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The twin-screw extruder has the same threebasic components as the single-screw extruderthe drive section, the process section and the die/ discharge section as shown in Figure 1. Thebiggest difference between the two is that thetwin-screw, as its name indicates, has two screwshafts inside the barrel rather than one. In addition,the twin-screw barrel and screw is usually builtwith modular components (Figure 2). Theprinciple exception is the counter-rotatingextruders used for prole extrusion.

Within the family of twin-screw extruders thereare several variations based on the way the screws

rotate relative to one another, the design of theights and how they interact with one another, andthe shape of the screw shafts. These variations are:

Co-rotating and counter-rotating designs

Fully intermeshing and non-intermeshingdesigns

Parallel shaft and conical shaft designs.

Figure 3 shows the typical twin-screwextruder designs based on these options.

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The terms co-rotating and counter-rotatingdescribe the screw rotation in the extruder. In theco-rotating geometry, both screws rotate in the

Figure 1. Basic components of twin-screw extruders.

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same direction, either clockwise orcounterclockwise. This is a machinerymanufacturer choice and does not inuence themachine performance. In the counter-rotatingdesign, one screw rotates clockwise while the other

rotates counter-clockwise. Again, from a designpoint of view, it makes no difference which screwrotates in what direction. However, from aperformance perspective there are consequences.These will be highlighted and discussed later in thecounter-rotating extruder portion of this chapter.

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The terms intermeshing and non-intermeshingrefer to the relative closeness of the screws. Innon-intermeshing geometry machines, the two

screws sit side by side, just as if they were twosingle screws sitting in a common barrel. Figure 4shows the cross section of a typical counter-rotating non-intermeshing machine. Fullyintermeshing means that the screws sit so closetogether that, except for enough mechanicalclearance between them to permit both screws torotate without touching each other, the crest or topof one screw nests in the root or bottom of theother (Figure 5). When the term intermesh orintermesh zone is used in this chapter, it is

referring to the area where the two screws overlapeach other as described earlier in this paragraph.

In co-rotating extruders, the screws are actuallytraveling in the opposite direction in theintermeshing region, while in counter-rotatingdesigns the screw are going the same direction in the

Figure 2. Modular barrel and screw components.

Figure 3. Typical twin-screw extruder designs.

Figure 4. Non-intermeshing twin-screw.

Figure 5. Fully-intermeshing(co-rotating) extruder.


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intermesh zone (Figure 6). This opposite directionmotion permits a self-wiping action for the co-rotating, intermeshing design extruder as the screwsrotate. The tip on one screw wipes (removes)material from the root of the neighboring screw.This action minimizes the potential for material tostay in one spot on the screw, which allows it todegrade and eventually re-enter the melt as acontaminant, such as a black speck.

The counter-rotating intermeshing geometrydoes not produce a self-wiping action in the

intermesh zone that is as effective as that

experienced in the co-rotating geometry. Asmentioned in the previous paragraph, both screwsare going in the same direction as they cometogether in the intermesh zone in a counter-rotating screw design. Therefore, the relativevelocity between the tip of one screw and root of the other is not signicantly different, and thewiping action is not as effective. Finally, as in thesingle-screw extruder, no wiping action exists fornon-intermeshing counter-rotating geometry.

Another impact of the co/counter,

intermeshing/non-intermeshing design variables is

Figure 6. Wiping action of twin-screw extruders.

Figure 7. Extruder conveying characteristics.


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the mechanism that actually moves materialforward or conveys material down the extruder inthe various designs. Figure 7 shows conveyingcharacteristics for single-screw, and both co-rotating and counter-rotating, fully intermeshingextruders. The single-screw extruder geometry

relies on drag ow. Drag ow is the action of arotating screw ight as it drags material thatwants to stick or adhere to the barrel wall down thelength of the extruder. If the material wants to stick to the screw more than the barrel wall, it will justremain at the same location on the screw. In thissituation, there will be no material movementdown the extruder. Figure 8 shows this principlewith a nut and bolt. If both the nut and bolt areallowed to freely rotate, then the nut will remain atthe same location on the bolt. However it the nut is

xed to a surface, it will move along the bolt asmaterial would move down the extruder channel.

Getting back to Figure 7, it shows that thecounter-rotating, fully intermeshing geometry doesnot rely on drag ow. The screws form individualC-shaped compartments that get pushed down theextruder channel by the screw rotation. This actionis called positive displacement ow. The co-rotating, fully intermeshing extruder relies on bothpositive displacement and drag ow. A small

portion of positive displacement action occurs inthe intermesh area where the two screws cometogether and drag ow in the rest of the barrelcircumference.

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The terms parallel and conical refer to therelative screw diameter at the beginning and endof the extruder. In parallel systems, the screwdiameter remains constant along the machine. Inconical extruders, the diameter at the feed end isgreater than at the discharge end (Figure 9).

In this introductory section, I have illustratedsignicant differences among the differentgeometry twin-screw extruders. In the nextsections, the components and working principlesof the co- and counter-rotating extruder designs

will be described in more detail.

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Overview

Before talking about the details of the co-rotating, fully intermeshing twin-screw extruder, itmight be interesting and useful to have a littlegeneral information. As I pointed out in the

Figure 8. Single-screw conveying mechanism.

Figure 9. Conical twin-screw extruder screw prole.


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introduction, the co-rotating, twin-screw extruderhas the same three basic components as thesingle-screw extruderdrive section, processsection, and die/discharge section. Also the twin-screw barrel and screw are not typically single-piece units, but rather are made up of modularcomponents. Depending on the application, the

extruder can be long or short (Figure 10). Just aswith the single screw, the length of the twin screwis typically described by its L/D ratio. This is theratio of the barrel length in inches, millimeters orother dimension of distance divided by thediameter of the screw (using the same units of distance). A typical L/D ratio for a compounding,twin-screw extruder (glass- or mineral-lledpolymer) would be between 30 and 40 L/D.Shorter 16 L/D machines are used to manufacture

many powder coating products (solvent-free spraypaint used on appliance surfaces, etc.). Extrudersof 40 to 50 L/D or even longer are used forapplications such as very complex compoundingprocesses with multiple ingredient additionsrequired along the length of the machine. Alsosolvent or monomer removal from a polymerrequires an extremely long machine.

