Abstract: Currently, automobile engine cylinder blocks are mainly divided into two structures in product design: split type and gantry type. Aluminum alloy die casting processes for these two structures differ, especially for gantry type cylinder blocks, which typically employ two schemes in gating design: transverse single-sided gating and longitudinal double-sided gating. This study investigates impact of different gating methods on die casting design and processes from both process scheme and mold flow analysis perspectives. Results show that there is no absolute advantage or disadvantage between two gating methods in terms of process; final choice of scheme requires comprehensive consideration of economic efficiency and other process capabilities. This research aims to provide process basis and scheme selection reference for die casting gating process design of engine cylinder blocks.
Introduction: In recent years, with rapid development of new energy vehicle industry, product design level and process technology capabilities of domestic automobile engine cylinder blocks have gradually improved. In terms of cylinder block structure, split structure technology is mature and widely used; however, in some application scenarios, gantry structure is also adopted to simplify product structure, control quality risks, and reduce production costs. This structure integrates crankcase into cylinder block, reducing number of crankcase connecting parts (such as bolts) and assembly steps, thereby eliminating risk of oil leakage at mating surfaces and lowering production costs. Two cylinder block structures are shown in Figure 1.

 

Figure 1 Different cylinder block structures
Two cylinder block structures differ in die-casting processes and implementation difficulty, especially in gating method: split structure often uses a transverse single-sided gating method; while gantry structure, based on production experience, can use either a transverse single-sided gating method or a longitudinal double-sided gating method. However, during trial production, these gating methods often result in die-casting defects such as shrinkage cavities, insufficient material, and air entrapment. Currently, with improvement of economic and technological strength of die-casting industry, mainstream professional die-casting mold flow analysis has been widely applied in R&D front end. Analyzing die-casting process through simulation can improve product quality, reduce production costs, and shorten development cycle, providing strong technical support for die-casting theoretical analysis and actual production. This paper focuses on gantry structure, studying impact of gating method on die-casting design and process.

Research object is a typical gantry structure, die-cast from aluminum alloy using ADC12 material. Its dimensions (length * width * height) are 380mm * 360mm * 270mm, with a net casting weight of approximately 17kg and a projected area of approximately 780cm². Main body wall thickness is 4mm, with a maximum wall thickness of 27mm and an average wall thickness of 6.6mm. To standardize theoretical and experimental conditions, this study is based on following fixed conditions: UBE1650T die-casting machine, 150mm punch, existing molds, and main body cooling structure, ensuring consistent experimental conditions. Gantry structure (3D view) is shown in Figure 2.

 

Figure 2. Gantry structure (3D view)

Based on product's structural characteristics and basic parameters of UBE1650T die-casting machine, two 3D models of gating schemes were designed in the early stages. Scheme A uses a transverse single-sided gating method, and Scheme B uses a longitudinal double-sided gating method. Different gating methods are shown in Figure 3.

 

Figure 3 Different pouring methods

Formula for calculating the total projected area Am is as follows: Am=a1+a2+a3+a4(1)
Where a1 is projected area of casting; a2 is area of gating system, generally set to (0.15～0.3)Am; a3 is area of overflow system, generally set to (0.15～0.2)Am; a4 is area of sprue, calculated according to punch diameter.
In actual design, design of expansion force needs to consider both forward and lateral components. Formula for calculating main expansion force Fm is as follows: Fm=Am*p(2)
Where Am is the total projected area; p is injection pressure.
Formula for calculating secondary expansion force Fs is as follows: Fs=As*p*tanθ(3)
Where As is lateral projected area of slider; θ is mold locking angle. Injection pressure 'p' refers to pressure per unit area. For load-bearing components like cylinders, its value typically ranges from 50 to 80 MPa. Projected area of casting is shown in Figure 4.

 

Figure 4 Projected area of casting
Clamping force Ft is calculated as follows: Ft = δ * (Fm + Fs) (4). Where δ is safety factor.
Clamping force parameters for two schemes are shown in Table 1.
Table 1. Clamping Force Parameters for Two Schemes

Based on parameters in Table 1, under premise of meeting clamping force requirements of UBE1650T die-casting machine, injection pressure of scheme A is 69 MPa, and injection pressure of scheme B is 61 MPa. Both are within theoretical allowable range of injection pressure for cylinder die-casting and meet process requirements. However, scheme B, due to its longitudinal arrangement, results in a larger projected area on upper side of mold, required expansion force is correspondingly increased, which can easily lead to aluminum fly-off during actual production.

Filling time t is calculated using following formula:

 

Where k is a constant; Ti is gate temperature; Tf is liquidus temperature; S is percentage of solids at the end of filling; Z is solids unit conversion factor; Td is mold temperature before filling; and T is average wall thickness.
Table 2 Filling Time Parameters

Filling time parameters are shown in Table 2. Based on parameters in Table 2, calculated filling time is 106.957 ms.

Based on product structure and structural characteristics of UBE1650T die-casting machine, and referring to existing process parameters, they are substituted into P-Q diagram for calculation. Calculation results show that for scheme A, working point filling time is 107.2 ms, filling speed is 40 m/s, and required injection speed is 4.1 m/s; for scheme B, working point filling time is 108.1 ms, filling speed is 45 m/s, and required injection speed is 3.8 m/s. Both operating points are within process window, indicating that injection force of current UBE1650T die-casting machine can meet process requirements of both schemes.

 

Figure 5 P-Q diagram
Process parameters are shown in Table 1, and P-Q diagram is shown in Figure 5.

Based on existing die-casting machine structure, molds for two schemes were designed. Both molds use a front fixed mold, rear fixed mold, moving mold, and four sliders (up, down, left, and right). Mold cooling design references existing cooling device configuration of UBE1650T die-casting machine and experience data from similar cylinder structures, and is optimized accordingly. Mold design scheme is shown in Figure 6.

