extrusiontheoryandproceduresfall2014

8/10/2019 ExtrusionTheoryAndProceduresFall2014

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Experimental Validation of Models forPlasticating Extrusion Process

1. Background and Theory

An extruder is a plastics manufacturing unit operation that is used to producethermoplastic polymers with a uniform cross section, such as pipe, hose, sheet, film andprofiles. Since extruders also produce the polymer pellets that are used by otherpolymer processing operations (such as film blowing, injection molding and blowmolding), almost all plastic material produced worldwide has passed through an extruderat least once. A single screw extruder, the most widely used type, is employed in thislab. The extruder consists of an auger-like rotating screw (with one or more 'flights'spiralling along its length) that closely fits within a heated barrel. Extruders can be fedwith solid plastic pellets, chips, beads or powder. The functions of an extruder are toconvey the solid polymer from the hopper, compact and melt the pellets, mix the

resulting highly viscous polymer melt, pressurize and pump it through a die thatproduces the shape of the plastic product. The overall goal of extrusion is to produce awell-mixed polymer melt at the proper conditions of flow rate, pressure and temperaturefor the next polymer processing operation, which usually forms the polymer melt into asolid product. Extruders range in size from the laboratory tabletop unit used in this labto much larger, high-volume, industrial extruders that can be more than 15 m long.Muccio (1994) provides more detailed descriptions of the extrusion process, the parts ofan extruder and types of extruded products. Figure 1 is a diagram showing the threeaxial sections of an extruder: the feed (or solids conveying) section, the compressionsection and the metering section.

Figure 1: Extruder Sections

The extruder barrel is usually divided into several heating zones, with a temperaturesensor and a controller for each zone. Each controller can supply heat to its zone usingan electric heater, or cool the zone using a fan, if needed. The heat to melt the polymeris partially supplied by electric heaters on the barrel, but the majority (60-90%) comesfrom friction generated by the shearing action of the rotating screw and the stationary


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barrel.

The feed section conveys the polymer pellets from the hopper to the screw channel,preheats and compacts them, and transports them down the channel. In thecompression section the pellets are molten and compacted. In the metering section thepolymer melt is homogenized, pressure is generated and the melt is pumped out of the

extruder and through the die, ideally at a uniform temperature, pressure and flow rate.Note that the channel depth between the screw and the barrel, H, varies along the lengthof the extruder, becoming smaller from the feed throat to the die. In the compressionsection, the channel depth between the screw and the barrel decreases gradually tofurther compact the pellets, expel air trapped between them, improve heat transfer, meltthe polymer and accommodate the density change of the material upon melting. In themetering section, the channel depth is again constant, but smaller than in the feedsection, so that the melt can be pressurized before being pumped out through the die.

Figure 2 shows the pressure profile along the length of the extruder and the die. At thebase of the screw, near the hopper, the pressure is atmospheric. The pressure

increases along the length of the screw and reaches a maximum at the end of the screwthat meets the die. The pressure drops across the die section, and once again returns toatmospheric pressure at the point where the extrudate exits the die. For effectiveextrusion, three points are important. First, the polymer should be completely moltenbefore it reaches the die. Second, the temperature within the die should be low enoughto prevent degradation of the polymer. Third, the pressure generated within the extrudermust be sufficiently large to be capable of pushing the molten polymer through the die.

Figure 2Pressure Profile along Extruder/Die


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Figure 3 is a diagram showing the geometry of an extruder screw, and Table 1 containsvalues for the geometry of the screw and die used in this lab.

