Based on structure of automotive engine oil pan, a die-casting process was designed. Numerical simulation was performed using CAE software. By optimizing high-pressure cooling point and flow channel scheme, combined with analysis and control of mold temperature field, problems of shrinkage cavities and leakage at hot spot of oil pan were improved, thus enhancing product quality and meeting mass production requirements.
Oil pan, as a crucial component of an automotive engine, is installed at the bottom of engine. Its main functions are to store engine oil and seal crankcase, while protecting oil passage components at the bottom of engine. Oil pan possesses strong wear resistance and impact resistance, acting as a buffer and dispersing impact force, preventing engine oil leakage, and maintaining normal operating temperature inside engine. Loss of any of oil pan's sealing, oil storage, and oil suction functions will cause serious damage to engine. New generation of automotive engine oil pan parts has a complex structure, uneven wall thickness, and high airtightness requirements. During die-casting process, eddy currents and uneven molten metal feeding can occur, leading to defects such as porosity and shrinkage cavities within casting. This results in high scrap rates and cost waste, necessitating in-depth analysis and continuous improvement of die-casting process.

Figure 1 shows a schematic diagram of an automotive engine oil pan structure. Material is aluminum alloy ADC12, with dimensions of approximately 427 mm * 310 mm * 165 mm, a weight of approximately 3.4 kg, and an average wall thickness of 3.3 mm. The overall structure of casting is relatively complex, being a six-sided machined body with interlaced reinforcing ribs throughout. Surrounding functional areas have local wall thicknesses ranging from 10 to 20 mm. In addition to moving and fixed mold cores, casting forming module includes three sets of sliding core-pulling mechanisms. Projected area of casting is approximately 1145 cm², requiring a 1650T die-casting machine for production. Surface of parts must be free of cracks, undercasting defects, mechanical damage, and any penetrating defects. Porosity of high-pressure oil passage sealing surface and filter mounting surface must be ≤0.3 mm, and porosity of other generally machined surfaces must be ≤0.5 mm. Internal porosity of casting must conform to ASTM E505 Class 2 standard. High-pressure oil passage holes and inner cavity must be able to withstand pressures of 0.4 MPa and 0.2 MPa for 25 s, with leakage values ≤1.0 mL/min and ≤4.0 mL/min, respectively.

 

Figure 1: Schematic diagram of an automotive engine oil pan structure

The thickest part of this component reaches 18.5 mm, located near pre-cast holes of high-pressure oil passage (see Figure 2). Significant difference in wall thickness in this localized area creates hot spots. During solidification, insufficient feeding leads to shrinkage cavities and porosity defects, resulting in part leakage. A common solution to this type of defect is to add a local extrusion pin for shrinkage compensation. However, due to limitations of product and mold structure, extrusion pin cannot be effectively designed. This means extrusion pin cylinder will interfere with inclined oil channel core-pulling cylinder. Locations of high-pressure oil channel hole and inclined oil channel core-pulling hole are shown in Figure 1(a). Therefore, to address risk of leakage from high-pressure oil channel hole in this oil pan, the only solution is to optimize and improve design of die-casting flow channel and high-pressure cooling scheme.

 

Figure 2. Cross-sectional view of thickened area
Since inner cavity of this oil pan is product ejection location, it is formed using a moving mold to facilitate product ejection, reduce clamping force, and thus ensure flatness of product. Furthermore, filter mounting surface, drain bolt hole, and rear end face are locations of three sets of sliders, as shown in Figure 1(b). Therefore, based on structure of this oil pan, parting line and feeding method are selected as shown in Figure 3: the overall feeding method is vertical feeding, with front end face of oil pan as feeding position. This avoids risk of flow divider cone getting stuck on slider. Ten sets of ingate feeds were designed, with rear end being filling end. Initial numerical simulations of gating system were performed. Simulation results showed that the overall temperature of casting was low when mold filling was complete, as shown in Figure 4. This is because gating system has a long flow path, resulting in significant heat loss during filling. Cold material easily accumulates at filling end, leading to increased porosity and inclusion defects after machining. Furthermore, two bottom flow channels feed preferentially, easily forming cold shuts, causing peeling after shot blasting, as shown in Figure 5.

