Casting

the running of melted metal in a mold so as to produce a piece of metal having a desired shape. Casting is to be distinguished from stamping. Coins, for instance, are struck out of a sheet of metal by a heavy punch, and pressed into form by a die. Casting is done for the most part in foundries. Small castings may be made in trays of sand; but for castings of size a bed of sand, often many feet deep, is spread over the floor of the casting room. Casting sand is fine, free from gravel, and has a faculty of retaining an impression as of a footprint. It must be dried out thoroughly to prevent an explosion through the sudden formation of steam when hot metal is poured into it.

In order to make a casting it is necessary, first of all, to have a pattern, usually of wood, the exact size and shape of the desired object, from which to form a mold. In case of a pattern of plain form, it is thrust simply into the sand bed. The sand is rammed to make it fit up closely and stand firm. The pattern is pulled out and molten metal is poured in from a ladle. In case of a large casting the metal is allowed to run through a gutter from the melting pot to the mold. As soon as the iron begins to harden it is covered with sand so that the cooling may proceed slowly.

A more complex pattern like that for the castings of a school desk, for instance, could not be drawn out without disturbing the sand. For such castings a mold is made by means of two frames called flasks. The first frame is laid on a table, filled level full of casting sand rammed tightly; the pattern is then laid flat on the sand and pressed down till half its thickness is pressed into the surface. An empty frame is then set on the first frame and sand filled in and rammed down as before. A small hole is made through the upper sand reaching to the pattern. The upper frame is then lifted off, bringing its sand with it. The pattern is taken out and the upper frame is set back carefully. If all has gone well, a cavity or mold the exact shape of the pattern has been formed between the sand of the upper and lower flasks. The molten metal is now poured into the cavity through the hole provided for the purpose, and is allowed to cool. The two flasks are then separated and the casting is removed.

Castings may vary in weight from a fraction of an ounce to many tons. A huge iron casting lately turned out in a Pittsburg foundry measured 26 X 8 X 12 feet and weighed 234,000 pounds--117 tons. In case of a hollow casting, such as a kettle or an iron pipe, an inside mold called a core, and an outside mold or jacket, must be made that the iron may be poured in between them. Some notion of the method of producing such a mold may be gained from the article on the BELL.

See Cast; Sculpture; Magnet; Iron; Steel