Charging pile bottom shell has external dimensions of 420mm * 263mm * 92mm and an overall thickness of 3.5mm. Material is a mixture of acrylonitrile-butadiene-styrene terpolymer (ABS) and polycarbonate (PC). Bottom shell serves as an assembly and fixing component for charging pile. Its main function is to prevent internal components from being damaged by external factors, to protect them from dirt and dust, and to isolate them from humid air. Simultaneously, back of bottom shell needs to connect to wall or column to ensure charging pile installation. Since sides of bottom shell are directly exposed, it is also an appearance component. It is necessary to ensure an aesthetically pleasing appearance after assembly, unaffected by external environment. Outer surface needs to be textured to prevent defects such as shrinkage, flash, impurities, and oil stains. As shown in Figure 1, back of bottom shell has four undercut positions A, B, C, and I. Position A has multiple reinforcing ribs on its back, and position I has a groove. Inside, there are four lugs D, E, F, and G with two reinforcing ribs below them, and an undercut position at H. Side of bottom shell has a full-circle groove at J with a rectangular hole in the center, a label groove at L, and seven reinforcing ribs at K on its curved side surface. Plastic part has an irregular shape and complex structure.

 

Figure 1. Bottom shell of charging station

To meet structural and production requirements of charging pile bottom shell, mold adopts a one-mold-one-cavity layout. A four-sided side core-pulling mechanism (driven by inclined guide pillars) is designed around molded part to handle external concave structure of plastic part. Simultaneously, inclined push mechanisms with both core-pulling and ejection functions are set at undercut positions of cavity and core. In addition, a hot runner needle valve gate, conformal cooling water channels, a side core-pulling mechanism, and a push rod composite ejection mechanism are configured to ensure high-quality production of plastic part.

Parting surface should be located as close as possible to maximum projected contour of plastic part to ensure smooth demolding and avoid sticking or ejection difficulties. Since a lateral core-pulling mechanism is required around perimeter of plastic part, parting surface is moved upwards, as shown in Figure 2. Upper part of PL is fixed mold section, and lower part is moving mold section.

 

Figure 2 Parting surface determination
In injection molding, core and cavity plate are key components of inner and outer surfaces of molded plastic part, and typically come in two forms: integral and modular. Given large size of plastic part, and considering its structural characteristics, as well as difficulty and cost of manufacturing mold parts, both core and cavity plate of mold adopt a modular structure.
Fixed mold section designs groove as a single unit, and arranges multiple small inserts in the middle area according to specific structure of plastic part. This design not only improves operability of CNC milling and metalworking polishing but also facilitates later maintenance and replacement, as shown in Figure 3(a). Moving mold also adopts a core structure combining integral and inlay design, is equipped with multiple characteristic small inserts to improve machining accuracy of mold parts and convenience of later maintenance, as shown in Figure 3(b). This modular inlay structure design effectively improves machining and maintenance efficiency of mold parts, while ensuring high-quality production of plastic parts.

 

Figure 3 Molded parts

To achieve high efficiency, energy saving, and quality control in injection molding process, a needle valve hot runner system is used for direct feeding. This design not only effectively shortens runner length and saves material, but also reduces injection pressure during molding process. To ensure equal flow length at each location, achieve sufficient pressure holding and reduce pressure difference, and ensure that weld line position is within an acceptable range, a configuration of 3 runners and 3 hot nozzles is selected. Diameter of needle valve gate is φ5mm, the total length of hot runner is 210mm, and diameter of runner is φ14mm. Feeding is done from middle of plastic part to be molded. This layout optimizes flow path, ensures uniform melt distribution, and improves molding quality of plastic part, as shown in Figure 4.

 

Figure 4 Gating system

To ensure temperature uniformity of melt during cooling in cavity and to prevent excessive local temperature differences from causing deformation of plastic part or inconsistent surface gloss of texture, design of mold cooling system is crucial. For fixed mold cavity plate, due to its flat surface and large area, nine independent φ8mm cooling water channels were designed to reduce heat transfer during flow of cooling water. φ18mm water wells were also installed at positions ③, ④, ⑤, ⑧, and ⑨ to optimize cooling effect. Meanwhile, center distance of cooling water channels was controlled within range of 3 to 5 times channel diameter, while distance from water channel to plastic part was maintained at 1.5 to 3 times channel diameter, as shown in Figure 5(a).
In moving mold section, due to presence of numerous inclined push blocks, push rods, and inserts, a conformal cooling water channel design was adopted to avoid these complex structures. By increasing number of water channels and ensuring their uniform distribution, cooling efficiency was effectively improved. Moving mold has a total of 8 water channels, 7 of which are straight-through, while water channel ② combines straight-through and well-type designs, as shown in Figure 5(b). A side-pulling core structure design is used for demolding side structure of plastic part. Given large size of sliders, each slider is equipped with a water channel, closely fitting plastic part to achieve optimal cooling. Four sliders have a total of 4 straight-through water channels. This design not only ensures effective cooling but also allows for precise temperature control of side mold, as shown in Figure 5(c).

