Calendering - 8

7/22/2019 Calendering - 8

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INTRODUCTIONThe calendering process isused to produce plastic films and sheets. Itmelts the plastic thenpassesthe pastelike melt through the nips of two ormoreprecision-heated, counterrotating, speed-controlled rolls into websofspecificthicknessandwidth,asshowninFigs5.1 and5.2.Theparallelrolls have extremely flat surfaces and rotate at the same speed or atslightlydifferentspeeds. The web may be polished or embossed (depend-ing onrollsurface), rigidor flexible (arangeofmaterial properties areused).Although plasticformingoccursin thecalenderitself,downstreamequipment isneeded toproduce the TP film orsheet. Upstreamof thecalender, a mixer blends the raw material, usually in powderedform,withthe desired additives, such asplasticizers or fillers (Chapter 1).Afterplasticizing, the pastelike melt passes through the multiple-roll mill.

Heavy-gauge products,0.02in.(0.6mm)off therolls,aredrawn downorstretchedat takeoff aslittleasconsistent with good stripping from thefinalcalender roll. Lighter-gaugefilm,0.007in.(0.18mm)offthecalender,istypically drawn downat aratioof1.2:1andwoundup at athicknessof0.006in.(0.15mm).

Calenders may consist of between two and seven rolls, possibly more.Theyarecharacterizedby thenumberofrollsandtheir arrangement:I, Z,orinvertedL.Most popularare thefour-rollinverted-L calenderand theZcalender.Zcalenders havetheadvantagesoflower heat lossin thesheet(becauseofshorter travel)and asimpler construction. Theyaresimplertoconstructbecause they need less compensation forroll bending; thisisbecause there are no more than two rolls in any vertical line (asopposedtothreein afour-rollinverted-L calender).

Other variationsinthese massive, multimillion-dollar calender linesaredictated by thevery highforcesexertedon therollstosqueezetheplasticmeltinto a thin web. High forcescan bend the rolls, producing a web

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thicker in the middle than at the edges. This can be counteracted bydifferentmethods:(1)crowned rolls, which haveagreater diameterin themiddle thanat theedges;(2)crossingtherolls slightly, thus increasingthenip opening at either end of the rolls; and (3)roll bending, where abending momentisapplied to the end ofeach rollbyhavingasecondbearing on each roll neck, which is then loaded by a hydraulic cylinder.Calenders callforhigh temperatures, with little variation acrosstherollsduringtheapplicationofhighpressures.

Thehigh forces arenecessarytosqueeze theplastic mass intoaverythinfilm;anyunevennessin theforcesalongtherollisreflectedasvaria-tionsin thefilmthickness.Onecauseofpressurefluctuationsis toomuchclearancein the bearings, which can be resolved either by pulling back therollsagainstonesideof thebearingor byusing tapered roller bearings.After forming, through the multiple rollsof the calender, the film orsheet iscooled by passing through precision-surfaced cooling rolls atprecisely controlled temperatures and/or a cooling tower. Thicknessgauges areusually located within this cooling sectionof the production

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Figure5.2Arrangementsofparallel rollsforcalender coating:(a)one-sided coat-ingwith vertical three-roll; (b)one-sided coating with inverted-L four-roll;(c)double-sided coating with inverted-L four-roll;(d)double-sided coating withtwothree-rolls; (e) one-sided coating with Z-configuration four-roll; (f)one-sidedcoatingwithS-configurationfour-roll;and (g)one-sided coating with modified-Sfour-roll.

line; thefeedbackthey provide makes thickness corrections automatically.Gaugecontrols can also be used in the neckdownsection,where the hotplastic leaves the calender rolls and transfers to the cooling rolls. Aftercooling, the plastic is trimmed at the edges and wound. Trim may accountfor up to 5% of thewidth, depending on theline'soperating efficiency,and most trim is immediately directed back through a granulator andblended with the virgin material (Chapter 16).

This is the typical sequence for a calender line, but there are manyvariationstosuitthe endproduct. Auxiliary equipmentcan beincluded,suchasannealing,quench tank, decorating, slitting and heatsealing,andorienting(uniaxialorbiaxial stretching).

