Axial screw temperature

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. 

Figure 10.19 Axial screw temperature profiles during a heating cycle. The screw was not rotating...

The axial screw temperature profiles for the screw speeds are shown in Fig. 10.21. These profiles were constructed from the data set shown in Fig. 10.20 by using the data collected at steady state. As shown in this figure, the temperature profile would approximate the model developed by Cox and Fenner [30], but the temperature distribution is more complicated than this simple model. 

Figure 10.21 Axial screw temperature profiles as a function of screw speed for barrel temperatures of 200, 220, and 240 °C for Zones 1, 2, and 3, respectively. The data point labels for the sensors were omitted for clarity...

Axial screw temperature profiles have been measured before, but they have been limited by the number of sensors and the quality of the data. The data presented here expand the existing knowledge of extrusion and also provide a new method for determining the ending location of the solid bed. 

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]. 

Target quantities power consumption P, axial screw force F, pumping pressure Ap, temperature of the extrudate expressed in temperature difference AT = T — To, and volumetric throughput Q... 

For neutral screws, that is, screws that are neither heated nor cooled through the shaft, it was found that the screw temperature at any given axial location equals that of the melt (le). 

This section describes the derivation of analytical equations of developing melt temperatures in screw extruders. The analytical equations for temperature as a function of axial distance are useful in predicting axial melt temperature profiles. The advantage of analytical equations is that the factors influencing temperature development can be easily identified and their effect determined in a quantitative fashion. Both the temperature and shear rate dependence of the viscosity strongly affect the developing temperatures in the extruder. 

The experimental flow curves obtained at constant injection pressure under given melt temperature, mold temperature and axial screw speed are given schematically in Figure 2 for a resin type at various spiral heists with melt flow index of the polymer brand as parameter. By comparing the flow lengths with one another at any spiral height also called wall thickness, the flowabihty of the resin brand in qnestion with reference to another brand can be inferred (1), (2). 

In contrast to screw temperature, since the barrel temperature is controlled and maintained to set values in the three zones of the extruder, the barrel temperature is usually assumed to be constant in most modeling and numerical analyses. However, it was demonstrated in an earlier paper [7] that even the barrel temperature can vary along the screw axis due to heat conduction in the axial direction. Moreover, since the thermocouples are positioned in the barrel and away from the barrel-polymer interface, the temperatures measured can be different than that at the interface, especially for processes that have an energy flux through the barrel wall. 

A number of issues relative to the prediction of solids conveying in smooth bore single-screw extruders are exposed from the theoretical fits to the data in Fig. 5.32. First, the data needed to carry out an effective simulation is difficult to take and is very time consuming, and only a few labs have the proper equipment that is, bulk density measurement, dynamic friction data, lateral stress, and solids conveying data. Moreover, care must be taken to develop an accurate representation of the surface temperature for the barrel and screw as a function of the axial position. This would be quite difficult in a traditional extruder with only a control thermocouple to measure the temperature at the midpoint of the barrel thickness. Second... 

Figure 7.37 Calculated axial temperature for the PC resin for barrel and screw rotation for a discharge pressure that caused the machine to operate at 60% of maximum flow (F = 0.4) [68]. The difference in the discharge temperatures for the two cases was about 6 °C...

Figure 9.2 Simulated axial pressure and temperature for the baseline process at 10.3 kg/h and a screw speed of 28 rpm. The solid lines are for the simulated profiles. The dashed line is the estimated pressure. The simulation predicts a discharge pressure and temperature of 5.8 MPa and 273 °C, respectively...

The metering section of the screw presented above was simulated at 120 kg/h and a discharge pressure of 6 MPa. The screw had to be rotated at a speed of 56 rpm to obtain 120 kg/h. The barrel temperatures were 160, 220, and 275 °C for the feed section through the discharge section, respectively. The simulated axial pressure and temperature are shown in Fig. 9.3. 

The baseline extrusion process was numerically simulated using the processing conditions in Table 9.4 and the method described in Section 9.2.1, that is, with a rate of 77 kg/h, a screw speed of 27 rpm, and a discharge pressure of 10.6 MPa. The iterative calculation process was used to estimate a bulk temperature of 160 °C and a pressure of 13.1 MPa at the entrance to the meter section. The axial pressure and temperature profile for the simulation is shown in Fig. 9.5. 

The temperature of the screw was measured by several investigators [29-32]. The measurements were performed by mounting thermocouples in an axial hole bored in the center of the screw or by protruding the thermocouples into the melt flow. The sensor signals were then transmitted to a chart recorder using an electrical rotary union. The technology available at the time of these measurements limited the number of sensors in the screw and the quality of the data. 

The response of the RTDs and the temperature of the screw were tested with the screw not rotating. For this experiment, the temperatures were first measured with the extruder at ambient conditions. Next, the barrel temperature set points were increased to 200, 220, and 240 °C for Zones 1, 2, and 3, respectively. The downstream die system was heated at the same time as the barrel and at a set point temperature equal to Zone 3 (240 °C). The temperature profile of the screw as a function of axial length is shown in Fig. 10.19 for select heating times. For heating... 

After the screw modification, the 148 kg/h rate was obtained at a screw speed of about 69 rpm with an extrudate temperature of 223 °C. Thus, the specific rate increased from 1.63 kg/(h rpm) before the modification to 2.14 kg/(h-rpm) after the modification, a specific rate increase of about 30%. At a screw speed of 69 rpm, the rotationai flow rate was calculated at 173 kg/h now the extruder was operating at about 86% of the rotational flow rate. The calculated axial pressure gradient required to maintain the flow of the extruder at the reported flow rate showed that pressures in the screw never decreased to zero, indicating that the channels were full as shown in Fig. 11.21. No adverse effects were experienced with the reduced discharge temperature (8 °C lower), no unmelted material was observed in the extrudate, and no gel showers occurred after the modification. A summary of the extrusion performance before and after the modification is shown in Table 11.5. 

Figure 13.9 Simulated axial pressure and temperature profiles for the original pumping screw at a maximum rate of 1590 kg/h and a screw speed of 15 rpm...

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