Screw diameter change

Figure 7.1 Extruder, change in screw diameter exaggerated...

There are various techniques used to increase the extraction capacity along the axis of a screw. The most influential is to increase the screw diameter, as the cross-sectional area increases as the square of this dimension. Other ways are to increase the pitch, reduce the centre shaft diameter, or reduce the friction on the face of the screw flight. These variations can be made in virtually any combination, or independently. The effect of these changes will be examined in detail. 

For mass flow hopper applications, these features limit the effective length of screws that vary in pitch only, to about five or six screw diameters. For non-mass flow applications, pitch changes are a useful means to reduce power and secure an improved extraction pattern, and much longer exposed sections of screws can be used. These benefits may not be essential, but offer advantages by avoiding excessive dead zones of storage. 

Considering that in most extruder screws the screw diameter is much larger than the channel depth (D/H 1, usually about 5), the change in channel width and helix angle over the depth of the channel will be rather small. If it is assumed that the channel curvature can be neglected, the screw channel can be unrolled onto a flat plane. The error that is made in this process may be acceptable considering the limited accuracy and reproducibility of most data on the coefficient of friction, as discussed in Section 6.1.2. Two simplifications resuit from this assumption. The first one is that now the channel width and helix angie are constant over the depth of the channei. The second simplification is that the extra force F can be determined directiy from a force balance in the cross-channei direction ... 

Change the values of screw diameter B, spiral pipe diameter d and the number of windings N, and repeat the above steps of the experiment. The values of three parameters are shown in Table 1. 

These features tend to limit the effective length of feeder inlet for a mass flow application to about four screw diameters when the pitch only is varied. This construction is, however, effective on long screws, where intermittent changes of pitch will relieve the shear under a dead region of flow and require less torque to shear through a length of flowing media. 

A version of this free helix device was also constructed that had a recycle system such that there was essentially no pressure change from the exit to the entrance of the extruder, as shown in Fig. 7.4. That device had a barrel diameter of 58 mm and it was used to measure the effect of the different parts of the screw on the extruder performance where pressure gradients could he imposed [5], The devices shown in Figs. 7.3 and 7.4 could be operated as a conventional extruder if hoth core and helix elements were driven at the same angular velocity with the barrel stationary. In another mode, only the helix was rotated in the same direction as the screw had been while the core and barrel were stationary. And finally the screw core was rotated in the same or different direction as the screw and the helix and barrel were stationary. 

The temperature increase for screw rotation was calculated using the same channel geometry and conditions as discussed above, except that the channel depth was changed from 11.1 mm to 3.8 and 18.4 mm. As expected, the temperature Increase for the process was the highest for the shallowest screw and the lowest for the deepest screw. The temperature increase as a function of time for the three channel depths is shown in Eig. 7.35. The channel with a depth of 3.8 mm is in the commercially important range for a machine of this diameter. 

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. 

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