Q: I have read every Wood Doctor column you have written and have gained a lot of practical information. But here is a question I have not seen addressed before. As background, our fairly large company has really gone into JIT, just-in-time, manufacturing, which means in-process materials cannot sit around very long at all. Well, this has recently translated into machining our glued up panels (edge-glued on a clamp carrier) within 24 hours after they are glued, or sometimes less. Of course, you know what the problem is: sunken glue joints that are obvious after finishing. My suggestion of waiting three days after gluing, as we have always done, has not been well-received. I am hoping that you have some help for us.
A: First, for those readers not familiar with your problem, the water-based glue used when edge-gluing the panel staves together causes localized swelling at the joints. If the panels are machined too soon after gluing, the swelling will go down after machining, giving a small depression at each joint. So, by waiting three days after gluing, the swelling will go down before machining and everyone is happy, except for the JIT people.
One possible choice for you would be a reactive polyurethane adhesive; there are two types and it would seem that the hot melt would not work well in a clamp carrier operation as you have.
The other type is available under a variety of labels; some of the names are " Gorilla Glue ", " Excel ", " PL Premium " or "RPA." They have 100 percent solids so there wouldn't be any swelling. They're also quite strong; nearly as strong as the wood itself, so they will be as strong as many glue joints.
The price for these adhesives (commercial level) has come down in the past few years, but they're still pretty expensive (about $4/lb). But it will solve your problem and they also do not require as high a spread rate, so they do glue more panels per pound than water-based adhesive.
But before you jump into this change, be aware that reactive polyurethanes can be difficult to clean up (especially from hands and skin). The uncured adhesive is also a carcinogen, so care in handling is essential. It is also necessary to provide good venting while the adhesive cures. Also, varying wood moisture content will give varying cure times.
All-in-all, I think you would be so much better off using your present adhesive and waiting three days. JIT is fine but not at the price of quality, as you already appreciate. I am not sure how to convince the JIT powers that this three-day timing is essential until they factor in quality.
Have something to say? Share your thoughts with us in the comments below.