Generally, ejecting a plastic part from a mold, whether using a single or multi-element ejection mechanism, is a one-time ejection action. However, due to special shape of plastic part or need for production automation, if a single ejection action is insufficient to remove plastic part from mold cavity or prevents it from automatically detaching, a second ejection action is required to eject plastic part out of mold. For thin-walled, deep-cavity plastic parts or complex shapes, sometimes secondary ejection is used to avoid excessive force on plastic part during a single ejection, thus distributing ejection force and ensuring quality of plastic part. This type of ejection mechanism is called a secondary ejection mechanism. In a secondary ejection mechanism, typically some or all of ejection elements initially eject plastic part together, then some ejection elements stop while others continue ejecting; or some ejection elements continue ejecting while others eject ahead at a faster speed—this is called accelerated ejection.
There are generally two scenarios for using secondary ejection:
① Plastic part exerts a strong clamping force on mold, and a single ejection would deform or break part;
② Plastic part has undercuts, and lateral core pulling is not possible; only elastic deformation of plastic part can be used for forced demolding.
Demolding action in second scenario consists of two steps: First, ejector element ejects undercut core and plastic part together, separating the other side of undercut from mold, creating space for elastic deformation of plastic part; second, undercut core stops moving, ejector element forcibly pushes plastic part away from undercut core, achieving forced demolding.
There are many types of secondary ejection demolding mechanisms, and their motion patterns are sometimes ingenious, but they all share a common feature: these mechanisms must have two or more sets of ejector parts with a certain difference in ejection stroke. Therefore, secondary ejection device must have a device to control ejection stroke.

Figure 6-38 shows an example of secondary ejection using a push plate and push rod. Plastic part is circular with a maximum outer diameter of 61.8 mm. It has a deep rib on moving mold side, 67 mm deep, and two raised ribs on its outer surface. A split-slider core-pulling mechanism is designed. Plastic part must be ejected using a combination of an ejector plate and ejector pins for demolding. After mold opening, split-slider opens. Ejection process: Ejector plate 17 drives ejector pins 19 and 10 to eject part. Ejector pin 10 pushes slider 14, which in turn pushes ejector plate 20 outwards. Plastic part remains encased in ejector plate insert 4, but has detached from moving mold core 5. Continuing ejection, inclined surface of slider 14 contacts inclined surface of hook body 13, causing slider 14 to slide into ejector plate 20. Ejector pin 10 no longer pushes ejector plate 20, and ejector pin 19 ejects plastic part from ejector plate insert 4 for demolding.
Figure 6-39 shows an example of a deep cylindrical plastic part undergoing secondary ejection via an ejector tube. Plastic part has a thin plastic layer, entirely within moving mold, and only one ejector tube. Ejection causes deformation of part, affecting dimensional stability. Therefore, a double-layer ejector plate secondary ejection mechanism was designed. Key design considerations for double-layer ejector plate include: necessity of guide pillars for guidance; designing ejection elements for different stages on corresponding ejector plates; clearly defining whether injection molding machine's ejector roller actuates upper or lower ejector plate.

 

Figure 6-38 Example of secondary ejection using ejector plate and ejector rod
1. Fixed mold base plate; 2. Cavity; 3. Half slider; 4. Ejector plate insert; 5. Moving mold core; 6. Core pressure plate; 7. Moving mold plate; 8. Guide pillar; 9, 11, 15. Screw; 10. Ejector rod; 12. Locating pin; 13. Hook body; 14. Sliding block; 16. Spring; 17. Ejector plate; 18. Ejector rod fixing plate; 19. Ejector rod; 20. Ejector plate
During ejection, injection molding machine's ejector roller first actuates upper ejector plate 2. Pull hook 13 pulls sliding block 12, simultaneously pushing out lower ejector plate 4. Ejector tube 9 pushes plastic part out of core. Continuing ejection, inclined surface of slider 12 contacts inclined surface of pull hook body 14, causing slider 12 to slide into lower ejector plate 4. At this instant, lower ejector plate 4 stops ejecting after hitting limit post 10, and upper ejector plate 2, driving ejector tube, pushes plastic part out of ejector tube's inner core.

Figure 6-39 Secondary Ejection of Deep Cylindrical Plastic Part
1 - Upper Ejector Pin Fixing Plate; 2 - Upper Ejector Plate; 3 - Lower Ejector Pin Fixing Plate; 4 - Lower Ejector Plate; 5 - Moving Mold Base Plate; 6 - Guide Pillar; 7 - Support Pillar; 8 - Ejector Pin; 9 - Ejector Tube; 10 - Limiting Pillar; 11 - Cooling Water Pipe; 12 - Slider; 13 - Hook; 14 - Hook Body
Figure 6-40 shows pull hook controlling secondary ejection. Plastic part is circular. A rib with a circumference depth is located on ejector plate insert 4 of moving mold. After mold opening, injection molding machine ejector pushes out insert 13. Under action of upper pull hook 17, upper and lower ejector plates eject together. Ejector plate insert 4 and ejector rod 12 together detach plastic part from core 7. After sliding block 18 touches inclined surface of lower pull hook 16, sliding block 18 moves, upper pull hook 17 disengages from sliding block 18, upper and lower ejector plates separate. Upper ejector plate 9 drives ejector rod 12 to detach plastic part from ejector plate insert 4.

 

Figure 6-40 Hook-controlled secondary ejection
1-A plate; 2-Cavity; 3-Push plate; 4-Push plate insert; 5-B plate; 6-Ejection rod; 7-Core; 8-Upper push rod fixing plate; 9-Upper push plate; 10-Lower push plate; 11-Support column; 12-Push rod; 13-Ejection insert; 14-Guide column; 15-Reset rod; 16-Lower hook; 17-Upper hook; 18-Sliding block; 19-Hook body
Figure 6-41 shows a spring-driven secondary ejection. Spring-driven secondary ejection is used when ejection force and ejection stroke are small, such as when forcibly removing undercut. Undercut ejector rod 7 of plastic part is fixed to lower ejector plate. Injection molding machine ejector first pushes upper ejector plate 9. Under action of spring 10, lower ejector plate 11 is pushed out together with upper ejector plate 9. Ejector 5, together with undercut ejector 7, detaches plastic part from core 4. Continuing ejection process, ejector 5 pushes plastic part off undercut ejector 7, achieving forced demolding.

 

Figure 6-41 Spring-driven secondary ejection
1. Fixed mold base plate; 2. Plate A; 3. Cavity; 4. Core; 5. Push rod; 6. Plate B; 7. Undercut push rod; 8. Square iron; 9. Upper push plate; 10. Spring; 11. Lower push plate; 12. Limit pin; 13. Moving mold base plate; 14. Return spring