Abstract: The runner-system design is of great importance for asuccessful injection molding process, particularly, for a family moldwith multiple cavities. This paper presents a methodology forautomatically balancing multi-cavity injection molds with the aid offlow simulation. After the runner and cavity layout have been designedand the gate size decided, one can adjust runner size iteratively,based on the outputs of flow analysis. A three-cavity family mold isused to illustrate this methodology.