Abstract: A screw conveyor which is particularly used for feeding in and compressing ligno-cellulosic material, such as wood chips, shavings, bagasse, sawdust and similar compressable material. The screw conveyor comprises a casing having a bore which tapers conically from a material inlet and to a material outlet end. A screw feeder having helical flights and an intervening spiral groove rotates within the bore to advance the material fed into the casing towards the outlet end while being progressively compressed into a plug. The casing has an opening in which stopper means move in a closed circuit to successively engage the spiral groove during the rotation of the screw feeder, thereby preventing the material from rotating and thus allowing it to be continually advanced while being progressively compressed.