Chaplets should be used to support parts of cores which cannot be entirely secured by their prints which are held in the sand of the mold. In Fig. 20 are shown the three principal forms of chaplets used, and how they are set in the mold; a is a stem chaplet; b is a double-headed or stud chaplet; and c is a form of chaplet made up of strip metal.
Fig. 20. Chaplets.
That portion of the chaplets which is to be bedded in metal is tinned to preserve it from rusting, because rusty iron will cause liquid metal to blow. For small cores nails are often employed for this purpose, but only new ones should be used. With the stem chaplets the tails must be cut off when the casting is cleaned - the stud chaplet becomes entirely embedded in the metal. There are now manufactured and on the market many different styles of chaplets. In selecting the size and form for a given purpose the head of the chaplet should be large enough to support the weight of the core without crushing into the sand and thin enough to fuse into the liquid metal. The stem must be small enough to fuse well to the metal and stiff enough, when hot, not to bend under its load.
In the section on Sands reference has already been made to gases which must be taken off from a mold when it is poured. There are three forms of these: (1) air, with which the mold cavity is filled before pouring; (2) steam, formed by the action of the hot metal against the damp sand during the pouring process; and (3) gases formed while the casting is cooling, from chemical reactions within the liquid metal and from the burning of organic matter, facings, core binder, etc., in the sands of the mold. It is of the greatest importance that these gases pass off quickly and as completely as possible. If they do not find free escape through the mold they are forced back into the liquid metal, making it boil or blow. This may blow the metal out through risers and runners, or simply form numerous little bubble-shaped cavities in the casting, called blowholes. These often form just below the skin of the casting and are not discovered until the piece is partially finished.
One cannot depend entirely upon the porosity of the molding sands, but must provide channels or vents for the escape of these gases. For light work a free use of the vent wire through the sand in the cope will answer all purposes.
On castings of medium weight, besides venting with the wire, risers are placed directly on the casting or just off to one side as shown in Figs. 19 and 21. These are left open when the mold is poured and provide mainly for the escape of the air from the mold.
Heavy castings that will take time to cool, and thus keep facings burning for a long time after the mold is poured, require venting on sides and bottom as well as top. Fig. 21 shows side vents aaaa connecting with the air through the channel bbb cut along joint and risers ccc passing through the cope. At the bottom the vents connect with cross-vents dd run from side to side between the bottom board and edge of flask. Fig. 22 shows a mold bedded in the floor; the side or down vents connect at the top, as in previous examples, and at the bottom with a cinder bed about 2 inches thick, rammed over entire bottom of pit. The gases find escape from this cinder bed through a large gas pipe.
In pouring, the gas from vents should be lighted as soon as may be. The burning at the mouths of vents helps to draw the gases from below and also keeps the poisonous gas out of the shop.
It is customary to keep risers closed with small cover plates when large castings are being poured so that the air in the mold will be compressed as the metal rises in the mold. This helps sustain the walls of the mold and forces the vents clear so that they will act more quickly when the mold is full. These covers are removed occasionally to watch the progress of pouring, and are entirely removed when the metal enters the risers.
Gating is the term applied to the methods of forming openings and channels in the sand by which liquid metal may enter the mold cavity. The terms sprues and runners are also used in the same connection in some shops.
There are practically three parts to all gates: pouring basin; runners; and gate, as seen in Fig. 21. The runner is formed by a wooden gate plug made for the purpose. The pouring basin is shaped by hand on top of the cope, and the gate proper is cut along the joint surface by means of a gate cutter. In all cases the gate section should be smaller than any other part so that, when pouring, the runner and basin may be quickly flooded; also that the gate when cold will break off close to the casting and lessen the work of cleaning.
The object of gating is to fill the mold cavity with clean metal - to fill it quickly, and while filling, to create as little disturbance as possible in the metal.
The impurities in liquid metal are lighter than the metal itself, and they always rise to the top when the melted metal is at rest or nearly so. Advantage is taken of this important property to accomplish the first of the objects mentioned.
Fig 23 shows a good type of gate to use on light work. For reasons given, the point a should have the smallest sectional area. This section should be wider than it is deep as shown at 6, because the hot iron necessary for light work runs very fluid.
The runner should not be more than 5/8 to 3/4 inch in diameter. The pouring basin should be made deepest at point c, and slant upward crossing the runner. When pouring, the stream from the ladle should enter at c, flood the basin at once, and keep it in this condition. The current of the metal will then tend to hold back the slag, allowing clean metal to flow down the runner.