Calendering was developed over a century ago to produce naturalrubber products; it is now used principally with TPs. Amajorproduct iselastomeric tire-fabric coating. Calendering is a highly developed art,recentlyelucidatedbycombining thecomplexityofmelt behavior withthemechanicsofrotating machines. With this understanding comes theabilityto make calenders more productive by increasing theirspeed,toproduce films with tighter thickness tolerances and greater uniformity,and to handle thicker sheets more effectively.

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MATERIALSAwide varietyofplasticscan beused;about80 wt% is PVC butothersareABS,PE, PP, and styrenes. When calendering other plastics, there is atrade-off in economy and quality. The basic plastic limitation of thecalendering process is the need to have asufficientlybroad melt index toallow a heat range for the process. This permits the material to have arelatively high viscosity in thebanksof the calender (banks indicatingwheretworolls meet,or the nip of therolls).As aresultof theviscosity,a shear effect can be developed throughout the process, especially be-tween the calender rolls. Thus, the calenderformsthe web as a continuous'extrusion'between the rolls. Unlike the process in an extruder (Chapter3)or aninjectionmolder (Chapter2), theplastic mass cannotbeconfinedwhen being calendered. Becauseof the lack of confinement, the sheareffect and abroad melt bandareessential aspectsofcalendering.

Theblending or compounding of theplastic with different additivesand fillers is acritical partof theprocess.Theblending must produceauniformlycoloredand stabilized product, inpowder form.After blend-ing,therateofconsumption dictatesthetemperatureof themelt. Becausethe plastic is processed between the process heat and its critical heat ofdegradation, the time at heat becomes extremely critical and an importantpartof theprocess.Forexample,theprocessor will minimizetheamountofmeltin thebank (nip)of therolls.Theresidence timeof theplastic fluxathigh heat mustbelimited.PVC isespecially heat sensitive.

Theplasticmix fed to thecalendermay be asimplehotmelt,aswithPE.ForPVC it isobtained bypremixingtheplastic, stabilizers, plasticizers,etc., in ribbon blenders, then passing the blend to a Banbury mixer wherethe mass is gelled for about 5-10min at 120-16O0C (248-32O0F). Thegelated lumps are made into a rough web on a two-roll mill and the webis fed to thecalender.Iffabric,paper,orother materialis fedthroughtherolls, plastic can bepressedinto thesurfaceof the web; then the calenderbecomesacoating machine (Fig. 5.2).

Ingredientsfor PVCfilm can include solid and liquid raw materials.Solids are weighed in batches and dropped into a high-speed mixer.Liquidsaremetered intothemixerbypistonpumpsorother volumetricmeteringdevices.Abatch mixing systemisgenerally inline withacon-tinuousfluxingsystem, starting at the holding bin. Some operations donotusebatch mixersforblending. Continuous metering systems, usingloss-in-weight feeders and pumps, meter directly into the fluxing ma-chine, acontinuous mixer.ilmsThePVCfilmindustry distinguishes betweentwocalender typesandrollarrangementsthatarerigidand flexiblePVC. RigidPVCmanufacturers


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preferthe Lconfiguration withfourtoseven rolls beingfedfromthe floorlevel. Since there are no disturbing vapors from lower calender rollswithinthepickoffarea,i.e.,condensationonrollsandfilm,it ispreferableto have thepickoffrolls on an elevated level.

Flexible PVC is commonly fabricated using a four-roll inverted-Lcalenderor an Fcalender. These systems enabletheplasticizer-saturatedvaporstoescapevia thesuction hood abovethecalender, where theyarefiltered beforebeing released intotheatmosphere.

Auniversalfive-rollLcalenderisusedforrigidand flexible PVCfilm.Itprovides heat stabilityand superior film controlforgood surface ap-pearance.Themajor difference between this universal calender and theother calendersis inmountingandplacementof the firstroll. Althoughthe firstrollisgenerally mounted horizontally besidethesecond roll,thelatest design stillhas thefirstroll mounted horizontallybut now it isnextto thethird roll, resemblingafour-rollLcalender.Thesecond roll remainsin action underneath the third roll, sofourgaps are still available. Roll/filmwrap length, however,isreducedsodwell timeis 40%less,decreas-ing PVC heat loss and shrinkage.

