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What is Extrusion...[continued]
In a single-screw extruder, the screw has a deep-channeled feed section that transports solids from the feed throat and compresses them, a transition or compression section where the channels become shallower and the polymer is melted, and a shallow-channeled metering section where the melt is conveyed to the adapter and die. Different polymers process best with different compression ratios (the ratio of the feed zone depth to the metering zone depth). Some resins melt better in a screw that has barrier flights in the transition section to separate the melt pool from the remaining solids bed. Some screw designs incorporate mixing devices such as pins or barriers to flow. Static mixers, which interrupt the melt flow, create distributive mixing and are used to make melt temperature more uniform. Single-screw extruders are used in simple compounding operations, such as mixing color and additives into a polymer, although Kneaders and twin-screw extruders are more often used for compounding extrusion that requires highly dispersive mixing. Relatively new mixing screws for single-screw extruders improve both distributive and dispersive mixing for more demanding compounding operations.
Single-screw extruders are widely used for extruding film, sheet, and other applications that use pre-compounded pellets.
The Buss Kneader is a special type of single screw extruder with a screw that both rotates and oscillates axially. The Kneader has a full spiral screw in the feeding and devolatilizing zones. In the mixing and kneading zone, the spiral is interrupted with kneading flights that intermesh with kneading teeth or pins fixed inside the barrel. The Kneader is widely used for compounding PVC pellets, because it operates at low shear and low temperature. It is also used for direct extrusion of PVC calendered film.
Several different types of twin-screw extruders (TSE) are used for different applications. Parallel twin-screw extruders have two screws that rotate in the same direction (corotating) or opposite direction (counterrotating), with non-intermeshing or fully intermeshing flights. Parallel twin screws are modular, and the screw design can be changed by rearranging feeding, venting, mixing, and pumping elements along the screw shaft. Corotating TSE are commonly used for compounding, while counterrotating TSE are often used for direct extrusion.
Conical, counter-rotating twin screws are tapered and made in one piece. Conical TSE are used primarily for processing PVC into pipes and profiles. The lower shear of conical TSE is an advantage for shear-sensitive materials like PVC.
While single-screw extruders are typically flood fed, with the feed filling the screw channel, twin-screw extruders are typically starve fed, with only a partially filled screw channel. Because the screw channel is not completely filled, downstream feedports or vents can be easily added. Adding shear- or heat- sensitive ingredients downstream, after the polymer is melted, helps keep them from degrading. Fibers can be added downstream to reduce screw wear and to preserve their aspect ratio. Vents are used to remove volatiles, such as water entrained in the mixture, from the melt. Compared to single-screw extruders, TSE are better at feeding and conveying, especially of powdered material or large amounts of fillers, and typically create more dispersive mixing.