Overview of Canlendering and supercallendering.docx
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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
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