The goal during runner balancing is to vary the diameters of the runner segments such that all the cavities, in a multi-cavity injection mold, fill at the same time. If the runner system is unbalanced, some cavities will fill before others, begin to overpack, and result in material wastage and inconsistent part quality. Numerical optimization methods and finite element mold-filling simulation are used to solve this nonlinear discrete variable problem. Approximation concepts are used to reduce the computational effort required for solving this iterative problem. This automated system has been successfully tested on a number of family molds.