The co-rotating, fully intermeshing twin-screw extruder is manufactured in many sizes.The smallest have screw diameters 16 mm or lesswith a 5 hp or smaller motor and an output in the

range of a pound per hour. The biggest machinesare 380 mm (Figure 11), with larger sizes on thedrawing board. They use motors up to 20,000 hpand currently produce product at rates

approaching 150,000 pounds per hour. The smallmachines are typically used in laboratories forproduct development, although they are used asproduction machines in some industries (e.g.,pharmaceuticals). The large-diameter machinesare predominantly used to convert polyethyleneand polypropylene powder that comes from thepolymerization reactor into pellets. The pelletscan then be used as feedstock for compoundinglines that mix the polyethylene and polypropylene(or other polymers) with ber glass for betterstrength, mineral ller (i.e., talc, calcium

Figure 10. Barrel layoutfor some typical applications.

Figure 11. 380 mm extruder(barrel and screws).

Figure 12. Compounding line.


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carbonate, and clay) for increased modulus,additives for improved processing, or pigments/ color concentrates. Most compounding lines arebased on co-rotating, fully intermeshing twin-screw extruders (Figure 12). Typically theseextruders have screws with diameters between 40

and 140 mm. The polymer pellets (unlled or thecompounded material) will then be used forinjection molding parts, melt spinning ber,extruding sheet for compression molding andthermoforming, or extruding proles for anapplication such as window casements. However,in some processes the polymer compounding stepis integrated directly with the nal shapingprocess, such as for 2x6 wood ber, compositedecking (Figure 13). However, each of thesemachines has a conguration that, in general, is

similar. Each will have a motor and drive, safetyclutch or slip mechanism, gearbox, processsection, and downstream equipment similar to theline shown in Figure 12. With the possibleexception of the very small-diameter (less than 20mm) units, all the machine process sections areconstructed from modular barrel and screwcomponents. The downstream equipment attachedto the end of the process section can be a singledevice such as the strand die shown on themachine in Figure 12, or be a series of devices.This series can contain some or all of thefollowing devicesstartup (divert) valve, throttlevalve, crossover valve, gear pump, screen pack and changer, die plate, and pelletizer. Downstreamdevices will be discussed in general in a latersection. They will only be incorporated in thischapter if they have a direct impact on one of themain extruder sections mention above.

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The typical extruder drive train consists of amotor and drive, a safety clutch, and a gearbox.Lets take a look at these three components but ina slightly different sequencemotor and drive,

then the gearbox, and nally the clutch.

Motor and Drive

Depending on the machine size, the motor (asmentioned previously) can be under 5 hp or over20,000 hp. Extruders up to 150 mm in diametertypically use a variable speed drive. A 150 mmunit can handle a 5,000 or possibly largerhorsepower motor/drive and produce product at arate of 20,000 plus pounds per hour. Today,extruders requiring 1,000 hp or less typically use

AC variable frequency motors. AC motors/drivesup to this size cost less (or no more) to purchase,less to run and maintain, and deliver constanttorque over the entire speed range. For extrudersabove 150 mm in diameter, single-speed ACmotors are typically used. The motor and driveshould have been specied to match the maximumpower transmission capacity of the extruder.Sometimes, if the material being compounded isone that does not require a lot of energy toprocess, the motor and drive may be specied to

be undersized. This action has an impact on thecontrol setup of the machine. Details about thisissue will be discussed in a later section.

Gearbox

The gearbox takes a single input rotation speedfrom the motor, modies it, and then, through aseries of gears, splits it to drive two output shafts.This is done in two stages. First the speed ismodied in what is typically referred to as thereduction stage. The split is done in thedistribution stage. By installing alternate reductiongear sets, one gearbox size can be ordered withdifferent maximum output speeds. Therefore themaximum extruder speed depends on the gear ratioof the gearbox and the maximum motor speed. Forexample, an extruder with a 3 to 1 speed reductionwould have a maximum speed of 600 rpm if amotor with an 1,800 maximum speed wereinstalled, but only 367 rpm if an 1100-rpm motor

Figure 13. Direct extrusion of wood composites.


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were installed. Today, some extruders can turn at1800 rpm. In these cases the reduction stage of the gearbox can actually be a step-up gear.

There are many designs for a twin-screwextruder gearbox. It is, therefore, not possible tocover this topic in the current chapter. Additionally,

technicians from the extruder or gearboxmanufacturer from whom the unit was purchasedtypically perform required maintenance. However,it might be interesting to take a look at a genericgearbox design (Figure 14). This is the top view of the middle layer of a gearbox.

Over the years, gearbox design has improvedto permit greater power transmission (morehorsepower or kilowatts) and allow them to run athigher rpm values. However, the power

transmission term used to compare gearboxes isnot hp or kW, but torque. A gearbox torque israted in newton-meters. Practically speakingtorque is a measure of the rotational energy thatcan be transmitted. The total power is the productof the rotational energy times the rpm. Figure 15shows a representation of the increase inpermissible rpm and gearbox torque. The

maximum allowable speed has increased by morethan a factor of ten and gearbox torque by a factorgreater than 2.5.

The size of the motor installed on an extruderdepends upon the gearbox rating and the

maximum output speed required. In any event, thekW or hp that a gearbox can transmit is directlyproportional to the rpm. Therefore, a gearbox thatruns at a max rpm of 600 needs to have twice asmuch installed motor power as a gearbox with anoutput speed of only 300 rpm. Today, asmentioned previously, gearboxes are capable of output speeds of 1800 rpm. Depending on themotor input speed, such a high-speed gearboxwould have a gear ratio of unity or less. That is, itis acting as a speed step-up device, not a speed

reduction device as has been its task up till now. Itcould also have a motor that is three times the sizeof the one on the 600 rpm max rpm gearbox.

Safety Coupling

A coupling is installed between the motor andgearbox to protect the extruder process section andthe gearbox from an over torque situation. Anover torque situation occurs when there is aresistance to the turning screw that is greater than ismechanically permitted for safe operation. This can

occur if too much material suddenly enters theextruder, or worsesome foreign object such as abolt falls in and jams the extruder causing it tocome to a sudden stop. The potential for damageoccurs because the motor can not be sized exactlyto match the gearbox. Therefore, the motor will beoversize or undersize with respect to the gearbox.If it is undersized, then there is no real problembecause the motor cannot drive the system beyondthe rated mechanical capacity. However, if it isoversized, then the gearbox and/or extruder shafts

could be damaged because the motor is morepowerful than the mechanical design capacity of the extruder. Under this circumstance, the safetycoupling would pop open and disengage the motorfrom the rest of the extruder.

In reality the safety coupling is the last line of defense against an over torque situation. Theextruder controls should be set up to have an

Figure 14. Gearbox design.

Figure 15. Development of power and speed.


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electrical shutdown before the mechanicaldisengagement of the coupling. Depending on theextruder manufacturer, the electrical shutdownoccurs when the gearbox torque registers 100105% of the rated capacity. These electricalcontrols will be covered in the later section onControls and Interlocks.

