Casting Sand
- Tuna Silica Sand
- January 01, 2019
Casting sands are generally mineral grains in the size of 0.05-2 mm. There are 2 types of casting sand. These; natural sands and synthetic sands. Natural sands are used in the same way as they are obtained from nature. So they are not subjected to any treatment. The most usable feature of natural mold sand is that it can retain moisture for a long time. The worst part is that its features are variable (unstable). But synthetic sand is often used in the market.
Synthetic sands are sands that have been processed (washed, impurities removed) and sieved to the desired grain size and shape. Therefore, less storage is needed. Since synthetic sands show the same feature everywhere, they do not separate and can coexist. For the metal to be cast, sand is taken from here and molded. This means savings. However, the most important feature of synthetic sands is that it has high gas permeability and more refractory properties. In addition, synthetic sands are more stable than natural sands and the process is controllable.
The most important feature expected from casting sand is gas permeability. If the sand mold does not transmit the gas, the gases coming from the metal during casting will create compression in the mold and cause porosities and explosions.
The second most important parameter expected is strength. Casting sand should have both wet and dry strength. If the required strength cannot be achieved, the mold is broken down during casting or after casting. In order to increase the gas permeability, angular geometry sand should be used so that the required gas discharge spaces are formed. However, if this gap increases too much, the strength will decrease. There are German (DIN) and Turkish (TS) standards related to this ratio. The decrease in grain size and globalization of the grain shape will decrease gas permeability. Therefore, it is necessary to avoid them.
Casting sands are of 4 types:
These are; silica sand, zircon sand, chromite sand and olivine sand. Silica (SiO2) sand is the most used sand type in the market. Because it is the cheapest and it shows high refractoriness (2073 K). Zircon sand (ZrSiO4) has 2 times higher density (specific gravity) and high thermal conductivity than silica sand. This is the highest parameter that makes zircon sand important. If you have a casting material that you want to solidify quickly, ie the grain structure you want homogenous everywhere, you will use zircon sand. But zircon sand is more expensive than silica sand.
Chromium sand (FeCr2O4):
Density is higher and its thermal conductivity is higher than silica like zircon sand. In other words, the liquid metal temperature coming from the casting can be given to the environment immediately. This ensures fast cooling. The difference from zircon sand is that the heat expansion is very low. Although almost to the melting point, a very high increase in the volume of sand is not observed. This reduces casting errors. Olivine sand (MgSiO4) is a basic character sand. As the amount of Mg in it increases, its basicity also increases. Because Mg is basic in normal conditions. Therefore, austenitic high manganese steels can be poured with olivine sand (not reacted). In addition, its thermal conductivity is higher than quartz.