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By Dr. Manoj Choudhary, Senior Technical Staff, Owens Corning, Granville,
OH
CFD is playing an important role in the development of environmentally
friendly building insulation products at Owens Corning, the world’s
leading insulation manufacturer. A decade ago, insulation manufacturers
moved away from chlorofluorocarbon (CFC) foaming agents because of the
damage they can cause to the earth’s ozone layer. The CFCs have been
replaced largely by hydrochloroflurocarbon (HCFC) blowing agents. While
the ozone depletion potential of HCFCs is considerably less than that
of CFCs, it is not zero and efforts are underway to find a replacement
for HCFCs. The challenge in developing new foaming agents is to maintain
the thermal and mechanical properties of the foam while keeping the manufacturing
process economically viable. Today, insulation manufacturers are rapidly
moving to foaming agents such as hydrocarbons or carbon dioxide because
removing chlorine from the foaming compound completely eliminates ozone
damage.
Pressure contours in a foam extrusion die
Polystyrene foam is one type of insulation that is produced
by saturating a polymer with a blowing agent
at high pressures and temperatures by means of an extrusion
system. At the extrusion die a rapid pressure drop
occurs, the solubility of the blowing agent in the polymer
melt is rapidly decreased, and the melt becomes
supersaturated. A large number of cells are nucleated
as the melt exits from the die. As the melt cools, the
blowing agent diffuses into these small cells, expanding
their size until the final product dimensions are achieved.
A major challenge in the manufacturing process is
to ensure that temperature, velocity, and pressure remain
relatively constant along the cross-section of the die to
maintain product uniformity. The traditional approach
to evaluating the performance of alternate foaming agents
is to perform experiments with dies. More recently, however,
Owens Corning researchers have turned to POLYFLOW
to simulate the process. Their model includes the complicated
rheology of the polymer materials in their full
complexity.
One of the benefits of using POLYFLOW is that it can
incrementally and automatically change the material properties
or system boundary conditions to obtain solutions
at intermediate steps during a solution procedure. To
simplify the simulation, the material mix, which is comprised
of the polymer and blowing agent, was treated
as a single homogenous melt rather than as two separate
species. Test data were combined with published
information to determine the complex system parameters
for the model. With the aid of manufacturing engineers
and the Fluent technical support staff, the Owens
Corning researchers were able to improve the efficien-
cy of the calculation process, reducing convergence time
to about 24 hours per test case.
The ability to accurately simulate the foam extrusion
process will dramatically speed up the process of evaluating
and optimizing new foaming agents in the future.
Instead of having to run a complicated series of physical
tests, engineers will be able to simulate the foaming
operation on the computer in a fraction of the time.
One big advantage of CFD is that researchers can obtain
critical flow, pressure, and temperature parameters at
any point inside the die. This information will help determine
the reasons for the good or poor performance of
a particular die and material combination, which in turn
will provide guidance for improving the design. The end
result is that researchers will be able to evaluate far more
potential foaming agents under a much wider range
of conditions, thus increasing the efficiency of the developmental
process. This should also make it possible to
substantially increase the yield of the material produced
with new foaming agents, resulting in reduced manufacturing
costs.
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