As a common fastening structure, screw thread has a wide range of applications in various fields. Because of special shape of screw thread, mold demoulding mode is different from that of ordinary mold. If mass of thread-forming mold is small in actual production, manual thread removal is generally used, active thread core or ring is used to remove mold together with plastic part after mold opening, and mold is manually or mechanically released; if it is mass-produced, it adopts automatic unthreading structure, which is convenient to improve production efficiency. There are many types of automatic unthreading structure, and the most widely used is rotary automatic thread removal structure, which has high thread removal efficiency and good thread quality. Principle is to drive transmission mechanism under a certain type of power, to make core or ring and plastic part relatively rotate to demold thread. In rotary thread removal structure, hydraulic cylinder rack unthreading and hydraulic motor unthreading are relatively common structures.

 

Figure 1 Protective cover

Plastic part shown in Figure 1 is a protective cover with a closed top, material is PBT, thread depth is about 1.5mm, height of thread part is about 20mm. It can be seen from Figure 1 that plastic parts have deeper threads and more thread turns, which is not suitable for forced demoulding. Thread precision of plastic parts is not high, but production efficiency is high. It is not suitable to use common rotary thread removal structure. Inspired by internal core pulling structure of lifter block, a 360° internal core pulling structure is designed to complete thread removal mold.

Since thread is 360°, sliding inner core core should also be 360°, which provides difficulties for space design of inner core pulling action. In addition, top of plastic part is closed, position of core on the top of molded plastic part also sets an obstacle for inner core pulling action.

Because outer part of internal core pulling structure is all threaded, top of plastic part is closed, and structure shown in FIG. 2 is designed.

 

Figure 2 Internal core pulling structure

1. Six-way dovetail oblique rail 2. Oblique core-pulling block 3. Oblique core-pulling block 4. Oblique core-pulling block 5. Oblique core-pulling block 6. Oblique core-pulling block 7. Oblique core-pulling block 8. Base

Six-direction dovetail inclined guide rail and base are fastened with screws. When mold is opened, oblique core pulling block moves upward. Six-directional dovetail oblique guide rail moves relatively downward to produce same movement effect as oblique top block core pulling. When mold starts to move, six oblique core pulling block move inward simultaneously. In order to ensure that 6 cores do not interfere with each other, moving speed of oblique core pulling block 3, 5, 7 must be faster than that of oblique core blocks 2, 4, 6, so as to generate a contracted space. Take oblique core pulling block 2, 3, 4 for motion analysis, as shown in Figure 3.

 

Figure 3 Movement analysis of oblique core pulling block

It can be seen from Fig. 3 that angle between S2 and S4 is 120°. Due to simultaneous movement of S2, S3 and S4, speed component of S3 in the direction of S2 must be greater than S2, so movement of S2 and S3 will not interfere. Decompose motion of S3, as shown in Figure 4.

 

Figure 4 S3 speed decomposition

It can be seen from FIG. 4 that component S3’ of S3 in direction of S2 must be greater than S2 to ensure that mutual movement during diagonal core pulling does not interfere. From trigonometric relationship, it can be concluded that speed of S3 must be faster than 2 times of S2, so inclination angle of inclined guide of S3 must be greater than 2 times of S2. Inclination angles of S3 and S2 inclined guide rails are respectively taken as 17° and 8° to ensure that they do not interfere with each other during movement.

Since lower part of core forms a thread, lower part must be movable. In this way, design space of movable core on the top of molded plastic part is limited, as shown in FIG. 5.

 

Figure 5 Top core of molded plastic parts

1. Top core of molded plastic 2. Oblique core pulling block 3. Oblique core pulling block
Due to upward movement of oblique core pulling block when the thread is removed, top core of molded plastic part needs to move upward simultaneously with oblique core pulling block, and top core of molded plastic part requires a six-direction dovetail inclined rail and a base to fix it. Top core of molded plastic part passes through six-direction dovetail inclined guide rail and base. Six-direction dovetail inclined guide rail and base only provide positioning in X direction and Y direction.

From above analysis, it can be seen that after core is pulled inside thread, mold action is not completed, and secondary ejection mechanism needs to continue to move to eject plastic part. Top core of molded plastic part needs to move synchronously with oblique core pulling block, so a synchronous ejection mechanism is needed. Movable mold structure is shown in Figure 6.

 

Figure 6 Movable mold structure

1. Top core of molded plastic 2. Oblique core block assembly 3. Fixing plate 4. Push plate 5. Support plate 6. Push plate 7. Push rod 8. Hook 9. Control block 10. Stop block 11. Push plate 12. Push plate assembly 13. Push rod 14. Push rod 15. Push plate

Top core 1 of molded plastic part is fixed on push plate 6 through oblique core-pulling block assembly 2. Push plates 4 and 6 are connected to push plate 11 through push rod 13, and push plate 15 is connected to push plate assembly 12 through push rod 14. Push plate assembly 12 and push plate 11 are fixed to each other by hook 8 and stopper 10, push rod 7 causes ejection force of injection molding machine to act on push plate assembly 12. When mold opening and ejection starts, push plate 11 and push plate assembly 12 move forward simultaneously, fixed plate 3, push plate 4 and push plate 15 move forward simultaneously. Plastic parts follow oblique core block assembly 2 and push plate 15 to move forward. While moving forward, because six-direction dovetail inclined oblique rail is fixed on support plate 5 and moves downward relative to oblique core pulling block assembly 2, pull oblique core block assembly 2 to move inward to release thread. When thread is released, push plate 15 continues to move forward. After push block 10 hits control block 9, it retracts inward, hook 8 falls off, push plate 11 loses power, and no longer moves forward, top core 1 and oblique core pulling block assembly 2 of molded plastic part also stop moving. At this time, push plate assembly 12 pushes push plate 15 through push rod 14 to eject plastic part to complete demolding action, thus mold demolding action is completed.