There are several designs for safety couplings.Desch, Autogard, and Brunell are a few of themanufacturers. You should consult your extrudermanual to see if you have one of these or anothertype on you machine.

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The process section of the extruder consists of a number of different components. There are

extruder barrels and screw elements that can beassembled in numerous sequences to accomplishspecic processing tasks. There are feed systemcomponents for introducing material into theextruder. These are in addition to the actualfeeders that are used to meter material at a specicrate into the extruder. There are also dischargecomponents for transporting the material from theend of the extruder and delivering it to a die forshaping the product as it leaves the extruder.

Feed System Components

Of the many feed system components, thedesign of the feed chutes and feed hopper can havesignicant impact on how the extruder operates.This system is designed to transport material fromthe feeder(s) to the feed opening in the extruder.Typically when the material being introduced is in astandard eighth-inch pellet form, there are noproblems. However, powders (particularly low-bulk density powders) can be more difcult to handle. If not handled properly, the powders can uidize andwill not feed well. The two most important points toremember when designing the feed system is tomake sure that the distance the material drops fromthe feeder to the extruder is as short as possible andto have an air-removal mechanism in the system. If either of these is not considered, then it is possiblefor the material to entrain a great deal of air whichis then vented back through the feed furtheruidizing the powder.

Barrel Components

As I mentioned in the introduction, co-rotatingtwin-screw extruders are constructed by takingvarious components and assembling them in aspecic sequence. In one sequence, they maymake a machine for compounding polypropyleneand glass ber. In another sequence, they maymake a machine for making breakfast cereal.

There are ve main barrel componentvariations. There are top open feed barrels forintroducing solid material, typically at thebeginning of the extruder. There are side feedbarrels used in conjunction with a device called aside feeder or stuffer. This barrel design is used tointroduce solid material to a middle section of theextruder that already contains molten material.

Most side feed barrels have an opening or vent inthe top to let air or moisture escape. If this vent isnot needed, it can be plugged. Vent barrels areused to remove volatile material from the system.These volatile materials can be as harmless as airor steam. They can also be harmful chemicals thatare still in the polymer from when it was originallymade, or were formed as part of the compoundingprocess. In either event they need to be removedfrom the system.

The vent section is typically located near theend of the machine. It may also be connected to avacuum system that helps remove the volatilesubstances and condense them back into liquidform for disposal. There are solid barrels that areused in melting, mixing, and conveying sections of the machine. Some solid barrels are drilled topermit a measuring or metering device to beinserted. Measuring devices might include a meltthermocouple or pressure transducer. Themetering device might be an injection nozzle foraddition of liquids into the melt. For all three typesof devices, the bores use 1

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inch threads.The length of each barrel section varies

depending upon machine manufacturer and/orextruder series. Typically, the barrels are between3 and 6 diameters long. That is, their length is 3 to6 times the diameter of the machine. The barrelcross section again varies by machinemanufacturer and extruder series. Today the cross


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section is typically rectangular. The other designoption is round.

Barrels are typically heated electrically andcooled with water. Each barrel is generally anindividually controlled heating/cooling zone. The

heaters are normally plates that are attached to theoutside of the barrel or heater cartridges that areinserted directly into the barrel body. Each barrelis usually also cored for cooling. Therefore, toheat up the machine, the electrical heaters areused. If the machine gets too warm, a controlledpulse of water is introduced into the barrel to coolit to the desired temperature.

It is important that the water be demineralized.Typically, the barrel being cooled is hot enough toconvert the water pulsed into the cooling channel tosteam. If the water contains minerals, they will bedeposited on the cooling bore surface. At somepoint, the cooling bores will become coated (orworse, clogged). This will make them less effective(or even useless) for cooling.

As an alternative design, especially on verysmall machines, some manufacturers combineheating and cooling in the same external clamp-on unit. In some situations, especially when themachine is operated in an explosion-proof environment, or at (or below) room temperature, asingle heating/cooling uid is used. For heatingabove room temperature, this is typically hotwater, steam, or oil. For running at (or below)

room temperature, chilled water or a water-glycolsolution is used.

Screw Components

As mentioned before, a screw conguration ina co-rotating, twin-screw extruder is composed of two basic types of elementsconveying elementsand mixing elements. Conveying elementstransport material from one section of theextruder (i.e., the feed section) to another (i.e., themelting section). Figure 16 illustrates the typicalsections in the twin-screw extruder designed forcompounding. Mixing elements are used to melt,disperse (break into smaller pieces), andhomogenize the material being processed. Theseveral different types of mixing elements will bedetailed later in this section.

All these different types of elements arearranged on a shaft in a specic sequence. Theexact sequence is determined by the requirementsof the task. Therefore, the arrangement requiredto process polymer composites is different fromthe one used to make wood-ber composite

Figure 16. Typical sections of a twin-screwextruder.

Figure 17. Screw-shaft element connection.


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decking which, in turn, is different from the oneneeded to produce lled polypropylene.

The elements are held in place on the shaft bytwo means. The rst is a screw tip which prevents theelements from falling off the end of the shaft. Thesecond is by designed mating geometry between theshaft and elements so that the elements do not rotate.Depending on the manufacturer and the age of themachine, this geometry can vary from round orsquare keys that t into grooves on both the shaftand elements to mating splines that are cut into boththe shafts and elements. Figure 17 shows examplesof several mating designs. However, it is important toremember that more mating grooves enable the shaftto transmit higher amounts of energy from the motor

and gearbox through the shaft and elements to thematerial being processed. Therefore, the design onthe right in Figure 17 can transmit much more energythan the design on the left.

Screw Bushings

Screw bushings are typically constructed withpitches ranging from approximately 0.5D (narrowpitch) to 2.0D (wide pitch) (Figure 18). The letterD refers to the diameter of the extruder. Bydening pitch in relation to diameter we can referto an element pitch independent of machine size.This means, for example, that the smallestconveying element pitch on a 40-mm twin-screw

would be approximately 20 mm and the largestapproximately 80 mm.

I have used the term pitch, but have notdened what it is. Pitch is the distance that istraveled along an element when the helix isfollowed through a 360 rotation (Figure 19).

Wide pitch elements might typically be usedin feed or devolatilization areas of the extruderwhere we want a low degree of ll. Medium(1.0D) pitch is used to convey material down theextruder, typically from one unit operation toanother. Finally, narrow pitch elements are suitedfor areas where compaction of material and/or100% ll is desired, such as before kneadingblocks or seals, or between unit operations (i.e.,feeding and vacuum devolatilization). You maywonder what the maximum allowable pitch is thatwill still convey material down the extruder. Froma theoretical point, 2.5D pitch provides thegreatest drag ow capacity. That is, in the absenceof any ow restrictions it will convey morematerial, exhibit a lower degree of ll, and have alower residence time than lower pitch elements.But it also has increased sensitivity to pressure

Figure 18. Conveying elements.

