Base sand is the material to be prepared in the first step for making sand molds. To bond the base sand together, a binder is needed. The most common types of base sand are:
1. Silica Sand
Silica (SiO2) sand is the most commonly used sand. It is made by crushing sandstone or from beaches and river beds. For high melting point castings, such as steel, at least 98% pure silica sand should be used. But for lower melting metals, such as cast iron and non-ferrous metals, lower purity sand (purity between 94% and 98%) can be used.
2. Olivine Sand
Olivine is a mixture of orthosilicates of iron and magnesium from pure peridotite in mines. It does not contain silica, so it can be used with alkaline metals (such as manganese steel). And because of its low thermal expansion, high thermal conductivity, and high melting point, it is safer than silica, so it is very popular in Europe.
3. Chromite Sand
Chromite sand is a solid solution of spinel. It is characterized by low silica content, a high melting point (1,850 ° C (3,360 ° F)), and high thermal conductivity. But because the material cost is generally high, it is often used for expensive alloy steel castings and cores.
4. Zircon Sand
Zircon sand is a mixture of approximately two-thirds of zirconia (Zr2O) and one-third of silica. It has the highest melting point of all base sands at 2600 ° C (4710 ° F), extremely low thermal expansion, and high thermal conductivity. Because of these good properties, it is usually used when casting alloy steel and other expensive alloys.
5. Chamotte Sand
Chamotte sand has a melting point of 1,750 ° C (3,180 ° F) and low thermal expansion. It is an affordable material in all types of base sand, but it is still twice as expensive as silica. Due to its coarse grain structure, it will result in poor surface finish and is limited to dry sand molding. Mold cleaning can overcome surface finish problems. Chamotte sand is usually used when casting large steel workpieces.