Automobile plastic parts generally have complex structures and require high surface roughness and dimensional accuracy. Left/right support bracket of a certain car is shown in Figure 1. According to UG software query, the overall dimensions of support bracket are 171.93 mm * 117.34 mm * 127.79 mm, the overall wall thickness is 3.5 mm,volume of left support bracket is 130 039 mm3, volume of right support bracket is 133 307 mm3, and molding material is PP+30%GF. Left/right support bracket is larger in size and has a complex overall shape, with many characteristic structures and side holes, side recesses, etc. Left support bracket has 2 more side holes in structure than right support bracket, and the other structures are symmetrical. Plastic parts are difficult to part and form.

 

Figure 1 Car left/right support bracket

In order to shorten mold design cycle, Moldflow is first used to conduct mold flow analysis. Moldflow can simulate filling time, overall temperature, transfer holding pressure, weld marks, warpage deformation, etc. of left/right support bracket to improve accuracy of mold design.

(1). Settings before Moldflow analysis. Before performing Moldflow analysis, set relevant parameters, including setting mold surface temperature to 45℃, melt temperature to 245℃, injection time to 1.6 s, and holding pressure to 80% of injection pressure. Analyze parameters set as shown in Figure 2 and Table 1.

 

Figure 2 Analysis parameter settings

Table 1 Molding parameter settings

(2) Material setting and thickness analysis. According to customer requirements, left/right support bracket molding materials are all PP+30%GF. Thickness of support bracket is analyzed through Moldflow software. Most of thickness of plastic parts is 3.5 mm, as shown in Figure 3. If wall thickness of plastic part is too thick, uneven shrinkage after injection molding will lead to defects such as plastic part deformation, shrinkage holes, dents or insufficient filling, so thickness of plastic part must be ensured to be uniform. In order to prevent parts of plastic parts from being too thick, parts with a thickness exceeding 3.5mm need to be modified. Generally, thickness of thermoplastic materials is usually 2~4 mm. According to analysis results in Figure 3, 5.532 mm part is appropriately thinned, and 2.8 mm part is appropriately thickened. Among them, 1.597 mm is reinforcing rib, which is used to strengthen structure of plastic part, so there is no need to increase or decrease thickness.

 

Figure 3 Thickness analysis

(3) Runner and gate settings. Because left/right support bracket is a symmetrical plastic part on the left and right sides of car, the overall shape of support bracket is complex and has many structures, in order to simplify mold structure and facilitate molding of plastic parts, a molding method of 1 mold and 2 parts is adopted, left/right support brackets are arranged symmetrically. According to customer requirements, there should be no gate marks on the surface of plastic part, so gating system uses a hot runner in the form of a main channel and a latent gate. Specific dimensions are shown in Figure 4, where diameter of main channel is φ12 mm, and diameter of branch runner is φ12 mm. φ6 mm, latent gate size is φ2 mm, and analysis conditions are set according to above parameters.

 

Figure 4 Cavity layout and flow channel settings

(4). Mesh division. Moldflow uses finite element analysis method. Before analysis, plastic part needs to be meshed. Left/right support bracket is meshed at two levels, obtained mesh quality is good. Average meshing aspect ratio is 1.74, and minimum is 1.16. Matching percentage and mutual percentage exceed 91%. Meshing matching rate and mesh situation are shown in Figure 5.

 

Figure 5 Mesh division

(5). Moldflow analysis. After setting up flow channel and dividing mesh, you can perform Moldflow simulation analysis on the left/right support bracket. It mainly analyzes filling stage, push-out stage and warpage deformation of left/right support bracket to provide reference for mold design. First, dynamic flow is analyzed, and Moldflow analysis results are shown in Figure 6. As can be seen from Figure 6, plastic melt can fill mold cavity evenly, and the entire injection filling process takes about 1.54 s. Left/right support brackets are evenly filled with melt, and there is no obvious blockage in mold.