Modern designs for rigid or flexible PVC lines currently show fewsignificant differences. Bothconsistof thesame rollgroups,but arrangedatdifferent elevation levels.Thepickoff sectionhas up to 15rolls, eachwithan OD of6in.(15cm),drivenandtemperature controlledinsections.Thisroll arrangement drops filmvertically into an embosser with threerolls (the embossing rollitself,acooled rubber roll,and acontact coolingrollto therubberroll).Fortemperingandcoolingthefilm,anotherset ofup to 19rollisarrangedfollowingtheembosser; they have diametersof6-10in. (15-25cm). These rolls are driven and temperature controlledin sections ofbetween two and five rolls. Temperature accuracycanbe controlled within 20F (1C).Abeta-gauge controls film or sheetthickness.RecycledHandling scrap and cold trim from the product line can pose a verydifficult problem.Thescrapand trim could represent 10-40wt%of themixwith virgin material.Theactual amount dependson thewidthof thecalenderin relation to the sheet width. The flux rate and energy requiredtoremeltthescrapareconsiderably less than requiredto flux thevirginplastic. With this differential, there is the potential danger ofmaterialdecomposition whenever scrap is processed. But optimum uniformityandproduct standardization requireablendof new and oldmaterial.

Reprocessed material is best added to the blender when a standard canbe established, although some plantsfeedit directly to the fluxing equip-ment.Carefulcontrolof thescrap percentagein thetotalmix isessential


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toobtain a quality-controlled product. These lines are expensive to set upand operate. They are most economically operated on longruns,wheretheycompetewith filmandsheet extruders (Chapter3).Extreme careisrequired to ensure the material or the processing area is not contaminated.Anyforeignmatter can severely damage the expensive and highly pol-ished rolls as well as causing other serious damage to the line.

ontaminationVitaltothis processis theremovalof anymetal, including microscopicmetallicparticles that usually occuringranulated scrap. These contami-nants would destroy the very expensiveprecisionsurfacesof the calenderrolls. Damage would also occur to the downstream cooling rolls. All kindsof precautionsaretakentoremoveanycontaminants, starting with theincomingraw materials, thefillers,and the additives. Materials arepassedthroughanextensive batteryofscreensandmetal detectors.

The compound is usually fed through an extruder with multiplescreens. Metal detectors tendto beused alongthe flowlineof theplasticon its way from the compounder to the first nip rolls. Detectors areincluded under the belt that conveys thefinalmelt,perhapsin a ropelikeformofdiameter2-4in.(5-10cm).

FLUXING AND FEEDINGFluxing is aplastic compositiontoimproveflow;fusionis theheatingofthevinyl compound to produce a homogeneous mixture. Fluxing unitsused in calender lines for preparing thermoplastics include batch-typeBanburymixersandFarrel continuous mixers(FCMs),Buss Ko-Kneaders(BKKs)andplanetary gear extruders(PGE).The dry/blend is fedintothemixer (extruder). Excellent mixing within a short dwell time and heattransfercontrol contributeto animproved product. Duringfluxing,eachparticlereceives the same'gentle'treatment, generating less heat history(Chapter 1) and producing more uniform feed rate, color, gauge, andsurface.Thefeedcandischarge ontoatwo-roll mill. Operating this way,itprovidesfor asecondfluxingdevice, mainlyforworkinginscrapor forconvenienceas a buffer .

