Nylon elbow joint has a 2-section elbow structure, material is modified nylon PA6, and shrinkage rate is 0.52%~0.58%, as shown in Figure 1. Outer diameters of two arc-shaped elbows are both φ23mm, but central arc of first arc-shaped elbow and central arc of second arc-shaped elbow are not in same plane. They are spatial structure curves. Front end of the first section is pipe head, tail end of second section is flange seat, and wall thickness of two sections of elbow is 2mm. Difficulties in mold design for molded plastic parts are as follows.
(1) Demoulding of inner wall of the first arc-shaped elbow requires design of a core-pulling mechanism for arc-shaped elbow, and movement route of arc slider of mechanism must be set according to center arc of first arc-shaped elbow.
(2) Arc elbow core-pulling mechanism also needs to be used to demould inner wall of second arc elbow section, and movement route of arc slider of this mechanism also needs to be set according to center line of second arc elbow section. Center arcs of two arc-shaped elbows are cross-linked in space. If core-pulling mechanism of the first arc-shaped elbow is arranged on plane where central arc is located, core-pulling mechanism of second arc-shaped elbow will not be easy to arrange.
(3) After flange seat is combined with second arc elbow, holes and grooves on flange seat can only be demoulded along center line of hole. Otherwise, mold needs to be designed with an additional demoulding mechanism featuring flange seat.
(4) Joint between flange seat and outer wall of second arc elbow forms a turning undercut area. If direction of hole centerline of flange seat is used as mold opening direction, turning undercut area here affects ejection of second arc-shaped elbow from mold cavity, and a special core-pulling mechanism needs to be designed to core-pulse this part first.

 

According to shape characteristics of plastic part, mold parting should first consider demoulding method of molded plastic part. Combined with demoulding method, mold is divided in following way: first, maximum outer edge contour of plastic part is used as main parting surface of outer wall parting, and two outer wall molding parts are separated, cavity plate insert 1 and core insert 2, as shown in Figure 2; then, for turning undercut area of plastic part, two secondary parting surfaces P1 and P2 are used, side sliders 3 and 4 are set at parting place to facilitate demoulding in turning undercut area of plastic part; finally, according to characteristics of 2-section arc bend on inner wall of bend, use P3 surface as inner wall parting surface to separate the first arc bend core 5 and the second arc bend core 6.

 

1. Cavity plate insert 2. Core insert 3. Side slider 4. Side slider 5. 1st arc elbow core 6. 2nd arc elbow core 7. Inclined push rod

 Figure 2 Parting and demoulding design
Demoulding of molded plastic parts after above-mentioned parting is completed according to following steps: ① PL surface of mold cavity is opened, and plastic parts are separated from cavity plate insert 1; ② Side sliders 3 and 4 pull out core on each side, molded plastic part is separated from side slide blocks 3 and 4; ③ Core 5 of the first arc bend tube completes core pulling; ④ Formed plastic part is pushed out by second section of arc elbow core 6, and plastic part separates from core insert 2 ; ⑤ Rely on a side inclined push rod 7 to push molded plastic part to automatically rotate and demould from second arc bend pipe core 6.

Mold uses a 1-cavity layout and a common runner structure for a two-plate mold, as shown in Figure 3. Side of fixed mold is mainly arranged with cavity plate insert 1, sprue sleeve 11, locking block 10 of demoulding mechanism and inclined guide pillars 8 and 9, etc. There is one cooling water channel (L1) for cavity plate insert 1, and pipe diameter is φ10mm. Core insert 2 and 6 demoulding mechanisms are mainly arranged on one side of movable mold. Among them, demoulding mechanism S1 is used to drive core 5 of the first arc bent tube to perform rotational core pulling, demoulding mechanism S2 is used to push out oblique push rod 7, demoulding mechanism S3 is used to push out second arc bend core, demoulding mechanisms S4 and S5 are used to drive side sliders 3 and 4 respectively to implement side core pulling; in addition to these five demoulding mechanisms, there is also a secondary push plate that comes with mold base to push out demoulding mechanism S6, which is mainly used to secondary drive demoulding mechanisms S2, S3 and push rod 12 to achieve complete demoulding of plastic parts. Core insert 2 is provided with a cooling water path L2 to cool it, pouring system is provided with two rectangular cross-section side gates G1 and G2.

