For plastic parts in Figure 1, setting of parting surface is mainly subject to following factors when designing mold: ① Annular ribs and multiple triangular ribs on outer wall of arc tube can only be demolded in a two-flap demolding method; ② Inner wall features of bracket sleeve can only be demolded along center line of bracket sleeve and center line of three screw columns, grid ribs on outer wall of bracket sleeve can only be demolded perpendicular to vertical direction of grid rib surface, and demolding of buckle feature can only be demolded in direction perpendicular to the bottom surface of buckle; ③ Demolding of inner wall of arc tube can only be demolded by core pulling in a circular arc rotation; ④ Inner wall of straight tube feature needs to be demolded by core pulling, and its core pulling direction can be set same as core pulling direction of inner wall of bracket sleeve.
From above analysis, it can be seen that plastic part to be molded can only be parted at PL shown in Figure 2, so as to facilitate demolding of molded plastic part, simplify demolding mechanism and mold structure. Under parting setting of PL surface, automatic demolding of molded plastic part can be achieved by setting up three slider mechanisms. Functions of corresponding three sliders are: ① Slider 1 forms clamping wall groove, straight groove, screw column and molding tube features; ② Slider 2 forms inner wall features of arc tube; ③ Slider 3 forms grid ribs and buckle features. Ring ribs and remaining outer walls of plastic part are molded and demolded by fixed mold insert and movable mold insert.

 

Figure 2 Parting and demolding design

Under parting state shown in Figure 2, determined molding part design is shown in Figure 3, slider 1 corresponds to straight tube slider, slider 2 corresponds to arc slider, and slider 3 corresponds to inner core-pulling slider. Straight tube slider drives core-pulling through hydraulic mechanism, arc slider drives rack through hydraulic mechanism, rack drives fan gear, and fan gear drives arc slider through rotating shaft to implement rotating core-pulling. Inner core-pulling slider can be locked and driven by a T-slot locking block according to its driving and locking requirements, and corresponding features can be implemented with an inward core-pulling.

 

Figure 3 Molding part design
Considering production efficiency of plastic parts, a 1-mold 2-cavity layout is adopted. In order to simplify mechanism and optimize mold structure, straight tube sliders of two cavities in Figure 3 are driven by a common hydraulic mechanism, and arc sliders in two cavities are also driven by a common rack slider mechanism. Inner core-pulling sliders of two cavities are also locked and driven by a common T-slot locking block.
When designing pouring system, in view of convenience of runner processing, a φ8 mm CNC milling cutter is used to process circular runner. Considering structural space setting, size of mold frame is reduced, and gate sleeve uses an oblique gate.

According to 1-mold 2-cavity structure set on parting surface, corresponding movable and fixed mold structures are shown in Figure 4. Combined with requirements of demolding mechanism and simplified mold structure, a two-plate mold frame is selected. In fixed mold setting, cavity plate is arranged eccentrically to ensure that center of gate sleeve is aligned with center of mold frame. Pouring of the entire mold cavity is eccentric pouring, and injection pressure is controlled. Cavity plate insert is installed in fixed mold plate. Material is 2344 alloy steel, and a single water channel is used for cooling.

 

Figure 4 Structure of movable mold and fixed mold
Three composite mechanisms are used in movable mold to synchronously implement core pulling and demolding of two cavities. Corresponding mechanisms are straight tube core pulling mechanism, curved tube core pulling mechanism and internal core pulling mechanism. Final demolding of plastic part to be molded is implemented by pushing out push rod. Core insert is installed on movable mold plate. Material is P20, a single water channel is also used for cooling.

Mold structure is shown in Figure 5.

 

Figure 5 Mold structure
1. Fixed mold base plate 2. Fixed mold plate 3. Arc slider 4. Arc pressure strip 5. Limit screw 6. Rotating shaft 7. Connecting screw 8. Rotating shaft 9. Fan gear 10. Rack 11. Rack slider 12. Pad 13. Moving mold base plate 14. Push plate 15. Travel switch 16. Moving mold pad 17. Moving mold plate 18. Limit glass beads 19. Inner core pull slider 20. T-shaped locking block 21. Core insert 22. Cavity plate insert 23. Oblique gate sleeve 24. Wall groove core 25. Straight tube core 26. Straight tube slider 27. Hydraulic cylinder 28. Reset rod 29. Reset rod spring 30. Limit block 31. Support column 32. Push rod 33. Push plate guide column 34. Guide column 35. Hydraulic cylinder
(1) Mold adopts a two-plate mold frame, and a movable mold pad 16 is added under movable mold plate 17 to facilitate installation of bending tube core pulling mechanism.
(2) In bending tube core pulling mechanism assembly, core pulling assembly of a single cavity mainly includes parts 3 to 11. Piston rod of hydraulic cylinder 35 drives rack slider 11 to move, and rack slider 11 then drives fan gear 9 to rotate around center of rotating shaft 8. Rotating shaft 8 then drives arc slider 3 located in arc pressure strip 4 on movable mold plate 17 to rotate, and inner wall of arc tube of molded plastic part is rotated to pull core. Position of arc slider 3 is limited by limit screw 5.
(3) Internal core pulling mechanism. Internal core pulling mechanism assembly mainly includes parts 18 to 20. When mold is opened, oblique T-shaped grooves on both sides of T-shaped locking block 20 simultaneously drive two internal core pulling sliders on both sides to implement inward core pulling. Limit of two inward core pulling sliders is respectively limited by their respective limit glass beads 18.
(4) Straight tube core pulling mechanism. Straight tube core pulling mechanism is a two-time core pulling mechanism, a delayed core pulling is set between two core pullings, and delay distance is 10 mm. Mechanism components include parts 24 to 27. During the first core pulling, wall groove core 24 is first pulled out 10 mm, then straight tube inner wall core is pulled out. Both cores are driven by straight tube slider 26, straight tube slider 26 is driven by piston rod of hydraulic cylinder 27.
(5) Pushing mechanism. Final demolding of plastic part is achieved by pushing out multiple push rods 32, which are pushed by push plate 14, push plate 14 is connected and driven by injection molding machine top rod through a connecting rod. In order to prevent push rod 32 from interfering with slider of straight tube core pulling mechanism and arc slider of bent tube core pulling mechanism during resetting, push rod 32 must be retracted before resetting. Therefore, push plate 14 is provided with a travel switch to detect its resetting status. Only after push plate 14 is reset, piston rods of hydraulic cylinders 27 and 35 can move to drive sliders on their respective mechanisms to reset.

When mold is working, mechanism's operating principle is as follows.
(1) Hydraulic cylinder piston rod first pulls core. Before mold is opened, because hydraulic cylinders 27 and 35 are in series, piston rod of hydraulic cylinder 35 moves first to complete core pulling of inner wall of bent tube, then piston rod of hydraulic cylinder 27 moves to complete core pulling of inner wall of clamping wall groove and straight tube.
(2) Mold has only a single parting surface. When mold is opened after injection is completed, slider of injection molding machine drives movable mold to move, and mold opens at parting surface.
(3) Push out. After mold is opened, movable mold continues to move, and ejector rod of injection molding machine drives push plate 14, which pushes push rod to push plastic part out of core insert 21, thereby demolding plastic part.
(4) Reset. When resetting, ejection mechanism is reset first, and travel switch 15 performs reset detection, which in turn controls piston rods of hydraulic cylinders 27 and 35 to drive their parts to reset.