Abstract: The quality of molding sand must be controlled. In this regard, the countermeasures to prevent sand inclusion are: select raw sand with dispersed particle size and irregular shape, use wet-type sodium bentonite or activate the calcium bentonite, appropriately increase the amount of bentonite and reduce the water content of the sand, add the appropriate amount “Buffering agent” such as coal powder, heavy oil, asphalt powder, fine wood chips, etc., remove the dust in the old sand, and ensure the mixing quality of the molding sand.
The quality of molding sand must be controlled. In this regard, the countermeasures to prevent sand inclusion are: select raw sand with dispersed particle size and irregular shape, use wet-type sodium bentonite or activate the calcium bentonite, appropriately increase the amount of bentonite and reduce the water content of the sand, add the appropriate amount “Buffering agent” such as coal powder, heavy oil, asphalt powder, fine wood chips, etc., remove the dust in the old sand, and ensure the mixing quality of the molding sand.
Whether the molding process is reasonable has a great influence on the sand inclusion of the casting. The casting time and casting position of the casting, the rising speed of the molten iron, and the type of mold must be selected appropriately.
(1) Using Rapid Casting
The surface of the sand mold always swells, so the decisive factor for preventing sand inclusion is whether the molten iron can quickly cover and touch the surface of the sand mold and exert a certain pressure on the sand mold. Rapid pouring can make the molten iron fill the casting mold before the “critical time” of sand inclusion in the casting, without giving the sand mold sufficient time to expand and form high water areas. Someone observed with a high-speed camera: If the hot metal can be touched immediately when the upper sand mold hangs down locally after being baked, the hot metal may return the hanging sand block to its original place. It can be seen that rapid casting can use the pressure of molten iron to cope with the expansion of the sand mold.
The speed of casting depends mainly on the size of the gate section. If the gate section of the gray cast iron is calculated by the following simple calculation formula, it can realize rapid pouring.
The casting M with larger plane is 0.8~1.2; the casting with large plane and thin wall is 1.2~1.5; the middle and upper limit of the wet type should be taken.
The production practice is verified. The above formula is reliable. If the casting has sand inclusion defects, it must be checked whether the cross-sectional area of the gate used by the casting is within the range of “quick pouring”. For large plane castings, larger gate cups, multiple lamellar inner gates or gap gates should be used. Make the molten iron cover the poured surface quickly, smoothly and uninterruptedly, and avoid local overheating of the sand mold. The gate ratio is usually semi-closed or open.
(2) Increase the Rising Speed of Molten Iron
The molten iron should have a higher rising speed in the sand mold, in order to reduce the baking degree of the upper sand mold. The rising speed of the molten iron is related to the pouring scheme. The oblique casting method of bottom-up filling (generally 3°–15°). It can avoid scattered hot metal flow, which is beneficial to the exhaust of the sand mold, reduces the heat radiation of the hot metal to the sand mold, and increases the rising speed of the hot metal. Pingaoli pouring technology can significantly increase the rising speed of molten iron.
(3)Choose Proper Feeder and Riser Location
The casting position of the casting must be conducive to the smooth filling of the molten iron and the elimination of the cavity gas, otherwise, it will cause the defect of sand inclusion.
(4) Adopt Suitable Sand Molding Type
Choose the appropriate mold according to the size of the casting. The wet type is generally suitable for small parts and middle parts with small planes and thick walls. For medium and large boards and thick wall parts, surface dry type and thousand type should be used. Some large flat plates can be made with small thermal expansion, good thermal conductivity and high thermal capacity, and graphite powder sand or refractory bricks, which can be reused and can effectively prevent sand inclusion.
(5) Increase The Sand Exhaust
The timely elimination of water vapor and other gases in the cavity can be beneficial to the rapid filling of molten iron and reduce the arching effect of high-temperature airflow on the sand surface layer, which is helpful to reduce the amount of water in the water condensation area and move its position inward. Therefore, it is very important to distribute air vents on the sand mold and disperse exhaust.
The manufacturing quality of sand molds involves the “critical time” for the generation of sand inclusions. How to carefully shape and improve the overall strength of the sand mold is the key to preventing sand inclusion.
(1) Clay Sand Should Be Firm And Even
The sand mould should be tight and even to avoid partial tightness and delamination. The wet type does not require excessively high tightness, while the surface dry type and the lower type should have sufficient tightness. The experience of large-scale iron castings to prevent sand inclusion is: “Manual use of 10-15 mm diameter steel brazing can not be inserted into the sand mold.” It can be seen that the prevention of sand inclusion should pay attention to the rigidity of the sand mold. Of course, increasing the tightness of the sand mold will affect the exhaust of the sand mold. The corresponding important means is to be more piercing and to be as close as possible to the surface of the sand mold. The hook and the hook should not be too close to the molding surface, and the core can not be too close to the surface of the sand core, because the first layer of sand filling should not be too thin when the sand is uneven, especially when the model surface board is thin, the board Elasticity delaminates the sand mold. The squeegee’s modeling operation should be especially careful. It is mainly formed by sand-shaving with a knife. The squeegee should not use excessive force when scraping the sand, so as not to delaminate the sand mold.
(2) Careful Repair And Coating
Don’t over-sharpen the sand mold when repairing the mold, so it will easily lead the water to the surface of the sand mold, form a hard block and separate from the body. The damaged part of the sand mold should be repaired after scratching, and it is not appropriate to brush too much water. Near the gate, the edge of the boss, the large plane and the part where the molten metal flows intermittently should be nailed and reinforced. The studs are plum-shaped, giving the sand mold an overall strength. The paint is a protective layer of sand and should be applied well. After repairing the mold, it is advisable to let the sand mold dry for a while and then apply the paint to facilitate the penetration of the paint. It is best to brush the paint twice, the first time to apply thick paint, and use the knife to press it again, and the second to leave a thinner paint.
(3) Control The Drying Range
Sand mold is also not easy to dry and easy to produce sand. To this end, the sand mold should have the correct drying range. The drying furnace can not be heated too fast at first, otherwise there will be a large temperature difference in the outer layer of the sand mold, causing cracking. There should be ample time for heat preservation to ensure that the sand mold is thoroughly dried. It should be poured as soon as possible after boxing to avoid moisture regain.
In order to prevent sand inclusion defects, in the casting process, we should strive to use a lower casting temperature on the premise of ensuring that no other defects occur. In the casting operation, we should avoid flow interruption and try to cast at a faster speed.
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