Emhart uses 3D visualization to fix
“belly bands” in spaghetti sauce jars
by Cecilia Galvin
Satisfying your customers is job one for a consultant. Seeing the positive
results of your work at the supermarket is an unexpected bonus. That’s what
happened to Matt Hyre, a PhD in computational modeling and a consultant
for Emhart Glass in Enfield, Connecticut, after he modeled the glass-making
process for a major spaghetti sauce manufacturer.
Almost 30 percent of the jars made by the manufacturer were plagued by
“belly bands,” a rippling in the glass around the middle of the jar, so the
company asked Hyre to determine the source of the problem. To do so, Hyre
modeled the process using a combination of traditional CFD tools and
advanced 3D visualization software.
In the beginning
Hyre started the project by requesting the CAD models for the glass-forming operation, including the
forehearth or cooling equipment; the feeder that drops the cooled gobs of glass into the mold; the mold itself;
the plunger; and the preform or parison, which expands inside the mold to produce the jar. The models,
created with Pro/ENGINEER, were exported in STEP format and assembled into files for CFD analysis.
“The geometry requirements for CAD and CFD models are different,” says Hyre. “In a CAD model, if there’s
a gap so small that you can’t see it with the human eye, it doesn’t matter. But in the CFD world, that gap will
kill you. You won’t be able to run the model because you have a tiny gap between lines in the geometry.”
Hyre used GAMBIT, Fluent’s CFD preprocessor and meshing package, to
make the necessary corrections to his 2D axisymmetric model. GAMBIT
“healed” some of the gaps automatically, but in areas where problems
remained, the geometry was recreated manually using the outline created by
the software and then deleting and connecting lines as required. The
corrective work took about a day to complete.
Meshing was also a day-long process, during which Hyre created highdensity meshes in the area where the glass meets the mold. The temperature
and viscosity gradients are highest there, so the mesh must be extremely
accurate.
“I had to use triangle meshes because I was using POLYFLOW as the solver,” said Hyre. “When the glass
surface begins changing and the mesh starts to distort, POLYFLOW will automatically call GAMBIT to remesh the surface and interpolate the results back onto the new mesh and start the computations again. That
will only happen if you’re using a triangle mesh, or a tet mesh for 3D models.”
The time sink
Before Hyre could finish the forming simulation of his problematic spaghetti jar, he had to determine how
much heat was going out of the glass and into the mold. To do so, he needed to know the mold temperature, so
he created another model in FLUENT to show the heat removal from the mold itself. He then coupled the
forming simulation in POLYFLOW with the simulation of the heat removal from the mold in FLUENT so that
boundary conditions could be passed between the two.
“You don’t know what the mold temperature is, so you pick a reasonable value,” he says. “You run your
simulation and get a flux profile over time from the glass to the mold, based on what you assumed. You then
take that flux profile and apply it to where the glass would be in your FLUENT model, and run that model
over and over until you get a mold temperature distribution.
“You then go back and start your POLYFLOW simulation again, using the
mold temperature distribution, and calculate a new flux between the glass
and the mold. You’ll get a different flux profile, which you put back into the
FLUENT model. You do this until the temperature in the mold doesn’t
change much.”
Hyre admits it’s a complicated and lengthy process—from three to five days
—but at the end he not only knows the glass temperature, but the
temperature of all the equipment surrounding the glass. That’s a huge
advantage because he has the necessary data should he need to look into
problems with cooling the mold, the size of the mold, or the mold temperature distribution.
The simulation generated about 70,000 time steps, each of which was analyzed to see what happened to the jar
over the length of the forming process. The time steps represented roughly 100 gigabytes of data. Some of
those were graphics files for post processing, but most were boundary conditions and the geometries needed
for subsequent time steps.
Animation is the key
Hyre created a file to define what he wanted to process and analyze—the geometry, temperature, viscosity,
pressure—a process that took about half an hour. He then imported the files into CEI’s EnSight to take
advantage of the software’s high-end visualization capabilities. EnSight read Hyre’s definition file and
retrieved the data in less than an hour.
“When we tried to post-process the data in FLUENT or POLYFLOW, it was horrendous,” says Hyre. “I
would have to pull in each individual data file, create a graphics file, save that file, then pull in the next data
set, and so on. Then I had to connect all the graphics files into an animation. It used to take almost as long as
the rest of the process—maybe three days.
“EnSight is able to read changing geometries and variables for huge data sets very quickly, which was the
number one thing I needed,” he says.
Hyre tagged elements in EnSight and looked at temperature gradients and viscosity levels that were higher
than a specified amount and might indicate a defect. Other areas were tagged and analyzed as well, based on
specific criteria.
“As soon as I hit the animation in EnSight, bang! In less than 10 seconds it was running,” says Hyre.
The animation showed that the middle section of the jar cooled too much, which meant its viscosity was too
high and it wasn’t blowing out as fast as the upper and lower parts of the jar. As a result, too much surface
area was being squeezed against the mold at the very end of the blowing process, causing the rippling effect.
“Once I saw that, there wasn’t a lot more I needed to do with the simulation, except share it with
management,” say Hyre. “EnSight has great animation—better than anything I’ve used. If your animations
look bad, or grainy, or jerky, people think they’re low-tech. And when you’re talking to management, you
need something that looks professional or it takes away from the work you do.”
A few days after finding the problem, Hyre was asked if he could fix it. So
along with the designers at Emhart, he came up with a redesign for the
plunger and blank mold, ran the model again, and created a new animation
that clearly showed the jar without defects.
The final steps were getting the modifications into production—and the sauce
to the supermarket.
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Cecilia Galvin is a writer for Cramblitt & Company, a marketing
communications company serving the CAD/CAM, computer graphics, IT and electronics industries.
copyright © 2005 CEI. All rights reserved.