Extrusion - Part I/14735_01.pdfP a r t 1 : S i n g l e S c r e w E x t r u s i o n

1 E x t r u s i o n P r o c e s s

The extrusion of polymeric materials to produce fin-ished products for industrial or consumer applications isan integrated process, with the extruder comprising onecomponent of the entire line. In some applications theproduction lines are very long with numerous operations,requiring operators to communicate and work together toproduce an acceptable finished product. If the extrudertemperature profile is set incorrectly, the product ingre-dients are not properly formulated, the cooling on theextruder feed throat is not running properly, the melt tem-perature at the end of the extruder is incorrect, the cool-ing bath temperature is not set correctly, the puller at theend of the line is running at the wrong speed, or any otherincorrect operating condition or combinations of condi-tions, the product may not meet customer specifications.Each step in the process adds value; consequently, the

desired product property profile. Some resin systemsmust be dried prior to extrusion to eliminate polymerdegradation due to moisture. Other resins, which do notnormally require drying, may have to be dried if they arestored in a cold warehouse and brought into a warm envi-ronment, causing moisture to condense on the surface ofthe pellets, flake, or powder. Once the polymer or blendis properly dried and ingredients mixed, the formulationis fed to the extruder, where it is melted, mixed, anddelivered to the die to shape the extrudate. After exitingthe die, the product is cooled and solidified in the desiredshape and pulled away from the extruder at constantvelocity to attain the appropriate cross section. Sec-ondary operations, i.e., flame treatment, printing, cutting,annealing, etc., can be done in line after the puller. Final-ly, the product is inspected, packaged, and shipped.

Figure 1.1. Basic extrusion process schematic.

product reaches its maximum value at the end of the line.An improper setting at the beginning of the process maycause the product to be unacceptable at the end of the lineafter significantly more value has been added. Speeds ofthe different process steps must be matched to ensureproduct compliance.

The extrusion process is shown in Figure 1.1. Poly-meric material is received, inspected, and stored. Prior toextrusion, the polymer may be blended with additives(stabilizers for heat, oxidative stability, UV stability,etc.), color pigments or concentrates, flame retardants,fillers, lubricants, reinforcements, etc., to produce the

The different parts of the process are discussed inmore detail in this chapter.

1.1 R a w Mater ia l Supp ly

Polymer resin is shipped in different size containersdepending on the quantity ordered, the processors' han-dling and storage capability, and the way the extruder isfed. Small lots are shipped in 50- or 55-pound bags, andlarge lots are shipped by tanker truck or rail. Table 1.1shows different shipping methods. Plastic pellets can be

PolymerReceived,

Stored, andInspected

MaterialBlended

with Additives

PolymerDried Polymer Fed

to Extruder

Extrusion Shaped andDrawnSolidificationand Cooling

Pul ler-Dimensional

Control

SecondaryOperation

DecorateInspect Package Ship

Table 1.1. Plastic Packaging

air- or vacuum-conveyed around the plant to storage con-tainers or the extruder hopper.

Pellets conveyed between storage silos, dryers, surgehoppers, and extruder hoppers must be in dedicated orproperly cleaned lines to prevent product cross-contami-nation. All lines must be properly grounded to eliminatestatic electricity build-up during the resin transfer process.

Raw materials stored in warehouses without environ-mental controls (lack of heat or cooling) need to be broughtto room temperature prior to extrusion. If the raw materialtemperatures vary between summer and winter, the poly-mer melting or softening point in the extruder will occur ata different location, leading to different melt viscosities,extrudate flow, and possible product inconsistency fromseason to season. Assume the raw material temperature is500F (100C) in the winter and 800F (26C) in the summer.Additional heat must be added to the raw material duringthe winter months, either by a hopper dryer, allowing thepolymer to come to equilibrium at room temperature, or byadding additional heat in the first zones to ensure the poly-mer is melting or being plasticated in the transition zone.Due to the insulative nature of polymers, a significant timeperiod is required to heat cold pellets that sat in a cold ware-house or in a cold truck to room temperature.

Storing raw materials in a hot environment over anextended time can lead to consuming the polymer stabi-lization package. Most thermal stabilization packages areconsumed over time as the polymer is heated. While ther-mal degradation happens fairly rapidly at elevated tem-peratures in the presence of oxygen, degradation contin-ues at a slower rate at elevated temperatures (above roomtemperature but below the melting or softening tempera-ture). Stock should be rotated to minimize long-termthermal degradation.

Many raw materials are accepted from vendors basedon a "certificate of compliance." Good procedures dictatethat incoming raw materials be periodically tested and adatabase of critical polymer properties be established.Most internal extrusion problems are not the result of rawmaterial variations; however, in the event the wrong raw

material is used, the processor should be able to identifyany raw material inconsistencies immediately to mini-mize operating losses. Critical raw material properties fora particular application need to be identified and charac-terized so incoming materials are tested only for theproperties that affect the final part performance. Criticalproperties may be viscosity, long-term heat aging, color,tensile properties, or other parameters, depending on par-ticular end-use application.

1.2 Raw Mater ia l B lendinga n d Mix ing

Depending on the product requirements, some pre-blending or ingredient mixing may be required prior toextrusion. (Blending and mixing are covered in more detailin Part 5, "Auxiliary Equipment.") Unless a single poly-meric material is being added to an extruder, the best wayto combine different raw materials and keep them uni-formly distributed prior to entering the extruder feed throatdepends on different factors. Some factors to consider are

Separation of powder and pellets Uniform distribution of additives introduced at low

concentrations Separation of ingredients in flood fed hoppers Proper mixing Introduction of different levels of regrind and/or

the effect of regrind particle size Addition of liquid additives to a single screw

extruder Uniform distribution of powder/powder blendsThe best way to meter materials and guarantee uni-

form component distribution is to gravimetrically feedeach component with different feeders directly above theextruder feed throat. Assuming there are enough spaceand feeders to accommodate the various components inthe formulation, gravimetric or loss-in-weight feedingensures each component is added in the correct propor-tion, while addition directly above the feed throat mini-mizes any ingredient segregation. The downside of thisapproach is the cost of gravimetric feeders, the spacerequired if there are more than four or five components,and if different size feeders are required. Assuming somecomponents are added in very low concentrations (< 1%)while other components are added in high concentration(>15%), the feeder size, feeder accuracy, and material(powder, pellets, flake, free-flowing versus compressivepowder, fiber, etc.) being fed are critical to the feeder per-formance. If all feeders are properly sized, designed forthe materials being fed (single screw feeder, twin screwfeeder, vibratory, weigh belt, etc.), and there is enough

Package Size, Pounds

50-55

300

1,000

4,000

40,000

150,000-220,000

Type Package

Bags

Fiber Pack

Gaylord

Bulk Pack

Hopper Truck

Rail Car

room to use a gravimetric feeder for each component,multiple feeders is the best method to ensure a repeatable,uniform formulation is being introduced to the extruder.

In many applications, a feeder is not available foreach ingredient, requiring preblending. Blendingdepends on the ingredients being mixed and the waymaterial is handled after blending and prior to extrusion.Assume pellets A and B are approximately the same sizeand are required to be premixed; proper concentrations ofA and B are individually weighed and added to low inten-sity blending systems. Typical low intensity blending sys-tems include tumble blenders (wide range of sizes), V-cone blender, ribbon blender, cement mixer, drum roller,or paint shaker for small lots. The same equipment can beused to mix pellets and powder. However, pellets andpowder are more likely to separate when transporting theblend or loading it to a feed or extruder hopper after theblending is complete. The powder can flow between thepellets; consequently, at the beginning of an extrusion runthe product may be rich in the powder component, whileat the end of the extrusion run the product may be rich inthe pellet component. One method to minimize this sep-aration is to coat the pellets with a small amount of liq-uid such as mineral oil to provide a surface to which thepowder can adhere. Of course, experimentation isrequired to verify the mineral oil does not affect the finalproduct properties or performance.

Powder/powder blends can be mixed either in lowintensity mixers described above or in high intensity mix-ers. High intensity mixers operate on the same principleas kitchen blenders. A mixing blade rotates at high speed,forming a vortex in the blender as it mixes the compo-nents. Due to the intense mixing, heat is generated andcare must be taken not to melt the blend components.High intensity blenders may be jacketed to heat compo-nents or remove heat during the blend cycle. With PVC,heat softens the particle surface, allowing the heat stabi-lizers and plasticizers to adhere to the surface. Powder/powder blends, once properly mixed, tend not to separateduring transfer, assuming the particle sizes of the differ-ent components are similar.

Uniform additive addition at low concentrations cre-ates a mixing and blending challenge. Obviously, the bestmethod is to feed each component directly into the feedstream with a small gravimetric feeder. However, this isnot always practical or feasible. An alternative approachis to mix the additive (assume it's a powder) with someresin powder being used in the formulation and producea masterbatch on a high intensity mixer. As an example,assume two additives, C and D, must be added at 0.5%and 0.08%, respectively, to resin B to produce a profile ofmaterial Z. A blend or masterbatch is produced by com-bining resin powder B with high concentrations of C andD and letting that blend down in an individual feeder. The

masterbatch is added using feeder #1, and pellets of B areadded via feeder #2 to produce the correct ingredientsratio in the final product. A 100-pound masterbatch isproduced containing 10 pounds (10%) of component C,1.6 pounds (1.6%) of component D, and 88.4 pounds(88.4%) of resin B. This masterbatch is let down in a 19:1ratio, with resin B feeding at a 190 lbs/hr rate and themasterbatch feeding at a 10 lbs/hr rate to produce the cor-rect additive ratio in the final product Z. If components Cand D were fed directly to the extruder at 0.5% and0.08%, the feed rate for each component would be 1.0lbs/hr for component C, 0.16 lbs/hr of component D, and198.84 lbs/hr of resin B to produce a 200-lbs/hr rate ofproduct Z. Using a masterbatch makes feeding small con-centrations of ingredients uniformly more practical.

