Material used for plastic parts is TN-3715B, and its main physical performance parameters are: shrinkage rate is 0.3%, ejection temperature is 91℃, maximum shear stress is 0.4MPa, maximum shear rate is 40000/s, melting temperature is 240~270℃, and mold temperature is 50~70℃. Figure 1 shows pressure-volume-temperature (PVT) curve, and Figure 2 shows viscosity curve, which reflects flow characteristics of material at different temperatures and pressures, and provides a basis for optimization of molding process.

 

Plastic part structure is shown in Figure 3. Dimensions are 195.69mm×119.36mm×4.16mm, average wall thickness is 1mm, and appearance requirements of plastic part are strict. Defects such as flash are not allowed. Plastic part is made of black plastic and surface is spray-painted. Color is pearl white plus leather paint, a two-coating and two-baking process is adopted.

 

Mold flow analysis is a computer simulation technology that aims to predict flow behavior of plastic melt in mold. This technology can effectively reduce number of mold trials, reduce development costs, and improve molding quality. Following will compare three different casting schemes to determine the best injection scheme.

Basic process of mold flow analysis: first establish geometric model of mold, then input performance parameters of material, set injection conditions, perform numerical calculations, finally generate filling diagram, pressure curve diagram, and warpage deformation diagram.

Three casting schemes were used for mold flow analysis. Process parameters are as follows: melt temperature is 260℃, mold temperature is 60℃, injection time is 0.6s, and holding pressure curve is shown in Figure 4.

 

(2) Scheme 1: Use 1 hot nozzle and 6-point side gate for feeding. Filling time is 0.6788s, and melt fills cavity without obstruction, as shown in Figure 5 (a). Pressure curve is shown in Figure 5 (b). Maximum injection pressure is 61.19MPa. When filling to gate position, pressure drops to 46.36MPa, and pressure drops to 5.16MPa at the end of filling cavity. Shrinkage and deformation trend show that deformation of plastic part is between -1.902 and 1.794mm, the overall flow and shrinkage conditions are good, but number of gates is large, as shown in Figure 5 (c).

 

(2) Scheme 2: Use 1 hot nozzle and 4-point side gate for feeding. Filling time is shortened to 0.5866s, melt fills cavity uniformly and has good fluidity, as shown in Figure 6 (a). Pressure curve is shown in Figure 6 (b), and maximum injection pressure is 64.25MPa; pressure at gate position is reduced to 52.14MPa, and end of filling cavity is 39.3MPa. Shrinkage and deformation conditions are similar to those of Scheme 1, as shown in Figure 6 (c), deformation is -1.805~1.668mm. Scheme 2 is more stable in pressure control and suitable for large-scale production.

 

(3) Scheme 3: Use 1 hot nozzle and 4-point bullhorn gate feeding. Mold flow analysis results are shown in Figure 7. Filling time is 0.579s, and pressure curve shows that maximum injection pressure is 82.85MPa, pressure at gate position is 53.06MPa, and end of filling cavity is 10.99MPa. Shrinkage and deformation conditions show that deformation is -0.8643~0.8379mm. High injection pressure and bull-horn gate feeding increase difficulty of mold parts processing and maintenance costs. Table 1 shows comparison of shrinkage of three casting schemes in XOY direction. Shrinkage of three schemes in X and Y directions is similar.

 

Table 1 Comparison of shrinkage results in XOY direction of three casting schemes

Through analysis and comparison of three casting schemes, it is found that Scheme 1 has more gates and a larger pressure drop at the end of filling; Scheme 3 has the smallest warpage, but injection pressure is higher, and pressure drop is relatively large when filling to the end, bull-horn gate increases difficulty of mold parts processing and subsequent maintenance costs. In contrast, Scheme 2 (using 1 hot nozzle and 4-point side gate feeding) shows the best melt filling performance and lower warpage, its pressure control is more stable and suitable for large-scale production. Therefore, Scheme 2 is determined as optimal casting scheme.

As shown in Figure 8, there are 4 through holes on the side of plastic part, which requires lateral parting. Since side belongs to second appearance surface, mold closing marks are not allowed. Peripheral slider structure is used to control mold closing marks in non-main appearance area, that is, 4 fillets. Figure 9 shows comparison of slider structures. In Scheme 1, side of slider rubs against side of cavity, that is, mold parts here are in interpenetration contact. This design is feasible in initial stage, but it is easy to cause flash in subsequent mass production process.

