1. Effects and changes of bentonite in clay binding sand
Foundry bentonite mainly includes sodium bentonite and calcium bentonite. Sodium bentonite is processed by natural sodium bentonite or artificial sodium bentonite, which is popular in foundry industry due to its good reuse and high wet compressive strength. Due to its good plasticity, it can prevent the occurrence of sand clip, scar, drop, sand collapse and other phenomena in the casting. In addition, with its strong forming ability and high cavity strength, it is convenient for the casting of wet or dry mold in the metal industry. It is the preferred sand binder for precision castings. Calcium sodium bentonite is processed from natural base calcium sodium bentonite. It is a kind of sand binder commonly used in light casting. It is welcomed by users for its utility and the convenience of casting sand cleaning.
Compared with other binders, bentonite has an important characteristic, that is, it has a certain heat resistance. As long as the heating temperature is not too high, the free water removed bentonite as long as the addition of water, still can restore the bonding capacity. Bentonite bond ability can only be shown after adding water. Bentonite''''s loss of bonding ability is also related to its dehydration. So far, it has been considered that there are three forms of water in bentonite: one is free water, that is, water adsorbed by bentonite particles. Free water can be removed when heated to more than 100℃. Bentonite of free water is removed, and its bonding capacity is not affected. The two kinds of water are firmly bound water. When heated for a long time at 110℃, bentonite can completely remove free water, but will not remove firmly bound water. Bentonite, which has completely removed free water, will continue to lose weight when heated at higher temperatures (e.g., 200 ° C, 300 ° C), indicating that there is still water loss. After heating and dewatering bentonite in this way, as long as water is added, the bonding ability can be completely restored. The three kinds of water are lattice water, or what some people call structural water. Lattice water can only be partially or completely removed at fairly high temperatures. When the lattice water of bentonite is removed, it loses its bonding ability and becomes dead clay.
The temperature at which bentonite loses its adhesion is different. It is proved by differential thermal analysis experiment that, generally speaking, the failure temperature of natural sodium bentonite and calcium bentonite is 638℃ and 316℃ respectively. Artificially activated sodium bentonite is lower than natural sodium bentonite due to different activation conditions and different failure temperatures. It can be seen that there are two kinds of bentonite in clay bonded sand, one is effective bentonite and the other is dead clay. The more dead clay is, the worse the cohesive property of clay sand is.
When sodium bentonite is heated below 600℃, its bonding capacity is basically unaffected. When the heating temperature exceeds 600℃, the adhesive ability is rapidly lost. When heated to above 700℃, the adhesive ability was almost completely lost. When the calcium bentonite is above 100℃, it begins to lose its bonding ability slowly. The higher the heating temperature is, the more obvious the loss of bonding ability will be. Artificially activated sodium bentonite, in the initial use, the failure temperature is only slightly lower than natural sodium bentonite, several times after repeated heating, and calcium bentonite similar.
After the molten metal has been 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 do not 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 this dead clay is attached to the casting surface along with the molding sand, which is carried away by the casting, and some is left in the recycled sand. Except for large castings, most molding sand does not reach this temperature during the casting process. But because different casting plants use bentonite produced by different manufacturers, the temperature of crystal lattice water removal is different, the rate of crystal lattice water removal is not the same. If bentonite, which is easy to remove crystal lattice water, is used, even in moulding sand that is not directly close to the casting, there will be more bentonite failure and become dead clay. If the use of clay lattice water is not easy to remove, the production of dead clay will be less. In conclusion, the less bentonite is added, the better; the higher the effective bentonite content in clay bonded sand, the more bentonite will fail due to heat.
2. Old sand control
After molding and casting with clay green molding sand, most of the molding sand can be recycled except that the active bentonite in the molding sand close to the casting fails to become dead clay after being heated. When preparing clay green molding sand, the amount of old sand is generally above 90%. If the old sand is not properly treated, no matter how to strengthen sand mixing, no matter what auxiliary materials are added, you cannot get good molding sand. Therefore, effective treatment of old sand is the premise to ensure the quality of molding sand.
