Under the same production conditions, the process yield and scrap rate of three different casting process schemes were obtained. Scheme 1 has high process yield and qualified rate, which is the ideal process scheme among the three schemes. First, the runner runner is placed above the inner runner, which is conducive to the foam and vaporization of the slag and the inclusion of gas and inclusions in the molten iron. It has good slag collecting effect and easy to obtain high-quality castings. There is no defect in the lost foam casting of the water table shell, as shown in Fig. 1. The runner length of the second plan, when the liquid metal enters the cavity, becomes slower and slower, filling time is longer, and more heat is needed to vaporize the runner and foam pattern, so that the temperature is reduced too much, resulting in incomplete evaporation of local patterns, and easy to cause defects such as wrinkle and slag. The pass rate is only 50%. Scheme 3 has only sprue and inner sprue, which shortens the flow distance of liquid metal and reduces the temperature drop. Under the same pouring conditions, it is conducive to the smooth discharge of vaporized products; However, because the inside of the water meter case is concave, the dry sand is not easy to fill and compact, and the paint at the depression is easy to crack and produce metal tumors during negative pressure pouring.
The main defects of lost foam casting for large-diameter ductile iron watch case are: liquid metal reverse injection during pouring, insufficient pouring, wrinkle and slag inclusion, cold shut-off and shrinkage, etc.
1. Choking fire
During the casting of lost foam casting, the metal liquid is sprayed back from the gate cup, resulting in the splashing of metal liquid everywhere, resulting in great hidden dangers of production safety. At the same time, the casting is prone to defects such as air hole, slag inclusion, cold shut and so on. It was found that the drying time of the foam mould did not meet the requirements of the process. The foam mould was not completely dried, and the retention time in the humid air before and after the sand molding was long and damp. When the water in the foam was vaporized, the huge pressure on the front of the alloy liquid was formed, resulting in the backwash. In addition, such as low negative pressure during pouring and poor air permeability of coating layer, liquid metal back spraying phenomenon occurs. In order to solve this problem, we can start from the following aspects: Foam white mold should be thoroughly dried after immersion coating and dip coating. A transition drying room shall be set in the sand embedding molding area to shorten the molding and pouring time as much as possible; Increase pouring negative pressure; Adopt the aggregate coating with larger particle size to improve the air permeability of the coating.
2. Insufficient pouring
Insufficient pouring (see Fig. 2) is one of the defects of lost foam casting of ductile iron watch case. The reasons are as follows: the pouring speed is slow, there is a gap in the sprue, and the external air is sucked into the cavity under the negative suction of negative pressure and vacuum during pouring, resulting in the increase of pressure at the front edge above the cavity and the inability of liquid metal to fill the cavity, resulting in the defect of insufficient pouring; The process design of lost foam casting is unreasonable, which is not conducive to the gas generated from the pyrolysis of the foam pattern from the cavity. Finally, the pressure of the filling part increases, resulting in insufficient casting defects. In addition, pouring temperature is low, and foam gasification and heat absorption reduce the temperature of molten metal, thus reducing the filling ability of molten iron and causing insufficient casting defects.
In order to avoid the occurrence of insufficient pouring defects, the following preventive measures can be taken: the pouring shall be stable, accurate and fast, ensure that the gate cup is filled instantaneously and the flow is not cut off quickly, and always keep the gate cup in the full state, so as to prevent the gap in the sprue from sucking the external air into the cavity; The bottom injection gating system is conducive to smooth discharge of molten iron and foam cracking. Increase the pouring temperature to ensure the fluidity and mold filling capacity of molten iron.
3. Wrinkle and slag inclusion
Another kind of common defects in lost foam casting of ductile iron watch case are wrinkle and slag inclusion (see Fig. 3). The main reason is that the nodular cast iron is paste and solidified during solidification, and the residue after pyrolysis of foam pattern can not be discharged in time and remains in castings.
In order to avoid wrinkle and slag inclusion defects, the following measures can be taken: raising the pouring temperature, completely vaporizing the foam pattern, reducing the amount of residue, and prolonging the liquid liquid holding time. A slag collecting riser is set at the top of each part of the casting to discharge the residue into the slag collecting riser; The bottom injection gating system is adopted to ensure the smooth filling of the molten iron, so as to ensure the smooth discharge of the slag after the vaporization of the foam. The foam pattern was made by using less copolymers of pyrolysis products (residues).
4. Cold shut-off and shrinkage
Due to the low pouring temperature, discontinuous pouring and cut-off flow, the metal liquid at the solidification front crystallizes before the metal liquid filled later, or mixed flow occurs due to unreasonable casting process design, which is easy to cause cold shut defects of castings (see FIG. 4A). In the test, it is also found that when nodular cast iron is used to produce large-diameter water meter shell, centralized shrinkage is easy to occur at the connection between slag collector riser and casting (see Fig. 4b). The main reason is that the pouring temperature is low, the riser neck is too small, and the riser neck solidifies before the casting, so that the shrinkage part of the casting can not be fed by the riser liquid metal, resulting in large concentrated holes in the final solidification part of the casting. In addition, the carbon equivalent of molten alloy is low, the fluidity of molten iron is poor, and shrinkage defects are easy to appear. The measures to prevent cold insulation are: increasing pouring temperature; Fast continuous pouring and continuous flow to keep the sprue always in a full state. Higher pouring temperature and higher carbon equivalent are effective measures to prevent shrinkage cavities in castings.
