Injection molding is one of the most exploited industrial processes in the production of plastic parts. The main stages in an injection molding process therefore involve filling, cooling, and ejection. Understanding the different stages is important for the design of the cooling system. It is known that more than 70% of the cycle time is spent in the cooling stage. The effect of neglecting the filling stage on the thermal analysis of the mold and the product is studied during injection molding process. A three-dimensional model including the mold, the cavity, and the cooling channels for the cooling process during the injection molding is presented. A finite volume numerical method is used for the solution of the physical model. A validation of the numerical model for the filling and solidification processes is presented. The results indicate that, there is a good agreement between the analytical solution and the numerical solution. They also show that the filling stage has a great effect on the temperature distribution through out the product and the mold. The results indicate that by neglecting filling stage, which represents about 7% of the cycle time, and assuming that the mold is initially filled, the solidification percent decreases by about 6%. POLYM. ENG. SCI., 49:993-1003, 2009. (C) 2009 Society of Plastics Engineers