Abstract: This paper addresses issues such as poor airtightness, core erosion, and excessive flash in initial process of a thermostat housing. CAE analysis was used to reveal causes of these defects. A symmetrical split-flow gating system was developed, gate angle and area were optimized, three independent cores were reconstructed into an integral slider. After optimization, gate velocity decreased from 62.1 m/s to 40.0 m/s, flow rate of molten aluminum at the mold front was more stable, and air entrapment was eliminated. Actual verification shows that airtightness and flash thickness of thermostat housing die-casting meet design requirements.
Thermostats are key components for improving energy utilization rate, driving range, and system reliability of new energy vehicles. Their housings are typically made of die-cast aluminum alloy. A certain model of thermostat housing requires no internal leakage under 0.2 MPa pressure and no porosity defects after machining sealing end face. However, during initial die-casting production process, after machining, thermostat housing die-cast part failed airtightness test due to porosity defects on sealing end face and inside, resulting in a product scrap rate as high as 35%. Die-casting mold also exhibited severe core erosion and aluminum adhesion in critical areas, which did not improve situation even with surface coating treatment. Furthermore, excessively thick flash increased subsequent trimming workload, significantly impacting production efficiency and costs. To address these issues, this study analyzes initial die-casting gating system design using CAE numerical simulation to identify causes of defects, optimize die-casting process parameters, gating system structure, providing a reference for solving similar die-casting defects.

Three-dimensional structure of thermostat die-cast part is shown in Figure 1. Its outline dimensions are 99.9 mm * 56.3 mm * 29.2 mm, blank net weight is 136 g, material is aluminum alloy A360, chemical composition is shown in Table 1, and average wall thickness is 3.2 mm. Casting has a relatively complex structure. Product surface and parting surface must be free of burrs, flash, and other defects. All external dimensions must conform to drawings and assembly requirements. Product must be leak-free under 0.2 MPa air pressure; therefore, internal cavity and sealing end face must be free of shrinkage cavities, porosity, and other quality defects.

 

Fig.1 3D structure of thermostat housing

Tab.1 Chemical composition of A360 alloy  %

Defects in die casting shell during trial production are shown in Figure 2. Most die castings were found to leak during pressure testing (Figure 2a). Obvious porosity defects were found on machined rear end face (Figure 2b). Porosity diameter was between 0.1 and 0.5 mm, and distribution was relatively dense, resulting in an airtightness test pass rate of only 65%. Two cores near gate showed severe erosion, with aluminum adhering to surface and turning white (Figure 2c). Surface quality of corresponding parts of product was poor (Figure 2d), affecting dimensional accuracy. Furthermore, due to erosion and adhesion of aluminum to mold core, mold fitting clearance increases, resulting in a flash thickness of 1.2~1.5mm on corresponding side of die-cast product (see Figure 2e), far exceeding design allowable range of 0.2mm. Subsequent trimming processes are time-consuming and labor-intensive, leading to low production efficiency.

 

Fig.2 Defects of thermostat housing in primary process
Simulation was performed using Supreium casting simulation software according to initial design. Model used unstructured tetrahedral mesh generation. Die-casting mold material was H13 steel. Heat transfer coefficient between die-cast part and mold was set to 20,000 W (/m²⋅℃), heat transfer coefficient between mold and mold itself was set to 1,000 W (/m²⋅℃), heat transfer coefficient between outer surface of mold and air was set to 100 W (/m²⋅℃), temperature was set to 20℃, heat transfer coefficient between cooling water and mold was set to 10,000 W (/m²⋅℃). Main die-casting process parameters are shown in Table 2.
Tab.2 Parameters for die casting process

Figure 3 shows initial mold filling flow simulation analysis. Results showed that internal leakage and end-face porosity in die castings were mainly caused by defects resulting from initial single-sided gating design, rooted in instability of flow field due to asymmetric flow. High-speed flow from position 1 (see Figure 3a) resulted in cold material accumulation at lower corner of right-side gating, leading to excessive cold material in right-side product. High-speed flow from position 2 (see Figure 3b) resulted in two starting points being out of sync. Due to lack of synchronization and gating angle biased towards product side, significant air entrapment and backflow occurred during aluminum liquid filling process (see Figure 3c), with air pressure in highlighted area indicated by arrow reaching 1.4~2.0 MPa. According to Bernoulli's principle, excessively high dynamic pressure in a local area leads to a decrease in static pressure in that area, potentially "drawing" gas from gaps such as venting channels to form secondary air entrapment . Simultaneously, asymmetric flow front caused far-end venting system to fail, resulting in high-pressure cavities within mold cavity. Gas could not be effectively discharged, leading to porosity defects during solidification.

