Screw metering zone

Attention also has to be paid to screw design. The screw metering zone should be long and shallow to ensure melt homogenisation but not so shallow that... 

Max. screw speed = 100 rev/min, screw diameter — 40 mm flight depth (metering zone) = 3 mm. 

As the screw turns, it conveys the pellets from the feed zone towards the melting zone. A combination of external heating and mechanical work melts the polymer as it is transported towards the metering zone. The metering zone pumps a uniform stream of molten polymer to a forming device, such as a profile die. Other types of extruders that employ two or more screws are commonly used for compounding polymer blends. The principles of twin screw extrusion will be discussed in Chapter 12. 

The build up of pressure is aided by the use of a breaker plate, which is a thick disk of perforated metal positioned just downstream of the tip of the screw. A screen pack, comprising layers of woven metal mesh, is placed on the upstream side of the breaker plate. The screen pack increases back pressure within the metering zone and acts as a filter to catch any extraneous material that may have entered the extruder. 

Why does the channel depth of the screw generally decrease as you move from the feed zone to the metering zone of an extruder ... 

K. Alemanskin and I. Manas-Zloczower, Color Mixing in the Metering Zone of a Single Screw Extruder Numerical Simulation and experimental Validation, Polym. Eng. Set, 45,1011-1020 (2005). 

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. 

Metering Zone This section is a relatively shallow portion of the screw at the discharge end with a constant depth and lead usually having the melt moves 3 or 4 runs of the flight length. 

The friction at the inner surface of the barrel is the driving force for the material feed, whereas the friction at the surface of the screw restricts the forward motion of the material. A high friction coefficient in the barrel and a low friction coefficient at the screw surface would contribute to a more efficient transfer of the materials in the feed section. Other properties of the feed stock, such as bulk density, particle size, particle shape, and material compactability, can also have an impact on the performance of the feeding section. The transfer of the material should be efficient in order to maintain an increase in pressure in the compression zone and the metering zone. The pressure rise in these... 

The polymer is fed into the feed hopper, which regulates flow into the screw. As the screw rotates, feed is drawn into the feed section and flows in the feed zone in the helical flow channel between the rotating screw and the barrel wall. Typically this channel depth decreases in the compression zone, which promotes increasing pressure in the extruder to promote mixing and expel air voids. In the flnal metering zone the channel depth is constant again, and the material is homogenized and supplied to the die zone at constant temperature and pressure to ensure consistent production. The die zone imparts the cross-section to the profile, screens out impurities and distributes the material evenly across the cross-section. 

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