Figure 1 shows a car engine hood with dimensions of 437 mm * 423 mm * 222.5 mm and a weight of 3.94 kg. Main structure consists of three parts: hood, engine bracket, and water pump support. Hood has an irregular shape and a complex internal structure. Engine bracket is located at the bottom center of hood and has three M10 bolt holes for connection. Water pump support is located on the side of hood, with a water pump outlet in the center and eight M8 bolt holes around outlet for fixing water pump. Besides sealing requirements at hood-cylinder block mating surface, there are sealing requirements at water pump connection and high strength requirements at bracket. Casting material is A380 aluminum alloy, with a typical wall thickness of 3.5 mm, and maximum wall thickness at bracket, ranging from 24 to 30 mm.
To meet sealing and strength requirements of front hood, internal quality of casting must be guaranteed. During mass production, X-ray inspection revealed obvious holes in the front hood bracket, accounting for over 70% of defects. After machining, M10 threaded holes in casting were exposed as defects, as shown in Figure 2.

 

Figure 1: Engine Front Hood
1. Bracket Area 2. Water Pump Connection

 

Figure 2: Location and Condition of Defects in the Front Hood
Aluminum alloy die-casting contains cavities. Mold design and process optimization are needed to control these cavities within an acceptable range to ensure casting quality meets requirements. Cavities are classified as gas cavities and shrinkage cavities. Gas cavities are characterized by a bright, smooth inner surface and relatively regular shape due to presence of gas. Shrinkage cavities are characterized by a dark color and irregular shape due to insufficient molten metal compensation during solidification, mainly appearing in thick-walled areas of casting. Sectioning defective front hood revealed that cavities were located in core of thick-walled area, with irregular shapes and dark inner walls, confirming that cavities in the front hood bracket were primarily shrinkage cavities.
In die-casting process, after molten metal is injected into mold cavity, surface cools first to form a hard shell, while internal molten metal, which cools later, solidifies and its volume decreases. If this reduced volume is not replenished by external molten metal, shrinkage cavities or porosity will form. Shrinkage defects in aluminum alloy die-castings are mainly related to casting structure, die-casting process, design of gating and overflow systems of die-casting mold: ① Casting structure: excessively thick local walls or hot spots. ② Die-casting process: excessively high alloy molten metal pouring temperature, insufficient final injection pressure, or excessively thin sprue. ③ Gating system: improper ingate location or excessively thin ingate. ④ Overflow system: incorrect overflow channel location or insufficient capacity.
First, die-casting mold and die-casting process are analyzed. Mold structure for engine front cover is shown in Figure 3. Casting is arranged longitudinally. To accommodate complex part structure, a U-shaped filling method is adopted, with an overflow channel and vent designed at convergence point of multiple molten metal streams at the end of filling. A slider core-pulling mechanism is installed above moving mold to accommodate lateral irregular shape and lateral bolt hole forming of front cover; an independent core-pulling mechanism is installed on the right side of moving mold for forming lateral holes; a core-pulling mechanism is installed above fixed mold for pulling three pre-cast bolt holes at the support. To simplify mold structure and facilitate sequential operation, hydraulic core-pulling is used in all three locations. Further analysis of casting structure reveals that wall thickness at front cover support is over 24 mm, approximately seven times typical wall thickness of a casting. This thick-walled area is final solidification point, which will generate shrinkage cavities during solidification. CAE analysis of casting was performed to simulate molten metal filling sequence. Results are shown in Figure 4, indicating that thick-walled area at support is located at the bottom of deep cavity and is final area filled with molten metal.

 

Figure 3: Mold Structure Scheme

 

Figure 4: Simulated Filling Sequence
Figure 5 shows structure of core-pulling mechanism in the front cover die-casting mold before optimization. Pre-cast core-pulling mechanism with three threaded holes at support is mounted on fixed mold. Three cores with a diameter of ϕ8 mm slide in engagement with fixed mold insert. Driven by a hydraulic cylinder, core insertion and removal are achieved. Mold structure is simple, but it cannot accommodate an overflow groove.
Optimized internal core-pulling structure is shown in Figure 6. Three cores with a diameter of ϕ8 mm are fixed to an internal core puller with a diameter of ϕ82 mm. Internal core puller slides in engagement with fixed mold insert. Driven by a hydraulic cylinder, core insertion and removal are achieved. Three ϕ8 mm cores are positioned at the top of internal core puller, and an overflow groove is located at the bottom. After core removal, shape of overflow groove inlet separates from forming area of internal core puller, achieving uninterrupted demolding of overflow groove.

