Closed Loop Pressure Control for the Extrusion Process

downstream equipment, no downstreamchanges will be required.Test MethodTest results are presented in Table 2 andplotted on graphs shown in Figures 3 through6, which demonstrate the effectiveness ofclosed loop feedback pressure control inminimizing output variations.Tests were made on a 2½” extruderprocessing 100% LDPE regrind at screwspeeds of 35, 50, 70 and 100 rpm. Use of100% LDPE regrind simulates the worstsurging conditions. The tests were madeutilizing a melt pressure transducer mountedin the die and a microprocessor-basedpressure controller. The PID output of thecontroller was fed to the extruder drive. TheTABLE 2. Test Resultsoutput of the controller was used tocontinually adjust the screw speed in order tomaintain a constant die pressure.• Graphs with broken linesindicate product linear weightover time with no pressurecontrol (fixed screw speed).• Graphs with solid lines indicateproduct linear weight over timewith control of die pressure(continually adjusted screwspeed).Extruder Speed withand without PressureControl35 RPM Nominalw/o pressure controlw/ pressure control% reduction50 RPM Nominalw/o pressure controlw/ pressure control% reduction75 RPM Nominalw/o pressure controlw/ pressure control% reduction100 RPM Nominalw/o pressure controlw/ pressure control% reductionPolymer Pressureat DieAvg Variation %Variation(B)16851870162516951775203026502850859027515544519016070114.856%179.146.4%25.19.462.5%6.02.558%Product Linear WeightAvg Variation %Variation(A)1.0751.141.92.091.9052.5752.533.220.3250.08750.50.290.8050.3750.320.132307.7774%26.313.947%40.614.664%12.64.266.7%Change in Productweight (A)Change in Pressure(B)2.701.621.551.531.621.552.101.68ResultsLinear weight output variation was 153%to 270% (1.53 to 2.70 times) of the pressurevariation. Based on the pressure variations,which occurred during the tests, outputvariations were not as large as theoreticallypredicted in Table 1. However, in all caseslinear weight output variation was greaterthan the pressure variation.

fluctuation, reducing the average inlet meltpressure, and increasing the safety ofoperation (2).The reduction in product linear weightoutput variation ranged from 47% to 74%(see Column A, Table 2). This suggests thatas a minimum, variations in output can becut in half using closed loop pressurecontrol.Closed Loop Pressure Control for MeltPump ExtrusionMelt pump assisted extrusion is gainingpopularity as a method of stabilizing theextrusion process. The literature states that agear pump helps to eliminate unexpectedsurges and drifting output rates from theextruder (2) (6-9).Gear pumps are positive displacementdevices that deliver polymer melt to theextruder die at a relatively constant rate. INgear pump extrusion, the primary function ofthe extruder is to act as a plasticating unit todeliver a homogenous melt to the gear pumpinlet.The gear pump which is mountedbetween the extruder and the die, buffers thedie from extruder surges and drifts in output,but the inlet of the gear pump itself is subjectto these extruder output variations. Since thepump requires a constant flow of plasticwithin certain pressure levels for lubrication,a prolonged low-pressure condition couldcause damage to the pump. Overpressurization at the pump inlet, caused by asudden surge of melt from the extruder, willchange the melt condition and in extremecases, can be dangerous to the equipmentand operatorFor these and other reasons, a closed looppump inlet pressure control system thatchanges the extruder screw speed to maintaina constant gear pump inlet pressure offers anumber of processing advantages.A study conducted by Dynisco and theUniversity of Lowell concluded that controlsystem of this type simplifies and improvesprocess operations by completely eliminatinglong term inlet pressure drift, significantlyreducing short term inlet pressureThe following is a brief summary of theresults:Short Term Inlet Pressure StabilityLarge fluctuations in the inlet pressuredictate that a higher average inlet pressureshould be used to avoid dropping below theminimum inlet pressure. Even a temporaryloss of inlet pressure will show on thedischarge side of the pump. The higheraverage pump inlet pressure increases theenergy input to the material, increasing theaverage melt temperature. Figure 7 showspump inlet and discharge pressures for bothmanual and closed loop operation at a pumpinlet pressure setpoint of 350 psi. Figure 8shows that short term inlet pressurevariations were reduced 70-80%.Long Term Pressure DriftWith manual control, it was necessary toperiodically adjust screw speed, producingthe result shown in Figure 9. With closedloop pressure control (Figure 10) some shortterminlet pressure instability was observedbut long-term pressure drift was completelyeliminated.Process Output ChangeA smooth, well-coordinated change inoutput rate was difficult to achieve withmanual control. An increase in pump speedrequired simultaneous and coordinatedincreases in extruder speed, often resulting inexcessive pressurization at the pump inlet.Closed lop inlet pressure control simplifiedthe operation considerably. An increase ordecrease in the pump speed was

