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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. It is shown how the simulation can be used in an iterative, forward-backward calculation, based on the time reversibility of the creeping gravity flow problem, to calculate the initial extru-date wall thickness 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 are used. Reduction of the cooling rate of the lower part 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.

Production of Pipe with Uniform Wall Thickness: How to Compensate for Gravity Sag