Split laminated glue joints

Q: We cold pressed several hard maple panels, 4/4 thick, then ripped them into 5-inch wide strips, and then we laminated these four 5-inch pieces to produce a post approximate 4 x 5 inch cross section. Then we sent the posts to the finish dept. The post began to split at the glue joints, usually the end, after the finish material was put on them and they were exposed to the hot box. How can we determine if the post are going to split before we apply finish material and heat?

 A: Without examining the posts and seeing your operation, I can only provide a good guess, based on years of experience, as to what is happening. In the finish room, I suspect the humidity in the hot box ovens is very low, much lower than the humidity in the plant or in storage. This low humidity causes the wood to dry on the surface and shrink, or try to shrink and open the joint.

One contributing factor may be that the heat in the ovens is also melting or softening the adhesive, causing the joint strength to plummet. Check with your adhesive manufacturer for the temperature sensitivity of the adhesive after a joint is made, but temperature sensitivity is common with cold press adhesives.

It is also possible that the joint itself is not strong initially. This would result because the surfaces are not perfectly flat at the time of laminating. To eliminate this possibility, take a finished post and cut a thin surface wafer from the near the end of the post on all four faces (1/4-inch thick and running about 1 inch up the post and either 4 or 5 inches long, depending on which surface was cut). If this wafer, which will include the glue joints, falls apart, then you know that there is a problem prior to finishing. In this case, check the raw lumber’s MC and the environment’s RH in manufacturing (and storage, if any). If the wood is 7 percent MC, then the environment needs to be 38 percent RH (called 7 percent EMC) to avoid changes in size. If 8 percent MC, then 42 percent RH; however, a home or office, especially in the wintertime will seldom be more humid than 30 percent RH (6 percent EMC). 

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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.