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A Basic Custom Plastic Injection Molding Machine


Custom Plastic Injection Molding Ejection and Core Pull Systems


Ejector System

The sole purpose of your ejection system is to remove the finished custom injection molded parts from the mold or die.  The ejection system is comprised of a hydraulically driven ejector plate, sometimes referred to as the butterfly plate, and some type of ejector rod or bar, which is used to drive the mold ejector plate forward, releasing the part from the cavity or core of the mold, or so that it can be removed by an operator or a robot.  Depending entirely on your mold and it’s requirements, there could be anywhere from 1 to 4 or more rods required to perform this task.  One critical thing to remember is that if you are using more than 1 rod, it is necessary that these rods are exactly the same length.  The reason for this is so that the ejector plate is moved forward in a uniform manner, to avoid causing the ejector pins to become scored and bound up.  If you are using one rod, it usually is placed in the very center of the mold.  If two or more are ejector rods are used, it is important that they a installed symmetrically into the mold, such as opposite corners, directly above each other, all four corners, or similar.  These rods can be installed as loose or “floating components” or tied-in.  As the term indicates when tied-in, the rods are attached at both the mold and machine ends of the rods, usually with bolts or threaded rod stubs in the rod.  Molds that have return springs can be used with loose or floating rods, as the springs are intended to provide the return stroke of the mold ejector plate when the machine ejector plate is retracted, allowing the rods to also be retracted out of the way.  This is especially useful if you run smaller molds with multiple ejection strokes and little or no actions built into the mold.


With more complex molds, the tie-in method is more appropriate and more often used.  The difference in using this method is that the machine is controlling the entire stroke and it allows you to safely control the ejection of parts from molds that have actions built into them such as slides, suicide core pins, or both.  This also helps prevent damage to the tool, should something go wrong with the injection molding process or machine.  When coupled with a robot or operator removal, the rods can maintain the ejector plate in the forward position until the robot or operator has removed the finished injection molded part from the mold.


Core Pulls

Core pulls do exactly what they sound like they would do, and that is to pull cores out of parts which otherwise could not be molded any other way.  For example, If you wanted to create a hole in the top flange of a part and the hole was not in “die draw”, you would be able to use a movable core to create this hole.  Just to clarify, die draw is anything that lies in the direction of “mold opening”, with sufficient draft for the part to be removed.  In the case of a rectangular box, by adding some amount of draft, you can mold this box without the aide of anything special besides the ejector system to remove it

from the cavity.  But, If you would like to put a hole in the side of the box, we need to use a core pull mechanism to do this.  The process is simple in the case of creating a hole.  Once the mold is closed, we will use the core pull system to drive a pin into the core wall, creating an area that plastic must flow around during the filling of the cavity.  This now creates a hole in the side of the plastic part.  Now if we were simply to open the mold up, we would rip or tear away all the plastic behind this core pin during ejection, leaving us with a useless part.  But, if we pull that core back up out of the way before we open the mold, we can safely eject the part with no damage to our hole.  This is just one of the myriad of uses of the core pull mechanism on the machine.  It can if desired, be used in certain mold design configurations be used as a hydraulic ejector system addition as well.

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Written by: WM8C, July 28th, 2006.  Not for use without written permission


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