Pouring system of die-casting determines flow direction, speed and filling time of molten metal. It also plays a certain role in regulating and controlling pressure conduction and mold temperature. It is a key factor in determining quality of die-casting parts. Overflow system includes an overflow tank and an exhaust channel, which is a channel for discharging gas, cold materials, and dirt during die-casting process. In design process of die-casting molds, pouring system and overflow system are often considered as a whole. This study takes engine front cover die-casting mold as an example to introduce design process and design method of its gating system and overflow system. After analyzing shape, structural characteristics and technical requirements of automobile engine front cover, product's gating system and overflow system were designed, calculation methods were used to design dimensions of inner gate, cross runner and overflow tank. On the basis of theoretical design, AnyCasting software was used to simulate die-casting process of engine front cover product. After actual mold trial, product quality was inspected and size of overflow groove was fine-tuned to provide a reference for development of similar products.

Figure 1 shows front cover developed by an automobile company. It is made of ADC12 alloy and complies with JIS-H5302-2006 standard. It has good fluidity and good mechanical properties. Maximum outer contour size of casting is 603mm*320mm*49mm, general wall thickness is 2.0mm, projected area is 1180cm2, and single piece mass is 1.72kg. Cavity sealing requirement is 100kPa, and maximum allowable leakage is 10mL/min. Front cover is a flat, thin-walled part with a large casting size and three large through holes in the middle, which is not conducive to filling with molten metal and is prone to casting defects such as flow marks, cold shuts, and deformation. Possible casting defects in castings need to be solved by rationally designing gating system, overflow system and adjusting die-casting process parameters.
According to structural characteristics of front cover, side runner is selected for filling. Side runner is arranged in long side direction of casting on parting surface, which can ensure a sufficient cross-sectional area of inner gate, shorten filling process and filling time. Since there are multiple bolt mounting holes on the side of front cover, in order to avoid impact on core when runner is filled, lateral runner is designed as a branch runner and is arranged between two adjacent bolt mounting holes of casting. Structure of pouring system is shown in figure 2. It has following advantages: molten metal filling process is short, it can quickly fill mold cavity and reduce heat loss; branch runner has a certain length, which can ensure flow direction and sequential filling of molten metal; avoid turbulence caused by impact of molten metal on the core; facilitate arrangement of overflow grooves and exhaust channels, which are conducive to exhaust; facilitate removal, and facilitate subsequent automatic trimming to prevent casting deformation; help balance mold temperature .

 

(a) Inner cavity           (b)External cavity
Figure 1 Front cover

 

Figure 2 Gating system and overflow system
1. Sprue 2. Cross runner 3. Inner gate 4. Overflow tank 5. Exhaust channel 6. Casting

 

Figure 3 Relationship between filling speed and filling length of castings with different wall thicknesses

 

Figure 4 Thickness and shape of internal gate
Based on clamping force calculated from projected area of casting and specific injection pressure required for injection, it was determined that front cover should be produced using a 20,000kN die-casting machine and melting cup diameter should be ϕ110mm. Thickness of sprue is thickness of material cake after injection, which is directly related to effective transmission of booster pressure. If material is too thin, pressure transmission is insufficient and internal quality of casting will be reduced; if material cake is too thick, material is wasted and explosions often occur due to insufficient cooling, seriously affecting production rhythm. Thickness of sprue is generally 25% of diameter of melting cup, and thickness of sprue of front cover is 30mm. Function of runner is to smoothly introduce molten metal from sprue into inner gate. Its shape and size depend on position, shape and direction of inner gate. Front cover uses branch sprues. In order to prevent molten metal from dispersing at corners, cross-sectional area of flow channels must be regularly reduced and fillet radius increased. Structural design of runner is shown in Figure 2. Cross-sectional area of runner is 3 to 4 times cross-sectional area of inner gate. Cross-sectional shape is trapezoidal, transition between runner and inner gate is a 60° slope R15mm fillet.
Overflow system is a channel that discharges air, accommodates residual paint, and initially fills cold alloy during process of molten metal filling mold cavity. It has a great impact on quality of castings. Overflow system mainly includes an overflow tank and an exhaust tank. In design of overflow tank and exhaust tank, location, structural form and introduction direction of molten metal must be fully considered. When designing location of overflow tank, you must first consider placing overflow tank at filling end of mold cavity and where multiple streams of molten metal converge. According to structural characteristics of front cover and location of branch sprues, overflow tank is designed at last filled position opposite front cover gate, and 7 overflow tanks are designed in different areas. At the same time, consider that there are two large through holes inside casting. Cavity structure here will hinder flow of molten metal and change flow direction of molten metal, causing molten metal to converge in local areas. In severe cases, incomplete filling will occur, so overflow grooves are designed at two large holes in the center of casting to facilitate filling of molten metal and feeding during cooling process.

Table 1 Recommended volume of overflow tank

Table 2 Die casting process parameters

 

(a) Quick shot switching position       (b) Fill 40%              (c) Fill 95%

 

(d) Coagulation analysis  (e) Casting temperature field analysis  (f) Air entrainment analysis
Figure 5 Simulation analysis results

 

(a) Grid diagram of casting defect locations    (b) Diagram of mold optimization plan
Figure 6 Improvement plan after mold trial

According to structural characteristics of front cover, numerical simulation is used to determine that front cover uses lateral branch runner for filling. Side runner is arranged in long side direction of casting on parting surface, overflow system is designed at filling end of molten metal and location where multiple strands of molten metal meet. The entire filling process is smooth, free of turbulence and air entrapment. Design sequence of gating system is to first design inner gate, sprue and lateral runner, then design overflow system according to runner and casting structure. After theoretical calculations, simulation analysis, and mold trial adjustments, specific location, shape, size of pouring system and overflow system were finally determined. It has been verified by mass production that product quality is high and mold operation is stable and reliable.