A Study on doubling in the offset printing process

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Thesis/Dissertation Collections

5-1-1989

A Study on doubling in the offset printing process

Amal A. Ba'adarani

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A STUDY ON DOUBLING IN THE OFFSETPRINTING PROCESS

by

AMAL A. BA'ADARANI

AThesissubmittedinpartialfulfillmentoftherequirements of

thedegreeofMasterofScience intheSchoolof_Printing andManagementSciences intheCollegeofGraphic Artsand_Photographyofthe

Rochester Instituteof_Technology

May1989

CERTIFICATE OF APPROVAL

MASTER'S THESIS

This is to certify that the Master's Thesis of

AMAL AHMAD BA'ADARANI

with a major

in

Printing Technology has been

approved by the Thesis Committee as satisfactory

for the thesis requirement for the Master of Science

Degree at the convocation of May, 1989.

Thesis Committee:

Franz Sigg

Thesis Advisor

Cliff Frazier

Thesi'S

Co-Advir,~r

Joseph L. Noga

6raduat\1 Program Coordinator

Miles Southworth

Iwouldlike to thank the

following

fortheirtime,

help

and support:

Mr. Franz

Sigg

oftheTechnical andEducation CenteroftheGraphic Arts fortechnical

advice and

help

inexperimental_planning; Professors Cliff FrazierandCharles

Layne;

Mr.

William EisneroftheTechnical andEducationCenteroftheGraphic Arts fordonationof

LIST OF FIGURES iii

ABSTRACT 1

CHAPTER ONE. INTRODUCTION 2

FOOTNOTES 7

CHAPTER TWO. THEORETICAL BASIS OF DIRECTIONAL DOT GAIN 8

FOOTNOTES 15

CHAPTER THREE. REVIEW OF THE LITERATURE 17

FOOTNOTES 21

CHAPTER FOUR. HYPOTHESES 22

CHAPTER FIVE. DESIGN OF THE EXPERIMENTAL RIT DOUBLING TEST TARGET 23

CHAPTER SIX. METHODOLOGY 29

FOOTNOTES 35

CHAPTER SEVEN. RESULTS 36

CHAPTER EIGHT. SUMMARY, CONCLUSIONS AND OBSERVATIONS 45

CHAPTER NINE. RECOMMENDATIONS FOR FURTHER STUDY 48

BIBLIOGRAPHY 50

APPENDIXA. CALCULATION OFTHECHANGE IN PRINT LENGTH

CAUSED BY A CHANGE IN PACKING 53

APPENDIX B. DERIVATION OF THE MURRAY DAVIES EQUATION 55

APPENDIXC. TABLE OF REGRESSION EQUATIONS AND ANALYSIS OF VARIANCE 57

APPENDIX D. PRESS RUN DOCUMENTATION 77

1. Percent Dot Area Increasedueto

Doubling

of 1 Unitat Various Angles 27

2. Tableof randtvaluesfor Magnitude of

Doubling

versus

Doubling

Angle 36

3. Tableofr andtvaluesfor Dot Gainon 50%tint_versusAverage

Dot Gain on Concentric Circles 43

4. Mean ValueandStandardDeviation for Magnitudeof

Doubling

and 50% Tint 43

5. Tableof r andtvaluesfor Magnitudeof

Doubling

and

1.

Hierarchy

of Dot Gain Classification 3

2. Enlarged Dots

Showing

theEffects_ofNo Dot Gain. Fill-in. Slur.

Fill-in and Slur ₃

3. The Appearance of

Doubling

_{in Three Tones 5}

4. The Effect ofLight

Scattering

on GrainedAluminum andOpal Glass 6

5. The GATF Star Target 18

6. _{The GATF Dot Gain Scale} and Slur Gauge 19

7. The GATF Ladder Target 20

8. The RIT Experimental

Doubling

Target 23

9. 50% Dot and line Tint 26

10. Plotof% Area Change dueto

Doubling

Angle fora50% DotandLine Tint 27

11. Layout of Test Form used on Web-Press Run 32

12. PlotofVerticalandHorizontal Registeron aCartesian System

of Coordinates 32

13. Scatter DiagramandStraight Line Approximation between

Magnitude of

Doubling

and

Doubling

Angle for Black 1 and2 38

14. Scatter DiagramandStraight Line Approximation between

Magnitudeof

Doubling

and

Doubling

angle for Magenta 1 and2 39

15. Polar Graphs of

Doubling

for Black and Magenta 1 and 2 ₄₀

16. Scatter DiagramandStraight Line Approximation between Dot Gain

on50% TintandAverage Dot Gainon Concentric Circles

for Black 1 and 2 41

17. Scatter DiagramandStraight Line Approximation between Dot Gain on50% TintandAverage Dot GainonConcentricCircles

ofthis _study wasto test theusefulness oftheExperimental RIT

Doubling

Target in

identifying

directional dotgainintheoffset_printingprocess.

Doubling,

aformof mechanicaldotgain,contributes_greatlytocolor variations while

printing.

Slur,

anotherformofmechanicaldot_gain,is oftenconfusedwithdoubling. Dot Gain Test Targets currently inuse oftendonotdistinguish between thetwo. A slurtest was carried out andtheresults showedthatsluris not acontributortodirectional dotgain.

