The rapid advancement of new energy electric vehicles has placed automotive lightweighting and extended driving range at the forefront of research for major manufacturers. As a crucial component of the body-in-white, reducing the mass of the door body is significant for achieving vehicle weight reduction. Unlike traditional doors formed by stamping and welding, this study proposes an integrated door body manufactured using the investment casting process. This work investigates the door body’s structural performance, wax pattern molding process, and casting process through finite element simulation software.
First, a structural performance analysis of the door body was conducted. Leveraging the powerful meshing capabilities of HyperMesh, a high-quality finite element model was established. This model was imported into Ansys software for sequential modal and stiffness analyses. The results indicate that the first-order natural frequency of the door body is far from common excitation frequencies, thus resonance is avoided. Under various loading conditions, the maximum deformation and calculated stiffness values all meet the standards of relevant automotive enterprises, confirming the design’s feasibility for the intended investment casting process.
Subsequently, the wax pattern molding process for the door body was designed. Using Moldflow software, gate location analysis was performed for four different gating schemes. The optimal gate location was determined based on simulation results, leading to the completion of the gating, venting, and cooling system designs. Single-factor experiments were conducted to study the effects of melt temperature, mold temperature, injection time, and holding pressure on wax pattern quality. Following this, a 4-factor, 3-level orthogonal experiment was designed to determine the optimal injection parameters: a melt temperature of 56°C, mold temperature of 33°C, injection time of 8 seconds, and a holding pressure at 60% of the filling pressure, resulting in a volume shrinkage of 3.863%.
Finally, the low-pressure investment casting process for the door body was designed and optimized. Process parameters for each stage of low-pressure casting were calculated. The gating system was designed and iteratively optimized using ProCAST simulation software. Single-factor experiments investigated the influence of pouring temperature, mold preheat temperature, and filling time on casting quality. A 3-factor, 3-level orthogonal experiment was then conducted to identify the optimal combination: a pouring temperature of 720°C, mold preheat temperature of 480°C, and a filling time of 16 seconds. To address shrinkage defects, a targeted cooling method using liquid nitrogen was designed and applied to critical sections. This optimization successfully reduced the final volume of shrinkage porosity within the casting to 4.9 cm³.
Structural and Performance Simulation Analysis of the Door Body
The door body is a complex shell-type component. To ensure accurate simulation, a mid-surface mesh was generated in HyperMesh. After model simplification and defect repair, a high-quality finite element mesh was created and assigned a thickness corresponding to the door’s design. The material for the integrated door body is casting aluminum alloy ZL114A, selected for its high strength-to-weight ratio, good fluidity, and low shrinkage. Its key properties are summarized below.
| Property | Value |
|---|---|
| Poisson’s Ratio | 0.3 |
| Density (g/cm³) | 2.65 |
| Elastic Modulus (GPa) | 69 |
Modal analysis was performed to ensure the door’s natural frequencies avoid excitation sources like the engine and road. The first six natural frequencies and their mode shapes were extracted. The first-order natural frequency was found to be 42.2 Hz, which is sufficiently higher than typical vehicle excitation frequencies (below 30 Hz), preventing resonance issues during operation.
Stiffness analysis under various loading conditions is critical for door functionality, sealing, and safety. Four primary stiffness types were analyzed: sagging stiffness (door supported by hinges and latch), torsional stiffness, window frame stiffness, and beltline stiffness. Constraints and loads were applied according to standard industry practices. The simulation results, including maximum deformation and calculated stiffness, are compiled in the following table. All values satisfy the performance criteria, confirming the structural integrity of the design for the subsequent investment casting process.
| Stiffness Analysis Type | Max. Deformation (mm) | Calculated Stiffness (N/mm) | Standard Value (N/mm) |
|---|---|---|---|
| Sagging Stiffness | 2.7 | 296.3 | >250 |
| Upper Torsional Stiffness | 4.8 | 187.5 | >91.5 |
| Lower Torsional Stiffness | 4.8 | 187.5 | >91.5 |
| Window Frame Corner Stiffness | 3.8 | 65.8 | >50 |
| Window Frame Middle Stiffness | 3.3 | 60.7 | >40 |
| Inner Panel Beltline Stiffness | 0.9 | 111.1 | >100 |
| Outer Panel Beltline Stiffness | 0.5 | 200.0 | >100 |
Wax Pattern Molding Process Design and Optimization
The quality of the wax pattern is fundamental to the final casting’s precision in the investment casting process. The door body model was prepared and a dual-domain mesh was generated in Moldflow for analysis. A medium-temperature wax (F28-448) was selected for its suitable properties. The parting line was defined on the inner panel side to facilitate core placement and part ejection.
