AbstractThe hydrodynamics and heat transfer that take place during molding of short fiber polymer composites have been analyzed. Consequently, the fiber orientation development has been qualitatively modeled and the flow mechanisms that contribute to fiber orientation have been identified: 1. Spreading radial flow in the vicinity of the cavity gate, originates transverse orientation of fiber with respect to the flow direction. 2. The fountain flow in the advancing front contributes to orientation of fibers in the surface layer in the flow direction. 3. Converging type flow, as the melt front flattens, causes some alignment in the flow direction. 4. Shear flow, which in the nonisothermal case, contributes to fiber orientation in the flow direction, at some distance from the wall. As a result of these mechanisms a layered structure is formed, having distinct fiber orientations. The experimentally observed fiber distributions in injection molding of thermoplastics, transfer, and compression molding of thermosets, agree with the qualitative predictions. The suggested orientation mechanisms may guide the mold and product designers as to the effect of gate location, mold dimensions, and processing variables on the final properties of the article.