In recent years a continuously rising demand for short-fiber-reinforced thermoplastics has been noted. The reason for the rising demand are the excellent weight specific mechanical properties compared to non-reinforced thermoplastics. The mechanical properties of the compound with high-strength fibers and the ductile matrix polymer are based on complex physical relations in which the fiber length is a major influencing factor. When processing short fiber reinforced thermoplastics with screw machines (injection molding, extrusion) the fiber length is a resulting material property that is, to some extent, influenced by the process parameters. The extent of the influence and the effects of the process parameters have been investigated in many different studies, mainly by parameter variations. Nevertheless, the influence of the process parameters on the fiber breaking is described with regression models that are valid for the studied parameter set. For a physical model of the fiber breakage during injection molding, simplified shear conditions are investigated. The pressures during plasticizing within the screw areas of injection molding machines are significantly lower compared to extrusion. For this reason the pressure flow within the screw channels can be neglected. The flow characteristic is basically influenced by the drag flow. To reproduce drag shear conditions, a test stand with a heated rotating piston and a stationary outer cylinder is used. In the gap between the piston and the outer cylinder the melted material faces the shear stress. The experimental results are used to find a correlation between the process parameters and the fiber breakage. This mathematical model is then transferred to the plasticizing process of injection molding. The validation of the model is done with a short fiber reinforced Polypropylene.