The kneading zone of the cooking extruder continues the compression started in the feeding zone, and the flow channels of the extruder screw have a higher degree of fill. As the degree of screw fill increases and pressure begins to develop in the extruder barrel, leakage flow ( flow over the outside diameter of the screw in a direction toward the extruder inlet ) and pressure flow both increase. The mechanism of shear does not begin to play a dominate role until the screw flow channel is full.