Q: Like everyone else, we have some end cracking at the glue joints during the wintertime in our glued-up panels. Our supplier says that the problem is the dry humidity in our plant (about 25 percent RH) and I wonder if it is the high MC in the lumber. What perplexes me is that when we have cracking, we measure the MC and it is always under 7 percent MC and sometimes under 6 percent MC. We are considering increasing the RH in our plant this winter; 45 percent RH has been suggested. Can you comment or direct us in the right direction, as this cracking is expensive?
A: First, let me state that not everyone gets end cracks in their panels in the wintertime. End cracking is totally preventable.
Second, remember that wood does not change size or shape unless its MC changes. When you see end cracks, we know that the MC has changed.
Third , a properly made glue joint is stronger than the wood itself and therefore should never be the location of a failure. (Stated another way, when you see end cracking at the glue joint, you know that the joint does not have the highest quality.) In most cases, a weak joint results because the pieces of wood are not close enough together at the time of gluing; this separation, which often results because the ends dry and shrink slightly before they are glued, creating a gap and a weak joint, needs to be only 0.006 inches or more.)
Fourth, if the wood is somewhat too wet for the plant's environment, dries out in response to this difference, shrinks, and opens a crack, and then if you measure the MC after the wood dries, shrinks and cracks, you will always see a low MC. You need to measure the MC before the problem happens if you want to find high MCs.
Fifth , the MC of wood in a customer's home is seldom over 7 percent MC in most of North America. This means approximately 38 percent RH. In the wintertime, many homes are 25 to 30 percent RH or about 6 percent MC in the lumber. We call the conditions in the home 6 percent EMC (equilibrium MC).
Sixth , we know that drying out a little is more of a problem than increasing MC a little.
Seventh , if you humidify your plant, it will result in the MC of the "slightly wet" wood and the EMC of the plant being close together. Therefore, the wood will not shrink and your problems in the plant will be ended. However, when the customer receives the items, they will shrink "in the field." I would rather have problems show up in the plant than in the field. Therefore, do not humidify the plant above 37 percent RH. In fact, 30 percent RH to 35 percent RH is probably a more prudent level. Measure RH with electronic sensors that are available from electronic supply stores (Radio Shack) for about $30.
Eighth , If your plant is 30 percent to 35 percent RH (6 percent to 7 percent EMC), then the lumber should be no wetter than 7.5 percent MC. Any lumber wetter than this will shrink and potentially cause problems. With a weak glue joint and such shrinkage, end cracking will result. You need to establish a MC measuring program to assure that all of the pieces of lumber are under 7.5 percent MC. (As a special note: Most kiln operations measure 12 samples pieces out of 10,000 pieces of lumber. Someone has to measure a few more pieces to assure that the MC of the lumber is correct.) In fact, just a few pieces of lumber over 8 percent MC will end up in many more panels when the lumber is cut into staves for gluing. At the least, measure the MC of no less than 30 pieces of randomly chosen lumber. Better would be to monitor MC every hour by measuring 10 pieces in production. Best would be to have an in-line MC meter that "kicks out" any wet pieces. You also do not pieces that have been over-dried (under 5.5 percent MC).
Ninth , a supplier of lumber will often insist that your RH is too dry, while a producer will insist that the lumber is too wet. The truth is that the wood is too wet for the dry conditions in the plant. At 30 percent to 35 percent RH, the plant RH is appropriate from a final-customer point of view, so you need to tighten up your purchase specifications, especially with respect to MC. It is possible to dry an entire kiln load to within + or - 3/4 percent MC using a process called equalization. This process should be standard operating procedure; if it is not, then find a new supplier that does proper equalization. Some people skip equalization and thereby save themselves $30 per MBF when drying. Certainly your end splits cost more than that!
Tenth , set a high standard. You can virtually eliminate wintertime cracking. I worked with one plant in Wisconsin that had over 200 cracking problems each winter in the field. After we developed a program to eliminate lumber over 7 percent MC, they had only 3. Another operation in Indiana had 17 percent of their panels with cracks. After controlling MC better, cracking dropped down to 4 percent, with further improvement as they became more involved with MC and RH measurement. Finally, a plant in Texas ran about 24 percent rejects and was able to reduce this to 4 percent.
Have something to say? Share your thoughts with us in the comments below.