Figure 1 shows housing of a car range extender, made of aluminum alloy ADC12, with outline dimensions of approximately 524 mm * 407 mm * 105 mm, a weight of approximately 9.4 kg, and a basic wall thickness of 3.5 mm. The overall structure of casting is relatively complex, requiring three-way sliding core-pulling molding on sides. Internal cylinder liner is integrally molded, flange face has narrow and densely packed grooves to reduce weight, and reinforcing ribs are densely interwoven. Functional areas of casting have locally large wall thicknesses. Porosity is controlled according to Q/XF B13008A-2024 "Standard for Porosity of Cast Aluminum Alloy Parts," and cavity is required to be leak-free under a pressure of 60 kPa. Based on calculated projected area of casting, a 25,000 kN die-casting machine is required for production.

 

Figure 1. Housing of a car range extender

Based on comprehensive analysis of casting structure and functionality, simultaneously meeting parting line and mold structure requirements, feeding direction and position are selected on the side near suspension hole and shaft hole inside cavity. Runner design is a combination of eagle-beak and fan-shaped gates. Venting design uses a mechanical vacuum valve for centralized venting. CAE simulation analysis was performed using Anycasting software, and results are shown in Figure 2. Leading edge of molten metal basically flows into overflow system, resulting in good overall filling effect and a clear casting outline.

 

Figure 2. Simulation analysis of filling process

Mold temperature control system consists of two parts: heating and cooling. Heating system preheats mold before production, and cooling system regulates temperature within a certain range during production. Purpose is to balance mold temperature field, maintain fluidity of molten metal during filling, effectively fill cavity, ensure uniform solidification of casting, facilitate pressure transmission, and improve internal forming quality of casting.
In mold design and development phase, results of casting wall thickness analysis, functional analysis, and simulated solidification analysis are generally used as design basis for mold temperature control system.
Risk area of casting is region with four suspension holes. Here, the thickest ribs reach 18.2 mm, and wall thickness of suspension hole bosses is also as high as 17 mm. Multiple reinforcing ribs converge here, forming a narrow, mass-reducing groove. Minimum width of groove bottom profile is 7 mm, and depth exceeds 35 mm, as shown in Figure 3. Large wall thickness difference in local areas of casting forms hot nodes. During solidification, insufficient feeding results in shrinkage cavities and porosity defects, affecting strength of casting area and failing to meet functional requirements of casting.

 

Figure 3. Cross-sectional structure of risk area of casting
Results of simulated solidification sequence analysis verified risk of defects in suspension hole region of casting. Figure 4 shows simulation analysis results of casting solidification sequence. When ingate has solidified, isolated liquid phase regions still exist in four suspension hole areas in fixed mold direction. To meet strength requirements after assembly, there is no room for optimization in casting structure; improvements can only be made during mold design. Since this area is close to runner, there is also room for adjustment in die-casting process during actual production. Mold is first improved through partial inlay, 3D printing of conformal cooling water channels, pre-casting of suspension holes, and design of a high-pressure spot cooling scheme.

 

Figure 4 Solidification Sequence Simulation Analysis
Based on die-casting process analysis and simulation results, designed mold temperature control system is shown in Figure 5. Fixed mold includes 14 sets of high-pressure spot cooling, 17 sets of atmospheric pressure cooling water, 2 sets of circulating atmospheric pressure cooling, and 1 set of mold temperature liquid channels; slider core pulling system includes 5 sets of circulating cooling water, 15 sets of high-pressure cooling water for slider, and high-pressure spot cooling for moving mold; moving mold includes 32 sets of high-pressure spot cooling, 2 sets of 3D printed water channels, and 19 sets of atmospheric pressure cooling water.

 

Figure 5. Mold Temperature Control System

Based on optimized process, die-casting mold for shell was designed using UG software, as shown in Figure 6. Mold base dimensions are 1480 mm * 1490 mm * 1150 mm. Mold includes four side core-pulling mechanisms, with a moving mold stroke of 100 mm, matching 420 mm eccentricity of die-casting machine.

 

Figure 6. Mold Structure
After die-casting mold was manufactured, trial production was conducted on a 25,000 kN bedroom cold chamber die-casting machine. Aluminum alloy grade was ADC12. During production on die-casting machine, four slide systems are sequentially fed in, moving and fixed molds quickly close. Molten aluminum enters mold cavity through barrel. Once molten aluminum reaches ingate position through runner, it rapidly fills remaining portion of cavity at a speed of 4.0 m/s. Then, under continuous pressure of die-casting machine, molten aluminum in cavity rapidly solidifies to form a dense die-casting. Finally, mold opens, casting is ejected by moving mold pusher.
During die-casting process, a mechanical valve is used to evacuate mold cavity to reduce amount of gas trapped and entrained during filling of molten aluminum, thus improving filling quality. X-ray non-destructive testing of produced castings revealed that most of hot spot areas of four suspension holes did not have internal defects, but some shrinkage porosity was observed. Figure 7 shows X-ray inspection results of suspension hole area of casting from first trial casting. Considering mold process and temperature control system design, an optimized scheme was determined to be to add local extrusion at corresponding shrinkage cavity defect locations in casting.

 

Figure 7. X-ray inspection results of castings

Local extrusion is a process where, after casting has been filled and molten metal has begun to solidify but not yet fully solidified, local extrusion is applied to areas of casting with thick walls where isolated liquid phase regions exist. A hydraulic cylinder piston rod pushes an extrusion rod to force molten metal in pre-stored space into mold cavity for forced feeding. This is an effective measure to solve shrinkage defects caused by thick walls in castings.
Extrusion mechanism for casting improvement is shown in Figure 8. Required extrusion amount is determined to be 6 times volume of defective area in casting. Designed extrusion rod diameter is φ9 mm, maximum extrusion stroke is 15 mm, and it is flush with surface of casting boss after extrusion. Hydraulic system pressure is 140 kg/cm². Based on calculation that local boost pressure is 20-30 times system pressure, local boost pressure is 4200 kg/cm². Therefore, required diameter of extrusion hydraulic cylinder is calculated to be φ50 mm. Extrusion design is located on moving die side. To facilitate maintenance of extrusion mechanism, it is designed as a quick-change structure that can be independently installed and directly disassembled, passing through ejection mechanism and moving die base plate.

 

Figure 8. Local Pressure Intensification Mechanism
1. Casting 2. Moving Die Insert 3. Moving Die Base 4. Push Rod Fixing Plate 5. Push Plate 6. Moving Die Base Plate 7. Extrusion Rod 8. Extrusion Sleeve 9. Extrusion Hydraulic Cylinder 10. Extrusion Mechanism Pressure Plate
After optimizing mold, trial production was conducted. Verification showed that adding local extrusion resolved internal shrinkage problem in casting's suspension hole area. Figure 9 shows X-ray inspection results after adding extrusion; casting showed no obvious internal defects.

 

Figure 9. X-ray Inspection Results of Casting After Adding Extrusion