A screw extruder for the continuous extrusion of materials with high viscosity, in particular metals such as aluminum and its alloys. The extruder includes an Archimedes screw (1) rotationally provided within a liner (2) of a screw housing (4) having an inlet (11) for the feeding of the material to be extruded, a compacting or extrusion chamber (5) and an extrusion die assembly with a die (6) which forms the shape of the extruded product (7). The required compaction takes place at the down-stream end of the screw towards the extrusion chamber (5) corresponding to up to 540° of the rotation of the screw, or up to 1.5 turns of the screw flight length. The solid plug of metal formed at the end of the screw and extrusion chamber (5) is restricted from rigid rotation to obtain the required compaction and extrusion pressure.