A detailed analysis of the fluid flow in the unwound single screw extruder model [1] is performed by combining numerical and analytical methods. While finite element analysis numerical models are used to extract the transversal velocity field, an analytical model in the limit of zero Reynolds number is used to determine the longitudinal component of the fluid velocities. The high resolution 3D model developed for the fluid flow in single screw extruders with aspect ratios (i.e. height/width) ranging from 1 to 0.1, allows the identification of the position and extent of Moffatt eddies [2] that impede the fluid mixing through the entire volume of the single screw extruder. This information is used to quantitatively analyze, using an entropic mixing measure [3], the extent by which the Moffatt eddies affect the distributive mixing of tracers in the extruder. Strategies to prevent the formation of Moffatt eddies are discussed. [1] Z. Tadmor and C. Gogos, Fundamentals of Polymer Processing, 2’nd Edition, Wiley-Interscience, 2006. [2] H. K. Moffatt, Viscous and Resistive Eddies Near a Sharp Corner, J. Fluid Mechanics, 18, 1-18 (1964) [3] I. Manas-Zloczower and M. Kaufman, Mixing Measures, chapter in Mixing and Compounding of Polymers: Theory and Practice, ed. I. Manas-Zloczower, Hanser Verlag, 2009.