 

Figure 6 Left/right support bracket filling analysis results
Injection pressure distribution results are shown in Figure 7. Maximum injection pressure is 16.3 MPa, switching and holding pressure is 13.1 MPa. According to set parameter analysis, injection pressure is within recommended range of Moldflow.

 

Figure 7 Injection pressure curve
Figure 8 shows resulting distribution of filling end pressure. At filling end of left/right support bracket, V/P converted pressure at the end of filling is 16.34 MPa, pressure at the end of cavity is 13.07 MPa. Above pressures are all less than 60 MPa (60 MPa is median of injection pressure, and analyzed pressure is smaller than median, that is, analysis result is appropriate), end pressure meets requirements.

 

Figure 8 Filling end pressure
Warpage is a defect caused by internal stress of plastic part after injection molding. Reason for warpage is uneven shrinkage of plastic part after cooling. In order to avoid warpage deformation of plastic parts, use Moldflow analysis to check whether designed runners and gates are reasonable, prevent warpage deformation of plastic parts or control deformation within a reasonable range in subsequent mold structure design. After Moldflow analysis, warpage deformation of left/right support bracket is shown in Figure 9. Analysis result in Figure 9(a) shows that the overall maximum deformation of plastic part is 1.138 mm. At the same time, left/right support bracket has large uneven shrinkage in X, Y, and Z directions, and volume shrinkage causes the overall warping of plastic part. Deformation amount of left/right support bracket in Z direction is shown in Figure 9(b). Amount of shrinkage and deformation at the top of left/right support bracket is about -0.48 mm, maximum amount of shrinkage and deformation is about 0.92 mm. The overall deformation is within allowable range of plastic part deformation tolerance.

 

Figure 9 Analysis results of warpage deformation of left/right support bracket
Analysis results of weld marks are shown in Figure 10. Formed weld marks are less than 75°. Weld marks will affect surface quality of plastic parts. Since weld marks on plastic parts cannot be avoided, in order to minimize impact of weld marks, an exhaust structure needs to be installed in the area where weld marks occur.

 

Figure 10 Location of weld marks
Based on above analysis results and actual production experience, it can be concluded that: ① Injection filling pressure is reasonable, and mold will not have filling problems; ② Weld marks cannot be avoided, and an exhaust structure needs to be installed at location where weld marks occur; ③ Plastic part has large uneven shrinkage in the X, Y, and Z directions, and the overall shrinkage deformation is within deformation tolerance range of plastic part. According to analysis results, designed shapes and parameters of runner and gate basically meet design requirements. Mold is designed according to Moldflow analysis results, pressure holding time is increased to avoid warpage and shrinkage deformation of plastic parts.

Four factors are usually considered when determining number of cavities during mold design: ① economic performance; ② rated clamping force of injection molding machine; ③ maximum injection volume of injection molding machine; ④ accuracy of plastic parts. In order to simplify mold structure, take into account production efficiency, ensure that plastic melt can reach gate and enter cavity through runner at the same time, comprehensively consider Moldflow analysis results, it is determined to adopt a balanced layout of 1 mold and 2 parts (see Figure 1(a)).

When designing an injection mold, position of plastic part to be formed in mold must be determined based on structure and shape of plastic part, that is, parting surface must be determined. Basic principle of designing parting surface: position with the largest cross-sectional profile of plastic part should be selected to facilitate smooth demoulding. At the same time, it should also take into account simplicity of mold structure and convenience for mold part processing and exhaust. Main parting surface design of left/right support bracket is shown in Figure 11.

 

Figure 11 Main parting surface design
After determining main parting surface, design core and cavity plate. In order to ensure mold closing accuracy, after core and cavity plate are separated, tapered boss positioning is designed on core and cavity plate respectively. Final designed core and cavity plate are shown in Figure 12.

