Getting a new extrusion die to shape a profile that meets design specifications has often been more art than science.
The idea is to design the die that provides a particular profile from a range of operating conditions, such as throughput, melt temperature, and heating or cooling rates. It's not as easy as it sounds.
"Getting a stable extrusion not long ago involved cutting several dies guided by manufacturing experience and sometimes guesswork," says Fluent Inc. Senior Consulting Engineer Hossam Metwally. "Simpler profiles might need only two dies. But more complex shapes might need 20 or more. Besides the cost of scrapped die heads, engineering time and machine downtime to test intermediate designs added to part costs."
CFD helps shorten die-design tasks with die balancing and reverse engineering. "Die balancing finds manufacturing conditions that generate a uniform flow at the die exit. And reverse engineering estimates the shape of the die-lip (die exit) cross section when the final profile differs from the die-lip-cross section," says Metwally. "This is needed for materials that expand or swell after leaving the die."
Die balancing is needed for parts with thin and thick sections that go through a calibrator — a downstream device that cools the extrusion in shape. "Flow at the die exit tends not to be uniform because the melt selects a path of least resistance and exits mostly through the profile's thickest section. The objective here is to give all paths through the die equal resistance. Thinner sections of the profile should be preceded by less resistance than thicker sections, and at the same time,thicker sections should be preceded by more resistance," adds Metwally.
Extrusion modeling software based on CFD supports multiple what-if scenarios. "The software becomes a virtual extrusion machine dedicated to testing and design. A die designer can test different heads, different extrusion speeds, draw-down rates, different die heating patterns, and extrusion cooling patterns to see their effects on the flow uniformity at the die exit prior to cutting the die," he adds.
Reverse engineering is typically applied to materials that swell after exiting the die. "Some expand by 300%. The opposite condition involves draw-down or pulling the extruded material as it leaves the die, as in fiber drawing," says Metwally. "In these cases, designers must specify a final profile shape from a CAD file or the profile of an existing part, the process operating conditions, and an initial guess for the die-lip-cross section."
Of course, the predicted die lip is valid only for the specified operating conditions and material. "CFD still lets users test the sensitivity of the process to changes in operating conditions. Increasing or decreasing temperature, for instance, lets the simulation show how the overall process will react, telling the online operator how tightly he must control the extruder's operating conditions," says Metwally.
Fluent Inc., fluent.com