A computational model for the description of polymer flow during the plasticating phase of the injection molding process is proposed. The polymer behavior is determined during the dynamic and static phases of the process. The model takes into account the backwards movement of the screw, the presence of a non-return valve and the conduction of heat during the idle times. Results for the dynamic and static phases of the plasticization are presented. The model is also used to study the influence of some important operative process parameters, such as, screw speed, backpressure, barrel temperatures and injection chamber length. The assessment of the computational results is made experimentally by comparing the average temperature calculated with measurements made in front of the screw nozzle using both, an IR camera and an IR thermometer. The differences between the computational and the IR camera measurements are below 5 %.