Introduction of Sand Casting Raw Materials
The basic raw materials for making sand molds are foundry sand and molding sand binder. The most commonly used foundry sand is siliceous sand. When the high temperature performance of silica sand cannot meet the requirements, special sands such as zircon sand, chromite sand and corundum sand are used. In order to make the sand mold and core have a certain strength and not deform or damage during handling, molding and pouring of liquid metal, a molding sand binder is generally added in the casting to bond the loose sand particles to become molding sand. The most widely used molding sand binder is clay, and various drying oils or semi-drying oils, water-soluble silicates or phosphates, and various synthetic resins can also be used as molding sand binders. The outer sand mold used in sand casting is divided into three types: clay wet sand mold, clay dry sand mold and chemical hardened sand mold according to the binder used in the molding sand and the way of building its strength.
clay wet sand
Clay and an appropriate amount of water are used as the main binder of the molding sand, and the sand mold is directly formed and poured in the wet state after it is made. Wet casting has a long history and is widely used. The strength of green sand depends on the clay slurry made by mixing clay and water in a certain proportion. Once the molding sand is mixed, it has a certain strength. After being pounded into a sand mold, it can meet the requirements of molding and pouring. Therefore, the amount of clay and moisture in the molding sand are very important process factors.
A casting method that uses molding sand and core sand as molding materials to make a mold and fills the mold with liquid metal under gravity to produce castings. Steel, iron and most non-ferrous alloy castings can be obtained by sand casting. Because the molding materials used in sand casting are cheap and easy to obtain, the casting mold is simple to manufacture, and can be adapted to the single-piece production, batch production and mass production of castings, and has been the basic process in casting production for a long time.
The mold used in sand casting is generally composed of an outer sand mold and a core. In order to improve the surface quality of castings, a layer of paint is often applied to the surface of the sand mold and core. The main components of the coating are powdery materials and binders with high refractoriness and high chemical stability at high temperature. In addition, carriers (water or other solvents) for easy application and various additives are added.
The advantages of clay wet sand casting are:
①Clay is rich in resources and cheap.
② Most of the used clay wet sand can be recycled and reused after proper sand treatment.
③ The cycle of making the mold is short and the work efficiency is high.
④The mixed molding sand can be used for a long time.
⑤ After the sand mold is pounded, it can still tolerate a small amount of deformation without damage, which is very beneficial to drafting and core lowering.
The disadvantages are:
①When mixing sand, the viscous clay slurry should be coated on the surface of the sand grains, and a high-power sand mixing equipment with a kneading effect should be used, otherwise it is impossible to obtain molding sand of good quality.
②Because the molding sand has a very high strength after mixing, the molding sand is not easy to flow during molding, and it is difficult to pound it. It is laborious and requires certain skills when molding by hand. When molding with a machine, the equipment is complicated and huge.
③ The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor.
④ Castings are prone to defects such as sand flushing, sand inclusion, and pores.
Clay dry sand mold The wet moisture of the molding sand used in the manufacture of this sand mold is slightly higher than that of the wet molding sand.
Clay Sand Cores Simple cores made from clay sand.