In order to ensure the stable performance of molding sand, the old sand temperature control should be kept below 50℃, more than 50℃ is considered as hot sand. The hot sand problem has been recognized as the biggest problem that clay green sand casting has to face. If the molding sand temperature is too high, the casting is prone to sand inclusion, surface roughness, sand flushing, porosity and other defects. At present, there are many types and specifications of molding sand cooling devices, including cooling roller, double disc cooler and cooling fluidized bed, etc., 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 water content of old sand. The water content of old sand entering into sand mixing is too low, and its influence on the quality of sand mixing may be no less than that of sand temperature. After casting, 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 restore its plasticity by adding water to make it absorb water. The lower the water content of the old sand, the longer the time needed to add water and grind it to achieve the required performance in the sand mixer. Because the mixing time in production is limited, the lower the water content of the old sand, the worse the comprehensive quality of the mixed sand. The water content of the old sand entering the blender is only slightly lower than that of the blended sand. Thus, there is a considerable amount of time before the sand cools and enters the blender, and water can fully moisten the bentonite on the surface of the old sand grains.
For cast iron made of clay green molding sand, finer grain size is preferred. As the amount of old sand is generally above 90% in sand mixing, the main factor determining the size of molding sand is old sand. It is impossible to change the grain size of green sand by adding new sand because the amount of new sand is very small. So, should often test the grain size of old sand: the sand on 140 mesh screen should be between 10% ~ 15%, 200 mesh screen, 270 mesh screen and chassis on the sum of fine sand should be as little as possible, the sum of fine sand generally should be less than 4%. The content of absorbent fine powder should be controlled between 2% and 5%. Too high content of absorbent fine powder will make the molding sand have higher moisture content, which will easily lead to the defects of pinholes, surface roughness and sand holes on the casting. If the content of absorbent fine powder is too low, the performance of molding sand (especially compactibility) is not easy to be stable. The absorbent fine powder is mainly dead clay, but also includes the coking fine pulverized coal and other fine powder.
3. The amount of bentonite added
In sand mixing, a certain amount of bentonite must be added to keep the necessary amount of bentonite in the molding sand. Bentonite content is usually determined by methylene blue method, and the amount of bentonite measured by methylene blue method is called effective bentonite.
4. Amount of new sand added
The new sand added should not be too much, in order to maintain the system sand total stability as a principle. The addition of too much new sand will have a negative effect on the sand quality. Foundry in our country generally scattered sand are more, many factories new sand supplement for 5% ~ 8%. Of course, there are many factors to consider, such as equipment conditions and core sand intake.
5. Pulverized coal supplementation
The amount of pulverized coal supplementation shall be determined according to the measured value of effective pulverized coal in the old sand. The content of pulverized coal in clay green sand should be controlled in the range of 3.5% ~ 5.5%. Of course, the quantity of pulverized coal should be determined according to the characteristics of the casting and the quality of the pulverized coal.
6. sand
The main function of sand mixing is to bond the clay to the sand and mix all the components evenly, so that the water can fully moisten the bentonite, and make the effective bentonite paste coated on the sand surface. The goal is to make the clay adhesive sand fit for the molding equipment. Since bentonite paste is a semi-solid viscous material, great energy is needed to achieve the above goal. If the power of sand mixing equipment is not high, or the mixing time is not enough, bentonite will not give full play to its bonding effect, the strength of molding sand is not high, other performance is not good. If increase the sand in the water, make the bentonite paste moisture increase, viscosity decrease, you can reduce the bentonite paste required energy, that is, the mixing time can be shortened. However, as the viscosity of bentonite paste decreases, the wet compressive strength of molding sand also drops sharply, this method is actually not desirable.
In order to reduce the energy required for sand mixing, it is important to use a reasonable feeding sequence. When mixing old sand, do not add dry materials (sand and bentonite) as usual, dry mix for a period of time, and then add water mix. Sand and water should be mixed first, followed by bentonite, because the water has dispersed, there is no larger water droplets, adding bentonite can only form a large number of smaller clay balls. Pressing open these small clay balls is relatively easy and requires less energy. That is to say, with the same sand mixing equipment, to get the same quality sand, the need for a shorter rolling time. In some foreign foundries, under the condition of intermittent type sand mixer, water is added to the sand mixer before sand mixing, and the operation is several seconds (of course, the equipment ensures water leakage). In this way, it not only has the above advantages of adding water first, but also can clean the grinding wheel and scraper before each mixing sand, improving the mixing efficiency.
7. Wet strength
The control target of wet compressive strength must be selected according to the modeling method, not just high strength. The higher the strength of the sand, the greater the energy required for shaping. Now, many foundry sand is too strong. The wet compressive strength of molding sand used by manual or vibration machine is as high as 130 ~ 170kPa. As a result, the surface quality of the casting is not good, and shrinkage defects are easy to occur.
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