 

Flash not only affects appearance of plastic part, but also easily accumulates paint during painting process, resulting in uneven coating, further reducing quality of plastic part. Therefore, Scheme 1 is not ideal in practical applications. In contrast, slider of Scheme 2 is in plane contact with cavity surface in mold closing direction, that is, through-molding. This design has higher stability and reduces probability of flash defects. After evaluation, Scheme 2 is the best solution.
Buckle inside plastic part adopts an oblique push molding scheme. Oblique push not only has lateral parting function, but also has function of pushing out. Its distribution is shown in Figure 10. Considering number and position of oblique push, ensure uniform force and stability of mold during molding process. Reasonable layout of oblique push can effectively avoid deformation problems during molding process, improve accuracy and consistency of plastic part. In view of large number of oblique pushers distributed around plastic part to be molded, in order to reduce the overall size of mold, improve compactness of design, facilitate disassembly and maintenance, a split oblique pusher structure is adopted, as shown in Figure 11. Root of oblique pusher is processed with a T-shaped groove and equipped with a pull rod to fix oblique pusher. This split structure not only makes installation and replacement of oblique pusher easier, but also provides convenient conditions for mold maintenance, extends service life of mold and improves reliability. In order to ensure that mold successfully completes molding of plastic part and can exit undercut structure, angle of oblique pusher must be accurately calculated. According to design experience, stroke of oblique pusher can be determined by following formula: oblique pusher stroke = undercut amount + shrinkage amount + safety value (1.5~2mm), where undercut amount refers to size of buckle extending out of parting surface. Formula for oblique pusher angle is: α=tan-1SY, where S is oblique pusher stroke, mm; Y is push-out distance, mm. According to measured undercut amount of about 2mm, push-out distance is set to 30mm, and final oblique pusher angle is 8°.

 

Cooling system is a key component in design of injection molds, affecting molding cycle, dimensional accuracy and surface quality of plastic parts. An effective cooling system can not only shorten cooling time, but also reduce deformation caused by uneven temperature, improve molding quality of final plastic parts and extend service life of mold.
Selection of cooling medium is crucial, and commonly used media include water and oil. Flow rate, temperature range and heat load of melt should be considered during design. In addition, layout of cooling channel should follow certain principles. As shown in Figure 12 (a), fixed mold water channel should minimize temperature difference between inlet and outlet water channels (controlled within 5℃). Since fixed mold has no ejection mechanism, water channel can be designed as a straight-through type. To facilitate connection of water pipes, water channel layout should be an even number. Movable mold needs to avoid ejection structure, and water channel should be arranged along periphery of plastic part, designed as a two-in and two-out method, as shown in Figure 12 (b).

 

In order to reduce mold temperature, it is usually achieved by passing "machine water" (about 20℃) through fixed mold and "frozen water" (about 4℃) through movable mold. Cooling simulation results are shown in Figure 13. Inlet and outlet temperature difference of cooling water circuit is less than 5℃. Since material used is a non-crystalline material, mold temperature fluctuates within ±10℃. The highest temperature of plastic part is 35.19℃, which is lower than the lowest ejection temperature of material of 91℃, indicating that cooling circuit design meets requirements.

 

The overall structure of mold is shown in Figure 14. It adopts a two-plate mold structure and a feeding method of hot runner to ordinary runner. Outer side of plastic part is formed by a full-circle slider, and slider adopts an integral structure to ensure its structural strength. In order to effectively remove air in cavity, an exhaust groove is designed on parting surface. Ejection mechanism adopts a push-tube ejection method, is assisted by an oblique push to improve ejection efficiency and stability. Working process of mold: After injection molding machine completes one injection, when mold is opened, plastic part moves synchronously with movable mold side along mold opening direction. At the same time, oblique guide column fixed on fixed mold side moves side slider to outside of mold to realize lateral core pulling. During ejection process of plastic part, relative sliding occurs between inclined push rod and T-slot of inclined push, and inclined push moves along inclined surface to complete action of ejecting plastic part.

 

1. Moving mold base plate 2. Pad 3. Push rod fixing plate 4. Oblique push rod 5. Screw 6. Moving mold plate 7. Core 8. Wedge block 9. Oblique guide column 10. Fixed mold base plate 11. Hot runner fixing plate 12. Temperature control system 13. Positioning ring 14. Hot runner assembly 15. Hot nozzle 16. Cooling water channel 17. Cooling water channel 18. Guide sleeve 19. Cavity plate 20. Fixed mold plate 21. Oblique push 22. Guide column 23. Reset spring 24. Reset rod 25. Push plate 26. Ejector rod
Figure 14 Mold structure