Drag induced solids conveying

Once the particulate solids have reached the feed port of the extruder, the material will flow down until it is situated in the screw channel. At this point, the gravity induced flow mechanism will essentially cease. In most extruders, the screw and barrel are placed in a horizontal direction and the role of gravity becomes a very minor one. In fact, in most analyses of solids conveying in single screw extruders. 

It has been found experimentally that, in most cases, polymeric particulate solids compact readily in the early portion of the screw channel. As a result, the solids form into a solid bed and the solids move down the screw channel in plug flow thus, at any cross-section of the solid bed all elements move at the same velocity. In other words, there is no internal deformation taking place inside the solid bed. This compaction of the particulate solids into a solid bed can occur only if there is a sufficient amount of pressure generation in the screw channel. 

Pressure generation cannot take place if the screw channel is not fully filled and consequently compaction and plug flow cannot occur. 

It has been observed [44] that even under normal extrusion conditions, Archimedean transport can occur for a short length. Observations of the filling action in the 

The first comprehensive analysis of solids transport in single screw extruders was made by Darnell and Mol in 1956 [43]. Later, numerous workers extended the work of Darnell and Mol however, the basic analysis has remained relatively unchanged. In order to come to a quantitative description of the drag induced solids conveying process, the following assumptions were made by Darnell and Mol  

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Drag-induced solids conveying occurs along the screw. Its purpose is to move material out from under the hopper so that pellets can continue to flow freely into the feed section of die screw. Additionally, the pellets are moved forward along the screw... 

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. 

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... 

The benefits of a grooved barrel section can be analyzed from the theory of drag induced solids conveying developed in Section 7.2.2. Figure 7.19 shows how the solids conveying rate varies with the coefficient of friction on the barrel 4, for a situation where the pressure gradient is zero. 

The unit can be fed polymer in the particulate solids form or as strips, as in the case of rubber extrusion. The solids (usually in pellet or powder form) in the hopper flow by gravity into the screw channel, where they are conveyed through the solids conveying section. They are compressed by a drag-induced mechanism in the transition section. In other words, melting is accomplished by heat transfer from the heated barrel surface and by mechanical shear heating. 

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. 

Saltation of solids occurs in the turbulent boundary layer where the wall effects on the particle motion must be accounted for. Such effects include the lift due to the imposed mean shear (Saffman lift, see 3.2.3) and particle rotation (Magnus effect, see 3.2.4), as well as an increase in drag force (Faxen effect). In pneumatic conveying, the motion of a particle in the boundary layer is primarily affected by the shear-induced lift. In addition, the added mass effect and Basset force can be neglected for most cases where the particle... 

Sections 9.2 and 9.3 discussed the elementary steps that make up the important and widely used screw extrusion process. The common and outstanding feature of all the elementary steps taking place in the screw channel is that they are induced by drag brought about by a single moving surface—that of the barrel. Solids are conveyed and compressed by the... 

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