Thetargetprovedtobea visualand quantitative measureofDirectionaland

Non-directional dotgain. Theconcentriccircles, oneofthemaincomponents ofthetarget, weremadewiththeintentof_matchingnon-directionaldotgain similarto thatofa50% tint madewitha 150line/inchscreen-ruling. A slurtestwascarriedout andtheresults showedthatsluris notamajorcontributortodirectional dotgain. Thevalues for dotgain

obtainedontheconcentriccircles werefoundtobe higher invaluethan thevaluesfor dot

gainobtainedonthe50% tint.

Nonetheless,

therewas a _strongcorrelationbetween the twovalues ofdotgain onboth targets.

Direction,

another aspect ofdirectional dot_gain, wasstudiedinrelation to the

doubling

magnitude. Itwasfoundthatadefinitepreferred

Theoffsetlithographicmethodisone ofthemajor_printingprocessesinuse. The term offsetdenotesaprocess wheretheink isnottransferred

directly

fromtheplateto the paper(as ischaracteristicofdirect

lithography)

buttoanotherintermediatecylinder covered with a rubberblanketandfromthere to thesubstrate. As a_printing process,offset

lithography

iscapableofexcellentprint_quality, both infine lineandtoneworkat alower costthanother_printingprocesses. This is_primarilydueto thefactthat therubberblanket cantransfer theinktoroughsurfaces sothat_{higher quality printing}canbedoneonthem.

However,

offset_{printing is}adelicateprocess. Inspectionoftheproduction runis

mandatory tomaintainstringent_qualityrequirements.

Traditionally,

some ofthedifferent qualitycriteriathathave beenobserved are solidink

density,

tonereproduction(dotgain), trapping, slurand

doubling,

registerandcolorbalance

Tomeasure and control printquality,it is bestto measureand control each separate component which contributestoprint_qualityanditsvariation. In_evaluatinga press sheet,welookattheprint_qualityofhalftonesandthatis observed

by

_noticingthe featureofhow sharpandclearthedetail is. One faultthatiseither unique tohalftonesor most troublesomewhenhalftonesare

being

printedis dotgain. Dotgainis theincrease whichtakesplaceinadot'sarea

during

thetransferstagesfrom filmtotheprintedsheet. Dotgaininitselfisnot regardedas a_{printing fault}sincea certain amount of

inkspread,

when_{transferring from blanket}to paper,is

unavoidable.1

Non Directional Fill-in

Directional

Slur

Doubling

Figure 1.

Hierarchy

ofDot GainClassification

Mechanical Dot Gain: Thereare

basically

twokindsof mechanical dotgain:

non-directional anddirectional.

Non-Directional Dot Gain iscalledfill-in (figure 2). The halftone dots suffera

generalincrease insizeinthat the dotsget_uniformlylargerdueto_spreadingofinkoverthe edgeof eachdot. Thiscanbecaused

by

toomuchpressurebetweentheblanketand

impression_cylinder,or

by

_printingwithtoomuchink (also_takingintoconsiderationthe

rheologicalpropertiesofthe

ink)

and_possibly

by

thecharacteristicsoftheblanket (whether

conventional orcompressible).

No Dot Gain Fill-in

Slur

[image:10.541.78.460.425.588.2]

Fill-in and Slur

distinct featureofsluristhat_{it usually}occursinthedirectionof cylinder rotation onthe press (figure 2).

Slurreduces shadow contrast. Halftone highlightareas andfine

lettering

are usuallynotaffected.2

A slurredimpressionwilloftenbeuniformfromthefrontto the backofthesheet.

Mechanically,

themajor causebehindslurisattributedtoadifference in surface speedbetweenthe two_printingcylinders (plate-blanketortheblanket-impression). This difference in surface speediscaused

by

adifference in diameterofthe_plate,blanket and/orimpressioncylinder. Aslurredimpressionwillshow_upas aresult ofexcessive impressionpressure. Itcan alsobe caused

by

the_{ink acting}as alubricant inthe

heavy

ink coverage areas_permittingthesheetto_{slip in}theimpressionnip.3

Other factorsthat produce other variations of slur intheformofstreaksare alooseor_slipping

blanket,

printingwith asoftink (notenoughtack); ortheuse oftoomuch_{ink especially}when printingon coated stock. Defectivepaperintheformof wrinkled orbulgedpaper can resultinslur. Ona single colorpress,thedefectivepaper willcauseslur, buton a multi-colorpress, itwill show _upasadouble.

paperareits_primary causes. It isa problemof register variationbetweenthe_printing

unitsofa multicolor pressbut itcan_conceivablyalsohappenon a single color pressdue to

varying positioningof plate andblanketcylinders. Itcanalsobecaused

by

an unstable automaticregister systemin thecaseof a webpress.6

On sheetfedpresses,

doubling

would sometimes occur_alongthe backedgeof asheetof paperdueto thepaper

"slapping"

theblanketprematurely. Aprematurecontact ofthepaper withtheblanketunitproducesa weaktransferofink fromtheblanketofdotsthatare out of register withthe true dots

transferred to thepaper

during

impression. Sheets with areas thatarenotflat

(wavy

or tight-edgedsheets)and static_electricitycancause premature contactofthepaperand blanket. Thisprematurecontact will causethepaperto_prematurelytouch thesurface of

the_{blanket causing}afaint impressionnextto thefull impression dot. Paperslippagein

thegrippers, andexcessive wearinthegeartrainorbearingsofthepress are other possible causes ofdoubling.

