Screw root

Vane Liquid ring Screw Screw Roots Trunk Crosshead Labyrinth Diaphragm... 

Thus the screw has a narrower normal distance between flights at the screw root because the helix angle is larger and because the lead remains the same. 

The z component of the screw velocity at distance E from the screw root is computed as... 

In this section, three models will be presented that don t force the reorganization of the solid bed and use screw rotation physics. These screw rotation models cause a significant portion of the energy dissipation to occur in the melt film between the solid bed and screw root. These models are for a conventional transition section, for a barrier melting section, and for a special case referred to as one-dlmenslonal melting. 

Figure 6.12 Schematic for the zones of the new melting concept Zone A is the solid bed. Zone B is the melt pool, Zone C is the melt film located between the solid bed and the barrel wall, Zone D is the melt film between the solid bed and the screw root, and Zone E is the melt film between the solid bed and the trailing flight. The cream color represents molten resin...

The thickness indicated by the red line in Fig. 6.18 is the gap between the solid bed and the screw root. The screw root is moving in the minus z direction while the solid bed is moving in the positive z direction. Melted polymer will thus be dragged into the gap, and there will be a negative pressure gradient dP/dz in the film. This topic will be presented in Section 6.3.1.3. 

Degraded Resin at the Screw Root Where the Flow Streams Merge... 

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. 

Vi-ji velocity of the screw root in the x direction for screw rotation velocity of the screw root in the z direction for screw rotation Vez average velocity in Film E in the z direction... 

Equation 7.21 is the literature expression for motion in the x direction for barrel rotation physics. The boundary conditions here are = 0 aty = 0 (screw root) and Erf = Kx aty = // (flight tip). Cross-channel velocity in the laboratory (Eulerian)... 

A three-dimensional simulation method was used to simulate this extrusion process and others presented in this book. For this method, an FDM technique was used to solve the momentum equations Eqs. 7.43 to 7.45. The channel geometry used for this method was essentially identical to that of the unwound channel. That is, the width of the channel at the screw root was smaller than that at the barrel wall as forced by geometric constraints provided by Fig. 7.1. The Lagrangian reference frame transformation was used for all calculations, and thermal effects were included. The thermal effects were based on screw rotation. This three-dimensional simulation method was previously proven to predict accurately the simulation of pressures, temperatures, and rates for extruders of different diameters, screw designs, and resin types. 

Transport of energy in the screws was modeled previously for single-screw extruders [30-32] and for twin-screw extruders [33]. In order to predict the axial screw temperature in a single-screw extruder, heat conduction along the screw has to be modeled. The model developed by Derezinski [32] included heat conduction from the barrel through the screw flights to the screw surface, heat conduction from the polymer to the screw root, and heat conduction in the axial direction. The model showed that the screw does not behave adiabatically and that the steady-state heat conduction in the screw depends greatly on the size of the extruder. 

The HIPS resin was extruded at screw speeds of 30, 60, and 90 rpm at barrel temperatures of 200, 220, and 240 °C for Zones 1, 2, and 3, respectively. The screw temperatures in Zone 3 as a function of time at the screw speeds are shown in Fig. 10.20. Because the RTDs were positioned within 1 mm of the screw root surface, they were influenced by the temperature of the material flowing in the channels. Prior to the experiment, the screw was allowed to come to a steady-state temperature without rotation. Next, the screw speed was slowly increased to a speed of 30 rpm. The time for the screw to reach a steady state after changing the screw speed to 30 rpm was found to be about 10 minutes. The temperature of the T12 and T13 locations decreased with the introduction of the resin. This was caused by the flow of cooler solid resin that conducted energy out from the screw and into the solids. At sensor positions downstream from T13, the screw temperature increased at a screw speed of 30 rpm, indicating that the resin was mostly molten in these locations. These data suggest that the solid bed extended to somewhere between 15.3 and 16.5 diameters, that is, between T13 and T14. When the screw speed was increased to 60 rpm, the T12 and T13 sensors decreased in temperature, the T14 sensor was essentially constant, and the T15, T16, and T17 sensor temperatures increased. These data are consistent with solids moving further downstream with the increase in screw speed. For this case, the end of the solids bed was likely just upstream of the T14 sensor. If the solid bed were beyond this location, the T14 temperature would have decreased. Likewise, if the solid bed ended further upstream of the T14 sensor, the temperature would have increased. When the screw speed was increased to 90 rpm, the T12, T13, and T14 temperatures decreased while the T15, T16, and T17 temperatures increased. As before, the solids bed was conveyed further downstream with the increase in screw speed. At a screw speed of 90 rpm, the solid bed likely ended between the T14 and T15 sensor positions, that is, between 16.5 and 17.8 diameters. These RTDs were influenced by the cooler solid material because they were positioned within 1 mm of the screw root surface. 

The axial screw temperature profiles are consistent with the general trends that would be predicted using the Cox and Fenner [30] model, but the temperature of the screw is obviously affected by all barrel temperature zones and not just the zone over the metering channel. The data shows that heat conduction from the barrel to the screw root is highly important. This conclusion is consistent with the observations and model by Derezinski [32]. 

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