Extruder screw having an axially extending extruding portion defined by a feed section, a barrier section and a metering section. At least one helical primary flight extends about and is coaxial with the screw body. The barrier section includes at least one helical secondary flight extending from the primary flight about the screw body which in turn defines a first surface of revolution between the primary and secondary flights. At least one helical tertiary flight is positioned between the primary and secondary flights and also extends about the screw body which defines a second helical surface of revolution between the primary and tertiary flights. A third surface of revolution is formed between the primary and tertiary fligths and extends axially along the barrier section. A series of circumaxially contiguous cam-like forms are created by a second and third surfaces of revolution.
