In design process of injection molds, for plastic parts with complex structures, shape, number, and location characteristics of core-pulling structure need to be considered to design demolding mechanism. A multi-directional circumferential core pulling mechanism was designed for double-layer side holes with a certain angle between direction of radial release of bearing ring and radial direction of bearing ring. Aiming at demolding requirements of multiple irregularly tapered holes, a parallel integrated core-pulling mechanism and a template-driven concave ring Huff slider core-pulling mechanism are designed. In above references, side core ejection mechanism is mainly designed for multi-directional buckle position on one side of circumference. Now combining multi-directional buckle structure of inner and outer sides of circumference of a plastic collar, core-pulling mechanism and mold structure are designed.

 

Figure 1 Plastic collar structure

Structure of ferrule is shown in Fig. 1 and its size is small. Its maximum external dimensions are ϕ21.8mm*14.2mm, its volume is 1.09cm3, and its average wall thickness is 0.9mm. Plastic structure consists of 4 parts: 360 degree slewing ring body, 36 4-layer outer bosses, 3 sets of inner buckles and 5 ribs. Height of outer bosses of four layers is about 0.3mm, which are aligned along axial direction. 36 bosses of each layer are equal in size and evenly distributed in radial direction. Its center normal direction is perpendicular to outer surface of main ring. Angle is 10 degree; three sets of inner buckles are evenly distributed along circumference of 18.8mm. Material of collar is acrylonitrile-butadiene-styrene copolymer (ABS), shrinkage rate is 0.5%, and density is 1.05 g / cm3.

 

Figure 2 Distribution of lateral core pull

From analysis of structure of plastic parts, it is known that demolding of plastic parts has following difficulties: ① Demolding of 36 4-layer bosses on outer side of plastic parts. Number of bosses is large and convex directions are radial. Multiple sliders can be used to form outer contour of plastic parts, and cores are pulled in different directions simultaneously. Take cross section of center of each layer of boss along axis of collar to obtain 4 groups of inner and outer contours. As shown in Figure 2, suppose that a certain slider forms outer contour of 9 sets of bosses at the same time, its core pulling direction is T1. Within 90 degree, oblique surface of K at uppermost outer set of bosses interferes with slider when core pulling. Similarly, bottom outermost group of bosses also interferes with slider when core pulling, that is, there should be less than 9 groups of bosses demolding in same core pulling direction. If 36 sets of bosses are evenly distributed, it is advisable to pull cores in same direction for every 6 groups; ② 3 sets of 0.52mm depth buckles evenly distributed along circumference of inside of plastic parts, and 3 sets of oblique top blocks are used for core pulling; ③ Consider ring-shaped thin-walled plastic parts, use a uniform pushing block to smoothly demold them, formed plastic parts are not easy to deform.

 

Figure 3 Mold structure

1. Pad plate 2. Movable seat plate 3. Pad block 4. Connecting rod 5. Push rod 6. Support plate 7. Bent pin 8. Slant top block 9. Core insert 10. Push block 11. Side slider 12. Cavity plate inserts 13. Fixed template 14. Hot nozzle 15. Hot runner plate 16. Fixed seat plate 17. Support plate 18. Guide bush 19. Guide column 20. Moving template 21. Reset lever 22. Push rod Fixed plate 23. Push plate 24. Inclined top block fixed plate 25. Push plate
Due to small size of plastic parts, mold adopts a 1-mold 4-cavity layout. Maximum external dimension of mold is 690mm*550mm*534mm, which belongs to a double parting surface injection molds. Mold structure is shown in Figure 3. Its main structural features are as follows: ① Adopt "hot runner+common runner" compound injection scheme, with latent gates for multi-point injection; ②Main molding parts are inlaid; ③outer core pull adopts "slider + bent pin" core pulling mechanism, inner core pull adopts connecting rod type lifter block; ④ Adopting "lifter block + push block" push-out method, which is automatically pushed out by a double-layer push-out mechanism; ⑤ Mold opening sequence and push-out sequence of two parting surfaces are controlled by external pull-mold buckle; ⑥Mold temperature is controlled by water cooling. Multiple cooling water channels are arranged on support plate, fixed template, movable and fixed mold inserts.

