A single or twin-screw device is used to incorporate additives in a thermoplastic plastics material melt. The or each barrel of the extruder includes a first feed aperture for the introduction of the melt and a second feed aperture downstream thereof for the introduction of the additives. A screw is mounted on a shaft in each barrel and includes a core and a helical flight. In the region of the second feed aperture, the flight is replaced by alternating disc members and spacer members mounted on the shaft. The diameter or major dimension of the disc members is approximately equal to the combined diameter of the screw core and its flight while the diameter of the spacer members is approximately equal to that of the screw core. The disc members have profiled downstream faces. The disc and spacing members increase mixing and consequently throughput in the region of the second feed aperture.