Basic Knowledge of Casting Sand Mixing
Aug. 11, 2020
Extra heavy iron manufacturer introduces you the basic knowledge of casting sand mixing.
The preparation of molding sand includes three aspects, namely, the preparation of raw materials, the mixing of molding sand, and the process links of mixing the mixed molding sand and loosening the sand. Some of the molding sand used in casting production is made by mixing recycled sand with an appropriate amount of new sand, clay and water, and some are all made of new materials. In order to ensure the quality of new sand, all raw materials must be accepted according to technical requirements before they can be used. For this reason, processing preparation must be carried out before sand is prepared.
(1) New sand New sand is often mixed with grass roots, coal chips, mud and other debris during the procurement and transportation process, and also contains a certain amount of fractions. The wet raw sand is not easy to screen, and it is not convenient to control the moisture of the molding sand when mixing sand. Therefore, in addition to the low water content, the green sand used for manual molding can be directly prepared, the new sand must be dried and sieved before use. Vertical or horizontal drying drums are used to dry fresh sand, and air drying can also be used. Commonly used sand screening equipment includes manual screens, drum screens and vibrating screens. (2) Clay Freshly mined clay often contains more water and is mostly lumpy. Therefore, it must be dried, crushed and ground into clay powder before use. It is mainly processed by specialized factories and supplied in ten thousand bags. In some factories, bentonite or clay and coal powder are made into a clay-coal powder slurry in advance to make the clay fully absorb water and expand. When mixing the sand, it is added to the sand mixer together with the raw sand and mixed evenly. This approach can simplify sand mixing operations, facilitate transportation, improve working conditions, and improve the quality of molding sand. However, the water content of the slurry must be strictly controlled, otherwise it will affect the sand performance.
(2) Additions Coal powder, boron compound, fluoride and sulfur must be crushed and sieved before use. (4) Old sand should be reused in order to save modeling materials and reduce the cost of castings. Old sand occupies a large proportion of molding sand, and it has a great influence on the composition and performance of molding sand. The old sand is often mixed with various impurities, such as nails, iron blocks, and sand masses. It must be treated before reuse, including crushing the sand, removing iron impurities from it with an electromagnetic separator, and then sieving. When cooling.
In a highly mechanized foundry workshop, the demand for molding sand is large and the turnover rate is very fast. The temperature of the old sand is often relatively high. The temperature of some recycled sand is as high as 60 degrees Celsius. Core box and sand bucket. Because the temperature of the molding sand is too high, the moisture will evaporate too quickly, the performance of the molding sand will be unstable, and the surface quality of the castings will be affected, and the productivity of molding labor will be affected. Therefore, ventilation and cooling must be strengthened in the process of casting sand falling, old sand sieving, transportation and sand mixing to reduce the temperature of molding sand.
(5) Sand mixing The task of sand mixing is to mix various raw materials uniformly so that the binder coats the surface of the sand grains. The quality of sand mixing mainly depends on the sand mixing process and the form of the sand mixer. 1. The form of the sand mixer. The commonly used sand mixing equipment in production includes roller type, balance wheel type and blade type sand mixer. In addition to the stirring effect, the wheel-type sand mixer also has the effect of rolling and kneading. The quality of molding sand is better, but the production efficiency is low. It is mainly used for mixing surface sand and single sand. The production efficiency of the pendulum sand mixer is several times higher than that of the wheel roller type, and it can be cooled by blasting while mixing the sand, and has a certain rubbing effect, but the quality of the molding sand is not as good as that of the wheel sand mixer, and it is mainly used for mechanization. Single sand and back sand are mixed in high-volume foundry workshops. The blade type sand mixer is a continuous operation equipment. One end of the various original sand mixers enters, and the mixed molding sand comes out of the other end of the sand mixer, which has high production efficiency. The blade type sand mixer has a mixing effect, but the kneading effect is very poor. It is mainly used to mix back sand and single sand with low clay content.
2. Feeding sequence and sand mixing time. The order of adding mixed clay molding sand is generally to add dry materials such as recycled sand, raw sand, clay powder and additives, dry and mix uniformly, then add water and wet mix, and then use it after uniformity.
If the molding sand contains residual oil and other liquid binders, water should be added to mix the molding sand evenly before adding oil binders. The disadvantage of this sand mixing and feeding sequence of adding dry powder first and then adding water is that some powder is left on the edge of the roller of the sand mixer, and these powders adhere to the wall of the sand mixer after absorbing water until the late stage of mixing or unloading. When the sand falls off, the molding sand contains unevenly mixed clay or coal lumps, which deteriorates the performance of the molding sand. At the same time, dust is flying during dry mixing and working conditions are poor. Therefore, some factories use the sand mixing process in which water is added to the recycled sand first, then clay and coal powder are added to mix uniformly, and finally a small amount of water is added to adjust to the required water content. The test results show that the latter feeding sequence can shorten sand mixing time, improve molding sand quality, and improve working conditions.
In order to mix the various raw materials uniformly, the sand mixing time should not be too short, otherwise it will affect the sand performance, but the sand mixing time should not be too long. Otherwise, the temperature of the molding sand will rise, excessive moisture will evaporate, the molding sand will agglomerate, performance will deteriorate and production efficiency will be low. Sand mixing time is mainly determined according to the type of sand mixer, clay content, and performance requirements for molding sand. Generally speaking, the more clay content, the higher the quality requirements of molding sand and the longer the sand mixing time. When using a roller type sand mixer to mix surface sand, the sand mixing time is generally 6-12 minutes, north sand is 3-6 minutes, and single sand is 4-8 minutes.
(6) Mix thoroughly
The blending of molding sand is also called reversibility and homogeneity, which means that the mixed molding sand is placed for a period of time without losing moisture, so that the water can evenly penetrate into the molding sand, and the clay can fully absorb water and expand to improve the strength and air permeability of the molding sand. Performance. The mixing time mainly depends on the type of clay and the amount of clay added. The more clay content in the molding sand, the finer the raw sand particles and the longer the mixing time. The mixing time should be appropriate, otherwise the molding sand performance is difficult to meet and needs to be injected. Single sand is generally 2-3 hours, and surface sand is 4-5 hours. The mixing of molding sand in the mechanized foundry is carried out in the mixing bucket of molding sand. In the non-mechanized manual molding workshop, the mixed molding sand is piled on the floor of the room and covered with wet sacks for mixing. The molding sand will be compacted after being mixed, rolled and evenly blended, and some are compressed into agglomerates. If this type of sand is used for direct modeling, the sand will have uneven firmness and poor air permeability. Therefore, the sand after mixing must be loosened or sieved before use. In mechanized foundries, round bar or blade loosening machines are generally used to loosen sand. In the mechanized manual moulding workshop, a mobile loose sand machine or a sieve with a screen hole of 5-8 mm is often used for sieving.