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Function and change of bentonite in green sand:
Foundry bentonite mainly includes sodium-based bentonite and calcium-sodium bentonite. Na-based bentonite is processed from natural sodium-based bentonite or artificial sodium 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 falling, sand mold collapse and other phenomena in castings. 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-based calcium-sodium bentonite, and is a commonly used molding sand binder for light castings. It is popular among users for its practicality and convenience of casting sand cleaning.
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 that has removed free water can still restore its bonding ability as long as water is added. The bonding ability of bentonite can only be manifested after adding water. The loss of binding ability of bentonite is also related to its dehydration. So far, it is believed that the moisture in bentonite has three forms: one kind of water is free water, that is, the water adsorbed by bentonite particles, and the free water can be removed by heating to above 100 ° C. Unaffected; the two types of water are firmly bound water, and bentonite can completely remove free water but not firmly bound water when heated at 110°C for a long time. The bentonite that has completely removed free water will continue to lose weight when heated at a higher temperature (such as 200°C, 300°C), indicating that there is still water loss. After bentonite is heated and dehydrated in this way, as long as water is added, the bonding ability can be fully restored; the three kinds of water are lattice water, and some people call it structural water. The 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. The differential thermal analysis experiment proves that, generally speaking, the failure temperature of natural sodium bentonite is 638°C, and that of calcium bentonite is 316°C. The artificially activated sodium bentonite has different failure temperatures due to different activation conditions, but they are all lower than natural sodium bentonite. It can be seen that there are two kinds of bentonite in green sand, one is effective bentonite and the other is dead clay. The more dead clay, the worse the greensand performance.
When sodium bentonite is heated below 600°C, its bonding ability is basically not affected. When the heating temperature exceeds 600°C, the adhesive ability is rapidly lost. When heated above 700°C, the bonding ability is almost completely lost. Calcium bentonite begins to lose its cohesive ability slowly above 100°C. The higher the heating temperature, the more obvious the loss of bonding ability. Sodium bentonite with poor artificial activation has an invalidation temperature only slightly lower than natural sodium bentonite when it is first used, and it is similar to calcium bentonite after repeated heating several times.
When the molten metal is injected into the mold, if the molding sand close to the surface of the casting is heated 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 molding sand will become dead clay. Some of these dead clays are attached to the surface of the casting along with the molding sand, and are taken away by the casting, and some remain in the recycled old sand. Except for the manufacture of large castings, most molding sands do not reach such temperatures during the casting process. However, because different foundry manufacturers use bentonite produced by different manufacturers, the temperature for removing lattice water is different, and the rate of removing lattice water is also different. If bentonite that is easy to remove lattice water is used, even in the molding sand that is not directly close to the casting, more bentonite will fail and become dead clay. If clay that is not easy to remove lattice water is used, less dead clay will be produced. In short, the less bentonite is added, the better. The higher the effective bentonite content in green sand, the more bentonite that will be invalidated by heating.
1. Control of old sand
After molding and casting with clay green sand, most of the molding sand can be recycled, except that the active bentonite in the molding sand close to the casting becomes dead clay due to heat failure. When preparing clay green sand, the amount of old sand is generally more than 90%. If the old sand is not handled properly, no matter how the sand mixing is strengthened, no matter what auxiliary materials are added, good molding sand cannot be obtained. Therefore, effective treatment of old sand is the premise to ensure the quality of molding sand.
In order to ensure the stable performance of the molding sand, the temperature control of the used sand should be kept below 50°C, and if it exceeds 50°C, it is regarded as hot sand. The hot sand problem has been recognized as the biggest problem that clay green sand casting must face. If the molding sand temperature is too high, castings are prone to defects such as sand inclusions, rough surfaces, sand washing, and pores. At present, there are many types and specifications of molding sand cooling devices, mainly including cooling drums, double-disk coolers, and cooling fluidized beds, all of which use water evaporation to cool molding sand. Among them, the effect of cooling fluidized bed is better.
