Magnesium alloys are increasingly used in automotive, aeronautic and electronic applications to produce high performance, light weight parts. In the thixomolding process the semisolid slurry is injected into a mold at controlled temperature such that the melt has specific flow behavior. This allows the fabrication of near net shape components with controlled microstructure and good mechanical properties. The numerical modeling of such applications presents unusual challenges for both the physical modeling and the solution algorithm. This paper presents 3D solutions of the injection molding of semi-solid AZ91 magnesium alloys. The methodology deals with the shear thinning, temperature dependent viscosity behavior and is able to accurately solve the high velocity flows encountered during semi-solid magnesium molding. The approach is applied to the injection of a tensile bar and the results compared with experimental data. The numerical solutions indicate that the material forms a jet at the exit of the gate and a swirling flow forms as the material advances along the first larger diameter section. The wall regions are filled first, leaving a void inside. This agrees very well with the experimental observation.