Figure 19. Pitch.

Figure 20. Backup lengths vs. pitch.


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ow. That is, as the pitch of an element increases,drag ow conveys material in the down-channeldirection at a faster rate. However, if there is arestrictive force placed in the ow path, (areverse-type screw bushing or kneading block, ora die) the higher pitch element is less effective inbuilding up the pressure necessary to pushmaterial past the restriction. That is why wetypically do not use wide pitch elements before adie. Figure 20 shows that the highest pitchelement has a greater backup length than themedium pitch element. It is also important topoint out that even though wide pitch elementsare more sensitive to pressure ow, within the

normal pressure range seen in the extruder, inmost instances, they still have greater pumpingcapacity than does a narrow pitch bushing. In allcases, the factors determining the optimumelement pitch are volumetric ow rate and theshear and temperature sensitivity of the materialbeing processed.

Screw bushings are typically used for

conveying material down the extruder, but thereare also reverse pitch elements. Reverse pitchelements are meant to restrict the ow of materialand, therefore, increase residence time and theamount of energy incorporated into the product.

In addition to conveying screw bushings,several elements have been designed for specialsituations. For example, elements have been

designed to accommodate hard-to-feed or low-bulk density materials. The self-wiping prole of the pushing ight has been transformed into asquare channel prole (Figure 21). Dependingupon machine size, the element can have as muchas 40% greater free volume. These elementstypically have either pitches of 1.5 or 2D. Otherspecial screw elements are SF bushings (singleight) (Figure 22). These are used in both feedand discharge sections of the extruder for creatingmore positive displacement ow. The wide crestscreate signicant barriers for leak ow. A narrowpitch version of the SF element has been usedmost frequently in reactive processing to conveywater-like viscosity feeds. Reverse pitch elementsare used to generate backpressure and thereforecreate sections of 100% ll that, for example, canbe used to separate unit operations, or totally ll amixing section (Figure 23).

Kneading Blocks

While screw bushings are constructed as acontinuous helix (pitch), kneading blocks are builtup from individual discs (Figure 24). Each disc hasan angular offset from the disc upstream anddownstream. Kneading blocks are characterized bytotal length, number of discs, and stagger angle

Figure 21. Standard vs. undercut elements.

Figure 22. Working principle of screw elements.

Figure 23. Forward and reverse conveying.


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between successive discs (i.e., KB45/5/40 is a unit40-mm long with 5 discs staggered at 45 withrespect to each other). Kneading blocks come in

three basic formsconveying, neutral and reverse(Figure 25). As shown schematically in Figures 26,kneading blocks provide both distributive(homogenization) and dispersive (breaking intosmaller pieces) mixing. The relative intensity of eachdepends upon individual disc width and the staggerangle between discs. Figure 26 also shows that forconstant stagger angle, an increase in disc widthresults in an increased dispersive mixing componentper unit mixing length. However, as a result,distributive mixing (stream splitting) is decreased.

In addition to mixing, disc width has anotherimpact on kneading block performance. A widedisc conveying kneading block has greater dragow capacity than a narrow disc KB. It is alsoless efcient at pushing material forward whenworking against backpressure. This relationship issimilar to the one exhibited by screw bushings aspitch is increased. However, the controlling

mechanism is totally different. As the individualdiscs on the kneading block element rotate, eachdisc tip pushes a nip of material in acircumferential path. As would be expected, thepolymer spreads itself downstream and

upstream perpendicular to the circumferentialow. The wider each disc, the more material isspread in the axial direction and the greater theratio of net down-channel to circumferentialmaterial ow. In addition to disc width, modifyingthe stagger angle between discs also inuences netdown-channel ow. Obviously changing thestagger angle from conveying to a neutral or a

reverse stagger has a signicant effect on down-channel ow as well as the magnitude of anelements dispersive/distributive components(Figure 27). Also, for either forward or reversepitch kneading blocks, the greater the staggerangle, the larger the angle opening between discsand the easier it is for leak ow to occur. In down-channel conveying units, this results in reducedconveying capacity and, therefore, higher degree-of-ll and increased residence time. For reversepitch units, a larger stagger angle means less

Figure 24. Kneading blocks.

Figure 25. Kneading blockperformance comparison.

Figure 26. Forwarding kneading blocks.

Figure 27. Operating principle of 3-lobe geometry.


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effective polymer restriction. Neutral elementshave the maximum leak ow opening. As theyrotate, they push an equal amount of material inpositive and negative directions. Figure 27 showspictorially the change in leak ow opening for30, 45, 60, and 90 (neutral) staggeredelements in a two-lobe system.

Special Elements

Three-Lobe Geometry for a Two-Lobe System

Special three-lobe kneading blocks (Figure28) have been developed and patented by Werner& Peiderer for applying stress more evenly tomaterial in a two-lobe system (e.g., thermoplasticmelt containing a substantial amount of un-melt).They provide a more uniform shear rate, withoutsacricing self-wiping. Both ow simulations and

extruder-testing have shown that these elementsfunction in a different manner than the standardtwo-lobe kneading blocks.

Distributive Mixing Elements

For distributive mixing beyond thecapabilities of narrow disc kneading blocks,toothed mixing elements, such as those shown inFigure 29, are used. The number of teeth aroundthe circumference, as well as the tooth angledenes these elements. The former contributes to

stream splitting for generation of interfacialsurface; the latter contributes to conveyingcapacity. The main function of both elements is toprovide the maximum amount of distributivemixing (little, if any, dispersive mixing) withminimal energy input. The self-wiping version isa design evolution developed to meet ever-increasing quality standards. The elements that donot wipe the entire barrel wall provide a low-velocity zone for material stagnation and potentialdegradation.

All the twin-screw extruder manufacturershave developed their own elements for specialapplications. It would be impossible to go into allthe variations. I recommend that you visit thewebsite for your particular brand of twin-screw to

get the most up-to-date and detailed information.

IN S T R U M E N TAT I O N C O N T R O L S A N D IN T E R L O C K S

A compounding extruder system is protectedfrom failure in two ways. The rst is through amechanical design that provides maximumlifetime of each component (i.e., application of proper safety factors for gears, bearings, andshafts) and proper sizing of the drive motor andsafety clutch. The second is through a system of

interlocks that provide warnings, alarms, andshutdown functions for the startup or running of the equipment.

