Present Situation of Lost Foam Casting
With the continuous rise of lost foam casting, how to reduce the production cost of castings, improve the surface quality of castings and improve the yield of castings is the goal of every foundry enterprise. It is well known that the lost foam casting adopts the binder free dry sand vacuum technology, and the common casting defects are sand inclusion (sand hole), porosity, slag inclusion, "carbon black", scarring, acupuncture, etc. To solve this problem, the selection of molding sand is very important.
Traditionally, in order to reduce the cost of sand, people generally choose cheap silica sand or magnesium olivine sand, but these types of sand has the problems of low refractoriness, poor fluidity, poor permeability and so on, which will produce a lot of casting defects in the pouring process, especially in the alloy steel casting. In addition, silica sand or Magnesium Olivine sand will produce a large amount of dust in the subsequent sand treatment process, resulting in a very bad environment in the production workshop, and workers are prone to silicosis; At the same time, the amount of waste sand increases, the effective sand decreases, the reuse rate of sand is low, and it is not durable. Therefore, from a comprehensive perspective, the cost of sand increases.
Application of Fused Ceramic Sand in Lost Foam Casting
Fused ceramic sand is a spherical sand made from high-quality calcined bauxite, which is melted in an electric arc furnace at high temperature and then sprayed by the compressed air to make it recrystallized. Fused ceramic sand has many advantages, such as high refractoriness, good fluidity, strong air permeability etc. It can effectively solve the casting defects such as sand inclusion (sand hole), porosity, slag inclusion etc.
Several Main Excellent Properties of Fused Ceramic Sand
1. High Refractoriness
Fused ceramic sand is spherical particles, mainly composed of alumina (Al2O3), and its refractoriness can reach 1800 ℃. The main component of silica sand is silica (SiO2), and its refractoriness is less than 1700 ℃; Silica sand will have many kinds of crystals at different temperatures, which will reduce the refractoriness of molding sand again in the pouring process. The use of fused ceramic sand can significantly reduce the mechanical and chemical sand sticking, greatly reduce the labor intensity of sand cleaning, and is not easy to cause defects such as sand inclusion, sand flushing, porosity, etc.
2. Good Liquidity
Fused ceramic sand is a spherical particle with an angle coefficient ≤ 1.1, and its fluidity is very good and superior to other existing types (silica sand angle coefficient 1.3 ~ 1.45). Under the same compaction conditions, the compactness of the mold is improved, and good air permeability still maintained, which is helpful to improve the quality of the casting.
3. Good Air Permeability
The air permeability of molding sand mainly depends on the grain size, particle size distribution, binder type and other factors. During the filling process of lost foam, if the air permeability of molding sand is poor, a large amount of gas generated by the powerful heat radiation inside can’t be discharged immediately, which will lead to choking fire, resulting in a lot of defects in the casting, and even scrapping.
4. Low Thermal Expansion Rate
During the high temperature pouring process of castings, the thermal expansion of molding sand will cause slight changes in the size of molding sand, which will affect the accuracy of casting size. The thermal expansion coefficient of fused ceramic sand is very small (only 0.13%), which is 1/10 of silica sand (1.5% of silica sand), and 1/3 of Magnesium Olivine sand (0.3 ~ 0.5% of Magnesium Olivine sand), which is equivalent to zirconium sand and can effectively prevent the expansion defects of castings.
5. Environmental Protection and Good Recycling Performance
As silica sand and Magnesium Olivine sand are polygonal sand with low strength, the medium sand is easy to be broken and produce a lot of dust in the process of molding and sand processing. These dusts also pollute the production environment. The working environment is very poor. Working in such an environment for a long time is prone to silicosis. In addition, a lot of waste sand will be produced, resulting in the sand being not durable and the recovery rate being low. Using fused ceramic sand as filling sand, the energy consumption of molding is greatly reduced, the dimensional accuracy of castings is significantly improved, and the durability is excellent.
The comprehensive properties and advantages of fused ceramic sand and the actual application results show that it has broad prospects as the material for lost foam casting.