- Internal Flow
- Film Casting
- Screw Design
Internal Flow
What's going on inside the die? Why study it?
By studying the flow
between the rotating screw and the barrel, you can gain valuable
insights on how to obtain better mixing or a more homogeneous temperature
distribution.By performing further analysis of the flow
inside the die, you can avoid recirculation zones, viscous heating and degradation
of the material or reduce pressure losses. It may also be important to balance
the flow at the die exit to minimize the extrudate deformations in the
free flow and finally to obtain the right shape of the extrudate after
the deformations.
POLYFLOW provides the tools to address these issues and the simulations
provide a wealth of information over the whole flow domain which no affordable
experiments could reveal. Data such as velocity and pressure distributions
in the fluid, temperature and residence times, particle trajectories and
stresses are typically obtained in a simulation. This vastly improved
insight into the flow provides a better understanding of the process itself
and points out improvements. These can then be tested on the computer
without interruption of the production line.
Simulation is sometimes the only way to have a quick answer to questions
like the reduction of pressure losses, the existence of stagnation zones
which would yield risks of degradation, unbalanced flows in flat dies
and exaggerated heat generation and transfer due to viscous heating. It
also becomes possible to reduce the number of necessary and costly experiments
and to reduce the global time-to-market.
Film Casting
Film casting is one of the processes widely used to produce plastic films.
The plastic is extruded through a narrow slit. After the die lip, the
resin is in contact with the air before touching the chilly roll rotating
at high speed. The rotational speed of the roll imposes the drawing velocity
at the end of the free surface. Depending upon the material and the operating
conditions, different widths and thickness profiles of the film are obtained.
The goal of the design engineer is to get a uniform thickness on the largest
width of the film, the side sections being cut and recycled.
POLYFLOW reproduces the necking, the thickness variation through the
film and the deformation of the shape of the film. The use of the differential
viscoelastic models (Phan Thien-Tanner, Giesekus-Leonov) available in
POLYFLOW gives a much better representation of the actual process than
a simple generalized Newtonian model would. Indeed, the extensional phenomena
are critical due to the high stretching undergone by the fluid.
In order to reduce the computation time and allow for the use of complex
rheological models such as the multi mode, non-isothermal viscoelastic model,
POLYFLOW has implemented a sheet approximation of the film. Due to the
large difference of dimensions (thickness << width), a uniform stress,
temperature and velocity field across the film is assumed. This does not
significantly affect the accuracy of the results while it strongly reduces
the computational time.
Multilayer film casting simulations can also be performed. The thickness
distribution for each layer is then calculated while taking into account
the different rheological behavior as well as different initial thickness
for each layer.
Screw Design
- Single Screw Extruder
- Twin Screw Extruder
The Challenges
Understanding the flow of material around rotating single screws, twin
screws or moving impellers is critical. Many defects of the final product
are due to a low mixing quality while the pellets are flowing around a
single screw; the pressure built by a given screw rotating at a specified
rpm can be too short considering the required pressured drop for a new
die that just has been cut; while mixing color or foaming additives with
a plastic melt through a twin screw extruder, a good quality mixing must
reach an additive dispersion as uniform as possible in order to avoid
regions of different color or lower foaming intensity. For batch mixers
as well, the quality of the mixing created by the rotating impeller is
critical to the quality of the final product.
In addition, the requested power to apply on the rotating impeller or
screw will depend upon the force and torque applied by the flowing material.
Getting an evaluation of the power requirement while designing the whole
device allows you to acquire the appropriate material. Furthermore, both the
cost and the mixing quality will depend upon the geometry of the barrel
and the moving impellers or screws, the flowing material and the operating
conditions. All these parameters can be modified on the computer in order
to select the best operating window.
Single Screws
Flow pattern, temperature field through the flowing material, pressure
increase along the screw, residence time, local shear rate, and local stress
are typical information that would be very useful to know while designing
or acquiring a screw.