The addition of a liquid colorant or other additive to asingle screw extruder can be difficult. If the liquid isadded in low concentration, it can be preblended with pel-lets in a tumble blender, ribbon blender, etc. Liquid can beintroduced into the feed throat with a liquid feed pump,assuming the liquid does not create extruder feed prob-lems due to pellet slippage on the barrel wall. It is criticalto monitor the liquid feed rate very carefully. When usinga gravimetric or loss-in-weight liquid feed pump, thepump rpm changes to keep the gravimetric feed rate con-stant. However, assume a volumetric liquid feed pump isbeing used (runs at constant rpm); the feed rate is depend-ent on the liquid temperature, which affects its viscosityand consequently the feed rate. Initially, a volumetric liq-uid feed pump must be calibrated and a graph generatedshowing motor rpm versus output rate in lbs/hr. On thesame graph, throughput rate curves versus motor rpmcurves need to be generated at different temperatures. Ifthe liquid temperature changes during the run, the feedrate will vary and the liquid concentration will changeover time. Assuming the liquid additive and feed pump areclose to the extruder, it is possible the liquid temperaturemay increase during the run as the ambient temperature inthe room increases due to the heat generated by theextruder. This results in an increase in the liquid feed rateand the wrong product formulation, unless the liquid feedpump rpm is changed to compensate for the temperaturechange or a gravimetric liquid feed pump is being used.

An alternative to feeding liquid into the feed throat isto pump the liquid into a two-stage extruder vent port.Modifying the vent port to accept a liquid injection noz-zle connected to the liquid feed pump and a two-stagescrew to accept the extra volume are the changes re-quired. This approach eliminates the potential for feedproblems associated with pellet slippage on the barrelwall in the feed zone due to the liquid. Feeding down-stream does minimize mixing in the extruder and re-quires an appropriate screw design or static mixer (dis-cussed later) to accomplish the mixing objectives.

1.3 Dry ing

Some polymers require drying prior to extrusion toprevent polymer degradation. Resins, e.g., nylon, poly-ester (polyethylene terephthalate [PET] and polybutyleneterephthalate [PBT]), and polycarbonate, are very hygro-scopic, absorbing moisture rapidly from the air. At extru-sion temperatures, moisture degrades these materials tolower molecular weight polymers, resulting in poorerproperty performance. Proper drying to eliminate mois-ture is critical to obtain the optimum property perform-ance in the final product. Other materials, e.g., acrylics,Ultem, polysulfone, Noryl, and acrylonitrile butadi-ene styrene (ABS), also absorb moisture from the air andmust be dried prior to processing. Any moisture in thepolymer is converted to steam in the extruder and,depending on the quantity present, can cause surfaceimperfections such as splay, holes in the product, or afoamy product. Some polymers, e.g., nylon, are shippeddry in moisture-proof containers. With proper handling,these resins do not normally require additional dryingprior to processing. However, if the seal is broken on thecontainer or the bag is not completely resealed afteropening, the product will absorb moisture and have to bedried prior to extrusion. Polyesters are particularly sensi-tive to moisture and must be dried in dehumidifying dry-ers, transported with dry air, and blanketed with dry airor nitrogen in the extruder feed hopper. Dehumidifyingdryers with -400F (-400C) dew points are recommendedfor drying most polymers. Dryers are covered in moredetail in Part 5, "Auxiliary Equipment."

Formulations requiring both a dry polymer resin plusblending with other ingredients can lead to special han-dling requirements. Once moisture-sensitive resin isdried, it picks up moisture when exposed to the atmos-phere. Additives or other components added to formula-tions containing hygroscopic resins need to be moisture-free. If the additives cannot be dried with the resin,special handling procedures or individual feeders arerequired to mix the dry resins and other additives or com-ponents at the extruder feed throat.

In some instances, resins containing moisture can beprocessed in a vacuum-vented extruder, with the mois-ture removed in the vent section. This does not work withall resins because some degradation can occur before themoisture is removed. One negative to this approach (dis-cussed later) is the extruder is effectively shortened byapproximately one-third its length, which may limitextruder throughput capacity.

Overdrying must be avoided to prevent resin degra-dation resulting in the loss of properties and/or the devel-opment of color bodies. Nylon 6,6 when overdried,becomes yellow and is accompanied with a loss of someproperties.

1.4 Feed ing Polymer to the Extruder

There are basically four ways to feed polymer to asingle screw extruder:

Flood feed Starve feed Crammer Melt feedOf these, the most common is flood feeding, where

resins or preblended materials are placed directly in a hop-per over the feed throat, allowing gravity and the screw tofeed the formulation to the extruder. A feeder with numer-ous hoppers to add different ingredients in the formulationsimultaneously can replace the single-feed hopper. Eachcomponent is metered in the correct ratio to the feed throat.With flood feeding, the extruder throughput rate is directlyproportional to the screw speed; higher screw speeds givehigher throughput. Figure 1.2 shows a flood-fed extruder.

Figure 1.2. Flood-fed extruder.Starve feeding is typically used in twin screw extru-

sion but can be employed with single screw extrusion.Feeders deposit the formulation directly onto the extrud-er screw, with the screw speed set to remove the formu-lation at higher rate than it is deposited on the screw.There is no material build-up in the extruder feed throat,and the throughput rate is determined by the feed raterather than the extruder screw speed. Starve feeding hasthe advantage of depositing all the formulation ingredi-ents in the proper ratio directly onto the extruder screw.Feed problems due to bridging and funneling in thefeed hopper or slippage on the barrel in the extruder are

Feeders on mezzanineabove extruder

Figure 1.3. Starve-fed extruder.

Figure 1.4. Crammer-fed extruder.as additional material is forced into the extruder. Initiallycare must be taken not to overfeed the extruder to theextent the extruder is unable to melt the polymer, forcingunmelted resin into the metering section. In extreme situ-ations overfeeding can break the extruder screw. Whilethis positive feed system provides a good mechanism toincrease throughput rates, caution must be used duringstart-up to minimize potential overfeeding problems.

Some extrusion operations use melt fed from anotherextruder, a Banbury, roll mill, or Farrel ContinuousMixer (FCM). A melt-fed extruder is normally shorterin length, because it is not required to melt the polymer.Basically, the extruder is a pump that generates a uniformpolymer melt temperature and pressure for the die. Fig-ure 1.5 shows a melt-fed extruder.

Feed

Figure 1.6. Akron Milacron single-screw extruder.

1.5.1 Shaping and DrawingThe last step in the extruder shapes the extrudate into

the desired cross section. As the extrudate exits the die, thepolymer molecules, which were oriented in the die landarea, relax and reentangle, causing die swell. If the extru-date is allowed to droll out the die, the cross section swells,becoming larger than the die opening due to polymerrelaxation. Pulling extrudate away from the extruder, witha puller farther down the line, orients the polymer molecu-lar chains in the machine direction. Neck down or extru-date draw down is induced by this pulling action. The drawdepends on the puller speed relative to the extruder output.Draw ratio is directly related to molecular orientation,resulting in higher tensile and flexural properties in themachine direction compared to the transverse position.

With a given die cross sectional area, there is onepuller speed at a given extruder throughput rate that pro-duces a product with the correct cross sectional dimen-sions. If the extruder throughput is increased, the pullerspeed must be increased proportionally to maintain thesame finished product dimensions. Likewise, if thethroughput is decreased, the puller speed must bedecreased proportionally to maintain the same finishedproduct cross sectional area. The draw ratio and molecular

FCM

Extruder

Figure 1.5. Melt-fed extruder.

Extruder

Crammer

typically eliminated. Feeders are normally setup directlyabove the feed throat or placed on a mezzanine above thefeed throat to deposit materials directly on to the screw.Figure 1.3 shows a typical starve-feeding setup with twofeeders on a mezzanine above the extruder.

Crammer feeding, shown in Figure 1.4, is a positivefeed system that works well with low bulk density materi-als, materials that tend to bridge, and other hard to feedmaterials. A screw mechanism inside the crammer posi-tively conveys material to the extruder. Extruder through-put rates are significantly increased with crammer feeding

1.5 Extrus ion

After feeding, polymers are melted or plasticated, con-veyed forward, melt mixed, and formed into a shape. Thesefive operations within the extruder will be discussed indetail in Chapter 4, "Plasticating Behavior in the Extruder."Proper operation in each stage will produce acceptableproduct at high yield with proper aesthetics and the correctproperty balance. Figure 1.6 shows a smaller AkronMilacron single screw extruder with control cabinet.

orientation can only be increased or decreased by changingthe die cross sectional area relative to the puller speed,assuming the final product dimensions are kept constant.This is easily done with sheet dies, cast film dies, or blownfilm dies that have adjustable die lips. Profile dies mayhave a fixed cross sectional opening that is not adjustable;at a given throughput rate there is only one puller speedthat yields a product with the correct final dimensions. Aproduct that tends to crack or break in the machine direc-tion (in the plant or in field applications) may have toomuch molecular orientation. A new die with a differentcross sectional opening is required to alter the draw ratioand change the molecular orientation to correct the prob-lem. Higher draw ratios increase the tensile and flexuralproperties and the tendency to crack or split in the machinedirection. Assuming most polymer molecules are alignedin one direction, it is easy to slit the product in that direc-tion, because the molecules oriented in the perpendiculardirection holding the product together are limited.

