Figure 1 Oxygen switch housing

Figure 1 shows emergency oxygen switch shell of an ambulance. Plastic part is composed of three parts. Main body is a cylindrical plastic part with a U-shaped opening in the middle. Upper and left ends are respectively semicircular shells. Total height of plastic part is 187mm. The largest fragment of cylindrical shell is ϕ76mm, and lower end of upper semicircular shell of plastic part is provided with a symmetrical concave on both sides. Plastic part is injection-molded. Difficulty in design of mold structure is demolding of two undercuts on inner wall of cylindrical shell of plastic part. Undercut has a width of 12mm, a depth of 3mm, and a height of 8mm. Design difficulty of demoulding mechanism with two undercuts is: if lifter mechanism is used to eject side core, head of lifter block is likely to interfere; if a relatively small lifter ejection angle and upper ejection distance are set, it exceeds total height of enlarged mold base and increases mold manufacturing costs.

 

Figure 2 Analysis of forming scheme

Reasonable gating system settings can effectively ensure filling effect and quality of molded plastic parts. Parting design based on layout of gating system and demolding mechanism is an important way to reduce mold manufacturing costs. According to appearance requirements of plastic part, gate position of plastic part is designed at G0 position shown in Figure 2. Use of latent gate can ensure automatic separation of runner aggregate and plastic part. Combined with demoulding of 2 undercut features on inner wall of cylindrical shell of plastic part, after analyzing structure and size of undercut, lifter mechanism is not used for demolding. Instead, PL1 and PL2 as shown in Figure 2 are used to separate molded parts of two undercuts from inner wall core of cylindrical shell. Therefore, plastic part to be formed adopts a horizontal layout in mold. As shown in Figure 2, parting surface is PL surface, so that horizontal space of mold can be used without increasing design height of mold.

 

Figure 3 Mold structure

1. Seal ring 2. Water nozzle 3. Guide sleeve 4. Guide column 5. Pull rod 6. Feed ejector rod 7. Push rod 8. First limit pull rod 9. First spring 10. First spring pin 11 .Spring 12. Screw 13. Slider seat 14. Locking block 15. Slider 16. Oblique guide column 17. Second spring pin 18. Second spring 19. Advancing drive core 20. Second limit lever 21. First inward core 22. Second inward core 23. Sprue sleeve 24. Positioning ring 25. Fixed mold seat plate 26. Cavity plate insert 27. Core insert 28. Fixed template 29. Moving Template 30. Cushion 31. Push rod fixed plate 32. Push plate 33. Movable mold seat plate 34. Support column 35. Reset rod 36. Spring
According to demoulding requirements of plastic part and setting of gating system, mold structure is shown in Figure 3, layout is carried out in the form of 2 cavities. Mold base uses LKM standard mold base LKMCI5060 two-plate mold base.

It is planned to use a latent gate for gating. Sprue sleeve 23 is directly installed on fixed mold seat plate 25. Runner diameter is ϕ8mm, it is designed as an S-shaped runner to ensure that material flow can be carried out at designated latent gate position. To ensure that runner aggregate stays in movable mold after mold is opened, a pull rod 5 is arranged at the end of runner, and a submerged gate is provided on feed ejector rod 6.

Two water channels are used to cool cavity plate insert 26 and core insert 27 respectively. Cavity plate insert 26 is cooled by two ϕ8mm water circuits C1 and C2, core insert 27 is cooled by two ϕ8mm water circuits C3 and C4 (presumption 3).

For two undercuts on inner wall of cylindrical shell, retractable core-pulling slider mechanism assembly includes pieces 8 to 22. First retractable core 21 and second retracted core 22 are formed parts of inner wall with 2 undercuts, which are respectively provided with corresponding T-shaped grooves. Corresponding T-shaped grooves are installed on retraction drive core 19 through T-shaped grooves to be slidingly fitted with it. Tail ends of first retraction core 21 and second retraction core 22 are respectively installed with a first limit rod 8, a second limit rod 20, and a first limit rod 8. Second limit rod 20 are driven by first spring 9 and second spring 18 respectively. Driving core 19 is fastened and installed on front end of slider 15 with screws. Slider 15 is guided by a guide bar and installed on movable mold plate 29. Its auxiliary drive and limit positioning are performed by spring 11, and core pulling limiting is limited by screw 12. Working principle of mechanism is: when mold is opened at PL surface, inclined guide column 16 drives slider 15 to extend core pulling. When slider 15 is pulled out, it drives retractable drive core 19 to pull core synchronously. At this time. Under support of first spring 9, second spring 18 and driven by T-slot of retracted drive core 19, first retracted core 21 and second retracted core 22 do not move in core pulling direction of slider 15, and performs an adduction lateral core-pulling movement toward center of cylindrical shell of plastic part, firstly complete 2 inverted lateral core-pulling and demolding. When two inverted core-pulling and demolding are completed, retracted drive core 19 meets first limit rod 8, screw head at right end of second limit rod 20 drives two screws to pull core together with slider 15, thereby driving first retraction core 21 and second retraction core 22 to pull core. Slider 15 moves until driving stroke of inclined guide column 16 ends, slider 15 is blocked by limit screw 12, and is stopped by spring against limit, so as to realize core pulling and demolding of cylindrical inner wall shell of plastic part.

Working process of mold is as follows: melt injection is completed and mold is opened at PL surface, inclined guide column 16 drives retractable core-pulling slider mechanism to complete side core-pulling ejection of inner wall of plastic part. Ejector rod of injection molding machine drives ejector element pull rod 5, feed ejector rod 6, and push rod 7 on plate 32 to push plastic part from core insert 27 to realize automatic separation and demolding of plastic part and the runner aggregate. When resetting, first push ejection mechanism back to its position, and then close the mold at the PL surface to start the next injection cycle.