Frame material of decorative strip is PP-GF20, and the overall dimensions are 955mm*62mm* 130mm. It is a non-appearance part. Product is long and arcuately curved, and there are a large number of reinforcing ribs on the surface to ensure strength of product. Decorative strip skeleton and another cover are pre-assembled, installed and fixed on instrument panel assembly in the form of components. There is a space for placing wiring harness on one side of product, forming multiple undercut shapes. Mold adopts a 4-cavity structure. Design difficulties are as follows: ① Product has a large number of undercuts and requires design of more core-pulling mechanisms; ② There are a total of 36 sets of designed slide blocks and 4 sets of hydraulic cylinder core pullers, all of which are easy to disassemble and assemble; ③ Mold has a large number of sliders, inclined push structures and hydraulic cylinder core-pulling mechanisms. Design must be verified by geometric methods to avoid interference from movement of mold parts.

 

Figure 2 shows front of product, and sliders need to be designed at S1~S9. Figure 3 shows bottom surface of product. An inclined push structure needs to be designed at L1~L5, and a hydraulic cylinder core-pulling mechanism needs to be designed at C1.

 

Product is in the shape of a long strip with uniform overall wall thickness. Every two adjacent products to be formed are distributed in mold cavity in a central rotation manner. Five groups of hot nozzles are installed on each of operating and non-operating sides of cavity, materials are fed using a combination of ordinary runners and hot runners. Ordinary runners use side gates. Arrangement and feeding method of products to be formed are shown in Figure 4.

 

Table 1 shows physical parameters of injection molding, Table 2 shows molding parameter settings, and Figure 5 shows flow result (contour) analysis. Cavity filling time is about 2.9s, and there is no underfilling phenomenon in cavity. Filling balance is complete, and contours of large area are uniform without retention.
Table 1 Injection molding physical parameters

Table 2 Molding parameter settings

 

Conduct flow analysis based on preset conditions and obtain simulation results: melt flow is balanced, there is no filling dissatisfaction, contours are uniform and there is no retention; melt front temperature is within molding temperature range, uniform without sudden changes and abnormalities; holding pressure switching percentage is between 95% and 99%. Holding pressure does not exceed 80% of allowable value of injection molding machine. Maximum clamping force does not exceed 80% of allowable value of injection molding machine; recommended molding temperature of material is 240~260℃. Pay attention to exhaust at the end of melt and reduce melt injection speed. Maximum clamping force required is about 7200kN, and injection molding machine can meet production requirements. Defect analysis was performed based on preset conditions and results were: there are pores in bones and ends of molded product, and exhaust must be considered; since temperature where welding lines may occur meets requirements, welding line defects are controllable; according to post-shrinkage characteristics of PP materials, analysis shows that volume shrinkage of gate area is too large, and pressure holding time can be extended to improve it. It is recommended that undercut area be designed with insert molding for subsequent adjustments.
Cooling analysis was performed based on preset conditions, and simulation results were obtained: when cooling water inlet temperature is set to 20℃, inlet water temperature is 19.34℃, outlet water temperature is 22.19℃, and temperature difference is within 3℃, as shown in Figure 6; When cooling time is set to 35s, temperature difference on the surface of product is about 10℃, as shown in Figure 7.

 

Layout of movable mold, fixed mold and slider is shown in Figure 8. It is equipped with a hot runner system. Piston rod of movable mold hydraulic cylinder is used to drive push rod fixed plate to push out molded product. Movable mold is equipped with a slider, an inclined push structure, and a hydraulic cylinder core-pulling structure. Mold is set to 4 cavities, and each two adjacent cavities are in a central rotation relationship with each other.

 

1. Slider S1 2. Slider S2 3. Slider S3 4. Slider S4 5. Slider S5 6. Slider S6 7. Slider S7 8. Slider S8 9. Slider S9
Figure 8 Arrangement of fixed mold, movable mold and slider
(1) Design of molded parts. Mold service life is 150,000 molds (product demand is 550,000). Since product material contains glass fiber (PP-GF20), moving and fixed mold plates are made of 718H material.
(2) Gating system design. Mold adopts a pouring system that combines ordinary runner and hot runner. Ordinary runner adopts side gate feeding, and gate adopts fan-shaped bottom style, as shown in Figure 9.

 

(3) Exhaust system design. Referring to mold flow analysis, there are air holes at the end of molded product. When designing mold, an exhaust channel is set up around fixed mold cavity. Exhaust is achieved with the help of core pulling mechanisms at undercut areas. Inserts are installed at other places where air is easily trapped to improve exhaust.
(4) Cooling system design. Design 5 sets of water channels in fixed mold and 3 sets of water channels in movable mold, as shown in Figure 10.

 

(5) Core pulling mechanism design. Mold core-pulling mechanism is divided into hydraulic cylinder core-pulling, slider core-pulling, and oblique push component core-pulling. They are all designed in movable mold. Among them, there are 4 groups of hydraulic cylinder core-pulling. Hydraulic cylinder piston rod is used to drive T-shaped block to achieve core-pulling. Single-group structure is shown in Figure 11. There are 36 groups of sliders in total, using slider + inclined guide column style. Single-group structure is shown in Figure 12. There are 20 groups of inclined push components. Single-group structure is shown in Figure 13.

 

1. Hydraulic cylinder 2. Limit block 3. Positioning block 4. Hydraulic cylinder piston rod 5. Core-pulling insert 6. Wear-resistant block 7. Positioning pin 8. T-shaped groove block 9. T-shaped block 10. T-shaped block Guide block 11. Limit switch pressure block 12. Fixed block 13. Pressure block 14. Protection block 15. Limit switch assembly
Figure 11 Core-pulling structure of a single group of hydraulic cylinders

1. Slider 2. Wear-resistant block 3. Inclined guide post 4. Limit screw 5. Wear-resistant block 6. Slider guide 7. Glass bead screw 8. Spring
Figure 12 Single set of slider core-pulling structure

 

1. Incline push block 2. Guide bush 3. Support sleeve 4. Positioning block 5. Connecting rod 6 Incline push block slide seat
Figure 13 Core-pulling structure of a single set of inclined push components
(6) Launch institutional design. Mold is driven by a hydraulic cylinder piston rod to realize movement and reset of push rod fixed plate. Push rods are evenly distributed in each cavity to push out molded products. In addition, inclined push component moves and pushes out molded products at the same time.
(7) Mold base structure design. Since mold has many inclined push seats and push rods, movable mold adopts style of push plate + push rod fixed plate to install and fix pushed parts. Mold structure is shown in Figure 14. Mold was actually tested and molded product is shown in Figure 15. Product meets usage requirements.

 

1. Lifting ring 2. Hydraulic cylinder 3. Inclined core pulling mechanism component 4. Bridge crane module 5. Fixed mold base plate 6. Hot runner system 7. Positioning ring 8. Hot runner plate 9. Fixed mold plate 10. Mold feet 11. Moving mold plate 12. Grease nozzle 13. Pad 14. Moving mold plate seat plate 15. Push plate 16. Push rod fixed plate 17. Limit column 18. Support column 19. Pressure sensor 20. Push plate guide column 21. Positioning ring 22. Inclined push block assembly 23. Cover plate 24. Pulling rod 25. Support column 26. Inclined core pulling mechanism travel switch
Figure 14 Mold structure