Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/421920
EXTRUSION One of the advantages of barrier screws is that for the most part they eliminate the problems associated with solids-bed breakup that are typical in conventionally fighted screws. Many conventional screws have to rely on restrictive mixing sections to complete the melting process because solids conveyed into the metering section are difcult to melt. With barrier screws this is essentially eliminated, as solids are trapped in the solids channel until they pass over the barrier and/or a high-shear section at the end of the barrier section. As a result, solids are completely melted by the time they reach the metering section (see Fig. 1). This Understanding Solids-Bed Breakup in Barrier Screws ensures that a more thermally homogeneous melt exits the screw without need for a mixing section. That said, solids-bed breakup can still be a factor in designing barrier screws. If the polymer in the solids channel somehow does manage to break up, it similarly becomes much more difcult to melt because it again becomes dependent on conduction for melting rather than on shear or viscous dissipation. Since polymers are efectively insulators, conductive melting is very inefcient and requires a high temperature diferential between the surrounding melt and the unmelted portion to have any efciency at all. Crystalline polymers, particu- larly polyolefns, are inherently much more susceptible to this issue because they do not appreciably soften as they are melted and require a higher diferential between the surrounding melt and remaining solid to overcome the heat of fusion using conductive heating. Most often, solids-bed breakup in barrier screws seems to occur when the compression section is too gradual. This allows the solid bed to be undermined by melt, destroying its structural integrity. In a conventional screw, the solids that have broken up are simply conveyed forward, creating no pressure spike. But in a barrier design, they remain trapped in the solids channel. As the solids channel cross-section is reduced by either depth, width, or both, the remaining solid-bed pieces collect again and completely fll the channel at some location. This creates an obstacle for further transport of the solids, and since the polymer is unmelted at that point and the channel is typically quite shallow, a very high localized pressure develops in the solid. This pressure is most often contained in approximately half of a turn. Since most of the screw circumference is flled with melt at Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION Most often solids-bed breakup in barrier screws seems to occur when the compression section is too gradual. By Jim Frankland Barrier screws all but eliminate problems associated with solids-bed breakup. But if they do occur, tremendous pressures can develop, causing screw wear. Conventional vs. Barrier Screw: Simulated Melt-Performance Comparison Melting in a conventional vs. barrier-fighted screw is visualized as if the screw fight were "unwrapped" into a continuous channel. The different views are "slices" across that channel along the screw in the direction of fow, shown by the arrow pointing downward. The channel depth decreases as you travel down the screw. In the ideal state, at the end of the barrier screw there is only melted polymer, but the conventional screw still has some solid lumps. FIG 1 Conventional Screw Unmelted Polymer Unmelted Polymer Melted Polymer Barrier Screw 28 DECEMBER 2014 Plastics Technology PTonline.com K now How