An aluminum alloy motor shell is shown in Figure 1. Material is AlSi12(Fe), shrinkage rate is 0.55%, outer dimensions of casting are 167 mm*161 mm*102 mm, volume is 277 cm3, weight is 748 g, and heat treatment is at (246±16) ℃ for 2.0~2.5 h. Leakage requirement is 30 Pa or less during a 50 kPa pressure test for 3 s.

 

Figure 1 Motor housing
Appearance of casting is mainly composed of two parts, one part is main body of motor housing; the other part is hollow flange. Basic wall thickness of main body of motor housing is 6 mm, thickness of hollow flange part is 3 mm, draft angle of pre-die casting hole is 1.5°, and rest of draft angle is 2°~3°, which meets demoulding requirements of die-casting mold.

During molding, side with the greater holding force of motor housing is inner cavity molding side. Inner cavity and hollow flange inner cavity are molded together. If inner cavity of motor housing is designed on movable mold side, a large push force is required when molded casting is pushed out, casting does not have enough strength and there is not enough space to design push rod, so inner cavity molding is designed on slider.

Parting design is shown in Figure 2. Parting lines of fixed mold and movable mold are selected on symmetry plane of motor housing, and sliders are designed on both sides to form two flange surfaces of motor housing respectively. Design position of runner is shown in Figure 2(b), and hollow flange is far away from runner. Inner cavity of casting is designed to be formed on slider, molded casting is fixed by core to reduce deformation caused by demoulding of inner cavity and ensure dimensional accuracy of outer contour of casting.

 

Figure 2 Parting design

Calculation of cross-sectional area of inner gate: picture, where V is volume of casting and overflow tank, cm3. Calculated picture is ≈202 mm2.
Based on cross-sectional area of inner gate, pressure of die-casting machine is initially selected to be 3.50*105 kN. Designed pouring and exhaust system is shown in Figure 3. Gate is designed in wide area of motor housing outline, slag bag and exhaust channel are designed on the other side. Because structural design of slag bag and exhaust channel is more flexible, they are designed on hollow flange side to meet filling requirements of die casting and obtain better quality castings.

 

Figure 3 Gating and exhaust system and bridge design
Middle window of hollow flange of motor housing is large, and at the filling end, structural strength is weak, which can easily cause poor molding. A bridge structure is designed here (see Figure 3) to enhance strength of casting.

When particles are set during filling process, software will automatically set N points in material liquid to observe flow distribution of material liquid during filling process. Particles can not only observe flow state during filling process, but also observe turbulence, vortex and other states of material liquid after it hits cavity wall.
Simulation analysis was conducted through Anycasting mold flow software. Filling effect is good, as shown in Figure 4(a). Particle display filling process meets expected requirements; solidification effect is shown in Figure 4(b). There are two hot spots at two ears of flange on one side. Position of hot spots needs to focus on cooling structure when designing mold structure.

 

Figure 4 CAE mold flow analysis

Structure of motor housing die-casting mold designed using UG software is shown in Figure 5. Size of mold base is 700 mm * 680 mm * 655 mm, and ejection stroke is 60 mm. In order to ensure airtightness requirements of castings, vacuum die-casting is used, and a vacuum structure is designed at the end of exhaust block; in order to ensure dimensional accuracy of castings, mold parting surface adopts step positioning and parting.

 

Figure 5 Mold structure

Side wall thickness of hollow flange of motor housing is relatively thin and there is no reinforcement rib support, which can easily lead to deformation of molded casting during demoulding process of slider. Size of slider is 210 mm * 155 mm * 217 mm. Reverse thrust structure is designed on slider to support two weak points of casting. As shown in Figure 6, when slider is pulling out core, reverse thrust structure resists casting to suppress its deformation.

