An extruder screw that enables mixing of viscous materials, such as rubber mixtures and thermoplastic materials, is disclosed. The screw includes a core extending from a sealing end to a nose cone end. At least one flight extends radially from the core in a quasi-helical pattern, wherein flights adjacent one another form channels therebetween. A plurality of elongate lugs extend radially from the core and are disposed in the channels. The shape as well as the number of lugs can be varied to ensure the requirements for a thorough and turbulent deflection and mixing of the material stream.