While injection molding simulation softwares are becoming more and more popular in thermoplastic industries due to their ease of use and advantages, many rubber molding companies have not yet fully benefit from this technology, known as reactive injection molding (RIM). This is mainly due to the complexity of material characterization, the lack of material data and appropriate description of the material behaviour. Specifically, the analysis of the filling stage of the RIM process requires the determination of the cure kinetics and the rheological, PVT, and thermal behavior of the material. Often one of the hurdles is characterizing and modeling the viscosity of rubber compounds reinforced with particulate fillers. This type of compounds usually does not obey to the Cox-Merz rule [1] due to the presence of fillers [2]. In addition, the cure rate dependence of viscosity has to be properly addressed. In this work, we have shown that the destruction of filler network at high shear rates makes the steady state shear viscosity, ç, measured by capillary rheometer, lower than the complex viscosity, ç*, obtained via RPA. In order to obtain an effective methodology to model compound viscosity, an engineering approach has been developed. It combines material characterization and modeling, numerical simulation, and experimental verification. By applying this methodology, numerical simulation results have shown good prediction of melt front time and sprue pressure for different injection molds and rubber compounds. References: [1] W.P. Cox and E.H. Merz, Journal of Polymer Science, 28, 619 (1958) [2] J. E. Mark, B. Erman and C. M. Roland, The Science and Technology of Rubber (Fourth edition), pp. 322