The basic principle of making parts from polymeric materials is in creating a melt from the solid material and forcing the melt into a die, the shape of which corresponds to the shape of the part. Thus, as Fig. 1.1 indicates, melt flow and heat transfer play an important role in the operations of polymer processing.
Extrusion is one of the most widely used polymer converting operations for manufacturing blown film, pipes, sheets, and laminations, to name the most significant industrial applications. Figure 2.1 shows a modern large scale machine for making blown film. The extruder, which constitutes the central unit of these machines, is shown in Fig. 2.2. The polymer is fed into the hopper in the form of granulate or powder. It is kept at the desired temperature and humidity by controlled air circulation. The solids are conveyed by the rotating screw and slowly melted, in part by barrel heating, but mainly by the frictional heat generated by the shear between the polymer and the barrel (Fig. 2.3). The melt at the desired temperature and pressure flows through the die, in which the shaping of the melt into the desired shape takes place.
The design of extrusion dies is based on the principles of rheology, thermodynamics, and heat transfer as described in the book by Natti Rao and Nick Schott, which is suggested for further reading, and listed along with the references at the end of this book. The quantities to be calculated are pressure, shear rate, and residence time along the die length. The most important parameter among these is, however, the die pressure as this quantity must match with the pressure created by the extruder.
Parametrical studies in polymer processing are of importance for determining the effect of significant parameters on the product quality. This section deals with those studies in blown film as an example.
The main features of the computer programs mentioned earlier for designing and optimizing extrusion machinery can be summarized as follows: VISRHEO: This program calculates the viscosity coefficients occurring in four different viscosity models, namely, Carreau, Muenstedt, Klein, and the power law, which have been treated in Section 1, on the basis of measured flow curves, and stores these coefficients automatically in a resin data bank.
In addition to the mechanical and melt flow properties, thermodynamic data of polymers are necessary for optimizing various heating and cooling processes that occur in plastics processing operations.
Heat transfer and flow processes occur in most polymer processing machinery and often determine the production rate. Designing and optimizing machine elements and processes therefore requires the knowledge of the fundamentals of these sciences. The flow behavior of polymer melts has been dealt with in Chapter 1. In the present chapter, the principles of heat transfer of relevance to polymer processing are treated with examples.
