The Krauss-Maffei system has been developed in collaboration with the Institute for Plastics Technology at Paderborn University, in Germany. It works by blowing cold air over the internal surface of the pipe, while conventional water cooling is used for the outer wall. The mandrel is 1 m long. The gap between the mandrel and the pipe is around 15 mm. Air is blown out of the die head into this gap.
Sag is a problem.
The key goal of the system is to stop the pipe from sagging as it exits the die, by stiffening the inner wall, but without affecting the pipe's mechanical properties. In normal HDPE pipe lines, the temperature of the inner wall of the pipe stays above that in the core of the wall. He says the new cooling system gets rid of the sagging without causing cavitation in the pipe wall. But the heat that is trapped in the middle of the pipe wall resoftens the cooled surface once the pipe is past the mandrel, preventing buildup of internal pores that are the natural result of a plastics melt shrinking between two solidified walls. But by this time, the overall temperature of the pipe is so low that sag is considerably reduced or even eliminated.