Amorphous and crystalline polymers were injected into different molds of disks, with different applied injection rates. The pressures and local volume flow rates were measured for different injection molding conditions. These measures show a difference between the average volume flow rate measured during the total filling time and the nominal applied volume flow rate. Moreover the local volume flow rate at the polymer front increases between the mold entrance and the end of the cavity. Effects of different parameters of the process (polymer, geometry of the part, injection molding conditions) on the evolution of these volume flow rates were examined. Interpretation of the observed phenomena neads to take into account the effect of two characteristics of the polymers which are generally neglected: the compressibility and the associated bulk viscosity.