Injection molding is a high-precision manufacturing process that injects molten plastic into a carefully designed mold, where the plastic cools and hardens into the specified part or product. The piece is then ejected from the mold, either as the final product or as a near-final product that is sent on for secondary finishing.
The injection mold consists of two parts: the mold core and the mold cavity. The space that these two parts create when the mold is closed is called the part cavity (the void that receives the molten plastic). Depending on production needs, “multi-cavity” molds can be designed to create multiple identical parts (as many as 100 or more) during the same run.
Designing the mold and its various components (referred to as tooling) represents a highly technical and often complex process that requires high precision and scientific know-how to produce top-quality parts with tight dimensions. For example, the proper grade of steel must be selected so components that run together do not wear out prematurely. Steel hardness must also be determined to maintain the proper balance between wear and toughness. Waterlines must be well-placed to maximize cooling and minimize warping. Tooling engineers also need to calculate gate/runner sizing specifications for proper filling and minimal cycle times, as well as determining the best shut-off methods for tooling durability over the life of the program.
During the injection molding process molten plastic flows through channels called “runners” into the mold cavity. The direction of flow is controlled by the “gate” at the end of each channel. The system of runners and gates must be carefully designed to assure even distribution of plastic and subsequent cooling. Proper placement of cooling channels in mold walls to circulate water are also essential for cooling to create a final product with homogeneous physical properties, resulting in repeatable product dimensions. Uneven cooling may result in defects called “hot spots”—areas of weakness that affect repeatability.
In general, more complex injection-molded products require more complex molds. These often must deal with features such as undercuts or threads, which typically require more mold components. There are other components that can be added to a mold to form complex geometry; rotating devices (using mechanical racks and gears), rotational hydraulic motors, hydraulic cylinders, floating plates, and multi-form slides are just some examples.