[image:12.541.79.324.411.473.2]

5% 1 5% 45%

Figure 3. The Appearanceof

Doubling

in Three Tones

Optical Dot Gain istheapparentexpansionofthedotwhenlight isreflectedfrom the paperthroughthe ink. The dots behaveasif_theyarelargerthan_theyreallyare. For example a40% dotpatterndoesnot absorb40%(reflect

60%)

oftheincident light. In

screen-ruling.

[image:13.540.109.354.188.267.2]

Grained Aluminum Opal Glass

Figure 4. The EffectofLight

Scattering

onGrained AluminumandOpal Glass.

Figure

(4)

shows a comparison oftwokindsofsurfacesthatreact

differently

in

termsoflightscattering. A doton an aluminum surface wouldhavea_sharpedge with no

spreadingoflight. Aluminum isnota translucent_surface, which meanslightcannot

diffuseandscatter withinaluminum. A 50% dotwouldabsorb50% ofthelightand reflect

50%. A doton atranslucentsurfacesuch asopal glass wouldbehave differently. The dot

patternis _completelydiffused before itemergesfromthesurface. Ifactualmeasurements

weretakenwithadensitometerforthis50%dot_pattern,itwouldread a reflection

density

of0.6 insteadof_{0.3 representing}anincrease inreflectionduetotheeffect oflight

scattering.7

Inthis chapter, thedifferenttypesofdotgainhave been definedand

briefly

described. The

following

chapter willinvestigatetheproblemof

doubling

andattempt at

FOOTNOTES

CHAPTERONE

^outhworth,

Miles

F.,

Dot Gain Causesand Cures.

Quality

Control

Scanner,

Vol.2 No.9 1982pp.1

2Anon.,

Makethe Halftone fitthePaper. Research Progress Report No.

15,

September- October

1949,

Graphic Atrts Technical

Foundation,

Pittsburgh Pa.

3IbM,

4Bureau,

William

H.,

WhatthePrintershouldknow about Paper. Graphic Arts

Technical

Foundation,

pp.190,215.

5Treff,

Ernie

H.,

An Engineer's ViewofDot Gainon aWeb Offset Press. A

Presentationmade atthePIA Annual

Meeting

in San Franciscoon

May

10, 1988,

p. 11

6Ibid.

7J.

A. C. YuleandW. J.

Nielsen,

ThePenetrationofLight into Paperandits

Slur

Thereare twokindsofdirectionaldotgain: SlurandDoubling. Slur is

theoretically

causedwhenthe surface speedoftheplate,blanketandimpressioncylinders

are notthesame. Ifthediametersshoulddiffereither

by

_overpackingor_underpackingthe plate and

blanket,

thesurface speedofthecylinderswill_varyand causeblanketslip.1

A

changein packingwillbeneededinordertocorrectforthisproblem. Anadvantage,if it

canbe describedas such,is that_{packing has}to bequitedifferent before sluris detected

and evenif itwere,itwouldnotbeconsideredmuch of a problemas

long

asitwas

consistent throughout theprocess.2

The first hypothesiswasformulated inanattemptto

investigatetheimportanceofslur(seepage24).

Theoretical AnalysisoftheEffectofSlur

Theeffectof_packingon slur was_{theoretically}analyzedwiththeuse of an

equationthatcalculates the changeinprintlength asa result of achangeinpacking. This

equation canbeusedtocheckforanincrease in dotareaof a50%squaredotof 150 line

screen ruling. The 50% squaredot hadasidelengthof120microns. The

following

formula3 D=2PrtC

/

100 was usedfor calculatingchangeinprintlength resulting froma

change_{in packing}whereDdenoteschangeinprint

length;

C is for %circumference

occupied

by

image;

P is for packingchange.

Upon obtainingavalueforthechangeinprint

length,

a ratioofincrease inprint

lengthto the50% dotsidelengthof 120micronswas usedtoshow the_resultingchangein

sizeas a resultofthischangeinpacking. Theoverallincrease in dotsize waslessthana

micron or_{approximately}.3microns(see Appendix A foractualcalculations). Asa ruleof

thumb, a 1 micron changein diameterof a50% dotof a 150linescreen causes a changein dotarea of almost 1% dotarea. Sincetheslurdoesnotoccurall aroundthe

dot,

the

increase inthedotsizewouldbe muchsmaller. Thisis dueto thefactthatit is_occurringin

that slurisnot afault_{in printing}and_{consequently in directional dot}gain. The only problem ofdirectional dotgainthatis significantinpractical workis doubling.

Doubling

Doubling

caused

by

inaccuraciesonthepresscanbest beunderstood

by

identifying

the typesofpressesinvolved. Pressesare classifiedassheet-_{fedand web-fed} accordingto the form inwhichthepaperis fed intoand throughthepress. Onasheetfed

press,sheetsofpaperarefed intothepressoneata

time,

theimpression ismade and each

sheetisremovedordelivered intoapile. On a webpress, thepaperis fed fromarolland

printing iscontinuous asthepaper passesbetweentheimpressioncylinderandblanket cylinder. Itis importanttounderstandhow

doubling

iscausedon eachofthesepresses

andtounderstandthevariablesthat

interplay

tocreatethephenomenon.

Primarily,

adoubleexists wheneverthereis a probleminregister

(however,

thisis

nota misregisterbetween 2colors,but betweenacolor andits

"ghost"

image).

Doubling

caused

by

misregisterwithinasingle unit canbea result of variationintherelative position

oftheplateandblanket fromimpressiontoimpression. Therearetheoriesthatindicatethat

sideways_playof theplate cylinderinadditionto_printingwithaloose blanketcanresultin

avariationintherelative positionoftheplate_{causing doubling.}

Theeasiest_waytodepict how

doubling

occurs_{is usually} onamulti-color press.