Using POLYFLOW, you analyze how a new screw
would perform in your own device before acquiring it, or
you can demonstrate to your customers that the new screw you have
designed will perform much better than the old one. Furthermore, you can
vary the operating conditions, such as the rotational speed of the screw,
and analyze the subsequent effects, such as pressure built or temperature
increase due to viscous heating. This allows you to identify the optimal working
window depending upon the device and the flowing material without deteriorating
the resin.
Twin - Multiscrew Devices
Understanding and simulating the flow around the twin screw has always
been challenging due to the complex geometry in the vicinity
of the interscrew region and the large deformations induced by the two
(or more) rotating screws. POLYFLOW has implemented a
Mesh Superposition Technique that superimposes meshes (for the flowing
materials and the screws itself). Also, non-isothermal flows around the
twin screw detecting the high temperatures induced by viscous heating,
quality of the mixing performed by particles tracking and statistical
analysis together with other many other quantities (pressure, velocity,
residence time, mixing index, local shear rate, stresses, viscosity) are
calculated. They provide you with a wealth of extremely
useful information about the flow of material around the screws.
Single Screw Extruder
- Process Description
- The Challenges
- Benefits Gained
- Advanced Numerical Aspects
Process Description
Melting the Material
Whether the resin is extruded, blow molded or even injected, the solid
pellets delivered by the resin provider have to be melted before being
formed. The solid pellets are fed through the feed hopper (2). Next the
solid material is conveyed by a single screw rotating inside a closed
barrel. The frictions induced at the interface between the flowing resin
and the wall increase the temperature through the viscous heating process,
locally melting the material.
Increasing the Pressure
The rotational motion of the screw not only conveys the melted material
but compresses it as well. So, the pressure of the plastic increases along
the screw. Depending on the material and the rotational speed, the pressure
at the end of the screw, i.e. at the inlet of the die, will be (more or
less) large. This pressure needs to compensate for the pressure drop appearing
inside the die. It is also very important that the screw builds up the
right pressure. Otherwise, a smaller flow rate, i.e. a smaller production
rate, will limit the whole process.
The Challenges
Understanding the flow of material around rotating single screws is critical.
Depending on the rotational speed of the screw, the shear rate can locally
reach unexpected values. This will create some peaks of temperature that
the material can not bear, leading to irreversible deterioration of the
polymer grade.
On the other hand, too-limited frictions due to too small rotational speed,
will lead to slow or even incomplete melting of the material which could
induce another dramatic failure of the final product.
Reaching the desired pressure is also a key point that will determine
the flow rate, i.e. the production rate. But the pressure increase is
not simply a linear function of the length of the screw or the rotational
speed. Small modifications of the screw geometry, the flowing material
or the operating condition (either thermal or mechanical) may lead to
a significant change of the final pressure and temperature map. Many defects
of the final product are due to a low mixing quality while the pellets
flow around a single screw; the pressure built by a given screw rotating
at a specified rpm can be too short considering the required pressure
drop for a new die that just has been cut.
Benefits Gained with POLYFLOW
Flow pattern, temperature field through the flowing material, pressure
increase along the screw, residence time, local shear rate, and local stress
are typical information that is very useful to know while designing
or acquiring a screw. These are invaluable pieces of information
that you obtain from numerical simulation. You can analyze how a
new screw would perform in your own device before acquiring it, or
demonstrate to your customers that the new screw you have designed will
perform much better than the old one. Furthermore, you may vary the
operating conditions, such as the rotational speed of the screw, and analyze
the subsequent effects, such as pressure built or temperature increase
due to viscous heating. This allows you to identify the optimal
working window depending on the device and the flowing material without
deteriorating the resin.
In addition, the requested power applied on the rotating impeller or
screw will depend upon the force and torque applied by the flowing material.
Getting an evaluation of the power requirement while designing the whole
device allow you to acquire the appropriate material. Furthermore, both
the cost and the mixing quality will depend on the geometry of the barrel
and the moving impellers or screws, the flowing material and the operating
conditions. All these parameters can be modified on the computer in order
to select the best operating window.
