Through the comprehensive analysis and judgment of the defects after the actual production of nodular iron castings and the CAE simulation results, the meat shortage problem of nodular iron castings is mainly caused by the excessive concentration and uneven distribution of the internal sprue in the gating system during the process design. Therefore, the process of the nodular cast iron is improved from the design of gating system in order to improve the quality of nodular cast iron. Specific measures are as follows:
(1) Since the nodular iron castings are provided with radiating rib plates along the circumference, in order to ensure that all rib plates can complete the mold filling, the number of inner sprues in the open gating system of the original process is increased, and the inlet position of the inner sprue is changed from the foundation to the bottom flange, so as to ensure the simultaneous inflow of molten iron at the inner sprue and the stable mold filling of molten iron, Avoid splashing when molten iron just enters the cavity.
(2) Judging from the defects of nodular iron castings in the original process, the lack of meat is mainly concentrated in the position of heat dissipation rib plate, where the structural thickness is only 4 mm, which is the limit value that sand casting can reach. Therefore, in order to fully ensure the filling temperature of molten iron in the mold, the original process is changed from two pieces in one box to one piece in one box. At the same time, by making full use of the characteristics of the barrel structure of this nodular iron casting, the flow of the sprue from the outer side of the nodular iron casting is optimized to the center of the shaft hole of the nodular iron casting, which can greatly shorten the flow of molten iron in the gating system, Reduce the temperature drop of molten iron in the mold and ensure the mold filling temperature. Figure 1 shows the optimized process scheme and gating system form.
The improved process is simulated by CAE, in which the material and pouring temperature of nodular iron castings are implemented according to the parameters of the original process scheme, so as to better carry out comparative analysis. The filling parameters after the process optimization are as follows: the pouring mass is 45 kg, the pouring temperature is 1420 ℃, and the filling time is 8 s, which is significantly shorter than that of two pieces in a box. Therefore, the temperature drop and temperature difference of molten iron in the pouring process will be reduced, which is conducive to the elimination of cold insulation.
Figure 2 and figure 3 respectively show the simulation results of molten iron flow rate and temperature field after process optimization. It can be seen that the optimized molten iron is filled stably and evenly in the mold, and the molten iron has no splash. Through the comparison between Fig. 4 and Fig. 3, it can be seen that the temperature of molten iron on the same plane of nodular iron castings when the mold filling is 80%. It can be seen that when 80% of the mold filling is completed, the temperature difference of the contour surface and the temperature of the nodular cast iron at the far end of the radiating rib plate of the optimized nodular cast iron are better than the original process scheme. At this time, the minimum temperature of the optimized process scheme of the nodular cast iron is 1139 ℃ (the original process is 1130 ℃), indicating that the optimized process scheme is feasible, It meets the process design requirements of structural characteristics of nodular iron castings.
After the feasibility of process optimization and CAE simulation of nodular iron castings, combined with the 3D printing sand molding technology of the author’s company, the production verification of the motor base is carried out. Figure 5 shows the nodular cast iron produced after process optimization. This nodular cast iron is only shot blasted. From the outside, through the process improvement and the application of 3D sand molding technology, the heat dissipation rib plate of nodular iron castings is well formed, the rib plate is formed at one time without any seam, uniform wall thickness and meat shortage. At the same time, the overall size of nodular iron castings is well controlled and the seam on the parting surface is small, The workload of post cleaning is greatly reduced. Through many production verification of the process, the nodular iron castings produced by this process scheme have been mass produced, with low scrap rate and stable product quality.