 

Figure 6 Backward structure
1. Slider seat 2. Spring 3. Reverse push plate 4. Reverse fixed plate 5. Push rod 6. Slider 7. Reverse guide column 8. Push rod 9. Reverse push reset rod 10. Slider assembly 11. Moving mold core
Working process of slider reverse thrust structure: before die-casting production, slider assembly enters mold cavity as a whole, and reverse push reset rod 9 first contacts movable model core 11. Driven by movable model core 11, reverse push reset rod 9 drives reverse thrust structure to move to the left. Reverse push plate 3, reverse fixed plate 4, push rods 5 and 8 move smoothly to the left under guidance of reverse guide column 7 and compress spring 2 until slide assembly 10 enters cavity.
After die-casting production is completed, slider assembly 10 and movable mold core 11 move to the right as a whole. Under action of spring 2, push-back reset rod 9 is still in contact with movable mold core 11. At this time, push rods 5 and 8 are stationary relative to movable mold and play the role of supporting casting to reversely support molded casting during demoulding process of slider assembly 10. When core pulling distance of slider assembly 10 is large enough, reverse thrust structure and slider assembly 10 move to the right simultaneously.

Cooling system is shown in Figure 7. Fixed mold and movable mold are respectively designed with one set of circulating cooling water lines and one set of mold temperature oil pipes. Mold temperature oil pipe is used to balance mold temperature. Mold temperature oil of about 150℃ is circulated in mold temperature oil pipe, which can heat mold before die casting and take away heat released by solidification of casting during die casting process; fixed mold and movable mold are also designed with one set of high-pressure point cold water pipes. Slider 1 (see Figure 2) is designed with 3 sets of high-pressure point cold water pipes. Slider 2 (see Figure 2) is designed with 7 sets of high-pressure point cold water pipes. High-pressure point cooling is a local cooling method. After die-casting, high-pressure point cold water pipes first pass high-pressure water to locally cool mold, then passes high-pressure gas to blow away high-pressure water to avoid local overcooling of mold. Among them, cold water pipe 2 at high pressure point of fixed mold and cold water pipe 5 at high pressure point of movable mold correspond to hot section area in CAE solidification analysis.

 

Figure 7 Cooling system
1. Fixed mold circulating cooling water pipe 2. Fixed mold high pressure point cold water pipe 3. Fixed mold warm oil pipe 4. Moving mold circulating cooling water pipe 5. Moving mold high pressure point cold water pipe 6. Moving mold warm oil pipe 7. Slider high pressure point Cold water pipe 8. Cold water pipe at high pressure point of slider

CAE simulation analysis shows that there are two hot spots in solidification process of casting. High-pressure point cooling is designed at corresponding positions of movable mold and fixed mold to improve solidification effect. Hollow area of casting flange is large, there are risks such as weak strength and poor filling when filling end is formed. By designing bridge and special-shaped slag bag structure, filling and internal quality of casting are improved while strength of casting is enhanced.
Hollow part of casting flange is prone to deformation during demoulding process. Reverse thrust structure is designed to suppress deformation of casting flange during demoulding process of slider. Push plate and push rod fixing plate of reverse thrust structure and reverse thrust guide column have sliding guide and positioning functions. They are made of H13 material, with a heat treatment hardness of 46~50 HRC and nitriding treatment to ensure stability and reliability of reverse thrust structure.
Nitriding surface of core or adding special surface coatings (such as titanium nitride aluminum nitride, alloy HC-FC series) to reduce erosion, adding high-pressure point cooling to increase cooling effect, reducing mold sticking and erosion of molded castings.
Adjusting screws can be added to reverse thrust structure. Service life of die-casting mold is generally about 100,000 mold times. Spring is selected according to use range of 500,000 to 1 million mold times. Mold does not need to be designed with adjusting screws; in addition, slider is small in size, easy to disassemble and replace.

By analyzing casting structure and CAE simulation, mold structure and temperature control system were optimized to ensure motion stability of reverse thrust structure. The first mold trial of mold was successful, and actual molded casting is shown in Figure 8. Successful application of reverse push structure shows that design of reverse push structure on small slider is feasible.

 

Figure 8 Actual formed motor housing