A motor support plate, shown in Figure 1, is made of ADC12, has dimensions of 251 mm * 251 mm * 60 mm, a volume of 483 cm³, a basic wall thickness of 5 mm, a local thickness of 15 mm, and a weight of 1.3 kg. Airtightness is required. This casting has a single φ5 mm inclined oil channel hole and cannot be pre-cast.

 

Figure 1. Motor Support Plate
(a) Exterior (b) Interior (c) Wall Thickness Analysis

Based on structural characteristics of motor support plate, parting line is located at flange surface with the largest outer contour of casting. This not only facilitates demolding of casting but also simplifies mold parting surface and mold structure, while also facilitating design of pouring and overflow drainage systems. Inner cavity side of complex structure casting is designed to be molded in movable mold, and casting is pushed out by ejection mechanism of movable mold; outer side of casting is designed to be molded in fixed mold. Specific parting design is shown in Figure 2.

 

Figure 2 Parting design

Casting system not only plays an important role in controlling flow direction and state of molten metal in mold cavity, overflow exhaust conditions, and casting pressure transmission, but also can adjust filling speed, filling time, mold temperature distribution, etc. Casting system design is shown in Figure 3.

 

Figure 3 Casting and overflow system
Cross-sectional area of inner gate is calculated according to flow formula (1):  

 

Wherein, ∑A is the total cross-sectional area of inner gate, mm2; G is weight of casting and slag bag (about 1.3 times weight of casting), g; ρ is density of alloy liquid, g/cm3; v is filling speed, m/s; t is filling time, s. According to 3D model, G is 1.3 kg, ρ is 2.4 g/cm3, v is 35 m/s, and t is 0.03 s. Cross-sectional area of ingode is calculated to be 670 mm2, and thickness of ingode is 3 mm, which is 60% of wall thickness of casting. If runner thickness is too small, molten metal temperature will be reduced. If it is too large, cooling rate will be slow, which will affect productivity and increase metal consumption. Runner thickness is determined by empirical formula: D=(5~8)T. Where D is runner thickness, mm; T is ingode thickness, mm. Runner thickness D is 22 mm, and demolding slope of runner is set to 10°. Cross-sectional area of runner is designed to be a flat trapezoid, and cross-sectional area maintains a uniform reduction from sprue to ingode. Casting is a regular disc with a hollow center that is relatively tall and thick, representing main functional area. Therefore, a clamp-type runner was used, with gate facing the central axis hole, as shown in Figure 3.

To improve casting quality and reduce defects such as air holes and slag holes, cavity should be filled with as much air and contaminated cold metal as possible during molten metal filling process. Therefore, a drain system structure is necessary. Because molten metal experiences significant heat loss upon reaching end of cavity, a slag bag can improve mold temperature balance and enhance casting quality. Complete gating and drain system design is shown in Figure 3.

Anycasting software was used to perform numerical simulations to verify designed die-casting process. 3D geometric model was imported into software in .stl format. Mesh cell size was set to 0.1 cm. Mold material was SKD61 steel, and casting material was ADC12. Process parameters are shown in Table 1.

Table 1 Process Parameters
Filling sequence and process are shown in Figure 4. At t = 0.050 8 s, two center streams of molten metal reached ingate and began filling cavity. At t = 0.055 1 s, four runners of molten metal reached ingate and began filling cavity. At t = 0.062 6 s, cavity was approximately 50% filled. At t = 0.073 8 s, molten metal completely filled cavity. At t = 0.080 8 s, slag bag and vent were completely filled. Overall filling process was relatively smooth, with no significant entanglement. Molten metal reached overflow system at the end of cavity almost simultaneously, indicating an ideal overall filling process.

 

Figure 4 Filling Sequence
(a) 0.050 8 s (b) 0.055 1 s (c) 0.062 6 s (d) 0.073 8 s (e) 0.080 8 s
As shown in Figure 5, solidification sequence analysis shows isolated hot spots concentrated around center shaft hole of casting. Location of these hot spots requires special attention during mold design, with a particular emphasis on cooling mechanisms. To reduce porosity and shrinkage defects and improve casting quality, vacuum die casting is recommended.

 

Figure 5 Solidification Analysis of Hot Spots

Die casting mold temperature is a key factor affecting casting quality. To ensure mold temperature remains within a reasonable range during continuous production, water cooling is typically used to cool mold cavity. Since basic wall thickness of casting is 5 mm, it is considered a thick-walled casting. Thickness of center bearing hole can reach 15 mm locally, generating significant heat. Based on solidification analysis results, cooling water circuits were designed for hot spots, as shown in Figure 6. Two cooling water circuits were designed for fixed die side, and one for movable die side. A cooling core circulation cooling structure was specifically designed for center shaft hole, where heat is concentrated.

UG software was used to design motor support plate die-casting mold, as shown in Figure 7. Mold frame dimensions were 870 mm * 780 mm * 670 mm, and movable die had a travel stroke of 50 mm.

 

Figure 7: Overall Mold Structure
After die-casting mold was completed, trial production began on a 9,000 kN horizontal cold chamber die-casting machine. During production, fixed and movable dies quickly close together, allowing high-temperature molten aluminum to enter through barrel. Under high pressure of die-casting machine, molten aluminum is guided by sprue bushing and diverter cone, filling cavity formed by fixed and movable die cores at high speed and uniformly, where it rapidly solidifies and forms under high pressure. Die-casting machine maintains pressure for 7 seconds, then opens mold, and casting is ejected by ejector. During die-casting trial run, mold cavity was vacuumed to reduce amount of air entrained and trapped during flow of molten aluminum and improve filling quality. X-ray nondestructive testing of resulting casting revealed significant porosity defects at the ends of bearing hole (on both sides of uncast inclined oil channel), as shown in Figure 8.

 

Figure 8 X-ray inspection results
Analysis of defective areas in casting revealed that primary cause was localized excessive mold temperature. High-pressure spot cooling was employed to cool mold. High-pressure spot cooling is a localized cooling method that offers a shorter distance to mold surface than conventional spot cooling, resulting in better cooling efficiency. High-pressure spot cooling also allows for real-time control of mold temperature field based on die-casting cycle. During die-casting, high-pressure spot cooling machine first supplies high-pressure cooling water to mold. After mold is cooled locally, cooling water is removed by high-pressure gas, ensuring that mold is not overcooled and that mold remains within a reasonable temperature range.
Six high-pressure spot cooling points were added to center bearing hole of casting, as shown in Figure 9(a). Temperature field at shaft hole was adjusted in accordance with die-casting parameters. During cavity filling, high-pressure spot cooling is used to rapidly cool mold interior, causing casting structure around high-pressure spot to solidify rapidly, forming a relatively thick, dense layer. This also reduces solidification time, thereby reducing precipitation and accumulation of gas in molten metal, reducing tendency of porosity in casting, and improving casting quality.

 

Figure 9 Process Optimization
Thickness of two central gates of casting was each increased by 0.2 mm, as shown in Figure 9(b). This increased flow rate of two central streams of molten metal, allowing greater energy to fill shaft hole area. This allows more molten metal to flow through shaft hole, which can carry away gas or slag flowing through area, improving internal quality of casting.
Based on optimized process, mold structure was modified and trial production was conducted on a 9,000 kN horizontal cold chamber die-casting machine. X-ray nondestructive testing revealed that porosity at the end of bearing hole (on both sides of uncast inclined oil channel hole) had disappeared, as shown in Figure 10. After processing, casting had no porosity defects in this area, meeting quality requirements.

 

Figure 10 Trial casting after mold optimization