A compressor end cover casting is shown in Figure 1. Material is ADC12, basic wall thickness is 4 mm, the overall dimensions are 146 mm×140 mm×36 mm, volume is 170 cm3, weight is 460 g, and there is an airtightness requirement. Casting integrates 3 independent cavities, as shown in Figure 1 (b). Each cavity has an oil channel hole. Hole 1 is a stepped hole, diameters of holes 2, 3 and 4 are φ3 mm, of which holes 3 and 4 are inclined holes.

 

Figure 1 Compressor end cover

According to structural characteristics of compressor end cover, parting line is selected at maximum outer contour of casting, that is, midline position of flange surface and core pulling, which is not only convenient for demolding of molded casting, but also convenient for simplifying mold parting and structure, at the same time convenient for design of pouring and overflow system. Inner cavity side of casting with complex structure is designed to be molded in movable mold, and molded casting is pushed out by ejection mechanism on movable mold. Outer side of casting is designed to be molded in fixed mold. Parting design is shown in Figure 2 (a). Three lateral holes are selected for core pulling precasting, as shown in Figure 2 (b). Precasting reduces difficulty of post-process machining, can also achieve requirements of uniform wall thickness, help reduce defects such as pores and shrinkage holes in casting, and improve internal quality of casting.

 

Figure 2 Parting design and core pulling direction

Casting system not only has an important control effect on flow direction and state of molten metal in cavity, overflow exhaust conditions, and casting pressure transmission, but also can adjust filling speed, filling time, mold temperature distribution, etc. Casting system design is shown in Figure 3.

 

Figure 3 Casting and overflow system
Cross-sectional area of inner gate is calculated according to flow formula (1):

 

Where: ∑A is the total cross-sectional area of inner gate, mm2; G is weight of casting, g; ρ is density of alloy liquid, g/cm3; v is filling speed, m/s; t is filling time, s.
According to 3D model, G is 460 g, ρ is 2.4 g/cm3, v is 35 m/s, and t is 0.015 s. Cross-sectional area of inner gate is calculated to be 365 mm2.
According to basic wall thickness of casting of 4 mm, 70% of wall thickness is taken, and inner gate thickness is selected as 2.8 mm.
If runner thickness is too small, molten metal temperature will be reduced. If it is too large, cooling speed will be slow, affecting production efficiency and increasing metal consumption. Thickness of runner is calculated by formula (2):
D=（5~8）T （2）
Wherein, D is thickness of runner, mm; T is thickness of ingates, mm.
Runner thickness D is 15 mm, demoulding slope of runner is set to 10°, and cross-sectional area of runner is designed to be a flat trapezoid, and cross-sectional area is uniformly reduced from sprue to ingates. Bedroom cold chamber die casting machine 350T is selected, and punch diameter is selected to be φ60 mm.

In order to improve molding quality of die castings, reduce defects such as pores and slag holes, gas in cavity, front cold and dirty metal liquid mixed with gas and contaminated by paint residues should be discharged as much as possible during process of metal liquid filling cavity. Therefore, it is necessary to design an overflow system. Since heat loss of metal liquid is large when it reaches end of cavity, design of slag bag can improve temperature field balance of mold and improve molding quality of casting. Design of overflow system is shown in Figure 3.

Anycasting software was used to perform numerical simulation verification on molding process. Three-dimensional geometric model was imported into software in stl format. Grid unit size was set to 1 mm. Mold part material was SKD61 steel, and die casting material was ADC12. Process parameters are shown in Table 1.

Table 1 Process parameters
Filling sequence is shown in Figure 4. It can be seen from Figure 4 that at t=0.043 7 s, molten metal reaches inner gate and begins to fill cavity; at t=0.047 6 s, cavity is filled by about 50%; at t=0.059 2 s, molten metal fills cavity; at t=0.064 5 s, slag bag and exhaust duct are all filled. Overall filling of cavity is stable, without a large range of wrapping, molten metal reaches overflow system at the end of cavity at the same time, and overall filling is ideal.