It is very important to strictly control the moisture content of the old sand. If the moisture content of the old sand entering the sand mixing is too low, the impact on the quality of the sand mixing may be no less than that of the sand temperature being too high. 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, and 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 properties. Since the sand mixing time in production is limited, the lower the moisture content of the old sand, the worse the comprehensive quality of the mixed sand. The moisture content 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 period of time from the sand cooling to entering the sand mixer, and the water can fully wet the bentonite on the surface of the old sand grains.
For iron castings made of clay green sand, the particle size of the sand should be finer. Since the amount of old sand used in sand mixing is generally more than 90%, the main factor determining the particle size of the molding sand is the old sand. The amount of new sand added is very small, and it is impossible to change the particle size of the 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 fine sand on the 200-mesh sieve, the 270-mesh sieve and the chassis should be as little as possible, and the sum of the fine sand should generally be less than 10%. Less than 4%. It is best to control the content of water-absorbing fine powder between 2% and 5%. If the content of water-absorbing fine powder is too high, the moisture in the molding sand will be high, which will easily lead to pinholes, surface roughness and sand holes on the casting. ; If the content of water-absorbing fine powder is too low, the performance of the molding sand (especially the compactability) will not be stable. The absorbent fines are mainly dead clay, and also include coked coal fines and other fines.
2. Add the amount of 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 determined by the methylene blue method, and the amount of bentonite measured by the methylene blue method is called the amount of effective bentonite.
3. New sand addition amount
The added new sand should not be too much, and the principle is to keep the total amount of system sand stable. Adding too much new sand will have a negative impact on the quality of the molding sand. The additional amount of new sand is 5% to 8%, and many factors should be considered in the additional amount, such as equipment conditions and the amount of core sand entering.
4. Addition of pulverized coal
The amount of pulverized coal added shall be determined according to the measured value of pulverized coal in the old sand. The coal powder content in the clay green sand should be generally controlled between 3.5% and 5.5%. Of course, the amount of pulverized coal added should be determined according to the characteristics of the casting and the quality of pulverized coal.
5. Sand mixing
The main function of sand mixing is to mix the components in the green sand evenly, so that the water can fully wet the bentonite, and apply the effective bentonite paste on the surface of the sand grains. The goal is to make the green sand adapt to the performance of the molding equipment. Since the bentonite paste is a semi-solid viscous material, a lot of energy is required 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 bonding effect, the strength of the molding sand is not high, and other properties are not good. If the water in the molding sand is increased to increase the water content of the bentonite paste and decrease the viscosity, the energy required for coating 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 sequence of feeding. When mixing old sand, do not add dry materials (sand and bentonite) first according to the habit, dry mix for a period of time, and then add water and mix well. Sand and water should be mixed first, and then bentonite should be added, because the water has been dispersed and there are no larger water droplets, and only a large number of smaller clay balls can be formed after adding bentonite. It is easier and requires less energy to press apart these little clay balls. That is to say, with the same sand mixing equipment, the molding sand with the same quality can be obtained, and the required mixing time is shorter. If a batch type sand mixer is used, add water to the sand mixer before sand mixing and run it for a few seconds. Sand mixing efficiency.
6. Wet pressure strength
The control target of wet pressure strength must be selected according to the modeling method, and it is by no means high strength. The higher the strength of the molding sand, the greater the energy required for compaction during molding. Nowadays, the strength of molding sand in many foundries is too high. The wet compressive strength of some molding sand used for hand molding or shaking machine molding is as high as 130-170kPa. It is difficult to pound the mold tightly with such molding sand. As a result, the surface quality of the casting is not good, and shrinkage defects are also prone to occur.
Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. specializes in the production of Green Sand Molding Line,Self-hardening sand molding line,foundry machines,Sand molding machine,Sand casting equipment,sand mixer,GS high efficiency rotary mixer,Resin sand mixer,no-bake resin sand mixer,Jolt squeeze molding machine/Jolt-squeezing moulding machines,Multi- Piston Moulding Machine/Hydraulic multi-piston moulding machine,sand casting molding machine,sand reclamation equipment，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.
