Plastic part structure is shown in Figure 1. Size is φ71 mm*47 mm, with a through hole in the middle. There is an internal thread at one end of plastic part. Model of internal thread is M64*3 mm, and its length is 18 mm. There are multiple protrusions on outer surface of plastic part, which can prevent plastic part from rotating with thread core when it rotates. Plastic part material is ABS, which has a high hardness. Internal thread cannot be demolded by forced demolding. Plastic part must be rotated out of cavity by rotating thread core.

 

Minimum diameter of inner hole of plastic part is located at small port, so inner hole of plastic part can be directly formed by thread core and fixed mold. Outer cylindrical surface of plastic part has a large size in the middle and small sizes at both ends. Therefore, parting surface is designed at maximum diameter of plastic part. Mold molding structure of plastic part is shown in Figure 2.

 

Size of plastic part is small. According to structure of plastic part, it is suitable to adopt a driving method in which one gear drives four threaded cores to rotate simultaneously. Therefore, mold is designed as a 1-mold 4-cavity layout. Considering that injection pressure of four cavities and force of gear transmission mechanism must meet uniformity requirements, cavity layout adopted is arranged in a square, distance between four plastic parts to be molded and center of mold is equal. Mold layout is shown in Figure 3.

 

According to requirements of plastic part structure and appearance surface allowing for gate marks, a two-plate mold structure and a fan-shaped gate are used to feed from outer cylindrical surface of plastic part to be molded. Influence of gate on plastic part can be ignored. In order to meet requirements of shortening length of runner and making injection pressure of each cavity uniform, runner used is "*" shaped.

Due to large production order volume of plastic part, threaded core of mold cannot be demoulded by manual rotation, and automatic rotation demoulding must be adopted. According to mold layout and structure, a demoulding mechanism is designed in which threaded core is driven by an inclined guide column to rotate. It is mainly composed of an inclined guide column, a rack, a driving gear and a driven gear. Driven gear is installed on threaded core and connected with a flat key. Driving gear is placed in the middle of four driven gears, and four core gears are driven to rotate at the same time. In order to prevent rack from interfering with threaded core, a transition gear is set between rack and driving gear. Automatic demoulding mechanism is shown in Figure 4.

 

Molded plastic part is ejected from mold by rotating threaded core. There is no need to use a push rod. Mold eliminates push rod, push rod fixing plate, push plate, reset rod and reset spring. Instead, a gear rack, gear transmission and other mechanisms are installed in mold. Rack is driven by inclined guide column to move, and active gear is driven by rack to rotate, then threaded core is driven by active gear to rotate.
In order to make threaded core rotate normally, thrust ball bearings and tapered roller bearings are installed on threaded core. In order to prevent plastic part from interfering with fixed mold when opening mold, mold is set with a delayed mold opening function, that is, inclined guide column starts to drive rack to move after fixed mold and movable mold are separated by a certain distance. In order to make rack run smoothly, a rack fixing plate is set on mold to prevent gear from being misaligned. Mold structure is shown in Figure 5.

 

1. Moving mold base plate 2. Pad 3. Gear shaft 4. Core gear 5. Thrust ball bearing 6. Moving mold plate 7. Tapered roller bearing 8. Guide column 9. Threaded core 10. Plastic part 11. Guide sleeve 12. Fixed mold base plate 13. Gate sleeve 14. Positioning ring 15. Fixed mold plate 16. Fixed mold insert 17. Moving mold insert 18. Inclined guide column 19. Wedge block 20. Flat key 21. Driving gear 22. Rack 23. Rack fixing plate
Figure 5 Mold structure

After injection is completed, mould is separated at fixed mould plate 15 and movable mould plate 6. Since movable mould part has threads, plastic part remains on threaded core 9 of movable mould. When movable and fixed moulds are separated for a distance, inclined guide pin 18 drives rack 22 to move horizontally, rack drives driving gear 21 to rotate, driving gear drives core gear 4 to rotate, and core gear drives threaded core 9 to rotate. Since there are multiple protrusions on outer cylindrical surface of plastic part 10 to prevent plastic part from rotating with threaded core, threaded core 9 will remove plastic part 10 from cavity of movable mould insert 17 when rotating. After plastic part is taken out, ejector of injection molding machine pushes movable mould and fixed mould to reset. During reset process, inclined guide pin 18 and wedge block 19 push rack to reset. When mould is completely reset, next plastic part can be injection moulded.
Following points should be noted during mold production: ① In order to balance injection of four cavities, active gear drives four threaded cores to rotate at the same time, and four plastic parts to be molded are arranged in a square; ② Inclined guide pin and rack must be equipped with a delayed mold opening structure. When opening mold, inclined guide pin cannot immediately drive rack to move. Otherwise, plastic parts will be screwed out without movable mold and fixed mold being completely separated, interference between plastic parts and fixed mold will easily damage plastic parts; ③ In order to make threaded core rotate normally, two different types of bearings should be installed on each threaded core. One is a tapered roller bearing, which bears axial pressure of threaded core to prevent threaded core from moving backwards during rotation; the other is a thrust ball, which can reduce friction between threaded core and other mold parts during rotation.