Injection molding is one of the most widespread processes for plastic manufacturing. Software packages are often used in several steps of the production cycle, from mold design to the determination of processing conditions. In this work a method is reported to improve the predictions of the description of pressure profiles made by injection molding simulation software. The method is applied to two polymeric materials (a PolyStyrene and a PolyCarbonate) injection molded into line-gated rectangular cavities adopting different molding conditions and cavity thickness. The method, that is independent of the software adopted, in this article is illustrated adopting the commercial code Moldflow (R). It is shown that the predictions of the description of pressure curves inside the cavity can be significantly improved by introducing the effect of pressure on viscosity and the effect of cavity deformation during molding. The former effect is taken into account by finding an appropriate value for the parameter which takes into account the effect of pressure in the equation which describes material rheology. The latter effect can be introduced by defining and implementing an apparent material compressibility. POLYM. ENG. SCI., 51:2542-2551, 2011. (C) 2011 Society of Plastics Engineers