 

Figure 4 Filling sequence
Solidification analysis is shown in Figure 5. Circled area is isolated hot spot area of casting. Hot spot position needs special attention when designing mold structure. Cooling structure should be designed with emphasis. In order to reduce pores and shrinkage defects, improve quality of castings, it is recommended to use vacuum die casting production.

 

Figure 5 Solidification analysis hot spot area

Temperature of die casting mold is one of important factors affecting molding quality of casting. In order to ensure that mold temperature remains in a reasonable range during continuous molding process, water cooling is often used to cool mold cavity. Water cooling is mainly divided into line cooling, normal pressure point cooling and high pressure point cooling.
Line cooling is a commonly used mold cooling method. Line cooling and normal pressure point cooling are forms of continuous cooling of mold, which mainly play a role in balancing overall temperature of mold. High pressure point cooling is closer to cavity surface than normal pressure point cooling. High pressure point cooling is a form of local cooling, and mold temperature field is controlled in real time according to die casting cycle. During die casting, high-pressure water is passed through high-pressure point cooling to cool mold locally. After die casting, high-pressure gas is passed through to blow away cooling water to ensure a reasonable mold temperature and avoid local overcooling of mold.
According to solidification analysis results, mold cooling water is designed for corresponding hot zone area, as shown in Figure 6. Gray position is designed with normal pressure line cooling, dark point position is designed with high-pressure point cooling, and light-colored core pulling structure is also designed with high-pressure point cooling. When cavity is filled, inside of mold is extremely cooled by high-pressure point cooling, so that casting structure around high-pressure point cooling is quickly solidified to form a thicker dense layer. At the same time, it can shorten solidification time, reduce precipitation and convergence of gas, reduce tendency of casting pores and shrinkage cavities, and improve molding quality of castings.

 

Figure 6 Temperature control system design

Local extrusion is to add an extrusion pin mechanism in mold. After cavity is filled and before it is completely solidified, extrusion pin is driven by a hydraulic cylinder to apply external force to molded casting locally, so that local shrinkage and shrinkage cavities disappear, and pores can also be broken up to improve molding quality of castings.
As can be seen from Figure 6 (b), structure of ellipse is too compact to directly design spot cooling, and there are 3 precast holes, of which 2 holes are designed for high-pressure spot cooling. However, cooling hole diameter on core is small, which has limited effect on improving shrinkage problem in thick-walled area of molded casting. Therefore, an extrusion mechanism is designed in this area, as shown in Figure 7. Generally, when solid phase ratio around extrusion pin is about 60%, extrusion is difficult to advance, and when solid phase ratio around is 19%~37%, extrusion is more ideal. Volume shrinkage ratio of molten metal from liquid to solid can be calculated in a simple way, that is, density change is used to express volume shrinkage ratio. ADC12 calculates volume of extrusion influence zone with a boost volume of 0.13 times. Volume of extrusion influence zone is measured by UG software as 17 cm3, a φ6 mm extrusion pin is selected, and extrusion depth is 10 mm. Volume shrinkage ratio D hydraulic cylinder/D extrusion pin ≈ 4.5~5.5, so extrusion hydraulic cylinder diameter is selected as D=φ40 mm, and stroke L=10 mm.

 

Figure 7 Local extrusion mechanism

Die structure is shown in Figure 8. Fixed die is designed with 2 core pulling mechanisms, movable die is designed with 1 slider and 1 extrusion mechanism. After die casting die is completed, it is trial-produced on a 350T bedroom cold chamber die casting machine. During die casting process, die cavity is vacuumed to reduce amount of inclusions and gas entrainment during metal liquid flow process, improve cavity filling quality. Actual molded casting is shown in Figure 9. After X-ray nondestructive testing and leakage detection, all meet requirements. There are no obvious pores and shrinkage defects in key areas, especially denser structure inside ellipse.

 

Figure 8 Die structure

 

Figure 9 Flaw detection results