It is worth noting that today, with the vigorous development of various chemical bonded sands, clay green sand is still the most important molding material, and its wide application range and large consumption are unmatched by any other molding material. It is reported that more than 80% steel castings in the United States are made of clay green sand. In Japanese steel castings, more than 73% is clay wet sand. The ability to adapt to molding conditions is extremely strong, which is also a major feature of clay green molding sand. 1890, the vibration compaction molding machine came out, and the clay green molding sand which has been molded by hand for a long time was very successful in machine modeling, which laid the foundation for the mechanization and automation of molding operations. Modern new technologies, such as high pressure molding, injection molding, air impact molding, static molding and non-impact vacuum pressure molding, are all based on the use of clay green molding sand. The implementation of various new technologies makes the status of clay green sand in casting production more important, and also makes clay green sand face many new problems, which makes our research on clay green sand continuously strengthen and deepen. Nowadays, with the rapid development of science and technology, the demand for castings in various industrial sectors is increasing, and the requirements for casting quality are also getting higher and higher. In modern foundries, the productivity of molding equipment has increased to an unprecedented level. If the performance of molding sand can not fully adapt to specific production conditions, or its stability and consistency can not be effectively controlled, the foundry may be buried in waste products in a short time. With the development of science and technology, foundries using clay green molding sand are generally suitable for their own specific conditions, including: the treatment of used sand, the addition of new sand and auxiliary materials, the mixing of molding sand, and the monitoring of molding sand performance. There are many changing factors in the clay wet sand system. If one or several key performances cannot be kept within the control range, problems may occur in production. An effective molding sand treatment system should be able to monitor the performance of molding sand and correct problems in time. It is impossible to formulate a general control method because of the different arrangement of sand treatment systems and different equipment used in various foundries. Here, I intend to propose some control points that have been widely recognized at present. After carefully understanding these points, each foundry can determine feasible control methods according to its own specific conditions. Moreover, with the progress of technology and the actual capacity of the factory (including personnel and funds), the control of the molding sand system should be continuously improved.

Calculation and comparison: the specific heat of molding sand is about 9.22× 102J/kg℃, the specific heat of water is 4. 19× 103J/kg℃, and the vaporization heat of water is 2.26× 106J/kg, 65000. The heat that can be taken away is 4.19×103×10× 30, that is, 12.57× 105J. When the temperature of 1 ton sand decreases 1℃, the heat dissipation needs to be 9.22× 102× 1000 J, that is, 9.22× 105 J, so 1% is added to the old sand. Evaporating water in 1 ton sand 1%( 10kg) can take away 2.26x107j of heat, but it can reduce the temperature of sand by 24.5℃. The above analysis shows that the cooling effect is poor simply by adding water to the belt conveyor or sprinkling water on the sand pile. Even blowing air on the sand surface after adding water can't improve much. After adding water, the water should be dispersed evenly in the molding sand, and then the sand should be blown loose to make the water evaporate quickly and eliminate the steam. At present, there are many varieties and specifications of molding sand cooling devices, mainly including cooling drum, double-disk cooler and cooling fluidized bed, all of which use water to evaporate and cool molding sand. The effect of cooling boiling bed is better.

2. Moisture control of used sand Almost all foundries check and control the moisture of mixed sand, but many foundry leaders and technicians still lack sufficient understanding of the importance of strictly controlling the moisture of used sand. The moisture content of used sand entering the mixed sand is too low, which may have no less influence on the quality of mixed sand than the sand temperature is too high. Experimental research and experience have proved that wetting dry bentonite with water is much more difficult than lubricating wet bentonite. Bentonite and water in molding sand are not simply mixed together, but rubbed to make them plastic. It's like making pottery with clay and water. The mixture of water and soil is loose and has no bonding ability. After each grain of soil is kneaded and fully absorbed, it becomes plastic, and then it can be shaped into a ceramic blank. After casting, due to the influence of molten iron, the soil-water bonding film on the surface of many sand particles is dehydrated and dried, and it is not easy to restore its plasticity by adding water. The water content of used sand is low, and the longer it takes to mix and grind with water in the sand mixer to achieve the required performance. Due to the limited sand mixing time in production, the lower the water content of used sand, the worse the comprehensive quality of mixed sand. At present, foundry workers all over the world have realized that the water content of used sand entering the sand mixer can only be slightly lower than that of mixed sand. It is best to add enough water in the cooling process of used sand, and the water content after cooling is slightly lower than that of mixed sand. In this way, there is still a long time from sand cooling to entering the sand mixer, and water can fully wet the bentonite on the surface of old sand particles. A better way is to set up a sand mixer in the system to premix the used sand and add water to the cooled used sand for premixing, so as to improve the mixing state of bentonite and water in the used sand. In foreign countries, some foundries add all the additives such as new sand, bentonite and pulverized coal when premixing. Tianjin Xinweixiang Foundry Company adopts German EiRich sand mixer for premixing. After the pre-mixed used sand enters the sand mixer, the amount of water added is very small, so it only needs to be adjusted slightly. Bentonite and water in molding sand are further mixed in the sand mixer, and the performance of molding sand is more stable and consistent.

3. The granularity of used sand for clay green sand casting iron parts and the granularity of molding sand should be fine. Because the amount of used sand is generally above 90% when mixing sand, the main factor determining the particle size of molding sand is used sand. The addition of new sand is very small, so it is impossible to change the particle size of molding sand by adding new sand. Therefore, the particle size of used sand should be detected frequently. When testing particle size, clean and remove mud after sampling (residual sand samples can be used when measuring mud content), dry and sieve. There are two requirements for particle size. The sand particles on the (1) 140 mesh sieve should be between 10- 15%. Retaining more fine sand can reduce the sand adhesion on the casting surface. Moreover, it will increase the number of bonding bridges between sand particles, thus reducing the brittleness of molding sand and avoiding sand washing defects. In addition, it is beneficial to improve the temperature strength, dry strength and humidification layer strength of molding sand after water migration. (2) The sum of 200 mesh sieve, 270 mesh sieve and fine sand on the chassis shall be as little as possible. Such fine sand has little effect on improving the surface quality of castings, but it will increase the water content of mixed sand and reduce the permeability of molding sand. The sum of fine sand should generally be less than 4%. 4. Content of water-absorbing fine powder The water-absorbing fine powder is mainly dead clay, including coking coal fine powder and other fine powders. The content of water-absorbing fine powder is not as low as possible, and it is better to control it between 2-5%. Water-absorbent fine powder will compete with bentonite for water when mixing sand, which will increase the water needed to achieve the target density of mixed sand. However, according to the current understanding, the water absorption capacity of water-absorbing fine powder is stronger than that of bentonite, but the water retention capacity is lower than that of bentonite. Therefore, the amount of water added in molding sand is slightly inappropriate, and the water-absorbing fine powder has a certain fine-tuning and stabilizing effect on the properties of molding sand. When the water content is high, fine powder absorbs water first, and bentonite can absorb water more stably and consistently; When the water in the mixed sand evaporates during transportation, the water absorbed by the water-absorbing fine powder evaporates first, while the water in the mud paste that binds the sand particles is relatively stable, so the performance fluctuation of the molding sand is small. Too high content of water-absorbing fine powder is not good, which will increase the water content of molding sand and easily lead to defects such as pinholes, rough surfaces and sand holes on castings. When the content of water-absorbing fine powder is too low, the properties (especially compactness) of molding sand are unstable.