The flow analysis network (FAN) method previously developed for die design is adapted to the problem of the cavity filling process in injection molding. The method is applicable to relatively narrow gap cavities of any shape. It permits the computation of the advancing front of melt at any time, as well as prediction of weld‐line location. The method was extended to nonisothermal flow in which solidification and “skin” formation during filling time was approximately accounted for. The nonisothermal analysis allows prediction of the possibility of a “short shot” situation. The analysis is applicable to any prescribed pressure or flow rate at the gate. Both can be arbitrary functions of time.