Complex structure of molded plastic part needs to design a special demoulding mechanism. Requirements of these demoulding mechanisms are to not only make molded plastic part demould smoothly, but also make demoulding action reliable, and mold structure should be simplified as much as possible. When determining parting surface of a molded plastic part, parting surface that can simplify mold structure and allow plastic part to be demolded should be selected first. For tube position structure of three different demoulding directions on breast shield shown in Figure 1, one of them is an internal thread cap, bottom of internal thread cap has a round tube and a small hole, and its demoulding structure complex. In order to simplify mold structure for forming plastic part, selected main parting surface is located in the middle of three pipe positions on plastic part, and all three pipe positions are demoulded by side core pulling mechanism.

 

Figure 1 Plastic structure

Breast shield is shown in Figure 1. Outer dimensions of plastic part are 97 mm * 113 mm * 56 mm, wall thickness is 2.0 mm, and material is modified PP material. There is 1 trumpet cover, 1 flat barrel (38 mm*29 mm*56 mm) and 1 internal thread cover on plastic part, internal thread size is M55 mm*22 mm, and there is a section of round tube at the bottom of internal thread cover (φ25 mm*14 mm), there is a small hole with a diameter of φ3 mm next to round tube. Difficulty in designing mold for this plastic part is demoulding of inner threaded cap, round tube at the bottom and small hole next to round tube.

Parting surface selected for plastic part is shown in Figure 2. Main parting surface is a plane, located in the middle of three pipe positions on plastic part, and gating point is located on main parting surface; three pipe positions adopt side core pulling, in which parting surface of flat barrel and inner screw mosquito cover is a stretched surface created along middle of glue plane, with a slope of 5°; parting surface of horn cover is created along middle of side wall glue plane vertical plane. On parting surface of the entire mold, there are no structures such as sharp corners and thin materials that affect service life of mold. In order to increase clamping accuracy of movable and fixed molds, positioning blocks are designed at four corners of main parting surface. In order to reduce wear of core of internal thread cap, sealing width of parting surface of internal thread cap is 20 mm. Set clamping clearance at the rest of parting surface.

 

Figure 2 parting surface design

Mold flow analysis is carried out according to one gating point shown in Figure 2, maximum injection pressure is set to 100 MPa, filling time of melt is set to 3 s, melt temperature is set to 180-210 ℃, and mold temperature is set to 60 ℃, results of mold flow analysis are shown in Figure 3. It can be seen from Figure 3 that time required for filling is about 3.686 s, maximum pressure at gate is 27.985 MPa, there are weld lines on plastic part, trapped air is mainly concentrated at inner thread and edge of plastic part.

 

Figure 3 Mold flow analysis
For injection defect shown by mold flow analysis, it can be solved by adjusting injection process or modifying mold structure. Weld line defect can be solved by adjusting mold temperature to 80 ℃ to improve fluidity of melt; for trapped air, change threaded core to an insert structure to make it easier for gas to be discharged from cavity and reduce trapped air.

In addition to internal thread on inner wall of plastic part, there is a round tube at the bottom, and a small hole beside round tube. Core-pulling structure is shown in Figure 4. In order to prevent injection of round tube at the bottom of internal thread cap from being insufficiently filled, and to facilitate modification of mold, core of round tube is designed as an insert structure; core of internal thread needs to be rotated to be demolded, and core of small hole at the bottom of internal thread cap cannot be rotated, so core of internal thread cap is divided into 3 parts: round tube core (located in the middle), threaded core (located on periphery), and core corresponding to small hole. The three cores are demolded in different ways, circular tube core and core corresponding to small hole are demoulded in a linear motion driven by piston rod of hydraulic cylinder, and threaded core is demoulded in a rotary motion driven by a hydraulic motor.

 

Figure 4 Core-pulling structure of internal thread cap
In order to make molded plastic parts can be demolded smoothly, a combined slider mechanism is designed, which is mainly composed of hydraulic motor, hydraulic cylinder, driving gear, round tube core, core corresponding to small hole, thread core, bearing, chute, hydraulic pressure cylinder fixed plate and slider seat, etc., internal structure of combined slider is shown in Figure 5(a), and the overall structure is shown in Figure 5(b).

