Screw temperature zone

Stream process operation. This type of machine is referred to as a plasticating singlescrew extruder. The barrel is usually heated with a minimum of three temperature zones. These different temperature zones are consistent with the three utilitarian functions of the screw solids conveying, melting, and pumping or metering of the polymer. 

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. 

Figure 10.20 Zone 3 screw temperature response to changes in screw speed. The barrel zones were 200, 220, and 240 °C for Zones 1,2, and 3, respectively...

Figure 10.22 Axial screw temperature profile at a screw speed of 60 rpm and as a function of barrel temperature. The barrel temperatures for the three zones are indicated in the figure. The data point labels for the sensors were omitted for clarity...

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]. 

Crahtree, S. L., Spalding, M. A., and PavUcek, C.L., Single-Screw Extruder Zone Temperature Selection for Optimized Performance, SPE ANTEC Tech. Papers, 54, 1410 (2008)... 

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F) and the screw temperature is approximately the same as the temperature of the feed zone.37 Processing aids are almost always required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers.37... 

PP and PB were dry-blended, and the mixture was fed into a 5.1-cm (2 inch) Rheotec extruder. For some of the samples a Berlyn extruder was used. The extruder was operated at a flow rate of 9 kg/h with a screw speed of 20 rpm and staged temperature zones to ensure a melt temperature of 244°C. The blends were extmded into 4 mil (0.102 mm) films on a chrome-plated casting roll in which the quench temperature was controlled at 12.2°C, so that the structure of PP is mesomorphous. The materials used for the various samples, and the processing conditions used, are given in Table 11.44. It is important to cool the extruded form of the blends rapidly so that mesomorphous PP is obtained. 

The 220 barrel temperature refers to the temperature of the last barrel temperature zone the actual profile is 150-185-205-220. For the 200 barrel temperature the actual profile is 140-170-185-200 and for the 180 barrel temperature the actual profile is 130-155-165-180. For the simple conveying screws, the barrel temperature profile (BTP) has little efi ect on the pressure variation, and the pressure variation is quite small. For the barrier screw, the BTP has a significant effect on the pressure variation and the pressure variation is much higher-as much as an order of magnitude. 

There are a number of possible solutions to air entrapment. The first approach should be to change the temperature in the solids conveying zone to achieve a more positive compacting of the solid bed. Often, a temperature increase of the first barrel section reduces the air entrapment however, in some cases, a lower temperature causes an improvement. In any case, the temperatures in the solids conveying zone are important parameters in the air entrapment process. It should be realized that both the barrel and screw temperatures are important. Thus, if a screw temperature adjustment capability is available, it should definitely be used to reduce the air entrapment problem. 

The barrel has to be cooled if the internal heat generation in the plastic raises the barrel temperatures above the set point. This is likely to occur when extruding high viscosity plastics and when running at high screw speeds. Cooling on single screw extruders is usually done with air. Blowers are placed imder the extruder barrel and temperature zones are partitioned, so that one blower cools only one temperature zone (see Fig. 11). 

Figure 7.7 Influence of screw temperature profiles on mixing with Maddock element at end of compression zone. Photomicrographs of screw tip channel section. (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 8. 1978, Rapra Technology)...

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