The cheapest quickest method of forming the general run of castings is by green-sand molding. Damp molding sand is rammed over the pattern, and suitable flasks are used for handling the mold. When the pattern is withdrawn the mold is finished, and the metal is poured while the efficiency of the mold is still retained by reason of this dampness. The mold may be poured as soon as made; and in case of necessity it may be held over a day or more depending upon its size. If the sand dries out, the mold should not be poured.
Core making supplements molding. It deals with the construction of separate shapes in sand which form holes, cavities, or pockets, in the castings. Such shapes are called cores. They are held firmly in position by the sand of the mold itself or by the use of chaplets. Core sand is of a different composition from molding sand. It is shaped in wooden molds called core boxes. All cores are baked in an oven before they can be used. The whole detail of their construction is so different from that of a mold, that core making is a distinct trade - a trade, however, that is generally considered a stepping stone to that of molding. Boys usually begin to serve their time in the core shop.
Dry-sand molding is the term applied to that class of work where a flask is used, but a layer of core sand mixture is used as a facing next to the pattern and the joint, and the entire mold is baked before pouring. This drives off all moisture and gives hard clean surfaces to shape the iron. It is used where heavy work having considerable detail is to be cast, or where the rush of metal or the bulk of it might injure a mold of green sand. Dry-sand molds are usually made up one day, baked over night, and assembled and cast the next day.
Loam work is the term applied to molds built of bricks carried on heavy iron plates. The facing is put on the bricks in the form of mortar and shaped by sweeps or patterns depending upon the design of the piece to be cast. All parts of the mold are baked, rendering the surfaces hard and clean. After being assembled, these brick molds must be rammed up on the outside with green sand in a pit or casing to prevent their bursting out under the casting pressure. Simple molds can be made up one day, assembled, rammed up and poured the next, but it usually takes 3 or 4 days and sometimes as many weeks to turn out a casting.
Loam work is used for the heaviest class of iron castings for which, on account of the limited number wanted, or the simplicity of the shape, it would not pay to make complete patterns and use a flask. In some cases the intricacy of the design makes a pattern necessary, and size alone excludes the use of sand and flasks.
No hard and fast rules exist for the selection of the method by which a piece will be molded. Especially with large work, the question whether it shall be put up in green sand, dry sand, or loam, often depends upon local shop conditions. The point to consider is: How can the best casting for the purpose be made for the least money, considering the facilities at hand to work with?