In the coinjection-molding process two polymers are injected into a mold sequentially or simultaneously. During the process one polymer melt forms the skin and the other forms the core, resulting in an encapsulated sandwich structure. In an attempt to develop a process model and simulation of simultaneous coinjection molding, along with the interface treatment a theoretical approach, physical model, and numerical algorithm were formulated based on the Hele-Shaw approximation. A detailed picture of the dynamics and kinematics of the interface evolution, providing a description of the multilayer flow and interface development during the multicomponent injection-molding process, information typically lacking in the available literature, is systematically presented. Based on the developed numerical simulation code, the moldability of various material combinations can be evaluated, which is required for process optimization. (C) 2003 Wiley Periodicals, Inc.