Design of an aluminum alloy shell die-casting mold

By analyzing structure and formability of shell parts, we focused on detailed analysis and research on main aspects of mold design; we analyzed and designed parting surface position of mold, structure of pouring and overflow system, and ejection mechanism, a structure of multi-point gates on thick wall on the side was adopted; a specific structural design was made for core and cavity of mold, using an ordinary two-plate structure with cavity in fixed mold and core in movable mold; moreover, core and cavity are made into movable inserts for easy repair and replacement. Mold structure is simple and practical, fully meeting design requirements of mold.
Aluminum alloy materials have lightweight characteristics. With continuous development of casting and forming process technology, die-casting process and mold design of aluminum alloy materials have developed rapidly. Design of die-casting molds is an important part of die-casting forming and has an important impact on cost, efficiency and accuracy of finished products of the entire processing process. Therefore, many scholars at home and abroad have conducted research and analysis on die-casting molds. Through experimental comparative analysis, Ma Dongwei found that main factors affecting size of aluminum alloy styles are residual stress and changes in solid phase crystallization; Shi Baoliang conducted relevant analysis on typical parts of structural parts used in automotive industry, focusing on performance characteristics of aluminum alloy castings under high pressure; Jin K C designed thin plate die-casting mold using two geometries, proposed a new overflow system based on numerical simulation, conducted an actual vacuum die-casting test of part of channel without backflow, and manufactured a high-quality sample using proposed optimized mold design; Péter Szalva compared high-cycle fatigue behavior of high-pressure die-casting and vacuum-assisted die-casting, described how casting defects affect fatigue failure, found that vacuum-assisted die casting significantly reduces pore size and volume, reduces occurrence of oxidized flakes, and thus increases number of failure cycles. Research of above scholars is all about microstructure of product parts after die-casting and structural performance analysis of aluminum alloy castings. Structural design and simplification of die-casting mold have not been mentioned yet. Therefore, direction of this research is to design an aluminum alloy shell die-casting mold. This design can effectively avoid defects such as cold insulation, slag inclusions, bubbles, looseness, and unformed heat sinks caused by die-casting process.

1. Die casting structure and process analysis

Figure 1 shows shell parts, which are made of aluminum alloy die-casting. Casting structure is relatively complex and wall thickness is different. Wall thickness of end face of shell is 5 mm, wall thickness of surrounding sides is 3 mm, and wall thickness of five bosses on one side is 15 mm. There are 25 heat sinks on the back, which are relatively dense. Width of narrow end is 1 mm, slope of one side is 1.5°, and depth is 9 mm. Wall thickness is 5 mm with four ribs and 1.5 mm with six ribs.

Figure 1 Shell parts diagram
After a comprehensive analysis of structural characteristics of casting, flow direction and characteristics of aluminum alloy liquid in mold should be considered when designing mold, material flow direction and relationship with direction of heat sink should be reasonably selected; due to uneven wall thickness of castings, casting defects such as slag inclusions and looseness are prone to occur during die casting. Therefore, location of inner gate should be selected reasonably so that casting can be fully formed during die casting. Considering structure of shell and actual production conditions, this die-casting mold design adopts a one-mold-one-cavity structure.

2. Mold structure design

2.1 Selection and design of parting surface

According to structural characteristics of casting and design requirements of parting surface, large end surface of shell is selected as parting surface of movable mold and static mold. In order to facilitate demoulding of casting, finished product should be left on the side of movable mold. Draft angle of inner surface of shell is 2.5° on one side and depth is 48 mm. Tightening force of forming part can keep shell on core side, so core is selected to be on movable mold and cavity is on fixed mold. Structural form is shown in Figure 2.

Figure 2 Parting surface settings

2.2 Design of pouring system and overflow system

According to design principles of die-casting mold pouring system, metal flow direction should be parallel to direction of heat sink to avoid defects such as cold insulation, slag inclusions, bubbles, looseness, and unformed heat sinks. In addition, position of inner gate should be set at thick wall, so that molten metal can fill thick wall first to avoid casting defects such as slag inclusions and looseness in thick wall. Therefore, in order to quickly fill mold cavity with molten metal, six ingates are provided on one side of shell boss. Runner adopts a stepped arc transition design to ensure sufficient filling speed. Sprue is equipped with a diverter cone, and diverter cone is designed with an arc transition structure, which can speed up filling speed of molten metal during die casting. Overflow tank should be set at the end of material flow direction. As shown in Figure 3, 13 overflow grooves and exhaust channels are provided on three sides of casting forming cavity.

