Theory of injection molding

The theory of injection molding can be reduced to four simple individual steps: Plasticizing, Injection, Chilling, and Ejection. Each of those steps is distinct from the others and correct control of each is essential to the success of the total process.

Plasticizing – describes the conversion of the polymer material from its normal hard granular form at room temperatures, to the liquid consistency necessary for injection at its correct melt temperature.
Injection – is the stage during which this melt is introduced into a mold to completely fill a cavity or cavities.
Chilling – is the action of removing heat from the melt to convert it from a liquid consistency back to its original rigid state. As the material cools, it also shrinks.
Ejection – is the removal of the cooled, molded part from the mold cavity and from any cores or inserts.
Repetition of these basic steps in sequence is the process of injection molding.

The practice of injection molding
The practice of injection molding varies from the theory, only in as much as process limitations and available equipment affect it. Step by step process is explained in ST’s Cutting Costs in Short-run Plastic Injection Molding guide.