Page 262 - Plastics Engineering

CHAPTER 4 - Processing of Plastics

4.1 Introduction

One of the most outstanding features of plastics is the ease with which they can
be processed. In some cases semi-finished articles such as sheets or rods are
produced and subsequently fabricated into shape using conventional methods
such as welding or machining. In the majority of cases, however, the finished
article, which may be quite complex in shape, is produced in a single operation.
The processing stages of heating, shaping and cooling may be continuous (eg
production of pipe by extrusion) or a repeated cycle of events (eg production
of a telephone housing by injection moulding) but in most cases the processes
may be automated and so are particularly suitable for mass production. There
is a wide range of processing methods which may be used for plastics. In most
cases the choice of method is based on the shape of the component and whether
it is thermoplastic or thermosetting. It is important therefore that throughout
the design process, the designer must have a basic understanding of the range
of processing methods for plastics since an ill-conceived shape or design detail
may limit the choice of moulding methods.
In this chapter each of the principal processing methods for plastics is
described and where appropriate a Newtonian analysis of the process is devel-
oped. Although most polymer melt flows are in fact Non-Newtonian, the simpli-
fied analysis is useful at this stage because it illustrates the approach to the
problem without concealing it by mathematical complexity. In practice the
simplified analysis may provide sufficient accuracy for the engineer to make
initial design decisions and at least it provides a quantitative aspect which
assists in the understanding of the process. For those requiring more accu-
rate models of plastics moulding, these are developed in Chapter 5 where the
Non-Newtonian aspects of polymer melt flow are considered.

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