Mixing in Single Screw Extruders

Extrusion is used for about half of all plastics product manufacture, mostly using single screw extruders. Very often part of their role is the incorporation of one or more of a very wide range of additives, mainly in the form of masterbatches. These range from very visible colours to the invisible traces of anti-block and slip additives. There are also continuing pressures to improve overall economics and these require increased mixing performance in many cases. The situation that mixing in conventional single screw extruders can easily be sub-standard for the application is illustrated in the book's first chapter which describes a number of real examples of inadequate mixing. The book explains why these typical shortcomings occur and the application of mixing principles plus various practical approaches to eliminating such problems. With the growing pressures to increase the amount of plastics recycling, both the limitations and success in blending the mainly incompatible polymer combinations are explained. The development of 'add-on' cavity mixers and floating ring mixers, together with their methodology are described, whilst the associated innovative techniques using liquid injection of colours, tackifiers, lubricants, crosslinking agents and foaming agents, (particularly carbon dioxide) are included. Developments in controlled levels of blending by 'chaotic mixing' to produce products with very specific properties such as barrier films is briefly described. Extrusion tests for carbon black dispersion are included and the book concludes with a practical guide to the preparation of microtomed plastics specimens for evaluation by optical microscopy.