Theactualmisregisteroccursasfollows: Thepresssheet entersthefirstunit ofthepress

andthe first layerofinkis laid down. Thepresssheetthen movesto thesecondunitofthe press andthestillwetinkofthefirstunit onthepaper, transfers to theblanketofthe

second unit. Whenit is time forthenext sheettoenterthesecondprintingunit, thisink

willbetransferredback fromthesecondblanketto thesecond sheet. This secondary transferofinkiscommonly knownasprint-back. Itis importanttounderstandthat this problemofmisregisterof

backtrapping

iswithina single color. Ifthe two sheets are not

from thesecond unit: theend resultis thatthedot increases insize_affectingcolor and contrast.

The EffectofPaperon

Doubling

Paper isa major sourceof_variabilitythatisresponsiblefor doubling.

Paper-relatedproblemsthatpromote

doubling

ona sheetfed pressareintheformofpaper wrinkling. Wrinkledpaperispaperthatlacks flatness. Paper_wrinklingcan becaused

by

animbalanceofmoistureinthepaper. Anexample wouldbe thatof paperbroughtintothe pressroomfroma storage area

having

adifferenttemperatureand/or

humidity

levelthan thatofthepressroom. _{Paper is usually} susceptibleto_wrinkling whenunwrappedand unprotected. Thepaper_quicklyreleasesorabsorbs moisture as itadjuststothenew environment. Thismoisture adjustmenttakesplace_primarilyaroundtheedgesofthe unwrappedpaper. Thiscreatesinternalstressinthe sheetsthatalmost guaranteespaper wrinkling. Moisture imbalanceoccursbetween theprotected areas

deep

insidethe stackof paper andtheunprotectedoutsideedges ofthesheets.

Depending

on whether paperis

losing

or_{gainingmoisture,} atight-edgedor wavy-edged conditiondevelops inthe sheet.4 A tight-edgedor_wavyedgedpaperdoesnot lie flaton_anysurface. Thisaffectsthe_way thepaperwill travelonthepress. Sometimes a slightbulgeor waveinthepaperwould causethepaperto touch the_{blanket prematurely resulting in}afaint impressionalongside themainimpression dot Thisis anothercause ofdoubling.

Paper-relatedproblems thatpromote

doubling

on a web presscanbest be

poorly set-uprollintheinfeedcanhaveconsequences_alongtheentirelengthofthe press.

Any

sidewaysmovementoftheweb(webweave)cancause aregisterchangefromone

revolutionto thenext. Thiseffectiscircumferential. Webweaveisconsidered as the

source ofthe_{largest doublesobservedinweb-offset}machinery.6

In addition,rollersin

theinfeed havea

tendency

toglaze_especiallywiththesmoothsteel rollers. Theurethane

rollers sufferthe samedisplacementasthatwhich occursbetweentheplate-blanket_nip

resulting intensionvariations. Webtensionhastobe heldconstant andthatcan_onlyoccur ifthe steel rollers remain clean. Failuretodosowillresultinpaperdust

build-up

onboth

thesteeland urethane rollers. Thiswillbe seenincolor variation onthepaperdueto the

mechanical shifts.

Tension inawebisalso affected

by

temperaturefluctuations inthedryer.

Tightening

and

loosening

ofthewebasthe temperaturevarieswill_consequentlychange

registerand cause_{doubling. Water pick-up}

by

thewebis anothercausefortension

variationbetweenthe units. Thistensionvariation can alsopromotedoubling.

Press CharacteristicsthatAffect

Doubling

Weband sheetfedpressesshare certaincomponentsinterms ofpressdesign. One

ofthesecomponents arebearers. Bearersarehardenedsteelrings foundattheendof

printingcylinder

bodies,

and areused onboth sheet andweb presses. The bearersofplate andblanketcylinders areincontactand_{supposedly facilitate} smoother_{rolling between}the

cylinders. Bearers foundon either sheet or web-fedpresses arethought

by

someto be

essentialfor high quality

/

highspeedprinting,butit isalsotrue that_manypresses have beenmadewithoutthem andrunquite _successfullyon similarkindsofwork.7

Externalfactorsthatfurtheraffectbearerperformance canbethepresenceofa

gum_coatingonthesurface ofthebearerswhileprinting. Gum coatingonbearersis

caused

by

gum

build-up

asaresult of_cleaningtheplate. Theeventualdepositof gum

coatingonthebearers is hardand cancause enough pressuretocause a

bump

whichforces thecylindersoutof paralleland could_eventuallyflattenaspotontheblanket.

Doubling

and Slurwouldresult,

depending

onthejob

layout,

anddueto theabsenceofparallelism between thecylinders.8

certainjobswouldhavecolor variationdueto thewearandduetothecylinders

being

out

ofparallel.

Theplate,

blanket,

andimpressioncylinders on an offset press aredriven

by

gears

and supported

by

bearers. Bearersarea common occurrence on web pressesbutnota

commondesigncharacteristiconsheetfed presses. Sheetfedpresses are_mostly

gear-driven. Evenwiththebestgear

drives,

cylinderswill_{vary minutely in}speed and as

themesh ofthegears move fromtooth to tooth. Dot Gain (thepercentof growth over

originaldot_size) anddirectional dotgain(slurand

doubling)

are caused

by

actions within

the_nip areasofthecylinders. _{The nip}areas arebetween the

inking

roller and_{plate, the}

plate and

blanket,

and

finally

betweentheblanketand paper.

