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3D FEM Analysis of a Wave Type Screw Channel

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The paper presents a fundamental study of the polymer flow within a “wave” type screw channel. The analysis is performed on an “unwrapped” form of a conventional screw channel and a “wave” type …

The paper presents a fundamental study of the polymer flow within a “wave” type screw channel. The analysis is performed on an “unwrapped” form of a conventional screw channel and a “wave” type channel of similar size. A 3D Finite Element Method
(FEM) simulation was used to simulate the flow field and flow characteristics of the wave channel are compared relative to the plain channel.

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  • 1. 3D FEM ANALYSIS OF A WAVE TYPE SCREW CHANNEL John Perdikoulias, Compuplast Canada Inc, Canada Jiri Svabik, Compuplast international Inc. Czech Repiblic Tim Womer, Xaloy Inc., USA Abstract any potentially un-melted particles. While these claims have been verified experimentally and in theThe paper presents a fundamental study of the field, the flow behavior is mostly based on perception.polymer flow within a “wave” type screw channel. This is exploratory study is performed in an effort toThe analysis is performed on an “unwrapped” form improve the understanding of the flow in a waveof a conventional screw channel and a “wave” type section and to determine whether the simulationchannel of similar size. A 3D Finite Element Method technique can provide a practical method of(FEM) simulation was used to simulate the flow field optimization.and flow characteristics of the wave channel arecompared relative to the plain channel. Geometry Introduction The geometry used in this study is based on a 90 mm diameter screw with a 100 mm flight pitch. TwoOne of the many developments in the area of screw turns of a conventional (simple) metering channeldesign is the addition of “wave” sections that have were compared with two turns of a “wave” channel.been introduced with claims of improved output, For simplification, the geometry is “unwrapped” andefficiency, homogenization, melt quality and overall will be studied in a planar co-ordinate system. Thescrew performance, in general [1-7]. There have geometries compared are shown in figures 3 and 4.even been some attempts at experimentallyquantifying the benefit of a “wave” section [8-12]. In this particular study, the channel depth variedHowever, there appears to have been very little or no between 10.6 mm and 5.3 mm with a 2.6 mm gapquantitative engineering analysis of the design. As between the center barrier flight and the “barrel” wall.such, it is believed the exact flow behavior of thepolymer melt in the wave section may not beproperly understood and the current designs not Simulationcompletely optimized. The simulations are performed using theThis initial investigation is focused on a particular Compuplast® Virtual Extrusion laboratory™ 3D“wave” or “undulating” channel design that is used in FEM module [13]. For simplification, we will assumethe so called Fusion™ screw [7]. Figure 1 shows a a stationary screw with a barrel rotating in the3D CAD drawing of this “wave” section and its opposite direction. In the planar co-ordinate systemposition on the screw. In this particular screw design, that we are using, this means that the upper surfacethe “wave” section is incorporated after the “Barrier” will move with a surface speed equivalent to 60 rpm.section where there would normally be a typical The material is assumed to be a 1 MI LLDPE flowing“Metering” section. The main channel is divided into at 200 kg/hr and a processing temperature of 200 C.two channels by a “barrier” flight with the depth of Figures 5 and 6 show the Pressure and Velocityeach channel oscillating out of phase. The “barrier” distribution, respectively, in the “Plain” channelflight is shorter than the main flights resulting in a while Figures 7 and 8 show the corresponding resultsgap between the top of the barrier flight and the for the “Wave” channel.barrel through which material can pass. The ideabeing, that the oscillating channel depth will force the The pressure distribution appears to be different formaterial to flow back and forth over the barrier, as each channel. The “Plain” channel appears to haverepresented in Figure 2. most of it’s pressure drop going across the channel in the range from 0.8 MPa to -1.4 MPa while theThe claims from the manufacturer are that this flow “Wave” channel has a strong pressure drop along thepattern helps to homogenize the melt and eliminate channel and in a range from 4.4 MPa to -0.9 MPa.
