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The trend plot shows a number of temperatures with a regular sinusoidal variation with the same frequency. The cycle time of the variation is about 6 min. The amplitude of the temperature variation varies considerably from 2° C up to 15° C (3.6 to 27° F). The IR measurement of tubing temperature after the die shows the largest variation (15° C/27° F). The melt-temper- ature variation measured with the immersion probe shows significantly less variation (2-3° C/4-5° F). The variation in tubing temperature is unacceptably large and indicates a problem. The die temperature at the discharge end varies about 4° C (7° F), and it would be tempting to conclude that the die- temperature variation is causing the tubing-temperature variation. However, this is not possible because the tubing- temperature variation is about three to four times greater than the die-temperature variation. Clearly, the die-temperature variation is caused by the melt-temperature variation and not the other way around. A similar argument can be used for the variation in barrel temperatures. There is little variation in motor load and no variation in screw speed. As a result, the tubing-temperature variation cannot be caused by changes in viscous dissipation. This leaves one other possible cause of the problem: variation in the feed material entering the extruder. Unfortunately, this extruder was not equipped with a pellet temperature sensor in the feed port. However, there was strong indirect evidence of pellet temperature variation. The polymer was dried before it was conveyed to the extruder. It turned out that the feed hopper was filled with hot, dry material every 6 min. The feed hopper itself was not heated. Therefore, the hot pellets from the drier would cool down in the feed hopper as they made their way to the feed opening of the extruder. The hopper was filled at 6-min intervals. This created a 6-min cycle in the temperature of pellets entering the extruder. Once the cause of the problem is known, the solutions are obvious. One possible solution is to install a hopper drier on the extruder so that the pellets stay dry and at constant tempera- ture in the hopper. It should be noted that the problem would have been noticed immediately if the extruder had been equipped with a pellet temperature sensor at the feed opening of the extruder. Such a temperature sensor provides important informa- tion. Unfortunately, extruder manufacturers generally do not provide a temperature sensor in this part of the machine. VARIATION IN FEED-HOUSING TEMPERATURE The extruder performance is dependent on the temperature of the feed housing. Thus, it is important to measure that temperature, but surprisingly, many extruders do not have a temperature sensor in the feed housing. That housing is generally water cooled because it is located up against the hot extruder barrel. Water cooling allows the feed housing to be maintained at temperatures much lower than the barrel temperatures. In most extrusion oper- ations, the barrel temperatures are quite high (over 150 C/302 F). If the feed housing cannot be maintained at a low enough temperature, extruder performance can deteriorate quickly. This can lead to unstable extrusion conditions. A high level of instability is called surging. Surging caused by overheating of the feed housing was discussed by J. Powers et al. at the Society of Plastics Engineers 2000 Annual Technical Conference (ANTEC). The overheating caused large variation in motor current, screw speed, and pressure in a large vented extruder. The pressure was controlled with a feedback control to the screw speed, which was varied to maintain constant pressure. However, this pressure-feedback control only works when the pressure changes very slowly and gradually. Pressure feedback does not work for rapid changes in pressure, which result in rapid screw-speed variation. The variation in motor current and screw speed is shown in Fig. 3. QUESTIONS ABOUT EXTRUSION? Visit the Extrusion Zone. If the feed housing cannot be maintained at a low enough temperature, the extruder performance can deteriorate quickly. Pressure is generally controlled with a feedback control to the screw speed. But pressure feedback does not work for rapid changes in pressure, which will result in rapid variations in screw speed and motor current, as shown here. The issue here turned out to be scale buildup in cooling channels. Time, min → 0 20 40 60 Screw Speed, rpm → 240 200 160 120 80 Screw Speed Current, amps → 800 400 0 Current FIG 3 @plastechmag 41 Plastics Technology E X T R U S I O N