Theplateandblanket nip isconsideredtobethemostcritical. Attheplate-blanket

nip,friction betweenthe twocylindersisreduceddueto the

lubricating

action oftheink

andwaterfilm. Astheinktravelsfromplatetoblanketandto_paper,this sameink issplit

in half betweenthe twosurfaces. Less inkbetweentheblanketand paperisa cause of

morefriction inthat_niparea. The resultingtransfer_occurringatthe_{printingnip, the}point

atwhichthepaperis actually_taking

ink,

is sodelicatethateven a slight variationinspeed can cause problemslikegear_streakinganddoubling. Thecylinderbearers

help

prevent

theseproblems

by

_smoothingout thedrivethrough_{rolling friction. For}maximum_effect,

theyarepreloaded, thatisthecylinderbearersarebroughttogetheruntilthereissubstantial

contactforcebetweenthebearerswhile theblanketiscompressedtoprovidethe_printing

pressure.9

Theradiusoftheblanketcylinderis constantly changingasitgoesthrough the

plate-blanketnip,becausetheblanketissoft whereastheradius ofthehardplate cylinder

remainsconstant. Becauseofthechangeintheradiusoftheblanketcylinder, we can

expectachange ora_tendencytochange ofthesurface speed oftheblanket. Thischange

insurface speed coupled with constant speed ofthesurfaceoftheplate cylinder produces a

situationinwhich_printingproblems such as_slurringand

doubling

are

likely

tooccur.

both sidesofthepaperinone pass. Theradiusofthe twoblanketcylindersin the_printing

nipare

simultaneously

changing inthe_niparea. Thischangeisexpectedtoproducea changeinsurface speedoftheblankets. Whetherthe_tendencyis forthesurface speed to increaseordecreaseisoflittlesignificanceinthat therewillbea similar_tendencyonboth sidesofthenip. _{Whateverhappensin the}

wayof

increasing

or

decreasing

speedonthe surfaceofthe

blankets,

the_elasticityofthepaper willallowittotrackthe twoblankets accurately

by

either_stretchingorcontracting. Thismakestheblanketto_{blanket nip} a_very stable operation and_printingproblemsare notexpectedto toarisethere. _{Instead any} problems

involving

the transferoftheimagesuchas _slurringand

doubling

are expectedto occurintheplatetoblanketnip.

Dr. A.

Ghany

Saleh found in his investigation intothecauses ofdotgain andits

effecton colorreproductionthat thepressureintheNIPareabetweenplate andblanketand

blanketand substratewasanimportantvariable. Thispressurewillresultin aninksquash whichcontributesto thedotgrowth. _{Machine geometry}ordesign andpressconditions caninduceotherformsofdotgainordotdefects suchassluranddoubling. Dotgain will ultimately be influenced

by

press_{configurations,}cylinder

diameter,

number of_gears, quality,dimensional stability,

finishing

and shape oftheteeth,inktrain,coolingsystem andthe typeof

bearing

in housing.10

Inkand water are twootherpressvariablesthat mustbe keptto aminimumto

maintaininkandwaterbalance.

However,

minimum water shouldberun without

promotingpilingonthe endsoftherollers. Excessiveamounts of watertend tobeused on

longer running

jobs;

howeverthiscanbe detrimentalto theprocess. Asmentionedearlier,

excessiveamountsof water means a greaterdepositofcoating, sizingandfiberfromthe

papercoating.This wetnessonthepapercancause_pilingand manifest itselfin a_grainy

print. Inadditionthiswatercan accumulate onthe

inking

rollers, vibrators, and

distributors andcan _consequentlypromote sluranddoubling.11

Blankettypeand_{quality influence}print quality. W. R. Grace & Co.U.S.A., a

leading

blanketmanufacturercarried outtestsof performance ofblanketsunder_varying

packingconditions. Itsstudiesrevealedthatdotgain and slur canbeminimized

by

proper

blanket packingandthatcompressibleblanketscan printwell undera much wider range of

packingthanconventionalblankets. Compressible blankets alsohavetheadded advantage

of

being

compressed_greatlyatthe_nipwhile_{maintaining relatively}light surface pressure

19

FOOTNOTES

CHAPTERTWO

1

Porter,

A.

S.,

Lithographic Presswork First

Edition,

Graphic ArtsandTechnical

Foundation,

1980,

p. 244.

2Ibid.

3Ibid.

4Anon.,

The Paper. Pressor_{Blanket may Cause Cut Sheets}toWrinkle. American

Printer,

Vol. 211 No.

5,

September

1988,

pp. 102.

5Crouse,

David B. Web Offset Press Operating. Second Edition Graphic Arts and

Technical

Foundation, 1984,

p. 121.

6Treff,

Ernest H. An Engineer'sViewofDot Gainon aWeb Offset Press. A

PresentationmadeatthePIAAnnual

Meeting

in San Franciscoon

May

10, 1988,

p. 25.

7Tyma

Louis

S.,

Ingo KoeblerandHerbert Stoeckle. Bearers A

Necessary

Evil?

TAGA AnnualProceedings

1982,

p. 402.

8Printing

IndustriesofAmericaWeb Offset Section

Meeting

Proceedings: 1987 Web Offset Section Annual

Meeting,

p. 244.

9Ibid.

10Saleh,

Abdel Ghany. The AnalysisoftheDot Gain Problemandits EffectonColor

11Ibid.

