Weighting Copes
This operation consists in placing as much weight upon copes as will hold them down securely, independent of any other means, such as bolting, binding, clamping, etc., when the pressure of molten metal tends to lift or force them upwards, such pressure (per square inch) being always proportionate to the depth from the metal's surface in the runner-basin to the cope's surface below. The total pressure is the amount per square inch multiplied into the area. For example, a cope covering a plate 4 feet square is 9 inches deep, and the runner-basin adds 9 inches more, making 18 inches in all. Now, as 18 inches pressure is exerted on every inch of the plate while the metal remains liquid, it only remains to ascertain what pressure a vertical column 1 inch square and 18 inches high exerts,--in other words, to find the weight of such a column, and multiply it into the area,--to discover what weight is required to hold the cope down; as, 18 X .26 (the weight of a cubic inch of cast iron) equals 4.68 pounds pressure on each inch, which, when multiplied into the total area, 48 X 48, equals 2304 inches; which, again, multiplied by 4.68 pounds, gives 10,782 pounds as the total pressure. Hence exactly that amount of weight, including the cope's weight, would be required to balance the pressure exerted against it, unless the head of pressure be cut off by a system of risers, to relieve it at some antecedent point below. When the full head of pressure is imposed some extra weight is needed, as fins, gate-surfaces, etc., act in proportion to their area, and the possibility of shock is a contingency that will always be adequately provided for by the wise moulder. See CUT-OFF; RISERS; PRESSURE OF MOLTEN METAL; HYDRAULICS; HYDROSTATIC BELLOWS.