As shown in Figure 1, aluminum alloy automotive shock absorber top rubber support casting is triangular in shape with a symmetrical structure. Central bearing hole is load-bearing hole, and three smaller holes are riveting connection holes. Central bearing hole is deeper on the front and shallower on the back. Casting material is ADC1, with dimensions of 150.3mm * 151.9mm * 51.5mm, a volume of 149cm³, and a weight of 403g. Wall thickness in the central circular hole area is approximately 10mm locally, with a basic wall thickness of approximately 3mm. Casting quality acceptance standard is: internal hole size < φ2mm.

 

Figure 1 Shock absorber top rubber support

Based on structural characteristics of aluminum alloy automotive shock absorber top rubber support, parting line is selected at position of the largest outer contour of casting. This simplifies mold parting surface and structure, also facilitates design of gating system and overflow system. Back structure of aluminum alloy automotive shock absorber top rubber bracket is relatively complex, so it is designed to be formed on moving mold side. The overall structure of top rubber bracket is relatively simple, but central bearing hole is deep, posing a risk of sticking to fixed mold during die casting. Therefore, shock absorber top rubber bracket is designed to be formed on fixed mold side. Mold design needs to focus on characteristics of central bearing hole and design a solution to address this. Specific parting design is shown in Figure 2.

 

Figure 2 Parting design

Based on dimensions and specifications of automotive bracket casting, market demand, and company's die casting machine resources, after technical review, a 3500kN horizontal cold chamber die casting machine and a 2-cavity technical scheme were selected; punch diameter is φ80mm. Casting is a triangular disc shape, with a hollow center that is relatively tall and thick-walled, representing main functional area. Therefore, gating system is designed with a clamp-shaped runner, with gate direction aligned with central shaft hole, as shown in Figure 3.

Gating system is a crucial component of die-casting mold. It not only determines flow direction and state of molten aluminum alloy within cavity, but also significantly impacts gas discharge, casting pressure transmission, and distribution of mold temperature field, as well as die-casting process adjustments. Gating system design for aluminum alloy automotive shock absorber top rubber support is shown in Figure 3. Cross-sectional area of ingate is calculated according to formula (1):

 

Where, ∑A_inner is cross-sectional area of ingate for a single part, mm²; G is mass of a single part and slag bag (approximately 1.3 times part mass), g; ρ is density of molten aluminum alloy, g/cm³; v is filling speed of molten aluminum alloy, m/s; t is filling time of molten aluminum alloy, s.
Using UG software to measure 3D model, with G = 403g, ρ = 2.4g/cm³, v = 42m/s, and t = 0.02s, calculated cross-sectional area of ingate is approximately 200mm², and ingate thickness is approximately 2.2mm, which is 70% of casting wall thickness.

Overflow system is designed to improve internal quality and appearance of casting and is an indispensable component of die-casting mold. Overflow system not only effectively reduces defects such as internal voids but also removes solidified molten metal at the front end during filling process. Because aluminum alloy melt loses a lot of heat when filling to the end of cavity, designing a slag pocket not only improves temperature field balance of mold but also improves casting quality. Overflow system design is shown in Figure 3.

 

Figure 3. Pouring and overflow system

Flow3D software was used to perform mold flow analysis on molding process. 3D model was imported into Flow3D software in .stl format, and mesh element size was set to ≤1mm. Parameters of Flow3D software were set sequentially according to operation procedure. Mold part material was SKD61 steel, and die casting material was ADC1. Die casting process parameters are shown in Table 1.
Table 1. Process Parameters

Cavity filling sequence is shown in Figure 4. When t=0.1998s, molten aluminum alloy reached vicinity of ingate and began to fill cavity; when t=0.2056s, cavity was about 50% filled; when t=0.2205s, molten aluminum alloy had completely filled cavity; when t=0.231s, slag pocket and venting channel were completely filled. Overall cavity filling was stable, with no large-scale air entrapment. Molten metal simultaneously reached overflow system at the end of cavity, indicating ideal overall filling. Solidification sequence analysis is shown in Figure 5. Isolated hot spots are concentrated around intermediate shaft hole of casting. Location of these hot spots requires special attention during mold structure design. Cooling system should be designed with emphasis to reduce porosity and shrinkage defects in aluminum alloy automotive shock absorber top rubber support, thereby improving its molding quality.

