A mechanistic model was implemented in order to simulate the fiber motion in molding processes. In this model, each fiber is represented by a chain of segments interconnected by articulations. A balance of forces and torques is considered in order to determine the velocity and position of each of these segments during the simulation. This balance includes hydrodynamic effects (drag forces and torques), fiber-fiber contact forces, and bending moments. The model was able to reproduce analytic results such as the Jeffrey orbits for a single fiber in a shear flow. Also, it was compared with experimental results for SMC (sheet molding compound process) and for a simple shear flow. In the case of the SMC, the model was able to reproduce the fiber orientation accurately and the phenomenon of fiber matrix-separation was captured by the simulations. For the case of a shear flow, the fiber orientation was over-predicted by the mechanistic model. The motion of fibers in the fountain flow region and the flow through the gate of a mold were also considered. In contrast with the research done by other authors, who have developed similar mechanistic models to study flows with simple kinematics (for instance simple shear) to predict bulk properties such as the viscosity of the compound, the work presented in this dissertation deals with complex flows and uses mechanistic models to study the phenomenon of fiber attrition and fiber matrix separation.