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246                                             Processing of Plastics

                         4.2 Extrusion
                         4.2.1 General Features of Single Screw Extrusion
                         One of the most common methods of  processing plastics is Extrusion using
                         a  screw inside a  barrel as illustrated in  Fig. 4.1. The plastic, usually in the
                         form of  granules or powder, is fed from a hopper on to the screw. It is then
                         conveyed along the barrel where it  is heated by  conduction from the barrel
                         heaters and shear due to its movement along the screw flights. The depth of
                         the screw channel is reduced along the length of the screw so as to compact the
                         material. At the end of the extruder the melt passes through a die to produce an
                         extrudate of the desired shape. As will be seen later, the use of  different dies
                         means that the extruder screwharrel can be used as the basic unit of  several
                         processing techniques.

                                                                Powder  or
                                                                 granules


                                               Heoter
                                 Die           bands

                                                                    I  .






                                      Filter       Rototing
                                      plate         screw
                                      . __      - -     _-.

                                        Fig. 4.1  Schematic view of single screw extruder

                           Basically an extruder screw has three different zones.
                           (a) Feed Zone The function of this zone is to preheat the plastic and convey
                         it to the subsequent zones. The design of  this section is important since the
                         constant screw depth must supply sufficient material to the metering zone so
                         as not  to  starve it, but  on  the other hand not  supply so much material that
                         the metering zone is overrun. The optimum design is related to the nature and
                         shape of the feedstock, the geometry of the screw and the frictional properties
                         of the screw and barrel in relation to the plastic. The frictional behaviour of the
                         feed-stock material has a considerable influence on the rate of melting which
                         can be achieved.
                           (b) Compression Zone In this zone the screw depth gradually decreases so
                         as to compact the plastic. This compaction has the dual role of  squeezing any
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