In general it is always well to make the gates as long as possible. Where a large amount of iron is to be run through a gate, it is well to protect the mould by placing cores against the pattern at the point where the metal will strike the sand. This may also be done by ramming a mixture of flour and facing sand against the patterns at weak points.
The size of the gate also depends upon whether the mould is to be poured rapidly or slowly. In the grate-bar Fig. 84, where there are a number of small cores, a a, the metal must be poured slowly. If it were to be poured in with a rush the cores would be washed away and the easting ruined. In Fig. 31 on the other hand, where then are no light cores the metal may be poured very rapidly. In Fig. 34 the iron should be poured hot and slowly. In Fig. 31 it need not be so hot and may be poured rapidly.
The point at which the iron enters the mould is of great importance. The method of gating shown in Fig. 33 is preferable to that of Fig. 31. In Fig. 33 the gate enters at the bottom of the mould, and sustains all the effect due to the falling of the molten metal. There is in this case little danger of washing away the sand of the mould. In Fig. 30 the iron falls from the point, where the gate enters the mould, upon the bottom c. The force of this falling metal will be likely to wash away the sand. This can be obviated in two ways; a dry sand core may be put in the bottom of the mould, or the gate may be led in at the lK)ttom as shown by the dotted lines. The latter is the better method. Where moulds are very deep, even a dry sand core would be in danger of being washed away.
Where two or more gates enter a mould they should do so in such a way as to promote a circulation of the iron. An example is given in the round plate Fig. 35. If there are two gates entering at a and b, it is evident that they will have a tendency to cause the metal to circulate in the same direction, as indicated by the arrows a' and b'. If, however, the gates are placed at b and c the:
In calculating the amount of metal to be melted or poured an allowance must be made for the weight of the sprues that till the gates and are attached to the castings. These weights must be added to that of the castings. Where there are cores they must be deducted from the calculated weight.
The gases formed by the molten iron must l>e allowed to escape from the mould. The gases are produced by the decomposition of the organic matter contained in the sand and cores. This is due to the heat of the molten iron. Unless the gases are allowed to escape they will hold back the iron and cause an irregular casting. A larger quantity of gas is formed from dry than from an equal weight of green sand. This is due to the presence of a larger amount of organic matter, such as flour, molasses, etc., in dry than in green sand.
Venting is usually accomplished by pushing a vent wire (Fig. 9) through the sand of the cope at several places. The larger the dry-sand core used the greater the number of vent holes. Venting is very important in moulding. When it is found that the gases pass up through the iron, causing it to bubble at the mouth of the gate, the venting is insufficient. If a mould is rammed solidly the number of vent holes must be large. When sand has been used a number of times the organic matter is largely decomposed. Hence, old sand will produce less gas than new, and therefore, requires less venting. The venting should be done while the pattern is still in the mould. If the venting is not properly done, the pressure of the gas may be sufficient to throw the metal out at the gate.
After the metal is melted it is drawn from the cupola into a large ladle. From this large ladle it is poured into smaller ones. These are taken to the moulds and emptied. They vary in size from the small hand ladle that may be carried when filled, by one man, to the ladle holding several tons that is moved by a crane. The iron as it comes from .the cupola is usually too hot to be poured immediately. Cast iron melts at about 1500° Fahr. It is usually much hotter than this when it leaves the cupola; also when poured. The temperature at which it should be poured depends upon the kind of easting to be made. No definite rule can be given, but it is customary to have the metal at higher temperatures for the thinner castings. Moulds having thin cores or projections should be poured slowly with hot metal. Heavy castings with few or no cores may be poured rapidly with dull metal.
As already stated, ladles are of all sizes. The ordi-nary ladle is made of cast iron and is freshly lined after each casting with fire clay. This lining is called daubing. The fire clay is mixed with water until it is in a plastic condition like putty. The interior of the ladle is then daubed to a thickness of from 1 to -inches. The daubing is thicker at the bottom than at the. top, and should be spread smoothly. After the daubing a wood tire is built within the ladle. This serves to dry the clay. If it were allowed to remain damp the steam formed by the molten iron would blow the latter out of the ladle. Many serious accidents have been caused in this manner. The ladles that can be carried by hand are usually held in a frame as shown in Fig. 37. The ladle thus hung is usually carried by three men. One takes hold of the two handles of the fork at a. The other two take hold of the single bar at b. The object of this arrangement is to give the moulder, who carries at a, complete control of tilting the ladle and pouring of the metal. The lengths of the arms are proportioned to put approximately the same weight on each man. At the same time the moulder carries more than one-third of the weight because he is in a better position than the others to do so. Ladles small enough to be carried by one man have a single handle as at h.
Fig. 37 .