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... 

As discussed in the previous section, it is convenient to consider the output from the extruder as consisting of three components - drag flow, pressure flow and leakage. The derivation of the equation for output assumes that in the metering zone the melt has a constant viscosity and its flow is isothermal in a wide shallow channel. These conditions are most likely to be approached in the metering zone. 

Note that the shear rate in the metering zone will be given by Vd/H. 

It should also be noted that in some cases correction factors, Fj, and Fp are applied to the drag and pressure flow terms. They are to allow for edge effects and are solely dependent on the channel width, T, and channel depth, h, in the metering zone. Typical values are illustrated in Fig. 4.11. 

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 final zone of an extruder is the metering section (also known as the pumping zone). The principal function of this zone is to ensure a steady output of molten polymer at a constant pressure. The longer the metering zone, the greater the pressure built up within it. Channel depth is constant to ensure a uniform transport rate, which helps reduce pressure fluctuations arising in the mixing zone. 

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). 

Fig. 11.11 The cumulative RTD function F(t) versus dimensionless time, t/1 for the metering zone of an SSE compared to plug flow, pipe flow (for Newtonian and isothermal conditions), and the very broad CSTR.

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