12Leslie,

Geoffrey.

BlanketsandtheControl ofDotGain. Australasian Printer

Magazine,

CHAPTERTHREE

REVIEW OF THE LITERATURE

Areview of pertinentliteraturerevealsthatfew studiesof mechanicaldotgain,

particularly

doubling

have beenundertaken. There have been indications in literaturethat

controlof mechanicaldotgain throughvarious measureshas beenattempted.

However,

beforecontrolis _possible,amethodofanalysis and measurementofdirectional dotgainis

needed.

Testtargets aretestimagesusedtodeterminethe_qualityof_{printing in}thevarious

reproductionstages. Sometest targetsare sensitivetodotgainand are capableof

detenriining

whether anincrease in dotareaisaresult ofdirectionalor non-directional

gain.

Doubling

is oftenconfusedwithslur. Directional dot_gain, whether

doubling

or

slur,cancause aregularhalftonetint togetdarkerandit may be hardtoassignthereal

causetoeither ofthe two. However ifatest targetisusedthatissensitivetodirectional

dotgain,it becomes easytodetermine

by

visualmeanswhetherdirectional dotgainhas

occurred. Oneofthefirstpapers publishedwheretherelationbetween directionaland

non-directionaldotgain wasstudiedwasthat

by

WarrenRhodes.1

He designedatest

pattern_consistingof parallelline tintsarrangedside

by

side;onewithhorizontalparallel

linesandtheother withverticalparallellines. Thispatternallowsvisualand objective

observationof

definition;

atermheusedtodescribesharpnessofprinting. He definedthe

term resolution asthe_abilityofasystemtoresolveordiscriminate between closely

spaced elements. Resolutiontestobjects perpendicularto thedirectionofsheettravelwere

affected

by

slur morethan thoseoriented paralleltosheettravel. Ifdirectional dotgain

exists,the twoparts ofthe targetwillbeaffected

differently

andonewill printdarkerthan

theother.Thisisavisualindicationof slur and/ordoubling.

Furthermore,

hisobjective

method ofevaluationwas conducted withtheuse of measurementstakenwitha

densitometer. Thismethod allowedhimtodistinguishbetween directionaland

termsofdensitometricmeasurements. The disadvantageofthis test targetwasthatit

wouldnotindicateadirectional dotgainif itshouldoccur at anangleof45 degrees.

The LTF Star Target (orwhatis known_today astheGATF Star

Target)

is an

adaptation and refinement of a similartarget thatwasdeveloped in 1957

by

RobertE. Woodof theWestern

Printing

and

Lithographing Company

Plantin Racine Wisconsin.

The WPLtargetconsisted of a solid one-eighthinch diametercentersurrounded

by

_ninety

pie shaped solid radial wedges. It isa visualindicatorof mechanicaldotgain; thecenter

increases in diameter ifink-spread

(fill-in)

occursandelongatesorstretches atrightangles

totheslurdirection ifsluroccurs.2

GATF designedthe Star Targetwith_{only 36}wedgeswhichresultedina much

smallercenter. Itprovedtobeaquickandeffective measure of

ink-spread,

slur and

doubling during

a pressrun. Thegeometricproperties ofthisarrangement makesitspress

sheetimage sensitivetoink-spread, slurand

doubling

_especiallywhen strippedinthe trim

areas at each corner ofthe_trailingedgeofthe presssheet. This is becausetheseareas are

usuallythemostaffected

by

slur ordoubling.

Doubling

willcause afigure 8to appearin thecentre ofthe star,while_slurringwillappearasan ovalinthe centrewithelongationsat

rightangles tothedirectionofsheet travel. Theproblem withthis targetis thatitcannotbe

usedtomakedensitometricmeasurementsfor dotgainbecause it is toosmall.

Originally,

itwas designed forvisual evaluation use.3

Figure5. The GATF Star Target

The GATF Dot Gain Scalewas anothertesttargetdesigned for

inspecting

and

evaluating halftones visuallyand withouttheuse ofthedensitometer. Italsowas a good

indicatorof changein dotsize atthevarious reproductionstageswhetherinthecontact

dotgain scaleis basedon_{the principle thattintsmade withfinescreens are}

much sensitive to changesin dotareathan_{are coarse screens. Itconsistedoftennumerical}

steps,starting from0to9. These_{numerical numbers were madewitha200-line screen}

whilethe

background

was of a coarser screenof65 tine.

During

thereproduction_stage, ifa

numberhasthe same

density

asthe

background,

itwouldbeanindicatorof anumeric

amountofgain. Whilethesenumbers are sensitivetodirectionaland non-directionaldot

gain,thereisalso aparallel-line typesectionwhichindicates directional dotgain.

Thisparallellinesectionis knownastheSlurGauge. Itconsistsof a barof

verticalline

background,

and a seriesofhorizontal lineswhichformthewordSLUR. The

lines havethe same_value, sothewordSLUR is invisiblewhen alllinesareprintedwith

equalthickness. But ifslur_occurs,thehorizontal linesthickenandthewordSLURshows

upasdarkerorlighterthan the background.

It2

.

p.

'-(,1

Figure 6. The GATF Dot Gain ScaleandSlur Gauge

GATFproceededtodesignanothertest targetlatertobe known astheLadder

Target. The Ladder Targetwasdesignedtoshow gripper-totailvariationinthevarious

reproduction stages.

