Each design for casting may be said to demand individual treatment, and the molder must select that method out of the many which alone, perhaps, can be successful. The subject is such a broad one that little will be here attempted further than to give by description and illustration the predominant points of the making of molds and castings. A simple, typical case of bench molding will be taken, that the relation of pattern, mold, core and casting may be clear.
The molding sands used are usually natural sands which contain greater or lesser amounts of clay, which, when moist, acts as a “binder” of the grains of sand. When used without drying, the mold is said to be a “green sand” mold; if dried, a “dry” or “baked” mold, as the case may be. The majority are “green sand” molds.
For the usual casting of which only a few or several duplicates are wanted, the “split” pattern is generally the most convenient.
The two halves of the mold, the “cope” (top) and the “drag” (bottom), are separately made in the two parts of the “flask” or molding box by “ramming” properly selected and “tempered” (moistened, mixed, and sieved) sand over the halves of the pattern. Of these, the drag is made first over the lower half of the separable pattern placed flat side down upon a bottom board. After “ramming,” i.e., packing the sand, just hard enough but not too hard, this half mold is reversed and the top half of the pattern placed upon the lower half, now at the upper face of the drag and flat side up. A little “parting powder” or fine, dry sand is sprinkled over the fresh surface of the half mold so that the upper half, next to be made, will not stick to the lower half, but can be lifted off at the proper time.
It May Be Bent Double Readily Without Breaking. (Fig. B)
The cope half of the two-part “flask” is now put on, filled and rammed with sand as was the drag. Any extra sand is scraped off with a straight edge and at the proper place a hole is cut with the “sprue cutter” straight down through the cope to the “parting.” More commonly, perhaps, this “sprue” hole is made by withdrawing a “sprue” stick (of wood) about which sand had been packed during the making of the cope. It is through this hole that the molten metal will be poured into the mold.
Because of Its Ability to Withstand Bending and 180–Degree Twists It Is Often Jocularly Referred to as the “Rubber Casting.” (Fig. C)
Lifting the cope or top half, it is turned upside down, and, after cutting in the drag the “runner” or “gate” connecting the “sprue” hole with the casting, the halves of the pattern are carefully drawn that the sand may not be disturbed. Now in the cavity left in the drag, to make the hole in the casting, is hung the baked “core” of sand, held together by flour or rosin or a “drying” oil. The cope is carefully replaced upon the drag, thus “closing” the mold.