Sand sticking of castings not only affects the appearance quality of castings, but also leads to scrap. Therefore, enough attention must be paid to the sand sticking of castings.
1、 Sand sticking
1. The metal liquid of mechanical sand sticking system infiltrates into the sand mold or sand core, sinters with sand and adheres to the casting surface. It can be a thin layer or a few millimeters thick. Sometimes, the molten metal will penetrate into the whole section of the sand core, resulting in the blockage of the inner cavity. This kind of sand sticking is often impossible to remove, so the casting has to be scrapped.
2. Chemical bonded sand is a kind of iron silicate dross with strong adhesion formed by metal oxide formed by chemical reaction of metal liquid and molding material. It often occurs in the gate or thick wall of the casting, especially when the sand mold or sand core is thin and the casting is thick.
3. The simple identification of chemical sand sticking and mechanical sand sticking lies in that the former sand sticking layer often does not contain metal iron.
2、 Cause of sand sticking
1. Enough pressure to make the liquid metal penetrate into the high hydrostatic pressure head between the sand particles. That is, the pressure formed by the pouring height of the casting and the pouring system. If the pressure exceeds the resistance pressure formed by capillary phenomenon between sand grains. That is, corpse hair = qcoso / R, where p hair is capillary pressure;. E is the wetting angle of the capillary, R is the radius of the capillary. Mechanical sanding will be formed. When the static pressure head is more than 500 mm and the sand for casting is coarse, most of the sand will be mechanically bonded unless coated. The above formula also shows that the larger the sand size is, the coarser the sand grain size is, and the smaller the body hair is, the more likely to produce mechanical sand sticking.
2. Dynamic pressure formed by liquid metal flowing in the mold.
3. Mold "burst" or "choke". That is, the dynamic pressure formed by the mixture of combustible gas released during casting and air and ignited by hot metal liquid.
4. As soon as the mechanical sand sticking starts, even if the pressure decreases, the liquid metal penetration will continue until the front of the liquid metal is solidified. That is, if the temperature of liquid metal is lower than that of solid phase line, the penetration can be stopped.
5. The common reasons for chemical sand sticking are the low fire resistance and sintering point of raw materials for wet mold and core making; impure quartz sand; insufficient addition of pulverized coal or substitutes; no coating or improper use; too high pouring temperature; improper pouring leading to slag entering the mold and other factors.