Plastic part is an antenna element in a communication equipment, and it is also a functional part. After injection molding, laser direct molding, also known as LDS[1], is required to upgrade plastic part into a conductive antenna. For this reason, plastic part must have certain strength, dimensional stability and LDS processability. Requirements for appearance of plastic parts are polished SPI B1, color is natural, and the overall dimensions are about 45 mm * 9 mm * 6 mm, as shown in Figure 1. Plastic part material is PC/ABS NX10302. Injection process parameters and properties of this material are shown in Table 1. These parameters are important technical basis for molding process and mold design to ensure quality of molded plastic parts and stability of subsequent production.

 

Figure 1 Plastic part structure

Table 1 PC/ABS NX10302 injection process parameters and performance
Through analysis of mold, according to requirement that molded plastic part has no push rod mark, designed mold opening method is shown in Figure 2. Shape of plastic part is similar to a trapezoid, most of visible area is formed on cavity side, and molding area at core is small, which will cause risk of plastic part sticking to cavity wall, which should be avoided. Molded plastic parts are required to have no push rod marks, and mold structure cannot be pushed out with a push plate. Therefore, a push rod must be designed, but push rod cannot directly act on plastic parts to be formed. Based on above analysis requirements, two Hough sliders are added to mold opening scheme. First, it can reduce molding area of plastic part on cavity side and avoid molding plastic part from sticking to cavity wall; second, core molding feature is designed on slider, which is convenient for push rod to be arranged under slider, push rod does not directly contact plastic parts, and push rod mark will not be left.

 

Figure 2 Schematic diagram of mold opening structure
In order to make molded plastic parts meet mold opening requirements, mold release angle is increased in the direction of slider during design of plastic parts, so as to prevent plastic parts from being pulled during mold release. As shown in Figure 3, dark part on the left is demoulding angle in the direction of slider 1, light-colored part on the right is demolding angle of sliding block 2 direction, and angles are all 1°. At the same time, these surfaces of plastic part are polished SPI B1, which is more conducive to molding and demolding of slider.

 

Figure 3 Draft angle analysis

Reasonable selection of mold parts materials determines service life of mold and quality of molded plastic parts. After plastic parts are formed, LDS treatment is required, and injection requirements are high. Visible surface of plastic parts is formed on slider. Each cavity of mold has 2 Hough sliders. Steel of slider should be well polished and resistant to S136 stainless steel with excellent corrosion performance, mold parts materials are shown in Table 2.

Table 2 Mold parts materials

Design of push-out system is the key point of mold. In order to avoid push-rod mark on molded plastic part, push-rod cannot be arranged in molding area, push-rod cannot be directly contacted with plastic part to be molded, and a certain distance must be kept, To this end, slider structure is used to supplement distance between cavity plate and push rod. Push rod is arranged under slider. After injection molding, slider is opened, leaving a push-out space and reducing holding force between plastic part and mold part. At this time, after push rod runs for a period of time, push rod contacts bottom surface of plastic part and pushes it out. Figure 4 shows schematic diagram of push-out method.

 

Figure 4 Push out method

Mold structure is shown in Figure 5.

 

Figure 5 Mold structure
1. Upper heat insulation plate 2. Fixed die seat plate 3. Hot runner plate 4. Cavity plate insert 5. Fixed template 6. Guide post 7. Core fixing plate 8. Reset rod 9. Spring 10. Push rod fixation Plate 11. Pad 12. Push plate 13. Moving die base plate 14. Lower heat shield 15. Pull rod 16. Limit block 17. Ejector rod 18. Straight push rod 19. Flat push rod 20. Screw 21. Core 22. Screw 23. Slider seat 24. Slider cooling external water pipe 25. Wedge block 26. Oblique guide column 27. Oblique guide column fixing block 28. Screw 29. Positioning ring 30. Hot runner assembly 31. Hot runner Guide protection pin 32. Dust-proof plate 33. Water pipe cover plate 34. Precision positioning block
(1) Fixed mold part. Upper heat insulation plate 1 is fixed on fixed die base plate 2, which prevents direct contact between die and panel of injection molding machine, avoids heat loss, so that die temperature can be maintained at normal set temperature. Hot runner assembly 30 is installed in hot runner plate 3. Hot runner is an extension of injection unit, which ensures that plastic melt extends from main gate to runner to cavity, improving material utilization and molding efficiency, cavity plate insert 4 is used to install hot runner nozzle and fix it in fixed mold seat plate 2, cooperate with slider seat 23 to slide and position, inclined guide column 26 is installed in fixed block 27 of inclined guide column to drive sliding block base 23 to open and close. Screw 28 is main part for connecting and tightening fixed mold part.
(2) Moving part. Slider seat 23 is installed on core 21, and cooperates with core 21 to form a plastic part, flat push rod 19 is fixed on push plate 12, cooperates with core 21 to guide push rod to push out and reset. Flat push rod 19 is not in direct contact with molded plastic part, and is separated by a distance and below the slider. Spacing distance is 8.0 mm, so as to ensure that no push rod marks are left on molded plastic parts, and at the same time, push rod can be used to push out. Straight push rod 18 is fixed on push rod fixing plate 10 and fits with core 21 in a clearance fit. Function of straight push rod 18 is to prevent movement of push plate 12 from driving flat push rod 19 to damage slider when mold is not opened.
Working process of mold: after injection molding machine is injected and cooled, mold is opened, movable mold part moves backward as a whole, main parting surface of mold is opened, molded plastic parts stay in core and leave cavity. When core moves a short distance, inclined guide post 26 touches slider seat 23, slider seat 23 is driven by inclined guide post 26 to pull core away from plastic part. After core pulling is completed, slider seat 23 stops at limit position, moving mold part continues to move backward and finally stops at set position, completing mold opening action. Push rod 17 pushes push rod fixing plate 10 when molded plastic part is pushed out. When flat push rod 19 fixed on push rod fixing plate 10 moves 8.0 mm, then contacts plastic part and continues to push out until limit block 16 on push rod fixing plate 10 touches bottom surface of core fixing plate 7, push-out action is completed, plastic part is taken out by manipulator and mold is closed, and next plastic part is formed.