Average side pressure at b = 2.48 lbs. per sq. in. Total side pressure at b (one side) = 138.69 lbs. Total side pressure at b (four sides) = 554.77 lbs. Downward pressure at c = 3.39 lbs. per sq. in. Total downward pressure on surface c = 216.9 lbs. Upward pressure on surface d = 4.69 lbs. In Fig. 30 let A B be the parting line between the cope and the drag; C be the cope and D the drag. It is evident that the pressure at b is sustained by the sand which is held by the sides of the drag. The pressure on the bottom c is carried by the moulding board and floor. Therefore, it is only the upward pressure at a and d that must be balanced by clamps or weights in order to prevent the cope from being lifted by the iron. In the case cited this amounts to 100.11 + 4.69 = 104.8 pounds. Hence a weight of at least 104.8 pounds must be placed on the cope to hold it down. Practically this should be at least 150 pounds.
If however this same pattern were to be cast as in Fig. 31 where 2 inches project up into the cope ; then h becomes 4 inches and the total upward pressure at a is 66.7 pounds.
The total upward pressure on the cope is obtained by adding that at d, which remains the same as before. Hence, total upward pressure on cope = 66.7 + 4.69 = 71.39 pounds.
The greatest care must be exercised in these cases to determine what may be the depth of the surface acted upon below the upper surface of the gate. The way in which a pattern is moulded sometimes has an important effect upon the pressure on the cope. Suppose, in Fig. 32, we have a hollow cylinder of 8 inches outside diameter, 8 inches long and with a shell and bottom 1 inch thick. It is to be cast bottom up, with the face a on the parting line A B. Let the depth h of the gate in the cope be 6 inches. Then the pressure per square inch at a will be 1.56 pounds. As the area at a is 50.266 square inches, the total upward pressure is, 50.266 X 1.56 = 78.41+ pounds.
The point at which the gate enters the mould is of no moment in calculating the pressure. It is the depth of the surface below the top of the gate that is important. In Fig. 32, the surface d is 7 inches below the parting line. Hence the upward pressure at that point, per square inch, is, .2607 X 13 = 3.39 pounds.
As the area is the same as in Figs. 30 and 31, the total upward pressure at d is 3.39 X 3 = 10.17 pounds. Total upward pressure on rope = 78.41 + 10.17 = 88.58 pounds.
If now the pattern is reversed and moulded bottom down, as in Fig. 33, a different pressure will be put upon the cope. The pressure at a is that due to the height h which is 0 inches, upon a ring 8 inches outside and 6 inches inside diameter. This is 34.3 pounds.
The pressure upon a' (the bottom of the cylinder) is that of a circle 6 inches in diameter, submerged to a depth of 13 inches below the upper surface of the molten metal." This amounts to 95.85 pounds. We therefore have:
Upward pressure at a
Upward pressure at a'
Upward pressure at d
Total upward pressure at cope
This is a difference of 51.74 pounds due solely to the method of moulding.
For light moulds no weight is needed. The two parts of the flask may be held together by clamps as in Fig. 26. In larger moulds, where the pressure would be apt to push the sand up out of the cope, weights must be used. The amount of weight to be used should be at least 50 per cent, more than the calculated pressure. The weight must resist not only the calculated or static pressure in the mould, but also that due to the momentum of the inflowing metal. This latter depends upon the height of the gate and the volume of the entering metal. For use an excess of weight fig iron is usually the most to use for weights.
It is as necessary that the mould shall be p'operly it it should be properly made. The gate is the passage left in the sand through which the metal flows to the mould. The following precautions must be taken in making the gate:
(a) It must be of such size that the metal shall flow freely through it without chilling.
(b) It must be of such shape that sand shall not be torn from its sides by the falling metal.
(c) It must allow the metal to enter the mould in such a manner that none of the sand constituting the latter shall be washed away.
In small castings one gate may be used. This gate may enter at the top of the mould or on the parting line between the cope and the drag. For heavier castings more than one gate must be used. The reason is that if only one were put in, it would have to be very large. This would involve a large inflow of metal which would wash away the sand. It is also desirable that the large moulds should be fed at different points. If the metal were poured into a large mould from one point only, it would be apt to chill before it reached all parts. This would produce a defective casting. The defect might consist of chilled parts or an incompletely filled mould. No fixed rule can be given for the number of gates. The number depends upon the size, shape and weight of the casting. The shape of the mouth of the gate also depends upon the weight of metal to be poured. For small castings it may be simply belled out as in Figs. 30 and 31. For larger castings or where the gate is deep, a basin should be formed in the sand of the cope at the mouth of the gate. Such a basin is shown at f in Figs. 32 and 33. The size of this basin depends upon the weight of metal to be poured.