A detailed simulation of the cooling process in the production of plastic pipes has been used to gain insights into the problem of gravity flow or sag, which can lead to troublesome wall thickness variations in thick walled pipes. A is shown how the simulation can be used in an iterative, forward backward calculation, bused on the time reversibility of the creeping gravity flow problem, to calculate the initial extrudate wall thick-ness profile that the die must deliver to precisely compensate for sag. For pipes with particularly thick walls, this calculation shows that no compensating initial profile exists whilst conventional cooling arrangements ave twee! Reduction of the cooling rate of the lower parr of the pipe promotes wall thickness uniformity and the existence of a compensating profile. Means for achieving the required initial profiles are discussed and the need for an integrated view of the design and operation of the die and cooling line in understanding and overcoming the effects of sag is emphasised.