Extrudate swell, commonly known as die swell, shownin Figure 1.7, is not always visible at the die exit becausethe extrudate is pulled away from the extruder, causingdraw down or neck down. If the extrudate is allowed todroll on the floor or is pulled from the extruder veryslowly, die swell becomes veryobvious. Polymer molecules in thedie land area are oriented in the flowdirection. The extrudate velocityprofile is higher at the center of theflow front and lower near the diewalls. Immediately after exiting thedie, the extrudate velocity profile isidentical across the entire cross section. Consequently, thevelocity at the extrudate surface outside the die is identicalto the velocity in the center of the extrudate. This changein the flow velocity profile gives rise to molecular relax-ation outside the die and the resultant extrudate swell.

As the extrudate exits the die, it is quenched and pos-sibly sized (drawn through a fixture) to maintain its finalshape. Depending on the extrusion process, differentmethods are available to quench the final product. Castfilm and sheet are quenched on rolls; blown film isquenched by air in a blown film tower; profiles, pipe, andtubing are quenched in calibration tanks filled with waterand in some cases connected to a vacuum system; strandsand monofilaments are quenched in water baths; wirecoating is done horizontally in air or water; and large partblow molding is quenched in molds.

1.5.2 Solidification and CoolingExtrudate cooling is normally accomplished with

water, air, or contact with a cold surface. Semicrystallinepolymers, i.e., polyethylene, polypropylene, nylon, poly-butylene terephthalate, etc., have very sharp melting

points and consequently very sharp solidification temper-atures. Amorphous polymers, on the other hand, do notmelt but enter a rubbery state above their Tg (glass transi-tion temperature, discussed later). As the temperatureincreases, polymer chain mobility continues to increaseuntil the polymer flows and is easy to process. Whenamorphous materials cool, the part temperature needs tobe below the material Tg to ensure the final part dimen-sions. Thick cross sections form a surface skin while thecenter is still molten. This allows the extrusion line to berun at higher rates; however, if product dimensional toler-ances are very tight, the entire product should be cooledbelow the melting point if it is a semicrystalline polymerand below the TQ if it is an amorphous polymer beforeexiting the cooling medium. Cooling from elevated toroom temperature after the product is completely solidi-fied results in additional product shrinkage and dimen-sional changes.

Proper part cooling is critical to produce warpage-free parts with the acceptable dimensions and perform-ance. Part warpage is caused by differential shrinkage. Tominimize differential shrinkage, the part must be cooleduniformly on all sides. If one side or area of the extrudatesolidifies before another, the part will warp, bending

toward the side that solidified last. If one side of theextrudate is dragged over an object in the cooling opera-tion, molecular orientation is induced on that side, caus-ing it to shrink differently from the other side, leading towarpage. Warpage is discussed in more detail in Part 4,"Troubleshooting the Extrusion Process."

Cooling rates with semicrystalline polymers are criti-cal to develop the correct amount and crystal size in thefinal product. Rapid quenching leads to small crystalgrowth development and low crystallinity. Heating orannealing later (heated for a specific time and temperatureabove its Tg) leads to additional crystal growth in the solidstate. Accompanying any increase in crystallinity is areduction in volume, a change in the part dimensions, andpossibly warpage. To maximize the crystallinity and crys-tal size, the extrudate should be cooled slowly. Coolingrates can be critical in maximizing product performanceand reproducibility. Cooling rates are determined bythroughput rates, part thickness, and cooling mediumtemperature (water bath, roll, or air temperatures).

Drawing products in the solid state (monofilamentproduction, oriented film, or biaxially oriented film)

Figure 1.7. Die swell and draw down.

Die Die Swell Draw Down

DieLand

Ext ruder

maximizes molecular orientation and directional proper-ties. In semicrystalline polymers, drawing can lead toadditional crystallinity development through molecularalignment.

In some extruded products, the cooling rate and treat-ment during cooling are critical to maximizing the finalproduct properties required by the customer. In sheet orcast film, roll stack temperatures and surfaces determineproduct aesthetes. Highly polished rolls run at relativelyhigh temperatures produce polished, glossy surfaces. Amatte finish on the product is obtained by using rolls witha matte finish, and a matte finish on one side and a pol-ished, glossy surface on the other are produced by usinga matte finish and a highly polished roll. A vacuum siz-ing tank is used for hollow profiles or pipe and tubing,where the extrudate is run through sizing rings underwater with a vacuum above the water. The fixturing andcooling required to maintain final dimensions dependson the application.

1.5.3 PullerThe puller controls the draw and tension on the mate-

rial from the extruder exit through the cooling and solid-ification steps. Final product dimensions are controlledby the extruder throughput rate and the puller speed. Witha fixed die opening and given throughput rate, there isonly one puller speed that producesthe correct product dimensions. Con-sequently, the puller speed must bematched to the extruder output rate. Ifperiodic variations occur in the pullerspeed or extruder output (due to surg-ing), the product dimensions continu-ously change. Slippage in the pullercan cause thicker sections in the partsthat do not meet finished productspecifications. A caterpillar typepuller is shown in Figure 1.8.

Pressure exerted by the puller mustbe sufficient to prevent product slip-page in the puller, but low enough toprevent part distortion or marks on theproduct surface. Extreme puller pres-sure can crush the final part, renderingit useless. The puller may be a longdistance from the extruder; however, itmust be properly aligned with theextruder to prevent the part from beingpulled in one direction or another,inducing molecular orientation thatmay lead to warpage.

Dimensional variations in the final product normallyresult from the extruder (surging, power input variations,slippage on the screw, poor feeding) or the puller (slip-page, improper compression on the part, or power outputvariations).

1.6 S e c o n d a r y Opera t ions

Numerous secondary operations are performed inline to minimize product handling and improve produc-tion efficiencies. Some secondary operations done in lineinclude cutting to length, drilling or punching holes,corona or flame treatment, decorating (painting, printing,gluing something to the surface, etc.), attaching adhesivelabels, welding, etc.

1.7 Inspect ion , Packag ing ,a n d Sh ipp ing

Visual inspection or gauging is done at the end of theline to verify all parts meet specification. Using statisticalprocess control (SPC) quality control guarantees productsmeet specifications, and visual inspection can be elimi-nated. The problem associated with visual inspection is

some defective parts always passthrough the system because of humanerror. Visual, subjective part inspectionshould be eliminated as much as possi-ble by installing other quality controlmethods to assure all parts meet cus-tomer specifications. In addition tovisual inspection, part weight and/ordimensions can be checked prior topackaging. Proper SPC procedureseliminate much QC work associatedwith product quality assurance.

Retained samples of each produc-tion lot should be stored, in the eventof a customer complaint. Physicalproperty verification or evaluation ofdeficiencies identified by the customercan be monitored or tested on both thecustomer's parts and the retained sam-ples to assist in the identification andcorrection of the problem.

The final steps in the extrusionprocess are to package the productaccording to customer requirementsand ship the parts.Figure 1.8. RDN puller.

Caterpil lar Puller

Review Questions

1. What controls product dimensions?

2. What part of the extrusion process contributes to final part warpage?

3. Name three factors that might lead to part warpage.

4. What are the different methods of feeding polymer to a single screw extruder, and whatcontrols the extruder feed rate in each method?

5. In the extrusion process, which steps affect physical properties and how?6. What materials need to be dried prior to extrusion?7. What are some methods of blending polymers? What are the best methods to use for

blending powder and pellets, pellets and pellets, powder and powder?

8. What is a masterbatch?

9. What is die swell?10. If the production rate is 300 pounds/hour and it is necessary to feed 0.07% of component X,

2% of component Y, and 1.2% of component Z with 96*73% of polymer pellets L, what isthe best way to mix and add the material to an extruder?

11. At what step in the extrusion process is the product worth the most money?

Front MatterTable of ContentsPart I. Single Screw Extrusion1. Extrusion Process1.1 Raw Material Supply1.2 Raw Material Blending and Mixing1.3 Drying1.4 Feeding Polymer to the Extruder1.5 Extrusion1.5.1 Shaping and Drawing1.5.2 Solidification and Cooling1.5.3 Puller

1.6 Secondary Operations1.7 Inspection, Packaging, and ShippingReview Questions

2. Extruder Safety3. Single Screw Extruder: Equipment4. Plastic Behavior in the Extruder5. Screw Design6. Processing Conditions7. Scale Up

Part II. Twin Screw ExtrusionPart III. Polymeric MaterialsPart IV. Troubleshooting the Extrusion ProcessPart V. Auxiliary EquipmentPart VI. CoextrusionPart VII. Extrusion ApplicationsIndex

Extrusion - Part I/14735_02.pdfSafety is each employee's responsibility, rangingfrom the janitorial staff to company president. It is theresponsibility of each associate to work safely and assistother employees to operate safely, endeavoring to elimi-nate all unsafe acts that lead to major accidents. Of allaccidents, 96 percent are caused by human error, care-lessness, or the attitude, "It won't happen to me." Conse-quently, our personal safety plus the safety of thosearound us is the responsibility of each employee. It isessential to obey all work area rules and be alert forunsafe acts and conditions. Besides working safely, it isimportant to encourage those around you to work safely.Before a job is started, it needs to be thought throughcompletely and determined if it can be done safely. If itcan't be done safely, don't do the job until you obtain theproper equipment or develop the proper procedure to doit safely. It is important to realize the hazards associatedwith each job and not take any shortcuts that might putyou or your associates in the way of potential danger andserious accidents.

