Flow simulation cuts long pellets short.

Easier-to-use software is making computer flow simulation more popular for designing dies for sheet, profiles, and film. Even the apparently simple case of a "spaghetti" die for pelletizing shows how flow simulation can be an essential tool for diagnosing and solving difficult processing problems.

Three years ago, Daikin Ltd. in Japan had a problem pelletizing FEP fluoropolymer. Pellets came out in different lengths and clogged hoppers in customers' processing lines. The FEP melt was extruded through a screen pack, around a torpedo, and through a pelletizing die with concentric rings of over 100 holes. Engineers in Japan tried changing the size and shape of the torpedo, but that didn't fix the problem.

Then Pierre Kuyl, a CAE engineer at Daikin America's Orangeburg, N.Y., technical center, tackled the problem with computer simulation. He chose Polyflow software from Polyflow S.A. in Belgium, later acquired by Fluent Inc. in Lebanon, N.H. Kuyl, who helped develop Polyflow as a graduate student, says the software models 3-D viscoelastic behavior, including die swell.

Kuyl's analysis showed that local hot spots in the melt caused strands in the middle of the die to flow faster and make longer pellets than the strands around the edge of the die. Kuyl modeled fluid velocity, pressure, and temperature, then simulated changes in die design and operating conditions to see what would even out the melt temperatures and flow velocities.

The first step was building a model of the extrusion die in a 2-D axisymmetric plane. Computation time was less than an hour on a Silicon Graphics Indy work station. The simulation showed a considerable amount of shear heat in the melt generated by the screw, causing the strands in the center to move faster and make longer pellets than those on the periphery. Engineers verified the temperature distribution by taking a picture of the die with an infrared camera. They found actual temperatures matched predicted ones within 10-20%.

Polyflow tried simulating a change in slope of the conical surface of the die plate to compensate for melt-temperature differences. By trying a large number of possibilities, Daikin engineers found one design that compensated for the temperature differences and made strands with uniform velocity. The engineers designed and built a die based on the simulation, and it worked, Kuyl says.

Processors can use it

Polyflow fluid-dynamic simulation has gotten a lot easier to use since Polyflow s acquisition by Fluent. The latest release of Polyflow 3.6 last October includes Fluent's Gambit preprocessor, which makes it easier to import existing CAD files and to create new ones, Fluent says. The preprocessor makes iterative changes, automatically updating finite-element meshes that previously had to be created manually. Fluent also added a powerful post-processor, which includes animation and video visualization.

Increased ease of use has broadened the appeal of flow simulation. "New clients are probably 60% processors and 40% resin companies," says Murali Kadiramangalam, business manager for Fluent's PolyFlow program.