Abstract: This paper introduces structural characteristics and defect forms of automotive engine hoods. Analysis shows that defects are caused by difficulties in casting at hood support, excessive local wall thickness, ineffective shrinkage compensation during cooling and solidification, leading to shrinkage cavities. By optimizing core-pulling structure within mold, designing large-capacity overflow grooves at locations where forming is difficult and shrinkage compensation is challenging, risk of shrinkage cavities is reduced. Use of hydraulic core-pulling and hydraulic-assisted mechanical locking mechanisms ensures reliability of core-pulling working position and movement process. Based on above mold optimization, local shrinkage cavity defect rate of product is reduced to below 2%. Mass production verification shows that hood product quality is stable, and mold operates smoothly, reliably, and durable.
With increasing demands for lightweighting and energy conservation in automobiles, automotive parts are developing towards lightweight alloys. Simultaneously, due to integration of part functions, their structures are becoming increasingly complex, and quality requirements are becoming increasingly stringent. Aluminum alloys have characteristics of low density, low coefficient of thermal expansion and good friction performance, are widely used in production of castings such as automobile engine blocks, engine covers, gearbox housings and automobile structural parts. At present, high pressure casting is an important process for achieving lightweighting of automobile parts. In order to achieve integration of parts, integrated die castings with multiple parts combined have become current development trend. Due to complexity of structure after integration, some die castings with uneven wall thickness or high sealing requirements have brought great challenges to design of die casting molds and die casting processes. This study takes an engine front cover as an example to analyze causes of shrinkage defects at its bracket and optimize mold design, aiming to provide a reference for production of similar products.

Figure 1 shows an automobile engine front cover with an outline size of 437mm*423mm*222.5 mm and a weight of 3.94kg. Main structure of part consists of three parts: front cover, engine bracket and water pump support. Front hood has an irregular shape and a complex internal structure. Engine mount is located at the center bottom of hood, with 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 securing water pump. Besides sealing requirements at hood-cylinder block mating surface, there are also sealing requirements at water pump connection and high strength requirements at support. Casting is made of A380 aluminum alloy, with a typical wall thickness of 3.5mm, and a maximum wall thickness of 24-30mm at support.

 

Fig. 1 Engine front cover
1. Bracket location 2. Water pump connection point
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 at the front hood support, accounting for over 70% of defects. After machining, M10 threaded holes in casting showed these defects, as shown in Figure 2.

 

Fig.2 Location and types of defects in the front cover

Aluminum alloy die castings have cavities inside. It is necessary to control cavities within an acceptable range through mold design and process optimization to ensure that quality of casting meets requirements. Cavities are divided into gas cavities and shrinkage cavities. Gas cavities are those with bright, smooth inner surfaces and relatively regular shapes formed due to presence of gas. Shrinkage cavities are those with dark colors and irregular shapes formed due to insufficient metal liquid compensation during solidification, which mainly appear in thick wall of casting. When defective front cover part is cut, it is found that cavities exist in the core of thick wall, with irregular shapes and dark inner walls. It is determined that cavities at the front cover bracket are mainly shrinkage cavities. During die casting process, after molten metal is injected into cavity, surface cools first to form a hard shell. Volume of molten metal that cools later in interior decreases during solidification process. If reduced volume is not replenished by external molten metal, it will form shrinkage cavities or shrinkage porosity. Shrinkage defects in aluminum alloy die castings are mainly related to design of casting structure, die casting process, gating and overflow systems of die casting mold: ① Casting structure: Casting has thick walls in some areas and hot spots. ② Die casting process: Alloy liquid pouring temperature is too high, final pressure of injection is insufficient, and remaining material cake is too thin. ③ Gating system: Ingate position is improper and ingate is too thin. ④ Overflow system: Overflow groove position is incorrect or capacity is insufficient.

First, die casting mold and die casting process are analyzed. Mold structure of engine front cover is shown in Figure 3. Casting is arranged longitudinally. In order to meet filling of complex part structure, a U-shaped filling method is adopted. An overflow groove and an exhaust channel are designed at the end of filling where multiple streams of molten metal converge. A sliding core-pulling mechanism is installed above moving mold to accommodate irregular side profile and lateral bolt holes of front cover. An independent core-pulling mechanism is located on the right side of moving mold for forming lateral holes. A core-pulling mechanism is installed above fixed mold for removing three pre-cast bolt holes at support. To simplify mold structure and facilitate sequential operation, hydraulic core-pulling is used in all three locations.

