In general, all polymer processing methods involve three stages- heating,
shaping and cooling of a plastic, processing However, this apparent simplicity
can be deceiving. Most plastic moulding methods are not straightforward and
the practical know-how can only be gained by experience, often using trial
and error methods. In most cases plastics processing has developed from other
technologies (e.g. metal and glass) as an art rather than as a science. This is
principally because in the early days the flow of polymeric materials was not
understood and the rate of increase in the usage of the materials was much
greater than the advances in the associated technology.
Nowadays the position is changing because, as ever increasing demands are
being put on materials and moulding machines it is becoming essential to be
able to make reliable quantitative predictions about performance. In Chapter 4
it was shown that a simple Newtonian approach gives a useful first approximation
to many of the processes but unfortunately the assumption of constant
viscosity can lead to serious errors in some cases. For this reason a more
detailed analysis using a Non-Newtonian model is often necessary and this
will now be illustrated.
Most processing methods involve flow in capillary or rectangular sections,
which may be uniform or tapered. Therefore the approach taken here will be to
develop first the theory for Newtonian flow in these channels and then when the
Non-Newtonian case is considered it may be seen that the steps in the analysis
are identical although the mathematics is a little more complex. At the end of
the chapter a selection of processing situations are analysed quantitatively to
illustrate the use of the theory. It must be stressed however, that even the more
complex analysis introduced in this chapter will not give precisely accurate
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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