Views: 0 Author: Site Editor Publish Time: 2022-10-08 Origin: Site
The preparation of molding sand includes three conditions: the preparation of raw materials, the mixing of molding sand, the adjustment of molding sand and the loosening of sand. The molding sand used in casting production is made up of recycled sand plus an appropriate amount of new sand, clay and water mixed evenly, and some are all made up of new materials. In order to ensure the quality of new sand, all raw materials must be qualified according to the technical requirements before use. Therefore, processing preparation must be carried out before sand preparation.
1, the new sand
New sand is often mixed with grass roots, coal dust and mud in the process of procurement and transportation, and contains a certain amount of ash. Wet sand is not easy to sieve, sand is not easy to control the moisture of sand. Therefore, in addition to low water content, for manual modeling of wet sand can be directly formulated, the new sand must be dried and sieved before use. The drying of new sand with vertical or horizontal drying drum, can also use air drying method. Commonly used sand screening equipment are manual screen, roller screen and vibrating screen.
Freshly mined clay often contains more moisture and is mostly lumpy, so it must be dried, broken and ground into clay powder before use, which is mainly processed by specialized factories. Some factories make clay - coal slurry in proportion to bentonite or clay and coal powder in advance, so that the clay is fully absorbent and expanded, and the sand is mixed with the original sand into the sand mixer and mixed evenly. This method can simplify sand mixing operation, facilitate transportation, improve working conditions, and improve sand quality. But the water content of slurry must be strictly controlled, otherwise it will affect the performance of molding sand.
Pulverized coal, α-starch, wood cellulose and other additives must be pulverized, sieved before use.
In order to save molding materials and reduce casting costs, the old sand should be reused. The proportion of old sand in molding sand is very large, and it has a great influence on the composition and performance of molding sand. Old sand is often mixed with various debris, such as nails, iron and sand, etc., must be processed before reuse, including grinding sand, removing the iron debris with a magnetic separator and then sifting, cooling if necessary.
In the casting workshop with a high degree of mechanization, the demand for molding sand is large, and the turnover speed is fast. The temperature of old sand is often relatively high, and the temperature of some recycled sand is above 60 degrees Celsius. If this molding sand is used, it is easy to adhere to the shape, core box and sand bucket. Because the temperature of molding sand is too high, the water will evaporate too fast, the performance of molding sand is not stable, and the surface quality of casting is affected, and the productivity of molding labor is affected. Therefore, it is necessary to strengthen ventilation and cooling in the process of casting sand falling, old sand screening, transportation and sand mixing to reduce the temperature of molding sand.
The task of sand mixing is to mix all kinds of raw materials evenly, so that the binder is coated on the surface of sand. The quality of sand mixing mainly depends on the sand mixing process and the form of sand mixing machine.
1) The form of sand mixing machine
The commonly used sand mixing equipment in production are roller type, swing wheel type and rotor type sand mixing machine.
In addition to the mixing effect, the roller type sand mixer also has the rolling and rubbing effect. The quality of molding sand is good, but the production efficiency is low. It is mainly used to mix surface sand and single sand.
The production efficiency of the tilting sand mixing machine is several times higher than that of the roller type, and it can mix sand while blowing and cooling, and has a certain rubbing effect. However, the quality of the molding sand is not as good as that of the roller type sand mixing. It is mainly used for mixing single sand and back sand in the casting workshop with high degree of mechanization and large production.
Rotor type sand mixer is a continuous operation type of equipment. All kinds of raw materials enter from one end of the sand mixer, and the mixed sand comes out from the other end of the sand mixer, with high production efficiency. Rotor sand mixer has mixing effect, but poor rubbing effect. It is mainly used for mixing back sand and single sand with low clay content.
2) Feeding sequence and sand mixing time
The order of adding mixed clay sand is generally the first to add sand, sand, clay powder and additives and other dry materials, dry mix evenly and then add water wet mix, uniform can be used. If the sand contains slag oil and other liquid binder, water should be added to mix the sand evenly before adding the oil binder. The disadvantage of this sand mixing feeding sequence of adding dry powder first and then water is that some powder is left on the edge of the roller of the sand mixing machine, which absorbs water and sticks to the wall of the sand mixing machine until it falls off at the later stage of rolling or unloading sand, so that the sand contains uneven mixed clay or coal powder clumps, which deteriorates the performance of the sand. At the same time, dust flying when dry mixing, poor working conditions. Therefore, some factories use water mixing in the reuse sand first, and then add clay and pulverized coal mixing evenly, and then add a small amount of water to adjust the water content of the sand mixing process. The EXPERIMENTAL results SHOW THAT THE latter feeding sequence can shorten the sand mixing time, improve the quality of molding sand and improve the working conditions.
In order to mix all kinds of raw materials evenly, the sand mixing time should not be too short, otherwise it will affect the performance of molding sand, but the sand mixing time should not be too long. Otherwise, the temperature of molding sand will rise, too much water volatilization, molding sand into block, performance deteriorates and production efficiency is low. Sand mixing time is mainly determined according to the form of sand mixer, clay content, sand performance requirements, etc.
6, to make
Molding sand homogenization, also known as return, infiltration, refers to the mixed sand in the condition of not losing water placed for a period of time, so that the water evenly permeated into the sand, so that the clay fully absorb water expansion, in order to improve the strength and permeability of sand and other properties. The time of mixing depends mainly on the clay type and the amount added. The more clay content in molding sand, the finer the original sand particles and the longer the mixing time. The adjustment time should be appropriate, otherwise the sand performance is difficult to meet the needs of injection. Single sand is generally 2~3 hours, surface sand is 4~5 hours. The molding sand homogenization between mechanized foundry is carried out in the molding sand homogenization bucket. The non-mechanized manual molding workshop is to pile up the mixed molding sand on the ground between the porch and cover the homogenization with wet sacks.
Molding sand will be compacted after mixing and rolling, and some will be pressed into clumps. If this type of sand is used directly, the firmness of sand is not uniform, and the performance of air permeability is poor. Therefore, the mixed sand must be loose sand or sieve can be used. In mechanized casting workshops, round bar or blade type sand loosening machine is generally used for sand loosening treatment. In the manual molding workshop, it is commonly used to sieve with a mobile loose sand machine or a sieve with a sieve hole of 5~8 mm.