Figure 3 Layout of pouring system and overflow system

2.3 Launch institutional design

This mold uses a push rod to push out casting. In mold design, position selection of push rod is crucial. Generally speaking, push rod position should be set at position where casting has the greatest tightening force on core and at thick wall of casting to prevent casting from being damaged when pushed out. After comprehensive consideration, all push rods adopt Φ8 mm round push rods. There are 6 push rods on the top surface of inner surface of casting and 12 push rods on the end surface of casting. In addition, 9 push rods are installed at sprue and runner, 13 push rods are installed at all overflow troughs. This design can fully meet launch requirements.

2.4 Cooling system design

Improving die-casting production efficiency, as well as quality and density of die-casting parts and reducing thermal stress, largely depend on adjustment of mold temperature. Considering that die casting is a thick-walled casting and is produced in small and medium batches, during continuous operation, in order to maintain high quality and high productivity of casting, a water cooling device needs to be installed in mold to allow heat to be quickly discharged with circulating flow of cooling water. Mold adopts a relatively simple cooling system, and cooling water channel is set in cavity with higher mold temperature (i.e., fixed mold insert). Six Φ10 mm cooling water channels are set up along length of cavity. There are six water nozzles on each side of fixed mold and fixed mold insert is threaded (sealed). Water inlet pipe and water outlet pipe are set on side opposite operator, water nozzles on both sides are connected with soft water hoses (tightened with a tightening reed) to form a complete water cooling circulation system, as shown in Figure 4.

1. Fixed mold plate 2. Fixed mold insert 3. Movable mold insert 1 4. Movable mold insert 2 5. Gate sleeve 6. Diverter cone 7. Movable mold plate 8, Push plate fixed plate 9. Push plate 10. Moving Mold base plate 11. Push rod 12. Reset rod 13. Guide sleeve 14. Guide column 15. Pad 16. Push plate guide 17. Push plate guide 18. Faucet 19. Limiting nail 20. Hexagon socket bolt
Figure 4 Mold assembly drawing

2.5 Mold structure and final assembly design

Figure 4 shows final assembly structure diagram of this mold. This mold adopts an ordinary two-plate structure. Considering complexity of casting structure and cost factors of mold production, cavity and core parts of mold adopt movable inserts, which are embedded in movable and fixed mold plates respectively. Moving and fixed mold inserts, moving and fixed mold plates adopt H7/K6 transition fit, are connected and fixed with bolts. This design facilitates processing of mold forming part, as well as repair, replacement and size adjustment of forming part. Mold closing of moving and fixed molds adopts combination of four guide pillars and guide bushes to ensure stable and accurate mold closing. In order to ensure that push rod can slide smoothly, push rod is fixed in push plate and push plate fixed plate. A structure of four push plate guide posts and guide bushes is used to support weight of push plate and push plate fixed plate to ensure that push rod operates smoothly and will not deform. Four Φ20 mm reset rods are installed in moving mold plate and fixed in push plate. After push-out action is completed, when mold is closed, reset rod in movable mold drives all push rods to complete reset.

3. Production verification

A domestic research institute currently has a 300 t cold chamber die-casting machine. Mold used is mold designed and developed this time. It adopts an ordinary two-plate structure with cavity in fixed mold and core in movable mold. Trial production of this aluminum alloy shell was completed by preparing aluminum alloy liquid and optimizing relevant parameters of die-casting process. Product produced after removing slag bag and sawing off gate is shown in Figure 5. This trial production effectively avoided defects such as cold insulation, slag inclusions, bubbles, looseness, unformed heat sinks caused by die-casting process, and achieved expected results.

Figure 5 Trial product sample

4 Conclusion

(1) Designing main forming part structure as an insert and processing it separately can not only control dimensional accuracy of casting, but also enable rapid repair, replacement and adjustment of wearing parts.
(2) Through structural analysis of casting, we chose to set up multiple internal gates at thick wall on the side of casting, and designed lateral runner into a stepped arc transition form, which not only ensures complete formation of heat sink, but also satisfies rapid filling of casting. requirements.
(3) By analyzing casting forming process, choosing to set up multiple overflow grooves at the end of metal flow direction can avoid casting defects such as cold shut, slag inclusions, bubbles, looseness, and unformed heat sinks during die casting.
(4) Structure of this mold is that core is in movable mold and cavity is in fixed mold. Main forming part adopts an inlaid structure, and mold adopts an ordinary two-plate structure. After mold testing, it was verified that mold operates smoothly and reliably, appearance quality and dimensional accuracy of die-casting parts fully meet product drawing requirements without any casting defects.

Design of die-casting mold for new energy vehicle aluminum alloy motor casing based on UG and Anycasting

1 Casting analysis

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.

2 Design of parting, pouring and exhaust systems

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.

2.1 Partial design

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

2.2 Design of pouring and exhaust system

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.

2.3 CAE mold flow analysis

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

3 Mold structure design

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

3.1 Backward structural design

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.

3.2 Cooling system design

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

4 Issues that need attention in design

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.

5 Conclusion

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