Undesirable or incidental orientation

There are essentially four types of process that may be used  

Two or more of these methods may sometimes be used together, either simultaneously or sequentially. 

To understand the conditions for orientation to be achieved by any of these processes it is necessary to consider the forms that may be taken by the load-strain curves for a polymer. These are considered briefly in section 6.1 and fig. 6.2 is reproduced here as fig. 10.1. Most thermoplastic polymers can exhibit behaviour ranging from (a) to (e) under the appropriate testing conditions, e.g. the correct temperature and strain-rate and whether the stress is apphed in tension or compression. The behaviour illustrated is as follows (for a sample initially of uniform cross-section). 

2 The appearance of a sample undergoing necking when drawn (a) the sample before drawing, with a reference grid of squares marked on it and (b) after drawing. The central section has increased in length by a factor of about five, i.e. the draw ratio is 5. (Courtesy of Dr J. Sweeney and Dr P. Caton-Rose.) 

Some of the processes of yield and fracture are discussed more fully in chapter 8. Only behaviour of the types shown in (c), (d) and (e) leads to significant orientation and, whichever of the processes (i)-(iv) is used to produce orientation, one of these types of behaviour is required. The individual processes are described briefly in the remainder of this section. 

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