Q: I know that it takes about 100 psi on a glue joint to get a good joint. This means when edge gluing two pieces together that are 1 inch thick and 10 inches long, the gluing surface is 10 square inches, so my press has to develop 1,000 pounds of force. If the pieces were 1 inch thick by 20 inches long, then it would require 2,000 pounds of force. Am I correct concerning these numbers? Now what about three 1 x 10 pieces edge-glued into a panel? The total glue area is 20 square inches (10 on each joint, and there will be two joints.) Is the pressure required still 1,000 pounds, or does it jump to 2,000 pounds? Or with four pieces, to 3,000 pounds?
A: Your numbers are indeed correct, although the 100 psi number will vary a little from species to species. In fact, in your first example, increasing the length of the pieces being glued from 10 to 20 inches means doubling the total pressure. What does this mean from a practical standpoint? It means that pressure needs to change as the piece size changes.
With a clamp carrier often this change is not done. I have seen the same screw pressure used for two clamps on 10-inch-long pieces as with two clamps on 20-inch lengths. Further, when gluing 3/4-inch material, you will need less total pressure than when gluing 1-inch or 1-1/4 inch, or thicker. This applies to panel presses as well as clamp carriers. In other words, total pressure needs to be evaluated for each particular size piece you are gluing.
The pressure that the clamps, or other pressure device, applies when gluing is transferred completely to each joint in succession. Therefore, if you are edge gluing two or ten 1-inch x 10-inch pieces, the 1,000 pounds of total force is all that is required in either case. However, this force is transferred only at the points where the adjacent pieces touch each other. If the pieces touch only in a few spots, the psi at these spots is really high, much higher than the 100 psi recommendation. This is why it is critical to make sure that the gluing surfaces are perfectly flat and square. Stated another way: gluing is an intimate moment!
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