Sand adhering to castings not only affects the appearance quality of castings, but may even cause them to be scrapped. Therefore, sufficient attention must be paid to the sand adhesion of castings to improve product yield. So how to prevent sand from sticking to castings?
“Cure” of course requires finding the root cause. First, let’s take a look at the reasons for sand sticking to castings.
1. Sufficient pressure allows the molten metal to penetrate into the higher static pressure head of the molten metal between the sand grains. That is the pressure formed by the casting pouring height and the pouring system. If the pressure exceeds the resistance pressure formed by the capillary phenomenon between the gaps between the sand grains. That is, corpse hair = QcosO/r, where P hair is the capillary pressure; is the surface tension of the molten metal; e is the wetting angle of the capillary of the molten metal; r is the radius of the capillary. Mechanical sticking sand will form. The static pressure head exceeds 500 mm, and the casting sand is relatively coarse, so mechanical sand sticking will occur in most cases unless it is painted. The above formula also shows that the larger the particle size, the coarser the sand particle size, and the smaller the corpse hair, the easier it is to produce mechanical sand sticking.
2.The dynamic pressure formed by the flow of molten metal in the casting mold.
3. The mold “explodes” or “chokes”. That is, the dynamic pressure formed when the flammable gas released when the mold is poured is mixed with air and ignited by the hot molten metal.
4. Once mechanical sand sticking begins, even if the pressure decreases, the penetration of molten metal will continue until the front edge of the penetrating molten metal solidifies. That is, the penetration can stop only when the temperature of the molten metal is lower than the solidus temperature.
5. The most common reasons for chemical sticking of sand are that the raw materials used for wet molding and core making have low refractoriness and sintering point; impure quartz sand; insufficient addition of coal powder or substitutes; no coating or improper use; pouring temperature is too high; Improper pouring may cause slag to enter the mold and other factors.
Now that we have found the cause of sand sticking, let’s talk about how to prevent it!
1.To prevent mechanical sand sticking, the following measures can be taken:
1) Avoid a high hydrostatic pressure head of the metal; do not allow the riser height to be too high when the casting feeding conditions are met; avoid pouring the ladle at a high position directly into the sprue. If necessary, a basin-shaped sprue cup can be used to buffer the metal flow and form a constant high static pressure head.
2) Try to use finer-grained casting sand.
3) The sand mold should be tight and well-formed. ‘Machine modeling should not be overloaded. The compressed air supplied to the molding machine should maintain the specified pressure. Avoid using molding sand that is too wet or has been stored for too long. Because it is difficult to compact, the core box vents (plugs) must not be blocked; use resin sand for modeling and manufacturing. The core cannot rely solely on the good fluidity of the molding sand. It must be tight and supplemented with vibration if necessary.
4) Prevent the mold from “exploding” or “choking”. Do not add excessive coal powder and moisture to the molding sand. Try to open air outlets and ventilation holes for the mold and core to increase the air permeability of the mold.
5) Reduce the dynamic pressure generated within the mold. The mold should be equipped with more air outlets and more air holes; an exhaust groove (ventilation groove or ventilation ditch) can be installed on the parting surface of the high-compactness mold.
6)Use effective coatings on molds or cores. That is, the filling type, the voids in the outermost layer of sand in the core. If the paint is too thick, it may crack, allowing the molten metal to penetrate into the sand. In this case, a thinner paint can be used in the first or second layer, and then normal or thicker paint.
2. To prevent chemical sand sticking, the following measures can be taken:
1) The sources of sand supply are different, and the purity, sintering point, and refractoriness of the foundry sand vary greatly. Low-purity silica sand with a sintering point below 1,200°C will promote sand sticking; high-purity silica sand or non-quartz sand with a pouring point above 1,450°C, such as quartz sand, chromite sand, etc., will reduce sand sticking.
2) Adding about 5% coal powder to wet clay sand can prevent sand sticking to small and medium-sized castings. The ash content of pulverized coal for foundry should be less than 10%. In order to prevent the accumulation of expired coal powder and dust in the molding sand system, some old sand should be eliminated and some new materials should be added in each production cycle. The amount of old sand discarded is generally about 10% to 15%, with a lower limit for the production of thin-walled castings and an upper limit for the production of thick-walled castings.
3) Due to the low sintering point of the mixture, water glass sand must be coated. There should not be too much sodium silicate and old sand in the sand mix. Adding 1% to 2% of pulverized coal to the sand mix can also help prevent sand sticking.
Post time: Sep-04-2024