Thestock delivered to the first calendernip needs to bewell fused,homogeneousincomposition,andrelativelyuniformintemperature.Theoptimum average temperatureforgood fusiondependson the formula-tion. A rigid PVCformula based on medium molecular weight resin(intrinsicviscosityof0.90-1.15[91)has atypical optimum temperatureof356-3740F(180-1900C)at thefirstcalender nip.Forbest calendering thereshouldbe nocold volume elements below3470F(1750C)and no hotspots


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Table5 1 Examplesof flexible PVCcalendering conditionsHeavy-gauge product Light-gauge product

Roll temp. Roll speed Roll temp. Roll speedRollno. F 0 C f tmin 1 mmin F 0C f tmin 1 mmin1 347 175 125 38 338 170 263 802 352 178 128 39 343 173 269 823 353 181 138 42 349 176 279 854 363 184 148 45 354 179 289 88

above 3830F(1950C).These typical temperature readings are necessaryfo r close control of fusion and mixing conditions; they are based onstudies offlow effects and their relationship with volume and elementsize. This interaction depends on stock temperature and in turn on theperformanceof PVCmelts.

Flexible PVC is normally calendered at temperatures 50-680F (10-200C)lower thanforrigid PVC. Typical calendering conditionsforunsup-portedflexible PVCfilmandsheetareshowninTable 5.1.In flexible PVCproduction,a short single-screw extruder, acting as astrainer,filtersoutcontaminantsfromstockbeforereachingthecalender. This methodis notapplicableforrigidPVCbecauseitdrastically increasesthehead pressureand theconsequent overheating would causethestocktodecompose.

PROCESSING CONDITIONSPreparationofstockforcalendering, conditionson thecalender,thetake-off, thickness measurements,andcontroltowindup mustall beadaptedto the plastic system being calendered. Other considerations includewhether the plastic is laminated to afabric,etc., on the calender; whethertheweb is embossed or slit inline; whatfinishis required (glossy, matt orsemimatt); and whether the product needs special properties such asopticalclarity, biaxial orientation (Chapters 1 and 3) or very low strainrecovery.General processing considerations include temperature control,calender speed, finish, gauge, orientation, and embossing. Significantprogress incalenderingis due toimprovementsincontrolof theoverallheating and cooling processes involved in the complete calender lines(trains), as well as the speed and tension of the web at each stageofprocessing.


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Design aspectsDesign improvementsincalenders have advancedthe art ofcalendering.Apreloading device eliminatesthemain bearing clearance(play)of thepenultimate calender roll.Afour-rollLcalender also eliminatesthethirdroll. Anotheraid is an airknifetofirmlyhold downthe filmon the firstsectionofpickoff rollsand tominimize neck-in, shrinkageand plate-out.Neck-inis thedifferencebetweenthewidthof the hot web as itleavesthehot rolls and the width of the cooler web downstream. This'shrinkage'dimension isused as a guide to ensure the processing conditions arecorrect. Plate-out is an objectionable coating gradually formed oncalendering rolls during processing. It is caused by the extraction anddeposition ofcertain components in the compound such aspigments,lubricants, plasticizers,andstabilizers;itrequires immediate cleanup.

Another improvement, the compact universal embosser, is used forhorizontalor vertical film drop, embossing on the top or bottom side.The air knife was developed to meet rising quality demands and in-creased production speeds. At highspeeds,more air is drawn between thefilm and the pickoff roll, preventingthe film from making contact withtheroll. Sincepickoff roll to temperatures are lower than calender rolltemperatures, liquids in the compound tend to condense and transferfromtherollto thefilm,causingsurfacedeficiencies.The airknifeensurescontactbetween the film and the pickoff roll.Air blown over the filmcreates pressure, preventing air entrapment between the film and thepickoff roll.

Surface finishingCalenders provide stylingorfinishingonfilmsand sheets.PVCsurfacecalendered goods require depthsofmattorprovideanevenroughness,whichcan bemeasured withadevice suchas aprofilometer. Differentmarkets usedifferent termstodescribe theirfinishes,e.g., satin,Sheffield,andlight matt.Aone-sided mattfinishmay beapplied withanembossingroll.Asharper, deeper,andmore precise mattfinishcan beappliedtofilmorsheeton thecalender.Tomake two-sided matt products, suchas the bigmarketfortwo-sided credit card stock,thelasttwocalender rollsarematt.Only the last calender roll is matt for one-sided matt goods meetingcriticalspecifications, suchasspecial print filmandone-sided credit cardstock.Toproduce these products, the last twocalender rollsare sand-blasted with aluminum oxide grit of controlled particle size, such as120-180 mesh. The heavier matts, coarser grit, and more passes of thesandblasting actionon therollsarerequired. Proper selectionofmetalforthe calender rolls is also necessary. Chilled cast-iron rolls take a sharpmatt,which wearsoffslowly.


