Drag-Induced Region

After solids have dropped through the base of the hopper, they enter the back of the screw in the feed throat. The solids must not begin melting at this point or no further solids would be able to enter. Therefore, the rotation of the screw must drag the solids out from under the hopper so that more solids can drop down. This mechanism is called drag-induced solids conveying. 

It seems logical that because screw rotation causes drag-induced flow, a condition for good solids conveyance in this region would be high interaction (friction) between the solids and the screw. But, this is a case for counter intuition In fact, when there is high friction between the solids and screw, the solids stick to the screw and simply rotate around and around, never moving toward the die. 

The optimum condition for drag-induced solids conveying is low friction between the pellets and the screw, and high friction between the pellets and the barrel. This condition 

Melting is defined here as the change of polymer from a solid to a liquid, with this definition holding for both amorphous and semi-crystaUine polymers. This function generally begins about three to five turns down the length of the screw. 

The heater bands have two more significant roles besides providing heat to melt the polymer. First, they supply the initial energy to heat up the hardware during start-up. An extrusion screw cannot be turned until the extruder is up to operating temperature. 

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The total drag-induced leakage flow in the apex region now becomes ... 

At speeds beyond 4000 m/min, inertial and air drag effects become the dominant contributors to fiber stress. Sufficient orientation can be induced so that significant crystallization occurs in the as-spun fiber. The structure begins to partition into either highly oriented crystalline regions, or amorphous regions of relatively low orientation. There is relatively less oriented-amorphous structure. 

The degradation ribbon at the merger of the flows occurs because of the crosschannel flow of material from the region between the solid bed and the screw root to the melt pool. As shown by Fig. 6.35, this flow is relatively large. As previously stated, the flow occurs because of pressure-induced flow and the dragging of fresh material under the solid bed by the backwards motion of the screw root. This process is consistent with the physics presented for screw rotation. The flow fields developed for a barrel rotation system would not create the low-flow region such as shown in Fig. 6.37. 

Fluorescence probes are frequently used to study changes in membrane organization and membrane fluidity induced by anesthetics, various drags, and insecticides. This technique measures fluidity as the rate and extent of phospholipid acyl chain excursion away from some initial chain orientation during the lifetime of the excited fluorescence state. Special techniques even allow the place of interaction to be localized, i.e. to the outer membrane region, the hydrophobic area, or the embedded proteins. 

For l-/im particles, the inertial force is quite small relative to the drag force, and rapid formation of a particle or gel layer is predicted. However, the particle layer does not grow steadily until it plugs the channel but seems to reach a steady-state thickness. One explanation is that the high shear rate near the wall causes a tumbling motion of individual particles, which expands the layer and leads to migration away from the wall. This shear-induced dispersion and the particle movement toward regions of lower concentration can be modeled with a particle diffusivity, which is proportional to the shear rate and the square of the particle size. More work is needed to understand the effects of particle shape and surface characteristics. 

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