Abstract. The High Speed-S-Truder (HSST) with floating screw sleeve is an alternative extrusion concept with solid melt separation for high speed operations. Inside the sleeve the material is plasticized and then discharged into the outer sleeve by hundreds of radial bores. Due to high rotational screw speeds the melt between sleeve and screw is subject to high shear loads. The resultant dissipation at the inner sleeve surface is one of the main sources of material heating in the HSST. For increasing screw speeds dissipation reduction is necessary to avoid exceedance of the material’s permitted processing temperature. It is expected, that the local enlargement of the gap between sleeve and screw by profiling the inner sleeve will lead to a decrease of the occurring shear rates. In this paper, concepts with radial, helical and axial grooved inner sleeve profiles are investigated with 3D-CFD-simulations. The evaluation of the numerical simulation results pay particular attention to the shear rate and dissipation in combination with the melt temperature development. Furthermore the effects of varying groove dimensions will be analyzed regarding the melting process and the fluid flow. Based on the results, a promising concept is recommended for experimental studies.