Figure 1 shows a plastic cover with internal threads. Material is ABS, internal thread specification is M54*2mm, and length is 7mm. There are 12 grooves evenly distributed on outer surface of plastic part. When thread core rotates, these grooves are buried in push plate to prevent plastic part from rotating with thread core.

 

Figure 1 Internal thread plastic cap

Since plastic part is a tubular structure with a through hole in the middle, parting surface of plastic part is divided into an inner parting surface and an outer parting surface. Inner parting surface is located at the step of inner surface, and outer parting surface is located at maximum contour of plastic part. Fixed mold core and moving mold core adopt a collision-through structure, fixed mold core adopts structure of plastic part to be molded, moving mold core adopts an insert form, and internal thread of plastic part is located on moving mold core. In order to make groove on outer surface of plastic part come out of push plate during demolding, taper direction of outer surface of plastic part must be opposite to taper of inner surface. Parting structure of plastic part is shown in Figure 2.
Plastic part has a simple structure, no buckles and ribs, and is suitable for production using two-plate molds. It adopts a single-point latent gate feeding, as shown in Figure 3.

 

Figure 2 Parting structure
1. Moving mold core 2. Push plate 3. Plastic part 4. Fixed mold plate

 

Figure 3 Latent gate
1. Moving mold plate 2. Plastic part 3. Gate sleeve 4. Fixed mold plate

Plastic part has an outer dimension of φ62mm*14mm, and mold is arranged in a rectangular array with a 4-cavity layout. A hydraulic motor is installed next to mold frame, as shown in Figure 4. This arrangement can not only ensure that injection pressure of each plastic part to be molded is same, but also balance force of gear when main gear drives 4 threaded cores to rotate. Internal thread of plastic part is screwed out of push plate by rotation of threaded core, so demolding can be achieved without use of a push rod and a push rod fixing plate in mold. Mold structure is shown in Figure 5.

 

Figure 4 Mold cavity layout
1. Plastic part 2. Core gear 3. Limit screw 4. Main gear 5. Hydraulic motor

 

Figure 5 Mold structure
1. Moving mold base plate 2. Pad 3. Gear pressure plate 4. Core gear 5. Main gear 6. Deep groove roller bearing 7. Moving mold plate 8. Spring 9. Limit screw 10. Threaded core 11. Plastic part 12. Fixed mold base plate 13. Gate sleeve 14. Positioning ring 15. Fixed mold plate 16. Fixed mold insert 17. Guide sleeve 18. Guide column 19. Push plate 20. Tapered roller bearing 21. Hydraulic motor 22. Motor bracket 23. Hydraulic sprocket 24. Chain 25. Driven sprocket 26. Deep groove roller bearing guide column 27. Main gear shaft

Mold transmission mechanism is shown in Figure 6. A hydraulic motor is installed on the side of mold, then main gear in the middle is driven to rotate through chain, and main gear drives four core gears to rotate at the same time, finally four core gears drive four threaded cores to rotate. There are grooves on outer surface of plastic part to prevent plastic part from rotating with threaded core. When threaded core rotates, plastic part can be pushed out of push plate.

 

Figure 6 Transmission mechanism
1. Hydraulic motor gear 2. Moving plate 3. Core gear 4. Main gear

(1) After injection is completed, mold is separated between fixed mold plate and push plate (at P1), completing first mold opening, as shown in Figure 7 (a). Since plastic part is held by threaded core and remains on push plate, push plate is held by plastic part and cannot be separated from movable mold plate.
(2) After fixed mold and movable mold are separated, hydraulic motor drives main gear to rotate through chain, and main gear drives four threaded cores to rotate. Since there are multiple grooves on outer surface of plastic part, plastic part can be prevented from rotating with threaded core. Threaded core rotates out of internal thread of plastic part and pushes plastic part out of push plate. At this time, under action of spring, push plate is separated from movable plate (at P2). Under action of limit screw, push plate stops moving after being separated from movable plate by 5 mm, completing second mold opening, as shown in Figure 7 (b).
(3) When plastic part is unscrewed from threaded core, force applied by threaded core to internal thread of plastic part disappears, and plastic part will automatically pop out of mold, as shown in Figure 7 (c).
(4) Travel switch sends a signal and hydraulic motor stops rotating. After plastic part is taken out, movable and fixed platens and push plate are reset under action of injection molding machine slider, then next injection cycle begins.

 

Distance between push plate and moving plate should be shorter than length of thread, otherwise there are two disadvantages: ① When thread core finishes rotating, force applied by spring to push plate will be concentrated on the end of thread, which is easy to cause damage to thread at the end and a high scrap rate; ② Even if thread is not damaged, plastic part will remain in push plate after thread core is separated from plastic part, and it is difficult for operator to remove plastic part from push plate.
Pitch of internal thread of plastic part is 2mm, and thread length is 7mm. A limit screw is set on push plate. Under action of limit screw, push plate stops separating from moving plate after moving 5mm. At this time, there is still a thread of 1 pitch in core. When thread core continues to rotate, elastic force of spring borne by limit screw is no longer borne by internal thread of plastic part. When thread core unscrews the last thread, thread is in a free unscrewing state and will not damage thread.