 

Fig.3 Filling simulation of aluminum melt in primary process
Figure 4 shows a schematic diagram of initial scheme's molten metal flow during filling. It can be seen that when aluminum liquid enters mold cavity at high pressure and high speed, due to single-sided gating system and downward angle of gate, there is an obvious flow deviation phenomenon during filling process, which has a strong impact on two cores near gate. At the same time, single gating area is only 70 mm², and speed at gate is too high, which further aggravates impact force of aluminum liquid. When aluminum liquid (about 680 ℃) fills mold cavity and comes into contact with mold steel, interface temperature will instantly exceed tempering temperature of mold steel, resulting in local softening. When temperature of molten aluminum when it comes into contact with mold steel is higher than critical temperature, Fe atoms and Al atoms will diffuse into each other, forming a series of intermetallic phases and aluminum-rich liquid face-centered cubic phases. Continuous shear stress generated by high-speed aluminum liquid on core surface causes brittle intermetallic compound layer to crack under thermal stress, and fatigue microcracks are generated on the surface of mold steel, which become channels for aluminum liquid to penetrate, resulting in "mechanical interlocking" type aluminum adhesion. Initial design's three independent cores provided more edges and gaps, exacerbating turbulence and heat accumulation of molten aluminum, thus accelerating aforementioned process. Due to erosion and adhesion of aluminum to mold cores, mold fitting clearance increased. Simultaneously, unreasonable design of gating system resulted in uneven distribution of filling pressure, causing flash thickness on corresponding side of die-cast product to reach 1.2~1.5mm (see Figure 2e).

 

Fig.4 Schematic diagram of metal liquid flow during filling process

Design of gating system typically needs to determine specific location and type of gate based on casting's external shape, internal structure, and dimensional accuracy requirements. Selection of gate for thermostat housing die-casting mainly follows principle of the shortest flow path, allowing for rapid filling of all parts of cavity, shortening molten aluminum filling time, thereby reducing risk of air entrapment or cold shut defects in molten aluminum within cavity. Simultaneously, its location should be ensured for easy subsequent removal. Based on above analysis and considering problems in initial gating system design, single-sided gating method was changed to a symmetrical split-flow gating structure (see Figure 5). Area of each gate was increased to 75 mm², and the total gating area increased from 140 mm² to 150 mm², ensuring smooth entry of molten aluminum into cavity. Gate angle was adjusted so that flow direction of molten aluminum into cavity was less than 45° from cavity wall, reducing direct impact of molten aluminum on critical parts of mold. A guide channel and buffer cavity were added to gating system to further slow down flow rate of molten aluminum, making filling process more stable. Optimized symmetrical split-flow design created a momentum-balanced flow field, eliminating momentum source of single-sided impact. Increased gate area directly reduced gate velocity, thereby reducing Reynolds number and dynamic pressure, weakening turbulence intensity and impact force. Addition of guide channel further guided streamlines, avoiding direct vertical impact on core.

 

Fig.5 Modified gating system

To address severe aluminum runoff problem in original structure during production, mold core structure was optimized by replacing three cores with a single integral slider, as shown in Figure 6. This structure simplifies mold parting surface, making slag venting easier and effectively improving aluminum runoff problem. It also increases core strength, thus extending service life of die-casting mold.

 

Fig.6 Mold structure before and after modification

CAE Simulation Verification: CAE numerical simulation was performed on optimized die-casting process and mold scheme. Results are shown in Figure 7. After molten aluminum enters cavity through symmetrical left-right split gating system, it achieves stable and synchronous filling with a consistent filling sequence, no obvious flow deviation, and no air entrapment or backflow. Filling process is more stable. Quantitative comparison with initial scheme shows that optimized punch speed decreased from 3.70 m/s to 2.55 m/s, and gate speed decreased from 62.1 m/s to 40.0 m/s, significantly improving filling stability. Gas content analysis results of improved scheme are shown in Figure 8. It can be seen that after filling, there is no red high-pressure area inside product, and maximum gas volume fraction in sealing end face area is less than empirical porosity risk threshold of 10%, effectively avoiding generation of defects.

 

Fig.7 Simulation results of filling flow process in modified scheme

 

Fig.8 Gas content during filling flow process in modified scheme
To verify reliability of numerical model and feasibility of improved scheme, an actual mold trial was conducted, test specimen was subjected to X-ray flaw detection and machining, as shown in Figure 9. As can be seen from Figure 9a, there are no visible porosity defects inside die casting and in sealing end face area under X-ray, and machined sealing end face also has no porosity defects, as shown in Figure 9b.

 

Fig.9 Thermostat housing die casting produced by modified scheme
While symmetrical gating and slider structure improvement scheme is effective, its application has specific boundary conditions. First, symmetrical gating system requires that casting geometry and thermal field be approximately symmetrical. For castings with strong asymmetric wall thickness or thermal points, simple geometrically symmetrical gating may lead to severe asynchrony of flow front, which worsens gas entrapment; therefore, an asymmetric gating design based on flow resistance balance is required. Second, the overall slider structure increases initial manufacturing cost and complexity of mold, which is not conducive to small-batch production. Furthermore, increased overall size of slider may compress space required for mold cooling water channel layout, necessitating a more refined thermal balance analysis. Future work could explore combining these active control technologies with this optimized structure to address more complex castings.

(1) CAE numerical simulation technology revealed problems such as mold filling deviation and air entrapment during die casting process of aluminum alloy thermostat housings, providing a foundation for process optimization and mold improvement.
(2) By optimizing gating system to a symmetrical flow distribution structure, expanding gate area, adjusting gate angle, adding a flow guide and buffer structure, and optimizing venting system, problems of aluminum liquid mold filling deviation and air entrapment were effectively solved, eliminating porosity defects in die castings.
(3) Improving mold core structure reduced erosion and aluminum adhesion phenomenon of mold core, reduced flash thickness of product, and extended mold's service life.