Extruded plastic pipe leaving an annular die is solidified in a long cooling tank by spraying the outer surface with cold water. The inside surface can take a long time to solidify as the solidification progresses radially inward. This results in flow of molten polymer down the inside of the pipe. This gravity flow of molten extrudate is called slumping, and it can cause serious nonuniformity in pipe wall thickness particularly in large diameter, thick walled pipes. It can also lead to another phenomenon known as “knuckle” formation where melt accumulates at specific locations. A simple numerical scheme to model this flow has been developed. Three-dimensional graphical illustrations of the slumping phenomenon based on this simulation are presented in this paper. The model predictions have been compared with commerical pipe data and they are in qualitative agreement.

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