In-Line Rheological Measurements for Extrusion Process Control

Screw extrusion lies at the heart ofmelt processing of polymers. The bulk of polymers pass through some form of melt extruder, for example: immediately on exiting a polymerisation reactor; in a compounding or pelletising operation prior to further melt processing; in extrusion of semi-finished or finished products such as pipes, profiles, films, fibres and coatings; and in reactive extrusion often for modification of the properties of the polymer. Examination of polymer processing equipment over the past two decades shows that extrusion has tended to lag behind injection moulding in terms of machine control technology and software. The microprocessor revolution of the late 1970s was relatively quickly embraced by injection moulding machinery manufacturers, bringing several benefits, including enhanced sequence control of machines with associated improvements in repeatability. This lag might have been expected because: (i) a much higher added value is generally associated with injection moulded products, particularly high precision products injection moulding technology has consequently enjoyed higher investment; (ii) the two processing techniques differ in nature there is less obvious gain from sequence control improvements in a continuous process such as extrusion (although resin handling and downstream devices may require sequence control) compared with the essentially batch injection moulding process. In recent years there has been an increasing recognition by extruder manufacturers that computer control of extrusion lines is important in the drive for improved quality. The most easily implemented control systems, such as barrel temperature control using PID or modified PID computer