Molding process for a two-color button with inserts is shown in Figure 1. Plastic part assembly is injection molded in two steps. First, insert is placed, and PC material is injected to form plastic part base and inner lining. Then, ABS material is injected onto inner lining to form outer plastic layer of plastic part, ultimately forming complete plastic part assembly.

 

Figure 1 Two-color button molding process

Vehicle stability control button structure is shown in Figure 2. It features a fully enclosed housing. To facilitate assembly and fixation of button to primary color substrate, two square hook holes are designed on either side. Maximum overall dimensions of plastic part are 33.8mm * 25mm * 18.8mm, with a base wall thickness of 1.2mm and a maximum rib wall thickness of 1mm. Primary wall thickness of primary color sub-part is 0.8mm, with a net weight of 2.458g. Primary wall thickness of primary color sub-part is 1.2mm, with a net weight of 2.230g.

 

Figure 2 Button structure
Insert used in primary color sub-part has a wall thickness of 0.7mm and a net weight of 0.084g. This plastic part is an exterior component, requiring high surface quality. After forming secondary color surface, it undergoes electroplating. Defects such as shrinkage, weld marks, and flash are not permitted. The first color sub-molded part is made of PC2805901510 (black), and second color sub-molded part is made of ABSPGGY419 (smoky gray). Both have a shrinkage rate of 0.5%. Insert is made of PC and requires manual placement.

Because two injection units and nozzles of two-color injection molding machine are independent, two colors of finished product are clearly defined. The key difference between a two-color injection molding machine and a conventional one lies in design of injection unit and movable turntable. Movable turntable provides 180° reciprocating rotation, enabling cyclic alternating motion in mold. This two-color injection mold is based on a 160-ton vertical two-color injection molding machine. Machine boasts an actual shot weight of 68g, a maximum injection rate of 44cm³/s, a maximum injection pressure of 250MPa, and a maximum injection stroke of 100mm, meeting production requirements of plastic parts. A two-shot injection mold consists of two sub-molds: one with same movable mold and one with a different fixed mold. After first injection molding, inner lining base is rotated 180° to form insert assembly for second injection of melt. Second injection molding then forms outer wall rubber layer, completing molded part.

Layout of molded part must consider both easy demolding and reduced part processing difficulty and mold structure complexity. Optimal mold structure is achieved by defining parting surface based on these considerations. Main parting surface is determined based on structural characteristics of part. As shown in Figure 3, structure is relatively simple, with main parting surface PL being a plane. Main molding components of mold are fixed and movable cavity plates.

 

Figure 3 Parting surface design

Following aspects should be considered in cavity layout: ① Number of cavities should be determined based on annual part production, part precision, and availability of injection molding machine. ② Distance from cavity to sprue should be kept as short as possible to reduce agglomerate and improve material utilization. ③ Distance from each cavity to sprue should be as equal as possible to ensure balanced material flow during injection. ④ Spacing between cavities should be reasonable to facilitate placement of ejectors and cooling water lines. In summary, cavity layout is one mold with two cavities, as shown in Figure 4. The first and second color cavities share same layout, resulting in a compact structure and flat parting, facilitating mold part processing.

 

Figure 4: Cavity layout

Two-color injection mold has two gating systems. To consider mold manufacturing costs, both liner substrate (in first injection molding) and outer wall encapsulation (in second injection molding) utilize a common runner gating system. Exhaust channels are provided at runner ends to facilitate smooth discharge of gases during injection and filling process. Gating system is shown in Figure 5. The first shot is fed from a three-plate mold using a point gate, while second shot is fed from a side gate. Considering that part is intended for electroplating, first-color liner substrate must be fully injected to prevent shrinkage that could lead to uneven thickness of second-color overmolding layer, potentially causing shrinkage defects. First-color liner substrate cavity is fed from two point gates to ensure full filling. Point gates minimize residual solids when separated from liner substrate, ensuring appearance quality of second shot and facilitating removal of runner solids from first shot.