 

Fig.3 Scheme of mold structure
Based on estimated clamping force of front cover, an 18,000 kN die-casting machine was selected. During trial production, adjustments were made to different pouring temperatures (665~682 ℃), different injection pressures (70~100 MPa), and different ingate speeds (35~50 m/s). Comparison of castings produced under different die-casting shifts and parameters revealed that optimizing die-casting process parameters did not significantly improve effectiveness of reducing shrinkage cavities. Further analysis of casting structure revealed that wall thickness at the front cover bracket exceeded 24mm, approximately seven times typical wall thickness of a casting. This thick-walled area is final solidification point, where shrinkage cavities will occur during solidification. CAE analysis was also performed to simulate molten metal filling sequence. Results, shown in Figure 4, indicate that thick-walled region at bracket is located at the bottom of deep cavity, representing final area filled by molten metal. Based on this analysis, root cause of shrinkage cavities at the front cover bracket is determined to be excessive wall thickness and its location deep within cavity, making casting difficult and hindering effective feeding during solidification. Furthermore, thick-walled region at bracket, located at the end of filling process, accumulates a large amount of cold sludge, preventing gas from escaping. Deep cavity also makes it difficult to design overflow channels in demolding direction, resulting in internal defects.

 

Fig.4 Simulated filling sequence

Based on above analysis, die-casting mold adopts an optimized internal core-pulling structure. A large-capacity overflow groove is added at the end of deep cavity filling, filling sequence is changed to concentrate cold sludge and gas during filling process in overflow groove. Simultaneously, casting is fed back, and gap of lateral core-pulling mechanism is used to increase local venting capacity, thereby solving local internal shrinkage cavity defect in casting.

Original structure of internal core-pulling mechanism of front cover die-casting mold is shown in Figure 5. Pre-cast hole core-pulling mechanism with three threaded holes at bracket is set on fixed mold. Three cores with a diameter of ϕ8mm slide with fixed mold insert, achieving core insertion and core pulling under drive of a hydraulic cylinder. Mold structure is simple, but it is impossible to set an overflow groove.

 

Fig. 5 Core-pulling structure before optimization
1. Casting 2. Mold insert 3. Core
Optimized internal core-pulling structure is shown in Figure 6. Three cores with a diameter of ϕ8mm are fixed on an internal core-pulling device with a diameter of ϕ82mm. ϕ82mm internal core-pulling device slides with fixed mold insert, achieving core insertion and core pulling under drive of a hydraulic cylinder. Three ϕ8mm cores are located at upper part of internal core-pulling device, and an overflow groove is set at lower part. After core pulling, shape of overflow groove inlet is separated from forming part of internal core-pulling device, achieving demolding without interference from overflow groove.

 

Fig. 6 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 die-casting mold is to expel gas from cavity, store front-flowing cold molten metal mixed with gas and paint residue. In mold design, overflow grooves are generally distributed by region. 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. According to Table 1, overflow groove volume accounts for more than 25% of adjacent cavity volume. Molten metal flows through gating and cavity for 250 mm. Overflow groove volume should be increased by 20% based on data listed in Table 1. Based on structural form of front cover support and its characteristics such as thick local walls and being located at the end of filling process, slag collection volume ratio is planned to be 50%.
Tab.1 Recommended value of overflow groove volume

Volume of front cover support was measured to be approximately 6,946 mm3 using UG 3D software. Core-pulling design diameter in fixed mold is ϕ82 mm, and its lower part is an overflow groove with a thickness of 7.5 mm. Calculated overflow groove volume is approximately 4,995 mm3, and overflow groove volume ratio is 56%, which meets design requirements. Three feed inlets are selected based on changes in part's shape. Feed inlet positions are relatively straight, making it easy to remove material in subsequent processes. Overflow inlet thickness is designed to be 1.2 mm, less than ingate thickness, to ensure that overflow outlet solidifies earlier than ingate, thus cutting off channel between solidifying metal and outside environment, achieving final injection pressure.

Core pulling mechanism mainly consists of forming elements, moving elements, transmission elements, limiting elements, and locking elements. Core pulling methods typically employ motorized core pulling and hydraulic core pulling. Based on structure of front cover, inner core pulling forming part is in fixed mold, requiring core pulling to be completed before mold opening. If motorized core pulling, such as bent pin core pulling, is used, a second mold opening is required, increasing difficulty and reliability of mold design. Therefore, fixed mold inner core pulling of front cover adopts a hydraulic core pulling mechanism. Hydraulic core pulling mechanism uses a dedicated hydraulic cylinder, achieving core pulling and resetting through the reciprocating motion of piston. This mechanism offers smooth transmission, high core-pulling force, and long core-pulling distance, with core-pulling action unaffected by mold opening time. However, a drawback is that when side core-pulling clamping force exceeds one-third of hydraulic cylinder's locking force, a separate clamping mechanism is required to ensure position and stability of lateral core-pulling during injection molding.

