Flash, a common injection molding defect, arises when melt flows from the cavity into thin gaps between parting surfaces. Besides rules of thumb for eliminating flash, there are few fundamental papers on flash analysis. Understanding flash as a transport phenomenon provides a systematic basis for solving flash problems. The governing equations for the gap flow are established and solved for an isothermal power law fluid, under constant pressure along the parting line where flash begins. Two shapes are investigated, rectangular and ring slits that respectively correspond to modeling flash from straight and curved parting lines. Our equation for flash length, the distance to which the melt penetrates the gap developed between the parting surfaces, is the main result of this work. Further, adimensionalizing not only unifies the results for straight and curved parting lines, but also provides insight into how rheology, pressure, temperature, and geometry govern flash. Our approach avoids tedious numerical simulation and mold structural analysis. Our theory is validated by our polycarbonate flash experiments.