Front bumper grille of a certain vehicle, shown in Figure 1, has a "banner" design and is made of PC+ABS. Its dimensions are 1846mm * 293mm * 475mm, with a uniform overall thickness of 2.5mm. Part is painted piano black, and exterior area is required to be free of sink marks, flow marks, and obvious weld lines. Mold for forming grille utilizes a single-mold, single-cavity structure. This grille molding presents several challenges: ① Large size, wide transverse span, numerous reinforcing ribs make melt filling difficult and result in high pressure; ② Complex structure, with its multiple rectangular holes, makes weld lines difficult to control and easily appears in visible locations; ③ Numerous T-shaped ribs create a risk of melt directly impacting exterior surface during filling, forming "spray marks."

 

Air intake grille was meshed using Moldflow software. Results are shown in Figure 2. After manually repairing poor elements such as free edges and aspect ratios, mesh contains 129,002 triangle elements and 64,271 node elements. Connected area is 1, maximum aspect ratio is 19.6%, and matching rate is 90.1%. Mesh quality meets analysis standard.

 

Air intake grille is made of PC+ABS material, brand HAC8250. This material combines advantages of PC and ABS, offering high impact resistance and maintaining good toughness even at low temperatures. Compared to pure ABS, PC+ABS has better heat resistance and can maintain its physical properties at higher temperatures. It also has good flowability and plasticity, making it easy to mold and suitable for various molding methods such as injection molding and extrusion. Main injection molding parameters are shown in Table 1. PVT curve and material viscosity curve are shown in Figure 3.
Table 1 PC+ABS Main Injection Parameters

 

 

 

Based on factors such as part size and material fluidity, two feed design options were analyzed and compared. Optimal gating system solution was selected based on a comprehensive comparison of various influencing factors. Plan 1: As shown in Figure 4, a 15-point needle valve hot runner to conventional runner feed method was used. G1, G2, G3, G4, G5, G10, G11, G12, and G13 used latent lap gates, as shown in Figure 5(a). G6, G7, G8, and G9 used lifter latent gates, as shown in Figure 5(b). G14 and G15 used side gates, as shown in Figure 5(c).

 

 

Option 2: As shown in Figure 6, a 16-point needle valve hot runner-to-conventional runner feed method is employed. G1, G2, G3, G4, G5, G6, G7, G8, G9, and G10 are side gates, G7, G8, G9, and G10 are top gates. Because part is essentially symmetrical, both options utilize nearly symmetrical feed positions, avoiding gates directly opposite holes whenever possible to prevent defects such as flash in molded part. Horizontal hot runner diameter at needle valve hot runner manifold in both options is φ20mm, vertical hot runner has an outer diameter of φ20mm, valve needle diameter is φ8mm, and hot nozzle diameter is φ5mm. Conventional runner is designed as a T-shaped runner, measuring 8.1mm * 6mm * 6mm. Conventional runner gate at the junction with part measures 12mm * 1.5mm. Due to large size of part, to reduce filling pressure and avoid sink marks, a fan-shaped gate was used for conventional runner gate at this location (see Figure 5(c)).

 

Select [Fill + Hold + Warp] analysis. Based on recommended molding temperature for material, process parameters were set to 65℃ for mold surface temperature, 250℃ for melt temperature, 3.5s for injection time, 98% for velocity/pressure (V/P) ratio, 30s for cooling time, 10s for hold time, and 60MPa for hold pressure. Hot nozzles of needle valves were set to open sequentially. After completing settings, Moldflow software was run to begin analysis.

As shown in Figure 7, grid filling time for both schemes is approximately 4.9s. Time it takes for melt to fill both ends of cavity is essentially same. Contour lines for both schemes are relatively even, indicating balanced filling, with no defects such as stagnation or underfill, meeting molding requirements. Comparing melt flow directions of two solutions reveals that melt flow direction for Solution 1, as shown in Figure 8(a), is from surface area, which is exterior surface. Melt flows from exterior area to non-exterior area, preventing direct melt flow onto exterior surface and thus avoiding jetting defects. In Solution 2, melt flow direction is partially from non-exterior area directly onto exterior surface, as shown in Figure 8(b). However, there is a risk of jetting in direct melt flow area, affecting exterior quality of part.

 

 

By fine-tuning gate position and adjusting gate opening timing, injection pressure was controlled within a reasonable range. Final injection pressure curve is shown in Figure 9. Maximum injection pressure for Solution 1 is 95.74 MPa, and for Solution 2 is 99.95 MPa. Both meet pressure requirements of injection molding machine. Solution 1 has a lower injection pressure than Solution 2, resulting in less filling resistance.

 

As shown in Figure 10, in Scheme 1, weld lines are largely confined to inconspicuous corners and grid lines, with minimal impact on appearance. In Scheme 2, weld lines are mostly inconspicuous in corners and grid lines, with one weld line on each side appearing on important surfaces, as indicated by circles in Figure 10(b), which somewhat impacts appearance quality of part.

 

Volumetric shrinkage of two schemes at launch is shown in Figure 11. After injection molding using both schemes, parts achieved good pressure retention. Volumetric shrinkage of the overall grille appearance area was approximately 3%, with difference within 3%, meeting analysis requirements and indicating no localized shrinkage.

 

As shown in Figure 12, maximum deformation for Schemes 1 and 2 was 7.009 mm and 6.802 mm, respectively. Deformation was primarily in +Z direction at both ends. Because grille has dense clips at both ends, this can be corrected through assembly. Company required a total deformation of 10 mm, and both schemes met requirements.

 

Based on Moldflow analysis results, Option 1 outperformed Option 2 in terms of weld lines, number of hot nozzles, and risk of jetting. Therefore, Option 1's feeding scheme (a 15-point needle valve hot runner to conventional runner feed) was adopted for mold gating system design, as shown in Figure 13. Based on this gating system, design of other structural modules was completed.

 

Mold design was completed based on Option 1's gating system, and a trial mold was conducted for verification. Trial mold used a Haitian JU2400011/19300 injection molding machine with a clamping force of 24,000 kN, a screw diameter of φ150 mm, and a theoretical maximum shot volume of 12,017 cm³. Based on CAE analysis, optimal solution was determined. Injection time, gate opening sequence, holding time, and holding pressure were set on injection molding machine. After simple adjustments, weld lines, shrinkage, and deformation of molded parts were generally consistent with moldflow analysis results, within company's acceptable tolerances, and parts met requirements for vehicle assembly. Actual plastic part is shown in Figure 14.