Figure 3: Screw Geometry

Table 1:Values for some geometric parameters of extruder screw and die


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2. Modeling of the plasticating extrusion process

The processes of solid polymer transport, compaction and melting that occur in the feedand transition sections of the extruder are quite complicated and are beyond the scope

of this lab. However, the flow of the polymer melt in the metering section of the extruderand in the die can be modeled with the aid of rheological theory and fluid mechanics.The flow rate in the metering section, as a function of the pressure increase, can bedescribed by Equation (1). The pressure inside the extruder increases from the hopperto a maximum value at the start of the die, which tends to force the material backwardsinside the screw channel. The drag flow attributed to the rotating screw, however, movesthe melt along the screw channel from the hopper towards the die. Overall, the extruderis operated such that the drag flow is greater than the pressure driven flow, and the meltflows in the proper direction. Equation (1) accounts for these opposing forces, as thetotal volumetric flow rate of polymer Q is expressed as the sum of the (positive) dragflow and the (negative) pressure flow. Physically, the opposing forces are beneficial asthey improve mixing within the extruder.

The explanations of all symbols used in the equations are provided in Table 2.

Table 2: Operating variables and coefficients for extruder characteristic equation

and are geometric factors that are constant for a given screw/barrel combination.They can be calculated from the geometry of the system (see also Figure 3) using thefollowing expressions:

(1)


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Details of the assumptions and derivation of equations (1) to (3) are given in Morton-Jones (1989), Osswald (1998) and Baird and Collias (1995). Polymer melts fall into acategory of fluids that exhibit shear thinning behaviour. This means that their apparentviscosity decreases with increasing shear rate. To estimate the viscosity of the polymermelt, we can use an empirical equation called the power law model:

1= nm

where m is the consistency index (Pa.sn-1

) and n the power-law index.

The shear rate in the metering section of the extruder (in units of 1/s) is given by:

H

DN =

Flow of the polymer melt through the tubular die used in this lab can be modeled usingan equation that incorporates the power law model for the polymer melt viscosity(Osswald, 1998):

Notice that equation (1) and equation (6) are expressions for the volumetric flow rate (Q)of the polymer melt through the extruder and the die (respectively) as functions of thechange in pressure. If we assume that the pressure at the start of the metering sectionis atmospheric, and the pressure at the die outlet is also atmospheric, then the pressurerise in the metering section of the extruder will equal the pressure drop in the die (so Pwill be the same for the extruder and for the die). Now, for a given extruder-diecombination at a fixed screw speed N we could predict the operating point (P op, Qop) bysolving equations (1) and (6) in the two unknowns. However, an easier way to solve

these equations is to use a graphical method. Equation (1) can be used to generate aplot of Q vs. P for the extruder, called the screw characteristic curve, and equation(6) can be used to generate another plot of Q vs. P for the die, called the diecharacteristic curve. The intersection point (Pop, Qop) of the two curves is thepredicted operating poin tof the extruder/die combination.

(2)

(3)

(4)

(5)

(6)


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Characteristic screw-die curves, such as the one shown in Figure 4 can be generated

experimentally by measuring the die pressure (P) and flow rate (m3/s or kg/h) as afunction of screw rotation speed (N) and type of die. Alternatively the theoreticalcharacteristic screw-die curves can be constructed using equations (1)-(6) above.

Figure 4: Characteristic screw and die curves for a conventional 45 mm SSEprocessing LDPE. Lines correspond to the extruder characteristic curve at differentrotation speeds, and curves correspond to dies having an ascending order of dierestriction, K.

3. Melt Flow Instabil ities

The shear stress experienced by the polymer at the walls of a cylindrical die is related tothe pressure drop by:

die

die

W RL2

P= (7)

It can also be expressed in terms of the flow inside the die as:

n

3

die

w

R

Q4

n4

1n3m

+= (8)


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The phenomenon of melt flow instabilities, or sharkskin, which appears when the shearstress at the wall exceeds the value of 0.14 MPa, is a significant problem, preventinghigh production rates in extrusion processing and imposes the upper limit of shear stressfor extrusion.

4. Deliverables

Your plant has recently received a million-dollar order for an underground piping systemfor water transportation, made of polyethylene. Your group has been assigned the taskof conducting some preliminary experiments on a small scale equipment, so that theoperation can be scaled-up accordingly. Currently there are two types of polyethyleneavailable in the Process Development (PD) lab of your company that you can use foryour tests. Initially it has been suggested that a low density polyethylene grade LDPEPetrotheneNA960000 by LyondellBasell is suitable for this task. The flow curve of thismaterial measured in the PD lab is provided in section 5.