 

Figure 4 Cavity filling sequence

 

Figure 5. Solidification analysis hot spot region

Die-casting mold structure for shock absorber top rubber support was designed using UG software, as shown in Figure 6. Overall mold dimensions are 760mm * 650mm * 550mm, with an ejection stroke of 40mm. To ensure smooth gas discharge from cavity, an venting block structure was designed at the end of venting channel. To ensure dimensional accuracy of casting, mold parting uses a concave-convex positioning parting method.

 

Cooling system of die-casting mold is a crucial factor affecting molding quality of casting. Cooling system is an important method for mold temperature control. To ensure continuity of die-casting production, heat generated during die-casting process must be removed through cooling system to maintain die-casting mold temperature within a reasonable range. Since shock absorber top rubber support has a basic wall thickness of 3mm, which is a conventional casting, and thickness of intermediate bearing hole is about 10mm, it generates a significant amount of heat. Based on solidification analysis results, cooling water channels were designed for corresponding hot spots, as shown in Figure 7. Intermediate shaft hole on fixed mold side generates a lot of heat, so a special cooling core circulation cooling structure was designed. One set of cooling water channels was designed near fixed mold flow channel, and two point cooling structures were designed at intermediate shaft hole on moving mold side. One set of cooling water channels was also designed near moving mold flow channel.

 

Figure 7 Temperature control system design

Fixed mold back-push mechanism is designed on fixed mold side to eject die-casting part. To address issue of high clamping force on fixed mold side, casting remains on moving mold side during mold opening, facilitating removal of die-casting part. Fixed mold back-push mechanism is shown in Figure 8.

 

1. Fixed mold frame 2. Reverse thrust block 3. Fixed mold insert 4. Fixed mold insert 5. Cooling pipe 6. Disc spring 7. Disc spring pad 8. Sealing ring 9. Water cooling core
Figure 8 Fixed mold reverse thrust mechanism
Fixed mold frame 1 and fixed mold insert 4 are fixedly connected. Fixed mold insert 3 can slide up and down a certain distance (4mm) within channel of fixed mold insert 4. Extreme position of slide is controlled by mounting platform of fixed mold insert 3. Counter-pressure block 2 is fixed to fixed mold frame 1, pressing down on disc spring 6. Disc spring 6 transmits pressure to fixed mold insert 3 through disc spring pad 7, pushing fixed mold insert 3 downwards a certain distance (4mm).
Before die casting production, when moving and fixed molds of mold are closed, fixed mold insert 3 is pushed upwards by moving mold, compressing disc spring 6 until upper end face of fixed mold insert 3 contacts fixed mold frame 1. During this process, cooling pipe 5, disc spring pad 7, sealing ring 8, and water-cooling core 9 move synchronously with fixed mold insert 3.
After die casting production, when moving and fixed molds of mold are opened, under action of disc spring 6, fixed mold insert 3, cooling pipe 5, disc spring pad 7, sealing ring 8, and water-cooling core 9 are simultaneously pushed downwards. At this time, fixed mold insert 3 pushes out casting and leaves it on moving mold.

After die-casting mold was developed and manufactured, trial production was conducted on a 3500kN horizontal cold chamber die-casting machine. Mold temperature control system was reasonably designed, and there were no obvious shrinkage cavities or sticking defects in locally thick areas near intermediate shaft hole of casting. Die-casting trial production was stable. Continuous batch production verification showed that mold parts moved smoothly and structure was reliable. Fixed mold push mechanism was reasonably designed. Actual formed aluminum alloy automotive shock absorber top rubber support underwent X-ray non-destructive testing, and no obvious porosity or shrinkage defects were found in key areas, as shown in Figure 9, meeting customer's casting quality acceptance standards.

 

Figure 9. X-ray non-destructive testing