Screw channel, cross section

Solids bed in an unwrapped screw channel with a screw channel cross-section. 

It is possible to simulate complete velocity and temperature fields through the screw channel cross-section and not simply be limited to mean fluxes. Here one generalizes Eqs. (119a), (119b), and (120) to... 

Figure 7.12 Photomicrographs of screw channel cross sections comparing screws with and without a Maddock element (continued opposite). (Reproduced with Permission from G.M. Gale, Masterbatch Flow Patterns in Polyethylene Extrusion, Rapra Members Report No.16, Rapra Technology, Shawbury, Shrewsbury, UK, 1978, Figure 13. 1978, Rapra Technology)...

The photomicrograph of a screw channel cross section in Figure 8.9 clearly shows the non-circulating layer at two-thirds of the channel depth. The overall trajectories will depend on the relative drag and pressure flow conditions as in Figure 8.10. 

Figure 8.9 Photomicrographs of screw channel cross-section showing noncirculating layer at two-thirds depth. (Reproduced with permission from R. W. Shales, Mixing of Thermoplastics in Single Screw Rextruders, Department of Chemical Engineering, University of Bradford UK, 1989. [PhD thesis])...

Figure 7. Screw channel cross-section temperature profile...

In the simplest and most often used form, the screw has a free channel cross-section that diminishes at a steady rate from the feed to the delivery end. The ratio of the channel depths from feed to die region along the screw is usually referred to as the compression ratio, since it gives a crude indication of the relative conveying capacities at feed and discharge. 

The effect of channel depth on solids conveying rate is shown in Fig. 5.26 for screw and barrel temperatures of 75 and 125 °C, respectively, and at a screw speed of 50 rpm. At zero discharge pressure, the solids conveying rates were nearly proportional to the depth of the screw channel (or cross-sectional area perpendicular to the flight). For example, the conveying rates were 91 and 125 kg/h for the 8.89 and 11.1 mm deep screws, respectively. For these screws, the cross-sectional areas perpendicular to the flights were calculated at 420 and 530 mm an area increase of... 

Figure 6.69 Schematic diagram of a channel cross-section in the melting zone of a single screw extruder.

As mentioned earlier, the melting mechanism in screw extruders was first formulated by Tadmor (29) on the basis of the previously described visual observations pioneered by Bruce Maddock. The channel cross section and that of the solid bed are assumed to be rectangular, as in Fig. 9.26. The prediction of the solid bed width profile (SBP), that is the width of the solid bed X as a function of down-channel distance z, is the primary objective of the model, which can be experimentally verified by direct observation via the cooling experiment of the kind shown in Figs. 9.20-9.25. As shown by Zhu and Chen (40), the solid bed can also be measured dynamically during operation by equipping the extruder with a glass barrel. 

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. 

Further trials using screw jacking following rapid barrel cooling compared sections from a conventional screw with the one fitted with the Maddock element. The feed material was LDPE with carbon black masterbatch. Figure 7.12 shows photomicrographs of microtomed channel cross sections taken every two turns and Figure 7.13 shows cross sections from the die adaptor. 

The fairly simple development which follows is for pedagogical reasons and is due to Werner (1976). A partially filled screw channel is shown in Figure 8.24. The degree of fill is given by fhe ratio of the filled channel cross-sectional area, Af, to the total cross-sectional area. A ... 

The constraint that the tip of one screw element wipe the flank of its mate in self-wiping, corotating twin-screw extruders leads to a unique relationship for the shape of the screw channel (Booy, 1978,1980). Figure 13 is an isometric view of this channel and Fig. 14 is a cross section of the channel in a plane that is perpendicular to the plane which defines the helix angle. Figure 14 shows the actual shape of the channel, which is described by the following expressions ... 

Figure 7.2 Transforming the Cartesian reference frame a) cylindrical cross section of the screw and barrel with flow out of the surface of the page, and b) the unwound rectangular channel with a stationary barrel and the Cartesian coordinate frame positioned on the screw, is the velocity of the screw core in the z direction and it is negative...

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

Figure 10.2 Photograph of a Maddock solidification experiment a) the screw is being pushed out of the barrel with the solidified resin, b) the screw with resin solidified in the channels, and c) a cross-sectional view parallel to the screw axis in the melting section...

Figure 14.7 Schematics of a Double Wave screw channel a) schematic of a Double Wave section. The unfilled flight is the secondary flight, and it is undercut with respect to the filled primary flight, b) and c) are cross-sectional views perpendicular to the flight. Section c) is about 1 diameter downstream from section b). The flow direction is out from the page surface...

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