Metering section

In the metering section of the screw, the channel is again a constant depth. This part of the screw is responsible for delivering molten polymer at a desired and uniform rate to the shaping device. Thus its function is primarily to convey the melted polymer. 

The fully melted polymer now enters the third zone of the extmder where it is pressurized. The buildup of pressure is required in order to pump the melt through the die at the end of the extruder. The pressurization of the melt is based on a viscous drag mechanism. We first illustrate how viscous drag can lead to a pressurization of the melt. This is followed by the development of a nonisothermal non-Newtonian model of the metering section. Because numerical methods are required to solve the equations generated in this model, we end the section by presenting the isothermal Newtonian case where an analytical solution is possible. 

This equation is solved using the following boundary conditions  

After integrating Eq. 8.70 and using the boundary conditions given above, the velocity field is determined to be 

Equation 8.72 is now integrated over the cross-sectional area to obtain the volumetric flow rate  

Q is seen to consist of two terms the first is called the drag flow, Qi, while the second is referred to as the pressure flow, gp. When there is no pressure buildup, the transport is due entirely to the drag flow term. However, if there is a significant pressure increase, then Q is decreased. In this case the pressure term can dominate to the point where flow can be in the opposite direction. (As we will see later this eannot happen in the extruder.) The main point is that as a result of viscous drag the fluid can be advanced against resistance due to pressure buildup. This is in essence the principle of operation of the metering section of the singlescrew extruder. 

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Tracer Type. A discrete quantity of a foreign substance is injected momentarily into the flow stream and the time interval for this substance to reach a detection point, or pass between detection points, is measured. From this time, the average velocity can be computed. Among the tracers that have historically been used are salt, anhydrous ammonia, nitrous oxide, dyes, and radioactive isotopes. The most common appHcation area for tracer methods is in gas pipelines where tracers are used to check existing metered sections and to spot-check unmetered sections. 

The screw consists of a feed section, a rapid transition section, and a metering section a rounded forward end prevents stagnation. The breaker plate that converts the rotary motion of the melt into smooth, straight flow should have as many holes as possible both ends of each hole should be countersunk for streamlined flow. 

Eig. 1. Parts of an extmder A, screw B, barrel C, heater D, thermocouple E, feed throat E, hopper G, thmst bearing H, gear reducer 1, motor , deep channel feed section K, tapered channel transition section and L, shallow channel metering section (15). 

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. 

The experimental system consists of three sections (i) a gas metering section with interconnected 4-port and 6-port valves, (ii) a reactor section including an in-situ diffused reflectance infrared Fourier transform spectroscopy reactor (DRIFTS) connected to tubular quartz reactor, (iii) an effluent gas analysis section including a mass spectrometer or a gas chromatograph (9). 

The gas metering section is designed to deliver controlled gas flows of C3H6 (Praxair, 99.99%), 02 (Praxair, 99.998%) and He (Praxair, 99.999%) to the reactor system via Brooks 5850 mass flow controllers at a total flow rate 40 ml/min. and latm pressure. Feed gas compositions are C3H6 (40%), 02 (10%) and He (50%) for the steady state reaction. Prior to each experiment, the catalyst was reduced in pure flowing H2 at 34 ml/min for 2 hours at 400 °C. 

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 set-up can be divided into three sections as shown in Fig. 3 the gas supply, the bubble forming section, and the gas metering section. 

Two driving forces for flow exist in the metering section of the screw. The first flow is due just to the rotation of the screw and is referred to as the rotational flow component. The second component of flow is due to the pressure gradient that exist in the z direction, and it is referred to as pressure flow. The sum of the two flows must be equal to the overall flow rate. The overall flow rate, Q, the rotational flow, 0 and the pressure flow, Qp, for a constant depth metering channel are related as shown in Eq. 1.12. The subscript d is maintained in the nomenclature for historical consistency even though the term is for screw rotational flow rather than the historical drag flow concept. 

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