Process controls in production

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Information about Process controls in production
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Published on January 28, 2016

Author: MarkBohan

Source: slideshare.net

1. 12.2015 The Magazine 2 WHAT’S HOT! Theuseofprocesscontrolsandinstrumentationiscoretoasuccessful qualityprintoperation.Itunderpinstheproductionlifecycleandprovides thebasisforproducingaccurate,consistent,andrepeatableprintsfor customers.Itisdifficult,ifnotimpossible,toeffectivelymanagethepro- cesstomeetcustomerexpectationswithoutmeasurementtoquantify theprocess. Therearemanyreasonstoemployprocesscontrols;themostobviousis toavoidcustomerrejectionsandrerunsofexistingjobs.Wecomeface tofacewiththisinourlabwhenwereviewprintingjobsthatcustomers haverejectedduetocolorvariation.Thisunhappinesscanbeminimized ifthecontractdocumentscustomerexpectationsandtheacceptabilityof colorcanbebasedonmeasureddataandnotdifferingopinionsofwhat iscommerciallyacceptable.Theabsenceofprocesscontrolswillleadto inconsistenciesinthecolorandincreasedworkandinterventionfrom thepressoperatorstomaintainquality.Thiscreateslongermakereadies, therebydecreasingtheoverallefficiencyoftheequipmentwhileincreas- ingwaste. Whenconsideringprocesscontrols,itisimportanttounderstand themagnitudeofthecolormeasurementendeavorandwherethechal- lengeslay.Ithasbeenestimatedthatahumancandistinguishbetween 10millioncolors;thisputsintoperspectivethechallengeinusingan instrumenttomeasurecolorwithmoreaccuracythenwecanseevisu- ally.Theinstrumentshaveimprovedsignificantlyovertheyearsinboth theirqualityandaffordability.Measurementswithdifferentinstrument configurations,suchasdensitystandard,observerangle,ormeasure- mentaperturewillproducedifferentresults.Usingdifferentinstruments withinthesameinstrumentfamily,betweeninstrumentfamilies,and betweenmanufacturerswillalsoproducedifferentresults.Itisimportant tonotethattherearestandardsthatdefinetheinstrument’sconfigura- tion,andthemostrecentversionsincludecapabilitiesfordealingwith opticalbrighteningagentsandfluorescents.Thesearesummarizedas: M0—Alllegacymeasurementdevicesfallintothiscategory.The UVcontentisnotdefined,anditnotrecommendedwithfluores- cence.Itisusedformostcurrentstandards,e.g.,GRACoL2006. M1(D50)—Thisisforusewithsubstratesthatcontainoptical brighteningagentssotheusergetsabettermatchbetween measurementsandvisualobservations.Itisusedinnewstandards, (e.g.,GRACoL2013).Therearetwospecificpartstothis,withpart2 usedonlywithopticallybrightenedpapersandnotcolorantsthat fluoresce. M2(formerly:UVcut)—InM2,theUVcomponentofalight sourceiscut,andthefiltercharacteristicsfortungstenlightsources aredefinedforthefirsttime. M3(formerly:Polarizationfilter)—Thisshouldbeusedwhere firstsurfacereflectionsneedtobeeliminatedusingpolarization. ThisalsousestheUVfilterindicatedintheM2definition. Itisnotonlythepresswhereprocesscontrolsshouldbeappliedbutalso onalloftheincomingmaterials,plates,andproofs.Thiscanbedonewith instrumentation,targets,andincertainapplications,avisualcheckwith atarget.Withoutconsistentinputtothepress,additionalcostsintime andmaterialswillbeincurred. Twoexamplescometomind,thefirstbeingwithincomingproofs.Inthis exampletheproductionjobwassentbythecustomeralongwithaproof andcolormanagedtoaparticularprintspecification.Theprintercolor managedthefilescorrectly,butthepressoperatorhadsignificantissues ingettingacolormatch.Thiswasaconsistentproblemwithonecus- tomeronlyandresultedinadditionalcostsandlessprofitfortheprinter. Whenthecustomer-providedproofswerechecked,theywereidentified asbeingincorrect.Thecustomerwashappywiththeirproofs,sothesolu- tionwasmadeinprepressforthenewcolortarget.Theproblemcould havebeenavoidediftheincomingproofshadbeenqualitychecked. Asecondexamplewaswithinkbeingrejectedattheflexopressfor beingtooweak.Uponinvestigationitwasidentifiedthecausewasthe operatingproceduresonpressasthewater-basedinkwasbeingreduced. Therewerenocontrolsormeasurementsinplace,andtheoperatorswere addingtoomuchwaterandthenrejectingtheink. Platesneedtobecheckedanditisimportantthat,whenpossible,the instrumentsareconnectedtoacomputerthatautomaticallysavesand analyzesthedata.Therearemanysystemsthatdothis,andtheycan eliminateproblemsbeforetheyoccuronpress.Inoneassignmentwe Dr. Mark Bohan and Jim Workman, Printing Industries of America

