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Some basics
Definition: mold. 1) A hallow form or matrix into which a plastic material is placed and which imparts to the material its final shape as a finished article. 2) To impart shape to a plastic mass by means of confining cavity or matrix.
The mould which in some cases is referred to as mold or tool is the primary form which creates a part within the injection molding process. While most are aware of this for those whom this is a new field or endeavor the mold is a form which is manufactured in a negative fashion to the final plastic part which it produces. An example is that a hole in the plastic part is created by a pin/core in the mold the opposite. Thus a great creative aid in designing a mold and it features is the ability to see things in the negative, or backwards of what they wish to be.
In a simple mold, for example a cup there would be a cavity (forming the outside features) and a core (creating the inside features). The cavity would be located in the A side of the tool and the core in the B side of the tool. The space or air gap between the cavity and core when the mold is closed is where the plastic is injected into and which forms the part (wall thickness). It is also important to remember that plastic typically shrinks (becomes smaller) and shall always move away from outside surfaces (cavities) and grip/collapse onto inside forming surfaces (cores).
In the abstract the mold is in fact a small factory which due to outside influences can produce plastic parts. It is this factory abstract which allows for understanding processing to a higher degree, basically by understanding what is happening within that factory.
Further the mold is also a heat exchanger; this is illustrated by the simple fact that molten plastic is injected into the mold; heat is extracted until such time as the plastic within the mold is rigid enough to be ejected out of the mold. It is important to keep in mind this heat exchanger concept as in all thermoplastic molding the plastic is always warmer than that of the mold setting, so that there is always a thermal transfer even with the use of high mold temperatures they are still lower than that of the molten plastic being processed.
Molds are a high capital cost and are precision factories, and come in various configurations. The mold itself is made of some basic components and the following illustrates some of those.
Please note that the mold itself should be constructed so as all is square, meaning that the ejector housing is parallel to the top clamp plate, thus when bolted into a molding machine platen area all is aligned correctly. This is critical in that the forces of the clamp (compressive) must hold the mold closed as plastic is injected into it and if not of correct clamp force and or aligned properly the parts may flash.
Also note that a mold has basic dimensions which have carried over from the start of injection molding. These are width, length and height of the mold. It is important to note the mold height is in the direction of opening and closing of the mold. Thus when a mold is placed into a machine which has a minimum mold height figure listed the mold height of the actual mold must be that and or preferably more so as to apply clamp force to the mold. The width figure is one which must fit between the tie bar spacing and the length must be contained within the platen space.
Are there exception to these rules yes, but the area of concern within the mold is always the locations of the cavities which are applying the force which hopefully are contained in the area outlined/contained between the tie bars on the press.
SLSILVEY
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