Figure 1 shows a sliding part in an electronic product, which is composed of an iron part and a plastic part. TIron part is made of stainless steel W304 to support and connect other parts; plastic part is made of polyoxymethylene POM 500P. It is main part of inner chute, which acts as a sliding guide rail, allowing parts to slide along guide rail to realize reciprocating movement of parts. The overall size of part is about 23 mm*5 mm*2 mm, and inner chute is high polished SPI A3. Effect of high polishing makes surface of contact surface smooth and reduces frictional resistance, so that part slides smoothly.

 

Figure 1 Part structure
Material of plastic parts is POM 500P, which has excellent self-lubricating properties, stability, and reliable manufacturability, especially dimensional stability, which can realize stability of dimensional fluctuations of plastic parts at different working temperatures, and ensure consistency of performance of parts when sliding. POM 500P injection process parameters are shown in Table 1.
Table 1 POM 500P injection process parameters

Layout of the overall structure of mold should be planned according to type of plastic parts to be formed and molding process. It should not only meet requirements of production process, but also have a compact layout to save manufacturing costs. Mold is an insert-type injection mold. When mold is closed and injected, iron parts must be placed first before injection. In order to facilitate placement of iron parts, this type of mold generally adopts layout of translation (meaning that layout of plastic parts is arranged next to each other), which is conducive to operator to conveniently place iron parts on fixed plate, manipulator will grab them and put them into mold.
Side of inner groove of plastic part is a functional area, and gates cannot be set in these areas. For this reason, side gates are installed on the outside of plastic part to be molded. Mold has a structure of 2 cavities, as shown in Figure 2. A hot runner gate is converted to a common runner to pour 2 cavities to ensure balance of pouring and easy maintenance. For injection molds with iron parts as inserts, mold parts must always match with iron parts, mold parts are easy to wear and tear, to avoid designing multiple cavities on one template. If a cavity wall is damaged, it will not be scrapped together, which can avoid greater losses.

 

Figure 2 Core and cavity structure

In order to realize sliding function, plastic part has a concave C-shaped groove in structure, so that parts in it can move back and forth. This groove presents an undercut feature when mold is formed, cannot be demoulded directly, and a special demoulding mechanism needs to be designed to achieve demoulding. After analyzing and judging way of undercut demoulding, core-pulling demoulding system in T-shaped slot is used to realize demoulding of C-shaped slot. Core-pulling structure in mold is shown in Figure 3.

 

Figure 3 Core-pulling structure in T-slot
By analyzing undercut direction and undercut amount of inner concave C groove of plastic part, undercut direction is parallel to horizontal plane, and undercut amount is about 1.3 mm. T-shaped oblique inner-drawing core-pulling can be used. Its structure and force analysis are shown in Figure 3. Principle is to realize movement through conversion of force. When T-shaped block moves up and down, conversion of inclined surface force of T-shaped groove drives slider to move left and right to realize demoulding of concave C-groove undercut. Each conversion stroke satisfies following formula [4].
Lw=Lc×tanß (1)
Lk=Lw×sinß (2)
In formula: ß——T-slot oblique core-pulling angle, (°); Lw——T-slot horizontal lateral movement, mm; Lc——T-slot oblique core-pulling axial movement, mm; Lk— —Mould-opening core-pulling stroke of T-slot core-pulling, mm.
It can be known from formulas (1) and (2) that when ß angle decreases, Lw and Lk also decrease, lateral force of oblique core-pulling of T-slot will also decrease, and lateral movement of Lw will also decrease. Although reduction of ß angle affects amount of lateral movement, its lateral force also becomes smaller, which is conducive to stability of movement of mold; on the contrary, if ß angle increases, Lw and Lk also increase, lateral movement of Lw also increases. Although efficiency is improved, lateral force of T-slot oblique core-pulling increases, which will affect movement of mold parts and cause sticking.
Structure adopts ß=10°, and stroke of T-slot core pulling is 15 mm. Through formula (1), it can be obtained that Lw=15×tan10°=2.376 mm, which is 1.5 mm larger than undercut of plastic part, and safety margin is 2.376 -1.5=0.876 mm, which meets demoulding requirements, so structural design is reasonable and reliable.

Mold structure is shown in Figure 4.