 

Figure 5 Cooling system design

A core-pulling mechanism I was designed to address curvature of top side of plastic part and 7 reinforcing ribs at point K. A core-pulling mechanism II was designed for groove at point J on one side. A side core-pulling mechanism III was designed for label-attaching groove L on side. A core-pulling mechanism IV was designed for undercut position at the bottom I. A total of four slanted guide pillar core-pulling mechanisms were designed, as shown in Figure 6(a). All slider mechanisms consist of slanted guide pillars, slider seats, sliders, wear-resistant plates, and limiting pillars.
Since weld lines that do not affect performance of plastic part are permissible on the surface, a structure with slanted guide pillars in fixed mold and sliders in moving mold was adopted to simplify mold structure. Four-sided sliders are relatively large, so an insert structure is used, meaning forming position of slider is fixed to slider seat with screws. Since core-pulling method of all slanted guide pillar core-pulling mechanisms is same, core-pulling mechanism I has the most complex structure, and its design will only be described as an example here.
As shown in Figure 6(b), slider is used to form concave structure of surface at point J. Slider 9 is fixed on slider seat 3, which enhances support and stability and facilitates later maintenance. Inclined guide post 6 is installed on inclined guide post seat 2 and fixed together on fixed mold plate 1. A wear-resistant block 10 is added to middle of guide groove of moving mold plate 8 and back of slider 9 to improve slider's movement accuracy, extend its service life, while also facilitating mold maintenance. In addition, a limiting block 4 and a limiting pin 7 are designed in mechanism to limit slider's stroke.

 

1. Fixed mold plate 2. Angled guide post seat 3. Slider seat 4. Limit block 5. Spring 6. Angled guide post 7. Limit pin 8. Moving mold plate 9. Slider 10. Wear-resistant block
Figure 6 Angled guide post core-pulling mechanism
Based on slider core-pulling distance of 28mm at hole at point J in Figure 1, an inclined guide post with a diameter of φ12.5mm and an inclination angle α of 20º is initially selected. According to core-pulling distance and inclination angle of inclined guide post, length of inclined guide post can be calculated to be approximately 82mm. Theoretical working length of inclined guide post is approximately 77mm. This scheme ultimately achieves successful demolding of concave structure of the entire surface at hole at point J. Specific Workflow: During mold closing, inclined guide post 6 inserts into inclined hole of slider seat 3, forcing slider to slide inward to forming position; during mold opening, fixed mold and moving mold separate, and inclined guide post 6, along with mold parting movement, pulls slider outward along inclined angle, removing lateral forming structure and allowing plastic part to be demolded smoothly.

Based on analysis of plastic part structure, it is known that there are undercut structures on the back of bottom shell A, B, and C, undercut structures also exist inside bottom shell D, E, F, G, and H. Considering all factors, an inclined push core pulling mechanism is used. Since undercuts are located in different positions on plastic part, it is divided into fixed mold inclined push core pulling and moving mold inclined push core pulling. Their operating principles are similar, but since inclined push core pulling mechanism is set in fixed mold, core pulling action cannot be directly completed by push-out action of standard mold base push plate. Therefore, a corresponding push-out and reset device needs to be designed in fixed mold. Considering that structure at point A is located in fixed mold and mechanism is relatively complex, this inclined push mechanism design will be introduced as an example.
Due to large size of lifter block, forming position and lifter rod of lifter block are separate structures. Furthermore, one lifter block is divided into two lifter devices, as shown in Figure 7(a). Lifter base consists of a T-slot and a T-block. Lifter rod and T-block are fixed to slider by a pin, as shown in Figure 7(b). When lifter rod begins its ejection movement, T-block slides within T-slot, and simultaneously lifter rod rotates around pin via the T-block, causing lifter block to demold. Lifter base is fixed to ejector plate with screws. When lifter block mechanism needs maintenance, mechanism can be removed simply by loosening screws through pre-drilled holes in fixed mold base plate, without needing to open the entire fixed mold, thus reducing labor intensity.

 

Figure 7 Angled push core pulling mechanism
Lifter core-pulling mechanism and key components are shown in Figure 8. Based on 5mm hole depth at position A (see Figure 1) requiring demolding, and considering safety distance for core pulling, proposed angled push core pulling distance is 10mm; initial design tilt angle θ is 15º, thus angled push distance is calculated to be 75mm. This scheme ultimately achieves smooth demolding of the entire concave structure at position A. Its working principle: During mold opening, parting surface PL separates first, fixed mold return spring 3 needs to reset, causing ejection fixing plate 4 to move downwards, while push plate 8 remains fixed. During this process, tilt angle of guide sleeve 13 works in conjunction with guide post 12, angled push seat 11 remains fixed, while T-block 10 rotates along pin 9, driving angled push rod 14 and angled push insert 15 to move downwards and to the left, simultaneously completing ejection of plastic part and side core pulling action; during mold closing, reset rod sequentially pushes moving plate 7 and side slider 6 to reset, linking ejection fixing plate 4 and other components back to their initial positions, ensuring angled push core pulling mechanism is fully reset. This design achieves timing control of complex demolding actions through precise mechanical linkage.

