An Improved Flow Channel Design for Film and Sheet Extrusion Dies¶
Masaki Ueda1, Makoto Iwamura2 and Hideki Tomiyama2
1 The Japan Steel Works, LTD., Hiroshima Research Laboratory, Hiroshima, Japan
2 The Japan Steel Works, LTD., Plastics Processing Machinery Dept., Hiroshima, Japan
Abstract
An improved flow channel design for a die for
polymer processing was devised. The design is based on
the combination of two conventional die designs to take
advantages of both types. The performance of the die with
the improved flow channel design was evaluated with a
flow simulation. It was expected that the improved die
could obtain more uniform flow rate at the die exit than
conventional dies without losing other performances such
as residence time.
Introduction
The extrusion die is one of the most important
equipment to determine the quality of film and sheet
products in polymer processing. Requirements for a die
have become more and more complicated and diversified
because the market of film and sheet products calls for
sophisticated functions. Because of this, extrusion die has
to fulfill the requirements such as uniformity in thickness
and physical properties of products, and productivity.
One of the basic requirements for a die is to distribute
the flow uniformly at the exit where its slit has a large
width to thickness ratio. It is mainly accomplished by the
balance of pressure drop across the entire width of a die.
Therefore, the flow channel geometry has to be designed
with the consideration of the material properties,
operating conditions and rheology. Although conventional
dies are also designed with considering those factors, they
tend to have limitations to control the thickness
uniformity. In some cases, a die may also be required that
it can achieve other capabilities such as short residence
time in addition to thickness uniformity. It means that a
geometry which can achieve two objectives at the same
time is needed.
This paper reports an improved flow channel design
of die which is devised to enable it to produce uniform
thickness products without losing other performances. The
concept of the design is to combine the advantages of two
conventional dies. Therefore, this die is named as
“Hybrid-type Die”. The performance of this die was also
discussed by comparison with conventional ones with a
flow simulation.
Geometry of Conventional Dies
Two of the conventional die designs are reviewed
below.
Coathanger Die
Figure 1 shows a structure of conventional
coathanger die. It is comprised of a manifold, whose cross
sectional area is tapered from larger at the center to
smaller at the edge, and a two step rectangular channel (a
preland and a lip land). The length of the preland is not
constant from the center to the edge. This design is so
popular that a lot of die manufacturers adopt it [1]. The
cross sectional area of the manifold and the length and
height of the preland assure the balance of the channel
resistance. These geometry factors affect the flow rate
distribution. In this type of die, it is possible to design a
die that can shorten the residence time to prevent melt
polymer from deteriorating by setting the taper of
manifold appropriately.
Since the coathanger die has a tapered manifold, the
channel area at the center of the die is larger than that at
the edge. If the inner pressure of the die is high, it will
cause larger deformation of the lip clearance at the center
of the die than that at the edge. As a result, the deviation
of the lip gap in the die across the width results in a
nonuniform flow rate distribution at the exit of die. In
particular, the tendency becomes significant with the die
width increasing.
Taper-Land Die
Figure 2 shows the structure of taper-land die. This
type of geometry is adopted in some companies [2,3]. It is
comprised of a straight manifold, and a 3-step rectangular
channel (a 2-step taper-land and a lip). The 2-step taperland
(rectangular channel) assures the balance of the
channel resistance, and the straight positioning of the
manifold establishes uniformity of the flow rate across the
width. Consequently, the deviation of the lip gap across
the width would become uniform even if the die was
subjected to inner pressure. It improves the problems that
occur in coathanger dies described above.
However, a taper land die gives a relatively long
residence time because relatively large cross sectional
area of manifold at the end causes a decrease in the
velocity of melt polymer. The geometry might cause a
problem; for example, in the case that a polymer
deteriorates easily by heat.
Improved Flow Channel Design
As mentioned above, each type of conventional die
has both merits and demerits. An improved flow channel
design was devised to combine the merits and compensate
for the demerits of both die types. The geometry of the
improved flow channel is shown by Figure 3. The die is
named “Hybrid-type Die” as the flow channel design is
based on the conventional die designs. It consists of a
manifold, which is straight in the width direction, and 3-
step rectangular channels like taper land die. However, it
differs from the manifold of a taper land die in that the
cross-sectional area is gradually decreased from center to
end like coathanger type. This design adds one more
design factor for balancing the flow resistance: an
improved geometry has 4 factors while conventional dies
have just 3. The factors for a coathanger die are the length
and height of preland, cross sectional area of manifold,
while those for a taper land die are the taper and height of
1st and 2nd step land. The increase of design factors is
essential to obtain more desired performances with
maintaining high uniformity of flow rate at die exit.
