Recently, the sandwich injection molding has drawn attention because it offers the flexibility of using the optimal properties of two different but compatible polymers and is also one of the most p romising methods in connection with recycling of thermoplastics. In this paper, a new particle tracing algorithm is presented in order to describe the advancement of core polymer melt during filling stage. The main advantage of this algorithm is the use of identity field information rather than tracking a set of fluid particles. In addition, to model the process accurately, especially to detect the possible