Paper to European Branch of the Institution of Chemical Engineers One-day Conference on Polymer Processing.

Published in The Chemical Engineer of March 1978, Issue no 330.
S F Bush with additional papers by G Schwarz, R Caloine and C Clayton

Introduction

For a number of reasons the mechanical and thermal processing of polymeric materials has developed in a largely empirical fashion over the last 30 years. One probable reason is that equipment manufacturers have not concerned themselves with the detailed properties of polymers, while until recently polymer manufacturers have relied on a few in-house specifications to characterise their materials.

In recent years, however, an effort to decrease processing times and improve quality in the manufacture of fibres, mouldings and films has led to a steady demand for improved quality and especially uniformity of polymer as supplied to the processor. Correspondingly there is now a greater interest in defining the limiting factors in the total polmer plus processing equipment system.

As its first technical venture the recently formed European Branch of the IChemE organised in conjunction with the Dutch engineering and chemistry societies a one-day conference to discuss the application of the basic principles of mass, momentum and energy transport to polymer processing. Accordingly, six of the seven contributions were oriented to specific operations, such as the drying and transport of polymer granules, injection moulding and the formation of laces (thick fibres) and foamed plastics, while the remaining paper, on bubble formation, dealt with phenomena of acute relevance to two of the others.

The basic lesson of the conference papers is that application of quite simple principles pays off at least as well on the polymer processing side as in the polymer manufacturing side. In particular, precision in mechanical layout, temperature control and setting is more important than in most mainstream chemical engineering operations. Both requirements are well illustrated by the paper on lace-forming. The individual components of a lace-casting line are relatively simple, but they must be precisely sized and positioned by being part of a larger system (e.g. to make granules and then fibres, films or mouldings), they can cause interruptions and out-of-specification production which can propagate through to the final customer. Temperatures at extrusion must be uniform within a few degrees on all co-extruded laces to avoid breakage and the excessive degree of line supervision then entailed, but once known, the required standard is readily obtainable in practical cases.

Two papers considered the drying, storage and pneumatic conveying of granules. Again the key sep in each case was the relating of well-understood theoretical ideas of momentum mass and thermal transport to equipment designed largely on a quite different basis. Three papers, however, touched on the less well understood phenomenon of bubble formation in polymer liquids and its crucial importance for foamed plastics. The final paper analysed the filling process in injection moulding and the relationships needed to maintain close control in practical cases.

Overall, the papers indicated the great scope for chemical engineering in the design of systems and equipment for polymer processing of all types.