Structure of cap of a drinking bottle for infants is shown in Figure 1. Height is about 75 mm. There are 2 handles on outer surface. Maximum distance between 2 handles is 127 mm. Main part of bottle cap is a circular structure with an outer diameter of φ66 mm. Inner surface has 2 turns of internal threads. Internal thread specification is M58 mm. Length of inner cylindrical surface of bottle cap is 14 mm. Length of internal thread is 6 mm. Material of plastic part is ABS. Since ABS material is relatively hard, internal thread of bottle cap must be demoulded in a rotating manner and cannot be forced to be demoulded.

 

Figure 1 Plastic part structure

Mold adopts a 2-cavity, three-plate mold structure. External structure of plastic part is formed by collision of two Huff sliders, and internal threads of plastic part are formed by a threaded core, as shown in Figure 2. Since demoulding distance of Huff slider is small, it is driven by inclined guide post. Inner hole of plastic part is formed using a structure in which fixed mold core and movable mold core collide.

 

Figure 2 Mold structure

Structure of plastic part determines that internal thread must be demoulded first, then two Huff sliders are separated, and gate is set on inner cylindrical surface of plastic part to be formed. Core is divided into two layers: inner and outer. Inner core is provided with runners, gates, push rods and pull rods. Outer core is provided with a structure for internal thread of molded plastic part. Core structure is shown in Figure 3.

 

Figure 3 Core structure

Set up a straight flow channel for each plastic part to be formed, align point gate with parting surface where fixed model core and movable model core collide with each other in inner hole of molded plastic part, then open a flow channel on the end face of movable mold core, material is fed in the form of an ear guard gate. There are two gates on each plastic part to be formed, as shown in Figure 4. This kind of gate has two advantages: ① Gate is relatively hidden and does not affect appearance of molded plastic parts; ② Gate condensate can be automatically trimmed. In order to avoid welding marks on molded plastic parts, both ends of flow channel on the end face of dynamic mold core are slightly extended to act as cold material pockets.

 

Figure 4 Gating system

In order to enable threaded core to rotate automatically, a transmission mechanism consisting of a hydraulic cylinder, a rack and pinion, a driven gear, a threaded nut and a tapered roller bearing is designed, as shown in Figure 5. In order to facilitate adjustment of position of threaded core when assembling mold, a gear adjustment device is provided in threaded nut mechanism. A tapered roller bearing is installed on the bottom surface of driving gear, and the two cores are driven by same driving gear. When thread core rotates, driven by thread nut mechanism, thread core also translates along axis direction at the same time, so that internal thread of molded plastic part is demoulded. In order to extend service life of threaded core and prevent its wear, a copper sleeve is installed on outer cylindrical surface of threaded core. Outer core consists of three parts. Thread structure of molded plastic part is set at the top, driven gear is set in the middle, and thread in thread nut mechanism is set at tail. Nut in thread nut mechanism is fixed on mold plate of mold base.

 

Figure 5 Transmission mechanism
1. Hydraulic cylinder 2. Rack 3. Gear meshing with the rack 4. Copper sleeve 5. Driven gear 6. Nut 7. Inner core 8. Driving gear 9. Gear adjustment device

Since internal thread demoulding mechanism demolds before mold is opened, after internal thread is demoulded, plastic part still adheres to core and needs to be pushed out with a push rod. Six push rods are set on inner core. Structure of push-out mechanism is shown in Figure 6.

 

Figure 6 Launch mechanism

Inner core of mold is a round shaft. In order to achieve better cooling effect, two straight-through cooling water channels are opened on inner core, then water pipe joints are connected through mold plate, as shown in Figure 7(a). There are 2 cavities on Huff slider. In order to ensure same cooling effect of each cavity, 2 independent cooling water channels are designed on each slider. Each cooling water channel consists of multiple straight water paths to form a three-dimensional tortuous water path, as shown in Figure 7(b). Cooling water path of fixed mold plate is simple and will not be described [6].

 

Figure 7 Cooling system

Since mold has a 2-cavity structure, material is fed from ear protector and a three-plate mold base is used. In order to install transmission mechanism consisting of a hydraulic cylinder, a gear rack, a driven gear thread core and a tapered roller bearing on mold, a non-standard mold base is used. On the basis of existing mold base, three additional plates are added between movable mold base and pad, namely gear fixed plate, core fixed plate and pad, as shown in Figure 8.

 

Figure 8 mold base structure
Mold structure is shown in Figure 9. Core of molded plastic part is divided into two layers: inner and outer. When outer core is demoulded, it rotates and translates at the same time. Therefore, a gap must be left between outer core and core fixing plate. Distance is set to 16 mm, and a distance should also be set between driven gears and nuts on two cores.

 

Figure 9 Mold structure
1. Moving mold base plate 2. Push plate 3. Push rod fixed plate 4. Pad 5. Backing plate 6. Core fixed plate 7. Pull rod 8. Gear fixed plate 9. Tapered roller bearing 10. Driving gear 11. Rack 12. Gear meshing with the rack 13. Outer core 14. Copper sleeve 15. Push rod 16. Inner core 17. Copper sleeve 18. Inclined guide post 19. Fixed template 20. Fixed mold insert Parts 21. Stripping plate 22. Fixed mold seat plate 23. Pulling rod 24. Positioning ring 25. Huff slider 26. Moving template 27. Nut 28. Gear adjustment device
After injection is completed, before movable mold and fixed mold are opened, hydraulic cylinder drives outer core 13 to rotate. Under action of threaded core mechanism, outer core 13 rotates and translates while demoulding from molded plastic part. After demoulding action of dethreading transmission mechanism stops, mold begins to open. Mold opening process is divided into two steps: the first step is to separate stripper plate 21 from fixed mold plate 19 to take out gate condensate; the second step is to separate movable mold plate 26 from fixed template 19. During second mold opening process, inclined guide post 18 drives Huff slider 25 to perform demoulding movement. When slider of injection molding machine stops movement of driven mold, ejector pin of injection molding machine drives ejection mechanism of mold (push plate 2, push rod fixed plate 3, pull rod 7 and push rod 15) to push out molded plastic part. After taking out plastic part, slider of injection molding machine drives movable mold part of mold to reset. Reset process is opposite to mold opening process. After mold is completely reset, next plastic part can be produced.