Screw output zone

In the plasticators output zone, both screw and barrel surfaces are usually covered with the melt, and external forces between the melt and the screw channel walls have no influence except when processing extremely high viscosity plastics such as rigid PVC and UHMWPE. The flow of the melt in the output section is affected by the coefficient of internal friction (viscosity) particularly when the die offers a high resistance to the flow of the melt (Chapter 3). Figure 5.2 shows the extruder s components where the following identifications are listed ... 

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

A portable electronic data acquisition system was transported to the plant site and connected to the extruder panel. All available instrument outputs from the panel were connected in parallel with the acquisition system. Data collected included barrel zone temperatures, screw speed, motor current, pressure at the entry to the pump, transfer line temperature, and gear pump temperature. Process data were collected at a frequency of once every five seconds. 

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. 

The processing zones in the extruder are the solid zone - feed section, transformation or plastification zone, the pumping or metering section (screw) and the mold. Feeding in the solid zone is the main determinant of machine output. 

This is the final zone of the screw, which is designed to further homogenize the material and ensure, that it has a uniform temperature. This zone imparts the pressure which is necessary for the melt to overcome subsequent resistance. The metering zone determines the output rate... 

When drag flow dominates, extruder output increases linearly with screw speed, and larger screws and deeper channels carry more melt. However, head pressure also rises with screw speed. Although this increases pressure flow, the actual effect depends heavily on melt viscosity. With high-viscosity melts, pressure flow may be minimal and have little effect on extruder output. In contrast, low-viscosity melts produce less head pressure but greater pressure flow. Thus, pressure flow will reduce the expected output. Deeper channels, neutral screws (Fig. 5.26), and shorter metering zones enhance these effects. 

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