  • 2. Another way of looking at it is that, under these (Figure 15) has a similar pattern except for theconditions, the “Plain” Channel has a pressure drop additional velocity change and correspondingof about 2.2 MPa while the “Wave” channel elongation deformation cause by the material flowingconsumes about 5.3 MPa. over the middle flight. Figure 14 shows a pathline that starts 4 mm from the “barrel” surface. It can beThe color contour plots of the velocity, shown in seen that this pathline experiences virtually noFigures 6 and 8 for each channel, respectively, fluctuation in velocity or elongational deformationcontain “2-D Cuts” to better show velocity gradients along the path. In contrast, the corresponding paththrough the depth and width of the channel. line in Figure 16 does show some fluctuations in velocity and hence, the material flowing along this PathLine Comparison path will experience some elongation deformation. It therefore appears that it mixing benefits of the wavePathlines provide a means of visualizing the motion channel are not a result of the material being forcedof the material within a flow field. The “seeds” or over the center channel but more like due to thestarting points of the path lines were specified in 2 oscillations in the velocity caused by the changingrows. The first row was 4 mm from the upper cross sectional area. It would then seem that moresurface (middle of channel depth) and placed at 5, 10, attention should be placed on this aspect of the design20, 30, 40, 50, 60, 70, 80, and 85 mm across the start and maybe study the effect of increasing theof channel. The second row was placed at 1 mm from frequency of the waves in the channels. Future workthe upper surface and the same horizontal positions will focus on this type of design optimization.as the first row. ConclusionsFigures 9 and 10 show the pathlines for the plainchannel while Figures 11 and 12 show the pathlines Firstly, this study demonstrates that a 3D FEMfor the wave channel. The plain channel pathlines in analysis can be applied to the study of a wavefigure 10 show the expected helical path, resulting channel with relative ease. Furthermore, the resultsfrom drag and pressure flow, along the channel. The can also provide a much better understanding of thepathlines starting in the middle of the channel, shown true nature of the flow field that exists within a wavein Figure 9, show much less helical flow. These channel. It was shown how the pathline analysis canresults helps to confirm that the simplifications used be used to quantify the deformation that the materialin this simulation provide reasonable results and experiences in a wave channel and how these resultswould then also be valid for simulating a wave can be applied towards a systematic improvementchannel. and optimization of the design based proper engineering principles rather than intuition andThe wave channel pathlines in Figures 11 and 12 traditional trial and error methods.appear to be somewhat more “chaotic” for both setsof rows. Most surprisingly however, is the relativelyfew times that any of these pathlines cross over the Referencesmiddle flight. In fact, virtually not of the pathslinesin Figure 11 cross over the middle flight. From theseresults, it does not appear that the flow path in the 1. G.A. Kruder, U.S. Patent 3,870,284 (March 11,wave screw has much similarity with the perceived 1975)flow path shown in Figure 2. 2. G.A. Kruder and W.N Calland, SPE ANTEC Tech. Papers, 36, 74 (1990)In an attempt to further quantify the flow field two 3. G.A. Kruder, U.S. Patent 4,173,417 (1979)representative pathline starting in the same position 4. C.I. Chung and R.A. Barr, SPE ANTEC Tech.on both geometries were studied in more detail. Papers, 29, 168 (1983)Figures 13 – 16 show the Velocity Magnitude and 5. C.I. Chung and R.A. Barr, U.S. Patent 4,405,239Elongation Rate along pathlines with corresponding (1983).starting points in both the plan and wave geometry. 6. R.A. Barr, U.S. Patent 6,599,004 (2003)Figure 13 shows the characteristic change in velocity 7. T.W. Womer, E.J. Buck, and B.J. Hudak Jr., USas the material changes direction when it reaches the Patent 6,672,753 (2004).flight wall. This velocity change is also associated 8. T.A. Plumley, M.A. Spalding, J. Dooley, andwith an elongational deformation which contributes K.S.Hyun, SPE ANTEC Tech. Papers, 40, 324to some degree of mixing in a conventional screw. (1994)The corresponding path line in the wave geometry
  • 3. 9. S.A. Somers, M.A. Spalding, J. Dooley, and 11. S.A. Somers, M.A. Spalding, J. Dooley, and K.S.Hyun, SPE ANTEC Tech. Papers, 41, 222, K.S.Hyun, SPE ANTEC Tech. Papers, 48, 307 (1995). (2002).10. B.A. Salamon, M.A. Spalding, J.R. Powers, M. 12. Meyers, J. and Barr, R, SPE ANTEC Tech. Papers, Serrano, W.C. Sumner, S.A. Somers, and R.B. 2002 Peters, R.B., Plast. Eng., 57, 4, 52 (2001). 13. Virtual Extrusion Laboratory™ Version 6.2., Compuplast Int’l Inc. 2007. “Wave” SectionFigure 1 The "wave" sction on a Fusion™ screw S Main Flight D D S S D D S S Barrier Flight D (undercut) D Main Flight S S DFigure 2 Perceived material flowpath in a wave section
  • 4. Figure 3 "Plain" un-wrapped screw channelFigure 4 "Wave" un-wrapped screw channel
  • 5. Figure 5 Pressure Distribution in "Plain" channelFigure 6 Velocity Distribution in "Plain" channel
  • 6. Figure 7 Pressure Distribution in "Wave" channelFigure 8 Velocity Distribution in "Wave" channel
  • 7. Figure 9 Pathlines starting at 4 mm from the barrel in the plain channelFigure 10 Pathlines starting at 1 mm from the barrel in the plain channel
  • 8. Figure 11 Pathlines starting at 4 mm from the barrel in the wave channelFigure 12 Pathlines starting at 1 mm from the barrel in the wave channel
  • 9. Figure 13 Velocity Magnitude and Elongation rate along a “surface” pathline in the plain channelFigure 14 Velocity Magnitude and Elongation rate along a "middle" pathline in the plain channel
  • 10. Figure 15 Velocity Magnitude and Elongation rate along a "surface" pathline in the wave channelFigure 16 Velocity Magnitude and Elongation rate along a "middle" pathline in the wave channel