 

Figure 4 Die structure
1. Upper insulation plate 2. Fixed mold seat plate 3. Hot runner plate 4. Cavity plate 5. Positioning insert 6. Shoulder bushing 7. Screw 8. Distance tie rod 9. Contour screw 10. Core Fixed plate 11. Guide column 12. Core support plate 13. Push plate 14. Pad block 15. Push rod fixed plate 16. Guide sleeve 17. Moving mold seat plate 18. Lower heat insulation plate 19. Screw 20. Ejector rod 21 .Push rod 22. Support column 23. Screw 24. Core 25. Distance tie rod 26. Cavity plate insert 27. Slide slot insert 28. Hot runner 29. Positioning ring 30. Screw 31. Hot runner guide protection rod 32. Guide sleeve 33. Screw 34. Precision block 35. Counter 36. Dustproof plate 37. Hanging mold lock block
(1) Cavity part. Upper heat shield 1 is installed on fixed mold seat plate 2, which blocks heat loss between mold and panel of injection molding machine, plays a role in maintaining mold temperature. Hot runner 28 is fixed in hot runner plate 3 , fixed mold seat plate 2 presses hot runner 28 and stabilizes it in hot runner plate 3. Hot runner guide protection rod 31 is also fixed in hot runner plate 3, its two ends respectively guide and position fixed mold seat plate 2 and cavity plate 4 to play a positioning role. At the same time, hot runner guide protection rod 31 is longer than nozzle of hot runner plate 3 , play the role of protecting hot nozzle, so that when cavity plate insert 26 and cavity plate 4 are installed on hot runner plate 3, hot runner guide protection rod 31 is first introduced to cooperate with cavity plate 4 to protect heat flow runner 28 was not damaged. Positioning insert 5 and chute insert 27 are installed and fixed on cavity plate 4 , while nozzles of hot runner 28 are installed in cavity plate 4 .
(2) Core part. The lower insulation board 18 is installed on movable mold seat plate 17, and is used for blocking heat loss between mold and injection molding machine panel and plays a role in maintaining mold temperature. Screw 23 connects movable mold seat plate 17, spacer 14 and core support plate 12 to become a whole, and leaves space for ejection motion of push plate 13. Guide post 11 is fixedly installed on core support plate 12, cooperates with core fixed plate 10 and cavity plate 4 to guide. Core 24 is installed and fixed on core fixing plate 10 through screw 7, core is main part of moving mold side of molded plastic part, distance tie rod 25 is connected to core support plate 12. There is a space between distance tie rod 25 and core fixing plate 10, which is travel space for movement of core fixed plate 10, core 24 utilizes this space to complete core-pulling action, and also completes demoulding of inner chute.

(1) Mold is closed for injection. After molded plastic part is cooled, mold is opened. Movable mold part of mold moves backward. Main parting surface of mold is opened first, so that plastic part and runner condensate leave fixed mold and stay on core side .
(2) Moving mold continues to move backward. When shoulder bush 6 on movable mold side touches bottom side of waist hole of distance tie rod 8 when it moves a certain distance, distance tie rod 8 is in place. At this time, fixed distance pull rod 8 drives core fixed plate 10 to move, there is a core-pulling mechanism in core fixed plate 10. Core-pulling mechanism provides driving force to complete core-pulling when core fixed plate 10 moves. After core fixed plate 10 hits fixed distance screw rod 25, movement of core fixing plate 10 ends, core pulling action is completed, and inner core pulling is decomposed by force on T-shaped groove slope to make chute insert 27 complete demoulding of inner chute, so that when molded plastic part is released, it will not be restricted by undercut of inner chute and cause demoulding.
(3) Ejector rod is installed at rear of injection molding machine, which drives ejector rod 20 to drive push plate 13 to move upwards. At this time, push rod 21 pushes out runner condensate, and molded plastic parts are pushed out by other push rods. When mold is pushing out, manipulator starts to move synchronously, enters mold to suck molded plastic part and clamps runner condensate to take out together, then manipulator moves up to place iron part on positioning insert 5 of cavity insert 26, then leaves mold, places molded plastic parts on conveyor belt, and runner condensate is placed in collection bucket.
(4) After molded plastic parts and runner aggregate are taken away by manipulator, push rod 21 and push plate 13 are pulled back by rear ejector pin of injection molding machine. After reset, mold starts to close mold and inject, and starts next cycle.