Sand and molding sand are the basic raw materials for casting. The most commonly used casting sand is siliceous sand. When the high-temperature performance of silica sand cannot meet the use requirements, zircon sand, chromite sand, corundum sand and other special sand shall be used. In order to make the sand mold and core have certain strength and will not be deformed or damaged during handling, molding and pouring of liquid metal, molding sand binder is generally added in casting to bond the loose sand particles into molding sand. The most widely used molding sand binder is clay. Various dry or semi dry oils, water-soluble silicate or phosphate and various synthetic resins can also be used as molding sand binder. The outer sand mold used incan be divided into three types: Clay wet sand mold, clay dry sand mold and chemical hardening sand mold according to the binder used in molding sand and the way of establishing strength.
1.Clay wet sand
Clay and an appropriate amount of water are used as the main binder of molding sand. After making sand mold, it is directly combined and poured in wet state. Wet mold casting has a long history and is widely used. The strength of green sand depends on the clay slurry formed 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 mold closing and pouring. Therefore, the amount of clay and moisture in molding sand are very important process factors.
The invention relates to a casting method for producing sand castings by making a mold with molding sand and core sand as molding materials and filling the mold with liquid metal under gravity. 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, and the mold manufacturing is simple, it can adapt to the single piece production, batch production and mass production of sand casting. For a long time, it has been the basic process in casting production.
The mold used in sand casting is generally composed of outer sand mold and core. In order to improve the surface quality of sand castings, a layer of coating is often applied on the surface of sand molds and cores. The main components of the coating are powder materials and binders with high fire resistance and good high-temperature chemical stability. In addition, carriers (water or other solvents) and various additives that are convenient for coating are added.
The advantages of clay wet sand casting are:
① Clay is rich in resources and cheap.
② After proper sand treatment, most of the used clay wet sand can be recycled.
③ The manufacturing cycle of mold is short and the work efficiency is high.
④ The mixed molding sand can be used for a long time.
⑤ After tamping, the sand mold can still tolerate a small amount of deformation without damage, which is very beneficial to the drawing and core.
The disadvantages are:
① During sand mixing, viscous clay slurry shall be coated on the surface of sand particles, and high-power sand mixing equipment with kneading function shall be used, otherwise it is impossible to obtain molding sand with good quality.
② Because the molding sand has high strength after mixing, the molding sand is not easy to flow and difficult to pound. Manual molding is laborious and requires certain skills, and the equipment is complex and huge when molding with machine.
③ The rigidity of mold is not high, and the dimensional accuracy ofis poor.
④ Sand mold castings are prone to sand flushing, sand inclusion, pores and other defects.
The wet state moisture of the molding sand used for making this kind of sand mold is slightly higher than that of the molding sand used for green molding. Clay sand core a simple core made of clay sand.
2.Clay dry sand
The wet moisture of the molding sand used for making this sand mold is slightly higher than that of the molding sand used for green molding. After the sand mold is made, the surface of the mold cavity shall be coated with fire-resistant coating, and then placed in the oven for drying. After it is cooled, it can be combined and poured. Drying clay sand mold takes a long time and consumes a lot of fuel, and the sand mold is easy to deform in the drying process, which affects the accuracy of sand mold castings. Clay dry sand mold is generally used to manufacture steel castings and large iron castings. Since the chemical hardening sand has been widely used, the dry sand mold has tended to be eliminated.
3.Chemically hardened sand mold
The molding sand used in this sand mold is called chemically hardened sand. Its binder is generally a substance that can undergo molecular polymerization and become a three-dimensional structure under the action of hardener. Commonly used are various synthetic resins and water glass. There are basically three ways of chemical hardening.
① Self hardening: both binder and hardener are added during sand mixing. After making sand mold or core, the binder reacts under the action of hardener, resulting in self hardening of sand mold or core. Self hardening method is mainly used for molding, but it is also used to manufacture large cores or cores with small production batch.
② Aerosol hardening: when mixing sand, add binder and other additives without hardener. After molding or core making, blow in gaseous hardener or liquid hardener atomized in gaseous carrier to disperse it in sand mold or core, resulting in sand mold hardening. The aerosol hardening method is mainly used for core making and sometimes for making small sand molds.
③ Heating hardening: when mixing sand, add binder and latent hardener that does not work at room temperature. After the sand mold or core is made, it is heated. At this time, the latent hardener reacts with some components in the binder to produce an effective hardener that can harden the binder, so as to harden the sand mold or core. The heating hardening method is mainly used for core making except for small thin shell sand mold.