Elimination of Sag in Plastic Pipe Extrusion

Plastic pipe is produced by extruding molten polymer through an annular die. As the pipe leaves the die, it passes through a sizing sleeve and then a cooling tank where it solidifies slowly. For thick walled pipe, the inside of the pipe remains molten for as long as ten hours causing downward melt flow called sag. Sag can cause serious non-uniformity in pipe wall thickness. The conventional way to reduce sag is by manually adjusting the die eccentricity, until an acceptable wall thickness profile is achieved. This tedious trial and error procedure can take up to four attempts to get the right profile. In this paper a new way has been proposed to reduce sag, by rotating the pipe during cooling. A numerical simulation of sag flow while pipe is rotating, predicts that for rotational speeds between 0.1 to 10 rad/s, sag is virtually eliminated for thick walled, large diameter HDPE pipe extrusion.