A medium to large motor housing mold with heat dissipation ribs is generally divided into four parts, which are combined with white film after molding. However, the size of the combined white mold is difficult to control and its stability is poor. The mold of our company is integrally formed, with a sliding block structure for both the outer shape and inner cavity, ensuring the smooth release of the overall white film from the mold, fundamentally solving the problem of casting deformation
And the issue of dimensional accuracy. The selection of shrinkage rate parameters is crucial in mold design, with a shrinkage rate of 1.35% in the length direction and 1.15% in the height direction for large lost foam motor casing castings.
The form of the lost foam casting pouring system is different from traditional processes. The size, dimension, shape, and direction of the sprue in lost foam casting directly affect the vaporization of the mold. If the size of the sprue is calculated according to traditional sand casting processes, the iron liquid capacity is small and the surrounding heat dissipation area is large, which will sharply reduce the temperature of the iron liquid entering the mold cavity and affect the vaporization speed of the mold. According to the traditional sand casting process, the total cross-sectional area of the inner runner is calculated and generally increased by 10% to 25%, which can be adjusted appropriately after testing. Due to the high height of the casting, a step is adopted The pouring method of the ladder is crucial, and the design of the inner runner position of the casting is crucial. Multiple points of molten iron are used for the inner runner. The pouring system and parameter design are as follows: the cross-sectional size of the sprue is 48mm, with a total of 4 layers of transverse and inner runners, evenly arranged in height and diameter directions. The top layer has 8 runners, and the remaining 3 layers each have 6 runners, for a total of 26 runners. Due to the hollow shape of the sprue, the molten iron directly enters the lowest sprue and rises with the liquid level of the molten iron.
Due to the 5mm thickness of the heat sink in the motor casing, the original EPS bead size should also be chosen to be smaller. Therefore, our company adopts the Dragon King p-s/p-4s model. Due to the numerous and relatively thin heat sinks, the pre emission density should be controlled at 25-26g/L. Electric heating is used for pre production to ensure uniform size and low moisture content of the pre produced beads. The special hydraulic semi-automatic forming machine for motor housing is used for foam forming. The equipment size is 2.6m long, 2.6m wide and 9m high. The maximum installation mold size is 2.3m long and 2.3m wide. After the automatic feeding is completed, the remaining processes are automatically completed by the equipment control system, which ensures the consistency of the quality of the white mold. After the model is produced, it cannot immediately enter the drying room and should be placed on the storage rack in the molding area. It should wait for the surface moisture to evaporate and be inspected before entering the drying room. The benefits of doing this are: firstly, the newly produced white mold has a high moisture content. If it is directly placed in the drying room, it will cause high humidity in the drying room; Secondly, during inspection, waste products can be picked out and directly scrapped, which will reduce the occupation of drying room space.
In addition to general performance requirements, there are three other points that deserve special attention when it comes to coating for motor shell castings: (1) good crack resistance and coating adhesion of the coating at room temperature. Due to the fact that almost 95% of the surface of the motor casing is vertical, it is difficult to ensure the thickness of each coating if the coating is not well applied. The heat dissipation ribs of the motor casing are numerous, the walls are thin, and there are threading grooves on the inner wall. The coating is prone to poor coating at this location, which can easily cause uneven coating. Thick water-based coatings are prone to cracking during the drying process, and these cracks are difficult to detect. If not handled properly, they can cause sand sticking during pouring, often making it difficult to clean and resulting in scrapped castings. author The company adopts a dip coating process. Before the dip coating operation, the coating is stirred for 1 hour with a mixer to ensure uniformity of the coating. Place the assembled white mold into a paint pool that is evenly stirred and meets the required process requirements. Slowly press down manually to bring the white mold into a range of 200-300mm, applying even force. Stop for 1 minute and rotate it 90 degrees, then stop for 1 minute and rotate again until it is completely hung. Then lift it out and place it upright on a rack. Use a brush to find any areas of white leakage. Transfer to the drying room after 2 hours. (2) High temperature crack resistance. Due to the large number of heat dissipation fins, there will be many deep and narrow areas on the outer wall of the casing, which will become hot spots after pouring. Therefore, these areas require very high coating requirements. If the pouring process cannot withstand the impact of molten iron, sand sticking defects will occur, as shown in Figure 4; If the fire resistance of the coating is insufficient or the adhesive is used improperly, surface scars and roughness will appear after shot blasting, as shown in Figure 5.
(3) The peeling performance of coatings. The inner cavity of the motor casing is threaded through the cable tray, and during the shot blasting process, the steel shot cannot enter the cable tray normally, requiring high stripping performance of the coating. The author’s company added additives to the coating, and after multiple adjustments, finally added 2% iron oxide red as aggregate, which met the usage requirements.
After using the lost foam casting process for motor casing castings, the yield rate of the castings is ≥ 97%, the metallographic structure and mechanical properties of the castings meet the standards, and the appearance quality of the castings is better than those produced by the original furan resin sand process.
The molds for medium and large motor shell castings are formed as a whole, with different shrinkage rates for radial and height dimensions; Adopting a stepped pouring system, strictly controlling the drying temperature and time of the white mold, with a focus on controlling the coating’s adhesion, room temperature/high temperature crack resistance, peel resistance, and other properties; The pouring temperature is between 1490 and 1500 ℃, and the pouring vacuum degree is controlled between -0.065 and -0.07 MPa. It is possible to produce castings with good surface quality, qualified dimensions, and satisfactory performance, and the production cost per ton of castings is 1400 yuan lower than that of furan resin sand process. Practice has proven that using the lost foam casting process to produce motor casing castings is feasible.