 

Figure 4 Latent gate

Design of gating system has a great influence on quality of plastic parts. Combined with structure of plastic parts and demoulding feature , 3 latent gates are evenly opened on inside of plastic parts to be formed, as shown in FIG. 4. Due to complexity of demoulding mechanism and distance between cavities, heat and pressure loss, waste generated by runner should be minimized. Composite feed structure of "hot runner + ordinary runner" is adopted, that is, melt enters diverter through hot nozzle, is injected into cavity through latent gate. Hot runner adopts an X-shaped four-point one-layer hot runner plate and a cylindrical hot nozzle, ordinary runner adopts a semicircular cross-section runner.

 

Figure 5 Core Pulling Mechanism

For outer contour of collar, six sets of bent pin side core-pulling mechanism are designed, as shown in FIG. 5, which are mainly composed of bent pins 7, side sliders 11 and pressure plates. Angle of inclination of bending pin 7 is 15 degree, distance that side slider 11 moves horizontally is 1.3+ (2 ~ 3) mm. Bending pin 7 is fixed on support plate 6 with screws, pressure plates on both sides of slider are fixed on movable mold plate 20 with screws to form a guide groove. When side core is pulled, six bending pins 7 move sliders 11 to move outward synchronously. When parting distance reaches 23mm, bending pin 7 is separated from side sliders 11 and each slider is limited by inner wall of movable mold plate 20.

Three inverted buckles on inside of plastic parts are evenly distributed along circumferential direction at 120 degree, and are formed with three sets of link-type inclined top blocks, as shown in FIG. 6. Depth of buckle is 0.52mm, inclination angle of lifter block is 5 degree. Bottom is connected to connecting rod 4, connecting rod 4 and lifter block fixing plate 24 are connected by a cylindrical pin. Guide hole on oblique top block 8 and core insert 9 is matched with H8 / f8.

 

Figure 6 Launch mechanism

Considering that plastic parts are thin and easy to deform during demolding, a secondary push mechanism is designed. Main push-out element is pusher block 10, which is threadedly connected to four pushers 5, pushers are fixed on pusher fixing plate 22 and pusher plate 23, as shown in Figure 6. Mating side of push block 10 and core insert 9 is designed as a tapered surface with a taper of 8 degree; taper of four outer sides of push block 10 and movable template 20 is 3 degree. Pushing process is divided into two steps: 1st step, pushing block 10 pushes plastic part away from core insert 9, inclined top block 8 synchronously completes inner pull of reversed buckle; 2nd step, inclined top block 8 stops moving and pushing block 10 continues to push plastic parts out of mold.

Mold temperature directly affects appearance quality, dimensional stability, deformation of plastic parts and molding cycle. Two planar circulation water channels with a diameter of ϕ10mm are provided on support plate 17 to cool hot runner. Four fixed-flow water channels with a diameter of ϕ6mm are provided on fixed template 13. In cavity plate insert 12 and core insert 9, plane circulating cooling water channels with a diameter of ϕ6mm are respectively opened. Maximum temperature difference between water inlet and outlet is 0.12 ℃, time for plastic parts to reach push-out temperature does not exceed 9s.

After injection molding is completed, mold is first opened at parting surface Ⅰ under action of die buckle, and parting distance is 25 mm, which is limited by limit nail. Six sets of bending pins 7 fixed on supporting plate 6 drive corresponding side sliders 11 to complete side core pulling at the same time. At this time, due to limiting effect of reset lever 21, push mechanism and plastic parts remain relatively stationary. Fixed platen 13 and movable platen 20 are opened along parting plane II. Ejector plate of injection molding machine pushes push plate 23, push plate 25 moves at the same time under action of die buckle. When pushing distance is 25mm, all three sets of lifter block 8 have completed inner core pulling, and core insert 9 has also been separated from plastic parts. Push plate 25 is blocked by pad 3, lifter block stops moving, push plate 23 continues to push out 10 mm, and push block 10 pushes plastic parts away from mold. After plastic part is automatically demolded, injection molding machine is closed, reset lever 21 drives push mechanism and inclined top block to reset, bending pin 7 drives slider 11 to reset, and mold continues to next injection molding.