Training new employees must include safety trainingplus the hazards associated with all equipment. In addi-tion to potential equipment hazards, new employees needto know the location of all safety equipment (fire extin-guishers, fire blankets, first aid, who to contact in anemergency, etc.) and understand the correct procedures tofollow in the event of an accident or injured employee.Who should be contacted? What is the procedure forreporting accidents? What is my responsibility? Where isthe muster point? What do different alarms mean, andwhat is my response?

Associates need to help each other. If you see fellowemployees performing unsafe acts, help them understandproper procedure and why what they were doing is unsafe.This is an act of caring and concern for our fellow employ-ees, not an act to belittle or make somebody look foolish.

The most important step in safety is to understand thepotential hazards, realize you are not invincible, and itcan happen to you. Follow procedures and think any jobthrough thoroughly before starting to evaluate the poten-tial for injury to yourself and others. Don't be the bull inthe china shop, charging ahead without thought. If a jobcan't be done safely, don't do it until procedures,methods, or equipment is available to do it safely.

2.1 H a z a r d s A s s o c i a t e dw i t h a n E x t r u d e r

The three biggest potential safety hazards associatedwith extruders are burns, electrical shock, and falls. With-out proper protective equipment, burns can be common-

place among employees working around extruders.Touching a hot die or handling extrudate without glovesnormally causes burns. Long sleeves with properlyapproved thermal gloves should be worn when workingaround the die, changing the die, tightening die bolts, orother functions performed on the die. If insulation isplaced around the die, make sure it is in good shape andproperly installed. Hot extrudate from the extruder willstick to your skin. Since polymeric materials are greatinsulators, after sticking to the skin they cool very slowly,continuing to burn the skin affected.

Never stand in front of a die when a single screwextruder is starting up. Air in the extruder and possiblygas from degraded products (if the extruder has been sit-ting at extrusion temperature with material in the barrelfor some time) is forced out of the extruder on start-up. Ifsome polymer is left in the barrel, trapped air can be com-pressed, blowing the hot polymer out of the die. Standingin front of the extruder creates an unsafe condition wheremolten polymer can be blown out of the die, land on you,and burn you. Polymer can stick to gloves, where it holdsheat for a long time, and can burn you through the glovesif the proper type of glove is not used. When removing thedie and/or screw from an extruder (they are normally hot),wear the proper protective equipment (heavy duty glovesand protective thermal sleeves) to prevent burns. Dies canbe heavy; therefore, a back brace or other equipment tolift and hold the die can prevent back injuries.

The potential for electrical shock exists whenimproperly trained employees remove the extruder cov-ers, thus exposing bare wires and electrical connections.Extruder heater bands are normally 220 or 440 volts andcan cause serious electrical shock. Check the wires to theheater bands on the die and adapters to assure there arenot frayed, bare, or exposed wires that can cause electri-cal shock. In some extrusion processes, water-coolingbaths are very close to the die, which can create addi-tional electrical hazard. Unless properly trained, opera-tors should never remove guards, exposing electrical ter-minals on heaters or open electrical cabinets, to solveelectrical problems.

The third major potential safety hazard aroundextruders is falls. Pellets spilled on the floor are slipperyand need to be cleaned up immediately. At start-up theextruder normally generates some scrap, which may beon the floor around the die. This creates tripping hazardsthat must be removed immediately. Occasionally pro-cessing issues arise at start-up, leading to too much mate-rial on the floor around the extruder. In these situations,the extruder should be shut down, the area cleaned, andthe extruder restarted. Some extrusion processes usewater for cooling. Water spills on the floor should be

2 E x t r u d e r S a f e t y

removed with a wet/dry vacuum or squeegeed to a drain.Wet floors are very slippery and can cause falls.

The most dangerous area around an extruder is theexposed screw turning in the feed throat. Never, neverstick your hands or fingers into the feed throat. If thescrew is turning, there is incredible power that can quick-ly remove a finger. If the feed throat is hot, you may alsoget burned.

The most dangerous time during extruder operation isstart-up. An extruder is a pressure vessel. Material isbeing fed into one end with a positive conveying mecha-nism (screw) operating at high horsepower. If the die endof the extruder is blocked with solid plastic or contami-nants, incredible pressure can build up very rapidly andblow the die off. Always start the extruder screw slowlyand monitor the die pressure closely until polymer isflowing continuously. Once die flow is established, thescrew speed can be safely increased. As mentioned pre-viously, never stand in front of an extruder during start-up in case molten plastic is blown out of the die underhigh pressure.

Extruders are equipped with rupture disks to preventhigh pressure form blowing the die off the end and pressuregauges (discussed later) to monitor the pressure in andbefore the die. Make sure the pressure gauges are function-ing properly. If the extruder does not have a rupture disk atthe extruder head to relieve high pressure, it should have apressure gauge with a feedback loop that automaticallyshuts the extruder down when a preset pressure is reached.Most modern extruders are equipped with both a rupturedisk and high-pressure sensor that will shut the extruderdown in the event of high-pressure situations. Having boththe rupture disk and pressure sensor will protect in the sit-uation where on heat-up, cold spots trap melting andexpanding polymer in the head and melt pipe area. It is pos-sible to exceed 20,000 psi in a pool of trapped polymer.

Each extruder should be equipped with a fume hoodat the die or vent port to remove any fumes generated.

2.1.1 Hazards Associatedwith Takeoff Equipment

The safety hazards associated with takeoff equipmentdepend on the extrusion process and takeoff equipment.Pinch points associated with nip rolls, pullers, and rollstacks are one potential safety hazard requiring carefuloperation. If two operators are running equipment con-taining nip rolls, they must communicate to verify alloperators are clear when nip rolls are closed. Loose-fittingclothing that can be caught in nip rolls or pullers must beavoided. Some lines have rolling knives or knives for edgeslitting. These should be guarded, and operators must usecaution when working in those areas.

High-speed rotating rolls present special hazards.Guards around all rolls and nip points must be kept in

place to prevent injury. Arms, fingers, and hands caneasily be pulled into high-speed rolling equipment, causingsevere personal injury or dismemberment.

Scrap from start-up lying on the floor poses a trippinghazard. This should be picked up and disposed as soon asthe line is running. If start-up problems continue and pre-vent clean up, the equipment should be shut down, the areacleaned, and the line restarted.

Noise above 80 dB requires that hearing protection beused by all people in the area. If the noise level is below 80dB, employees may still want to wear hearing protection toprevent long-term hearing loss.

Like the extruder, identify potential safety hazardsassociated with the takeoff equipment. Form a plan toavoid potential hazards. Know where all emergency stopbuttons are and verify that they work. Don't take theapproach, "It Won't Happen to Me."

2.1.2 Personal Protective EquipmentPersonal protective equipment exists to make your

job safer. Determine what equipment you need to do yourjob safely and use it. Following is some of the personalprotective equipment available:

Safety glasses with or without side shields Safety shoes Ear protection Gloves Thermal insulated gloves for hot applications Long sleeves Hard hats Face shield Goggles Back brace Wrist brace Floor mats

2.1.3 Lock-Out, Tag, and Clear ProcedureAnyone working on equipment should have a person-

al lock with his or her name on it and the only key. Priorto doing maintenance or other work on the equipment,turn off the power switch and lock out the switch withyour personal lock. Employees working on the line mustattach their own lock. After locking out and tagging theequipment prior to doing any work, each worker attemptsto start the equipment to verify it is off and cannot beturned on. Once the maintenance or other work is com-pleted, each worker removes his own individual lockbefore the equipment can be restarted. This procedureprevents somebody from getting hurt while working onequipment when another person inadvertently starts theequipment.

2.2 Proper Training

Don't run equipment without proper training andunderstanding the potential safety hazards associatedwith the operation. Know where you can get hurt andunderstand how all the equipment and controls operate.Training includes start-up and shutdown procedures,understanding the caution or warning signs on themachines, and operating time on the equipment with anexperienced operator.

2.3 Inspect ion a n d H o u s e k e e p i n g

Before each shift, evaluate the operating area andplant in general, looking for unsafe conditions, e.g., trip-ping hazards, exposed wires, water on the floor, etc.Determine what you are going to do on your shift andreview the operation for safety.

Good housekeeping is directly related to safety. Acluttered, dirty area will lead to accidents and reflects onyour attitude toward the job. A proper storage area for all

tools and equipment makes the job easier and the plant abetter place to work.

2.4 Mater ia l Safety

Understand the materials you are using by reviewingthe Material Safety Data Sheets (MSDSs). Improper oper-ating conditions or purging with the wrong materials canhave serious consequences. Overheating polyvinyl chloride(PVC) generates hydrochloric acid (HCl), which attacks thelungs and rusts plant equipment. Never mix acetal withnylon, PVC, fluorinated polymers, or ionomer in an extrud-er, as they will react and give off formaldehyde.

PVC has limited thermal stability and should not beleft in a hot extruder. PVC degrades in an autocatalyticreaction, generating HCl. Proper purge material should beavailable to remove PVC from the barrel if the extruder isgoing to be down for an extended time. Operators whohave the flexibility to change extruder temperature pro-files need to understand the upper processing limits whenextruding PVC or other temperature-sensitive polymers.

