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

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.

Introduction

One of the many developments in the area of screw

design is the addition of “wave” sections that have

been introduced with claims of improved output,

efficiency, homogenization, melt quality and overall

screw performance, in general [1-7]. There have

even been some attempts at experimentally

quantifying the benefit of a “wave” section [8-12].

However, there appears to have been very little or no

quantitative engineering analysis of the design. As

such, it is believed the exact flow behavior of the

polymer melt in the wave section may not be

properly understood and the current designs not

completely optimized.

This initial investigation is focused on a particular

“wave” or “undulating” channel design that is used in

the so called Fusion™ screw [7]. Figure 1 shows a

3D CAD drawing of this “wave” section and its

position on the screw. In this particular screw design,

the “wave” section is incorporated after the “Barrier”

section where there would normally be a typical

“Metering” section. The main channel is divided into

two channels by a “barrier” flight with the depth of

each channel oscillating out of phase. The “barrier”

flight is shorter than the main flights resulting in a

gap between the top of the barrier flight and the

barrel through which material can pass. The idea

being, that the oscillating channel depth will force the

material to flow back and forth over the barrier, as

represented in Figure 2.

The claims from the manufacturer are that this flow

pattern helps to homogenize the melt and eliminate

any potentially un-melted particles. While these

claims have been verified experimentally and in the

field, the flow behavior is mostly based on perception.

This is exploratory study is performed in an effort to

improve the understanding of the flow in a wave

section and to determine whether the simulation

technique can provide a practical method of

optimization.

Geometry

The geometry used in this study is based on a 90 mm

diameter screw with a 100 mm flight pitch. Two

turns of a conventional (simple) metering channel

were compared with two turns of a “wave” channel.

For simplification, the geometry is “unwrapped” and

will be studied in a planar co-ordinate system. The

geometries compared are shown in figures 3 and 4.

In this particular study, the channel depth varied

between 10.6 mm and 5.3 mm with a 2.6 mm gap

between the center barrier flight and the “barrel” wall.

Simulation

The simulations are performed using the

Compuplast® Virtual Extrusion laboratory™ 3D

FEM module [13]. For simplification, we will assume

a stationary screw with a barrel rotating in the

opposite direction. In the planar co-ordinate system

that we are using, this means that the upper surface

will move with a surface speed equivalent to 60 rpm.

The material is assumed to be a 1 MI LLDPE flowing

at 200 kg/hr and a processing temperature of 200 C.

Figures 5 and 6 show the Pressure and Velocity

distribution, respectively, in the “Plain” channel

while Figures 7 and 8 show the corresponding results

for the “Wave” channel.

The pressure distribution appears to be different for

each channel. The “Plain” channel appears to have

most of it’s pressure drop going across the channel in

the range from 0.8 MPa to -1.4 MPa while the

“Wave” channel has a strong pressure drop along the

channel and in a range from 4.4 MPa to -0.9 MPa.

Another way of looking at it is that, under these

conditions, the “Plain” Channel has a pressure drop

of about 2.2 MPa while the “Wave” channel

consumes about 5.3 MPa.

The color contour plots of the velocity, shown in

Figures 6 and 8 for each channel, respectively,

contain “2-D Cuts” to better show velocity gradients

through the depth and width of the channel.

PathLine Comparison

Pathlines provide a means of visualizing the motion

of the material within a flow field. The “seeds” or

starting points of the path lines were specified in 2

rows. The first row was 4 mm from the upper

surface (middle of channel depth) and placed at 5, 10,

20, 30, 40, 50, 60, 70, 80, and 85 mm across the start

of channel. The second row was placed at 1 mm from

the upper surface and the same horizontal positions

as the first row.

Figures 9 and 10 show the pathlines for the plain

channel while Figures 11 and 12 show the pathlines

for the wave channel. The plain channel pathlines in

figure 10 show the expected helical path, resulting

from drag and pressure flow, along the channel. The

pathlines starting in the middle of the channel, shown

in Figure 9, show much less helical flow. These

results helps to confirm that the simplifications used

in this simulation provide reasonable results and

would then also be valid for simulating a wave

channel.

