The use of online profilometers upstream near the die (hot location) and at the end of the line (cold location) enables faster and more accurate control of extrudate dimensions. cascade control procedure presented here uses the end-of-line profile dimensions to update the upstream (hot) set points periodically during a run. n more frequent adjustments to the extruder(s) or die(s) or feed composition are made to bring the upstream hot dimensions to their upstream hot set points.
This control concept is illustrated by a procedure for a rectangular-shaped profile being off spec because of deviations in extrudate swell and in profile cross-sectional area (drawdown error). Correcting the area deviation by adjusting screw speed causes both the thickness and width dimensions to change in the same direction with thickness changing at a faster rate than width. With swell deviation, the thickness and width dimensions change in opposite directions. A high swell deviation means the extrudate thickness is increased and the width is decreased while the cross-sectional area of the extrudate remains constant.
Area deviations are easily corrected by adjusting screw speed to get the area of the profile on spec. Correcting swell deviations requires a shape change in which the thickness and width dimensions change in opposite directions. Several ways to bring about this shape change are discussed; for this presentation an adjustable die will be the means for shape control.
To make the needed adjustments to both screw speed and the adjustable die simultaneously, models are used to relate thickness and width dimensions to die adjustments and to changes in screw speed. When these calculated adjustments are made to screw speed and to the die, the upstream (hot) dimensions are expected to meet the upstream hot specified dimensions. The hot profile specification had been set previously to make the downstream cold dimensions correspond to the end-of-line specification.