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Basic Custom Plastic Injection Mold


Custom Injection Molding Process


The Injection Mold | It’s Main Components and Function | Injection Molds Explained

The function of the “mold” or “mould” depending on which spelling you subscribe to in the injection molding process, is simply to be the vessel within which a plastic part is born.  Think of the mold as the reverse image of your finished part.  What I mean by that is this…if I took a ball and I covered the entire surface of the ball with plaster and let it harden, and I then cut it exactly down the middle into two halves and removed the ball, what I would be left with is the basis of an injection mold.  It is a void or space of some shape that is split into two sections so that we can inject plastic into that space to create a plastic part when it is removed from this mold.  We refer to the halves of the mold as the cavity and core, or cover and ejector.  The cavity and core can be either the front or back half of a mold depending on the design.  Most molds will have the cavity or cover of the mold mounted on the stationary platen and core or ejector of the mold on the movable platen.  In the case of molds that are built with cavity or cover ejection this would be reversed, and then we often would use the core pull system to run the ejectors forward.  We will stick with the more common configuration of the ejector half of the mold being located on the movable platen and using the standard injection molding machine ejector system.


The “ejector” or “ejection” half of the mold is what will ultimately cause the finished plastic part, to be ejected from the mold. It is comprised of a number of components, some of which will be the ejector housing, ejector plate, ejector pins, and water (cooling) lines.  As design requires it, it may also have slides, cores, lifters, and other moving components that make up the the ejection system.  It also is usually the core half of the mold.  Besides being a critical part of the molded part, it’s only other purpose in life is part removal or “ejection”.  Plastic shrinks to varying degrees, and because of this we must build molds with sufficient draft to allow the part to easily slip off of the core of the mold without damage to the parts plastic surfaces.  Draft needed for a given material or resin can vary greatly and you should consult your material vendor for the specifications for the material you are using for your specific application.  Draft can be as little as a couple of degrees, to something much more substantial depending the part design and material requirements.  When a part is molded and then cooled, the ejector plate of the mold is moved forward causing the ejector pins and/or lifters to push the part free of the mold, allow it to be removed by an operator or robot, or to simply fall onto a chute or conveyor to be moved to it’s next operation.


The cover or core of the mold, except when the part is cover ejected, does not typically have any moving parts.  It does however usually house the entry channel for the plastic into the mold.  This is done through a number of different methods, such as a sprue bushing and sprue, or a hot runner manifold.  The channels of plastic flow can be directly into the part itself or flow through a “runner” system, which is the most commonly used  method.  The runner system can be part of the hot runner manifold or a cold runner system that is cut into the surface of the mold itself.  We need a separation point

between the part and the runner and this is called the “gate”.  There are many “gate” configurations and again, this all depends on your parts design and requirements as how this will be performed.  This half of the mold will also require sufficient draft, as the part must release from the cavity as the mold opens which separates the two halves prior to ejection, with the part remaining on the ejector half of the mold.  This half of the mold the mold will also contain the horn pins which move any slides the mold might have into place as the mold closes. The guide pins, which will located in two up to all four corners of the mold, perform the task of keeping the two halves aligned as the mold closes.  When the two halves are closed together, they create a void that will be filled with plastic to create the finished molded part. 

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


Up ] Choosing a Machine ] Quick Mold Change ] Basic Injection Molding ] Basic Injection Molding II ] Basic Injection Molding III ] Basic Injection Molding IV ] Basic Injection Molding V ] Basic Injection Molding VI ] Basic Injection Molding VII ] Basic Injection Molding VIII ] [ Basic Injection Molding IX ] Basic Injection Molding X ] Basic Injection Molding XI ] Basic Injection Molding XII ]


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