Screw transition zone

Z-N catalyst See catalyst, Ziegler-Natta. zone A location in a plasticating screw. See screw feed zone screw metering screw transition zone, zone axis A group of crystal planes that are all parallel to one line is called the zone axis. 

As the polymer granules enter the melting zone (which we also refer to as the transition zone), their temperature rises and they begin to melt, The heat required to melt the polymer comes from two sources, external heaters and mechanical work from the action of the screw. When we extrude viscous polymers at high rates we may need to cool the barrel to remove some of the heat induced by working the molten polymer. 

The melting zone. The melting or transition zone is the portion of the extruder were the material melts. The length of this zone is a function of material properties, screw geometry, and processing conditions. During melting, the size of the solid bed shrinks as a melt pool forms at its side, as depicted in Fig. 3.10 which shows the polymer unwrapped from the screw channel. 

Transition Zone-. It is the section, also called the compression zone, of a screw between the feed zone and metering zone in which the flight depth decreases in the direction of discharge. In this zone the plastics starts in both solid and molten state with target to have all molten upon leaving this zone. 

For PCITPU or TPUIPC blends (Texin tm), the screw should have a long gradual transition zone and a long metering zone. 

Helical transition n. In the transition zone (compression zone) of an extruder screw, the root surface of the screw describing an advancing spiral surface of increasing radius. 

Transition section (transition zone, compression section) n. In a metering-type screw for a single-screw extruder the section of decreasing channel volume per turn between the feed and metering sections, in which the plastic is changing state from a loosely packed bed of particles-cwm-voids, to a void-free melt. The transition may be... 

In metering screws, the flighted section is divided into three zones feed, transition or compression, and metering. The feed zone has a constant channel depth as does the metering zone. However, the channel depth gradually decreases in the transition zone. Since molten polymer requires less volume than the solid particles, the metering zone channel depth is shallower than the depth of the feed. The squeezing of the polymer is quantified by the compression ratio (CR) ... 

The recommended screw design is one having three zones of approximately equal length. Screws with sharp transition zones are not recommended. 

The recommended L/D ratio of the extruder is at least 24 1. The preferred screw design should have compression ratio of 3 1 and should feature a long, gradual transition zone and a long metering zone. Temperature setting should be between 200 and 230 C. 

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