The objective of this paper is to present a methodology for automatically balancing multi-cavity injection molds with the aid of flow simulation. After the runner and cavity layout has been designed, the methodology adjusts runner and gate sizes iteratively based on the outputs of flow analysis. This methodology also ensures that the runner sizes in the final design are machinable. To illustrate this methodology, two examples are used wherein a 20-cavity mold and a 3-cavity mold are modeled and filling of all the cavities at the same time in both cases is achieved. In the case of the 20-cavity mold, the effect of changes in the processing conditions on the balance of the flow is also studied using a 3 3 -factorial test. The examples indicate that the described methodology can be used effectively to balance runner systems for multi-cavity molds.