Screw flight pitch

Screw flight pitch in the melting zone, variations in pitch and changes in pitch of one flight relative to the other. 

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

Fig. 6.4-38 sho vs the functional components. At the inlet on top of the equipment, the slightly pre-mixed feed components enter first a mixing and predensification screw. Pressure builds-up by the changes in shaft diameter, sometimes variable pitch of the screw flights, and by a collar at the end of the screw shaft or a pressure piece , both forming an annular space with reduced area through which the material must pass. Such a screw conveyor, mixer, and processor is often called expander. 

Screw flights increase from 2/3 to full diameter. Used as feeder screws for uniform withdrawal of lumpy materials. Generally equivalent to and more economical than variable pitch flight. 

Cyclic variations The face of the screw flight moves material forward and there is a tendency for a gap to develop behind the working face. This is more pronounced when the pitch of the screw is expanded... 

There are various techniques used to increase the extraction capacity along the axis of a screw. The most influential is to increase the screw diameter, as the cross-sectional area increases as the square of this dimension. Other ways are to increase the pitch, reduce the centre shaft diameter, or reduce the friction on the face of the screw flight. These variations can be made in virtually any combination, or independently. The effect of these changes will be examined in detail. 

---