Influence of high sand temperature on green sand casting

Hot sand is defined as any high temperature sand that adversely affects sand preparation, molding and casting quality. Specifically, it refers to recycled sand at a temperature of 49 °C to 71 °C, which is hot enough to cause non-uniformity and control problems in sand mixing.

1. Why does hot sand cause problems
The bentonite in hot sand does not disperse and coagulate the same as the bentonite in cold sand. The data also show that the viscosity of the bentonite increases with the temperature of its binder. It is assumed that when the temperature of the binder increases, the thin layer of bentonite suspension particles arranges itself from the edge to the middle, forming an open structure. Compared with the binder solution in the cold water state, the thin layer of bentonite suspension particles remains face to face. The arrangement is quite different, with an open structure forming as the temperature increases, leading to several negative effects on the sand system. Most importantly, bentonite in hot sand is less able to retain water than cold sand, causing it to lose water more easily and degrade physical properties. According to this study, an interesting phenomenon occurs when hot sand is used: hot metal is poured into the hot sand mold, producing castings with defects associated with high water content in the sand, and the hot sand mold is cooled before pouring In the hot metal, the castings produced showed defects related to the low moisture content of the molding sand.

2. The influence of molding sand temperature on the physical properties of molding sand.
The moulding sand tests were carried out in a closed temperature and humidity controlled room and at temperatures of 21°C and 60°C, respectively. Except for the temperature change of the molding sand, all other conditions remained the same. The results show that the properties of molding sand are greatly degraded when tested at 60 °C. These studies show that: all process studies and data collected so far clearly define the green sand molding process.The critical temperature of moulding is 49 °C (120 u), problems related to hot sand. Hot sand affects every aspect of green sand moulding work and results in higher scrap rates, increased consumption of bentonite, or even complete loss of the system control.

3. The influence of hot sand on the rejection rate of castings
As far as the waste is concerned, most of the defects related to the molding sand have a great correlation with the high temperature of the molding sand. These defects include sand inclusion, surface roughness, metal infiltration, expansion box, sticky sand, formed by gas. of pinholes, pores, succulents, and mold breakage. Many of these defects are caused by the rapid loss of moisture from the mold surface. Hot molding sand has a number of adverse effects in terms of sand handling system operation. The hot sand that is usually reused, when entering the mixer, fluctuates greatly in temperature and humidity. The tests conducted by Sanzhuji in the iron foundry showed that the sand temperature fluctuated between 32 °C and 193 °C in different periods of the large hopper. Sudden changes in temperature during sand preparation
Will cause changes in the amount of water evaporation. This variation makes precise addition of moisture to the mixer and control of compaction difficult. The instability of the backup sand temperature will exacerbate the variation in physical properties between batches of molding sand. Uncontrollable dryness of the sand is also critical when transferring hot spare sand over long distances to multiple molding machines.

4. Influence of hot sand on sand regeneration system
4.1 The tendency of hot sand to cause moisture condensation on cold surfaces also creates several special problems when entering and exiting a sand handling system. First, the hot sand tends to stick to the cooler hopper and silo walls and form a hopper shrinkage tube. When the hot sand enters the top of the hopper, it directly passes through the shrinkage tube in the middle of the silo. The result is that only a small fraction of the provided system sand is used frequently. Since there is less active sand in the system, the turnover rate of sand increases rapidly, which increases the temperature of the sand and exacerbates the problem of hot sand. The second serious problem caused by moisture condensation is the cold core placed in the hot cavity. Excessive moisture on the core surface can reduce core strength and produce casting defects such as gas-related pores and pinholes. Metal foundry workers may also experience problems such as sand sticking to the model due to condensation. In general, hot sand problems will become more severe due to the natural tendency to reduce the amount of available sand.

4.2 Cooling the hot green sand keeps the sand system stable and reduces fluctuations and changes, which requires not only a balance of input and output materials, but also a balance of energy. When adding new raw materials, it is necessary to ensure that there is energy to make the clay in the mixer react. The heat generated by the solidification of the casting must be removed from the sand to keep its system in energy balance. Recycled sand from falling sand fluctuates in temperature, moisture, particle size, clay content, and other key physical properties. The incompatibility of old and new sand is a problem for sand handling equipment (sand mixers), whether it is automatically or manually controlled. Ideally, the sand cooling system mixes sand with different temperatures and other incompatible properties into uniform sand. After the casting is shaken out, the sand is mixed in an appropriate way. The system sand (due to homogenization) will change gradually over time rather than in unexpectedly abrupt ways. However, just adding water to the hot sand cannot effectively cool the sand to produce uniform sand. In order to be able to cool effectively, the water must have sufficient time to be in contact with all the sand particles. At the same time, it is necessary to try to remove the water vapor formed by the conversion of water from liquid to gas. For these two reasons, watering the sand on the conveyor belt is not effective in cooling the sand below 49 °C. It is important to emphasize that if the surrounding air humidity of the hot sand and water mixture is saturated, no vaporization will occur. In order for the sand cooling system to effectively perform vapor cooling, it is necessary to inject unsaturated air that can absorb moisture. Since it is ineffective to pass unsaturated air to the top of the wet sand mass, it is better to pass unsaturated air into the interior of the sand mass.

4.3 With regard to the cooling of molding sand, another major consideration is the stagnation time in the cooling vessel. If the unsaturated air can effectively remove the steam, then the sand can be rapidly cooled to 100 °C by the continuous water vaporization method. To bring the sand temperature down to 100. Below C, the cooling time will be extended, and the process is no longer instantaneous. The effective measure is that the sand cooling system must be able to provide sufficient unsaturated air and allow the molding sand to have sufficient stagnation time in the cooling vessel to fully utilize the advantages of water vaporization and drying. In order to vaporize the hot sand, water must be added, but the addition of water must be controlled within a narrow range. The amount of water added should be appropriate, that is, it will help to cool and strictly control the moisture of the molding sand, so that the moisture of the molding sand is as close as possible to the percentage required by the molding. Also, when possible, it can be helpful to add some or all of the required bentonite to the sand cooling system. Due to the homogenization in the silo, adding water and bentonite at this stage is useful to improve system efficiency. Strictly controlling the moisture of the system and the effective homogenization of the sand have a positive effect on ensuring the uniformity of the spare sand delivered to the molding process. The ability to effectively mix and control the temperature and moisture of the shakeout prior to sand mixing increases the capacity of the compaction control device on the production line.

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