an extruder screw for use in a barrel of an extruder for forwarding thermoplastic polymer from a first end to a second end of the extruder . the extruder comprises a transition section and at least one flight extending substantially along the length of the screw . a dam is positioned at the point where the rapid melting of the solids starts to slow down and the rate of solid bed width decrease approaches zero . the dam provides the screw with a high output of extrudate and close control of the residence time of polymer particles as they are transported through the screw , resulting in an extrudate which is uniformly melted and which is substantially free of degradation products and / or entrapped air .