a cooling system for quenching extruded thermoplastic tubing which moves in a vertically upward direction from a die to a cooling mandrel comprises a seal means between the die and the mandrel and the introduction of cooling fluid such as water between the seal and the mandrel to flow upwardly as the tube passes along the mandrel . a water vapor atmosphere below the seal provides a controlled atmosphere leaking past the seal which condenses immediately upon passing the seal and coming into contact with the cooling fluid . this controlled atmosphere avoids air leakage past the seal and consequent imperfections in the thermoplastic material tube .