This paper demonstrates how a 2D Finite Element Method (FEM) flow simulation is used to analyze and optimize a co-extrusion die for the production of coaxial 3D printer filaments. Via flow simulation and the rheological properties of the materials used, this study also shows how this 2D FEM Module can provide valuable design information which can avoid interfacial instabilities in coaxial flows of ABS/CaCO3 and PLA/CaCO3.