Gate location analysis is crucial for ensuring complete filling and minimizing defects. Four gating schemes with varying numbers and positions of gates were simulated. Key results including fill time, flow front temperature, weld lines, switching pressure, and volumetric shrinkage were compared. Scheme 2, featuring five gates, was selected as optimal. It offered a good balance, with a relatively low and uniform switching pressure (32.3 MPa), manageable weld lines, and acceptable volumetric shrinkage (5.180%), while ensuring stable filling. The gating system was designed with a hot runner approach. The sprue, runners, and pin gates (2mm diameter) were laid out on the parting plane. Cooling channels (20mm diameter) were designed around the mold cavity, and venting holes were strategically placed on the outer panel thick sections, which would later also serve as attachment points for ceramic reinforcing rods.
To determine the optimal wax injection parameters, single-factor experiments were conducted. The effects of melt temperature (A), mold temperature (B), injection time (C), and holding pressure ratio (D) on volumetric shrinkage were studied. Based on the trends observed, reasonable ranges for each parameter were identified. Subsequently, a L9(3^4) orthogonal array was designed to find the best combination. The factors and levels are shown below.
| Level | A: Melt Temp. (°C) | B: Mold Temp. (°C) | C: Inj. Time (s) | D: Hold Pressure (%) |
|---|---|---|---|---|
| 1 | 56 | 30 | 6 | 50 |
| 2 | 58 | 33 | 8 | 60 |
| 3 | 60 | 36 | 10 | 70 |
The volumetric shrinkage for each experimental run was obtained via simulation. Range analysis (R) was performed on the results. A larger R value indicates a greater influence of that factor on the shrinkage.
| Exp. No. | A | B | C | D | Vol. Shrinkage (%) |
|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 3.947 |
| 2 | 1 | 2 | 2 | 2 | 3.863 |
| 3 | 1 | 3 | 3 | 3 | 4.015 |
| 4 | 2 | 1 | 2 | 3 | 4.351 |
| 5 | 2 | 2 | 3 | 1 | 4.402 |
| 6 | 2 | 3 | 1 | 2 | 4.531 |
| 7 | 3 | 1 | 3 | 2 | 4.267 |
| 8 | 3 | 2 | 1 | 3 | 4.235 |
| 9 | 3 | 3 | 2 | 1 | 4.547 |
| K1 | 3.942 | 4.188 | 4.238 | 4.299 | – |
| K2 | 4.428 | 4.167 | 4.254 | 4.220 | – |
| K3 | 4.350 | 4.364 | 4.228 | 4.200 | – |
| Range (R) | 0.486 | 0.197 | 0.026 | 0.099 | – |
The analysis reveals the primary influence order as A (Melt Temperature) > B (Mold Temperature) > D (Holding Pressure) > C (Injection Time). The optimal level combination is A1B2C2D2, corresponding to a melt temperature of 56°C, mold temperature of 33°C, injection time of 8s, and a holding pressure at 60% of the fill pressure. Under these parameters, the simulated volumetric shrinkage is minimized to 3.863%.
Low-Pressure Investment Casting Process Design and Optimization
The door body is a large, thin-walled component with a complex structure, featuring thicknesses ranging from 1.5mm to 3mm. Integrating low-pressure casting with the investment casting process offers advantages like smooth filling, effective feeding under pressure, and high dimensional accuracy. The combined process parameters were calculated and designed.
The gating system is designed for a two-cavity mold to improve production efficiency. The total cross-sectional area of the ingates is calculated based on the mass of the casting and the desired filling velocity to ensure smooth filling. The formula used is:
$$
A_{\text{inner}} = \frac{G}{\rho \cdot v \cdot t}
$$
Where $A_{\text{inner}}$ is the total ingate area (cm²), $G$ is the casting mass (13.6 kg), $\rho$ is the molten alloy density (2.7e-3 kg/cm³), $v$ is the gate velocity (1200 cm/s), and $t$ is the filling time (s). For an open gating system, the area ratios are typically $A_{\text{sprue}} : A_{\text{runner}} : A_{\text{ingate}} = 1 : 1.6 : 2.15$. Based on the fill time calculation, the required areas were determined.
