a sizing tool for extruding plastic tubing is disclosed . the tool includes a peripheral wall having an inner surface that defines the outer dimension of a plastic tubing advancing from an extruder through the tool under influence of an internal pressure exceeding the pressure prevailing in the tool . the tool also includes a lubricating water channel system in the peripheral wall and extending along the tool length and having at least one opening communicating with the inner surface of said peripheral wall . the tool also includes a cooling medium channel system in the peripheral wall and extending along the length . the cooling medium channel system being substantially closed within the peripheral wall and being separate from the lubricating water channel system .