1. The small laboratory scale extruder is to be used for pilot runs for preliminarydeterminations of operating conditions. Generate screw-die characteristic curves similarto the ones shown in Figure 4, using equations (1)-(6). Design a set of experiments toobtain experimental data points, which you can compare with the theoretical predictions.How do the experimental operating points compare to the theoretical ones?2. Previous investigations done in the PD lab have found that the model outlined aboveis inadequate, since it overpredicts the pressure values. Malfunction of the pressuretransducer has been ruled out. Leakage flow over the flights has been cited as a

possible reason for the discrepancy. Leakage flow occurs when the clearancebetweenthe screw flight and the barrel is not negligible. The amount of leakage flow rate, Q L,must be subtracted from the total flow rate shown in equation (1) and can be estimated

by:

L

P

e12

DQ

322

L

= tan (9)

Refer to Figure 3 for explanations of the symbols.If needed further, suggest any other improvements that are needed in the model.

3. One of your managers insists that a linear low density polyethylene, is more suitablefor this task. He proposes a linear low density polyethylene grade from the samesupplier, LLDPE (PetrotheneGA601030), with similar melt flow index as the LDPE. Theflow curve of this LLDPE is shown in Section 5. Figure out the maximum throughput thatyou can achieve with this resin, without the presence of sharkskin or other instabilities. Inyour report explain what would be the advantages of using LLDPE instead of LDPE andwhy there is a difference (if any) on the processing pressures and melt flow instabilities.

4. Using the information on melt flow instabilities provided in Section 3, together withexperimental data gathered for the LLDPE material, investigate further the discrepancybetween experimental pressure measurements and theoretical predictions.


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5. Estimation of power-law parameters from viscosity data

The measured viscosity curves for the LDPE grade, Petrothene NA960000 and theLLDPE grade PetrotheneGA601030, both from LyondellBasell are shown in Figure 5.The corresponding Excel worksheet is provided separately. This data can be used to fitthe power-law model, equation (4). Please note that the power-law model can only be

used successfully to describe a straight line dependence of the viscosity vs. shear ratedata (on a log-log scale). This can only be done using the data at higher shear rates,typically above 1 s-1. The data obtained at lower shear rates typically do not obey thepower-law, therefore should be discarded.

100

1000

10000

100000

0.1 1 10 100 1000

Viscosity

(Pa

s)

Shear rate (/s)

PetrotheneLDPE

PetrotheneLLDPE

Figure 5:Viscosity vs. shear rate at 190C for the two polyethylenes


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6. Experimental Equipment and Procedures

The single screw extruder to be used in this lab is located in the pilot plant in thebasement of Dupuis Hall. As shown in the photograph in Figure 6, the equipmentconsists of a hopper for the polymer pellets, the extruder itself, a die and a control panelto measure and control the barrel and die temperatures and the screw speed. Thepolyethylenes available are an LDPE, Novapol LF-Y819-A and an LLDPE, Novapol PP-0118-F. Their material data sheets are provided separately.

Figure 6:Single-screw extruder by Wayne Machines located in DupuisHall

7. Safety

Safety glasses and hardhats must be worn at all times in the pilot plant. The extrudate isat extremely high temperatures immediately after leaving the die, so thermally insulatedsafety gloves MUST be worn when sampling the polymer and handling it. The extruderand die are also at a high temperature, so caution should be taken to avoid skin contactwith it. If the exiting stream becomes blocked, the exit pressure can quickly increase tounsafe levels. Accordingly, operators must not stand in front of the die during extruderoperation. MSDS sheets should be read in order become familiarized with all dangers ofthe materials. Degradation temperatures should be avoided as polyethylene maydecompose, or crosslink, thus plugging the extruder.


