Multiple injection cavities are automatically balanced by adjusting runner and gate sizes based on an iterative redesign methodology integrated with computer-aided engineering (CAE) packing simulation. For runner balancing, each cavity must be filled simultaneously at uniform pressure. In addition, the time-pressure history of the polymer melt over the entire molding cycle should be considered. Based on the proposed methodology, a multicavity mold with identical cavities is balanced to minimize entrance pressure differences among various cavities at discrete time steps of the molding cycle. The results have shown more than a 95% reduction of the entrance pressure differences over other related studies, and also have demonstrated increased searching performance over other optimization techniques. A family mold with different cavity volumes and geometries is also balanced to minimize pressure differences at the end of the melt flow path in each cavity on a basis of discrete time steps of the molding cycle. The methodology has shown uniform pressure distributions among the cavities during the entire molding cycle.