Air pressure is the reading on the pressure regulator that you set in order to deliver the desired force to the electrodes on the part being welded. Air volume is the ability to deliver this air and force within the time desired by the weld schedule. If the air manifold feeding the resistance welder cannot fill the cylinder in the squeeze time of the weld cycle you will be welding at a low force and in an uncontrolled condition.
In some cases a group of machines in a cell may run off one air manifold. Any one machine may be fine but when they all start cycling, they could tax that manifold to deliver and maintain full consistent air flow and pressure to the total cell. Very inconsistent weld quality could be expected from this cell. Expulsion could be expected around the cell.
There are two fixes either a larger air feed line or a surge tank installed in the cell to cover the momentary surges in air demand.
Reference: RWMA Resistance Welding Manual 4th Edition