Exploring Sand Molding
In the metalcasting process, molten metal is poured into a mold and solidifies into a component. Sand is the most widely used material for mold making. To create the mold, sand grains are packed around a metal pattern that is roughly the shape of the desired metal part. Sand is available to the metalcasting facility in various sizes, shapes and chemical compositions, and metalcasting sand systems must conform to a standard of sizes and shapes in order to maximize mold properties and casting quality.
Silica sand is the most common type used in metalcasting due to its availability and price, but specialty sands such as zircon, chromite and olivine also are used. These sands provide better mold properties, which lead to higher quality castings.
To produce a mold, metalcasters can use one of two basic methods of making sand grains stick together. The less expensive of the two, “green sand,” mixes clay and water with the sand. Other constituents are added in small quantities to boost performance and minimize defects.
Sand grains also can be chemically bound by resins that harden in the presence of a catalyst. Chemically bound molds are stronger and typically produce castings with more precise dimensions than green sand molds. Most cores (which create hollow spaces in castings) and large mold assemblies are made using chemically bound sand.
Sand is defined as decomposed or crushed rock ranging from 0.002 to 0.083-in. in diameter. Sand grains can be round, angular or sub-angular in shape. Silica sand from lakes and dunes tends to be angular to sub-angular, while pure silica sand is typically round. Round sand grains have minimal surface area and need less binder to coat the surface than angular grains, but they expand more when a metalcasting mold is filled with liquid metal, which can lead to casting defects. Strength and permeability are other competing factors that must be measured in the molding process.
-The Cast Metals Institute, Schaumburg, Ill.