Mechanical Design

A twin-screw extruder is designed to transmitenergy from the motor by way of the gearbox andprocess section to the material. The amount of energy that can be transferred depends onmachine design and allowable rpm. However,upset conditions in an operation can result inoverload situations that need to be compensatedfor by safety factors in the mechanical design of the machine components. The drive motor is sizedso that its power rating is equal to or greater than120% of the rating of the gearbox and shafts. Themotor amperage is calibrated to display the torquebeing transmitted through the shafts as percent of the maximum allow value. When torque reaches105%, a warning light is activated, the feed

Figure 28. Operating principle of 3-lobe geometry.

Figure 29. Gear mixing elements.


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system is shut down, and a timer is initiated(typically about 2 minutes). If the torque remainsabove 105% but below 110% for two minutes (ourexample), the main drive motor shuts down. If thetorque drops below 105% within this two-minuteperiod, the timer is reset. If the torque exceeds110% the motor shuts down instantly. Themechanical clutch between the motor and thegearbox input shaft is the nal safety interlock.This clutch is set to disengage or slip at a torquelevel equal to 115% of the torque rating for thecombination of the two shafts divided by thegearbox ratio.

System Interlocks

Co-rotating twin-screw compoundingextruders typically have ve interlock systems to

permit safe operation and prevent damage to anyof its components.

1. Extruder Startup Permissive Interlocks

2. Extruder Immediate Trip Interlocks

3. Extruder Timed Trip Interlocks

4. Feed System Trip Interlocks

5. Downstream Equipment Trip Interlocks

These systems utilize current sensors, straingauges, tachometers, thermocouples, levelsensors, switches, proximity sensors, and otherdevices to monitor the extrusion line and provideinput to the logic system that controls it. Thepurpose and physical signicance for eachinterlock in the process is described below.

Extruder Startup Permissive Interlocks

The following drive startup conditions must allbe satised. These conditions are inputs into anAND logic block and all must be a logic 1(i.e., yes) to permit startup.

Barrel temperatures up to set point for 1 to 4hours for heat saturation

No thermocouple breaks Extruder lube oil pump running and oil

pressure is normal (forced lubricationgearboxes only)

Speed indicator set to 0

Downstream equipment running

Cooling water pump running (for closed-loop cooling systems only)

All trip signals off

A heat-up saturation period is required toensure that the process section has reachedthermal equilibrium. A bypass for this is providedin case there is a shutdown after the processsection is heated up. If any of the thermocouplesare broken, the heater for the respective processzone will not operate properly. This condition willprevent startup but will not shut down the extruderwhile running. On gearboxes with a forced lubeoil system, the lube oil pump must be running andhave a normal oil pressure reading. The

potentiometer for main drive speed adjustmentmust be set to 0 speed set point to prevent themain drive motor from starting at high speed. Alldownstream equipment must be running to acceptextrudate when it reaches the pelletizer or die.Cooling water pump must be running for allclosed-loop cooling systems. All other trip signalsmust be off so that the machine may not be startedunder alarm conditions.

Extruder Immediate Trip Interlocks

Immediate trip interlocks are triggered byconditions that threaten to damage the extruder ina short period of time. Some of these trips requirea response time of less than one-tenth of a second.Any one of the following conditions will causeextruder shutdown:

Greater than 110% torque

High discharge pressure

High extruder thrust bearing load

High main drive motor temperature

Low main drive motor cooling airow forair-cooled motors

Extruder coupling guard not in place

Clutch disengagement or slippage

Extruder DC drive malfunction Downstream equipment not running.


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Exceeding 110% of the rated torque of thesystem will shut down the main drive motor. Thiscondition is sensed by a current transmitter on theextruder motor SCR drive power supply. Apressure transducer, after the last barrel, is used tomonitor extrusion pressure and, above a certainpoint, shuts down the extruder. This is to avoiddamage to downstream equipment and to thethrust bearings. A strain gauge thrust-sensingsystem on the thrust bearings will also causeshutdown if there is too much pressure. If thedrive motor overheats or the airow stops on anair-cooled motor, it is shut down to prevent burnout. For operator safety, the extruder couplingguards must be in place to prevent shutdown.Torque in excess of 115% will cause the coupling

to slip or disengage; this will be sensed and themain drive motor will shut down. If thedownstream equipment shuts down, the maindrive motor stops also, in order to prevent damageto the downstream equipment, and/or over torqueof the screw shafts and couplings.

Extruder Timed Trip Interlocks

There are extruder conditions that do notrequire immediate shutdown, but cannot betolerated for more than a few minutes. Whenthese conditions are encountered, warningindicators are turned on and timers are trippedthat start a shutdown sequence. If the conditionsare corrected, the shutdown sequence iscancelled. Otherwise the main drive trips off atthe end of its timing interval. The following arethe standard conditions for tripping a timedinterlock:

Low extruder torque (10%

Extruder gearbox lube oil pressure low (forforced lube systems)

Extruder gearbox lube oil pump notrunning (for forced lube systems)

Extruder gearbox lube oil temperature high

Extruder main drive motor torque greaterthan 105%, but less than 110%.

Running at less than 20% torque and greaterthan 10% screw speed indicates loss of feedingand activates a two-minute timer. If theseconditions persist, the two-minute timer activatesan under-load alarm and starts a four-minute triptimer. If feeding is not restored (thereby, returningtorque to normal levels) within this period, theextruder main drive motor is shut down to preventthe wear that can result from operating theextruder empty at high speeds.

To protect the gearbox from poor lubrication,a two-minute trip timer is activated when the lubeoil pressure is low, or the pump is not running (forforced lubrication systems), or when the extrudergearbox temperature is high.

The extruder torque may run up to 105%, but

if it exceeds this value, a two-minute trip timer isactivated. If the torque returns to below this valuethe timer is reset. If the machine is run between105% and 110%, it will shut down after twominutes. When any of the above gearbox ortorque two-minute trip timers is activated, thefeed system shuts down.

Feed System Trip Interlocks

The feed system trip interlocks are designedto prevent damage to the extruder from running

dry, feeding at low speed, or feeding ller in aside feeder when there is no polymer. In order tostart and operate the main feeder the followingconditions must be met:

All two-minute trip timers must be off Extruder drive motor must be running Extruder screw speed is greater than 10% Side feeder must be running (where

applicable).

If a two-minute timed trip sequence isoperational, then the feeder will not start orcontinue to operate. This allows the machine toempty itself when it is in a warning condition.The main feeder is tripped and, if it exists, so is thedownstream feeder. If a side feeder (side stuffer) isbeing used, it remains running (unless the extrudershuts down) to prevent backow of the melt intothe side feeder. If the extruder shuts down, all


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feeders are shut down. However, unlike theprevious conditions, the side feeder (stuffer) is alsoshut down to prevent packing the barrel with a highpercentage of ller or reinforcement material. If thescrew speed is below 10%, the same action is takenas with the two-minute timers. If the side feeder(stuffer) shuts down or is not running, its feeder istripped, as is the main feeder (unless bypassed).

