by R. Bruce Hoadley
The object of clamping a joint is to press the glue line into a continuous, uniformly thin film, and to bring the wood surfaces into intimate contact with the glue and hold them undisturbed until setting or cure is complete. Since loss of solvent causes some glue shrinkage, an internal stress often develops in the glue line during setting. This stress becomes intolerably high if glue lines are too thick. Glue lines should be not more than a few thousandths of an inch thick.
If mating surfaces were perfect in terms of machining and spread, pressure wouldn’t ‘ t be necessary. The ” rubbed joint, ” skilfully done, attests to this. But unevenness of spread and irregularity of surface usually requires considerable external force to press properly. The novice commonly blunders on pressure, both in magnitude and uniformity. Clamping pressure should be adjusted according to the density of the wood. For domestic species with a specific gravity of O. 3 to 0. 7, pressures should range from 100 psi to 250 psi. Denser tropical species may require up to 300 psi. In bonding composites, the required pressure should be determined by the lowest-density layer. In gluing woods with a specific gravity of about 0. 6, such as maple or birch, 200 psi is appropriate. Thus, gluing up one square foot of maple requires pressure of (1 2 in. x 12 in. x 200 psi) 2 8, 800 pounds. Over 14 tons! This would require, for an optimal glue line, 1 5 or 20 cee-clamps, or about 5 0 quick-set clamps.
Conversely, the most powerful cee-clamp can press only 10 or 1 1 square inches of glue line in maple. Jackscrews and hydraulic presses can apply loads measured in tons. But since clamping pressure in the small shop is commonly on the low side, one can see the importance of good machining and uniform spread. But pressure can be overdone, too. Especially with low-viscosity adhesives and porous woods. too much pressure may force too much adhesive into the cell structure of the wood or out at the edges, resulting in an insufficient amount remaining at the glue line, a condition termed a starved joint. Some squeeze-out is normal at the edges of an assembly. However, if spread is well controlled, excessive squeeze-out indicates too much pressure; if pressure is well controlled, undue squeeze-out suggests too much glue. Successful glue joints depend on the right correlation of glue consistency and clamping pressure. Excessive pressure is no substitute for good machining. Panels pressed at lower pressures have less tendency to warp than those pressed at higher pressures. Additionally, excessive gluing pressure will cause extreme compression of the wood structure.
When pressure is released, the cells spring back and add an extra component of stress to the glue line.
The second troublesome aspect of clamping is uniformity, usually a version of what I call ” the sponge effect. ” Lay a sponge on a table and press it down in the centre; note how the edges lift up. Similarly, the force of one clamp located in the middle of a flat board will not be evenly transmitted to its edges. It is therefore essential to use heavy wooden cover boards or rigid metal cauls to ensure proper distribution of pressure.
Clamp time must be long enough to allow the glue to set well enough so that the joint will not be disturbed by clamp removal. Full cure time, that is, for development of full bond strength, is considerably longer. If the joint will be under immediate stress, the clamp time should be extended. Manufacturer’ s specified clamp times are established for optimum or recommended shelf life, temperature, wood moisture content, etc… If any of these factors is less than optimum, cure rate may be prolonged. It’s best to leave assemblies overnight.
Most glue specifications are based on ” room temperature” (70 · F). Shelf life is shortened by storage at above-normal temperature, but may be extended by cold storage. Normal working life of three to four hours at 70· F may be reduced to less than one hour at 90· F. Closed assembly at 90· F is 20 minutes, against 50 minutes at 70· F. A curing period of 10 hours at 70· F can be accelerated to 3 – 1 / 2 hours by heating to 90· F.
Finally, cured joints need conditioning periods to allow moisture added at the glue line to be distributed evenly through the wood. Ignoring this can result in sunken joints.
When edge-gluing pieces to make panels, moisture is added to the glue lines (1), especially at the panel surfaces where squeeze-out contributes extra moisture. If the panel is surfaced while the glue line is still swollen (2, 3), when the moisture is finally distributed the glue line will shrink (4), leaving the sunken joint effect.