Plastic Mold Design Principles: Secondary Ejection Mechanism
Secondary ejection is a forced demolding mechanism, primarily used when ejector pin cannot be disengaged by lifters or slide blocks. Two-stage ejection isn't always limited to forced demolding; sometimes, it's used to create deeper molded products or prevent gate damage.
Generally, removing finished product from mold, whether using a single or multi-ejector mechanism, is a single-stage process. However, due to special shape of finished product or requirements of mass production, if product remains in cavity after the first ejection or cannot fall automatically, a second ejection is necessary. This type of ejection design is called secondary ejection.

 

Secondary ejection
Or, to obtain a reliable demolding effect, demolding resistance of plastic part is reduced, and plastic part is demolded through a secondary demolding action. Mechanism that completes demolding of plastic part is called a secondary demolding mechanism, as shown in Figure 8.5.1.

 

Example of a secondary demolding mechanism: (1) As shown in Figure 8.5.2, plastic part has a semi-circular recess between two bones, which is tightly wrapped by rear mold cavity. Demolding mechanism is shown in Figure 8.5.3. The first demolding causes plastic part to be ejected from rear mold cavity, providing space for strong demolding deformation; second demolding is performed by ejector pins, and semi-circular recess of plastic part is forcefully ejected from core push block. Movement process of this mechanism is as follows: four ejector plates move during the first demolding, along with ejector pins and core push blocks, and demolding distance is ≥h, causing plastic part to be ejected from rear mold cavity, and the first demolding is completed. When movement continues until swing block touches upper limit surface, swing block swings and causes two upper pin plates to move quickly, driving ejector pins to be ejected from plastic part, completing secondary demolding. Note that for this mechanism: h1>h, H>10mm+h1+(secondary demolding movement distance).

 

Since sprue must be located on lifter block and pass through lifter block to insert sprue, mold needs to ensure that sprue ejects from lifter block first. Mold adopts a two-stage ejection mechanism as shown in Figure 8.5.6. During first ejection, pull rod keeps sprue stationary, ejector pins and lifters disengage from part by a distance M, separating part from sprue. Sprue deforms and ejects from lifter block, completing the first ejection. During second ejection, all four ejector plates move, ejecting part and sprue out of mold cavity. Note that to ensure sprue ejects from lifter block, M > S (sprue length).
(2) Sprue and parting line on part are shown in Figure 8.5.5.

 

Delayed Ejection
Delayed ejection solves problem of product not automatically disconnecting from gate (gate type: overlapping or side gate). Delayed ejection only ejects from runner.
Delayed ejection can be achieved by changing position of ejector pin.

 

Use of a two-section ejector pin is solely for increasing pin's strength and preventing bending deformation during ejection.
Using this type of ejector pin to achieve two-stage ejection is the simplest in mold structure; it only requires work on ejector plate. Example given here uses this mechanism to cut off a submarine gate.

 

During ejection, left ejector pin moves first, and right ejector pin only moves when lower fixing plate reaches ejector pin head circled in red below it. This two-stage ejection structure prevents damage to finished product caused by gate being pulled away during ejection, which is common with submarine gates.
Side gates can also be ejected using a similar method to avoid damage to finished product at gate location.
Of course, this ejection method is not only used for ejections near gate. Because starting position of its second ejection stroke can be achieved simply by changing length of column below ejector pin, and mold manufacturing cost is relatively low, this ejection method is also suitable for ejecting deeper box-shaped objects mentioned earlier.