Melt Conveying Zone

For simple melt conveying, screw elements with a pitch of 1 D are commonly used. If other components are to be added, e.g., via a side feeder, the pitch should be 1.5 D in order to increase the feed capacity. 

As will be shown in subsequent chapters, melt conveying can be calculated very well provided that sufficient material data is available. 

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This represents the volumetric throughput of the melt conveying zone. The first term after the equal sign in Eq. 7.198 is the drag flow term. It represents the flow rate in pure drag flow, i.e., without a pressure gradient in the down-channel direction ... 

Figure 7.57 Fluid motion in melt conveying zone...

As the melt conveying zone develops more pressure the spirals are pushed more closely together. In the extreme case of closed discharge (zero output) there is no axial component to the flow. In this case fluid elements move tangentially close to the barrel surface and in the opposite direction close to the screw surface the screw flights act as a plough, so there is mixing but no forward flow. 

The power consumption in the melt conveying zone is an important parameter to consider in screw design and in the analysis of actual extrusion operations. The power consumed for pumping in the channel is ... 

Similar problems occur in the melt conveying zone. A polymer element at about 2/3 of the height of the channel will have no cross-channel velocity component and as a result will have a short residence time in the melt conveying section and little mixing... 

With this expression the distributive mixing process in the melt conveying zone of a single screw extruder can he evaluated quantitatively. The total shear versus normal distance for several values of the throttle ratio is shown in Fig. 7.130. 

The simplicity of Eq. 8.39 makes it a useful and convenient expression for optimizing the geometry of the melt conveying zone of an extruder. For polymer melts with a power law index in the range of 0.3 to 0.4 and typical values of the flight width, the optimum helix angle is about 22 to 24°. 

This screw is more appropriate for a plasticating unit of an injection molding machine. The zero-meter screw is used to reduce the temperature build-up in the material by deepening the depth of the channel in the melt conveying zone of the extruder. The obvious drawback is that the pressure generating capability of the screw will be adversely affected, but this is not a major concern in injection molding applications. In other applications, however, the approach outlined in Section 8.3 is recommended. An extension of the zero-meter screw is the zero-feed zero-meter screw shown in Fig. 8.34. 

Rauwendaal [84] developed a finite element method (FEM) program to determine temperature profiles in the melt conveying zone of extruders. This FEM program allows the calculation of three-dimensional velocities and temperatures at any point in the screw channel. The program is based on a 2 A-D analysis, which means that the velocities are assumed to change little in the down-channel direction. 

M0lt TGmpGraturGS. Because of the low thermal conductivity of the plastic melts, the temperatures in the melt conveying zone can vary substantially. Local temperatures are difficult to measure because it hard to put a probe in the... 

DiStribUtiVG Mixing. The extent of distributive mixing can be determined from the total shear deformation of the plastic melt in the melt conveying zone. The total shear deformation is also called the total shear strain-, it is determined by the product of the shear rate and the length of time that the fluid is exposed to the shear rate. For instance, if the plastic melt is exposed to a shear rate of 100 s for 15 s, the resulting shear strain is 100 x 15 = 1500. The units of shear strain are dimensionless. The shear rate is determined by the velocity profiles in the extruder. 

It should be noted, however, that in plasticating extruders WATS defines mixing only in the melt-conveying zone of the extruder and neglects the very substantial contribution to... 

If the kneading block is placed in the melt conveying zone, the polymer is already melted wh reaching the inlet of the kneading block. The melt mixture is expected to be further compounded in the kneading blocks. 

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