Primarily

itwasdesignedtogive a qualitative measure

by

visual

inspection,

butnumerical measures of slur and

doubling

couldbeobtained

by

taking

densitometricmeasurements incertainareas. Thetargetwasthree-quartersofaninchwide

andtwenty-five inches long. Thecenter portionconsistedofa50%percenthorizontal line

screen. Thetwoouterstripsarethesameexceptthat thelinesrun vertically. Slurand

doubling

are measured_{quantitatively}asthedifferencebetweenthe two_{neighboringareas.}

The Laddertarget_{is very}sensitivetodirectional dotgainand manifestsitself

by

a

perceptible

darkening

ofthecenter ofthetargetinthedirectionofprinting.

Doubling

can

also causethecenter ofthe target todarkenifthe

doubling

was_{occuring in}the_printing

direction. Ifthe

doubling

was_occurringsideways, the sidesofthe targetwill_{be darkened.}

of_{showinganydirectional image displacementif itshouldoccur atanglesof45degrees.}

[image:27.541.132.373.159.214.2]

Thereforealaterversionofthis targetalsoincludedabarwithlines at a45angle.5

Figure 7. The GATF Ladder Target

The UGRA Plate Wedge isanothertarget thatcantestfor directional dotgain.

Since itsconceptionin

1962,

the targethasundergone some changes. The 1976version

oftheUGRA-Scaletargetconsistsoffiveelements_namelyacontinoustone_grayscale, a

60and 120

line,

ahalftone stepwedge,circularpatchesusedasresolution targets,aline

patchand aslurtarget.6

The slurtargetisusedtotestfor directional dotgain. It consists

ofconcentric circleswhosedistance isequivalenttotheirwidth(50% area). Itis usedto

observeslurand

doubling

on thepress. Thistargetwas an actual refinement oftheparallel

linetinttargetbecauseitwillshowdirectional dotgain at_anyangle. Slurwould manifest

itselfas a_spreadingoflines runningacross the_printing

direction;

_{lines running}parallelto

the_{printing direction}are not affected. Slur is indicated

by

twodarksegments parallel with

the_{printing direction.}

Doubling

also manifestsitself_visuallyasdarksegments asinthe

case ofslur.7

Howeverthesesegmentscould_{be in any direction}andtherecan be 2or

more segments

depending

onthe_severityof misregister. Neitherparallelline tintsnor

concentric circles candistinguish between slur and

doubling

though_theyservethe same

purpose of

indicating

directionaldotgain.

Test Targets suchas

Gretag,

RIT Color Control Bars andtheKodakCustomized

Color Targetuseatestpatch comprised of concentric circles.

Generally,

theirdisadvantage

lies inthefactthat_theycould_onlygivequalitative or visualindicationofdirectional dot

FOOTNOTES

CHAPTERTHREE

Rhodes,

Warren

L.,

StudyofObjectivemethodsfor

Evaluating

Sharpness in

Lithography

TAGAProceedings

1955,

p.109

2Jorgensen,

George

W.,

The GATF StarTargetforInkspreadandResolution

Measurements.Research Progress Report No.

52,

February

1961,

Graphic Arts

Technical

Foundation,

Pittsburgh,

PA.

3Anon.,

GATF Scales Detect Dot Gain. Canadian Printerand

Publisher,

April

1986,

p. 37

4Ibid.

5Hull,

Harry

H.,

The GATFLadderTarget- A New Test Image. Graphic Arts Technical

FoundationResearchProgress Report#

99,

October 1973

6Sigg,

Franz. A fewthings aboutMicrolinesthatmost peopledonotknow, TAGA

Proceedings

1988,

p. 434

CHAPTERFOUR

HYPOTHESES

1. The dotareadifference betweenthedarkandlightsegment ontheExperimental RIT

Doubling

Target isnot significantwhen 12mil_{packing is}addedto the blanketwhile

simultaneously removingthesameamountfromtheplate packing.

2. There isno_{statistically} significant_relationship betweenthedirectionof

doubling

and

themagnitudeofdoubling.

3. Thereis nodifference between Dotgain measured onthe50%tintoftheExperimental

RIT

Doubling

Targetandtheaveragedotgain calculatedfromthedotgain

CHAPTERFIVE

DESIGN OF THE EXPERIMENTALRITDOUBLING

TEST TARGET

Design oftheTarget

The RIT

Doubling

Target is 1 1/4 inchessquarein area. Themajor part ofthe

targetiscovered

by

aseriesofconcentric circles. Atthe cornersarelocated four 5mm

square patchesthatconsist of:

1. 3% dotpatch usedtoobservetheeffect of

doubling

on smalldots

2. Solidpatch usedtoindicate ink-filmthicknessinordertocalculatedotgain

3. 50% tinttocorrelatebetweentheaverageoflightanddarkarea values ofthe

concentric circles andthe50% tint

4. Aclear patchtozeroout thedensitometer.

Ineachhalfofthecircumferenceofthe circle,a_scale,

indicating

anglesfrom 0to 180of

directional dotgainisavailabletofacilitate

labeling

thedirectionofdirectionaldotgain.

RITDOU 160

LING 0

TARGET 20

1

'*-120_^^1 ^.60

100-j^H

30JH

eo^^H W12

w.-dfl^^^ 0 160

ol Techn

140

j

20

[image:30.541.167.259.476.572.2]

Rochester In ogy|

AspectsOfnewdesinnas comnarpritothe parallel linetint :

Itwas madeintocircles_{allowing it}tobesensitive toalldirections.

