Closed Loop Pressure Control for the Extrusion Process

Extrusion is a continuous process and successful economic production depends on maintaining stable output and melt quality at an accurately controlled rate. Current screw design technology, the use of DC drives, computerized controllers and raw material testing help to deliver the melt to the extrusion die at relatively constant temperature, pressure and viscosity. However, even the slightest variation in the raw material or process will affect product dimensions. The manufacturing of small diameter tubing which must be held to ± 0.001” or less dimensional tolerances, is particularly sensitive to process instabilities. In the production environment, variations in the raw material and process do occur. Mechanical parts wear, monitors vary in speed, temperatures go above and below setpoint and ambient conditions change. Some plastic materials are sensitive to moisture and storage temperature. All these conditions are subject to change and they all affect the stability of the extruder output and the product quality (1). The extruder screw is the one item that most influences the quality and quantity of the melt produced by the machine. The most common extruder screw design has a feed zone, transition zone and metering zone. Each of these zones within the extruder must perform a specific function simultaneously to achieve stable output.