 

Figure 5: Gating system
Since gate marks are unacceptable on exterior surface of second shot, a side gate is used to feed part from underside of skirt. During electroplating process, plating overflow can occur on the edges of part, requiring cathodic protection. Side gates and runner solids from second shot are removed after plating is complete.

When designing core and cavity plates, following considerations should be taken into account: ① Core and cavity plates should be easy to manufacture. A patchwork treatment should be applied at deep glue point (see Figure 6) to facilitate degassing during injection and filling, facilitate fabrication of mold components. ② To ensure mold's longevity, core and cavity plates should be heat-treated to a hardness of 48-52 HRC. ③ To prevent rust on cavity surface, cavity plates should be made of corrosion-resistant steel such as S136. ④ After trial molded parts have been visually confirmed to be free of defects, second-color cavity surface can be coated to extend mold's service life. ⑤ To facilitate insert assembly, all non-mating and non-parting surfaces should be rounded to avoid gaps. ⑥ Inserts should be accurately positioned, and positioning holes should be provided on core according to their characteristics.

 

Figure 6: Moving Mold Cavity Plate Assembly Structure

Mold uses a standard mold base. The first-color sub-mold uses a point gate and a three-plate mold base. Second-color sub-mold uses a side gate and a two-plate mold base. Movable mold and mold base of two sub-molds are identical in height after closing. After designing one sub-mold, the other was completed by copying and modifying it, as shown in Figure 7.

 

Figure 7 Mold base structure

Side hook holes of plastic part require a lateral core pulling mechanism for demolding. As shown in Figure 8, sliders of two sub-molds are located in movable mold. Based on part structure, two slider core pulling mechanisms are designed for left and right movable mold sliders, for a total of four sliders to drive side core pulling. Since liner substrate of first injection molding is required to remain on core, this prevents lateral core pulling insert from crushing part structure during second injection molding due to mold part machining and assembly precision errors. As shown in Figure 9, locking block and pressure block of first sub-mold only serve a locking function and do not have a lateral core pulling function. Therefore, lateral core pulling slider does not move for demolding when mold is opened after first injection molding. Second-color mold features a T-shaped pressure block and slider core-pulling assembly. T-shaped pressure block and locking block are fixed to fixed platen of second-color mold, as shown in Figure 10. Under injection molding machine's mold opening force, T-shaped pressure block forces spring-driven slider to slide outward along track, allowing plastic part's hook hole and movable mold gate to solidify, completing side core-pulling operation. Left and right sliders, locking blocks, and side cores are identical for ease of manufacturing. For cost and efficiency reasons, standard parts are used for pressing strips.

 

Figure 8 Button button hole core-pulling structure

 

Because body stability control button has a narrow demolding angle and a large stroke, and is surrounded by a fully enclosed shell structure, plastic part cools and shrinks after injection, resulting in significant holding force. Therefore, a combined secondary ejection mechanism consisting of a push rod and movable platen is required, as shown in Figure 11. Movable mold pre-pull insert is secured to movable mold support plate via a pre-pull core retaining plate, movable platen is secured to movable mold support plate via limit screws and limit bushings.

 

1. Push block 2. Push plate 3. Push rod fixing plate 4. Push block 5. Spacer 6. Limit bushing 7. Pre-extraction core fixing plate 8. Pre-extraction insert for movable mold 9. Spring block 10. Fixing block 11. Movable mold cavity plate 12. Movable mold plate 13. Plastic part 14. Pin 15. Spring
Figure 11: Secondary ejection structure
After second injection is complete, mold opens to a certain position under action of injection molding machine's slider. Ejector pushes push plate through ejector block, driving push plate to act on spring block in mold opening direction. At this point, secondary ejection mechanism (push block 4, spring block 9, fixed block 10), movable platen, movable mold cavity plate, and plastic part move synchronously in mold opening direction, moving relative to movable mold pre-extraction insert to achieve first ejection. When ejector fixed plate and movable platen move upward to a certain position, interaction between inclined surfaces of fixed block and spring block causes spring block to retract, making way for push plate. Ejector fixed plate then drives ejector to continue moving in mold opening direction, achieving second ejection, and plastic part is successfully demolded.

