Long fiber-reinforced thermoplastic (LFT) materials have gained importance due to their exceptional lightweight properties and cost-efficient manufacturing processes. The mechanical performance of the finished part depends on the material properties, process characteristics and part design. The orientation of the fibers, the fiber content and the fiber length distribution are the three main characteristics that dictated the mechanical properties. Experiments have shown that all three properties can vary tremendously. A fundamental understanding of the physics behind fiber motion in LFT processing is necessary to achieve an accurate prediction of the fiber properties. In this work, a mechanistic model was developed that represents each fiber as a chain of elements interconnected by articulations. The model takes into account hydrodynamic effects, elastics deformations, fiber-fiber interactions and excluded volume effects, as illustrated in Figure 1.