Abstract: Structure and forming characteristics of produced motorcycle hood were analyzed using UG software. The key points of mold design were analyzed, layout of one mold cavity and parting surface of mold were determined. Based on deep cavity structure, a gating system with a horizontal runner uniformly surrounding casting and an independent point cooling system were designed. Feasibility of process was verified by simulation test using AnyCasting software.
As a core load-bearing component of motorcycle body, motorcycle hood needs to have both lightweight and high strength characteristics. Its dimensional accuracy is generally required to be higher than ±0.1mm, and surface roughness needs to be controlled within Ra1.6μm. Currently, high silicon aluminum alloys (such as ADC12) have become preferred material for such thin-walled shell parts due to their excellent fluidity and corrosion resistance. However, complex geometric features (such as deep cavity and dense hole system) make traditional mold design prone to defects such as flow marks and cold shuts, resulting in a high scrap rate. According to customer requirements, mold design and optimization were carried out for production of a certain model of motorcycle engine cover using aluminum alloy die casting process, and AnyCasting software was used for mold flow analysis to verify rationality of mold design, in order to obtain a product with excellent performance and no surface defects.

Designed motorcycle engine cover is made of ADC12 aluminum alloy, chemical composition of which is shown in Table 1. Si content reaches 10.6% (mass fraction, same below), which has good fluidity and filling properties, significantly improves high temperature strength and wear resistance. At the same time, it has relatively high corrosion resistance compared with A380 alloy. Figure 1 shows structural diagram and dimensional diagram of casting. Shape of casting is relatively complex, and whole is shell-shaped. It has a variety of difficult-to-form structures such as holes (A), deep cavities (B), and strong ribs (C). There are many holes, which are mostly distributed on side walls. Deep cavity area is large and there are many strong rib structures. During forming, micro-defects such as shrinkage cavities and porosity are easily generated. Casting has a maximum width of 208.4 mm, a maximum length of 252.2 mm, a maximum hole diameter of 27.3 mm, and a minimum hole diameter of 4.1 mm.
Tab. 1 Chemical composition of ADC12 aluminum alloy %

 

 

Fig.1 3D model of shell components
Figure 2 shows wall thickness distribution cloud map of casting. It can be seen that wall thickness distribution of casting is extremely uneven. A boss with a thickness of 18 mm exists in 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 system and cooling system.

 

Fig.2 Nephogram of wall thickness distribution of casting

2.1 Parting Surface Design
A reasonable parting surface design can simplify process flow, extend service life of mold, and reduce costs. Design of parting surface should be combined with structural characteristics of blank to meet requirements of venting and inserts. Lettering surface of this product is appearance surface, it must be free of parting marks and step differences; parting surface has two pillow positions, which are used as through-hole surfaces; design of parting surface 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 surface contour.

 

Fig.3 Parting surface design
2.2 Draft Design
Demolding within tolerance is performed based on parting surface. All holes in product require demolding, and a suitable demolding angle needs to be designed. To avoid product damage, draft angle on each side is greater than 1°. Figure 4 shows demolding simulation results, and demolding effect meets expected requirements.

 

Fig.4 Draft design

3.1 Gating System Design
A reasonable gating system can not only control filling speed but also improve fluidity and temperature distribution of molten metal in mold cavity, avoiding air entrapment while reducing generation of micro-defects such as slag inclusions and shrinkage porosity in thick-walled areas. This product is a hemispherical casting with a large area for molten metal filling. To ensure rapid filling of cavity, multiple side gates are placed around casting to facilitate flow. Product exhibits significant local wall thickness variations, making thicker sections prone to shrinkage cavities and porosity defects. Therefore, ingate is placed at thicker wall sections to ensure adequate feeding of molten metal. A well-designed corrugated plate can control flow direction of molten metal, reduce flow velocity, evenly distribute pressure. Figure 5 shows gating system design. Gating system adopts a "ring-shaped horizontal runner + fan-shaped ingate" structure, with multiple overflow strips evenly surrounding casting, a well-placed corrugated plate. This ensures smooth molten metal flow without affecting parting, preventing defects, improving product quality.

 

Fig. 5 Gating system design
1. Overflow strip 2. Corrugated plate 3. Sprue
3.2 Point Cooling System Design
Figure 6 shows location of point cooling water channels in fixed and moving molds. This product is a hemispherical part with a thin shell and a large area. Deep cavities have many strong ribs and uneven wall thickness, which is unfavorable for design of an integrated planar cooling system. To address thermal imbalance problem easily caused by wall thickness gradients, 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 and moving molds, especially in central thick-walled areas. This promotes a uniform temperature distribution throughout mold, further reducing internal stress in casting and improving density of die-casting.

 

Fig.6 Spot cooling system design
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.

 

Fig.7 Assembly diagram of spot cooling systems

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 600mm * 630mm * 290mm. This mold adopts a two-plate structure, assembling designed mold base, sleeve plate, core, base plate, sprue bushing, support pillars, ejector pins, support plates, ejector plates, and core-pulling structure. The overall assembly drawing is shown in Figure 8.

 

Fig.8 Assembly diagram of mold
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 traces were 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 pores.

 

Fig.9 CAE mold flow analysis

(1) Embedded cores for all holes in motorcycle hood casting simplify mold structure while ensuring airtightness of hole system and quality of casting.
(2) To address issue of uneven wall thickness distribution in casting, a side-gating system is used, with multiple ingates at thick-walled sections. This achieves rapid filling while ensuring sufficient feeding of molten metal in thick-walled sections.
(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.