 

Figure 3 Schematic diagram of gating system structure

 

Figure 4 Temperature field distribution diagram

 

Figure 5 Schematic diagram of filling process
Problem of heat loss in flow path due to excessive length of bottom and side flow paths can be addressed by appropriately increasing filling speed of ingate, thereby increasing temperature of aluminum liquid entering mold cavity, according to formula (1):

 

Where: T is temperature rise of molten metal, ℃; v is ingate speed, m/s; c is specific heat capacity of metal, J/(kg·℃); K is thermal equivalent, kgf·m.
When specific heat capacity of ADC12 die-cast aluminum alloy is 0.9 J/(kg·℃) and thermal equivalent is 100 kgf·m, relationship between ingate speed and temperature rise of molten metal is described by formula (1), as shown in Figure 6. When ingate speed is 40 m/s, temperature of molten aluminum alloy entering mold cavity will rise by about 8 ℃; while when ingate speed is 60 m/s, temperature of molten aluminum alloy entering mold cavity will rise by 20 ℃. Therefore, the higher ingate speed, the higher temperature of molten metal when passing through ingate. Ingate speed of oil pan is selected as 50 m/s. According to formula (1), ingate temperature rise is 14 ℃, which can compensate for temperature loss of molten aluminum alloy before entering mold cavity to a certain extent, maintain fluidity of molten aluminum, and ensure filling at the end. At the same time, a slag bag of a certain volume is set at all filling end positions of mold wall to accommodate cold material at the front end of molten aluminum, reduce occurrence of end porosity and slag inclusions. Hot spot of high-pressure oil channel is unavoidable and requires forced cooling by high-pressure spot cooling.

 

Figure 6 Relationship between ingate speed and molten metal temperature rise

Principles for selecting slow injection speed: (1) To minimize heat loss when molten aluminum alloy is poured into injection chamber and injected into ingate, ensuring sufficient fluidity for easy filling; (2) To minimize tumbling and surging of molten aluminum alloy as punch moves forward, minimizing amount of gas entrained; (3) To prevent molten aluminum alloy from splashing out of gate. Therefore, slow injection speed should not be too fast or too slow, according to formula (2):

 

Where: Vc is theoretical maximum critical speed of injection punch, m/s; D is inner diameter of injection chamber, m; g is acceleration due to gravity, m/s²; h is initial height of molten metal in injection chamber, m.
Oil pan casting was produced using a 1650T die-casting machine with an inner diameter of 110 mm, a dry stroke of 816 mm, a casting weight of 8.34 kg, and a casting volume of 3147 cm³. Initial height of molten metal in injection chamber was 50 mm. Based on formula (2), theoretical maximum critical velocity of injection punch was calculated to be 0.6 m/s. Numerical simulation was performed using AnyCasting software to observe flow pattern of molten aluminum alloy before it completely filled chamber. Results are shown in Figure 7.

 

Figure 7: Simulation results of chamber filling
Simulation results show that temperature of molten aluminum alloy in chamber remained above liquidus line +30 ℃. During low-speed filling process, molten aluminum filled smoothly as punch advanced, without any air entrapment, thus avoiding entrainment of gas in chamber.

Principles for selecting fast injection speed are: (1) Molten aluminum alloy must have good fluidity before filling mold cavity to ensure smooth filling; (2) Molten aluminum alloy should be able to quickly, orderly fill mold cavity and discharge gas outside mold cavity; (3) High-speed metal flow should not be formed to scour mold cavity or core, avoiding occurrence of mold sticking. Ingate speed of aluminum alloy die castings is generally 40~60 m/s. The total cross-sectional area of ingate of oil pan casting is 771 mm², and cross-sectional area of pressure chamber is 9498.5 mm². Ingate speed is initially selected as 50 m/s. According to Bernoulli's equation: ingate speed * ingate cross-sectional area = high speed * pressure chamber cross-sectional area, high speed is calculated to be 4 m/s.