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Theprinciple used inembossing PVCfilm and sheet is todeliver arelatively hot web to a relatively cold embossing roll. This action impartsthe desiredsurfacefinish to the web andfreezesit by rapidly cooling theembossment pattern. Development continues for embossing a wide rangeof rigid,semirigid, and flexiblePVCcompounds. Film temperature, rolltemperature,andpressureareoptimized empiricallyforeach compoundformulationand pattern to be embossed. The steel embossing roll and therubber pickup rollareinternally cooledby aliquidmedium; bothofthemrequire a veryuniform surface temperature to develop the embossingpattern.But therubber pickup roll tendstostickto a hot PVCfilm unlessit is cooled externallyas well as internally. External cooling in a wettrough is required because most rubbers have poor heat-transfer proper-ties, so poor that a nonexternally cooled backup rollsurface would risealmostto theprocessing temperatureof the PVCfilm.Thesurfaceof thesheetisdeterminedon thelast rolland may beglossy,matt, or embossed. After leaving the calender, the sheet iscooled bypassing it over cooling rolls then through a thickness gauge (e.g., a beta-raydevice) prior to being wound up. Quick roll-change is used for the lasttwocalender rolls. Changesfromhigh glosstomatt,or todifferent pro-files,can now bemadeinabout1Ohorless;in thepastittookatleast8Oh.Embosserrollscan bechangedinminutes insteadofhours.

CoatingOne specialand important applicationof the calenderis the coatingofpaper, wovenandnonwoven textiles, plastic filmand sheet (Fig. 5.2).Acalenderwith three rollsisusually sufficient for one-sided coating,butfourrollsareusedforextremely thin coatings. Double-sided coatingcanbe applied simultaneously on both sides of a fabric, using afour-rollcalender, or sequentially by two three-roll calenders.

RollconfigurationsareshowninFig. 5.2.Themost popular configura-tion is theinvertedL.However, theseandotherconfigurationsareusedtomeetdifferent product requirements.

So-called frictional calender coating takes a material such as anelastomerand forces itintothe intersticesof thewovenorcord fabricswhile passing throughtherollsof thecalender.

CONTROLSManycontrolfeaturesin calenders provide versatility, better quality, andhigher operating rates. Complete control of friction ratios gives goodtracking ofstockviaindividual driveson allcalender rolls. Close toler-anceson filmprofilesareobtained fromaxiscrossing,rollstraightening,


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and gapprofilingofcalender rolls; greater profiling accuracyisobtainedby hotgrinding ofcalender rollsatoperating temperatures. Bettercon-trolover stock temperature is achieved by using drilled rolls with heat-transferfluid circulated through them at high rates and by using accurate(3-40F,20C)systemstocontrolthe fluidtemperature.

Automaticweb-thicknessprofilecontrol, well known sincethe1940sinextrusion,can beused incalendering plastics suchasPVC.As anexam-ple,perhaps 64-76 individually temperature-controlled airnozzles, ar-rangedin arow, createaseriesofzones acrossthewidthof thesheet.Theweb thickness can be controlled by adjusting the coating thickness appliedtoeach zone.

Calender bowl deflection can occur. It is the distortion suffered bycalender rolls resulting from the pressure of the plastic or elastomerrunning between them.If notcorrected,thedeflectionproduces film orsheets thickerin themiddle thanat theedges.Themachinesare set up sothe required adjustments can be made to correct this distortion. As anexample, lessdeflectionat high operating conditions can be achieved bythe use of stiffer rolls, based on higher-modulus steels or dual-metalconstruction.

