Computer simulation is one of the most efficient ways to assist engineers to find a good design solution and to produce high quality plastic parts. The prediction of the parameter evolution during material forming requires a fair understanding of the interaction between the material properties and the process. One of the problems encountered in numerical simulation of the injection molding process is the tracking of the polymer-air front or interface during the filling stage (Haagh et al., Int Polym Proc 1997, 12, 207). This article presents a numerical simulation of a nonisothermal molten polymer flow in a cavity as in the injection molding process. The continuity and complete Navier-Stokes equations are coupled with the level set convective equation to predict the flow front and the fountain flow effect. The fluid behavior is modeled by the Cross-Arrhenius model. Thanks to the use of the level set method, a special focus is made on the polymer-mold interfacial heat transfer, and the effect of a variable thermal contact resistance is thoroughly investigated. A new interpretation of the flow marks defect causes, based on the interfacial heat flux analysis, is then suggested. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1579-1592, 2011