An improved extruder (10) is provided which permits successful introduction of very high quantities of injected steam into material being processed, on the order of 6-8% or more by weight steam. The extruder (10) includes an elongated extruder
barrel (12) having at least one elongated, axially rotatable, helically flighted
extrusion screw (16,18) therein. The
barrel (12) is equipped with obliquely oriented
steam injection ports (44, 46) along the length thereof, housing steam injectors (48, 50). The
barrel (12) includes relatively high free volume
steam injection heads (32 and 38, 40) having therein screw sections (78, 82) of relatively long
pitch length, together with steam restriction heads (30, 34, and 42) on opposite sides of the injection heads (32, and 38, 40) having therein relatively short
pitch length screw sections (76, 80, 84).