Contrary to the case for the melt conveying zone, no adequate description exists as yet for the solids conveying zone of a co-rotating twin-screw extruder. A physico-mathematical model of the solids conveying zone, however, is a prerequisite for determining the location of the plasticizing zone and for establishing the optimum screw configuration that goes with a selected point of operation. This paper presents a physico-mathematical model to describe the processes taking place in the solids conveying zone. The conveying zone is divided into a partially-filled feed zone and a completely filled compression zone. Force and torque balances are drawn up for three different volume elements and, with the solutions obtained for these, it is possible to calculate the maximum conveyable throughput, the pressure profile over the length of the solid material accumulations and the drive power that is required or the solids conveying zone.