Rotor zone cooling apparatus channels liquid coolant flow in rotors in continuous mixers of plastic materials for causing predetermined zones along the lengths of the rotors to be cooled as may be desired and for other predetermined zones to remain neutral, i.e. without significant cooling. A sequence of alternating cooling/neutral/cooling/neutral/cooling/neutral zones can be arranged, including zones of various possible length, various possible locations and various possible numbers of such zones. These zones are tailored or customized for optimizing temperature conditions along a rotor depending upon the mixing requirements and characteristics of plastic materials being mixed. Within an axial bore in a rotor is inserted a novel elongated cooling tubing assembly which rotates with the rotor. For creating a neutral zone, coolant flow is diverted inwardly into an isolation channel within an isolation tubing section spaced inwardly from the rotor bore wall. For creating a cooling zone, coolant flow is diverted outwardly from an isolation channel into a cooling channel located adjacent to the rotor bore wall. Coolant flow in such a cooling channel is located outside of a cooling booster tubing section. Isolation tubing sections and booster tubing sections are shown assembled in a versatile arrangement along a central tube. Rabbeted ring fittings have shoulders overlapping lips on adjacent support rings welded to respective tubing sections and hold seals in contact with the bore wall. A rotary joint near the bore entrance allows the whole coolant tubing assembly to rotate with the rotor.