Q. We have just had two panels of cedar with glue line failure. Normally, we have excellent results. We are using a two-part cross-linking PVA emulsion adhesive with a catalyst. We are checking the glue edges for squareness and flatness; we are positive that we have a good, even glue spread; we are keeping the open time to less than ten minutes; we are making sure we have a four-hour clamp time; and we do not machine for 24 hours. Our press has manual pressure. What should we be looking for?
A. It is hard to diagnose a problem like yours in which you have already done an excellent job of assuring good joints. So, we need to look for something unusual which is hard to do without being there. Here are three possibilities.
First, one reason is that the edges were old and had aged a bit (contaminated perhaps with cedar oils or dust or oxygen, etc.), making them less receptive for gluing. As you are probably aware, the joint strength should be 1-1/2 times stronger than wood, so normally a slight bit of aging will not cause a problem; perhaps your edges were a bit older than normal.
Second, one rare reason would be that the pressure drops slightly after initial pressure application. Because the initial momentary application of higher pressure squeezed out the glue, then when the pressure drops and the joint opens a tiny bit, there is a starved, weak joint. It might be worth checking to see if a pressure drop is possible, but only rarely.
A final possibility is that the temperature of the wood or adhesive was quite cool or exceptionally hot. Temperature does affect glue bonding, so uniform temperatures from day to day are important. Many operations will keep glue in a room with a constant temperature, year round.
In all the years that I have been a consultant, I have had only two cases where the adhesive itself was the issue, so there is no need to change adhesive brands or types as almost all the time you have no problems. I also assume that the catalyst was properly mixed. Let me know what you find.
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