 

Figure 5. Internal Core Pulling Structure Before Optimization
1. Casting 2. Fixed Mold Insert 3. Internal Core Pulling

 

Figure 6. Internal Core Pulling Structure After Optimization
1. Casting 2. Fixed Mold Insert 3. Core 4. Internal Core Pulling 5. Overflow Groove
Main function of overflow groove in a die-casting mold is to expel gas from cavity, store molten metal mixed with gas and paint residue. In mold design, overflow grooves are generally distributed by region, and volume of each overflow groove is related to volume of adjacent cavity. Ratio of overflow groove volume to adjacent cavity area is shown in Table 1. Front cover generally has a wall thickness of 3.5 mm. Referring to Table 1, overflow groove volume accounts for more than 25% of adjacent cavity volume. Since molten metal flows 250 mm through gating system and cavity, overflow groove volume needs to be increased by 20% based on data listed in Table 1. Considering structural form of front cover support and its characteristics such as thick local walls and location at the end of filling process, planned design slag collection volume ratio is 50%.
Calculations show that locking force of inner core pull in the front cover is much greater than core pulling force, necessitating design of an independent locking mechanism. Since inner core pull is located in fixed mold, it needs to be completely pulled out of cavity to a safe position before mold can be opened. Therefore, locking block needs to be designed in fixed mold, arranged perpendicular to direction of inner core pull's movement. Locking mechanism for inner core pull in front cover primarily uses a hydraulically assisted mechanical mechanism, achieving mechanical locking through locking block and a fixed plate, with locking block moved by a hydraulic cylinder. Structural diagram of inner core pull in fixed mold and its locking mechanism is shown in Figure 7. Sequence of actions is: mold closing – inner core pull mechanism inserts core → inner core pull mechanism locks → injection → inner core pull mechanism locking block retraction → inner core pull mechanism pulls core → mold opening → part removal.

Table 1 Recommended overflow channel volume

 

Figure 7 Structure diagram of core-pulling and locking mechanism within fixed mold
1. Core-pulling within fixed mold 2. Fixed mold sleeve plate 3. Side locking cylinder 4. Fixing plate 5. Core-pulling cylinder 6. Core-pulling cylinder fixing plate 7. Wedge block 8. Inner core-pulling connecting plate 9. Limit switch connecting plate 10. Core-pulling cylinder support plate 11. Limit switch assembly
After optimizing core-pulling structure within die-casting mold of front cover using above measures, production was carried out using a 18,000 kN die-casting machine. Casting pressure was 75 MPa, slow injection speed was 0.15 m/s, and high-speed stage punch speed was 4.2 m/s. 2,000 pieces of optimized mold were produced in batches, all of which underwent X-ray flaw detection. No obvious shrinkage cavities were observed at bracket, indicating a significant improvement in product quality. Appearance and internal quality of front cover are shown in Figure 8. After machining, local shrinkage cavity defect rate of mass-produced products was reduced to below 2%. Mass production verification showed that front cover product quality was stable, and operation was smooth, reliable, and durable.

 

Figure 8: X-ray inspection of the optimized front cover
Conclusions
(1) Due to casting difficulties at bracket area, local wall thickness of front cover product was excessive, making it impossible to effectively compensate for shrinkage during cooling and solidification, leading to shrinkage cavities.
(2) Adding large-capacity overflow channels locally can effectively improve filling and molding, and compensate for local shrinkage, reducing risk of shrinkage cavities.
(3) For areas with large local thickness in deep cavity of casting, especially where conventional overflow channel design methods are not feasible, overflow channels can be designed using side core pulling, thereby changing demolding direction of overflow channel. After core pulling, overflow channel can be easily demolded.
(4) For hydraulic core pulling mechanisms with high clamping force, using a hydraulically assisted mechanical clamping mechanism can effectively reduce volume of clamping mechanism, locking is stable and reliable.