An automobile engine cylinder group frame developed by JMC is made of AlSi9Cu3 (Fe) alloy, and its chemical composition is shown in Table 1. Outline size of casting is 485 mm * 364 mm * 148 mm, total weight is about 7.1 kg, structure is complex, wall thickness is 4.5 mm, there are a large number of bosses and holes around it, resulting in local wall thickness of casting being too thick. Internal pores of casting are implemented in accordance with "ASTM E505" Level 2 specification, and maximum hole is less than 1.5 mm; sealing surface is required to be less than 0.4 mm. Casting seal test requires that cavity pressure is 0.1 MPa and leakage volume is less than 10 mL/min; oil channel pressure is 0.3 MPa and leakage volume is less than 10 mL/min.

 

Figure 1 Cylinder group frame
Chemical composition of A1Si9Cu3 (Fe) alloy

According to structural characteristics of cylinder group frame, in order to ensure internal quality of thicker parts of functional area around cylinder group frame, castings adopt a U-shaped pouring system, which can quickly fill castings and effectively feed thick parts on both sides.  Cylinder group frame U-shaped sprue is shown in Figure 2.

 

Figure 2 Cylinder group frame U-shaped sprue
Mold structure is shown in Figure 3. It consists of dynamic and static mold inserts, 4 sliders and static mold oblique core pulling. This mold is a large die-casting mold. In order to prevent normal guiding accuracy from being affected by thermal expansion during production process of mold, a square guide pillar and guide bush structure is adopted and arranged at four corners of mold. Core pulling of slider adopts hydraulic cylinder core pulling, mold design is simple, stable and reliable.

 

l. Moving mold sleeve plate 2. Moving mold insert 3. Upper core-pulling cylinder assembly 4. Upper slide block 5. Square guide bush 6. Vacuum valve plate 7. Right core-pulling cylinder assembly 8. Right slider 9. Lower block 10. Lower core-pulling cylinder assembly 11. Left slider 12. Left core-pulling cylinder assembly
Figure 3 Cylinder group frame die-casting mold structure diagram (moving mold)

During production process, castings were sealed and tested after processing, it was found that there was a slight leakage in M6 threaded hole on machine filter mounting surface. Leakage location is shown in Figure 4, and scrap rate reached 8.3%. X-ray inspection of leaking parts revealed shrinkage cavities and shrinkage porosity inside castings. Defective part was sectioned and it was found that there was overheated material on hole wall of blank. Cause of leakage is existence of internal shrinkage holes and shrinkage porosity. After machining, internal small shrinkage holes penetrate oil channel hole and bolt hole, causing leakage volume of oil channel to be greater than 10 mL/min when oil channel is pressurized to 0.3 MPa.

 

Figure 4 Casting leakage location
Shrinkage cavities in castings exist in installation area of machine filter. Local wall thickness in this area is too thick and there are hot spots. Cooling rate during solidification process of castings is slow, resulting in shrinkage cavities here. Therefore, it is necessary to increase local cooling of mold, increase local cooling rate, balance the overall temperature of mold to effectively reduce occurrence of shrinkage cavities.

Defect occurs in left slider. Mold slider uses series water cooling. Cooling water channel is far away from hot joint of local wall thickness of casting. It is impossible to achieve single-point independent forced water cooling and cooling effect is not good. Now heat generated locally on mounting surface of casting machine filter is calculated, and local point cooling is re-optimized to ensure that left slider is within a reasonable mold temperature range during continuous production process.
Original mold design scheme is that left slider forming part has an integral structure, and cooling water is a series structure, which cannot achieve local single-point cooling; M6 bolts are pre-molded with a core diameter of ∅4.8 mm without cooling. Mold is optimized by changing structure and cooling method of slider forming part. Optimization plan is shown in Figure 5.

 

Figure 5 Structure diagram of left slider optimization plan
Special-shaped parts adopt an insert structure, and a Φ7 mm cooling water channel is added inside. It adopts nozzle type independent water cooling, with built-in stainless steel nozzle Φ4 mm. Water enters inner hole and returns water to outer wall. See Figure 6.

 

Figure 6 Left slider special-shaped insert
Cooling water is added to small and thin core to force-cool core. Core is connected to core nozzle assembly at rear end through threads, is sealed with a high-temperature-resistant O-ring, which allows quick replacement without disassembling mold. Use an electric spark drill to drill a water channel hole with a diameter of Φ2 mm at the front end of core. Core adopts nozzle type independent water cooling, with a built-in stainless steel nozzle Φ1.2 mm. Water enters inner hole and returns water to outer wall. Structure diagram is shown in Figure 7.

 

 

Figure 7 Cooling scheme for small core of left slider
Core and inserts adopt an independent point cooling method, and mold temperature can be controlled at a single point by adjusting flow of cooling water. Since diameter of built-in nozzle is too small, pure water is used as cooling water, which is cooled at a high pressure of 1 to 1.5 MPa, and small core is forced to cool.
After calculation, optimized mold cooling capacity Q2’ at hot section of casting is greater than heat that mold needs to be taken away by mold water channel. During use, flow of cooling water can be adjusted to achieve a thermal equilibrium state of mold.

Through optimization and improvement of mold, local temperature of mold can be effectively controlled to avoid defects such as shrinkage cavities, mold sticking, strain in casting due to local overheating of core and cavity at hot spots of casting. Sealing test rejection rate after machining was reduced from 8.3% to 0.9%. Figure 8 shows X-inspection comparison picture near machine filter installation surface before and after improvement.

 

Figure 8 X-inspection comparison pictures near machine filter installation surface before and after improvement