Screw Simulation techniques

For the coarse estimation of extruder size and screw speed, simple mass and energy balances based on a fixed output rate can be used. For the more detailed design of a twin-screw extruder configuration it is necessary to combine implicit experience knowledge with simulation techniques. Theses simulation techniques cover a broad range from specialized programs based on very simple models up to detailed Computational Fluid Dynamics (CFD) driven by Finite Element Analysis (FEA) or Boundary Element Method (BEM). 

Very effective ways to minimise downtimes of crucial system components, for example hall screws, ball bearings and drives, are condition diagnostics and condition monitoring. Furthermore grouping of maintenance activities can either rednce downtimes or maintenance costs. Since many analytical models of technical systems have significant limitations and sim-phfications, simulation techniques are applied very often, see Bertsche (2008). 

With the development of modern computation techniques, more and more numerical simulations occur in the literature to predict the velocity profiles, pressure distribution, and the temperature distribution inside the extruder. Rotem and Shinnar [31] obtained numerical solutions for one-dimensional isothermal power law fluid flows. Griffith [25], Zamodits and Pearson [32], and Fenner [26] derived numerical solutions for two-dimensional fully developed, nonisothermal, and non-Newtonian flow in an infinitely wide rectangular screw channel. Karwe and Jaluria [33] completed a numerical solution for non-Newtonian fluids in a curved channel. The characteristic curves of the screw and residence time distributions were obtained. 

A three-dimensional simulation method was used to simulate this extrusion process and others presented in this book. For this method, an FDM technique was used to solve the momentum equations Eqs. 7.43 to 7.45. The channel geometry used for this method was essentially identical to that of the unwound channel. That is, the width of the channel at the screw root was smaller than that at the barrel wall as forced by geometric constraints provided by Fig. 7.1. The Lagrangian reference frame transformation was used for all calculations, and thermal effects were included. The thermal effects were based on screw rotation. This three-dimensional simulation method was previously proven to predict accurately the simulation of pressures, temperatures, and rates for extruders of different diameters, screw designs, and resin types. 

The three-dimensional FDM technique provided an excellent prediction of the pressure at 5.6 diameters from the start of the screw, as shown in Fig. 7.16. The method, however, is difficult to use and requires relatively long computational times on a fast computer. This example is an excellent test case for determining the acceptability of a simulation code. 

For these types of simulations, the rate 0 is set at a specific screw speed and entry pressure Pg. The simulation is then progressed until the discharge pressure at the end of the metering section is obtained. If the calculated discharge pressure is not correct, the screw speed is adjusted until the correct discharge pressure is obtained. In many cases, P is known based on experiments or from baseline simulations. The baseline technique is presented in detail in Chapter 9. 

To reduce this effort, the software Polyflow (Fluent, Lebanon, USA) contains a special module to avoid the remeshing of the flow channel for every single timestep. This is called the Mesh Superposition Technique , where the inner barrel and the screw are meshed separatly. The discrete meshes are overlayed to create one system where the surfaces of the screw define the channel boundary. A major issue with this method is that the flow channel volume varies as the intersection of the surface elements leads to unequal sums over all elements. This is compensated by a compression factor on which the simulation results react very sensitively. 

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. 

A new type of test methodology for characterizing engineering plastics has been developed. These techniques simulate the extrusion and molding process to show what a polymer would undergo in terms of shear, temperature, pressure, and residence-time deformation. The behavior of a compound can be accurately predicted prior to processing. An online-type rheometer continuously measures the viscosity of the polymer from the die on a real-time basis, and the data is used to make screw speed adjustment to keep the viscosity consistent (10). 

In this paper, the numerical simulations are applied to the rotor section with different process conditions. The software we used is originally developed 3-D flow analysis code based on FEM technique. The marker-particle tracking analysis is also applied in order to estimate the marker-particles pass, residence time distribution (RTD), and stress history. The kneading block (ICB) section of co-rotating twin screw extruder is investigated additionally in comparison with the mixing in... 

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