The low-pressure casting process consists of several stages: lifting, filling, pressurization, pressure holding, and pressure relief. The pressure and time for each stage are calculated to ensure controlled filling and solidification. The key parameters are summarized below.
| Process Stage | Time (s) | Pressure (kPa) |
|---|---|---|
| Lifting | 8 | 22 |
| Filling | 9 | 50 |
| Pressurization | 5 | 100 |
| Pressure Holding | 600 | 100 |
| Pressure Relief | 2 | 0 |
Numerical simulation of the casting process was performed using ProCAST. An initial gating scheme led to turbulent filling. The system was optimized by adjusting the number and placement of ingates to achieve a steady, progressive fill front. The optimized filling process was smooth, and the temperature gradient was favorable for directional solidification towards the ingates. However, simulation predicted shrinkage porosity in isolated hot spots at rib intersections and parts of the window frame, with a total volume of 14.68 cm³.
To further improve casting quality, key process parameters were optimized. Single-factor experiments studied the individual effects of pouring temperature, mold preheat temperature, and filling time on shrinkage volume. Based on the results, a L9(3^3) orthogonal experiment was designed with the following factors and levels.
| Level | A: Pour Temp. (°C) | B: Mold Preheat Temp. (°C) | C: Fill Time (s) |
|---|---|---|---|
| 1 | 710 | 450 | 12 |
| 2 | 720 | 480 | 14 |
| 3 | 730 | 510 | 16 |
The shrinkage porosity volume for each trial was simulated. Range analysis of the results is shown below.
| Exp. No. | A | B | C | Shrinkage Vol. (cm³) |
|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 14.62 |
| 2 | 1 | 2 | 2 | 14.32 |
| 3 | 1 | 3 | 3 | 14.56 |
| 4 | 2 | 1 | 2 | 13.92 |
| 5 | 2 | 2 | 3 | 12.34 |
| 6 | 2 | 3 | 1 | 12.87 |
| 7 | 3 | 1 | 3 | 14.21 |
| 8 | 3 | 2 | 1 | 14.01 |
| 9 | 3 | 3 | 2 | 13.99 |
| K1 | 14.50 | 14.25 | 13.83 | – |
| K2 | 13.04 | 13.56 | 14.07 | – |
| K3 | 14.07 | 13.81 | 13.70 | – |
| Range (R) | 1.46 | 0.69 | 0.37 | – |
The order of influence is A (Pouring Temperature) > B (Mold Preheat Temperature) > C (Filling Time). The optimal combination is A2B2C3, i.e., a pouring temperature of 720°C, mold preheat temperature of 480°C, and a filling time of 16s, yielding a minimum shrinkage volume of 12.34 cm³ in simulation.
A final process refinement was implemented to address the residual isolated hot spots. Instead of traditional chills or risers, which are difficult to incorporate in this investment casting process, targeted cooling using liquid nitrogen spray was proposed. The high heat capacity and latent heat of vaporization of liquid nitrogen make it an effective chilling medium. By applying it to specific hot spots on the ceramic shell during solidification, localized cooling is enhanced, promoting directional solidification and feeding. Simulation of this optimized process showed a significant reduction in shrinkage defects, with the final predicted volume decreasing to 4.9 cm³.
Conclusion
This study successfully designed and optimized an integrated car door body using a combined low-pressure investment casting process. The key findings are summarized as follows:
1. The structural design, utilizing ZL114A aluminum alloy, meets all required modal and stiffness performance criteria for automotive application, validating its suitability as a lightweight alternative to traditional stamped and welded doors.
2. The wax pattern molding process was systematically designed and optimized. Through gate analysis and parameter optimization via orthogonal experiments, the optimal injection parameters were determined, achieving a low volumetric shrinkage of 3.863% for high-precision wax patterns.
3. The low-pressure investment casting process was meticulously planned. The gating system was designed and simulated to ensure smooth filling. Process parameters were optimized through orthogonal experimentation, minimizing predicted shrinkage. The innovative use of targeted liquid nitrogen cooling as a process enhancement further reduced the simulated shrinkage porosity volume to 4.9 cm³.
The proposed methodology demonstrates the feasibility of producing a complex, thin-walled, integrated automotive component via this advanced investment casting process. It offers a promising route for significant weight reduction, part consolidation, and potentially lower manufacturing cost compared to conventional multi-part assembly methods, contributing directly to the goals of vehicle lightweighting.