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.