With advancement of automotive manufacturing technology, higher demands are being placed on vehicle energy efficiency. To reduce vehicle weight and save manufacturing costs, automotive interior components generally adopt plastic structures. Door panels, a key component of automotive interiors, are subject to increasingly stringent quality requirements. In addition to high strength, they must also blend harmoniously with vehicle's interior décor.
When injection molding automobile door panels, rational design of gating and cooling systems directly impacts quality and strength of plastic part. Using Moldflow analysis platform, this paper simulates and analyzes gating and cooling systems for a door panel with a leather-like surface, a complex shape, and a speaker mesh. To ensure that surface defects such as weld marks, white tipping, and flash are not permitted, plastic melt is analyzed for flow, cooling, and warpage. Results are then compared to identify optimization solutions.

 

Figure 1: 3D Model of an Automotive Door Panel
3D model of an automotive door panel is shown in Figure 1. Material is PP, with a shrinkage rate of 1.65%. The overall dimensions are 950mm * 599mm * 116mm, with a wall thickness of approximately 2.3mm and a minimum wall thickness of 0.7mm. Part has a complex external structure, including features such as speaker mesh holes, which hinder melt flow, easily cause weld marks and air pockets. High surface precision is required, defects such as whitening, scratches, and flash are not permitted.
Automotive door panel contains numerous and densely packed speaker mesh holes. To save simulation time, simplification is required before importing into CAE software. Because the overall CAE mesh finite element model has a large aspect ratio, contains free edges, pre-processing is difficult and time-consuming, resulting in a low mesh matching rate. Therefore, equivalent speaker mesh wall thickness is required before importing model to simplify model structure, shorten pre-processing time, and improve mesh quality. Specific method is to treat each hole as a simple basic unit and construct a mesh by splicing them together (only one basic repeating unit is considered during simplification process). Simplification process can be divided into two stages: isovolumetric conversion and isothermal conductivity conversion, i.e., equivalent thickness = isovolumetric wall thickness / equivalent thermal conductivity factor. Using Moldflow's high-aspect-ratio mesh and pre-processing, including free edge, intersection, and overlapping element correction, optimized mesh unit for plastic part is obtained. All speaker mesh locations are removed, and equivalent wall thickness at speaker mesh locations is manually defined, as shown in Figure 2.

 

Figure 2: Equivalent Wall Thickness Processing at Speaker Mesh Location
Before simulation analysis, ensure that geometric mesh model is essentially consistent with actual plastic part to prevent loss of key features. Ensure accurate mesh matching and thickness, complete gating and cooling systems, consistent dimensions and parameter definitions with actual mold processing to ensure consistency between analysis results and actual mold trials. During simulation, position and size of valve gate and hot runner were created according to designer's requirements. A one-mold, two-cavity structure was used, and mold flow analysis was performed, as shown in Figure 3.

 

(a) Cavity Layout

 

(b) Mold Flow Analysis
Figure 3 Cavity Layout and Mold Flow Analysis
Large dimensions of plastic part present challenges such as difficulty in melt filling, easy deformation, and significant runner material waste. Based on experience, a hot runner system was used for casting. Each part has four hot nozzles (see Figure 3(b)), for a total of eight hot nozzles, as shown by numbers 1 to 8 in Figure 3(a). Hot nozzle diameter is 4 mm, runner diameter is 22 mm, and valve gate diameter is 10 mm. Mold is water-cooled, with four cooling water circuits designed for both movable and fixed molds. Cooling water temperature is maintained at a constant 25℃.

Moldflow primarily simulates molding process, specifically flow and molding of plastic melt within mold cavity. It utilizes a finite element numerical solver to calculate and analyze changes in velocity, displacement, temperature, and pressure during molten plastic molding process, identifying and predicting defects in actual production process. This simulation and analysis visualizes and clarifies injection process, reducing mold modification time during subsequent mold trials and improving production efficiency.

