Plasticator feed zone

K. Schnieder, Technical Report on Plastics Processing in the Feeding Zone of an Extruder, Institute of Plastics Processing (IKV)TH, Aachan (1969). 

The modified screw was placed back into the injection-molding press and evaluated for performance. The barrel temperatures were maintained at 245, 255, 260, and 260 °C for the feed zone through the last barrel zone, respectively. This temperature setting was lower than that used for the original screw. The screw was rotated at a speed of 235 rpm, and the back pressure was set so that the pressure at the tip was 10 MPa. The 0.244 kg part and runner system was plasticated in 4.2 s for a specific rate of 0.89 kg/(h-rpm). All parts produced were completely free of the splay defect. The modifications were able to eliminate the bubbles and the unmelted material. 

Feed Zone This is the part of the screw that picks up the plastic at the feed opening (throat) plus an additional portion downstream. Many screws, particularly for extruders, have an initial constant lead and depth section, all of which is considered the feed section. This section can be welded onto the barrel or a separate part bolted onto the upstream end of the barrel. The feed section is usually jacked for fluid heating and/or cooling. 

Transition Zone-. It is the section, also called the compression zone, of a screw between the feed zone and metering zone in which the flight depth decreases in the direction of discharge. In this zone the plastics starts in both solid and molten state with target to have all molten upon leaving this zone. 

The plastic feed may contain chlorine in the form of PVC or PVDC. Preferably, a substantial portion of any chlorine in the feed is removed by the addition of a chlorine scavenger compound to the feed, for example, sodium carbonate or slaked lime (calcium hydroxide) to the feed. It reacts in the pyrolysis zone with the hydrogen chloride to form sodium chloride or calcium chloride which becomes part of the residue at the bottom of the pyrolysis vessel. Preferably, the chlorine content should be removed to less that about 5 ppm. 

The feed zone is the area where the moistened formulation is first introduced into the extrusion device. It includes a hopper to channel and distribute the flow of material into the chamber containing the screw(s). Most screw extruders will be operated with only a slight excess of feed or even in a somewhat starved state. Because, for extrusion, the material must be plastic, too much feed tends to build up over the screws and bridging is likely to occur. 

Z-N catalyst See catalyst, Ziegler-Natta. zone A location in a plasticating screw. See screw feed zone screw metering screw transition zone, zone axis A group of crystal planes that are all parallel to one line is called the zone axis. 

During the plasticating process, the screw is rotated by a hydrauhc motor. The material descends from the hopper and enters the feed zone. The rotation of the screw then conveys the material toward the screw tip and through the open non-return valve. The screw is forced backward by the material accumulating in the injection chamber (ante-chamber) in front of the screw tip. As soon as the introduced melt has pushed the screw back to trigger the adjustable feed stroke limit switch, screw rotation is stopped. 

Therefore, the plasticating extruder has three zones (1) the feed zone, wherein the conveying of the solid particle is carried out (it is usually cooled in order to eliminate sticking) (2) the compression zone, in which the principal melting takes place and (3) the metering zone, in which the liquid melt... 

When single-screw extruders are starve fed (Fig. 5.12d), plastic particles do not immediately fill the screw channel. As a result, the first few channels of the feed zone lack the pressure required to compact the polymer particles. Particle conveyance in the imfilled channels is not as steady as transport with filled channels. Consequently, metered feeding is seldom used with single-screw extruders. Such feeding can be used to reduce the motor load, limit temperature rises, add several components through the same hopper, improve mixing in singlescrew extruders, control flow into vented barrel extruders, and feed low-bulk-density materials. 

Once the plastic particles reach the feed zone, they are compacted into a solid bed. The solid bed is conveyed down the extruder via viscous drag or drag flow. In this process, the solid bed adheres preferentially to the barrel wall and is pushed down that surface by the rotational motion of forward edge of the screw flights. As the solid bed moves down the feed zone, pressure on the solid bed rises gradually. 

The feed zone has the most effects on solid conveying of the resin. Three coefficients of friction take place, namely friction between barrel and pellet, between pellet and pellet, and between root of the screw and pellet. Solid conveying is the process wherein the material must stick to the barrel and slip on the screw. Hence, cooling root of screw will reduce the coefficient of friction between the screw and the plastic pellets. Screw cooling should be installed in the core of the screw in the feed section to give another zone of temperature control on the extruder. 

The feed zone conveys plastic forward to the transition zone and has a constant root diameter. The root diameter tapers through the transition zone, which compresses the granules forcing air back toward and out of the feed throat and hopper. The main purpose of the transition zone is to compress the plastic and provide the shear heating to drive the melting process. Ideally, by the end of the... 

Once a feed strategy is identified, appropriate screw configuration and barrel design for commercial extrusion must be identified (Schenck ef al. 2011). As a process is scaled from pilot to commercial scale, often a need for more efficient moisture removal is identified. This requires optimal placement and design of melt seal sections near the feed zone to prevent premature plasticization of the feedstock by water vapor. In addition, vents must be placed accordingly along the extruder barrel with... 

Modylen 2-5112) - Preferably twin-screw extruder with L/D = 25-30, in plasticizing zone having the screws equipped with kneading elements. Feeding zone temperature 160-180°C Die temperature 240-270°C... 

The compaction zone has an increasing screw diameter from the small diameter in the feed zone to the larger diameter in the metering zone. The gaps between the screw and the barrel are reduced in the compaction zone. The plastic is melted in the compaction zone due to increased temperature and shear heating from the small clearance between the screw and the barrel. 

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