Overview of Canlendering and supercallendering.docx

Overview of Canlendering and supercallendering.docx

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OverviewofCanlenderingandsupercallendering

OverviewofCanlenderingandsuper-callendering

A.Introduction

Amajorandimportantphaseofconversionistheprocessdealingwiththesurfacequalitiesofthepaperbycalendaringtoproduceasmoothcompactedsurfacesuitableforprintingorwritingandforappealtotheartisticsense.

Althoughtheprocessofcalendaringpaperisprobablyasoldastheartofpapermakingitself,interestinthisphasehasbeenbroadenedandintensifiedwiththeadventofon-the–machinecoatingofpaperandthenow-establishedpracticeofcalendaringthisgradeofpaperatspeedsandpressuresmuchhigherthanthoseformerlyused.

Inthepasttheresultofcalendaringactiononthepaperhadbeenacceptedwithoutconsiderationastothecause.Itistheaimofthischaptertodealwithbasicfundamentalsinvolvedincalendaringpaperandtofurnishinformationthatwillbeofassistanceinthecontrolofthevariableinvolvedintheprocessoffinishing.

Toassistthereadertoformulateaclearunderstandingofthesubject,itishelpfultodefinethetermsused.

1.Terms

Thetermcalendaringisthebasicactionoccurringwhenawebofmoreorlessplasticmaterialissubjectedtopressureandfrictioninitspassagebetweentwoormoresetsofadjacentnips,theeffectonthematerialbeingachangeinthesurfacepropertiesusuallyimprovedsmoothnessandhigherdensity,aswellasgloss.

Thetermcalendarappliestoamachineanditsauxiliariescapableofimprovingsurfacecharacteristicsandincludesallmachinesdescribedherein.Thetermsmachine,breaker,smoothing,superfriction,hot-plating,andembossingcalendersindicatethemeansusedorfunctionperformed.Withtheexceptionofthelast-mentionedmachines,allothersfunctiontoimprovethesurfacequalitiesofsmoothnessandgloss.

2.Fundamentals

Thefundamentalsinvolvedintheprocessofcalendaringorironingpaperthatneedtoberecognizedareasfollows:

1.Calenderingpaperisamechanicaloperationbywhichthewebiscompactedandthesurfacepropertiesareimproved.Calenderingthereafterreflectsthestandardofqualitiesincorporatedinthepapermakingandcoatingprocess.

2.Calenderingactioncomprisesthreeelements:

pressure,allieddeformationinbothpaperandfilledrolls,andtemperature.

3.Theefficiencyofconversionbycalendaringismatterofbalancebetweentheresponsivenessofthecoatedpaper,inturn,ispredeterminedbythecharacteristicsofpaperanditscoating.

4.Thereislimitedintheamountofcalendaringactionthatthepaperand/orcoatingpropertiescanwithstand.Beyondthislimit,calendaringactionusuallycausesadecreaseinthequalityofthepaper,withacorrespondinglossofprintability.

5.Coated-paperpropertiesarethesumoftheinternalpropertiesofthepaperitselfplustheexternalpropertiesofthecoating.

6.Ofthenumberofvariablesencounteredintheproductionofasuitablecalenderedpaper,onlyfiveareavailableformanipulationincalendaringaction:

pressure,moisture,speed,temperature,androlldesign.

B.Calenders

Acalendar,orstack,usuallyconsistsofacomplementofall-metalrollsrangingfrom2to14innumber,arrangedoneabovetheothervertically,housedbetweentwouprightframes,andheldbetweenthenbysuitablebearings.Pressureisappliedtorolls,aswillbedescribedlater.Usuallythenumberanddiameterofrollsaredependentonthekindandwidthofpapertobecalendered.Thebottomroll（calledthekingroll）isdriven,andtheotherareturnedbycontactfriction.

1.Mechanicsofcalendarnipaction

Themetalrollsaremadeofchilledcastiron,oralloysteel.Whensubjectedtonormaloperatingpressure,theyretaintheiroriginalcontouratthepointofcontact（thenip）andtherefore,withinthepressurelimitsofoperation,thenipcontactareremainsconstant.

Thisnipactionisaccountableforthelowerfinishvaluesinacalendarascomparedwithsupercalender.Anysheen,orgloss,resultingfromcalendaractionoriginatesfromsuchphysicalpaperpropertiesasplasticityorbulkiness,whichtendedtoenhancereplicationofrollsurface.Thereforeacalendarpossessesonlythepowertosqueezeorcompressthepaperandchangeitscaliperandsmoothness.

2.Typesofcalenders

Mostcalendersaresimilaranddifferolyinthenumberanddiameterofrollsandthenippressuresused.Thenamesofsuchtypesasmachine,breaker,andsmoothingcalendersrefertotheirlocationinthepapermachine.Allthreetypesareusedinthelineofpaperproduction.

3.Machinecalendar

Themachinecalendarislocatedinthelineofpaperflowimmediatelyfollowingtheoutletendofthedryersectionorfollowingabreakercalendar.