Moldflow's Dual Domain Mesh Pre-Processing Analysis and Flow Analysis modules simulate and analyze melt flow and packing process. Filling analysis (contours) is shown in Figure 4. Cavity filling time is 6.94 seconds. During injection, hot nozzles open sequentially, ensuring uniform contour spacing on molded part's exterior surface, a uniform filling speed, and smooth material feeding without short shots or stagnant flow.

 

Figure 4: Filling Analysis

 

Figure 5: Melt Flow Front Temperature
Distribution of melt flow front temperature (temperature at which melt front reaches a specific point in the center of cross-section of molded part) is shown in Figure 5. Material temperature during injection simulation was 220℃ (injection temperature for PP material is 180-260℃). As shown in Figure 5, melt front temperature is mostly within injection temperature range of PP material, but localized front temperature is lower at horn mesh. In actual mold structure design, separate cooling water channels or heating rods should be installed at horn mesh.

 

Figure 6: Instantaneous Pressure Distribution at V/P Switching Point
Figure 6 shows instantaneous pressure distribution at V/P (velocity/pressure) switching point. When cavity fill volume reaches 98%, pressure shifts from velocity control to pressure control, with a switching pressure of 80.89 MPa.

 

Figure 7: Clamping Force
As shown in Figure 7, clamping force analysis shows that projected area of plastic part in mold opening direction is approximately 8400 cm². Required clamping force during filling phase is 2.739 * 10⁶ kN, and clamping force during holding phase is 2.903 * 10⁶ kN. Depending on actual environment, actual required clamping force may need to be increased by 20%.

 

Figure 8: Air Cavitation Distribution
As shown in Figure 8, air cavitation occurs at the end of fill, at ribs, and at columns, especially in speaker mesh area. Venting slots are required to prevent air entrapment, which can affect surface quality of part.

 

Figure 9: Weld Mark Distribution
Weld marks occur where two low-temperature melt streams meet, affecting part strength and causing cosmetic defects on part surface. Weld mark analysis in Figure 9 shows uniform melt flow, with no noticeable weld marks on main surface. Only small weld marks are observed at the edges of part, which can be eliminated with subsequent spraying.

Mold temperature directly affects melt filling, pressure-holding cooling, part shape, and precision. To ensure uniform cooling and increase cooling rate, both movable and fixed molds utilize four evenly distributed circulating water circuits for cooling. (a) Analysis of Fixed Mold Cooling Effect (b) Analysis of Moving Mold Cooling Effect
Figure 10 Cooling Effect Analysis
Figure 10 shows mold cooling effect analysis. Inlet and outlet temperature difference of fixed mold cooling water channel is 1.48℃ < 3℃ (see Figure 10(a)), inlet and outlet temperature difference of moving mold cooling water channel is 1.02℃ < 3℃ (see Figure 10(b)). Cooling effect on plastic part is ideal, with no obvious overheating or undercooling areas. (a) Upper Surface (b) Lower Surface
Figure 11: Plastic Part Surface Temperature
Figure 11 shows plastic part surface temperature. It shows that upper and lower surface temperatures of plastic part are approximately 50℃, with a temperature difference of less than 10℃ between front and rear surfaces, indicating a balanced temperature distribution.

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Figure 12: Time it takes for plastic part and runner to reach ejection temperature
Figure 12 shows time it takes for plastic part and runner to reach ejection temperature. It shows that temperatures during molding cycle are lower than recommended ejection temperature of 118℃ for PP plastic, indicating that cooling effect within a single molding cycle meets ejection requirements.

 

(a) X-Direction Warpage

 

(b) Y-Direction Warpage

 

(c) Z-Direction Warpage
Figure 13: Warpage of Plastic Part in Various Directions
Analysis results of warpage of plastic part in various directions are shown in Figure 13. Maximum deformation of plastic part is approximately 6mm in X and Y directions, and approximately 3mm in Z direction. Severe warpage occurs at opening of automobile door panel. Deformation compensation should be implemented in mold design to improve warpage.

 

Figure 14: Total Warpage Effect
The total warpage effect is shown in Figure 14. A comprehensive comparison of analysis results shows that main factors affecting plastic part warpage are shrinkage and molecular orientation. In actual production, pressure during holding stage should be increased.