Theoretical high-speed switching position for die-cast parts is when molten metal fills to ingate. For this oil pan casting, theoretical high-speed position is 590 mm. However, because ingates are arranged vertically, bottom ingate will feed material earlier, leading to cold shuts at the bottom of casting. Therefore, high-speed switching point is set to 550 mm. Specific die-casting process parameters are shown in Table 1.

[Table 1] Die-casting Process Parameters

In small-batch die-casting production, main problem is shown in Figure 8: leakage from high-pressure oil passage holes (unmachined), with a scrap rate of 30%, and visible cracks inside holes in some leaking products.

 

Figure 8. Anatomical diagram of oil pan defects
High-pressure oil channel leakage problem was caused by internal shrinkage cavities. In some leaking products, these shrinkage cavities were exposed, leading to cracks. Oil channel is located at product's hot spot. High-pressure cooling point of oil channel casting pin could not completely cool this area. Thermal imaging was used to detect mold temperature at this location; the overall temperature at oil channel casting pin location was nearly 290 ℃, as shown in Figure 9. Adjusting water flow rate and extending water flow time at high-pressure cooling point of casting pin did not significantly improve situation, suggesting other influencing factors. Repeated confirmation of CAE simulation results revealed that oil channel casting pin was located at confluence of two flow channels at the end of operating side, as shown in Figure 10. Two flow channels directly impacted oil channel casting pin and failed to fuse properly, resulting in excessively high casting pin temperature, causing shrinkage cavities and even their exposure.

 

Figure 9. Mold temperature field before improvement

 

Figure 10. Simulation analysis before improvement

 

Figure 11. Mold temperature field after improvement
Based on location of defect, mold was optimized and adjusted. Last flow channel on operating side was blocked, as shown in Figure 12. Simultaneously, a new high-pressure spot cooling system was added to fixed mold core to spot cool bottom wall thickness of high-pressure oil passage, enhancing cooling at hot spot, as shown in Figure 13.

 

Figure 12. Simulation analysis after improvement

 

Figure 13. Optimized mold spot cooling scheme
After optimization, mold temperature at oil passage location dropped to 170 ℃. Batch production verification was conducted, and no leakage or cracks were observed during pressure testing. Casting had a bright appearance, with no cold shuts, flow marks, or other defects. Surface roughness after shot blasting met requirements, with no obvious shot blasting peeling, as shown in Figure 14. X-ray inspection of product showed good internal quality, with overall porosity meeting ASTM E505 Class 2 standard requirements, as shown in Figure 15. After dissection of oil passage, a targeted inspection of cross-section and interior was conducted. No obvious porosity or shrinkage cavities were found, as shown in Figure 16. Following this process, mass production resulted in a leakage scrap rate of 0.48%, and the overall product qualification rate reached over 98%, meeting requirements for mass production.

 

Figure 14: Actual photo of oil pan

 

Figure 15: Overall X-ray inspection

 

Figure 16: Dissection and X-ray inspection of oil passage

(1) For shrinkage cavity defect in thick-walled area of die-cast aluminum alloy oil pan, optimization was achieved by improving high-pressure spot cooling scheme. At casting pin location with a larger diameter, high-pressure spot cooling was used to simultaneously force-cool casting pin body and opposite mold core, improving solidification conditions in thick-walled area and effectively improving shrinkage cavity at hot spot of oil pan.
(2) For pre-cast pins near inner runner of oil pan, pre-cast pins are easily affected by direct impact of nearby runner, which can cause sticking to mold and lead to an increase in temperature of casting pins. In the case of frequent cleaning and replacement, gating system can be optimized, improved by combining CAE numerical simulation to ensure that multiple flow channels do not converge and merge poorly at casting pin position. This can effectively improve problem of product leakage caused by internal shrinkage cavity due to high temperature caused by casting pin sticking to mold.