 

Figure 5 Internal thread cap combination slider mechanism
Demoulding process of combined slider: driven gear and screw are arranged on threaded core. Driven gear, screw rod and threaded core are integrated, and nut is set on slider seat, pitch of screw nut is same as that of internal thread cover. When fixed mold is separated from movable mold, piston rod of hydraulic cylinder shrinks, driving the entire combined slider to perform demoulding movement, so that round tube core and small hole core are demoulded. When piston rod of hydraulic cylinder stops shrinking, hydraulic motor drives threaded core to rotate through gear. Under action of screw nut mechanism, threaded core rotates while doing translational movement, so that threaded core is demoulded; then piston rod of hydraulic cylinder drives combined slider to do a translational movement, so that round tube core and small hole core are demolded.

Flat barrel of plastic part has a large volume, and oblique guide column + slider mechanism is used for demoulding. In order to prevent vacuum from forming between plastic part and core when flat barrel is pulled out of mold, a side push rod is installed inside side core, as shown in Figure 6.

 

Figure 6 Flat barrel demoulding mechanism
1.Moving mold insert 2.Plastic part 3.Fixed mold insert 4.Side push rod 5.Side drawing core 6.Spring 7.Press plate 8.Moving platen 9.Wedge 10.Moving platen
Working process of demoulding mechanism of flat barrel: there is a section of vertical surface on the side of wedge 9 with a height of 8 mm. When mold was fully closed, side push rod 4 was positioned under action of spring 6 and vertical surface of oblique wedge 9, and now end face of side push rod 4 was flush with end face of side pumping core 5. When distance between movable and fixed molds is 0-8 mm, side-drawn core 5 moves sideways under action of inclined guide column, but side push rod 4 cannot move sideways under action of vertical side of inclined wedge 9. At the same time, side push rod 4 stretches out from side-drawn core 5 to separate plastic part from side-drawn core, and at the same time, air enters cavity from around side push rod 4 to prevent vacuum in cavity. When distance between movable and fixed molds is greater than 8 mm, contact surface between side push rod 4 and wedge 9 is an inclined plane, and under elastic force of spring 6, side push rod 4 moves laterally with side-drawn core 5. When mold is closed, movement process of flat barrel demoulding mechanism is opposite.

Mold is a one-mold, one-cavity, two-plate mold structure, as shown in Figure 7, using a common runner for pouring, specifically main runner + runner + circular gate, and using a single gate for gating.

 

Figure 7 Mold structure
(a) Sectional structure of core-pulling along internal thread cover (b) Sectional structure of core-pulling along horn cover and flat barrel
1. Moving mold base plate 2. Push plate 3. Push rod fixing plate 4. Internal thread cover slider seat 5. Bearing 6. Chute seat 7. Internal thread cover slider seat 8. Gear 9. Hydraulic motor 10. Hydraulic pressure Cylinder 11. Hydraulic cylinder fixing plate 12. Side core pulling fixing plate 13. Nut 14. Thread core 15. Intermediate core 16. Round tube core 17. Sprue sleeve 18. Fixed mold insert 19. Plastic part 20. Movable mold insert 21. Push rod 22. Pull rod 23. Inclined guide column 24. Inclined wedge 25. Horn cover slider 26. Fixed template 27. Fixed mold seat plate 28. Flat barrel slider 29. Spring 30. Side Push rod 31. Slanted wedge 32. Slanted guide post
Mold working process: After injection is completed, movable mold and fixed mold are separated at main parting surface PL, inclined guide post 23 drives horn cover slider 25 to realize demoulding of horn cover, and inclined guide post 32 drives flat barrel slider 28 to realize demoulding of flat barrel, and after movable mold stops moving, combined slide block realizes demoulding of internal thread cap. After combined slide block is demolded, push rod 21 pushes plastic part 19 out of mold cavity, and then mold parts start to reset. Reset movement process is opposite to mold opening process. After mold is completely closed, next injection cycle begins.