Extruder temperature

Formation of carbonyl groups which may occur when polyethylene is overheated, such as in an extrusion coating operation. Here a proper balance between extrudate temperature and exposure time to air is required. 

A complete extrusion hne for the tire industry also consists of various downstream machinery, including the cooling hne which reduces the extrudate temperature from about 100°C-115°C at the head exit to ambient plus 5% at the windup stations or at the tread-cutting station. 

MERCURE TRAINOR Extruder Temperature Control Experimental... 

The extruder temperature profile for a single-screw extruder is set such that the functions of the process convert the polymer from a solid to a fluid. These two words are in quotation marks because for noncrystalline glassy (or amorphous)... 

The temperature of molten polymer process streams is commonly measured using a thermocouple positioned through a transfer line wall and partially immersed in the polymer stream. Process stream temperature measurements that use an exposed-tip thermocouple, however, can be misleading since the temperature of the thermocouple junction is a balance between the heat transferred from the polymer stream and from the thermocouple assembly [39]. Due to the low heat transfer rate between the polymer and the exposed tip and the high thermal conductivity of the thermocouple sheath, the temperatures measured can be different by up to 35°C depending on conditions. Extrudate temperatures, however, can be accurately measured using a preheated, handheld thermocouple probe. This method minimizes thermal conduction through the probe sheath. 

A profile extrusion line was required to increase its rate from the current 75 kg/h to about 230 kg/h in order to meet business demand. The extruder was 88.9 mm in diameter and was running a HIPS resin. Operation of the existing extrusion equipment, however, caused the extrudate temperature to be too high at rates higher than about 80 kg/h. The objective of this project was to increase the rate of the profile line from the current 75 kg/h to a maximum rate of 230 kg/h while maintaining the extrudate temperature below 195 °C. 

A handheld thermocouple measurement device and an IR temperature sensor are very important to have for understanding the performance of the extruder. These devices have been discussed previously in this chapter. In many of the case studies presented later, the extrudate temperature was measured by immersing a handheld thermocouple probe into the extrudate. The entire probe length is immersed or draped in the extrudate to minimize heat conduction to cooler parts of the probe. The highest temperature measured is the reported value. If it is unsafe to measure the extrudate temperature using a handheld device, then an IR temperature sensor is used to estimate the discharge temperature. 

At the startup of the line, the extruder was operated at 91 rpm to produce the required rate of 148 kg/h for a specific rate of 1.63 kg/(h-rpm). The temperature of the extrudate was measured through the transfer line wall at 232 °C. Due to process safety constraints the extrudate temperature could not be measured using a handheld temperature sensor. The extrusion rate was required in order to maintain the downstream take-away equipment at its maximum rate. At first the extruder appeared to be operating well except that the specific rate was lower than predicted. That is, the screw was rotated at an rpm that was higher than expected to produce the 148 kg/h. At 91 rpm, the rotational flow rate was calculated at 228 kg/h the specific rotational flow rate was calculated at 2.51 kg/(h-rpm). Thus, the line was operating at only 65% of the rotational flow rate. A barrier design... 

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 12.14 Screw speed, extrudate temperature, and barrel zone temperatures for a large-diameter extruder running stably and unstably...

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