The wave channel pathlines in Figures 11 and 12

appear to be somewhat more “chaotic” for both sets

of rows. Most surprisingly however, is the relatively

few times that any of these pathlines cross over the

middle flight. In fact, virtually not of the pathslines

in Figure 11 cross over the middle flight. From these

results, it does not appear that the flow path in the

wave screw has much similarity with the perceived

flow path shown in Figure 2.

In an attempt to further quantify the flow field two

representative pathline starting in the same position

on both geometries were studied in more detail.

Figures 13 – 16 show the Velocity Magnitude and

Elongation Rate along pathlines with corresponding

starting points in both the plan and wave geometry.

Figure 13 shows the characteristic change in velocity

as the material changes direction when it reaches the

flight wall. This velocity change is also associated

with an elongational deformation which contributes

to some degree of mixing in a conventional screw.

The corresponding path line in the wave geometry

(Figure 15) has a similar pattern except for the

additional velocity change and corresponding

elongation deformation cause by the material flowing

over the middle flight. Figure 14 shows a pathline

that starts 4 mm from the “barrel” surface. It can be

seen that this pathline experiences virtually no

fluctuation in velocity or elongational deformation

along the path. In contrast, the corresponding path

line in Figure 16 does show some fluctuations in

velocity and hence, the material flowing along this

path will experience some elongation deformation. It

therefore appears that it mixing benefits of the wave

channel are not a result of the material being forced

over the center channel but more like due to the

oscillations in the velocity caused by the changing

cross sectional area. It would then seem that more

attention should be placed on this aspect of the design

and maybe study the effect of increasing the

frequency of the waves in the channels. Future work

will focus on this type of design optimization.

Conclusions

Firstly, this study demonstrates that a 3D FEM

analysis can be applied to the study of a wave

channel with relative ease. Furthermore, the results

can also provide a much better understanding of the

true nature of the flow field that exists within a wave

channel. It was shown how the pathline analysis can

be used to quantify the deformation that the material

experiences in a wave channel and how these results

can be applied towards a systematic improvement

and optimization of the design based proper

engineering principles rather than intuition and

traditional trial and error methods.

References

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1975)

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Papers, 29, 168 (1983)

5. C.I. Chung and R.A. Barr, U.S. Patent 4,405,239

(1983).

6. R.A. Barr, U.S. Patent 6,599,004 (2003)

7. T.W. Womer, E.J. Buck, and B.J. Hudak Jr., US

Patent 6,672,753 (2004).

8. T.A. Plumley, M.A. Spalding, J. Dooley, and

K.S.Hyun, SPE ANTEC Tech. Papers, 40, 324

(1994)

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13. Virtual Extrusion Laboratory™ Version 6.2.,

Compuplast Int’l Inc. 2007.

Figure 1 The "wave" sction on a Fusion™ screw

S

S

D D

S

S

D D

S

D

S

S

D D

Main Flight

Barrier Flight

(undercut)

Main Flight

“Wave” Section

Figure 2 Perceived material flowpath in a wave section

Figure 3 "Plain" un-wrapped screw channel

Figure 4 "Wave" un-wrapped screw channel

Figure 5 Pressure Distribution in "Plain" channel

Figure 6 Velocity Distribution in "Plain" channel

Figure 7 Pressure Distribution in "Wave" channel

Figure 8 Velocity Distribution in "Wave" channel

Figure 9 Pathlines starting at 4 mm from the barrel in the plain channel

Figure 10 Pathlines starting at 1 mm from the barrel in the plain channel

Figure 11 Pathlines starting at 4 mm from the barrel in the wave channel

Figure 12 Pathlines starting at 1 mm from the barrel in the wave channel

Figure 13 Velocity Magnitude and Elongation rate along a “surface” pathline in the plain channel

Figure 14 Velocity Magnitude and Elongation rate along a "middle" pathline in the plain channel

Figure 15 Velocity Magnitude and Elongation rate along a "surface" pathline in the wave channel

Figure 16 Velocity Magnitude and Elongation rate along a "middle" pathline in the wave channel