automatically followed by the extruder(Figure 11).SafetyClosed loop inlet pressure controlincreases operator and equipment safety. If,during manual operation, the pump ceasesrotation and the extruder continues to pumppolymer, pressure at the pump inlet reachesdangerous levels in a matter of seconds.Mechanical rupture plugs, electricalinterlocks or high inlet pressure limit relays(a feature of some controllers) should beused.SUMMARYAlthough modern extruder drives andtake-off devices provide constant, drift-freeoperation, variations in the raw material andprocess occur which causes the output of theextruder to change. The output instability ofthe extruder is related to the melt pressure atthe die. The output variations of theextrudate will be a multiple of the pressurevariation at the die. A closed loop feedbacksystem, which adjusts screw speed tomaintain a constant die pressure, is the mosteffective method to minimize variations inextruder output.Closed loop inlet pressure control for gearpump extrusion reduces inlet pressureinstability, as well as the average minimuminlet pressure. Reducing the inlet pressurevariations assures a more uniform melttemperature at the die, thus improving theconsistency and quality of the extrudate.

Figure 1. Output and Pressure Relationship of the Extruder DieFigure 2. Effect of Changes in Extruder Output on Die Pressure

Figure 3.Figure 4.

Figure 5.Figure 6.

Figure 7. Short Term Inlet Pressure Stability @ Pump Speed: 35 RPM (Approximately 105 RPMExtruder Screw Speed): Manual and Closed Loop Operation.Figure 8. Short Term Pump Inlet Pressure Stability vs. Pump Speed: Manual and Closed LoopOperation.

Figure 9. Long Term Pump Inlet Pressure Drift: Manual Adjustment in Extruder Screw Speed.Figure 10. Long Term Inlet Pressure Drift with Closed Loop Inlet Pressure Control.

Figure 11. Closed Loop Pump Inlet Pressure Response to a Pump Speed Change.REFERENCES1. Dynisco, Inc. “An Economical Method of Stabilizing the Extrusion Process”.2. Malloy, Robert A., SPEC ANTEC, p. 604, (1984).3. Steven, M. J., “Extruder Principle and Operation”, Elsevier Applied Science Publishers,pp. 101, 211, 237-248, (1985).4. I. Patterson, T. DeKerf, SPE ANTEC, p. 483, (1978).5. Frados, J. “Plastics Engineering Handbook, SPI, Fourth Edition”, pp. 156-174, (1976).6. Fox, Steve A., Extrusion Technology for Tubing and Profiles Seminar, Killion Extruders,Inc. (1988).7. Kruder, George, PM&E, p. 24, Vol. 18, No. 5, (May 1989).8. Kramer, William A., SPE ANTEC, p. 23 (1985).9. Rice, William T., “The Case for Gear Pumps: What’s Behind the New Interest”, PlasticsTechnology, pp. 87-91, February 1980.10. Maddock, B. H., “Measurements and Analysis of Extruder Stability”, Union CarbideTechnical Information.11. Plastics World “Extruders Gear Up for Tighter Gauge Limits”, pp. 42-45, July 1985.