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Courtesy of Forsheda
The shaping of rubber compounds by extrusion is a widely used process
for producing finished products such as profiles for automotive applications
and construction. The transition from the flow in the die to the freely
streaming polymer gives rise to changes in the local velocities, which
induce deformations in the extruded profile. When a designer's knowledge
is combined with the insight provided by numerical simulation, several
designs can be rapidly evaluated so as to yield the required product.
This results in a reduction of the number of trial dies, with subsequent
reductions in costs, time-to-market, and scrap material.
Numerical result of the die design task: die (gray), deformed extrudate
(red)
In an attempt to reduce die design costs, engineers at Forsheda in Sweden
collaborated with the POLYFLOW product team in Belgium for analyses of
dies for extruded rubber products. A particularly helpful feature of the
POLYFLOW software is the availability of an "inverse extrusion"
function, an automatic die lip design functionality that uses an advanced
deforming mesh technique. The result of a calculation that uses this function
is a die design that takes material deformations into account so as to
produce the desired finished product.
As a part of this project, two synthetic
rubber materials were simulated, each of
which was characterized by a power law
viscosity. Constraints on the pressure drop
across the die forced the length of the die
to not exceed a pre-defined value for a
given extrudate speed. A larger pressure
drop would require an investment in
equipment which was not in line with the
scope of the project.
The complexity of the die geometry resided in the juxtaposition of thin
and thick parts, leading to a strongly unbalanced velocity profile. A
first simulation revealed that the velocity through the thin slit region
was 10,000 times slower than that across the wider body opening. The next
analysis, therefore, involved two stages: a die balancing task to get
a satisfying velocity profile across the die lip and an "inverse"
simulation to automatically calculate the die lip shape to compensate
for the residual deformation.
The first attempt to improve the product led to encouraging results,
even though experimental work revealed some discrepancies with the targeted
profile. An analysis of these results suggested that advanced physics
such as partial slip and thermal effects (such as viscous heating) were
playing a significant role. By comparing the flow pattern revealed by
the simulation and the actual deformation of the material, a better understanding
of the most efficient techniques to modify a given die was reached.
The steel die used for the extrusion process
With limited information regarding the flow pattern and material behavior,
the numerical task for the engineers at Forsheda was a real challenge.
Sliding behavior, geometric details, and thermal effects proved to play
an important role in this process. As a result of the simulations, the
rheological and sliding behavior of the material became better understood,
leading to fundamental understanding that could be applied to other similar
applications.
This text is a summary of the work published at the ESAFORM conference,
Liege, Belgium, April 2001.
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