Extruders in which the working component is a screw with continuously decreasing channel depth (Fig. la) are the most widely used. In such a screw, the mixing process begins after loading zone I in melting zone II and ends in feeding zone III. However, mixing in the melting zone occurs only in the melt region and does not affect the solid core, which exists along virtually the entire length of the zone; therefore, significant melt inhomogeneities are possible, even in the end of the melting zone. Thus, the resulting evaluation of the quality of mixing should be carried out with respect to the feeding zone.