Ten gluing tips

Q: We have a clamp carrier type of edge-gluer and have been getting about half of our panels with joints that are separating. Is this high? Is there something we should be looking at? If you are ever in Denver, please stop in and we can show you around.

A: You should be running at 98 to 100 percent successful gluing in the clamp carrier. In other words, you should not be having any defects, except in rare instances. So, there is something wrong in your operation.

Here is a list of questions about any gluing operation. The answers will help shed some light on what is wrong. Anyone experiencing gluing problems can run through this list.

1. Are the open joints mainly at the ends or not? End cracks at the glue joint are common when the individual strips are too wet for the environment in which they are being exposed. The ends, which dry out quickly compared to the rest of the piece, achieve equilibrium with the dryer conditions in the plant. With this drying comes shrinking. As a result, when you go to glue the strips, the ends will be too narrow, leading to gaps in the joint that are too wide to be adequately filled or bridged by the adhesive. In other words, the joints at the ends of the panels are not adequately bonded or glued together. You must have the MC of each piece of lumber (throughout the piece and not just an average) within 2 percent of the EMC of the air. (Recall 6 percent EMC is 30 percent RH; 9 percent EMC is 50 percent RH; 12 percent EMC is 65 percent RH; and so on.)

2. Are the problems worse at any certain time of the year or any day of the week? The plant RH varies throughout the year and perhaps even daily, so we do, from time to time, indeed see problems related to the calendar. If you answer yes to this question, refer to the first question. Do you know the RH in your plant when running and when shut down? In lumber and part storage? You really should have this information so you know the EMC at all times. Do not run your business on luck. Run it on knowledge!

3. Do you measure the MC of the pieces before gluing, after gluing, when there is a problem, or when? It is critical to know the MC of the wood when it is first cut into strips, when it is being glued and after a problem has occurred. Unless you have all these values, you will not know if MC is a problem. Refer to the first answer for more detail. (Too often, MC tests are random and seldom done. Yet they are critical if quality and profitability are important. When I visit a plant, I always ask to see the moisture meter. It is amazing how many times the meter is not in working condition, or the meter is 10 years old but shows no wear (meaning usage) at all!

4. How much time between edge preparation and application of glue? In addition to shrinkage that occurs when pieces are stored for a while (hours, days, weeks) and when the MC of the lumber does not equal the EMC of the lumber (refer to the first question), wood surfaces also become less reactive, chemically speaking. That is, the surface is not able to chemically attach itself to the glue as well as the surface ages. It depends on how dusty and moist the atmosphere is, but significant degradation can occur within a day or two in bad situations. (For veneer, it can be minutes!)

5. How do you prepare the edge-is it sawn? It is not uncommon to have the saw blade give you a poor quality (very bumpy and un-true) edge. Remember that it is the sides of the saw tooth (not the top) that prepare the surface you will glue. Each tooth should stick out the same from the side of the blade. The process of getting them all the same is called side dressing. The sides of each tooth should be as sharp as the top and also should be flat and parallel to the blade.

6. How much time between application and pressure? As glue sits on the surface of wood, it begins to lose moisture into the wood and become thicker. Eventually (minutes), the glue may be too thick to properly bond to the wood, fill in the microscopic nooks and crannies, and spread well to the opposite face being glued. The time for this to happen is much shorter when the wood and glue are warm (summertime) than when they are cool. Do you change formulations between winter and summer?

7. Does the failure run primarily lengthwise and on one edge of the joint? This can happen when the edges are not 90 degrees to the faces. The result is that you have a larger-than-desired gap along the one edge; most glues cannot bridge a gap well. (Exception: hot melts.)

8. Does the operator of the clamp carrier hit the panels (after the pressure is applied) with a hammer to get them flat? If so, often this results in shifting of the panels and the loss of strength. Further, the need to do this may indicate that the edges are not square to the faces. Avoid this practice.

9. Does the clamp carrier have the correct pressure? Too little pressure and the wood pieces are not close enough to each other. Too much pressure and all the glue is squeezed out.

10.  Is there some squeeze-out along the entire joint? This indicates that there was enough glue. Areas without squeeze-out are likely to be "starved joints" that will often have little strength.

I suggest that as an experiment you saw the strips to be glued and then apply the glue and pressure within 10 minutes after sawing. There should be a significant amount of squeeze-out. This rapid processing will eliminate many potential problems. Make about 50 panels. If the results are good, then we can look at EMC problems, storage problems, and so on. If the results are still poor, we might double check the adhesive (Only once in 40 years has the adhesive been the problem, however!), saw blade problems or rare wood contamination problems (oils in teak for example).

You did give me one very large clue as to what could be happening - you mentioned that you are in Denver. We know that the RH in Denver (and therefore the EMC) is very low. It would not be unusual to see humidities under 20 percent RH (under 5 percent EMC). This is exceptionally dry and it would be uncommon to see lumber dried to an MC low enough for this exceptionally dry climate. So, I suspect that your lumber's MC is not within 2 percent of the air's EMC. However, lowering the MC is not the answer in most cases as lower MCs mean more chipped grain, tear out and so on when machining.

Your approach would be three-fold. First, make sure that the lumber is as dry as reasonable. I would suggest 5.5 to 6.5 percent MC. Any pieces over 6.5 percent MC should not be used, but should be returned, dried further or stored until you have more humid weather. Second, glue the strips as quickly as possible after ripping them. Third, explore the opportunity to humidify the plant to about 30 to 35 percent RH. (If you go more humid than this, then when the customers receive the product in their dry home or office, the wood will shrink after they receive it, potentially resulting in failure of the finished product!)


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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.