The basic raw materials of sand mold manufacturing are casting sand and molding sand binder. 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 in sand casting can 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.
Clay green sand mold takes clay and an appropriate amount of water as the main binder of molding sand. After making sand mold, it is directly combined and poured in the 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 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. Disadvantages: ① when mixing sand, 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 the mold is not high, and the dimensional accuracy of the casting is poor. ④ Castings are prone to sand flushing, sand inclusion, pores and other defects.
At the beginning of the 20th century, the foundry industry began to mix sand with roller mixer, which greatly improved the quality of clay green sand. The new high-power sand mixer can make the sand mixing work achieve high efficiency and high quality. The appearance of shock compaction molding machine mainly based on vibration has significantly improved the compactness and uniformity of the mold. With the improvement of dimensional accuracy and surface quality requirements of castings, high-pressure molding machine based on compaction appears again. Making clay wet sand mold with high-pressure molding machine can not only improve the dimensional accuracy and surface quality of castings, but also simplify the action and shorten the cycle of compacting the mold, and realize the high-speed and automation of the whole process of molding and mold closing. Using the thixotropy of clay slurry, the new molding machine with gas impact pressurization can obtain a very tight mold by instantaneously applying a pressure of 0.5 MPa. These developments are important conditions for clay wet sand casting to meet the requirements of modern industry. Therefore, this traditional process has been used to produce a large number of high-quality castings.
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. 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 casting accuracy. 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.
Chemical hardening sand mold the molding sand used in this sand mold is called chemical hardening 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.
The characteristics of the chemical hardening sand mold casting process are: ① the strength of the chemical hardening sand mold is much higher than that of the clay sand mold, and after the sand mold is made, the film is removed after hardening to a fairly high strength, so there is no need to repair the mold. Therefore, the mold can accurately reflect the size and outline shape of the pattern, and it is not easy to deform in the later process. The dimensional accuracy of the casting is high. ② Because the viscosity of the binder and hardener used is not high, it is easy to mix with sand particles. The sand mixing equipment has the advantages of light structure, low power and high productivity. The work part of sand treatment can be simplified. ③ The mixed molding sand has good fluidity before hardening, and the molding sand is easy to pound during molding, so there is no need for a huge and complex molding machine. ④ When molding with chemically hardened sand, pattern materials such as wood, plastic and metal can be selected according to production requirements. ⑤ The content of binder in chemically hardened sand is much lower than that of clay sand, and there is no powdery auxiliary material. If raw sand with the same particle size is used, the gap between sand particles is much larger than that of clay sand. In order to avoid metal infiltration between sand particles during casting, the surface of sand mold or core shall be coated with high-quality coating. ⑥ The chemically hardened sand using glass as binder has the advantages of low cost and odorless working environment. However, after pouring metal, the molding sand is not easy to collapse; The used old sand can not be recycled directly, but must be recycled, and the regeneration of sodium silicate sand is more difficult. ⑦ The cost of chemically hardened sand with resin as binder is high, but the casting is easy to be separated from molding sand after pouring, the workload of casting cleaning is reduced, and most of the used sand can be recycled.
Core in order to ensure the quality of castings, the core used in sand casting is generally dry core. The clay core and oil core sand are divided into several kinds of binder according to the resin core.
Clay sand core a simple core made of clay sand.
The oil sand core is made of core sand with dry oil or semi dry oil as binder, which is widely used. The viscosity of oil is low, the fluidity of mixed core sand is good, and it is easy to compact when making core. However, the strength of the newly made core is very low. Generally, it should be carried out with a profiling core support plate, and then dried in an oven at 200 ~ 300 ℃ for several hours. The oil is oxidized by air to harden it. The disadvantages of this core making method are: the core is easy to deform in the process of demoulding, handling and baking, resulting in the reduction of dimensional accuracy of castings; Long baking time and high energy consumption.
Resin sand core various cores made of resin sand. After the core is hardened in the core box, it can be taken out to ensure the correct shape and size of the core. According to different hardening methods, the manufacturing of resin sand core is generally divided into two methods: hot core box core making and cold core box core making. ① Hot box coremaking: it appeared in the late 1950s. Furan resin is usually used as core sand binder, and latent hardener (such as ammonium chloride) is also added. When making the core, keep the core box at 200 ~ 300 ℃, after the core sand is injected into the core box, ammonium chloride reacts with free formaldehyde in the resin at a higher temperature to form acid, so as to harden the core quickly. It takes about 10 ~ 100 seconds to establish the demoulding strength. Using hot core box to make core, the dimensional accuracy of core is relatively high, but the process device is complex and expensive, the energy consumption is much, the irritant gas is discharged, and the working conditions of workers are also very poor. ② Cold box coremaking: appeared in the late 1960s. Urethane resin is used as core sand binder. When this method is used, the core box is not heated, and the core can be hardened by blowing amine steam into it for a few seconds. This method is superior to the hot core box method in energy, environment and production efficiency. In the mid-1970s, the furan resin cold core box method hardened by blowing sulfur dioxide appeared again. The hardening mechanism is completely different from the urethane cold box method, but the process characteristics, such as fast hardening and high core strength, are roughly the same as the urethane cold box method.