The reliability of numerical simulations of the injection molding process depends mainly on the rheological characteristics of polymers. The rheological characterization of a polymer for injection molding is commonly carried out by a capillary rheometer. However, in this method the measurement is carried out on a sample of unprocessed polymer, neglecting the influence of the plastication phase. This influence is demonstrated in the first part of the paper by comparing the rheological characterizations of a polycarbonate both in the virgin and processed state. To improve the accuracy of rheological data by rejecting these simplifications, a laboratory test method has been developed for the physical simulation of polymers rheology under industrial process conditions. The method is based on an in-line slit-die rheometer which has been mounted as a mold on an injection molding machine. By means of this test it is possible to relate a polymer viscosity not only to its temperature and shear rate but also to those parameters that characterize the plastication process: screw rotation speed, residence time, temperature and geometry of the system.