Paperisreceivedfromaspring-supportedroll（toachieveproperpapertension）situatedbetweenthelastdryerandthecalendarataconvenientheightandisledintothetopnipofthestackbythrowingorblowingthetailintothenip.Metalfingersguidethepaperintosucceedingnips,andfinallythroughalowerniptothereellocatedafterthestack.Allrollsareequippedwithdoctroostokeeprollsurfacescleanandbrightandtodoctoroffpaperwraparound,i.e.,paperthatgoesaroundaroll.Doctorsarearrangedtocontactrollindividuallyorasagroup.Nippressureisusuallyappliedbyaweighted-compound-leversystem,apowerscrewmethod,orpneumaticand/orhydrauliccylinders.

4.Waterboxes

Toimparthigherfinishandgreatersmoothness,shallowboxescontainingwaterheldattheconstantlevelarearrangedsothattherollorrollssoequippedarewettedontheuptakesideoftheniptocontactthepaperinthezoneofpressure.Wettingthepaperinthismanneraddstoitsplasticflowproperties,resultinginahigherdegreeoffinish.Waterboxes,whenused,aremorecommonlyplacedonpapermachinecaleners.Thesurfaceproducedinthismanneriscalledwaterfinish.Materialsotherthanwater,suchaswax,starch,etc.,arealsoappliedbyawaterbox.

5.Breakercalender

Thebreakerstacklocatedinthedryertrainfunctionstogivetopaperorpaperboardalevelingactionorlumpsoffillerstockinboard.Thecomplementofrollswillvaryfromtwotoniceormore,andtheirdiameterswilldependonthekindofactionrequired.Breakerstacksarealsousedonuncoatedbondpaperoftheloft-driedtypeandintheboardindustry.Breakerstackeffectivenessisaffectedbysheetmoisture,andbreakerstacksmayloseeffectivenessasspeedsorgradesarechanged.

6.Smoothingcalender

Smoothingcalenders,asthenameimplies,areusedtolevelandsmoothouthighplacesandlumps.Theyarelocatedbeforethedryersandcalledsmoothingpresses.Theyusuallyconsistoftworollsofsimilardiameter,theweightofrollsusuallybeingsufficienttoproducethepressurenecessarytothedegreeofsmoothness.Suchstacksareusedtoadvantagewithlaminatingmachinesandon-boardmachines.

7.Operatingspeedsandpressures

Speedsofmachinecalenderstacksconformstopapermachinespeedsandnippressuresinaccordancewiththekindofpaperanditspropertiesandthedegreeoffinishdesired.

Currenttrendistowardmachinecalendersusingafewernumberofrollsandatleastoncrowncompensatingrollsandatleastoncrowncompensatingrollwithmorerefinedloadingsystems.Therollsinmoderncalendersarelargerandtheopenframeconfigurationallowsformoreconvenientrollremoval.Useofcrowncompensatingrollspermitsoperationatvariedpressurelevelstoobtainoptimumchangeinsurfacecharacteristicsandcaliperreduction.Whentwoorthreecrowncompensatingrollsareincludedinafourorfiverollcalenderthereisawiderangeofflexibilityavailableasstopressureapplied.

Sincedensifiedcalipershouldbeaslevelaspossibleinthecross-machinedirection,seriouseffortmustbemadetomaintaineventemperatureandmoistureacrosseachcalenderroll.Thisisdifficultbecauseofcross-machinevariationsevenwiththeflowofheatedoilinthecrowncompensatingrollsandwithcontrolledtemperaturefluidpassingthroughtheotherironrollsinthemachinecalender.Manymachinesalreadyoperatewithautomatedsystemstoselectivelyblowcoolairononeormorecalenderrollstochangethediameterandconsequentlychangethepressureappliedtothepaper.Thesesystemshavemanyairblowersmountedacrossthemachineandacomputerizedsystemthatcansensecaliperafterthecalenderandcancontrolthelocationandtheamountofairappliedtooverpowerabadbasisweightprofileorabadmoistureprofileanddeliverasheettothereelwithacceptablecalipervariations.

Tishouldbenotedherethatthismethodofinducinggoodcaliperisunacceptablewhenthepaperisgoingtoanoff-machinecoaterortoasupercalenderbecauseitcauseslocalizeddensityvariations,whichwillreactdifferentlytoasupercalender.

C.Supercalenders

Theonlyphysicaldifferencesbetweencalendersandsupercalendersisthesubstitutionofalternateintermediatemetalrollswithfilledrollsofmaterialpossessingresilientorelasticproperties.Suchfillingisobtainedbycompressingfabrics,papers,ornonwovenmatsofcellulosefiberunderhighpressure.

1.Mechanicsofsupercalenderaction

Manyyearsagoitwasgenerallyacceptedthat,bysomeforce,thepapersurfacewasrubbedagainstthesurfaceofthechilledironrollduetotheresilientnature,andperhapsthelargerdiameter,ofthefilledroll.Thisrequiredarollarrangementthatwouldpresendonesideofthepapertotheironrollintheupperpartofthesupercalenderandtheothersidetotheironrolls,inthelowerpart.Hencefinish,notablygloss,couldbeequalonbothsides.Thetheoryheldfirmuntilmillsproductiontwo-sideco