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The variation in screw speed and motor current are quite severe. The screw speed varies between 80 and 160 rpm with a variation in motor current from 400 to 800 amps. Calcium deposits in the cooling channels of the feed housing will reduce heat transfer and lead to higher temperatures. It is important to measure the inlet and outlet temperatures and the flow rate of the water. With these three data points, the amount of cooling can be quantified. If the amount of cooling declines over time, this is likely caused by calcium deposits. The problem of scale buildup in cooling channels is quite common. Calcium scale is a hard, thick coating of calcium carbonate that forms on the internal walls of cooling channels. Water that contains minerals that cause scale is called "hard" water. It is important to make sure that the water is properly treated with a water softener to avoid scale buildup. There are other ways to prevent scale, such as use of an electronic water descaler. For water cooling of the extruder feed housing it is good practice to use a closed-loop circuit. This reduces the chance of getting hard water in the cooling system. This applies not only to the water-cooled feed housing but also temperature zones along the extruder barrel that employ water cooling. VARIATION IN MELT PRESSURE In the first part of this article we discussed how the screw flight creates a sawtooth pattern in the melt pressure measured at the discharge end of the extruder. A similar situation occurs with a gear pump (Fig. 4). Gear pumps are used to closely control the flow from an extruder in order to achieve less variation in flow rate and extrudate dimensions. At the discharge end of the gear pump, the polymer melt is expelled by the intermeshing action of the gears. This creates pressure pulses as shown in Fig. 5. This figure shows short-term fluctuations at about 4-5 pulses/sec caused by the meshing of the gear teeth. The pressure pulses from the gear teeth are small (about 4-6 psi) but clearly distinguishable. The figure also shows longer-term fluctuations with a cycle time of about 10 sec and with amplitude of about 10-12 psi. The source of the longer-term fluctuation was not known; however, this fluctuation certainly should be investigated and eliminated if possible. The pressure pulsing of the teeth of a gear pump can be reduced by using helical gears rather than straight gears. CYCLIC VARIATION IN DISCHARGE PRESSURE The last example deals with a new extruder whose discharge pressure showed a regular sinusoidal variation, as shown in Fig. 6. The barrel temperatures also showed a regular sinusoidal variation (Fig. 7). The barrel-temperature trend plot shows that the variation in barrel temperature was much too large—about 10-15° C (18-27° F). This suggested poor tuning of the temperature controllers for the Barrel temperatures can be optimized to achieve the lowest pressure fluctuation. This can be done by changing the setpoint and following the transient response of the extruder. Mounting Flange Seal Drive Shaft Suction Port Drive Gear Idler Gear Case Seal Bushings Pressure Port FIG 4 Gear pumps are used to closely control the flow from an extruder to achieve less variation in flow rate and extrudate dimensions. At the discharge end of the gear pump, the polymer melt is expelled by the intermeshing action of the gears. FIG 5 The intermeshing action of a gear pump creates both short- and long-term pressure fluctuations. The latter must be investigated and solved. The pressure pulsing of the teeth of a gear pump can be reduced by using helical gears rather than straight gears. 3655 3645 3635 Time, sec → 55 56 57 58 59 60 Gear Pump Discharge Pressure, psi → 42 JULY 2017 Plastics Technology PTonline.com T ips & Technique s