Extrusion molding is a widely utilized technique for processing polymers—particularly plastics and rubbers. Despite its broad industrial application, several aspects of the transport behavior of polymers during extrusion remain inadequately understood. Undesirable and unexpected phenomena such as sharkskin instabilities, turbulent flow transitions, and flow fractures frequently occur during the process. However, definitive explanations for these phenomena are still lacking. Numerous studies suggest that the transport behavior of polymers plays a crucial role in the emergence of these instabilities. To date, several empirical investigations have examined the rheological properties of conventional polymers to better understand these transport dynamics. Nonetheless, significant limitations persist, and a comprehensive theoretical model that can predict and explain these behaviors remains elusive.

To contribute toward resolving this issue, the present study employs both single- and double-concentric cylinder models to simulate the screw mechanism within an extruder. These models are used to derive and analyze the transport characteristics of polymers during extrusion molding.

This study was divided into two main sections. The first section focuses on the transport behavior of solid polymer particles, typically occurring in the feed zone (zone 1) of the screw. The second section examines the transport of polymer melts, which predominantly takes place in the melting and metering zones (zones 3 and 4). Key boundary conditions—including velocity profiles, flow rates, shear rates, and shear stresses—are systematically investigated and discussed to provide deeper insight into the mechanisms governing polymer transport in extrusion molding.