse will average closer to 12 percent EMC, so I would expect to see much higher than 8 percent MC in your panels. Also, if your tops are 5.2 percent MC, then they wouldn't be shrinking when they got to Arizona. They would already be dry enough.
You need to lower the plant conditions to 33 percent RH, provide dryer storage in your warehouse (33 percent RH) and replace your moisture meter.
You might also consider a more vapor resistant, thicker coating to retard the drying that might occur in Arizona when the table first arrives.
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