Over the past three decades, the classic Lipscomb constitutive equation of fiber suspensions in the Newtonian fluids has often been used to discuss the fundamental effects of fiber orientation on flow fields. However, the numerical methods of the Lipscomb flow-fiber coupling employed to simulate anisotropic flows have always been unsatisfactory since convergent results have not been easily obtained, especially in industrial applications for high fiber concentrations. To derive a new informed isotropic (IISO) constitutive equation as an alternative Lipscomb form, particularly introduced with the strain-rate dependent parameter of the Trouton ratio, is a significant requirement. For the fiber-filled polymer melts, the IISO constitutive equation was used herein to predict the rheological behaviors from simple flow to complex flow, including an overshoot of the transient viscosity in shear flow and a dramatic change of the corner vortex in axi-contraction flow. For the practical injection molding simulation consisting of hybrid shear/extension flows, a peculiar filling pattern and a broader core orientation distribution were investigated, and experimental validation was realized. (c) 2019 The Society of Rheology.