As a core load-bearing component of motorcycle body, hood must possess both lightweight and high strength characteristics. Its dimensional accuracy generally requires a tolerance higher than ±0.1 mm, and its surface roughness must be controlled within Ra 1.6 μm. Currently, high-silicon aluminum alloys (such as ADC12) are preferred material for such thin-walled shell parts due to their excellent flowability and corrosion resistance. However, complex geometric features (such as deep cavities and dense pore systems) easily lead to defects such as flow marks and cold shuts in traditional mold designs, resulting in a high scrap rate. Based on customer requirements, this paper describes mold design and optimization for production of a specific motorcycle hood using aluminum alloy die casting technology. AnyCasting software was used for mold flow analysis to verify rationality of mold design, aiming to obtain a product with excellent performance and a defect-free surface.
Designed motorcycle engine cover is made of ADC12 aluminum alloy, whose chemical composition is shown in Table 1. Si content reaches 10.6% (mass fraction, same below), exhibiting good fluidity and filling properties, significantly improving high-temperature strength and wear resistance, while also showing relatively higher corrosion resistance compared to A380 alloy. Figure 1 shows schematic diagram and dimensions of casting. Casting has a relatively complex shape, generally shell-like, with various difficult-to-form structures such as holes (A), deep cavities (B), and strong ribs (C). There are numerous holes, mostly distributed on sidewalls, and deep cavities are large. Numerous strong ribs make it prone to microscopic defects such as shrinkage cavities and porosity during forming. Maximum width of casting is 208.4 mm, maximum length is 252.2 mm, maximum hole diameter is 27.3 mm, and minimum hole diameter is 4.1 mm.
Figure 2 shows wall thickness distribution cloud map of casting. It can be seen that wall thickness distribution of this casting is extremely uneven. A boss with a thickness of 18 mm exists in the central area of casting, forming a significant gradient difference with surrounding thin-walled area (1.12 mm). Furthermore, wall thickness changes drastically at strong rib structure, making continuous solidification difficult during subsequent cooling and resulting in challenging forming. Therefore, thermal balance control needs to be achieved through coordinated design of gating and cooling systems.

 

Table 1 Chemical Composition of ADC12 Aluminum Alloy (%)

 

Figure 1 3D Model of Shell Part

 

Figure 2 Cloud Map of Casting Wall Thickness Distribution
A reasonable parting surface design can simplify process flow, extend mold's service life, and reduce costs. Parting surface design should consider structural characteristics of blank, meet requirements for venting and inserts. Lettering surface of this product is appearance surface, requiring no parting marks or step differences; parting surface has two pillow positions, which are used as through-hole surfaces; parting surface design must ensure that product remains in moving mold during production and does not stick to fixed mold. Comprehensive analysis suggests that parting surface should be selected at maximum outer contour in demolding direction. Red curve in Figure 3 represents parting line profile.
With parting line as reference, internal mold release is performed. All holes in product require mold release, necessitating a suitable mold release angle design. To prevent product damage, draft angle on each side is greater than 1°. Figure 4 shows mold release simulation results, and mold release effect meets expected requirements.

 

Figure 3 Parting Line Design

 

Figure 4 Mold Release Design
A reasonable gating system can not only control filling speed but also improve fluidity and temperature distribution of molten metal within mold cavity, avoiding air entrapment, reducing microscopic defects such as slag inclusions and shrinkage porosity in thick-walled areas. This product is a hemispherical casting with a large molten metal filling area. To ensure rapid filling of cavity, multiple side gates are set around casting to facilitate molten metal flow. Product exhibits significant local wall thickness variations, making thick-walled areas prone to shrinkage cavities and porosity defects. Therefore, ingate is placed at thick-walled areas to ensure sufficient molten metal feeding. A well-designed corrugated plate can control flow direction of molten metal, slow down flow rate, and evenly distribute pressure of molten metal. Figure 5 shows gating system design. Gating system adopts a "ring-shaped horizontal runner + fan-shaped ingate" structure, and multiple overflow strips are designed to evenly surround casting. Corrugated plate positions are also reasonably set, ensuring smooth flow of molten metal without affecting parting, avoiding defects, and improving product quality.
Figure 6 shows location of cooling water channels in fixed mold and moving mold. This product is a hemispherical part with a relatively thin shell and a large area. Deep cavity has many strong ribs and uneven wall thickness, which is not conducive to design of an integrated planar cooling system. To address thermal imbalance problem easily caused by wall thickness gradient, a "zoned point cooling + dynamic control" strategy is proposed. An independent point cooling system is used. Multiple deep cooling points are set in deep cavities of fixed mold and moving mold, especially in central thick-walled area. This is beneficial for uniform distribution of the overall temperature field of mold, further reducing internal stress of casting and improving density of die casting.
Figure 7 shows a schematic diagram of point-cooling system assembly. A high-pressure water pump injects cooling pressure into cooling points embedded in mold. Embedding depth varies at different locations, with thick-walled and deep-cavity cooling points requiring deeper embedding.

 

Figure 5 Gating System Design
1. Overflow Strip 2. Corrugated Plate 3. Sprue

 

Figure 6 Point-Cooling System Design

 

Figure 7 Schematic Diagram of Point-Cooling System Assembly
Considering numerous and complex hole systems in this casting, all holes are designed as moving mold inserts, simplifying mold while ensuring airtightness of hole system and quality of casting. Based on product structure, mold dimensions were calculated while ensuring mold strength, resulting in a final design of 600 mm * 630 mm * 290 mm. This mold adopts a two-plate structure, assembling designed mold base, sleeve plate, core, base plate, sprue bushing, support pillar, ejector pin, support plate, ejector plate, and core-pulling structure. The overall assembly drawing is shown in Figure 8.
To verify rationality of mold design, filling process of casting was simulated and analyzed using AnyCasting software. Pouring temperature was 680 ℃, initial mold temperature was 200 ℃, initial injection speed was 5 m/s, and acceleration ratio was 10. Material trace was observed from the moment molten metal entered gate until cavity was completely filled, as shown in Figure 9. It can be seen that casting filling process was smooth, cavity was fully filled, and the overall filling time was 0.2558 s. Molten metal flow was relatively stable during filling process, and there were no obvious slag inclusions or air entrapment around holes.

 

Figure 8 Mold assembly diagram

 

Figure 9 CAE mold flow analysis
Conclusion
(1) All hole cores of motorcycle engine cover casting are embedded, which simplifies mold structure, ensures airtightness of hole system and quality of casting.
(2) To address issue of uneven wall thickness distribution in castings, a side-gate gating system is adopted, with multiple ingates set at thick-walled sections. This achieves rapid filling while ensuring sufficient feeding of molten metal in thick-walled areas.
(3) Selecting an independent point-cooling system can improve the overall temperature field of mold, reduce internal stress in casting, prevent defects such as cracks, shrinkage cavities, and porosity.