Numerical Simulation and Thermal Analysis of the Filling Stage in the Injection Molding Process: Role of the Mold-Polymer Interface Rabie El Otmani, 1 Matthieu Zinet, 1 M’hamed Boutaous, 1 Hamda Benhadid 2 1 CETHIL, UMR 5008, CNRS, INSA Lyon, Universite ´ Lyon1, F-69621 Villeurbanne, France 2 Laboratoire de Me ´canique des Fluides et d’Acoustique, UMR 5509, CNRS, Ecole Centrale de Lyon, Universite ´ Lyon1, INSA Lyon, F-69134 Ecully, France Received 16 June 2010; accepted 3 November 2010 DOI 10.1002/app.33699 Published online 4 March 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: 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 thor- oughly investigated. A new interpretation of the flow marks defect causes, based on the interfacial heat flux analysis, is then suggested. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1579–1592, 2011 Key words: injection molding; rheology; computer modeling; level set method; heat transfer INTRODUCTION Simulation of injection molding has become an essential tool to assist the engineers in optimizing the part and mold design, as well as in identifying the most suitable process parameters for productiv- ity and quality enhancement. So far, various kinds of models have been developed to predict the behav- ior of molten polymers during the filling step in the injection molding process. We can quote the pioneer works of Kamal and Kenig, 1 Tadmor et al., 2 and Lord and Williams. 3,4 Their analyses, in simple geo- metries, have focused on the prediction of tempera- ture and pressure fields. Austin 5 was then the first to extend these approaches to real parts. More recently, Coupez and coworkers 6 have proposed and implemented into the Rem3D finite element software a level set method to capture complex moving inter- faces, as those occurring in fluid buckling. Most injection-molded parts are three-dimensional, with complex geometrical configurations, and the rheological behavior of the molten polymer is gener- ally non-Newtonian and nonisothermal. 7 Due to the several coupled phenomena and the difficulty of their numerical analysis, especially during the filling pro- cess, several simplifications are usually introduced. Hieber and Shen, 8 using the Generalized Hele- Shaw (GHS) flow model, introduced simplified governing equations for nonisothermal and non- Newtonian flows in mold cavities. The Hele-Shaw model neglects the inertia and the gapwise velocity component for polymer melt flow in thin cavities. A review of the use of the GHS based methods can be found elsewhere. 9,10 However, because of their sim- plifying assumptions, 11 these models are not suitable for accurate parametric analyses. Other works 12–14 take advantage of the two-dimensional Navier- Stokes equations in Eulerian formulations. Such simplified models fail to predict the flow behavior in parts showing large curvature and complex geo- metries. Generally, a weak coupling between the energy and the flow equations is used, and the heat convection in the transverse direction of the flow is neglected. This leads to inaccurate predictions of the interfacial phenomena occurring between the flowing polymer and the mold. The wavelike flow mark phenomenon is one of the surface defects that can occur in the injection Correspondence to: M. Boutaous (mhamed.boutaous@ insa-lyon.fr). Journal of Applied Polymer Science, Vol. 121, 1579–1592 (2011) V C 2011 Wiley Periodicals, Inc.