The side feeder (stuffer) is normally set up toauto-start when the extruder is above low speed(>10%). The downstream feeder for the sidefeeder (stuffer) is started by a one-minute timeractivated by the main feeder running. This ensuresthat polymer has reached the side feed barrel.

Downstream Equipment Trip Interlocks

As stated earlier, the main drive will trip off if the downstream equipment is not running.Generally speaking there is a signal which themain interlock system receives from eachdownstream equipment system. The status of thissignal depends on the state of all the variousdownstream components such as gear pumps,diverter valves, water pumps, conveyors, screen-packs, pelletizers, and so on. Due to the variety of downstream equipment, it is beyond the scope of this article to discuss each system.

When the downstream equipment systeminterlocks are all satised, a logic 1 or an ONsignal is transmitted to the Extruder ImmediateTrip Interlock System and the extruder is allowedto run. Any condition in the downstreamequipment system that causes a logic 0 signalwill shut down the extruder.

Control and Measurement

The success of a compounding operation isnot only dependent on obtaining the desiredproduct quality initially, but to maintain consistentquality. In order to assure consistent quality, theprocessing parameters on the machine must beaccurately measured, recorded, and, if appropriate,controlled. The typical parameters measured in thecourse of extruder operation are screw rpm,temperature (both barrel and material), motorpower consumption (either in the form of amps orkilowatts), and, nally, pressuretypically at the

die. Also, material parameters such as feed rateand recipe ratio must be measured and recorded.

Screw rpm, as the term indicates, is ameasurement of how fast the screws are turning.As mentioned in the previous section, this value

could be anywhere from 100 to 1,800 rpm. Mosttypical compounding systems run between 600and 1,200 rpm. The primary reason that screwspeeds are measured is to have a rst lineindication of the energy being introduced into thematerial. The higher the screw speed, the greaterthe energy input. Remember, the greater theenergy input, the higher the material temperature.

Barrel temperature is set to either introduce orremove heat from the barrel. Interestingly enough,the feed barrel typically does not have atemperature controller but cold or cool water iscontinuously circulated through the barrel tominimize the possibility that polymer will melt asit enters the feed throat. For barrels justdownstream from the feed zone, the temperature isset to a temperature at or above the melting pointof the polymer. The objective is to introduce heatinto the barrel and also the polymer. Once thepolymer has been melted, the barrels are set to atemperature close to the discharge temperature of the polymer. This sets up an approximatelyadiabatic operating condition such that thematerial in the extruder is being neither heated norcooled by the barrels.

Material temperature is typically measured inthe die or transfer pipe at the end of the extruder.The most accurate measurement is with animmersion thermocouple. An immersionthermocouple is placed in the melt stream, so thatit is only measuring the melt temperature ratherthan the temperature of the metal equipment

surrounding it.Energy is introduced into the extruder through

both the motor and the heaters. However, onproduction machines 50 mm and greater in screwdiameter, the most signicant amount of energy isintroduced by the motor through the conversion of electrical energy to mechanical energy as thescrew rotates. This energy is best measured by


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recording the kilowatts (kW). Kilowatts are theproduct of amps (current) times volts used inrunning the motor. For a DC (direct current)motor and drive, the voltage varies directly withscrew speed. As the screw speed is increased, thesystem voltage increases proportionately. Theamps measure the amount of current being used toturn the screw. The more difcult it is to turn thescrew (maybe because there is more material init), the more current is required. Therefore, at aspecic rpm (voltage is constant), the ampsmeasure the relative energy required to processthe product. You may have heard of the term %torque. The amp consumption for a xed rpm,tracks the % of available power being consumed.To describe how energy is introduced to the

material, the value is typically represented askilowatt-hours per kg of material or horsepowerper pound of material.

Discharge pressure is monitored for a numberof reasons. First, it is interlocked with theoperating system to assure the safety of thegearbox thrust bearings. Thrust bearings are ratedfor a specic maximum pressure. If that pressureis exceeded, they are more likely to fail.Therefore, to protect the gearbox, the machinewill shut down if the pressure gets too high.Another reason to measure pressure is to monitorthe status of downstream equipment such as thescreen changer and die. If the pressure builds up,then the screens may need to be changed, or someof the die holes may be plugged.

Downstream/Discharge Components

Between the end of the extruder and the nalpolymer discharge, there are often several piecesof additional equipment. In the simplest setup,there is only a die at the end of the machine; butmore complicated processes may requireadditional pieces of equipment such as screens,screen changers, gear pumps, or other devices forthe specic process.

On the large twin-screw extruders (250380mm, rates up to 70 tons/hr.) used to convertpolyethylene and polypropylene powder resin topellets, there are other pieces of downstreamequipment. There can be a discharge throttle valveto introduce additional energy into the system.Also, many times there is a gear pump installed to

efciently generate the pressure (sometimesseveral hundred bar) needed to have the materialow through the screens and the underwaterpelletizer.

The type of discharge components depends onthe material being extruded and the chosenprocess. For example, pellets produced duringcompounding may be pelletized through anunderwater pelletizer or through another processsuch as a quench bath. Processes producing nalextruded product such as this will feed theextrudate through sizing equipment. Yourapplication will dene your discharge equipment.

Thanks to Cincinnati Milacron and Polymer Processing Institute for photo and graphiccontributions to this chapter.


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G ENERAL C HARACTERISTICS OF C OUNTER -R OTATINGTWIN-S CREW E XTRUDERS

Counter-rotating twin-screw extruders areused for a variety of plastic products andprocesses. It is interesting to note that closelyintermeshing, counter-rotating twin-screwextruders can be designed to pump materials in anon-drag ow manner in locked C-shapedchambers. Only this device (and ram extruders)can convey via positive displacement, as compared

to the drag ow single-screw extruder and semi-drag ow co-rotating twin-screw extruder.

There are two distinct and separate families of counter-rotating twin-screw extruders:

High-speed and energy input, with bothintermeshing and non-intermeshing designs,melt the polymer early and are designed asmass-transfer devices, with the primaryapplications being mixing, devolatilization,and reactive extrusion (HSEI).

Low-speed, late fusion, intermeshing witheither parallel or conical screws are designedto avoid energy input and do not melt thematerials (typically PVC) until the middle orlatter part of the process section (LSLF).

High-speed, energy input (HSEI) counter-rotating twin-screw extruders can be intermeshingor non-intermeshing. The co-rotating

intermeshing mode, as previously discussed,dominates the compounding market, havingcaptured over 90% of current installations.Counter-rotating designs are primarily used forspecialty applicationssuch as high-leveldevolatilization and reactive extrusion.

