Smoothing Out the Wrinkles

By Cecilia Galvin

Satisfying customers is job one for a consultant. Seeing the positive results of your work at the supermarket is the unexpected bonus. That’s what happened to Matt Hyre, a Ph.D. in computational modeling and a consultant for Emhart Glass in Enfield, CT, after he modeled the glass-making process for a major spaghetti-sauce manufacturer.

The Problem

Almost 30 percent of the bottles made by the manufacturer were plagued by “belly bands,” a defective rippling in the glass around the middle of the bottle. 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.

       

The Solution

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 liquefied gobs of glass into the mold; the mold itself; the plunger; and the preform or parison, which expands inside the mold to produce the bottle. The models, created with Pro/Engineer, were exported in STEP 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 the Pro/E 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 high-density 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 Fluent’s POLYFLOW as the solver,” says Hyre. “When the glass surface begins changing and the mesh starts to distort, POLYFLOW will automatically call GAMBIT to re-mesh 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-sauce bottle, he had to determine how much heat was going out of the glass and into the mold. 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—lasting from three to five days—but at the end he not only knows the glass temperature, but the temperature of all the equipment around the glass. That’s an advantage because he has the necessary data should he need to examine problems with mold cooling, 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 bottle over the length of the forming process. The time steps represented roughly 100GB of data. Some of those were graphics files for postprocessing, 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 postprocess 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 bottle cooled too quickly, which meant its viscosity was too high and it wasn’t expanding within the mold as fast as the upper and lower parts of the bottle. As a result, too much surface area was being squeezed against the mold at the very end of the blowing process, ultimately the cause of 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,” says 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 bottle without defects.
The final steps were getting the modifications into production—and the sauce to the supermarket. n

Cecilia Galvin, a former editor of Popular Science, often writes about CAD/CAM, computer graphics, IT, and electronics. Send your comments about this article through e-mail by clicking here. Please reference “Smooth Wrinkles, September 2006” in your message.

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Company Information

EnSight
CEI
Apex, NC

Emhart Glass USA
Elmira, NY

GAMBIT, POLYFLOW, FLUENT
Fluent, Inc.
Lebanon, NH

Pro/Engineer
Parametric Technology Corp.
Needham, MA