With development of computer technology, tablet computers are favored by users because of their advantages such as light weight, powerful functions, and convenient portability. Different brands and different types of tablet computers are emerging endlessly. In order to ensure its overall rigidity, portability, and aesthetics requirement, cover of tablet computers are usually injection molded from high-strength plastic. Now take back cover of a tablet computer as an example, combine use and performance requirements to analyze process, then use UG and MoldFlow software to carry out three-dimensional modeling, simulation analysis and optimization for problems generated, finally design injection mold structure based on this.

 

Figure 1 Plastic parts

Structure of plastic parts is shown in Figure 1. Its external dimensions are 197mm*114mm* 7.5mm, wall thickness of main body is 1mm. As an assembly, its overall size and shape accuracy are required to be high. In order to improve strength, there are more ribs on inner side, thickness is 0.3mm, 18 mounting buckles (shown in rectangular frame in Figure 1) are distributed on inner side, 2 through holes are on each of 2 vertical sides. As appearance part, its outer surface requires clear and beautiful patterns, without defects such as welding marks.

Considering size, shape, performance and use requirements of plastic parts, mold adopts a 1-mold 1-cavity structure. According to actual situation of plastic parts, there are two difficulties in injection mold design: optimization, design of gating system and special side core pulling mechanism.

 

(A) Three-dimensional structure

 

(B) Two-dimensional structure

Figure 2 Gating system

Due to thin wall thickness of plastic parts, pressure drop and temperature drop of melt during filling process are large, it is prone to insufficient filling, welding marks and insufficient welding strength, resulting in warping deformation of plastic parts. Plastic parts are appearance parts. Gate marks are not allowed on outer surface. It is preferred to use a small gate to pour from inside of plastic parts to be molded. Hot runner has advantages of improving melt transfer efficiency (reducing pressure drop and temperature drop during filling), shortening molding cycle and being easy to automate, and also improving freedom of gate position of plastic parts. Therefore, mold adopts gating system structure shown in Figure 2, I-shaped hot runner plate divides melt through two hot nozzles to long side parting surface on both sides of plastic parts, and then flows through ordinary runner, latent gate is opened along long side of plastic parts, injected from push rod on inner bottom surface of plastic parts.

 

Figure 3 Filling time

 

Figure 4 Flow front temperature

Due to appearance of welding marks in the middle of plastic parts (see Figure 1), which affects appearance and overall quality, position of gate of existing mold is adjusted appropriately. Results obtained after filling simulation are shown in Figures 3 and 4. Injection time is 0.7533s, filling time at each distal end is relatively uniform. Temperature of middle weld of plastic parts is relatively uniform, maximum temperature difference with melt is only about 3.5 ℃. Therefore, gate position of this solution is more reasonable, which can improve welding marks in the middle.

 

Figure 5 Inclined push mechanism

 

Figure 6 Layout of Inclined Fader

Plastic part has two structures that need to be pulled sideways, one is 18 mounting buckles distributed on inner side of plastic parts, the other is 2 vertical sides each with 2 through holes (see Figure 1). Because distance of lateral core pulling of inner side mounting buckle is small and number is large, in order to simplify mold structure, oblique push mechanism is used to achieve side core pulling. Oblique push mechanism is shown in FIG. 5. T-slot cooperation between oblique push rod and base realizes side sliding of oblique push rod to complete side core pulling.

 

Figure 7 Slider structure

 

Figure 8 Slider layout

For through holes on the side, although core-pulling distance is wall thickness, it is too close to buckle structure. If an oblique push mechanism is used, interference will be easily caused, so oblique guide column side core pulling mechanism is used to achieve core pulling. Structure and layout of slider are shown in Figures 7 and 8.

 

Figure 9 mold structure

1. Fixed mold base plate 2. Nozzle 3. Fixed template 4. Guide sleeve 5. Guide post 6. Push rod 7. Pull material rod 8. Moving template 9. Pad block 10. Push rod fixing plate 11. Push plate 12. Movable base plate 13. Slider baffle 14. Slider 15. Slant guide column 16. Water nozzle 17. Locating ring 18. Screw 19. Hot runner junction box 20. Cavity plate 21. Core 22. Inclined push rod 23. Reset lever 24. Guide block 25. Spring 26. Base 27. Guide post 28. Guide sleeve 29. Limit nail 30. Slant guide post 31. Slider 32. Sealing ring

According to specific requirements of plastic parts, combined with design of above main parts, mold structure is shown in Figure 9. Working process of mold is: when melt is diverted from main nozzle of hot runner center to nozzle 2 through hot runner plate, then cavity is filled through ordinary runner and latent gate, after cooling and solidification, moving mold part begins to be separated under action of mold opening mechanism of injection molding machine, hot runner nozzle condensate is disconnected from ordinary runner. During parting process, sliders 14, 31 are moved outward along guide groove under action of inclined guide columns 15, 30 to achieve a side core pull. After oblique guide columns leaves slider, sliders 14 and 31 are respectively positioned by slider baffle 13 and screw ( slider 31 is installed below). After mold is separated, push plate 11 is pushed by injection molding machine’s top rod, so push rod 6, pull rod 7, reset rod 23, and oblique push rod 22 are moved forward. During pushing process, oblique push rod 22 not only completes side core pulling, but also plays role of pushing plastic parts together with push rod 6. When push rod 6 is pushed out, latent gate is forcibly cut off, pulling rod 7 is pushed out of ordinary runner condensate. When mold is closed, reset lever 23 and spring 25 work together to complete reset of ejection system and enter next injection cycle.