Pouring system of die casting determines flow direction, speed and filling time of molten metal, plays a certain role in regulating and controlling pressure conduction and mold temperature. It is a key factor in determining quality of die castings. Overflow system includes overflow trough and exhaust channel, which is channel for discharging gas, cold material and waste material during die casting. In design process of die casting mold, pouring system and overflow system are often considered as a whole. This study takes die casting mold of engine front cover as an example to introduce design process and design method of its pouring system and overflow system, analyzes shape, structural characteristics and technical requirements of automobile engine front cover, designs pouring system and overflow system of product, uses calculation method to design size of inner gate, runner and overflow trough. On the basis of theoretical design, AnyCasting software is used to simulate die-casting process of engine front cover product. After actual mold testing, product quality is checked and overflow groove size is fine-tuned to provide a reference for development of similar products.

Figure 1 shows front cover developed by a certain automobile company. Material is 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, projection area is 1180c㎡, and single piece weight 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 3 large through holes in the middle, which is not conducive to metal liquid filling and is prone to casting defects such as flow marks, cold shuts, and deformation. It is necessary to solve casting defects that may occur in casting by reasonably designing pouring system and 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 sufficient inner gate cross-sectional area and shorten filling process and filling time. Since there are multiple bolt mounting holes on the side of front cover, in order to avoid impact of runner on core during filling, side runner is designed as a branch runner arranged between two adjacent bolt mounting holes of casting. Pouring system structure is shown in Figure 2. It has following advantages: metal liquid filling process is short, cavity can be filled quickly, and heat loss can be reduced; branch runner has a certain length, which can ensure flow direction and sequential filling of metal liquid; avoid turbulence and sticking caused by metal liquid impacting core; conveniently arrange overflow troughs and exhaust channels, which is conducive to exhaust; easy to remove, and convenient for subsequent automatic trimming to prevent casting deformation; conducive to balancing mold temperature.

 

Picture 1 Front cover

 

Picture 2 Casting system and overflow system
1. Sprue 2. Horizontal runner 3. Ingate 4. Overflow trough 5. Exhaust channel 6. Casting

 

Picture 3 Relationship between filling speed and filling length of castings with different wall thickness

 

Picture 4 Thickness and shape of ingate
According to clamping force calculated based on projected area of casting and injection pressure ratio required for injection, it is determined that this front cover is produced using a 20000kN die-casting machine and cup diameter is φ110mm. Thickness of sprue, that is, thickness of sprue after injection, is directly related to effective transmission of boost pressure. If sprue is too thin, pressure transmission is insufficient and internal quality of casting will be reduced; if sprue is too thick, it wastes materials and often explodes due to insufficient cooling, which seriously affects production cycle. Thickness of sprue is generally 25% of cup diameter, and thickness of sprue of front cover is 30mm. Function of runner is to smoothly introduce molten metal from sprue into ingate. Its shape and size depend on position, shape and direction of ingate. Front cover adopts a branch runner. In order to prevent molten metal from dispersing at corners, cross-sectional area of runner should be regularly reduced and fillet radius should be increased. Structural design of runner is shown in Figure 2. Cross-sectional area of runner is 3 to 4 times cross-sectional area of ingate. Cross-sectional shape is trapezoidal, and fillet transition with ingate is a 60° slope R15mm.
Overflow system is a channel for exhausting air, accommodating residual coatings and initially filling cold alloys during process of molten metal filling cavity, which has a great impact on quality of casting. Overflow system mainly includes overflow grooves and exhaust grooves. In design of overflow grooves and exhaust grooves, position, structural form and introduction direction of runner should be fully considered. When designing position of overflow groove, the first thing to consider is to place overflow groove at the end of filling of molten metal in cavity and position where multiple strands of molten metal converge. According to structural characteristics of front cover and location of branch runner, overflow trough is designed at the last filling position opposite front cover gate, and 7 overflow troughs are designed in different areas. At the same time, considering 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 local area of molten metal to converge, and in severe cases, incomplete filling will occur. Therefore, overflow troughs are designed at two large holes in the center of casting, which is conducive to filling of molten metal and shrinkage compensation during cooling.

Table 1 Recommended values of overflow trough volume

Table 2 Die casting process parameters

 

Figure 5 Simulation analysis results

 

Figure 6 Improvement plan after trial mold

According to structural characteristics of front cover, numerical simulation is applied to determine that front cover adopts lateral branch runner for filling, side runner is arranged on 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 converge. The entire filling process is stable without turbulence and air entrapment. Design sequence of pouring system is to design gate first, then sprue and runner, then design overflow system according to runner and casting structure. After theoretical calculation, simulation analysis, and trial mold adjustment, specific position, shape and size of pouring system and overflow system are finally determined. After mass production verification, product quality is high, mold operation is stable and reliable.