Abstract: A plasticizing screw which has a feed zone, a plasticizing zone, and a metering zone. In the feed zone the screw has a single flight which forms a feed channel and which feeds solid material to the plasticizing zone. At the beginning of the plasticizing zone, a secondary or barrier flight is introduced to divide the feed channel into a solids channel and a melt channel. The solids channel is cut deeper than the feed channel at the beginning of the plasticizing zone so as to be able to accommodate all of the solids contained in the feed channel even though its width is substantially less than the width of the feed channel. The depth of the solids channel gradually decreases as it approaches the end of the plasticizing zone.The depth of melt channel is shallow at the beginning of the plasticizing zone and gradually deepens as it approaches the end of the plasticizing zone so as to accommodate all of the plasticized material.