Review Questions

1. What is the most dangerous time during extrusion and why?

2. Where are the most dangerous locations around an extruder and why?

3. What are some potential hazards associated with extrusion?

4. What is the "lock out, tag, and clear" procedure, and when should the procedure be used?

5. Why is housekeeping important?

6. What is a near miss?

7. What hazards are associated with takeoff equipment?

8. What is some personal protective equipment?

9. What materials should not be mixed with acetal in an extruder?

10. What happens if PVC is overheated?

Front MatterTable of ContentsPart I. Single Screw Extrusion1. Extrusion Process2. Extruder Safety2.1 Hazards Associated with an Extruder2.1.1 Hazards Associated with Takeoff Equipment2.1.2 Personal Protective Equipment2.1.3 Lock-Out, Tag, and Clear Procedure

2.2 Proper Training2.3 Inspection and Housekeeping2.4 Material SafetyReview Questions

3. Single Screw Extruder: Equipment4. Plastic Behavior in the Extruder5. Screw Design6. Processing Conditions7. Scale Up

Part II. Twin Screw ExtrusionPart III. Polymeric MaterialsPart IV. Troubleshooting the Extrusion ProcessPart V. Auxiliary EquipmentPart VI. CoextrusionPart VII. Extrusion ApplicationsIndex

Extrusion - Part I/14735_03a.pdfI n t r o d u c t i o n

What are the objectives or goals to be accomplishedwith the extruder in the overall extrusion process? Thestandard answer is to produce a quality product thatmeets the customer specifications 100% of the time.While this is true, the extruder has five distinct goals orobjectives to achieve in the extrusion process that willresult in a quality product if done correctly:

Correct polymer melt temperature Uniform/constant melt temperature Correct melt pressure in the die Uniform/constant melt pressure in the die Homogeneous, well-mixed productThe next two chapters focus on the extruder equip-

ment and polymer behavior in the extruder. To optimizean extrusion process, it is not sufficient to simply under-stand the equipment, how it operates, and how it func-tions; it is essential to understand the polymers and theirreaction and behavior in the various process steps. Toeffectively troubleshoot an extrusion process, the equip-ment and its interaction with the material, along with thematerial properties, must be understood. This chapterfocuses on single screw extruder equipment, and Chapter4 focuses on the plasticating behavior in the extruder.

3.1 E q u i p m e n t

The simplest extruder is a ram extruder, shown in Fig.3.1. Pressure is applied to a piston, forcing the extrudateout the die. Heat applied to the barrel melts the materialand lowers its viscosity. With the correct combination ofheat and pressure, extrudate is forced out the die in thedesired shape. There are many problems associated withthis extruder. First, it is a discontinuous process. Second,plastic is a great insulator; consequently, it takes a rela-tively long time to heat the billet uniformly from the bar-rel surface to the center. Using high barrel temperature toincrease the melting rate tends to degrade the resin at thebarrel wall before the billet is molten. Lowering heater

Force Applied to Piston

Figure 3.2. Single screw extruder.The drive system comprises the motor, gear box, bull

gear, and thrust bearing assembly. The feed system is thefeed hopper, feed throat, and screw feed section. Thescrew, barrel, and heating systems are where solid resin isconveyed forward, melted, mixed, and pumped to the die.Extrudate is transported and shaped in the adapter and die,respectively. Finally, the control system controls theextruder electrical inputs and monitors the extruder feed-back. Computer-designed extrusion controls not only runand monitor the extruder, but also can control the entire

extrusion process withfeedback loops that auto-matically change feedersettings, puller speeds,screw speeds, etc., tomaintain product quality.

Extruders are soldbased on screw or barreldiameter and the length (L)to diameter (D) ratio,called L over D or L/D, ofthe barrel. In the UnitedStates, extruder diametersare measured in inches;other countries use mil-limeters. Typical diametersare shown in Table 3.1.

3 S i n g l e S c r e w E x t r u d e r : E q u i p m e n t

band temperatures to minimize degradation requires longheat soak times to soften the billet properly. Third, shearheating from the ram movement is minimal.

The key components in a single screw extruder areshown in Fig. 3.2. A single screw extruder has five majorequipment components:

Drive system Feed system Screw, barrel, and heaters system Head and die assembly Control system

ScrewCooling

ThrustBeanng Screw

HeatersPressureGauge

Die

BreakerPlateMotor

CoolingFans Barrel

Table 3.1. Inch and mmExtruder Size

Piston

Barrel

Die

Billet (Materialto be Extruded)

ExtrudateFigure 3.1. Ram extruder.

< 4" (100mm)inch0.50.751.0

1.251.51.752.02.53.03.5

mm1520253035

4550607090

> 4" (100mm)inch4.56

810121416182024

mm120150180220250300350400450500600

Note that the match-up betweeninch and mm is only approximate.There are several standard sizesup to 10 inches (250 mm) in dia-meter. Larger sizes are usuallycustom built according to cus-tomer requirements.

Extruder L/D describes therelative length of the screw andextruder barrel. The L/D ratio isgiven by Eqs. (3.1) and (3.2):

T / _ Flighted Length of Screw' D

Outside Diameter of Screw

T / _ Flighted Length of Screw Axial Length of Feed Pocket'

D Outside Diameter of Screw ^ '2^

Which L/D definition is used depends on the equip-ment manufacturer. Some manufacturers include theaxial feed pocket length as part of the barrel length, whileothers do not.

Throughput is directly related to the extruder L/D.Two extruders with the same diameter but different L/Dshave different throughput capacities. Longer extruders(higher L/D) have more melting and mixing capacity,allowing the extruder to be run at higher rates. Short L/Dextruders have the following advantages:

Less floor space required Lower initial investment cost Lower replacement part cost for screws and barrels Less residence time in the extruder when process-

ing temperature-sensitive materials Less torque required Less horsepower and corresponding motor size

Longer L/D extruders have the following advantages: Higher throughput because of screw design More mixing capability Can pump at higher die pressure Greater melting capacity with less shear heating Increased conductive heating from the barrelSome typical extruder L/Ds are 18:1, 20:1, 24:1,

30:1, 36:1, and 40:1.If one knows the L/D, one can calculate how long the

barrel is. If we have a 2.5-inch extruder with a 24:1 L/D,the length is calculated as follows:

Calculate the length of the barrel on a 4.5-inch diam-eter extruder with a 30:1 L/D.

Throughout rates are directly proportional to thescrew diameter. Larger diameter extruders have greateroutput. Figure 3.3 shows typical throughput charts fordifferent size single screw extruders.

3.2 Drive

The drive turns the screw at a constant speed over alarge speed range while supplying sufficient torque toprocess the polymer being used. Screw speed variationsare directly proportional to throughput variations, whichcan cause changes in product dimensions. In practice,screw speed variations are observed either by the screwrpm or the extruder motor load readout (normally givenin torque, percent load, or amps). Screw speed can bemonitored with a hand-held tachometer or read from thecontrol panel.

The most popular drive on large extruders is a DCmotor connected to speed reducers that convert motorrpm to screw rpm. To generate maximum torque, DCmotors are run at maximum rpm (1750 rpm). Twooptions for motors are standard DC motors and brush-less DC motors. At one time, all extruder drives werepowered with DC motors; however, AC motors are nowfound on smaller extruders. AC motors don't have to berun at maximum rpm to obtain maximum torque, andthe new flux vector AC drives can achieve torque andspeed control superior to DC brushless motors andmore economically. Flux vector AC drives have solidstate power switching, with a microprocessor control-ling both magnetizing current and torque-producingcurrent through vector calculations. The other option isfor a variable frequency AC drive with tachometer andencoder feedback.

Direct and indirect drive systems are used to transfermotor power to the screw. A direct drive system, shownin Figure 3.4, uses a quick-change gearbox to convert themotor rpm to the desired screw rpm. Changing the gear

Figure 3.3. Nominal extruder throughput.Single Screw Diameter, inches Single Screw Diameter, inches

Output, pounds/hour (Thousands)Output, pounds/hour

Figure 3.4. Direct drive.

ratio modifies extruder screw speed range. Before chang-ing the gear ratio to increase the extruder screw speed,verify that the motor has sufficient horsepower to gener-ate enough torque to process the quantity of plastic resinintended for use on the extruder at the higher screwspeeds. Changing gears without determining if the motorhorsepower is sufficient to process the new volume canresult in high screw speed with insufficient torque orpower to turn the screw when the extruder is full. Screwtorque or motor power is critical in melting the polymer(discussed in Chapter 4).

An indirect drive extruder, run with pulleys, is shownin Figure 3.5. While Figure 3.5 shows the location foronly two pulleys, an indirect drive extruder normally hasthree to five pulleys and belts. Belts are shipped inmatched sets and must all be changed simultaneously.

Figure 3.5. Indirect drive.

Belt slippage can result in throughput variations causedby screw speed fluctuations. To change the screw speedrange, the pulley sizes are modified. When changing pul-leys, make sure all pulleys are properly aligned beforerestarting the machine. As with the direct drive system,verify that the motor has sufficient horsepower to meetthe extruder torque requirements at higher throughputsbefore changing any pulleys.

The thrust bearing is located between the screw shaftand the drive output shaft. As the extruder screw rotates,it is attempting to twist itself out the back of the extrud-er. Combining this with the die head pressure, the screwis generating high forces on the thrust bearing. Figure 3.6shows the thrust bearing assembly. For every action thereis an equal and opposite reaction; in an extruder the loadon the thrust bearing is directly proportional to the headpressure and the screw diameter. Force on the thrust bear-ing is obtained by multiplying the screw cross sectionalarea times the extruder head pressure.

Figure 3.6. Thrust bearing assembly.