 

Figure 6 Mold Design Scheme

This paper uses mold flow analysis software to establish a mold flow analysis model, utilizes its mesh generation module to generate mesh and perform calculations. Mesh size is set to 1.5 mm³, number of mesh elements in X, Y, Z directions are 600, 258, and 586, respectively. Solver parameters are set in visual interface of MAGMA software, boundary conditions and main process parameters are referenced in Table 3.
Table 3 Process Parameters

Based on above model, following section will use mold flow analysis software to conduct a comprehensive mold flow analysis of velocity field, temperature field, air entrapment field, filling degree, oxides, surface solids content, solidification sequence, shrinkage cavities of two schemes.

Velocity field analysis is shown in Figure 7. Analysis results show that filling process of both schemes can meet product molding requirements. However, Scheme A has a higher filling speed inside product, which leads to a higher risk of mold erosion and casting scratches, less room for adjustment of injection speed and filling time in subsequent processes. In contrast, Scheme B performs better than Scheme A in terms of filling speed control.

 

Figure 7 Velocity field analysis

In actual production, there is a temperature loss during gating, so actual gating temperature is 20-30℃ lower than gate temperature. Currently, simulated gating temperature is 680℃, the overall surface temperature difference between Schemes A and B is controlled within 50℃ and 70℃, respectively. Scheme A has the lowest temperature region at the end and on moving mold surface, ranging from 610 to 620℃; Scheme B has the lowest temperature region at the end, around 605℃. Both schemes have minimum temperatures higher than material's liquidus temperature (582℃). Both schemes require controlling mold temperature and adjusting overflow channel position to ensure confluence point is on overflow channel for effective slag removal.

 

Figure 8 Temperature field analysis
Temperature field analysis is shown in Figure 8.

A vacuum valve was used for evacuation, and upper limit for gas entrainment analysis was set at 0.34 kg/m³. Normally, areas exceeding this upper limit are not allowed in product (marked by circles in Figure 9). Gas entrainment field analysis is shown in Figure 9. Results show that Scheme A has poor gas entrainment due to molten metal entrainment within barrel, requiring adjustment of injection parameters to control molten metal flow for optimization. Scheme B has severe gas entrainment at circled area, as this is final filling position. Adding a new flow channel here can optimize gas entrainment by achieving a layout of four flow channels on each side.

Filling degree analysis is typically used to observe whether there are any incompletely filled areas at the end of product filling and to identify final filled area. Figure 10 shows filling degree analysis results. Results indicate that in Scheme A, final filled area is where gate avoids threaded hole, posing a risk of air leakage. Gate design needs adjustment at this location. In Scheme B, final filled area corresponds to area identified in air entrapment analysis; no incompletely filled areas were found, and the overall filling requirements are met.

Figure 11 shows oxide analysis results. Results indicate that in Scheme A, oxides are mainly concentrated at the end of product and not concentrated in any one area, flowing smoothly to overflow channel at the end. Remaining oxides can be removed by optimizing gating method. In Scheme B, oxides are mainly concentrated at the front of product. This is because oxides mainly enter cavity with the first branch of runner and accumulate there. Furthermore, there is insufficient overflow channel at the bottom of this area for slag removal, resulting in a higher local oxygen content. Optimization by modifying gating method is necessary.

Surface solidity analysis was used to observe solidification of different areas on product surface, while solidification sequence analysis was used to observe solidification of different areas inside product. Surface solidity and solidification sequence are shown in Figure 12. Analysis results show that highlighted areas indicate solidified portions, while slower-solidifying areas are risk points requiring focused cooling. The overall solidification process of both schemes is relatively synchronized, with intermediate baffle solidifying more slowly due to its greater thickness.

 

Figure 12 Analysis of surface solidity and solidification sequence

Shrinkage cavity analysis is shown in Figure 13. Analysis results show that the overall wall thickness of products in schemes A and B is uniform, shrinkage cavities are distributed at connection between baffle and main body, where wall thickness is greater.

 

Figure 13. Analysis of shrinkage cavity

Based on above process schemes and mold flow analysis, research results on two gating methods show that each scheme has its advantages and disadvantages. Based on above main process, soft mold manufacturing and small-batch trial production were carried out. Pass rate of trial production parts for scheme A was 65%, significantly lower than 90% for scheme B. Main defect of Scheme A is internal casting holes (shrinkage cavities), especially concentrated near 2J and 4J water-oil passage areas. Reason for this is that release agent easily accumulates in lower slider area of transverse mold structure, and existing box-type spraying system has a long blowing time after spraying, leading to a decrease in mold temperature. Subsequently, spraying and blowing system was optimized, a more efficient and precise adaptive spraying technology was developed and adopted, ultimately increasing pass rate of Scheme A to 90%. Trial production and defects are shown in Figure 14.

 

Figure 14. Trial Production and Defects
(1) Through mold flow analysis, cylinder filling and solidification process can be simulated in the early design stage, predicting and identifying die-casting defects in advance, clarifying their causes, then verifying advantages and disadvantages of different schemes, making targeted optimizations to determine more economical process scheme.
(2) In actual production, spraying and blowing processes have a significant impact on mold temperature and product internal quality, and need to be combined with mold flow analysis.
(3) In actual production, economy (cost) is one of key factors that enterprises need to consider. Two schemes show a significant difference in process yield: Scheme A yields 63%, while Scheme B yields 73%. Difference in material recycling costs (for molten aluminum) due to non-forming parts such as sprue, overflow trough, and gating system is approximately 2%.
Die casting design and process is a systematic engineering project that requires evaluation from multiple dimensions, including product design, process design, manufacturing process control, and cost, to select the most suitable process scheme based on specific conditions.