the invention relates to a single screw extruder comprising a barrier screw and a barrel in which the barrier screw is held in a manner that permits to rotate and comprising at least one feed zone longitudinal section and at least one melting zone longitudinal section . the extruder is characterized in that the barrel , on the inner wall thereof in the area of the melting zone longitudinal section , has at least one groove which runs in a longitudinal direction . the invention also relates to a method for extruding plastic material using a single screw extruder comprising a barrier screw which is held inside a cylinder in a manner that permits it to rotate , whereby the extruder comprises a feed zone and a melting zone , and the barrier screw has at least one solid matter channel and a melting channel . the method is characterized in that solid plastic material is transported in the area of the melting zone in a defined quantity out of the solid matter channel and into the melt channel .