 

Figure 12 Core and cavity plate

(1) Core-pulling structural design of inclined guide column slider. Because left/right support bracket has side holes and underside concave structures, lateral parting needs to be designed during molding. Depth of side hole is 6.07 mm, and depth of side recess is 2.74 mm. An inclined guide pillar slider core-pulling mechanism is used. In order to facilitate processing and molding, lateral core adopts an integral core, diameter of inclined guide column is designed to be φ24 mm, and angle is designed to be 12°, as shown in Figure 13.

 

Figure 13 Inclined guide column slider core-pulling structure

(2). Design of core-pulling mechanism on the inside of inclined push block. Left/right support bracket has a 4-hole structure, as shown in Figure 14. Left support bracket has 3 holes, and right support bracket has 1 hole. Because location is far away from inclined guide column core-pulling structure, if inclined guide pillar core-pulling mechanism is used for molding, core-pulling distance will be longer, core-pulling distance cannot be combined with side hole and side concave structure. If another inclined guide pillar core-pulling mechanism is designed, it will interfere with inclined guide pillar core-pulling mechanism in Figure 13. In order to avoid interference and simplify mold structure, a core pulling mechanism on the inside of inclined push block is used to form 4 holes. As shown in Figure 15, inclined push block not only functions as inner core but also plays the role of pushing out plastic parts. Since thickness of plastic part is about 3.5 mm and inner core-pulling distance is short, tilt angle of inclined push block is designed to be 4°, push plate stroke is 75 mm, and maximum core pulling stroke is 75×tan4°≈5.244 mm>3.5 mm. Core pulling stroke is sufficient.

 

Figure 14 Left/right support bracket 4-hole structure

 

Figure 15 Core-pulling structure inside inclined push block

(1) Cooling water channel design. Impact of mold temperature adjustment system on production efficiency is mainly reflected by cooling time. Plastic melt temperature is generally around 200℃. Temperature of plastic parts taken out of mold is below 60℃. About 5% of heat released by plastic melt during molding is dissipated into air in the form of radiation and convection, remaining 95% is taken away by cooling system. Because the overall structure of left/right support bracket of mold is relatively complex, in addition to need for cooling water channels for core and cavity plate, large volume of side parted core also requires establishment of cooling water channels. Diameter of the overall cooling water channel of mold is φ10 mm, which is evenly distributed in core, cavity plate and side core. Due to complex shape of plastic part, cooling water channel is also complex. The overall cooling water channel is shown in Figure 16.

 

Figure 16 The overall cooling water channel of mold

(3). Launch institutional design. Since outer surface quality of left/right support bracket is high and inner surface is relatively low, use of a push rod push-out mechanism can simplify mold structure. Although 4 inclined push blocks have been designed in mold, push force is uneven. In order to make push force more uniform, push rods are evenly placed at the bottom of plastic part. Diameter of push rods is selected from 4 specifications according to different parts. Diameters are φ10, φ8, φ6, φ4 mm, push rod arrangement is shown in Figure 17.

 

Figure 17 Putter layout
After design is completed, final mold structure is shown in Figure 18. Mold is poured through a hot runner. Working process is relatively simple. First, mold is opened from parting surface, wedge block 5 drives inclined guide column to pull slider 6 outward. After core pulling is completed, ejector pin 13 pushes push rod fixing plate 11, and plastic part is pushed out through inclined push rod 18. Molds assembled after processing have been verified by actual production. Injection molded plastic parts meet customer's requirements and have been delivered to customer for production use.

 

Figure 18 Mold structure
1. Positioning ring 2. Fixed mold base plate 3. Heat insulation plate 4. Cavity plate fixing plate 5. Wedge block 6. Slider 7. Side core 8. Core fixing plate 9. Pad 10. Push rod Fixed plate 11. Push plate 12. Moving mold base plate 13. Ejector 14. Moving mold positioning sleeve 15. Inclined push rod holder 16. Inclined slide block 17. Guide block 18. Inclined push rod 19. Core 20. Type cavity plate