By contrast, low-speed, late fusion (LSLF)counter-rotating twin-screw extruders areprimarily used for PVC and other shear-sensitiveformulations that benet from a design thatminimizes energy input combined with pumpinguniformity. These devices are often inadequate toperform energy-intensive processing. As impliedby the category, the LSLF counter-rotating twin-screw extruder operates with lower rpm than itshigh-speed cousin.

Just like any extruder, control parameters forthe counter-rotating twin-screw extruder includescrew speed, feed rate, temperatures along theprocess section, and vacuum level. Monitor-onlyparameters include melt pressure, melttemperature, and motor amperage. The motor (ACor DC) inputs energy into the process viainteracting twin screws imparting both shear andenergy. Higher screw speeds result in more shearfor a given screw design. Barrel sections areelectrically heated and cooled by liquid or air,depending upon the machine conguration andheat-transfer requirements of the process.

C OUNTER -R OTATINGT WIN -S CREW E XTRUDERSC HARLIE M ARTIN


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As in co-rotation, the gearbox transmits powerfrom the motor to the screws, reduces the motorspeed to the desired screw rpm, maintains theangular timing of the screws, and takes the thrustload from the screw set. The gearbox is usuallyseparated into two distinct partsthe reductionsection and the distribution section. The reductiongearbox is a conventional helical gearbox, whichreduces the motor input speed (1,800 rpm, forexample) to the screw output maximum speed,which may be anywhere from 10 to 500+. In theprocess of reducing the speed, the torque ismultiplied by the same ratioso all the shafts,gears, and bearings have to be designed tocontinuously transmit the torque rating of eachpart of the gear system. The distribution gear

section takes a single-shaft input from thereduction gear and transmits it to two (2) paralleloutput shafts. As a safeguard, a mechanical overtorque coupling is utilized that connects the motorshaft to the gearbox input shaft and automaticallyuncouples/disengages the motor if the torqueexceeds a preset level.

The feed system is a critical component in anycounter-rotating twin-screw extrusion system.Various delivery mechanisms are used for feeders,including screw-augers, vibratory trays, and belts.

Liquid feed streams typically use piston or gearpumps to set the rate to the extruder system,depending upon the viscosity of the liquid, and canutilize a heated or ambient reservoir/piping.Feeders can be either volumetric or loss-in-weight,depending upon the nature of the installation.

For low-speed counter-rotating twin-screwextruders, the feed system can be a ood fedhopper, where the hopper sits over the extruderfeed throat, which relies on gravity to move thematerials into the machine. In this case, the screwrpm determines the throughput rate. For PVCmaterials, the formulation is generally pre-mixedin a high-intensity mixer and the hopper is lled.

Another option is to take the same PVC pre-mix,but to meter it to the extruder feed throat via starvefeeding, making the feed rate independent of thescrew rpm. Crammer feeders can be used forhighly lled or low bulk density formulations.

The sequence of process operations in acounter-rotating twin-screw extruder is almostidentical to the co-rotating design. Flighted screwelements push material forward past barrel ports,through mixers, and out of the extruder to the die.Zoning elements isolate operations within theextruder. Mixing elements can be distributive ordispersive in nature. Screw designs can be madeshear intensive or passive, based upon theintended range of applications.

H I G H -SPEED , E N E R G Y INPUT (HSEI) C O U N T E R -R O TAT I O N There are various types of HSEI counter-

rotating twin-screw extruders with thecommonality being that the machine is primarilydesigned to input energy into the process. Thefollowing is a description of different models thatare available.

Traditional intermeshing and parallel design

The traditional counter-rotating intermeshingtwin-screw extruder designed for mass transferoperations was embraced in the 1970s for themanufacture of color masterbatch and similarproducts (Figure 1). Looking into the feed throat,the screws rotate outward to facilitate feeding of the material on both screws. In the screwintermesh region, the ight of one of the screwspenetrates the ight depth of the second screw andthe velocity of the screws intermesh is in thesame direction. This region is referred to as thecalender gap. Screw rotation forces materials upand through the calender gap (Figure 2) tofacilitate melting and mixing, as the processedmaterials experience an extensional shear effect.Essentially, the entire length of the screw can

Figure 1. Traditional counter-rotating intermeshing compounding screw design.


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function as a mixing device as materialscontinually experience the extensional mixingand shear associated with the calender gap. Inaddition to calender gap mixing, gear mixers canbe utilized for distributive mixing, as well asblister rings for planar shear mixing, and/or toprovide a seal for vacuum venting. At thedischarge end of the screws, the traditionalcounter-rotating intermeshing can be designed topump in a C-locked chamber (Figure 3).

Screw diameters for this type of twin-screwextruder range from 18 to 135 mm. A typicalprocess length is 20 to 30 to 1 L/D. Because of screw deection inherent with the materialstraveling through the calender gap, the screw rpmis typically limited to 150 or below. Barrels andscrews can be either one-piece or modular, and

the relatively low screw rpm allows either air orliquid cooling to be considered.

Counterflight intermeshing and parallel design

A new approach to mixing in counter-rotationwas introduced by Leistritz in the 1990s, referred

to as counteright. Counteright technologyshifts the mixing from the calender gap to lobalmixing elements, as in co-rotation. In co-rotationthe rotational clearances typically limit the lobecount to two, hence the term bi-lobal. Incounter-rotation, up to six lobes are possible atthe same ight depth (Figure 4). This translatesinto more mixing events for each screw rotation.

For instance a bi-lobal twin-screw extruder

operating at 100 rpm would have 200 mixingevents as compared to 600 for a hexa-lobalcounter-rotating mixer. To allow for higher screwspeeds, open ighted elements are utilized todrive material over the counteright mixers. Theredistribution of mixing to the counterightelements combined with the minimization of acalender-gap effect allows signicantly higherscrew rpm without screw deection, as comparedto the traditional counter-rotating designsdescribed above. Interestingly, counter-rotatingscrew designs that integrate both traditional andcounteright have been successfully employed formany specialty applications. Figure 5 shows threedifferent areas of the screw and describes theeffect achieved at each of the areas.

For the counteright designs, screw diametersrange from 18 to 135 mm. A typical processlength is 32 to 52 to 1 LD and screw rpm of up to

Figure 2. Top view counter-rotating, intermeshingcalender gap and gear mixer.

Figure 3. Example c-locked chamber.

Figure 4. Example hexa-lobal counter-rotatingmixers.


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liquid cooling. Screws are also segmented andassembled on high-torque, splined shafts.

