Different Types of Foundry Sand
In the 17th century, China had used silica sand as a molding material to make castings such as bells, mirrors, pots and artillery. However, the early used mostly natural clay-containing silica sand, namely mountain sand and river sand. The natural sand has good plasticity and can be directly used to make molds and cores, which is suitable for the conditions of manual production at that time.
After entering the industrialized large-scale production, especially the modeling mechanization, the natural clay-containing silica sand showed several problems.
①Poor performance uniformity;
② The quality of the molding sand is difficult to control;
③ It cannot meet the process requirements.
Therefore, the natural silica sand with low mud content and artificial silica sand made by crushing silica were adopted. At the same time, the use of various non-siliceous sands has also been expanded.
The application and development of resin sand modeling and core making technology puts forward higher requirements on the quality of foundry sand, such as less fine powder, small specific surface area, and low acid consumption. In addition, there are new requirements for the size, shape and particle size distribution of sand particles. Some countries lacking high-quality sand sources have also developed silica sand washing technology to improve the grade and quality of silica sand.
Foundry sand should meet the following requirements:
1. Higher purity and cleanliness. Taking silica sand as an example, the content of SiO2 for cast iron sand should be more than 90%. And for larger steel castings, the content of SiO2 should be more than 97%.
2. High Refractoriness and thermal stability.
3. Suitable particle shape and particle composition.
4. Not easily wetted by liquid metal.
5. Cheap and easy to obtain.
The particle shape and particle composition of foundry sand have an impact on the fluidity, compactness, air permeability, strength and resistance of liquid metal permeability of the sand. Both the partible shape and composition are important indicators of the quality of foundry sand.
There are generally three types of particle shapes of foundry sand.
1. Round sand: The particles are round or close to round, with smooth surface and no prominent edges and corners.
2. Polygonal sand: The particles are polygonal, and most of them have obtuse angles.
3. Sharp-angled sand: The particles are sharp-angled, and most of them are acute.
generally, the angle coefficient (the actual specific surface area of the sand/theoretical specific surface area of the sand) decide the particle shape of foundry sand.
The sieve number determine the particle composition of sand. The measurement method is to pour the dry water-washed sand to remove the mud and dry it into a standard sieve, and then put it on the sand sieve for sieving. After separation, weigh the sand remaining on each sieve separately. Usually, the first and last sieve numbers of the three adjacent sieves with the most concentrated sand after sieving with a standard sieve represent the particle composition.
According to the different mineral composition, foundry sand consist of two categories: silica sand and non-siliceous sand.
Silica sand of whose main mineral composition is SiO2.
1. Natural silica sand: used for molding sand and core sand for non-ferrous alloy, iron and medium or small steel castings.
2. Selected natural silica sand: used for all kinds of molding sand and core sand using organic matter as a binder.
3. Artificial silica sand: molding sand and core sand for steel castings.
Non-siliceous sand: there are many types of non-siliceous sand, with different uses.
1. Limestone sand: Made of crushed limestone, the main mineral composition is CaCO3. Mainly used for molding sand and core sand of steel castings.
2. Zircon sand: The main mineral composition is ZrO2·SiO2. Mainly used for the core sand or surface sand of large steel castings and alloy steel parts. Its powder can be used as a coating.
3. Magnesia sand: The main mineral composition is MgO. Mainly used for the surface sand and core sand of high manganese steel castings. Its powder can be used as a coating.
4. Chromite sand: The main mineral composition is FeO·Cr2O3. Mainly used for the surface sand and core sand of large or special steel castings. Its powder can be used as a coating.
5. Corundum sand: The main mineral composition is α-Al2O3. Mainly used as a shell material for investment and ceramic mold casting.
6. Olivine sand: The main mineral composition is (MgFe)2SiO4. Mainly used for molding sand and core sand for iron castings, non-ferrous alloy castings and high manganese steel castings.