Conveying zone

The goal of this preliminary investigation was to gain more knowledge of the Interaction between solid polymer physical properties and their relation to extruder rate and energy dissipation characteristics In the solids conveying zone. The data presented here will focus on the effects of temperature and polymer type on solids... 

The solids conveying zone. The task of the solids conveying zone is to move the polymer pellets or powders from the hopper to the screw channel. Once the material is in the screw channel, it is compacted and transported down the channel. The process to compact the pellets and to move them can only be accomplished if the friction at the barrel surface exceeds the friction at the screw surface. This can be visualized if one assumes the material inside the screw channel to be a nut sitting on a screw. As we rotate the screw without applying outside friction, the nut (polymer pellets) rotates with the screw without moving in the axial direction. As we apply outside forces (barrel friction), the rotational speed of the nut is less than the speed of the screw, causing it to slide in the axial direction. Virtually, the solid polymer is then "unscrewed" from the screw. To maintain a... 

It is easy to visualize the mechanisms that take place in the solids conveying zone of a screw extmder, and to develop appropriate mathematical models describing these mechanisms. This was done by Darnell and Mol (28) in 1956, and their model is discussed later in this section. 

However, the melting or plasticating process that follows the solids conveying zone is a relatively complex one, and unlike the melt extrusion and solids conveying processes, the detailed physical melting mechanisms cannot be easily visualized, predicated, and modeled from basic principles without experimental investigation. 

A more common situation, however, is that before there is any significant frictional heating, an axial position is reached where the barrel is heated to well above the melting point, forcing the creation of a him of melt. In either case, this marks the end of that portion of the process in the extruder called the solids conveying zone, where only solids are present and the only elementary step that occurs is handling of solids. 

Finally, the total power consumption in the solids conveying zone is obtained by taking the product of the force between the barrel surface and solid plug Pi and the barrel velocity in the direction of the force nNDj, cos cj>... 

Fig. 9.30 Graphical representation of the various components of the total power consumption in the solids conveying zone of screw extruders for H/Di, = 0.15 and a constant fs/fb ratio as a function of the angle (f> Pwb, which is the major component of the power, is the power dissipated into heat at the barrel plug interface Pws and Pwf are the power dissipated on the root of the screw and flights, respectively and Pwp is the power consumed for the pressurization of the plug. [Reprinted by permission from E. Broyer and Z. Tadmor, Solids Conveying in Screw Extruders, Part I. A Modified Isothermal Model, Polym. Eng. Sci., 12, 12-24 (1972).]...

Calculate (a) the base pressure of the hopper, (b) pressure at the end of the solids conveying zone, and (c) power consumption in the solids conveying zone. 

Assume isothermal operation and that the inlet pressure to the solids conveying zone equals the base pressure of a fully loaded hopper. Bulk density of the feed is 595 kg/m3, the static coefficient of friction in the hopper is 0.3, and the dynamic coefficients of friction on barrel and screw are 0.45 and 0.25, respectively. 

Thus, the exit pressure from the solids conveying zone is 3.776 x 103 x 60.9 = 2.3 x 105N/m2 (33 psi). This result indicates that the solids conveying section functions properly and that higher outputs could be obtained at this screw speed before solids conveying limitations (e.g., starving ) were encountered. We should note, however, that the analysis of the solids conveying zone is very sensitive to the values of the coefficient of friction. 

Vpi Axial velocity of the solids in the solids conveying zone of a single-screw... 

Kneading block assembly Solid conveying zone... 

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