Counter-rotating, non-intermeshing design(CRNI)

The CRNI twin-screw extruder has non-intermeshing screws, which allows for uniquedesign capabilities because each screw can bedesigned using congurations similar to a single-screw extruder. Normally, the design of eachscrew is mirrored on the other screw, but this is notalways the case. The screws can have forward orreverse ights, different helix angles, thick or thinight thicknesses, multiple screw starts, and othersingle-screw design features. A small rootdiameter can be specied in the feed area to

facilitate a large free volume for low bulk densityfeedstocks and the root diameter can be tapered upafter the feed section to compress and melt thepolymer. Screw elements can be matched orstaggered at different points along the processlength to facilitate pumping and/or mixing (Figure6). Different types of mixing elements areavailable for distributive and dispersive mixing.

Screw diameters for the CRNI twin-screwsystem range from 20 to 250 mm. A typical processlength for this twin-screw conguration is 30 to 54to 1 L/D and screw rpm of 500 or less are used. Thescrews are segmented and historically have beenconnected by triple start threaded studs. Barrels arealso modular and are typically liquid cooled.

Due to absence of an intermesh and theassociated geometric limitations, the non-intermeshing mode may be specied at 100 to 1 L/ D or more. This can be benecial for processesthat require a long residence time (for instance,some specialty reactive extrusion applications).

Figure 5a. Feeding and melting.

Figure 5b. Calender gap and hexa-lobal mixing.

Figure 5c. Hexa-lobal mixing, degassing, thermal homogenization and discharge.

Figure 6. Example CRNI mixed and staggeredscrew ights.


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L O W S PEED , L A TE F U S I O N (LSLF) C O U N T E R -R O TAT I O N

The LSLF counter-rotating twin-screwextruder shares some of the characteristics of thecounter-rotating intermeshing design described

above; however, this mode is characterized by agentle melting effect and narrow residence timedistribution, in combination with high-pressurepumping capabilities. Comparatively low screwrpm, late fusion screws are designed to avoidimparting too much energy to the process. Theseeffects are of particular importance whenprocessing a thermally sensitive material, such asPVC. As compared to the HSEI counter-rotatingtwin-screw extruder, LSLF mode is used formaterials and/or applications where shear- or

temperature-sensitive materials are being

processed, high head pressures are desired, and/orthe materials do not convey well by drag ow.Historically, 90%+ of this format twin-screwextruder has processed PVC materials. Currently,there is a push to expand the market applicationsfor this mode of twin-screw extruder intoalternative materials, so that in the future 70 to80% usage for PVC may become the norm.

The LSLF counter-rotating twin-screwextruder is primarily a positive displacement pumpthat conveys material with controlled melttemperatures. Various ight pitches, as well asmulti-start screws are available. The calender gapsare sized for gentle mixing and minimal friction.

Lower melt temperatures help minimize sizing andcooling problems of complex shapes, which is atypical end product in the PVC prole industry.

In its solid state, PVC powder has acomparatively low bulk density. The performance

of the feed zone determines the uniformity of themelt over the full length of the screw. As thematerial passes from the feed zone into the pre-heating/pre-compression zones, a transformationfrom a solid to a viscous melt begins to occur. Asthe material enters the compression zone, ow isrestricted and the nite ight volume in thecompression zone generates a backpressure thatcompresses the material, while shear increasesand the viscosity of the melt stream decreases.Devolatilization occurs late in the process,

typically using multi-start screws to increase thesurface area of the melt stream, and just beforenal pumping. In the metering zone, the primaryprocess functions are to complete the plasticizingprocess and generate pressure to pump thematerial through a die at high pressure.

There are two types of LSLF counter-rotating,intermeshing designsconical and parallel.These two designs (Figure 7) are dened by thediameter of the screws at the feed section versusat the tips. The parallel screw has no change in

diameter as you travel down the screw, whileconical screws decrease in size as you approachthe tips.

In conical screws, the large-diameter feedzone has a continuous taper to the discharge end(or tips) of the screws. For example, a 55-mmmodel has a feed zone diameter of 114 mm and adischarge diameter of 55 mm. The conical screwdesign provides a natural compression over theentire length of the screws. Less dramatic pitchchanges are required in the ight geometry toachieve a homogeneous melt, as compared to aparallel design. The large diameter in the feedzone provides a larger area for maximizing heattransfer and facilitates rotational shear to beapplied to the incoming material. The smalldischarge diameter minimizes rotational shearand heat generation as the screws pump thematerial through the die.

Figure 7. Schematic of conical (top) and parallel(bottom) low speed, late fusion counter-rotating

twin-screw extruders.


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A unique feature of the conical design is thatthe radial clearance between screws and barrel canbe altered to assist in desired changes to theprocess. Moving the screws forward tightens theradial clearance and improves the pumpingefciency of the extruder for high head pressureapplications. Moving the screws backwardincreases or opens up the radial clearance,increasing back ow and mechanical shear from thescrews and improves mixing capability for highlylled or lubricated compounds. Conical twin-screwextruders have shorter processing sections andfootprint as compared to parallel designs.

The parallel counter-rotating LSLF twin-screw extruder is differentiated from the conicalextruder in that the screws are cylindrical and have

a constant diameter. Radial clearance cannot beadjusted. The screws are typically longer thanthose of a corresponding conical design, and thescrew geometry relies solely on dimensionalchanges to the gaps between the screw ights, theight count, and changes in pitch to achieve thedesired compression ratio to transform thefeedstock from a solid to a melt. The additionallength of the screws provides more versatility forsequential process tasks to be performed.

Since the diameters of the screws are constant,

the same circumferential speed is provided overthe entire length, and wider screw ights aretypically incorporated into the design. This serves

to reduce surface pressure and helps to minimizescrew and barrel wear. Once wear has occurred,barrels and screws are more easily rebuilt due tothe constant diameter and tolerances throughoutthe length of the processing zone.

LSLF counter-rotating twin-screw extrudersrange from 25 to 170 mm screw diameters, withmotors from 10 to 300 hp. Maximum rpm varieswith size, but is generally below 50 rpm and thetypical process length is 20 to 28 to 1 L/D. Screwsand barrels are normally one-piece, which makesthe cost signicantly lower compared to modulardesigns. Heating of the barrels is via electric bandheaters, and cooling is external to barrel by eitherliquid or air. Screws are typically internally coredfor liquid cooling, which is a preferred designfeature for thermally sensitive PVC.

S U M M A RYThere are many counter-rotating twin-screw

extruder types from which to choose to performvarious polymer-processing applications. The endproduct can be a pellet or extruded part.Continuing developments in counter-rotation,sometimes drawing upon co-rotating technologies,will continue to expand and improve the range of

products that can be successfully manufactured,taking advantage of the unique geometriccapabilities inherent with counter-rotating designs.

paul andersen

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