Green sand casting is currently the most commonly used sand casting method in foundries. Among them, bentonite and coal powder are the most used for sand mixing. The uniformity and composition ratio of sand mixing determine the quality and quality of the entire casting. Sanzhuji combines green sand The working principle of the sand mixing equipment and the pressure feedback of the green sand molding equipment summarize the role and changes of bentonite in clay-bonded sand:
Bentonite for foundry mainly includes sodium-based bentonite and calcium-sodium bentonite. Sodium-based bentonite is processed from natural sodium-based bentonite or artificial sodium-based bentonite. It is popular in the foundry industry for its good reusability and high wet compressive strength. Because of its good plasticity, it can prevent the occurrence of sand inclusion, scarring, block dropping, sand collapse and other phenomena in the casting. In addition, it has strong formability and high cavity strength, which is convenient for casting wet or dry molds in the metal industry. It is the first choice for precision castings. Sand binder. Calcium-sodium bentonite is processed from natural-base calcium-sodium bentonite and is a commonly used sand binder for light castings. It is popular with users for its practicality and convenient casting sand removal
Compared with other binders, bentonite has an important feature, that is, it has a certain heat resistance. As long as the heating temperature is not too high, the bentonite from which free water has been removed can still recover its bonding ability by adding water. The bonding ability of bentonite can only be shown after adding water. Bentonite loses its binding ability, which is also related to its dehydration. So far, sanzhuji believes that the water in bentonite has three forms:
1. Water is free water, that is, water absorbed by bentonite particles. When heated to a temperature above 100°C, free water can be removed, and the bentonite from free water can be removed without affecting the bonding ability;
2. Water is firmly bound water, 110 When heated at ℃ for a long time, bentonite can completely remove free water, but it will not remove firmly bound water. Bentonite, which has completely removed free water, will continue to lose weight if heated at higher temperatures (such as 200°C, 300°C), indicating that there is still water loss. After the bentonite is heated and dehydrated in this way, as long as water is added, the bonding ability can be completely restored;
3. Water is lattice water, which is also called structured water. Lattice water can only be partially or completely removed at a relatively high temperature. After the lattice water of bentonite is removed, it loses its bonding ability and becomes dead clay.
Different bentonites have different temperatures at which they lose their bonding ability. It is proved by differential thermal analysis experiment: Generally speaking, the failure temperature of natural sodium bentonite is 638℃, and calcium bentonite is 316℃. Artificially activated sodium bentonite has different activation conditions and different failure temperatures, but they are all lower than natural sodium bentonite. It can be seen that there are two types of bentonite in clay-bonded sand, one is effective bentonite and the other is dead clay. The more dead clay, the worse the performance of clay-bonded sand.
Sodium bentonite is heated below 600°C, and its bonding ability is basically not affected. If the heating temperature exceeds 600°C, the bonding ability will be rapidly lost. Heating above 700°C almost completely loses its bonding ability. Calcium bentonite begins to slowly lose its bonding ability above 100°C. The heating temperature increases again, the more obvious the loss of bonding ability. The artificially activated sodium bentonite has a failure temperature only slightly lower than that of natural sodium bentonite when it is first used. After several repeated heating, it is similar to calcium bentonite.
After the molten metal is injected into the mold, if the molding sand close to the surface of the casting is heated to above 800°C (some non-ferrous alloys cannot reach this temperature). No matter what kind of bentonite you use, the bentonite in this part of the sand will become dead clay. Part of this dead clay is attached to the surface of the casting along with the molding sand, and is taken away by the casting, and part is left in the recycled sand. Except for the manufacture of large castings, most of the molding sand cannot reach such a temperature during the casting process. However, because different foundries use bentonite produced by different manufacturers, the temperature of removing the lattice water is different, and the rate of removing the lattice water is also different. If the bentonite is easy to remove the lattice water, even in the molding sand that is not directly close to the casting, more bentonite will fail and become dead clay. If the clay is not easy to remove the lattice water, the dead clay will be less. In short, the smaller the amount of bentonite added, the better, the higher the effective bentonite content in the clay-bonded sand, the more bentonite will fail due to heat.
Control of used sand:
After modeling and casting with clay green sand, most of the sand can be recycled except that the active bentonite in the sand close to the casting becomes dead clay due to heat failure. When preparing clay green sand, the amount of used sand is generally more than 90%. If the used sand is improperly treated, no matter how the sand is strengthened, no matter what auxiliary materials are added, good sand will not be obtained. Therefore, effective treatment of old sand is a prerequisite for ensuring the quality of molding sand.
In order to ensure the stable performance of the molding sand, the control of the temperature of the used sand should be kept below 50℃, and it is considered hot sand if it exceeds 50℃. The hot sand problem has been recognized as the biggest problem that must be faced in clay green sand casting. If the molding sand temperature is too high, the casting is prone to defects such as sand inclusion, rough surface, sand washing, and pores. At present, there are many varieties and specifications of molding sand cooling devices, mainly cooling drums, double-plate coolers and cooling boiling beds, etc., all of which use water evaporation to cool the molding sand. Among them, the effect of cooling the boiling bed is better.
It is very important to strictly control the moisture of the old sand. If the moisture of the old sand entering the sand is too low, the impact on the quality of the sand may be as high as the sand temperature. After casting the mold, due to the influence of hot metal, the soil-water bonding film on the surface of many sand grains is dehydrated and dried. It is not easy to add water to make it absorb water and restore plasticity. The lower the moisture content of the used sand, the longer it will take to add water to the sand mixer to achieve the required performance. Since the time for sand mixing in production is limited, the lower the moisture content of the old sand, the worse the overall quality of the mixed sand. The moisture of the old sand entering the sand mixer can only be slightly lower than that of the mixed sand. In this way, there is still a long time from when the sand is cooled to entering the sand mixer, and the water can fully wet the bentonite on the surface of the old sand.
