Part Ejection in Injection Moulding Mold
– By Zafar Kamal
A very important step in the construction of the injection mold is part ejection. It depends upon the geometry of the part and the material from which it may be molded, and a number of other factors, the ejection system can vary in design and the method of the operation.
The method which is commonly used is knockout pin method which contact the molded part at its edges, or a flat area, or both, and are mechanically operated when the mold opens to eject the part. As the part becomes more detailed, the may be limited to contacting the runner system and not the part itself. Sometimes pins have blades or bar to provide more contact area, or the part may be lifted from the core by means of the stripper rings.
Some molded part requires very finish outlook therefore, air blast is used to remove the molded part from the cavity or core section of the mold. It is also important to consider the type of mold finish being used or may be tolerated, as an assist to proper part ejection. Jobs which are running at the high speeds or high production rates may employ some combination of these available methods.
Part design, mold design, mold finish, cycle conditions, and mechanical ejection devices may all be incorporated within a given molding system to allow the job to run at the rates specified and to reasonably assure the continued operation of the machine without fear of parts hanging up in the mold. Obviously, any mechanical movement for actuating knockout devices, must also include a system for the resetting of the pin plate or return of the pins prior to the next shot, usually accomplished by use of knock-out-plate return pins. They are round pins slightly longer than the knockout stroke which contact the mold base on the closing prior to any interference of the pins with the cavity or core members.
