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gearbox combination is able to supply. Limitations in the down- stream equipment include high discharge pressure in the case of high-viscosity materials and restrictive dies, and high pellet temperature in the case of inadequately sized cooling equipment. Only once the limitation is understood can steps be taken to remove it. This process is one of continuous improvement, where if one limitation is removed, another will be uncovered at a higher throughput rate. FEED SYSTEM To achieve the highest throughput rates, feeding equipment must deliver an accu- rate and consistent feed to the extruder. To do this, the feed system must be designed according to the materials and throughput rates being fed (see Fig. 1). Easily fluidizing powders are handled well by a feeder with twin, narrow-pitch, con- cave augers, as the smaller clearances and longer path to the discharge abate the fluidization behavior. On the other hand, solid polymer pellets may become impinged in the tight clear- ances between these screws, so pel- lets benefit from being fed with a coarse-pitch, single auger. Other materials, such as wet or cohesive aggregates, tend to be best suited for a belt feeder rather than a screw feeder. Consult your feeder OEMs for their specific recommendations. For materials that tend to bridge, rat hole, or stick to the hopper, agitation is needed to keep the product flowing consistently. Classically this has been done by some type of a mechanical stirrer. One downside of this method is that it can compact sticky materials, and increase the cleaning time when changing over to different materials. An alternative to mechanical stirring is the ActiFlow system, which works through an intelligent vibratory drive attached to the outside of the feed hopper. It continuously optimizes frequency and amplitude based on changing material flow condi- tions and provides feedback to the feeder controller to remove the vibratory noise from the load-cell signal. Air It's vital to know the characteristics of the material in order to select the right feeder screw type. Here, a colored field in the table means essentially suitable. Feeder Screw Types Based on Material Characteristics Flow of Powders and Air into Feed Intake Section of the Extruder When feeding powders, air becomes entrained as the powders drop from the feeder into the extruder. Since the melt zone of the extruder is fully filled, the entrained air cannot move downstream past the melt seal with the molten polymer. Instead, it is forced to escape through the nearest upstream opening, which is usually the feed opening. FIG 1 Material Characteristics *Requires Agitator Twin Concave Profile Screws Twin Auger Screws Twin Spiral Screws Double Spiral Screws Spiral Screw Auger Screw Auger Screw, Larger Tube Very Free Flow Free Flowing Relatively Free Flowing* Poorly Flowing* Dusty* Sticky* Lumpy* Greasy* Damp* Hygroscopic* Bridging* Rat-Holing* Flooding Compacting* Fluidizing Plasticating* Powder Pellets Granules Fibers Flakes Melt Seal Powder and Entrained Air FIG 2 @plastechmag 57 Plastics Technology T W I N - S C R E W C O M P O U N D I N G