Inolder calenders severe casesof'rollfloat'causedbychangesin thebalance offorceson aroll were sometimes encountered. Preloading de-vices reduce roll floating and thus provide better gap control betweenpairsofrolls, especially during startup. Film thicknesscan bemeasuredautomatically with improved feedback control to the calender anddrawdown system. Programming temperature changes at rates of4.50FnIm 1(2.50CmIn-1)or less will avoid roll distortion caused by ther-malshock.

Withvertical stacksofrolls,theseparatingforcesexertedby thestockatthenipsmay beadjustedby theoperating conditions,so themiddle rollsdefect very little.As a first approximation, it may be assumed that thethird roll on afour-roll Lcalender or an inverted-L calender does notdeflectinbalanced operation.Deflectionof thefourthroll primarily deter-mines gauge variation across the width of the web. For thin plastic webs,roll-separating forces in the final nip may be as high as 6000Ibin.'1(1072kgcm 1), which may cause considerable deflection of the finalcalender roll. Unless this is compensated, gauge variation will occuracrossthe web.

Good drawdown and pickoff control is achieved by using tightlyspaced multirollpickoffs with grouped roll-speed controls. Helical cavi-tiesineachpickoff roll allow high circulation ratesofheat-transferfluid.Quick-opening devices on all nips reduce damage to rollsfrom runningtogether in the event of power failure or stock runouts. Mechanical,square-cut devices with automated tension controlandroll changeas theroll reachesapreset diameter ensure better windup.


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Compensation fordeflectionofcalender rollsisnormally achievedbycombining the three methods of axis crossing and skewing, roll straight-ening (also called roll counterbending), and roll crown (also called rollprofile).Rollcrownmay beground orblocked.

Axis crossing increases the roll separation at the ends but does notchange it in the middle. Roll straightening involves the applicationofexternalbending momentstoboth endsof theshaftof acalender rollinordertocompensatefordeflection(+) orexcessive roll crown(-).Withtheusualapproach,positive straightening isused tocompensate for deflec-tion caused by roll-separating forces in the nip of acalender; negativestraightening is used to reduce excessivefixedcrown on a roll, in which ithas been ground.

Radiationfilm-thickness gauges are now used with computer systemsthatprocess the raw data and print out detailed production reports. Theyalso control the gap in the last calender nip by varying the opening of therolls, the axis crossing, and the roll straightening on thefinalcalender roll.In order for these thickness gauges (also called beta-gauges) to workeffectively, the webcomposition mustbeexactly knownandaccuratelycontrolled.The gauges measure attenuation ofradiation, which is pro-portional to the quantity of matter in the measured area. For knownformulacompositions, this can be translated into web thickness. Althoughgood results have been reported, these gauges cannot compensate fordeficienciesinolder calendersor forimproper controlof the compound-ingformulations.

ADVANTAGESOF CALENDERING OVER EXTRUSIONCalendered sheet isusually lessglossy than extruded material (Chapter3).Calenderingmay be preferableforcertain applications requiring itshigher tensile propertiesandunusually close gauge control. Extrusionofcolored films or sheets requires the extruder to be cleaned and purgedwhen changing colors.A calender requires aminimumofcleaningbe-tween color changes. Calenderingisdefinitelyused forlong productionruns to be economicallyprofitable,producing smooth and other finishesat higher speeds. Applications include window-shade stock, electricaltape,PVCtile, PE-coatedfabric, tire-fabriccoating,and swimming-poolliner.

COSTINGMostoutputinplasticsiswith PVC. Flexiblefilmand sheet accountsforthe greatest volume. The capital cost for a new calendering line willaverage $4-5 million, not including building costs. The sum of the fixedcostsand thevariable costsforoperatinga newlinein theUnited States


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Table 5.2 Examples fo rproducingPVC film andsheetby different manufacturing meFlexBlown lmExtruder-calenderalender

40high0.3-750 0.001035h5-30no30-615(

90higher0.3-0.6600 4 V 2in.)0.001-0.0031032h5-30minno30-60min15(20)

2low1-2.5500-15000.002-0.0053 (1-5)510minsecondsyes10-40min60(80)

155lowest1-6800-80000.002-0.0503 (1-5)62-5 minsecondsyes5-30min80(150)

