Sand casting

Sand casting is a method for shaping metal by pouring it into a mold made of sand. This sand, known as casting sand, serves as the mold material, and the term can also describe the finished object produced this way. The work happens in foundries, which are factories dedicated to casting. As of 2003, more than 60% of all metal castings worldwide were made using this technique.

Sand molds are inexpensive and can withstand the high temperatures needed for metals like steel. The sand is mixed with a binder, usually clay, and moistened with water or other substances to give it strength and flexibility. This mixture is packed into a frame called a flask, which holds the mold together. The mold’s cavity and channels are formed by pressing sand around a pattern, carving directly into the sand, or using 3D printing.

The basic process has five steps: first, a pattern is placed in sand to create a mold; second, the pattern and sand are set up with a gating system, and the pattern is removed; third, molten metal is poured into the mold cavity; fourth, the metal is left to cool; fifth, the sand mold is broken away to reveal the casting.

Patterns are built by a pattern maker from wood, metal, or plastic like expanded polystyrene. Because metal shrinks as it cools, and cooling can be uneven, the pattern must be larger than the final product—this extra size is called contraction allowance. Different metals have different shrinkage rates, so separate scaled rules are used for each. Patterns often include core prints, which are marks that hold sand cores in place inside the mold. These cores, sometimes reinforced with wire, create hollows or undercuts that cannot be formed by the main mold halves, such as the internal passages of valves or engine blocks. The runner system—consisting of a sprue, feeders, and in-gates—guides molten metal into the cavity. Gases and steam escape through the porous sand or through risers, which may be part of the pattern or added separately.

Molders also use tools to create holes in the sand, in addition to using patterns. The molding box, or flask, has two halves: the top is the cope, and the bottom is the drag. For a simple object flat on one side, the drag is filled with sand, packed down through a vibratory process called ramming, and leveled. The surface may be stabilized with a sizing compound. The pattern is placed on the sand, then the cope is added and more sand is rammed over it. A cover is put on, the box is turned and unlatched, the halves are parted, and the pattern, sprue, and vent patterns are removed. Any defects from removing the pattern are fixed, and the box is closed. This creates a “green” mold, which must be dried before receiving hot metal—otherwise, a steam explosion could fling molten metal. Sometimes sand is oiled instead of moistened, so casting can happen without drying. Chemical binders, like furane or amine-hardened resins, can also bond the sand. Additive manufacturing (3D printing) can produce the sand mold directly, skipping the need for a pattern and reducing lead times. It can also be used alongside traditional methods, for example, to make printed cores for a cavity formed by a conventional pattern.

To control how the metal solidifies, metal plates called chills can be placed in the mold. These cause rapid local cooling, creating a finer grain structure and sometimes harder metal in that area. In ferrous castings, this is similar to quenching in forge work. For instance, the inner surface of an engine cylinder is hardened with a chilling core. In other metals, chills promote directional solidification, helping to prevent internal voids or porosity.

Cores are used to create hollow cavities or internal features that a pattern alone cannot form. They are usually made of sand, though some processes use permanent metal cores. After the pattern is removed from the mold, cores are inserted into the casting box. Designers try to avoid cores when possible, because they add setup time, weight, and cost. Once the mold has the right moisture content, it is positioned for filling with molten metal—commonly iron, steel, bronze, brass, aluminum, magnesium alloys, or pot metal alloys containing lead, tin, and zinc.