Injection molding three basic operations

The term IM is an oversimplified description of a quite complicated process that is controllable within specified limits. Melted or plasticized plastic material is injected by force into a mold cavity (Figure 4.1). The mold may consist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels or runners which direct the flow of the melted plastic to the individual cavities (Chapter 17). The process is one of the most economical methods for mass production of simple to complex products. Three basic operations exist. They are the only operations in which the mechanical and thermal inputs of the injection equipment must be coordinated with the fundamental behavior properties of the plastic being processed. These three operations also are the prime determinants of the productivity of the process since manufacturing speed will depend on how fast we can heat the plastic to molding temperature, how fast we can inject it, and how long it takes to cool (or solidify) the product in the mold. 

The standard metering extrusion screw with its three zones (conveying, compression, and metering) basically operates like a conventional injection molding (IM) screw, as reviewed in Chapter 2. The nomenclature is the same for each (Fig. 2-3), except that no valve is used at the end of the extrusion screw (Fig. 3-7). As reviewed, extrusion screws operate at lower pressures and in a continuous mode (IM is repeatable with abrupt, completely on-off pressure changes and very fast cycles). Even though many... 

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