 

1. Cavity plate insert 2. Core insert 3. Side slider 4. Side slider 5. 1st arc elbow core, 6. 2nd arc elbow core 7. Incline push Rod 8. Inclined guide column 9. Inclined guide column 10. Locking block 11. Gate sleeve 12. Push rod
Figure 3 Mold structural layout

(1) Demoulding mechanism S1 is shown in Figure 4. Mechanism S1 is used to drive the first arc-shaped elbow core 5 to perform rotational core pulling. Tail end of the first arc-shaped bent tube core 5 is set in the shape of a slider and is installed on the top of rotating swing arm 16. Front end of hydraulic cylinder piston rod 17 and rotating swing arm 16 are connected through movable pin 13, lower end of rotating swing arm 16 is positioned on support plate 15 with movable pin 14, rear end of hydraulic cylinder 18 is also positioned on supporting plate 15 with a movable pin. When hydraulic cylinder piston rod 17 is retracted, swing arm 16 will be driven to rotate clockwise, and the first arc bend core 5 will be extracted from plastic part. When reset, piston rod 17 only needs to be extended.

 

1. Cavity plate insert 2. Core insert 3. Side slider 4. Side slider 5. 1st arc elbow core 6. 2nd arc elbow core 7. Inclined push rod 8. Inclined guide column 9. Inclined guide column 10. Locking block 11. Gate sleeve 12. Push 13. Movable pin 14. Movable pin 15. Support plate 16. Rotating swing arm 17. Piston rod 18. Hydraulic cylinder 19. Movable pin 20. Hydraulic valve 21 once pushes out the combination plate 22. Secondly pushes out the combination plate 23. Separation control mechanism 24. Fixed mold plate 25. Moving mold plate 26. Pad 27. Moving mold plate seat plate
Figure 4 Mold structure
(2) Demoulding mechanisms S4 and S5 are commonly used inclined guide pillar slider mechanisms, which will not be described here.
(3) Demoulding mechanism S6 is a secondary push-out mechanism added to mold base, including a primary push-out combination plate 21, a secondary push-out combination plate 22 and a separation control mechanism 23. Separation control mechanism 23 is a commonly used rod-type secondary push-out mechanism.
(4) Setting of demoulding mechanism S2 is an important measure to solve problem of difficult molding of plastic parts, as shown in Figure 5. Lower end of arc bend core 6 of second section is pushed out with push rod 29 and pushed by secondary pushing combination plate 22. Lower end of inclined push rod 7 is provided with an inclined push seat 28 and is pushed by primary pushing combination plate 21. Effect of this setting is that when secondary push-out combination plate 22 and primary push-out combination plate 21 are pushed out simultaneously, oblique push rod 7 and second arc elbow core 6 are also pushed out simultaneously, molded plastic parts will be pushed out from core insert 2 by second arc bend pipe core 6. After being pushed out for a certain distance, secondary push-out combination plate 22 stops pushing out, but primary push-out combination plate 21 will continue to push oblique push rod 7 to push out. At this time, inclined push rod 7 will push plastic part out from bottom of flange seat of molded plastic part according to arc section of second arc bend pipe core 6 until molded plastic part automatically falls. During the first push, push rod 12 cooperates with push rod 29 to push out molded plastic parts due to excessive tightening force of flange seat.

As shown in Figure 4 and Figure 5, working principle of mold is as follows.
(1) Mold clamping and injection. After mold is installed and debugged on injection molding machine, mold is closed and prepared for mold opening after filling, pressure holding, cooling and other processes.
(2) Demoulding mechanism S1 core pulls. Before mold is opened, hydraulic cylinder 18 first drives the first arc bend core 5 to complete core pulling.
(3) Mold opening. Slider of injection molding machine drives movable mold to retreat, and mold is opened at PL surface. During opening process, inclined guide posts 8 and 9 of demoulding mechanisms S4 and S5 drive side sliders 4 and 3 to complete side core pulling.
(4) Push out mold. Movable mold retreats a certain distance and then stops. Ejector pin of injection molding machine pushes combination plate 21 for one push and combination plate 22 for second time to push out simultaneously. During the first push, plastic part will be pushed out from core by second arc bend pipe core 6, then second push of combination plate 22 will stop, the first push of combination plate 21 continues. Oblique push rod 7 is pushed to push molded plastic part out of second section of arc elbow core 6, and plastic part is completely demoulded.
(5) Reset. Reset process is opposite to mold opening process.

For nylon joints with two sections of arc elbows, a pair of 1-cavity and two-plate injection molds was designed, which solved problem of forming and demoulding two sections of different arc elbows in plastic parts. The first arc bending mechanism in mold realizes arc core pulling by driving slider with a hydraulic cylinder. The second arc bending demoulding mechanism uses a secondary push method and relies on an oblique push structure to push plastic part from one side to eject from the core of the second arc bend pipe, solving problem of ejection from inner wall of second arc bend pipe. Mold structure is ingeniously designed, mechanism is practical and reliable, and has a certain reference effect.