Commercial injection mold filling simulations use the lubrication approximation to model pressure and temperature during flow. This approximation is valid when flow streamlines ale parallel to a cavity surface, such as when the cavity thickness is much smaller than the flow length. Yet this approximation may not be valid in complex cavities where the streamlines ale not parallel. Flow through junctions, gates and sprues ale always accompanied by a velocity component transverse to the mold surface which is not included in the model. Neglecting transverse flow through use of the lubrication approximation can result in anomalous changes in bulk mean temperature, filling patterns, and molecular and fiber orientation. This paper presents a mass balance algorithm for approximating transverse flow through gates, sprues and junctions. This new algorithm is presently suitable for only relatively simple geometries. Future work will attempt to make a similar algorithm for transverse flow through complex mold shapes with negligible penalty in additional computational run time.