Generally, plastic parts can be taken out of the mold cavity by only one ejection, yet due to their special forms, it is still difficult to take out the parts from cavity or the parts can not fall off freely from the mold upon the completion of an ejection movement. Under such case, another ejection must be applied to make the plastic parts fall off; sometimes a secondary ejection is also adopted to avoid exceeding force on plastic parts through one ejection, such as plastic parts with thin walls and deep cavity or those of complicated shapes, which can easily get cracked or distorted upon one ejection due to the large contact area between the parts and the mold, and hence, secondary ejection is applied to such plastic parts to detract ejection force, thereby ensuring the quality of plastic parts.