We theoretically developed the design formula for
this improved flow channel geometry, and it enables us to
decide the precise geometry quickly.
Flow Simulation
In order to compare the performance of the improved
die and conventional dies, flow simulation based on FAN
(Flow Analysis Network) method [4, 5] was conducted.
The flow channel geometry of each die used for the
simulation is shown by Figure 4. The geometries had been
optimized so that the flow channel can provide uniform
flow rate at the die across the width under the conditions
that the manifold size at the center and the pressure of
manifold at the center become comparable. The property
of polypropylene, whose melt index (MI) was 5 g/10 min
(230
oC, 2.16 kg) and melt density was 0.75 g/cm
3, was used for the simulation. The shear viscosity is shown by
figure 5 and was modeled using a six constant curve
equation as follows:
where K is the shear viscosity (Ps-s), J is the shear rate
(1/s), T is the temperature (
oC), A’s are curve fit
coefficients. The constants are provided in Table 1. The simulation was performed with the throughput of
150kg/hr, the isothermal condition at 230
oC.
Results and Discussion¶
The simulation result of flow rate deviation at the exit
of die is shown in Figure 6. It shows that all three dies
give so highly uniform flow distribution that the deviation
is within ± 3%. In particular, the hybrid type gives the
most uniform flow distribution that the deviation is less
than ± 1%.
Figure 7 shows the simulation result of residence time
along each flow pass from inlet of the die to each position
at the exit of die across the width. It shows that the
residence time at the end of taper land die is 1.5 times
longer than that of coathanger die. This is attributed to the
straight manifold structure of taper land die that the
sectional area of manifold does not decrease form center
to end. That structure causes decrease of velocity and
longer stay of melt polymer near the end of the manifold.
In contrast, the die with improved flow channel exhibits
the comparable residence time to the coathanger type. The
short residence time indicates that the shear strain rate
near the end of manifold is high enough to shorten the
process time of resin replacement and color change, and
the prevent degradation of melt polymer.
Table 2 shows the comparison of each dies based on
the simulation. The results are summarized below.
1. The improved die gives more uniform thickness
than conventional one.
2. It is possible to shorten the residence time of
polymer from entrance to exit of die. That prevents
deterioration of polymer in continuous run.
3. It is possible to prevent wall adhesion of polymer
at the manifold because the shear strain ratio at the
area can be kept high.
4. It is possible to ensure the highly uniform thickness
in actual operation because it has resistance against
clam shelling that the deviation across the width of
the lip gap change by inner pressure is uniformized.
Conclusion
A “Hybrid-type Die” with an improved flow channel
design for polymer processing was devised. From the flow
simulation, it was expected that high uniformity of flow
rate across the width and the prevention of polymer
degradation could be achieved in the die with the
improved flow channel design. The main reason of the
better performances is that the improved flow design has
an additional design factor for balancing the flow
resistance compared to conventional designs. This will be
more valuable when dies become wider.
References
1. W. Michaelli, Extrusion Dies for Platics and Rubber,
Hanser Publishers, New York (2003)
2. H. Kometani, T. Miki, T. Nakamura, T. Goto and Y.
Kitauji, Mitsubishi heavy Industries, LTD Technical
Review, 38, 1,17 (2001).
3. Peter F. Cloeren, U.S. Patent 5,256,052 (1993).
4. Z. Tadmor, R. Broyer, C. Gutfinger, Polym Eng Sci,
14, 212 (1974).
5. M. Booy, Polym Eng Sci, 22, 7, 432 (1982).
Figure 1 Structure of coathanger die (conventional die)
Figure 2 Structure of taper land die (conventional die)
Figure 3 Structure of Hybrid die (improved flow channel design)
Figure 4 Each flow channel geometry (1/2 symmetrical model) used for the flow simulation (dimensions are in mm)
Figure 5 Shear viscosity for the 5 MI PP and the fitting curves.
Table 1 Shear viscosity fitting parameters.
Figure 6 Result of flow simulation for the flow rate distribution
Figure 7 Result of flow simulation for residence time distribution.
Table 2 A comparison of each die.
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