2. 12.2015 The Magazine 3 WHAT’S HOT! werebroughtintosolvemakereadyissuesonpress.Operatorshadto alterpressconditionstogetacolormatch,andattimesplatesneeded toberemade.Uponinvestigationitwasdeterminedthatprepresswas taskedwithmeasuringtheplates,butsincetheywereconsistentoverthe firstcoupleofweeks,theyhadstartedtomarkthecontrolchartswithout measuringthem.Hadtheprepresspersonnelproperlymeasuredthe plates,theywouldhaveidentifiedtheissue. Controloftheprintedproductonpresscanbecarriedoutbydiffer- entmeans,andeachhastheirownadvantagesanddisadvantages. Traditionalprocesscontroliscarriedoutbymeasuringthedensity,tone valueincrease(TVI),andtrappingcharacteristics.Thenaturalvariation inanyprocessneedstobeunderstoodbeforetolerancesareputinplace. Themeasurementscanalsobeusedtohelpidentifyissueswiththe pressitself.Thiscanoccurregardlessoftheprocess—onadigitaldevice recentlyweidentifiedissueswiththetransferbeltandreplacedthisprior toacolormanagementexercise.Inanothercaseacolorvariationwas identifiedasonlyoccurringoncertainjobswiththecolorshiftsbeing visibleonsequentialsignatures.Analysisofthedensitydatashowedthe problemoccurredonallthejobs,butthatyoucouldonlyseeitsomeof thetimedependentupontheimagecomposition.Astheconditionsof thepresschangedensity,TVIandtrappingcanbeusedtoidentifythe changesandthepropercorrectiveaction. Thereisincreaseduseofneutraldensityorgraybalancecontrolonpress tocalibrateandthenmaintaincolor.Theneutraltonesaresome ofthefirstinwhichhumanswillseeacolorshiftandcanbeusedinpro- ductioneitherasavisualguideorwithinstrumentation.Itisimportant whenusingtheneutraltonesforcontrolthatthereisagoodunderstand- ingofwhytherehasbeenacolorshift.Thereproductionofsolids,tones, overprintingandregistershouldbeinvestigatedpriortoanychanges. WehaveseenamovetowardtheuseofCIEL*a*b*colorspaceinprocess controlswithmanycontrolpatchesenablingitsuse.Inaddition,the modernstandardsandspecificationshavemovedawayfromdensityto definingthecolorofthepatches/overprintsandsettingtolerancesusing CIEL*a*b*.Thisprovidesoneofthemostcompleteformsofprocess controlsandcanbeusedbothwithcontrolbars,spotcolors,andwithin theimageitselftoensurethatoptimalreproductionisasachieved.When changesareidentified,itisthenagainsteppingbackandlookingateach ofthecomponentsthatmakeuptheimagetoidentifythetruecauseof thecolorchange. Processcontrolisfundamentaltoobtainpredictableandconsistentprint. Theuseofinstrumentationprovidesthemetricsandthefoundationofa well-designedcontrolsystem.

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