Below is a thrust bearing load calculation. Calculatethe typical force on the thrust bearing of a 4.5-inchextruder running at 5000 psi die pressure.

Screw Cross Sectional Area nxr2

= 3.1416x(2.25)2= 15.9 inch2

Force on Thrust Bearing = Pressure x Area= 5000psixl5.9inch2

= 79,500 pounds force

Thrust bearing life is rated in B-10 life, which ismeasured in hours. B-10 life assumes that 90% of thethrust bearings running at 5000 psi head pressure and 100rpm screw speed will exceed 100,000 hours or 10 yearsbefore failure. Ten percent of the thrust bearings runningunder these conditions fail within 10 years. If extrudersare run at higher pressure or speeds, the anticipated thrustbearing life decreases. Likewise, thrust bearing life isincreased at lower speeds and pressures.

Figure 3.7 shows a Davis Standard gear box with atransparent top, exposing the bull gear, other gear reduc-tion from the motor, part of the motor, and the belt driveunder the protective cover.

Shows one beltExtruders normallyhave multiple belts.

Figure 3.7. Davis Standard gear box, drive motor, and belts.

Motor

GearBox

BeitDrive

ThrusiBearing

Tapered Roller or Ball Bearing

Key

BearingBearings

Thrust Bearing

Motor

Bull Gear

Direct DriveQuick-Change Gear Box

Coupling

3.3 Feed

The two solid feed systems that rely on gravity areflood and starve feeding. Both feed systems have a hop-per sitting directly over the extruder feed throat with thehopper opening size matched to the feed throat opening.All dead spots in the hopper and feed throat are eliminat-ed to prevent polymer or additive build-up that can latercause cross-contamination or bridging. The feed throatsection, attached directly to the extruder barrel, is waterjacketed for cooling. In operation, water flow can bemeasured with a flow gauge on the cooling line returnfrom the feed throat or simply by feeling the feed throatarea and verifying it is not overheated. The feed throatshould feel warm to the touch but not hot. Figure 3.8shows a typical flood-fed extruder and the cooling chan-nels in the feed throat area. The purpose of water coolingis to prevent feed materials from softening, becomingtacky, and sticking together in the feed throat, causing

Cooling ChannelsFigure 3.8. Feed throat section of flood-fed extruder.bridging or premature melt blockage in the feed section.A good insulative barrier is installed between the barreland feed section to minimize heat transfer from the barrel.

Feed throat and hopper geometry allow material toflow freely into the extruder with minimum restriction.Standard feed throat design for pellets or powder is shownin Fig. 3.9, geometry A. Feed throat geometries B and C inFig. 3.9 are more appropriate for melt-fed extruders. Pel-lets fed in configuration B can wedge between the barreland screw, causing the screw to deflect.

Figure 3.9. Feed throat configurations for flood-fedextruders.

Grooved feed throats are popular in blown film andother applications to increase extruder output. Figure3.10 shows a grooved feed section from a large DavisStandard feed section. Notice how the grooves are deepin the beginning of the feed section under the feed hop-per and disappear just prior to going into barrel zone 1.

Figure 3.10. Davis Standard grooved feed throat.

The cooling channels around the feed section removefrictional heat generated by the rotating screw and pelletcompression into the screw channels, preventing the pel-lets from premature melting. The grooves shown in Fig.3.10 are in the axial direction but can also be helicalaround the feed section. The advantage of a grooved feedthroat is increased friction between the pellets and thebarrel wall, resulting in higher throughputs. Extruderswith grooved feed sections require three alterations:

Excellent feed throat cooling to dissipate the fric-tional heat generated and the capability of handlinghigh pressures (15,000 psi plus) in the groovedfeed section

A good insulative barrier between the barrel andthe feed section to minimize heat transfer

Lower compression ratio extruder screws to handlethe increased throughput rate

Normal screw compression ratios (discussed later)are approximately 1.5:1.

Other ways to feed (crammer and melt) are discussedin Chapter 1.

3.4 Screw, Barre l , a n d Heaters

The screw conveys material forward, contributing tothe heating and melting, homogenizing and mixing themelt, and delivering the melt to the die. The barrel andheaters help heat and melt the polymer by controlling thetemperature in the different zones, preventing materialfrom overheating and degrading. The screw, in combina-tion with the barrel, feeds polymer to the die, buildingpressure in the die.

Barrel hardware is shown in Fig. 3.11. Heaters arelocated along the barrel, with thermocouples in each zoneto control the heaters and barrel temperature. The heaters

Hopper Feed Throat

A B C

cover as much barrel surface area as practical,minimizing hot and cold spots along the barrellength. In an individual extruder temperaturezone, there may be one, two, or three heaterbands with one thermocouple controlling them.Assume the heater band closest to the thermo-couple burns out; the other two heater bands haveto supply all the external energy required, creat-ing the possibility that the area is hotter near thetwo heater bands that are working. In the eventthe band farthest from the thermocouple burnsout, the barrel area under the burnt-out heater isanticipated to be cooler than areas where the heaters arefunctioning properly near the controlling thermocouple.Burnt-out heater bands should be replaced as soon as pos-sible to assure uniform heat input. Thermocouples placedin the barrel wall penetrate as close to the barrel liner aspractical. Water- or air-cooling in each zone is used to con-trol the barrel temperature. At the extruder head prior tothe breaker plate, there is a pressure transducer to measurehead pressure and a rupture disk for safety, in case there isa sudden and/or unexpected pressure rise. Barrels may belined with a bimetallic liner to increase service life.

Barrels are fabricated from solid carbon steel or othermaterial. Nitriding about 0.3 mm deep hardens the insidebarrel surface. However, nitriding is not particularly effec-tive when running abrasive fillers such as glass, mineralfillers, or other fiber reinforcements. Stainless steel bar-rels with hardened interiors are an option for small extrud-ers. However, hardening stainless steel lessens the corro-sion resistance, and stainless steel is not a particularlygood heat transfer medium. A second approach toimprove corrosion or abrasion resistance in the barrel is touse a bimetallic coating. Coating is thicker (1.5-3 mm)than nitriding, providing better wear resistance. Table 3.2shows some coatings and their wear properties. The thirdTable 3.2. Bimetallic Coating

approach to improve abrasion or corrosion resistance is abarrel liner, which is a thin-walled tube of stainless steel,nickel-based alloy, or hardened carbon steel, inserted inthe barrel. Heat transfer may suffer slightly if there is anair gap between the liner outside diameter and barrelinside diameter. The barrel inside surface should be hard-er than the screw to prevent barrel wear. Screws tend towear faster than the barrel because the barrel surface areato screw surface area is about a 10:1 ratio, meaning thescrew flights come in contact with only 10% of the barrelwall during each revolution.

In a new barrel or extruder installation, the barrelshould be bore-scoped to define the extruder centerlineand verify that the thrust bearing and shaft are properlyaligned with the feed throat and barrel. Leveling anextruder barrel does not necessarily mean the extrudercenterline is completely level. Bore-scoping verifies thatthe center support and end support for the barrel areproperly aligned with the feed section and thrust bearing.Proper alignment of the barrel, feed throat, and thrustbearing allows the screw to slide easily in and out whenthe extruder is cold. If the extruder barrel must be heatedto insert or remove the screw or to turn it easily, some-thing is misaligned or the screw is bent. Running theextruder without proper alignment can result in seriousdamage. Bore-scoping can be done with lasers that areattached to the extruder, or an outside contractor can bebrought in to your facility to bore-scope a new extruderinstallation.

Barrel wear is measured with a cylinder gauge thatmeasures the barrel inside diameter (ID) versus the bar-rel length. Starrett and Sunnen produce two acceptablecylinder gauges.

High pressures in extruder barrels can be very dan-gerous. Consequently, a rupture disk is installed at theextruder head for safety purposes. In the event there is anincrease in melt pressure in the barrel, the rupture diskbreaks, relieving the pressure. Barrels are normallydesigned to withstand 10,000 psi pressure. Rupture disksare bought with specific pressure ratings, e.g., 7,500 psi,8,000 psi, etc., that will fail below the 10,000 psi barrelpressure rating. The rupture disk screws into a standard

Figure 3.11. Screw, barrel, heater band configuration.

Cooling Fans

ZoneHeaters

Barrel withBimetallic Liner Pressure

Transducer

BreakerPlate

RuptureDisk

BaseElements

Fe

Ni/Co

Ni/Cr

OtherElements

Ni, Si, B, Cr

Cr, Si, B, Fe

W, B, Fe, Si

Rockwell CHardness

50-65

45-60

60-65

Comments

Excellent wearresistance,

no corrosionprotectionGood wear

resistance, bestfor corrosion

protectionBest for wear

resistance, bestfor highly filledmaterials, verygood corrosion

protection

Figure 3.12. Fike rupture disk.

Three heater styles are available to heat the extruderbarrel and adapters: mica, ceramic, and cast. Heaters mustcover the maximum barrel area and be tightly clampedaround the barrel to prevent hot spots and provide uniformheating. Large extruders normally have cast heaters, andsmaller extruders use band heaters. Ceramic heaters aredesigned for higher temperature than mica, and bothheaters have a wide operating temperature range.

Barrel cooling is accomplished with either water or air.Water is a better cooling medium with better heat transfercharacteristics than air and provides better control. How-ever, water costs more to install and requires a recirculat-ing system or once-through water. Water lines can becomedirty and clogged, solenoids must be maintained so theywork properly, and a recirculating water system requireswater treatment. Water has the advantage that it does notforce hot air back into the room, heating the extrusion area.If a water-cooling system is properly maintained, it is veryefficient and works well. Cooling systems are shown inFig. 3.13 for both air and water. The ribbed spacers aroundthe barrel in the air-cooled system provide additional sur-face area for heat removal and increased cooling efficien-cy. Air-cooled systems have a damper valve above the fanto adjust the airflow, providing maximum efficiency withdifferent polymer processes.

