Sand casting can be broken down into three general processes: green-sand molding, dry-sand molding, and pit molding. Green-sand molding, in all probability, produces the largest tonnage of castings but is limited in that long, thin projections are very difficult to cast. Dry-sand molds, on the other hand, can produce castings of any desired intricacy (within the normal dimensional limits of the process) because the sand is baked with a binder, increasing the strength of the thin mold sections. Pit molding is the sand process used to make very large, heavy castings.
The Process Green-Sand Molding
The essential parts of a green-sand mold are shown in Figure S.1. The pattern is placed in the flask (consisting of the cope and drag) and molding sand is tightly rammed around it. Usually, the mold is made in two halves, with the pattern lying at the parting line. Removing the pattern from the rammed sand leaves the desired cavity from which the casting is produced. The molder then produces a sprue for pouring metal into the cavity and an opening for a riser to permit air in the cavity to be expelled. Both the sprue and riser also act as a source of hot metal during solidification and help eliminate shrinkage cavities in the casting. Cores can be added to the mold cavity to shape internal casting surfaces.
FIGURE S.1 Cross-section of a sand casting mold.
Among the advantages of green-sand molding are the following: one pattern can be used to produce any number of castings; most non-ferrous alloys, gray irons, ductile irons, malleable irons, and steel can be cast; and finally, the fragility of the sand cores permits them to collapse after metal is cast around them, thereby eliminating or reducing casting stresses and tendency for hot tearing.
The limitation of the process is its inability to support long, thin projections in the sand.
Core boxes, not patterns, are used to make the various parts of the mold. A mixture of sand and a binder is formed in a core box acid baked at 204 to 260°C to harden the sand. The various pieces are then assembled into a mold.
Dry-sand molds, because of the way in which they are produced, can be used to make intricate castings. The baking operation strengthens the sand and permits thin projections to be cast without danger of collapsing the mold walls. Cores used in dry-sand molding are collapsible and help reduce hot tearing tendencies.
If large, intricate parts must be produced, pit molding is considered the most economical production method. (Castings weighing up to 230,000 kg have been produced in pit molds.) Pit molding is a highly specialized operation and the equipment used — sand slingers, molding machines, etc. — precludes the use of much hand labor. The mold usually is dried, increasing sand strength and, consequently, the ability to resist mold erosion during pouring as well as the weight of the casting being poured.