After our discussion here, I wrote to technical support at Titebond regarding our discussion of clamp pressure. I got a next day reply from a very knowledgeable and helpful gentleman, Mr. Zimmerman. I'm posting it here.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I am writing in response to your question about clamp pressure. First, your calculation and understanding is correct. If you wanted to produce
200 psi over an area 12" x 12", you would need 28,800 pounds of force. On the other hand, it is not clear whether you often, or ever, fell short of the actual, required clamp pressure.The actual required clamp pressure for any bond involving a wood glue is a combination of the small amount of pressure required to squeeze the glue into a thin, consistent layer, and the pressure necessary the compensate for any distortion or lack of fit in the wood stock being used. That means when the surfaces of the pieces being joined are true, and there is no gap between the pieces when they are dry fit, very little pressure is required. If, however, the same assembly is being made using pieces which are bowed, twisted or ill-fitted, the required pressure is much greater, and is largely the pressure required to straighten the wood and pull it into position. Thus, the actual required pressure for a bond also reflects the thickness, or fight, of the wood involved, with much more pressure obviously required to straighten a very thick piece of maple or oak than to straighten a thinner piece of the same species.
In many applications, then, pressure, serves to compensate for some lack of diligence in wood preparation. That being the case, good wood preparation lessens the need for, or dependence on, pressure. In the case of our literature, the high suggested pressures reflect the fact that those individuals being addressed include those who, at least on occasion, are trying to bond thick, poorly fitted pieces of wood, and for those readers, the high, suggested values are, indeed, necessary.
Finally, because the bond strength produced in a joint is the result of the entanglement of the glue particles which have been drawn into the pores and anchored to the wood on the two sides of the joint, there is rarely any concern for applying so much pressure that the glue is all squeezed out. In fact, the bond strength achieved increases as the bondline or layer of glue becomes thinner. Given that fact, there are only two situations in which high pressures may be counterproductive. First, there is always a concern that the pores of wood at the bonding surfaces not be crushed, and that is the reason that our listed pressures are lower for the softer woods. The second situation deals with bonds involving end grain or other open grain. There the concern is that the open grain is prone to suck up a large amount of glue and, if that thirst has not been quenched before clamping, that excessive absorption of glue may result in a starved, and weak joint. Because most bonds involve face or edge grain which is relatively straight, that particular risk is rarely a concern. I hope this response is helpful, and ask that you feel free to write again or to call me at 1-800-###-#### if I can be of any further assistance.