Figure 3.14. Extruder screw stages.

section during the melting process. Metering is the lastscrew section and has the shallowest flight depths. Screwnomenclature is defined below and shown in Fig. 3.15:

Channel depth: Distance from the top of the flight tothe root

Channel: Space between flightsTrailing flight flank: Back edge of flightPushing flight flank: Front edge of flightPitch: Distance between consecutive flightsHelix angle: Angle flights make from a line perpen-

dicular to the screw shaftScrew diameter: Distance between furthest flights

across the screw shaftKeyway: End of screw containing the key that fits

into the shaft surrounded by the thrust bearingRoot diameter: Distance from the channel bottom on

one side to the channel bottom on the opposite sideLength: Distance from hopper to screw tipL/D ratio: Screw length divided by diameterCompression ratio: Ratio of the feed channel depth

to the meter channel depth

pressure transducer hole in the barrel flush with theinside barrel wall to assure no dead space is present forpolymer to collect and degrade. Figure 3.12 shows a Fikerupture disk that screws into an extruder barrel.

A single screw extruder screw typically has three dif-ferent sections, as shown in Fig. 3.14. The feed sectionhas deep flights to transport powder or pellets away fromthe feed throat. The transition section changes graduallyfrom deep flights with unmelted pellets to shallow flightscontaining the melt. Resin is compressed in the transition

Figure 3.15. Definition of screw elements.The screw compression ratio is critical in processing

different polymeric materials. While it is desirable tohave one general purpose screw that will process allmaterials efficiently at high rates, in practice this doesnot occur because different polymers have different vis-coelastic properties. Some polymers run better on screwsFigure 3.13. Water and air cooling on extruder barrel.

Fluid Out

Heater RibbedSpacer

Cooling

Fluid In

AirAdjustment

Fan

PushingFlight

Diameter

Channel FlightTrailing FlightChannel Depth

Material Flow

Helix Angle

Pitch

Root

Feed Transition Metering

with a 2.5:1 compression ratio, while other materialsprocess better on screws with a 3.5:1 or 4:1 compressionratio. For this reason it is important to be able to measurethe screw compression ratio and know which screwworks best with different polymers. Figure 3.16 showshow to make screw measurements with gauge blocks andcalculate the compression ratio. F is the root diameter in

Figure 3.16. Screw compression ratio calculation.

the feed zone, and M is the root diameter in the meteringzone. FD is the screw outside diameter including thegauge blocks in the feed zone, and MD is the outsidediameter including the gauge blocks in the meteringzone. If the gauge blocks are the same size, MD and FDshould be equivalent. Use the following information tocalculate the compression ratio of a hypothetical screw:

Root diameter in the feed section = 2.688" Gauge blocks thickness = 0.250" Distance between the outside of the gauge blocks

in the feed section = 3.994" Root diameter in the metering section = 3.250" Distance between the outside of the gauge blocks

in the metering section = 3.995"The compression ratio of the screw is calculated as

follows:

Feed Zone Channel Depth = 3.994 - 2 x 0.250 - 2.688 = 0.806Metering Zone Channel Depth = 3.994 - 2 x 0.250 - 3.250 = 0.245n n , / ^m Feed Depth 0.806 _

on

Compression Ratio (CR) = = = 3.29Meter Depth 0.245

Compression Ratio (CR) = 3.29:1Gauge blocks are used to measure the outside screw

diameter to determine screw wear. Gauge blocks are placedacross the top and bottom of the flights (as shown in Figure3.16) using calipers or a micrometer to measure the totaldistance. The gauge block thickness is subtracted from themeasurement to obtain the outside screw diameter. Diame-ters of new screws should be measured and recorded priorto initial use to compare with the specifications and to useas a benchmark later when determining the screw wear.

An easier way to inspect a screw and get the data tocalculate the screw profile and compression ratio is with

a dial indicator mounted in a bar, as shown in Figure3.17. The bar should be 2.2 times the length of the screwdiameter. This allows zeroing the dial indicator by span-ning three flights and zeroing the dial indicator on themiddle flight.

Figure 3.17. Screw channel depth measuring tool.

Clearance between the screw and barrel wall forsmall extruders is normally 0.001 inch (0.025 mm) timesthe screw diameter. For large extruders the normal clear-ance between screw and barrel wall is 0.004 inch (0.1mm). These are screw clearance guidelines only; eachindividual extruder should be benchmarked prior to theinstallation of a new screw or after installation of a newbarrel or liner.

A screw with a square pitch has a 17.66 degree helixangle; for each complete flight revolution around thescrew, the pitch is ID. Therefore, for a 4.5 inch extruderscrew with a 17.66 degree helix angle, the pitch is 4.5inches. With a square pitch, the number of flights is equalto the screw L/D, i.e., a 30:1 L/D has 30 flights or chan-nels. (The actual number depends on whether the feedpocket is included in the L/D.) Flight width is normally0.1D. This makes the flight strong enough to preventchipping or breaking while still leaving sufficient roomin the channel to process polymer. Unless the screw con-tains a barrier section, most screw designs have one par-allel flight. Screws are usually between 2OD and 30Dlong with four to eight flights in the feed section, six to10 flights in the metering section, and the remainingflights in the transition section. Feed channel depth isnormally 0.10D-0.3OD, with the compression ratiobetween 2 and 4:1.

Early screw designs were determined empirically bytrial and error. Today extruder screws are designed bycomputer programs based on polymer rheological datacombined with the desired throughput rates, machinehorsepower, mixing required for the application, and thepolymer and additives being processed to generate theoptimum screw design. If a current screw design is to bemodified to a different design when purchasing a newscrew, understand the objective or reasons for changingthe screw design. What are the deficiencies in the currentdesign? What is expected from the new screw design thatcan't be achieved with the existing screw? Some poten-tial reasons for changing screw design are higher

BT = Gauge Block Thickness

FDF M

MD

C o m p r e s s i o n R a t i o = D e p t h F e e d Z o n e / D e p t h M e t e r i n g Z o n e

D e p t h F e e d Z o n e C h a n n e l = ( F D - 2 B T ) - F

D e p t h M e t e r i n g Z o n e C h a n n e l = ( M D - 2 B T ) - M

C o m p r e s s i o n R a t i o = [ ( F D - 2 B T ) - F ] / [ ( M D - 2 B T ) - M ]

throughput requirements; polymer supply has changedfrom pellets to powder; the need for more mixing to dis-tribute or disperse additives, colorants, fillers, etc.; cur-rent screw generates too much or too little shear heat; thedesire for better melt temperature control; running twodifferent polymers that require two radically differ-ent screws and seeking a general purpose screw thatruns both products reasonably well; the need to run adifferent polymer with substantially different rheolo-gy than current production; etc. Before making rad-ical screw design changes, run trials with the newscrew geometry in the vendor's facility to verify thatit meets all processing criteria. Changing to a newscrew design solely for the purpose of doing some-thing different without a specific extrusion objectiveis not smart.

Screws can be cored for either a heating or cool-ing fluid during processing to add additional heat to thepolymer or remove excess shear heat, respectively. Heattransfer fluid is pumped into or removed from the screwthrough a rotary union at the screw shank. Cooling isadded to the feed zone to assist polymer feeding by pre-venting the screw or material from overheating, whichmay cause the polymer to stick to the screw root in thefeed section (discussed in more detail in Chapter 4). Fig-ure 3.18 shows a cored screw. Cooling required on both

Figure 3.18. Screw cooling.

the barrel and the screw may indicate improper screwdesign and/or operating conditions. Heat removal fromthe extruder through screw cooling is not an energy-effi-cient operation.

Specifications required when purchasing a new screwinclude the following: polymer to be processed, through-put requirements, screw diameter, extruder manufactur-er (needed to determine if LID includes feed pocket andthe keyway design), L/D, compression ratio, flightdepth in feed or metering zone, barrier screw (discussedin Chapter 5), mixing elements (discussed in Chapter4), number of flights in feed zone, number of flights intransition zone, number of flights in metering zone,helix angle, screw cooling, and number of screw stages(single or two-stage screw).

Figure 3.19. Breaker plate and seal at end of extruder.containing many holes that is placed between the extruderand adapter. The breaker plate has the following functions:

It stops the spiraling action of the polymer meltcoming off the screw by forcing the polymer instraight lines as it passes through the breaker plate.

It provides a seal between the extruder and thedie/adapter. If the sealing surface is damaged or ifthe surface has been ref inished a number of timesto remove nicks and/or dents and the breaker plateis now too thin to provide a good seal, molten poly-mer will leak around the gate between the die/adapter and the extruder. High pressure is generat-ed in this area, so the sealing surfaces must besmooth and pressure evenly applied around the sealto prevent polymer leakage.

Screens in the breaker plate filter contamination fromthe polymer melt and create pressure at the extruderhead. The screen and breaker plate combinationassists in providing thermal homogeneity in the poly-mer melt. Screens clogged with contaminants causehigh pressure at the extruder head and reduce the ex-truder throughput. If the formulation contains fillersor reinforcements, all screens must be removed.

