256 IDENTIFICATION OF DYNAMIC MODELS FOR PLASTICS EXTRUSION PROCESSES IN RELATI ON TO CGMPUTER CONTROL STUDIES A.K. Kochhar, Research Fellow in Computer Aided Manufacturing Control Systems University of Bradford Bradford, U.K. ABSTRACT This paper first discusses the way in which, under the twin pressures of high raw materials costs and increasing demands from customers for higher quality of products, the plastics industry is being forced to improve extrusion process control systems. Alternative methods of determining extrusion process models, suitable for high level control, are examined and the importance of time- series techniques for feedforward control is demonstrated. The results of identification experiments, carried out on a single screw used for processing polyethylene, are described. The models are validated using statistical diagnostic checks and by testing the models with measured process data. INTRODUCTION The process feedstock for plastics manufacture is oil. Recent instabilities in the oil pricing structure have caused unprecedentedly large changes in the economic structure of the plastics industry and the machinery manufacturing system supplying the plastics Before these changes occured it was not untypical for plastics processolli to have 15% - 30% of scrap or re-cycled scrap product. The current very high cost of raw materials is now providing the stimulus for develop- ing and interlinking the two essentially complemen- tary aspects a) improvements in plastics processing machinery design b) improvements in process control. There is also the social issue, i.e. process contrm should be improved anyway to minimise process scrap rates and improve product tolerance so as to con- serve scarce world oil resources. It is probably true to say that in relation to many market areas the plastics processing industry must improve its process control systems if it is to survive. Because of the complexity of the processffi involved this will inevitably involve the use of on-line computers. Between 1950 - 1970 the world consumption of plastics grew at an annual rate of 16%(1) and simultaneously an increasingly sophisticated J. Parnaby Professor of Manufacturing Systems Design University of Bradford Bradford, U.K. market for the products demanded higher and h igher product quality. As a consequence there has been an evolutionary development within the industry of control systems of the interactive low level single-input/single- output type. This has occured together with the implementation of simple logic control functions. There is now an urgent need for research work aimed at developing high level control systems using modern theoretical techniques. Typical of the high product tolerances demanded are those of B.S.S.3505 relating to extruded plastics pipework. Also, as plastics have been increasingly used in engineering applications re- placing precision made metal components such as gears, the demands made of the process control systems have increased. The majority of the high tonnage industrial plastics such as polyethylene, P.V.C., polypropy- lene and polystyrene pass at some stage through an extrusion process. This means that the plastics extruder has a central and most important role in the plastics processing industry and the research work related to identification and computer control of plastics extrusion described here is readily justified. 2 THE EXTRUSION PRO CESS There are two basic types of plastics extruder; the screw extruder and the twin screw ex- truder. The experimental aspects of this paper are entirely associated wit h the single screw plastica- ting extruder although it is believed that the techniques applied will be equally suitable for twin screw control studies. The single screw extrusion process is shown in figure 1. A screw rotates in a heated cylindrical barrel having a die at the outlet end. Solid polymer granules are added to the feed hopper and are transported by the screw to the die. As a consequence of shearing in the screw and heat trans- fer through the barrel the polymer is melted. The melt is extruded through the shaped die at high pressures up to, typically, 1000 atmospheres. Ideally the screw should deliver melt to the die at accurately controlled temperature Td and