Poor accuracy, lonstream time, low rdegradation, reducversatility

Low investment,multiplant capabilthin gauge (0.003iandunder)and hegauge (0.050-0.125

Lowerrate,versatilityproblem

Accuracy,gaugeadjust,reduced cost

High capitalcost, heattime

Versatility,high rate,accuracy,ease and,adjustmentease atreprocess

Lines installed,USARelativeresin costMachinecost ($m illion)Rate andrange (Ibrf1)Prod uct gauge range (in.)Sheet accuracy(% )Time toheat (hr)Time for'onstream'Gaugeadjust timeAu togauging capabilityColor or product change timeWindup speed (ydmin 1):average(max.)Limitations

Applicationsand advantages


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amount to$450-550perhour. TakingPVC as anexample, these lines havea throughput of 90-6000IbrT1 (400-270ORgIT1); the average range ofcomponding costs for calenderingPVC isabout $1.08-2.47per pound($0.49-1.12perkilogram).

Notincluding material costs,theexpenseofoperatingalarge calenderline may be$350-560per hour, depending on the degree of automation,investment, labor, power costs, and overheads. For PVC sheet 7.1ft(2.13m) wide and 0.02in. (0.51mm) thick traveling at 91ftmin4(27.4mmin 1) and woundup at arateofabout1076IbIT1(489kgh 1),theconversion cost is about$0.46per pound($0.21per kilogram). These costsare based on PVC resin at $1.70 per pound ($0.77 per kilogram) andplasticizerat$2.42per pound ($1.10perkilogram). Table5.2providesageneral manufacturing comparison of calendering with combinationextruder/calender, extrusion blown film, extruder with flexible-lip die,plastisol-cast,and melt roll.

Calendering costs that willaffectproduct costs include(1)bank marksandshiny patcheson thesurface;(2)cold marks (crow's feet)causedbythestock beingtoocold;(3)blisteringdue tohigh temperatureor alargebank ofrolls; thicker sheetsaremore vulnerabletoblistering;(4)pinholescausedbyforeignmatteror by thepresenceofunplasticized plastic parti-cles;and (5)watermarking usuallydue tolubrication contamination.

Toreducethecostofproducts,aim tocalender under conditions thatyieldthehighest windupspeedconsistent with good quality. However,there are numerousfactorswhich limit calenderspeed.At a givenspeed,more frictional heat is generated by the more highly filled and rigidstocks,and by reducing web thickness. Excessive speed produces degra-dationorstickingofmany productsonwhichthestock temperatureislimited. Optimal speeds of the final calender roll for double-polished,clearmelt viscositiesat agiven temperature degradeataboutthesamemaximumtemperatures as the rigid stocks. Air occlusion limits the speedfo r some heavy-gauge products, whereas the matt quality or micro-roughnessof thefilm,ormeltfracturein thelast calender nip, limitsthespeedonothers.

Bank marksare minimized byoptimizing formulations, calenderingspeeds, and roll temperatures so as to obtain the most orderly behavior ofthe rolling banksofstockat the calender-nip entrances. Properuse ofdrawdown permits windups to be run substantiallyfasterthan the finalcalenderroll on many thin, unsupported film products. Calenders andtakeoffsare runalmost synchronouslyonheavy-gauge products.

Films and sheets with a high gloss takenoff ahighly polished finalcalender roll tend to stick to the roll more than their matt counterparts.Very soft webs also tend to stick to the final calender roll.The fastestcalender speeds are generally obtained using mediumstiffness, moder-atelyfilled,matt products in a median thickness range.


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Theamountofedge cutting influences costs. Edge cutsinexcessof3in.(76mm) arenormaltoavoid undesirable edgeeffects.Oncritical productsitisrealized that close controlof therework ratioisnecessaryforuniform-ity ofoutput.Excessreworkfromcritical productsisusedinthose whosespecificationspermit this practice[9].

TROUBLESHOOTINGEarliersectionsofthis chapter coveranextensive rangeofcalenderingproblems and their remedies; collectively they should provide an ad-equate troubleshooting guide.

Calendering - 8

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