Many common production problems result because of non-uniform viscosities that naturally develop through the cross-sections of the runner system as rubber flows from the injection molding machine through the mold. These non-uniform material properties create many problems for molders including excessive flash on the cavities first to fill as well as a significant disparity in cure rates between cavity groups. Therefore, to maximize the efficiency of an injection mold and to minimize post-molding labor, it is imperative to control the material properties as the rubber flows from the injection molding machine through the mold. This cannot be accomplished at the injection molding machine because, during filling and packing, the machine control is limited to linear speed control of the advancing plunger or screw. Consequently, the machine has limited influence on the material properties and limited control on pressure development within each cavity. However, these non-uniform material properties can be controlled within the mold's runner system and evenly, or otherwise, distributed within the mold cavities by implementing in-mold rheological control systems. Controlling the material conditions can eliminate flash, reduce the cure time difference between cavities, improve part quality, and widen the process window to ensure the mold runs at peak manufacturing efficiencies. This paper will explain why non-uniform material properties develop as it flows and how patented rheological control systems work, explaining the various types of systems and discussing the many benefits of implementing these systems into a mold's runner system so that balanced material properties are delivered to each cavity, controlling the melt property distribution within the cavities.