To understand the extrusion process, it is necessary to describe the energy balance of an extruder and its related processes as a first step. It is helpful to view the extruder as a black box, analyzing the energy and mass input and output. Previous work with energy balancing has resulted in a model that allows mathematical simulation, yielding a good correlation between measured and calculated values. Unfortunately, these calculations are not precise enough for today's requirements. An intimate knowledge of the behavior of the material as it passes through the extruder with its rheological description and residence time distribution allows the development of a more precise model of the extrusion process. With the help of this knowledge, the extrusion process can be predicted and controlled according to 6 Sigma strategy.