Abstract: Material to be colored is fed from an extruder to a positive displacement means such as a gear pump. The output from which is split into at least two streams one of which is fed to a mixer which is preferably a cavity transfer mixer. Colorant is injected into one of the streams either before or at the mixer and the two streams are recombined downstream of the mixer preferably in a diehead. The arrangement provides for reliable coloration whilst premitting flexibility in production.

Abstract: An extruder comprising a cylindrical extruder housing and an extruder screw extending inside the housing to define an intake section, a mixer section, and a discharge section, the screw having a basic flight which extends in an uninterrupted helix from the intake section, through a mixer section to the discharge section and the basic flight is of one diameter in the intake and discharge sections so as to be a running fit in the housing but have a diameter in the mixer section less than the one diameter so as to define in use a clearance fit in the housing and, in the mixer section, a flow advancer and diverter between appearances of the basic flight which over at least the greater part of its length is of the one diameter so as also to be a running fit in the housing so that material fed to the mixer section is caused, in the mixer section, to pass over the basic flight.