The screw extruder serves to form a coating on welding electrodes and comprises a feed hopper, two feeder screws, which are disposed in the feed hopper and mesh with a backlash, a precompacting screw, which is disposed below the feed hopper, a first housing surrounding said precompacting screw, two highpressure screws, which mesh substantially without backlash, and a second housing surrounding the two high-pressure screws. The feeder screws are operable to deliver coating composition to the precompacting screw and the latter is operable to deliver coating composition to the two high-pressure screws.