Analysis of Flow in Extruder

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. 

For the small element of fluid ABCD the volume flow rate dQ % given by 

Substituting in (4.1) and integrating over the channel depth, H, then the total drag flow, Qd, is given by 

This may be compared to the situation in the extruder where the fluid is being dragged along by the relative movement of the screw and barrel. Fig. 4.8 shows the position of the element of fluid and (4.2) may be modified to include terms relevant to the extruder dimensions. 

In both cases, AB =dz, element width = dx and channel width = T Fig. 4.7 Melt Flow between parallel plates 

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C.J. Rauwendaal andj. Anderson, Finite Element Analysis of Flow in Extruders, 52nd SPE ANTEC, 298-305, San Francisco, CA (1994)... 

Expressions for the limiting shape factors when the width of the channel is small relative to the depth (W H ) are given hy Booy [29]. However, this type of channel geometry is generally not encountered in commercial twin screw systems. Numerical simulation of the flow and heat transfer in twin screw extruders is covered in Chapter 12. Section 12.3.2 discusses 2-D analysis of twin screws, and Section 12.4.3.3 deals with 3-D analysis of flow and heat transfer in twin screw extruders. Since 2000, major advances have been made in the numerical methods used to simulate twin screw extruders. The boundary element method now allows full 3-D analysis of flow in TSEs. A significant advance in the finite element method is the mesh superposition technique that allows analysis of complicated geometries with relative ease. This is discussed in more detail in Chapter 12. 

We have seen how the screw extruder pump is synthesized from a simple building block of two parallel plates in relative motion. We have also seen how the analysis of the screw extruder leads in first approximation back to the shallow channel parallel plate model. We carried out the analysis for isothermal flow of a Newtonian fluid, reaching a model (Eq. 6.3-27) that is satisfactory for gaining a deeper insight into the pressurization and flow mechanisms in the screw extruder, and also for first-order approximations of the pumping performance of screw extruders. 

B. H. Maddock, A Visual Analysis of Flow and Mixing in Extruder Screws, Tech. Papers, Vol. V, 15th Annu. Tech. Conf., Society of Plastics Engineers, New York, January 1959 also, B. H. Maddock, A Visual Analysis of Flow and Mixing in Extruder Screws, SPE J., 15, 383-389 (1959). 

Virtual Reality for the Analysis of Flow Phenomena in Compound Extruders... 

The most common way for three-dimensional simulations is the application of the FEA. The 3D analysis of flow processes in co-rotating twin-screw extruders is very time-consuming, because for transient geometrical adjustment in the intermeshing zone a new volume mesh must be generated for each new screw position (see top of Fig. 5.26). 

JL White, S Montes, JK Kim. Experimental study and practical engineering analysis of flow mechanisms and starvation in a modular intermeshing corotating twin screw extruder. Kunstoffe 43 20-25, 1990. 

Campbell G A, Sweeney P A and Felton J N (1992), Experimental investigation of the drag flow in extruder analysis Polymer Engineering and Science, 32, 1765-1770. 

This effect is very important during the preparation of the nanocomposite via melt blending, as observed by Gilman et al. for a PS nanocomposite, where gel permeation chromatography analysis of the samples, extruded without a nitrogen flow in the extruder, showed some evidence of degradation in the form of lower molecular weight. ... 

B. H. Maddock, A Visual Analysis of Flow and Mixing in Extruder Screws , SPE Journal, May, (1959), pp383-389... 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

In order to understand the mechanisms for fluid flow in the metering channel, it is important to understand the reference frames used in the mathematical analysis of the section. As discussed in many chapters leading up to this point, the extruder is a cylindrical structure, as shown in Fig. 7.1(a), that has a helical channel formed... 

As far as heat transfer is considered, Fenner [27] made a detailed comparison of the thermally fully developed flow and thermally developing flow. He indicated that the thermally developed flow will not be achieved when heat conduction effects become significant [34]. Bruker et al. [35] experimentally verified that the thermally developing flow analysis provided a more accurate description of the flow in the extruder. 

Figure 7.9 Laboratory (Eulerian) frame boundary condition for the analysis of extruder fluid flow. The observer is away from the channel. is the velocity of the screw core in the z direction and it is negative...

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