Raw materials for green sand in sand casting

Green sand is composed of original sand, clay, additives and water in a certain proportion. The commonly used feeding sequence is to mix the recycled sand, new sand, clay powder, coal powder and other dry materials, and then add water to mix them to the required water content. The formula of molding sand shall determine the performance range of molding sand according to casting alloy type, casting characteristics and requirements, molding method and process, cleaning method and other factors. Then, it shall be determined according to the variety and specification of various molding raw materials, sand treatment method and equipment performance, sand iron ratio and other factors.

(1) Quartz raw sand (please refer to casting technical data – casting technology – casting process parameters – casting molding materials – casting sand)

The most widely used raw sand in foundry production is natural silica sand with quartz as the main mineral component. Natural silica sand is rich in resources, widely distributed, easy to exploit, low in price, and can meet the requirements of most cases in foundry. In production, the chemical composition and mineral composition of the original sand are usually selected according to the alloy type, quality and wall thickness of the casting. For example, the casting temperature of the cast steel is as high as 1500 ℃, the carbon content of the molten steel is relatively low, and there is no strong reducing atmosphere in the mold cavity which can prevent metal oxidation. The metal on the interface contacting with the mold is easy to oxidize to form FeO and other metal oxides, so it is easy to react with the impurities in the molding sand and cause chemical sand sticking. Therefore, it is required that the SiO2 content in the original sand should be high and the harmful impurities should be strictly controlled. The higher the pouring temperature and wall thickness of the steel casting, the higher the SiO2 content of the original sand is required.

(2) Non quartz primary sand

Non quartz primary sand refers to the primary sand with no or only a small amount of free SiO2 in the mineral composition. Although silica sand is widely used because of its wide source and low price, which can meet the production requirements of general cast iron, cast steel and non-ferrous alloy castings, there are still some disadvantages of silica sand: large thermal expansion coefficient, low thermal diffusivity, low thermal storage coefficient, easy to play a role with iron oxide, etc. All of these will have an adverse effect on the interface reaction between mold and metal. In the production of high alloy copper castings or large-scale steel castings, the use of sand mold made of silica sand is easy to cause sand sticking defects, which makes the cleaning of castings very difficult. In the process of sand cleaning, workers are prone to silicosis if they inhale silica dust for a long time.

In order to improve the surface quality of castings and improve the working conditions, some non quartz raw sand has been gradually used in the production of cast steel to prepare molding sand, core sand or coating with inorganic and organic chemical binders. Compared with silica sand, most of these materials have the advantages of high fire resistance, thermal conductivity, thermal diffusivity and heat storage coefficient, low coefficient of thermal expansion and even expansion, low reaction ability with metal oxide, etc., which can obtain castings with high surface quality and improve sand cleaning conditions. However, some of these materials are expensive and scarce, so they should be selected reasonably. At present, the available non quartz primary sands include olivine, zircon, chromite, limestone, magnesia, corundum, ilmenite, bauxite and so on.

(3) Clay (binder data query)

Clay is the main binder of green sand. The clay has cohesiveness and plasticity after being wetted by water, and hardens after drying with dry strength. However, the hard clay can restore the cohesiveness and plasticity after adding water, so it has better reusability. Clay is widely used because of its rich resources and low price.

Clay is mainly composed of fine crystalline clay minerals. According to the crystal structure, it can be divided into kaolinite group, including kaolinite, pearl clay, dickite, halloysite, etc.; montmorillonite group, including montmorillonite, bedite, chlorite, saponite, etc.; illite group, including illite, glauconite, etc.

According to the different types of clay minerals, foundry workers usually divide the clay into two types: fireclay and bentonite. Bentonite is mainly composed of montmorillonite minerals, which is mainly used as molding sand binder for wet casting. The clay for casting mainly contains kaolinite or illite minerals.

(4) Accessories (auxiliary material data query)

In addition to raw sand, clay, water and other materials, some additives such as coal powder, residual oil and starch are usually added to the molding sand, so as to make the molding sand have specific properties and improve the surface quality of castings.

Adding pulverized coal to green sand for cast iron and non-ferrous alloy can prevent sand sticking on the surface of castings and improve the surface smoothness of castings. The content of pulverized coal in the sand used for green cast iron is usually in the range of 3-8% (mass fraction), which varies according to the size and thickness of the casting.

Substitute materials of coal powder include solid asphalt or its emulsion, residual oil or coal tar or its emulsion, mixed powder or slurry of bentonite and asphalt or other petroleum derivatives, synthetic polymer and starch dispersed in solid or water, etc. The starch content of cast iron surface sand (calculated by mass fraction) is generally about 0.5%, and that of cast steel surface sand is about 0.5-1.0%.

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