When particularly clean castings of medium weight are required, some form of skimming gate should be used. Fig. 24 illustrates one of several practical forms. They all depend for their efficiency upon the principle cited. In the illustration, a is the pouring basin and runner, b is a good sized riser placed about 3 or 4 inches from a, and c is a channel cut in the cope joint, connecting these two. The gate d should be cut in the drag side of the joint, just under the riser but at a right angle with the direction of c. The metal rushing down the runner is checked by the small size of the gate and so washes any dirt or slag up into the large riser 6. The level of metal in this riser must be sustained by sufficiently rapid pouring until the mold is filled.
Fig. 23. Gate.
In bench work and floor work, the greatest care must be used to have all parts of the gate absolutely free from loose sand or facing which would wash into the mold with the first flood of metal. On heavy work special skimming gates are not used, for the capacity of the pouring basin is very much greater than that of the runners which can be quickly flooded and thus retain the slag. Besides this, large risers are set at the sides or directly upon the casting, to receive any loose sand or facing that washes up as the mold is being filled. Fig. 22 illustrates this type.
As regards the filling of the mold quickly and quietly, these two conditions are closely allied. The shape and thickness of the casting are the important factors in determining the number and position of the gates. Aside from the fact that the gate should never be heavier than the part of the casting to which it attaches, the actual size of the gate opening is something that the molder must learn from experience.
In arranging gates with regard to the shape of the pattern, the following points should be borne in mind: Place gates where the natural flow of the metal will tend to fill the mold quickly. Usually gate on the lighter sections of the casting. Select such points on the casting that the gates may be broken and ground off with least trouble - the greater the number of castings to be handled, the more important this point becomes. A study of the molding problems given will illustrate this point.
Fig. 24. Skimming Gate.
Provide enough gates to fill all parts of the mold with metal of uniform temperature. This depends upon the thickness of the work, as is illustrated in Fig. 25 by two molds having the same shape at the joints but of different thicknesses. In thin castings the metal tends to chill quickly, so it must be well distributed. In this illustration, a is a plate 1/4 inch thick, and should have several gates. [A piece having the same diameter but heavier, would run better from one gate as at b, while if a bushing of this diameter is required, the best results would be obtained by gating near the bottom, as in Fig. 22. For running work at the bottom as shown in Fig. 22, the gate piece 6 is separate from the runner, and is slicked into the mold after the pattern is drawn. The runner r should extend below the level of the gate to receive the force of the first fall of metal, which otherwise would tend to cut the sand of the gate.
Melted metal shrinks as it cools, and this process begins from the moment the mold is filled. The surfaces next to the damp sand are the first to solidify, and they draw to themselves the more fluid metal from the interior. This process goes on until the whole casting has solidified. This shrinkage causes the grain in the middle to be coarse and sometimes even open or porous.
The lower parts of a casting are under the pressure or weight of all the metal above, and so resist these shrinkage strains. The top parts, however, require the pressure of liquid metal in gates or risers to sustain them until they have hardened sufficiently to hold their shape, or they will sink as indicated by the section, Fig. 26. Risers, mentioned in connection with securing clean metal, are also required on heavy pieces to prevent this distortion and to give sound metal. When used in this way they are called shrinkage heads or feeders. They should be 6 or 8 inches in diameter, so as to keep the iron liquid as long as possible, and should have a neck 2 or 3 inches in diameter, to reduce the labor required to break them from the casting in cleaning. To prevent the metal in this neck from freezing, an iron feeding rod is inserted, as in Fig. 27, and is churned slowly up and down. This insures fluid metal reaching the interior. As the level in the feeder lowers, hot metal should be added from a hand ladle. Pressure in Molds. The subject of the pressure of liquid iron mentioned repeatedly in the foregoing pages, must be dealt with by the molder, in weighting his copes, strengthening flasks, securing cores, etc., and most frequently in connection with the first-of these.
Fig. 25. Use of Gates.
Molten iron acts in accordance with the same natural laws that govern all liquids - as, for example, water (see Mechanics, Part II); iron, however, is 7.2 times heavier than water. The two laws applicable in foundry work are these: (1) Liquids always seek their own level; (2) pressure in liquids is exerted in every direction.