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8. Standard Operating Procedures

Extruder Start-Up Procedure

1. Turn the cooling water valve on the wall so that it is full open (i.e. so that the handle is

parallel to the pipe).2. Turn on the power switch on the wall.3. Turn the main power switch on the back of the extruder on.4. Turn the temperature controllers on for the die and barrel zones (5 controllers in total).Eachtemperature indicator should light up with two red numbers at this point.5. Adjust the temperature controllers to the desired temperature set points that you wishto heat the extruder to.6. Let the extruder heat up until the actual set points are reached (about one hour).(Note: Even after one hour, barrel zone three may not reach the desired set point. If theother four zones are at the desired temperature, commence the experiment anyway). Donot turn the screw on until the extruder has reached the desired temperature as polymerleft over from previous runs may have solidified in the barrel.7. Place a bucket full of cold water on top of a scale directly below and in line with theextruder die. This will be used to catch and weigh the polymer.8. When activating and operating the screw, never place yourself in front of the die aspolymer may become stuck and discharge with such a great force as to injure someonein front of it.9. Once the temperature has reached the desired set point, turn on the screw speedcontroller. Adjust the controller to 5 rpm and press the green button to start the screw.Once the screw has been started, adjust it to the desired speed set point.10. Fill the hopper with polymer pellets.11. Let the screw run for two to three minutes, or until steady state is reached i.e.polymer is flowing continuously from the die and the pressure indicator shows that asteady state pressure has been reached.11. Adjust the bucket so as to catch the molten polymer.

Suggested Flow Measurement Procedure:1. Using scissors, cut the stream of polymer at the die, and place the cut end in theweighing bucket on the scale.2. Hold the waste bucket underneath the stream of polymer while the scale is tarred.3. Once the scale has been tarred, cut the polymer at the die again. At this point,simultaneously start the stopwatch, and place the end of the previously extrudedpolymer strand in the waste bucket. Move the waste bucket out of the way so that thenew polymer strand falls into the weighing bucket on the scale.4. At the end of a known time period, cut the polymer strand at the die, halt thestopwatch, and place the end of the extruded polymer strand in the weighing bucket to

measure its mass.5. While the polymer strand is being weighed, catch the new polymer strand in the wastebucket.6. Record the temperature of each die zone, the time for the run, and the mass readingon the scale.7. Cut a cold piece of polymer from the waste bucket. Label it and save it for futureobservation.8. Repeat steps 2 through 7 for following runs.


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9. Add polymer to the hopper and empty the buckets as necessary.

Extruder Shut-Down Procedure:1. Scoop any excess polymer pellets from the hopper using the pellet dispensing bucket.Do not place your hands into the hopper.2. Adjust the screw speed to a high RPM to flush the remaining polymer out of the

hopper and barrel.3. Once the polymer has been flushed of the barrel (i.e. - the extrudate flow rate slowsdownconsiderably and the pressure indicator significantly decreases) adjust the screw speedcontroller to the lowest RPM possible.4. Press the red button on the control panel and turn off the screw speed controller.5. Adjust the temperature controllers to room temperature. Leave the controllers on untilthe extruder has cooled to room temperature.6. Save your polymer samples and throw out any extra polymer.7. After the extruder has returned to room temperature, shut off the cooling water.8. Turn off the main power switch on the back of the extruder.9. Turn off the power switch on the wall.

References

1. Baird, D.G. and Collias, D.I. Polymer Processing Principles and Design,Butterworth-Heinemann, Boston, 1995.2. Middleman, S. Fundamentals of Polymer Processing, McGraw-Hill, New York,1977.3. Morton-Jones, D.H. Polymer Processing, Chapman and Hall, London, 1989.4. Muccio, E.A. Plastics Processing Technology, ASM International, 1994.5. Osswald, T.A.

Polymer Processing Fundamentals, Hanser Publishers, Munich,1998.6. Tadmor, Z. and Gogos, C.G. Principles of Polymer Processing, Wiley, New York,1979.

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