For iron castings made of clay green sand, the particle size of the sand should be finer. Since the amount of used sand during sand mixing is generally more than 90%, the main factor determining the particle size of the molding sand is the used sand. The amount of new sand added is very small, and it is impossible to change the particle size of molding sand by adding new sand. Therefore, the particle size of the old sand should be checked frequently: the sand on the 140-mesh sieve should be between 10% and 15%, the sum of the 200-mesh sieve, 270-mesh sieve and the fine sand on the chassis should be as small as possible, and the sum of fine sand should generally be Less than 4%. The content of water-absorbing fine powder is best controlled between 2% and 5%. If the content of water-absorbing fine powder is too high, the moisture content of the molding sand will be higher, which is easy to cause pinholes, rough surface and sand holes on the castings. ; If the content of water-absorbent fine powder is too low, the properties of the molding sand (especially compactability) will not be stable. The water-absorbing fine powder is mainly dead clay, but also includes coked coal powder and other fine powder.
Add bentonite:
When mixing sand, a certain amount of bentonite must be added to keep the necessary amount of bentonite in the molding sand. The content of bentonite is usually measured by the methylene blue method, and the amount of bentonite measured by the methylene blue method is called the effective bentonite amount.
New sand replenishment amount:
The additional new sand should not be too much, and the principle of keeping the total amount of system sand stable. Too much new sand will have a negative impact on the quality of molding sand. Foundries in our country generally have a lot of scattered sand, and in many factories, the new sand supplement is 5% to 8%. Of course, many factors must be considered for the new sand replenishment amount, such as equipment conditions and the amount of core sand entering.
Addition of pulverized coal:
The amount of pulverized coal added must be determined based on the measured value of effective pulverized coal in the old sand. The content of pulverized coal in the clay green sand should generally be controlled between 3.5% and 5.5%. Of course, the amount of pulverized coal supplementation must be specifically determined according to the characteristics of the casting and the quality of the pulverized coal.
Sand mixing:
The main function of sand mixing is to mix the components of the clay-bonded sand uniformly, so that the water can fully wet the bentonite, and the effective bentonite paste is coated on the surface of the sand grains. The goal is to make clay-bonded sand suitable for molding equipment. Since bentonite paste is a semi-solid viscous material, it requires a lot of energy to achieve the above goal. If the power of the sand mixing equipment is not high, or the sand mixing time is not enough, the bentonite cannot fully exert its binding effect, the strength of the molding sand is not high, and other properties are not good. If the moisture in the molding sand is increased to increase the moisture content of the bentonite paste and decrease the viscosity, the energy required to coat the bentonite paste can be reduced, that is, the sand mixing time can be shortened. However, due to the decrease in the viscosity of the bentonite paste, the wet compressive strength of the molding sand also drops sharply. This method is actually not advisable.
In order to reduce the energy required for sand mixing, it is important to adopt a reasonable feeding sequence. When mixing old sand, do not add dry materials (sand and bentonite) first, dry and mix for a period of time, then add water and mix well. Sand and water should be added first to mix well, and then bentonite is added. Because the water has been dispersed and there are no large water droplets, only a large number of smaller clay balls can be formed after adding bentonite. It is easier to press these small clay balls and requires less energy. In other words, using the same sand mixing equipment to obtain molding sand of the same quality, the required mixing time is shorter. In some foreign foundries, under the condition of using intermittent sand mixers, add water to the sand mixer before mixing and run it for a few seconds (of course, the equipment must ensure that the water does not leak). In this way, not only the above-mentioned benefits of adding water first, but also the grinding wheel and scraper can be washed before each sand mixing, which improves the efficiency of sand mixing.
Wet compressive strength:
The control target of wet compressive strength must be selected according to the modeling method, and it is by no means a high strength. The stronger the sand, the greater the energy required for pounding. At present, the strength of molding sand in many foundries is too high. Some moulding sands used for hand moulding or shaking machine moulding have wet compressive strength as high as 130～170kPa. It is difficult to pound the mold very tightly with such molding sand. As a result, the surface quality of the casting is not good, and shrinkage defects are prone to occur.
Therefore, an automated sand mixer and feeding equipment can be used to set the proportions of bentonite, coal powder, and sand, and mix in equal proportions to improve uniformity; the modeling adopts a vertical parting boxless sand injection molding machine or a fully automatic horizontal Box-type molding machine, the sand mold produced is high in strength and compact; the iron beans in the old sand are removed by magnetic separation before the old sand is regenerated, and the fine hexagonal screen is used to control the mud content in the old sand, thereby controlling the entire molding sand The ratio of bentonite to pulverized coal can ensure the quality and output of the entire casting.
Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. specializes in the production of sand reclamation equipment,foundry machines,Sand casting equipment,GS high efficiency rotary mixer,no-bake resin sand mixer,Jolt squeeze molding machine/Jolt-squeezing moulding machines,Multi- Piston Moulding Machine/Hydraulic multi-piston moulding machine,foundry molding machine,flaskless moulding machine,shot blasting machine,dust collector,according to the amount of old sand recovered Carry out plan customization, and provide sand reclamation equipment installation, commissioning, and training. Welcome guests to visit the factory.