Filtration is accomplished with wire mesh screens(square or twill weave), sintered powder, or metal fibers.Table 3.3 compares various filtration media and their char-acteristics. A square weave screen, Figure 3.20, has every

3.5 Die a n d Adapter

The extruder head assembly includes breaker plate,adapter to connect the die assembly to the extruder, anddie. The breaker plate, shown in Fig. 3.19, is a round disk

Die Clamp

Adaptor

Die

Breaker PlateScreens

Table 3.3. Characteristics of Different Filtration Methods

Characteristics

Gel CaptureContaminate

CapacityPermeability

SquareWeaveScreen

Poor

Fair

Very Good

Dutch TwillWeaveScreen

Fair

Good

Poor

SinteredPowder

Good

Fair

Fair

MetalFiber

Very Good

Very Good

Good

RotaryUnion

BreakerPlate

other wire over and under,while twill weave has everysecond wire over andunder. Screen mesh meas-ures the number of wiresper inch; the higher themesh, the more wires perinch, resulting in finer holesize and better contamina-tion removal. Screen place-ment in a breaker platestarts with a coarse screen

closest to the screw, followed by a finer and finer mesh,with the last screen in the group being a coarse mesh to actas a support for the fine mesh screens. The last coarsemesh prevents holes from being blown in the fine meshscreens in front of the breaker plate holes when high pres-sure is present. A typical 20/40/60/20 screen pack has thescreens ordered from the extruder screw as 20 mesh, 40mesh, 60 mesh, and finally a 20-mesh screen to supportthe 60 mesh.

Applications with high contamination requiringnumerous screen changes may need an automatic screenchanger to run economically. Many screen changerdesigns are available. Their sophistication depends on theapplication, the running time between screen contamina-tion, and the expense associated with shutting down andrestarting the process. Continuous-operation screenchangers index as the screens become clogged, replacingthe dirty screens with new screens, and never shuttingdown the process. Noncontinuous operation requires theprocess to be shut down, the screens changed, and theprocess restarted. Figure 3.21 shows a two-position

Extruder behind

Hydraulic unitpulls or pushesnew screen intostream

Screen Pack and Breaker PlateFigure 3.21. Hydraulic screen changer.hydraulic screen changer with one screen in the polymerstream and the second clean screen waiting to be trans-ferred into the polymer stream once the first screenbecomes contaminated. Hydraulic screen changers maymove slowly, requiring the process to be stopped, or veryrapidly, where they can be changed on the fly with onlyminimal loss in product and time. Other screen changersavailable for continuous processes are rotary screenchangers and double bolt screen changers. As screenpacks become contaminated, the head pressure increasesand the output decreases. If the pressure increases sub-stantially during a run, either the extruder screw speed

must be increased or the puller speed decreased to main-tain product dimensions. Screen changers are discussedin detail in Part 5, "Auxiliary Equipment."

The die is attached either directly to the extruder or toa transfer pipe or adapter that is connected to the extrud-er. Polymer melt temperature in the adapter must bemaintained. Transfer pipes, like extruder barrels, shouldhave heater bands covering as much area as possible tominimize hot or cold spots. Remember, two extruderobjectives are to provide uniform melt temperature andthe proper melt temperature. Transfer pipes normallyhave smaller diameters than the extruder barrel, with aconverging hole as material enters the transfer pipe fromthe extruder and a diverging hole as it exits the transferpipe to the die. This helps maintain good polymer veloc-ity "through the transfer pipes and adapters.

Many die designs are available, depending on theextrusion process and the product being produced. Com-pounders use strand dies to make continuous strands thatare chopped into pellets. Sheet and cast film producersuse flat dies. A film or sheet die design is shown in Fig.

3.22. This pro-duces a flat web ofa specific thick-ness that passesthrough a three-roll stack or laysflat on a cast roll.Tubing and pipedies are similar to

that shown in Fig. 3.23, where extrudate exiting the dieenters a vacuum sizing tank to set the product dimensions.Profile dies are all shapes and types and depend on thecomplexity of the product profile. Wire coating dies aresimilar in configuration to tubing dies, except a crossead

Figure 3.23. Pipe or tubing die.die is used running perpendicular to the extruder. Dies arediscussed in more detail in Part 7.

Extruder melt pressure is the pressure generated bythe extruder screw pumping melt through the breakerplate, transition or adapter, and die. Small die openingscombined with high throughput rates generate very highdie pressure, resulting in high extruder backpressure,which reduces the throughput rates. Extruder through-put is the drag flow rate created by the screw rotation

CausesWeld Lines

Screw Adjustment

Spider SupportScrew AdjustmentAir

Breaker Plate

Extruder

Figure 3.22. Sheet/film die.

Choker BarFlex Lip

Fixed Lip

Figure 3.20. Squareweave wire screen.

minus the pressure flow generated by the die and/orscreen pack resistance. If the die is removed from theextruder, backward pressure flow is absent and maxi-mum output is attained. With the breaker plate, screens,and die attached to the extruder, the material forcedthrough the die openings combined with the viscoelas-tic properties of the resin at a particular melt tempera-ture determine the pressure generated. The polymervelocity profile in the metering section is depicted inFig. 3.24. Polymer flow in the metering zone is accom-plished by drag flow. Drag flow has high velocity alongthe barrel wall as material is scraped off the barrel with

Figure 3.24. Plastic throughput profiles.

the screw flight, and zero velocity at the screw root. Pres-sure flow is the result of die pressure or head pressureforcing the material backward into the metering zones.The velocity profile is similar to a plug flow velocity pro-file with high velocity in the center and zero velocity atthe barrel wall and screw interface. Combining these twovelocity profiles forms the throughput velocity profile inthe metering section. The throughput velocity profileshows high polymer velocity at the barrel wall, zerovelocity at approximately two-thirds the depth of thechannel (from the barrel wall), and a negative velocitynear the screw root. Extruder throughput is given byEq. (3.3):Plastic Output (Q) = ,3

3x

Drag Flow (QD ) Pressure Flow (Qp ) Leakage Flow (QLF )

Leakage flow is the flow over the top of the extruderflights. Leakage flow is normally negligible and disre-garded except for a worn screw, where leakage flow canbe significant.

3.6 Contro ls

Without good process controls plus an understandingof the extrusion process, the extruder becomes nothingmore than a black box where pellets are put in one endand an extruded shape exits the other. What occurs in theextruder? How do you know the system is in control?How do you know the extruder is running properly? Theanswer to these questions is process control, with both

input and feedback loops to verify that the process isoperating properly and at equilibrium.

Control is making a measurement, determining ifsomething needs to be changed, making a decision, and tak-ing action. If the system is operating at equilibrium, thedecision is that everything is running properly and nochanges are necessary. If the product is borderline accept-able or unacceptable, or a process step is outside the SPCcontrol chart limits, the decision is that something is wrongand a change is required to get the system back into control.

With improved computer capability and durability,extruder controls have become more sophisticated, result-ing in better process control. The entire process can becontrolled from a central location, ranging from the indi-vidual feeder throughput to the puller speed and every-thing in between. Feedback loops are available to plotSPC data every second (or your chosen interval) for alltemperature controls, screw speed, melt temperature,melt pressure, motor load, puller speed, on-line gaugesfor thickness or dimensions, feed rates, water tempera-ture in cooling tanks, vacuum levels, windup speeds, rollpressures, and so forth. With the entire process instru-mented, it can be monitored from a remote site. Engi-neers sitting in an office can monitor each plant line, ver-ifying that all lines are running properly and theprocesses are in control based on SPC data.

In comparison with other plastic processes, anextruder has very few independent control variables thatcan be changed by an operator to alter the process.Assuming the correct screw is in the extruder, the properdie is installed, the screen pack is clean, and the equip-ment is operating properly (all heater bands and thermo-couples are functioning properly, air or water cooling oneach heating zone is working, and cooling on the feedthroat is operating properly), the only extruder variablesthat can be changed are the temperature setpoints andscrew rpm. Several parameters are monitored to assurethe process is in control and running properly: barreltemperatures (actual and setpoint), extruder load (percentload, torque, or amps), screw speed, melt temperature,melt pressures (prior to the screen pack, in the die, and ina two-stage extruder at the first stage metering section),feed throat cooling water, temperature of raw materialsentering the extruder, vacuum level if vacuum venting isbeing used, and cooling on the individual barrel zones.Other parameters to be monitored prior to the extruderinclude blend ratio, feed rates, moisture content of hygro-scopic and moisture-sensitive materials, liquid feed rates,and raw material lot numbers. Downstream equipmentparameters to be set and monitored include

Roll and/or puller speeds Roll gaps Cooling bath temperature

Output DirectionBarrel Wall Barrel Wall Barrel Wall

Screw RootDrag Flow

Screw RootPressure Flow

Screw RootThroughput

Plastic Velocity Profile

Next Page

Front MatterTable of ContentsPart I. Single Screw Extrusion1. Extrusion Process2. Extruder Safety3. Single Screw Extruder: EquipmentIntroduction3.1 Equipment3.2 Drive3.3 Feed3.4 Screw, Barrel, and Heaters3.5 Die and Adapter3.6 Controls3.6.1 Temperature Zone Control3.6.2 Melt Temperature3.6.3 Pressure Measurement3.6.4 On-Line Measurements3.6.5 Control Summary

3.7 Extruder Devolatilization3.8 Vertical ExtrudersReferencesReview Questions

4. Plastic Behavior in the Extruder5. Screw Design6. Processing Conditions7. Scale Up

Part II. Twin Screw ExtrusionPart III. Polymeric MaterialsPart IV. Troubleshooting the Extrusion ProcessPart V. Auxiliary EquipmentPart VI. CoextrusionPart VII. E