Three-dimensional structure of mold is shown in Figures 12 and 13. After mold is installed on machine, fixed mold is connected to oil temperature controller, and movable mold is connected to cooling water. After mold is opened and closed, dry run is pushed out, insert is put into mold and first color mold starts to be injected. After plastic melt is filled, movable mold moves downward under drive of injection molding machine slider. Under action of hook 15 and pull rod 21, runner push plate 19 and fixed mold plate 16 of first color mold are separated at PL2 parting surface, and runner condensate is separated from plastic part; mold continues to open, under action of hook 15 and pull rod 17, fixed mold plate 16 drives runner push plate 19 to push out runner condensate at PL1 parting surface; mold continues to open, fixed mold plate 16 and movable mold plate 12 are separated at PL3 parting surface. When first color mold is opened in place, lateral core pulling does not cause displacement and demolding. Under action of injection molding machine's turntable, movable mold rotates 180° before closing mold. After first and second color sub-molds are injected simultaneously, movable mold moves downward, driven by injection molding machine's slider. Second color sub-mold separates at parting plane P1, and slider's core-pulling mechanism completes side core-pulling action. Movable mold continues to descend and open into position. Injection molding machine's ejector pushes against pusher block 2 and moves upward. Secondary ejection mechanism separates movable platen 8 from movable platen 12, completing core-pulling of movable mold. Pusher block 2 continues to push pusher plate, driving pusher 23 to eject molded part, achieving complete demolding. This repeated operation achieves semi-automatic production. Mold closing process is opposite of mold opening process.

 

1. Movable mold base plate 2. Push block 3. Push plate 4. Push rod fixing plate 5. Spring 6. Push block 7. Limit rod 8. Movable mold plate 9. Pre-extraction core fixing plate 10. Movable mold pre-extraction insert 11. Fixing block 12. Movable mold plate 13. Limit pin 14. Hold-down strip 15. Draw hook 16. Fixed mold plate 17. Pull rod 18. Screw 19. Runner push plate 20. Fixed mold base plate 21. Pull rod 22. Spacer 23. Limit column 24. Support column 25. Clamping bar 26. Movable mold cavity plate 27. Slider 28. Locking block 29. Locking block 30. Fixed mold cavity plate 31. Nozzle 32. Sprue bushing
Figure 12: Structure of the first color mold

 

1. Movable mold base plate 2. Push block 3. Push plate 4. Push rod fixing plate 5. Spring 6. Push block 7. Movable mold stopper 8. Movable mold plate 9. Pre-extraction core fixing plate 10. Movable mold pre-extraction insert 11. Fixing block 12. Movable mold plate 13. Stop pin 14. Hold-down strip 15. Draw hook 16. Fixed mold plate 17. Fixed mold cavity plate 18. Screw 19. Nozzle 20. Fixed mold base plate 21. Positioning ring 22. Spacer 23. Push rod 24. Locking bar 25. Movable mold cavity plate 26. Slider 27. Locking block 28. Locking block 29. Screw
Figure 13: Second color mold structure

Surface quality of molded part is excellent, as shown in Figure 14(a), with no molding defects such as sink marks, air marks, flow marks, weld lines, deformation, or strain, meeting appearance requirements. After electroplating and laser engraving, the overall effect of plastic part is good, as shown in Figure 14(b), all features such as dimensions and appearance meet expected design requirements. After verification in small-batch production, no molding problems such as smoothness and strain occurred during mold injection, and injection process parameters were reasonable, which can achieve stable and reliable production, ensure stable size and good appearance of plastic parts, and meet design requirements.

 

Figure 14 Actual plastic part and electroplating effect