Advanced Numerical Aspects
The combination of non-isothermal simulation and long geometries typical
for a screw requires the use of appropriate interpolation techniques
in order to keep the computational time low enough. Using specific interpolation
for both the temperature and the velocity field allows for very accurate
results in a reasonable time (a few hours in some cases for complex screws).
Twin Screw Extruder
- Process Description
- The Challenges
- Benefits Gained
- Advanced Numerical Aspects
Process Description
The goal of the twin screw extruder is both to convey the melted material
and to mix the flowing resin together with additives or tyres, by far
its most important task. Melted material enters the barrel containing
the twin screws. Here, the resin flows around different screw elements
such as conveying elements, kneading blocks, and reverse elements.
At the interscrew region, a narrow, very complex flow section
appears in front of the flowing particles. This will induce the shear
and extensional flows that are required for the mixing quality.
The Challenges
Understanding the flow of material around rotating twin screws is critical.
Many defects of the final product are due to a low mixing quality while
the pellets are flowing around a twin screw : the pressure built by a
given screw rotating at a specified rpm can be too short considering the
required pressured drop for a new die that just has been cut; while mixing
color or foaming additives with a plastic melt through a twin screw extruder,
a good quality mixing must reach an additive dispersion as uniform as
possible in order to avoid regions of different color or lower foaming
intensity. For batch mixers as well, the quality of the mixing created
by the rotating impeller is critical to the quality of the final product.
In addition, the requested power to be applied on the rotating impeller
or screw will depend upon the force and torque applied by the flowing
material. Getting an evaluation of the power requirement while designing
the whole device allow you to acquire the appropriate material. Furthermore,
both the cost and the mixing quality will depend upon the geometry of
the barrel and the moving impellers or screws, the flowing material and
the operating conditions. All these parameters can be modified on the
computer in order to select the best operating window.
Benefits Gained with POLYFLOW
Understanding and simulating the flow around the twin screw has always
been challenging due to the complex geometry in the vicinity
of the interscrew region and the large deformations induced by the two
(or more) rotating screws. POLYFLOW has
a Mesh Superposition Technique that superimposes meshes (for the flowing
materials and the screws itself). Also non-isothermal flows around the
twin screw detecting the high temperatures induced by viscous heating,
quality of the mixing performed by particles tracking and statistical
analysis together with other many other quantities (pressure, velocity,
residence time, mixing index, local shear rate, stresses, viscosity) are
calculated. They provide you with a wealth of extremely
useful information about the flow of material around the screws.
In order to accurately evaluate the quality of the mixing, statistical
analysis is performed by POLYFLOW. By tracking the particles during their
journey around the twin screws, the software collects a wealth of local information
of the temperature, flow pattern, extension, stretching, and residence time.
Next, all this information is treated using statistical tools to summarize
the data into a few numbers or a curve. Now, you can evaluate
and compare the quality of different equipment, material and/or operating
conditions.
Advanced Numerical Aspects
The simulation around the twin screw is a very challenging task due to
the motion of inner solid tools (the screws) and the complex intermeshing
regions between the two screws. This usually leads to excessively complex
mesh generation tasks and requires unrealistic remeshing techniques. POLYFLOW
has worked around these difficulties using the
Mesh Superposition Technique. The basics of this technique
assume that you define one mesh for each screw and a mesh for the
barrel. These meshes being generated separately are easier to set up.
Also, you do not need to worry about the complex mesh usually requested
at the complex interscrew section.
Next, POLYFLOW superimposes the different meshes either from the screws
or from the barrel and through an intelligent algorithm, the code is able
to detect whether the node is inside the rotating screw or represents
a flowing particle. Also, either the Navier-Stokes equations or the heat
conduction problem is solved respectively.
No remeshing technique is required. Fully transient simulations including
the inertia terms and the viscous heating are performed, revealing all
the detail of the flow of particles.
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