Up to now, all the theories on solids conveying in single-screw extruders (Darnell and Mol, 1956; Broyer and Tadmor, 1972; Campbell and Dontula, 1995; Hyun and Spalding, 1997) considered the solids to move as a plug flow (i.e., there is no relative movement in the granular solid, the velocity of the granules is constant in time and uniform in space). Therefore, the solids were treated as a rigid body which never experienced any deformation. But our experiments have shown that in corn meal extrusion, the solids move as a plug only in the loose powder region. After the start of compaction, there is a helical flow in the extruder channel. The flow in this region is not a plug flow at all, and the convective heat transfer is not negligible. Based on our experiments, the whole extrusion process can be divided into four zones, each one having different characteristics. Each zone should be treated with different motion models.