 

1. Screw 2. Hot runner plate 3. Return spring 4. Ejection fixing plate 5. Fixed mold plate 6. Side slider 7. Moving mold plate 8. Push plate 9. Pin 10. T-block 11. Angled push seat 12. Guide column 13. Guide sleeve 14. Angled push rod 15. Angled push insert
Figure 8 Angled push core pulling mechanism

Due to clamping force of moving mold cavity on outer surface of plastic part, ejection force of fixed mold inclined push core-pulling mechanism during mold opening, and additional force of moving mold inclined guide post core-pulling mechanism, plastic part gradually detaches from fixed mold core after mold opening. To eject plastic part from moving mold cavity, a suitable ejection mechanism must be set in moving mold. Moving mold cavity is relatively deep, plastic part material has good rigidity, and demolding resistance is relatively large; therefore, mold ejection parts must be adequately and reliably designed. Considering structural characteristics of plastic part, ejection parts are set in positions with greater clamping force and sufficient ejection area. Waterproof ribs are located deeper, and 23 φ7mm ejector rods (a) are designed, while 27 φ8mm ejector rods (b) are designed at the bottom plane. Since there are several cylindrical holes on inner surface of plastic part, and size of these holes is small, their cores are prone to breakage. To facilitate mold repair, ejector rod assemblies are used for demolding, and 17 sets of φ9mm ejector tubes are designed, as shown in Figure 9. Ejector assembly is a standard part with good interchangeability. If ejector tube breaks or there are flashes at cylindrical opening, ejector assembly can be directly replaced, resulting in less maintenance work.

 

Figure 9 Push-out part

Mold's external dimensions are 900mm * 840mm * 830mm, as shown in Figure 10. It adopts a split-type structure design. Fixed mold section integrates a lifter core-pulling mechanism and its matching ejection system. Each forming mechanism of the moving mold is also equipped with a precision ejection mechanism consisting of a pad 11, a support column 29, and a reset rod 30. Various auxiliary functional components are arranged on outer surface of mold. Specific working principle is as follows.

 

1. Fixed mold base plate 2. Hot runner plate 3. Guide sleeve 4. Guide pillar 5. Fixed mold plate 6. Wear-resistant plate 7. Slider 8. Nylon buckle 9. Moving mold plate 10. Guide rod 11. Pad block 12. Moving mold base plate 13. Hot runner system 14. Ejector plate 15. Ejection fixing plate 16. Reset rod 17. Main cavity plate 18. Wear-resistant plate 19. Core insert 20. Core 21. Angled ejector rod 22. Ejector tube 23. Ejector rod fixing plate 24. Ejector plate 25. Wear-resistant plate 26. Slider 27. Spring 28. Ejector rod 29. Support pillar 30. Reset rod 31. Pin 32. Guide pillar 33. T-block 34. Angled ejector seat 35. Angled ejector rod 36. Guide sleeve 37. Wear-resistant plate 38. Slider 39. Spring protective sleeve 40. Lifting ring 41. Ejector rod 42. Anti-collision protective block
Figure 10 mold structure
(1) Forming Stage: Driven by injection pressure, melt in hot runner flows through hot runner system 13, passes through manifold, reaches needle valve gate, and smoothly enters cavity. Within cavity, melt undergoes pressure holding and cooling before finally completing forming process.
(2) Mold Opening Stage: During mold opening, fixed mold system drives ejection mechanism via a spring, which in turn drives inclined push rods to push main cavity plate 17 and internal core pulling mechanism. Moving mold system, guided by inclined guide pillars, simultaneously completes external core pulling. Moving and fixed molds work together to ensure complete demolding of plastic part.
(3) Ejection Stage: Injection molding machine ejector pushes ejector plate 24, which in turn drives ejector fixing plate 23. During movement of ejection mechanism, various ejector rods 28 and ejector tubes 22 work together to eject molded plastic part, allowing it to smoothly detach from moving mold.
(4) Mold Closing and Reset Stage: Under action of the reset rod 30, ejector fixing plate 23 and ejector plate 24 drive ejector rods 28 and ejector tubes 22 to eject molded plastic part, and moving mold ejection mechanism completes reset action. At the same time, each inclined guide pillar drives slider, causing each inclined guide pillar core pulling mechanism to reset. Moving mold plate 9 pushes fixed mold reset rod 16, causing fixed mold to eject fixing plate 15, push plate 14, and inclined push seat 34, inclined push rod 35, and other parts to reset as a whole until mold is completely closed. The entire mold's operation is coordinated and orderly, with each stage closely connected, ensuring efficient production and high-quality molding of plastic parts.

Designed charging pile bottom shell mold was actually manufactured in enterprise, as shown in Figure 11. Mold structure is reasonable, plastic parts have good molding effect, high dimensional accuracy, good surface quality, and no obvious defects, as shown in Figure 12. In continuous production testing, mold operates stably, molding cycle is short, meeting needs of large-scale production. There were no problems such as uneven ejection, uneven mold temperature, or difficulty in demolding on the bottom shell surface.

 

Figure 11 Actual mold

 

Figure 12. Actual plastic part