With rapid development of automobile industry, there are more and more aluminum alloy die-casting parts. In order to achieve energy conservation and emission reduction, design of die-casting parts has developed in direction of multi-function and multi-part combination. Therefore, die-casting parts often have excessive wall thickness in thick-walled areas, which is prone to casting defects such as shrinkage cavities and shrinkage porosity. In order to meet functional requirements of parts, methods such as local pressurization, semi-solid die casting, and insert die casting are often used to improve internal quality of castings. This study takes automobile engine chain cover as research object, introduces casting structure and development difficulties of automobile engine chain cover, rationally designs casting pouring system and overflow system through process analysis and theoretical calculations, bidirectional multi-branch runner is used to achieve rapid filling to meet appearance and internal quality requirements of casting. This article introduces design of pouring system and overflow system of flat castings, and uses embedded casting method to solve shrinkage cavity defect at lifting hole of chain cover, aiming to provide a reference for production.

Figure 1 shows a certain type of automobile engine chain cover. Outer contour size of casting is 265mm * 133mm * 49mm. Weight of single piece is 480g, and projected area is 261cm2. The overall structure of casting is flat and long, with a general wall thickness of 2.5mm. There are 12 bolt mounting bosses on periphery, one through hole with a diameter of φ49.5mm on each side of middle, an arched bridge with a minimum width of 11mm in the center, and a lifting hole with a diameter of ϕ20mm in the center. Casting material is Al-9Si-3Cu (Fe). Internal pores require maximum pore at lifting lug to be less than 2mm and other parts to be less than 0.8mm. Sealing performance of casting requires that leakage volume is less than 3mL/min under a pressure of 0.04MPa.
On the premise of meeting minimum wall thickness requirements of mold cavity, maximum core pulling size of ϕ10mm will cause shrinkage cavities near lifting lugs due to excessive wall thickness. Abroad, production process of lateral core pulling ϕ10mm and machining to ϕ20mm is adopted. Due to small size of lateral core pulling, there are a large number of shrinkage cavities inside casting. Machining allowance on one side of machined expansion hole is 5mm. After processing, internal shrinkage cavities are exposed on the surface of lifting hole, which affects both strength of casting and appearance quality, so yield is low. In view of above development difficulties, failure mode analysis was conducted in the early stage of development, and effective measures were taken. By rationally designing gating system and adopting embedded casting process, internal quality problems of castings were effectively solved.

 

Figure 1 Chain cover

 

Figure 2 Gating system and overflow system

1. Sprue 2. Cross runner 3. Inner gate 4. Overflow tank

Table 1 Recommended volume of overflow tank
Die-casting mold overflow system mainly includes an overflow tank and an exhaust channel, which can control filling and flow state of molten metal. A reasonable arrangement of overflow system can effectively reduce casting defects such as cold shuts, flow marks, and pores. Design of overflow trough and exhaust duct is related to factors such as casting structure, pouring system structure and size. Position of overflow tank is generally arranged on parting surface, first impact position of molten metal, last filling position and position where multiple streams of molten metal gather are selected. Exhaust channel is set at rear end of overflow tank to enhance overflow and exhaust effects.
Structure of chain cover mold is shown in Figure 3. Casting is vertically discharged in one mold and two cavities. Through calculation of clamping force, Aluminum Taiwan Precision Machinery 9000kN die-casting machine was selected. Pressure chamber is located 200mm eccentrically below the center of mold. In order to meet requirements for formation of lateral lifting holes in castings, a hydraulic lateral core-pulling mechanism is used, and a hydraulic cylinder is arranged above mold to position lifting hole inserts. Since shape of casting cavity is relatively simple, mold forming parts adopt an integral insert structure, which simplifies mold structure and facilitates arrangement of cooling water channels. Moving and fixed mold sets are carriers of formed parts, guide parts and lateral core-pulling mechanisms, and can withstand impact of injection force during die-casting. Integral structure is adopted to make mold structure simpler and more compact while ensuring functionality and strength. Ejection mechanism uses a push rod to push out. Since casting is a flat casting, layout of push rod should try to balance pushing force on each part of casting to prevent deformation of casting. Therefore, on the premise of meeting push force, number of push rods is increased, 30 push rods with a diameter of ϕ8mm are installed on each cavity of movable mold insert, including 16 cavities, 9 sprues, and 5 overflow grooves. Cooling system adopts circulating water cooling and single-point water cooling according to changes in casting structure and position.

 

Figure 3 Chain cover die-casting mold structure diagram (moving mold)
1. Sprue 2. Casting 3. Dynamic mold cavity 4. Upper hydraulic cylinder 5. Square guide column 6. Exhaust channel insert 7. Moving mold plate 8. Splitting cone

 

Figure 4 Insert casting structure
1.Dynamic model cavity 2.Hydraulic positioning rod 3.Insert

 

Figure 5 Insert
Local volume near φ20mm lifting hole at lifting lug is thick and mold filling is slow, which can easily cause air stagnation and shrinkage defects. In order to reduce shrinkage cavities and porosity at thick wall of lifting lug and improve internal quality of casting, insert casting process is adopted. Insert casting is to place metal parts (inserts) in die-casting mold first, then cast them together with die-casting parts to eliminate local hot spots in castings, reduce wall thickness of castings, and prevent shrinkage cavities. Since shape and structure of chain cover is relatively simple, after casting is molded at maximum contour, there is no obvious difference in wrapping force on both sides. Therefore, after determining parting surface, forming part of side core-pulling structure that needs to be designed at lifting eye hole of chain cover is designed in movable mold. At the same time, pre-cast hole core on peripheral flange surface is also designed in movable mold, which can increase tightness of movable mold part and prevent casting from remaining in fixed mold cavity after mold is opened. This type of split design results in insert being placed in movable mold cavity. In order to ensure that insert is stable and reliable when movable mold moves during die-casting process, a reliable positioning and pressing method must be designed.

 

Figure 6 X-ray inspection picture
Chain cover is a flat casting with a through hole in the center. It can be filled quickly through use of two-way multiple sprues. Combined with a reasonable design of overflow system, casting can achieve excellent surface and internal quality. For castings with thick-walled structures, when core pulling and local pressurization processes cannot be effectively implemented due to influence of casting structure and mold structure, insert casting process can be used for die casting. Use of inserts for die-casting production can eliminate local hot spots in castings, reduce wall thickness of castings, and effectively prevent shrinkage cavities.