Feed Zone Material Flow

In the screw, the cross-section of the screw channel is open, enabling the material exchange to take place from one flight to the other. Normal leak flow (mechanical clearance) between screw crests and barrel is required for material flow. In a fully inter-mesbing screw, the screws are open lengthwise. 

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Figure 288(a) depicts on the left side the conditions in a machine with a narrow roll face and on the right side the wide-faced machine, both with gravity feed. Since there is little cross-flow in the nip the starved feed conditions in the roll border zones result in less compaction and, in extreme cases, excessive leakage at the cheek plates. It is obvious that this effect is more noticeable if the roll face is narrow because, although the absolute amount of less densified border zone material is constant for a given material, roll diameter, and feeder geometry, the relative proportion is smaller if wide-faced rolls are used. 

The polymer is fed into the feed hopper, which regulates flow into the screw. As the screw rotates, feed is drawn into the feed section and flows in the feed zone in the helical flow channel between the rotating screw and the barrel wall. Typically this channel depth decreases in the compression zone, which promotes increasing pressure in the extruder to promote mixing and expel air voids. In the flnal metering zone the channel depth is constant again, and the material is homogenized and supplied to the die zone at constant temperature and pressure to ensure consistent production. The die zone imparts the cross-section to the profile, screens out impurities and distributes the material evenly across the cross-section. 

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. 

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. 

A low-viscosity feed stream also influences the working of the feed zone. In this partially filled zone, material with low viscosity is not likely to be dragged properly along the channels of the screw but tends to flow on the bottom of the extruder because gravity forces predominate rather than viscous forces. A dimensionless parameter that relates gravity and viscous forces is the Jeffreys number ... 

There are two effects that hinder throughput starved conditions in the feed zone and too much squeezed air from the particle mass that flows upward and against the powder flow, reducing the supply of material to the nip area (25). 

Nozzle designs with positive shut-off devices have been successfully used. The gas must be free to escape through the nozzle. Material freeze off in the nozzle or malfunction of the positive shut-off device could develop pressure to cause blow-back of the material through the feed zone and hopper or create hazardous conditions. In such cases, conventional free flow and reverse taper type fitted with a heater band for temperature control of the nozzle prevents nozzle drool or freeze off of the material and is used and in nylon processing. Sprue cutter is associated with the nozzle to help the process. A nozzle forms a seal between the injection system and the mold. 

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