Formula for calculating core pulling force of die castings is:

 

Where, F_pull is core pulling force required for lateral demolding of die casting, N; F_pack is clamping force of die casting on the side of core pulling, N; F_resistance is frictional force between die casting and core during core pulling, N; p is extrusion stress, perpendicular to core surface, generally 10-12 MPa for aluminum alloys; A is lateral area of die casting encasing core, inner core pulling of front cover includes three ϕ8mm cores and surface area of slag collection bag wrapped between core and cavity, the total lateral wrapping surface area is 3730mm2; μ is friction coefficient of die casting alloy on cavity, taken as 0.20-0.25; α is draft angle, taken as 1.5°. Substituting above data into formula (1), core pulling force of inner core pulling is calculated to be approximately F_pull = 11190 N. Locking force of side core pull is mainly to overcome reaction force during injection and prevent displacement of side core pull. Formula for calculating locking force of side core pull is:

 

Where, Flock is clamping force required for die casting to be demolded laterally, N; S is projected area of molten metal perpendicular to core pulling direction, mm2; P is injection pressure, MPa. Diameter of core pull in the front cover fixed mold is ϕ82 mm, and injection pressure is taken as 80 MPa. Substituting into formula (2), we get Flock = 422 264 N.

When using hydraulic core pulling, initial core pulling force includes core pulling force of inner core pull and frictional resistance of motion mechanism when core is detached. Since environment for core pulling in die casting mold is relatively harsh, core pulling force of hydraulic cylinder is generally selected to be more than 3 times core pulling force. As calculated above, locking force required for core pulling within fixed mold of front cover is much greater than core pulling force. Therefore, design of core-pulling hydraulic cylinder only considers core pulling force. Hydraulic cylinder is used to pull out core, and a mechanical locking mechanism is used to lock core within fixed mold, preventing displacement caused by core pulling during injection. This design uses a hydraulic cylinder with an inner diameter of ϕ120 mm and a piston rod diameter of ϕ45 mm, with a maximum core pulling force of 116,570 N, meeting requirements for lateral core pulling force.

Calculations show that locking force for core pulling within front cover is much greater than core pulling force, necessitating design of an independent locking mechanism. Since core pulling within front cover is located in fixed mold, it needs to be pulled out to a safe position completely detached from cavity before mold can be opened. Therefore, locking block needs to be designed in fixed mold, arranged perpendicular to direction of core pulling movement. Locking mechanism for inner core-pulling mechanism of front cover is mainly achieved using a hydraulic auxiliary mechanical mechanism. Mechanical locking is achieved through a locking block and a fixed plate, with locking block moved by a hydraulic cylinder. Structural diagram of inner core-pulling mechanism and its locking mechanism in fixed mold is shown in Figure 7. Its action sequence is: mold closing – inner core-pulling mechanism inserts core → inner core-pulling mechanism locks → injection → inner core-pulling mechanism locking block retraction → inner core-pulling mechanism pulls core → mold opening → part removal.

 

Fig. 7 Structure diagram of core-pulling and locking mechanism in fixed mold
1. Core pulling mechanism in fixed mold 2. Fixed mold sleeve plate 3. Lateral 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 components
Working principle of inner core-pulling mechanism for front cover is as follows: After moving and fixed molds are closed, oil enters rodless chamber of core-pulling cylinder. Piston rod drives inner core-pulling connecting plate and inner core-pulling mechanism forward, realizing core insertion action. When inner core-pulling mechanism is inserted into designed working position, lower stop rod in limit switch assembly touches lower limit switch and sends an electrical signal. Upon receiving signal, locking cylinder begins to operate, oil enters rodless chamber, and piston rod drives wedge block forward to lock. Locking block is designed as a double-sided wedge with a locking angle of 5°. During mold closing, one side of locking block wedges tightly against inner core-pulling connecting plate, while the other side wedges tightly against fixed plate fixed to fixed mold sleeve plate. Before mold opening, oil is first introduced into rod chamber of lateral locking cylinder, and piston rod drives wedge block to retract to a safe position. Then, oil is introduced into rod chamber of core-pulling cylinder, and piston rod returns, driving inner core-pulling cylinder to return to required position to complete core-pulling action. After core pulling is completed, upper stop rod in limit switch assembly touches upper limit switch to send an electrical signal, mold opens and part is removed. Structural features of this design are: core-pulling action of inner core-pulling mechanism is completed by core-pulling cylinder; mold structure is simple; core-pulling action sequence and core-pulling distance are not limited; lateral mold closing is achieved through a mechanical locking structure, which is suitable for locking side cores with high locking force; locking position is reliable and not easily retracted; displacement action of locking block is completed by locking hydraulic cylinder, and usage position, action sequence, displacement distance are flexible and controllable.

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. A batch of 2,000 pieces of optimized mold were produced, 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 defects in mass-produced products were reduced to below 2%. Through large-scale production verification, front cover product quality is stable, mold operates smoothly, reliably, and durablely.

 

Fig.8 X-ray inspection of optimized front covera