An extruder for degassing a thermoplastic plastics material melt comprises at least one screw mounted for rotation in a barrel. The barrel includes a first feed aperture for the introduction of the melt and a degassing port downstream thereof connectable to means for degassing the melt. The at least one screw is mounted on a shaft and has a helical flight. In the region of the port, the flight is replaced by alternating disc members and spacer members mounted on the shaft. The diameter or major dimension of the disc members corresponds substantially to the combined diameter of the screw core and its flight whilst the diameter of the spacer members corresponds to that of the screw core. The disc members have profiled downstream faces and the thickness of each disc member and each spacer member lies in the range of 0.1 to 0.5 times the diameter of the screw.