method for dispersing a fluid additive within only a portion of a stream of thermoplastic material comprising the steps of flowing said stream of thermoplastic material into a channel having walls of substantially circular cross section , into which there has been placed a rotating mixer head having in the surface thereof a plurality of alternate lands and grooves each of said lands and grooves having an upstream end and a downstream end , said lands alternating with said grooves and being arranged in pairs , each of said pairs having a land in loose clearance with said wall followed by a land in close clearance with said wall , said mixer head having an unobstructed internal passage positioned therein over a substantial portion of its terminal length and a plurality of conduits severally connecting elongated zones between said lands pairs and said hollow internal passage , and injecting said fluid additive into said channel at a position between said upstream and downstream ends of said lands and grooves .