Its largesize allows oneto takeminimum and maximum measurementsinthe

lightestanddarkestsegmentswithinthepatch withanormaldensitometer.

Itsuse ofthe

Murray

Daviesequation_{allowing it}toobtain valuesfor directional

gain as opposedto theuse of visualevaluation methodsor

density

measurements.

Small dots maymakeiteasiertodifferentiate betweensluranddoubling.

Characteristicsoftheconcentric circlesdesign:

The basic advantageofthenewdesign isthelargesizeofthe concentric circles.

Thisenables onetomakedotarea measurements atthelightanddarksegmentswhich

gives anindicationofdirectional andnon-directionaldotgain. Moreoverthedesignofthe

concentric circlesisanadvantageinthatitsolvestheproblem ofdirectional dotgain

occurringat45 degrees whichwasimpossibleto takecareof with parallelline tints.

Theprincipleof_{measuring dot}gaininthisfashion isnotinquestion and_using

Murray

Davies is simplyan_existingmethodforthisspecific application. Wherethecircle

is

lightest,

thereisnodirectionaldotgainbecausethe

darkening

ofthe linefallson_topof

itself

indicating

nodirectionaldotgain. Theactual_readingwould_{only indicate fill-in}and

opticaldot-gain. Atthedarkestspot,we measuretheworstpossible caseofdirectional dot

gain aswellasfill-inandopticaldotgain. Hencethedifference betweenthelightanddark

segmentsis due to thecontributionofdirectional dotgain.

Problemswiththe method:

If

doubling

should occur over several_printingunits, theremay beacomplex

moirepatternthathasnot_onlyoneminimumandmaximumsection. This maymakeit

impossible tomeasure. This

is, however,

aproblemforall

doubling

_{measurements,} and

Aproblem arisesinthat

doubling

isa complex phenomenon. It isunpredictable

andcannot_onlyoccurinonedirectiononone_printingunitbut inadifferentdirectionon

another_printingunit.

Furthermore,

thecolorprintedonthe_{last printing}uniton apress,

normallydoesnothave any

doubling

of

dots,

since no more_printingunits for ittogo

through, exist.

Itwas_previously statedthereisthe_possibilityof

having

slur on_topof

doubling

andbothofthemcanoccurin different directions. Inorderforthismethodtowork,we

thereforeneedto_relyonsimplified assumptionswhichare:

1. Slur isnotconsideredmuch of aproblem;packing has tobeoff

by

a sizable

amountbeforesluroccurs.

2.

Doubling

occurs moreatthefirstand second units

following

theoriginal

impression thanbetweenthe second andthirdunits. _{The farther away}the

sheetis fromtheoriginal

impression,

theweaker willbetheprint-back and

thereforethe

doubling

andtheeffectofdoubling. The inkatthispointis

dilutedand weak and would not contribute_greatlytotheproblem.

Screen Ruling:

1)

Whatscreen_rulingshouldbeusedfor makingtheconcentric circleinorderto

obtain,ontheaverage, theapproximatesamevalue ofdotgainaswouldbe

obtained ona150line/inch50% dot tint ?

50% Dotof 150 Line/inch Screen

Ruling

_{Line Tint}

mlsiwulOOSSwu

14.14 units

10units

5uiii5

10units

100% Dot Area= (14.14 units)2=₂₀₀ Units2= Unit Square

50% Dot Area=(10.0units)2

= 100Units2= _{50% Dot}

Figure9. 50% DotandLine Tint

Ifwechosethegeometric relationshipsbetween dotandline screens such that

both showthesameareaincrease foradoublethat occurs at45_angle, thenthe

relationshipoffigure

(9)

results. Note thatthedotscreenis shown at a45 angle. Under

these conditions,thescreen_{ruling for}thelinetinthastobe 1.41 times finerthanforthe

dottint. Inotherwords theequivalent screen_{ruling for}a 150 lines/inch dot halftone isa

linetintwith 150x1.414=212 lines/inch_{screenruling.}

Belowis adiagramoftwo sinefunctions thatindicatesthepercent areachangedue

todoubling. Inotherwords, ifadoubleis afunctionofacertaindirection (or_angle),we

cancalculatetheexpected% changeinareafor boththedotandline tint. Tables 1 and2

arethecalculationsthatwere usedto_{illustrate graphically} thesinefunctions for boththe

[image:33.541.65.432.135.294.2]

3 O Q 3 o a> D) c m -C O (0 4) 10 n 8 -4 -2 -LineScreen T" T

90 180 270

AngleofDouble

Average

[image:34.540.90.404.118.321.2]

360

Figure 10. Plotof% Area Change dueto

Doubling

Angle fora 50% DotandLine Tint

Table1. Percent Dot Area Increase due to Doublingof 1 UnitatVarious Angles

Dot Screen

Percent Increase in Dot Area dueto Doubling Ancle

l*10*(sin(0)

+_cos(0)) = ₁₀ units2

l*10*(sin(22.5)

+_cos(22.5))- 13.06units2

l*10*(sin(45)

+_cos(45)) = 14.14

units2

l*10*(sin(67.5)

+_cos(67.50))= 13.06units2

l*10*(sin(90)

+_cos(90))= ₁₀ units2

= 5%_area

Line Screen

Percent Increase in l _{ine Area due to}

Doubling

Angle

2* 10*₁

*sin(0)

=0_units =0% _area

2*10*l*sin

(22.5)

=_7.65

units =_3.837%

2*10*l*sin

(45)

= 14.14 _units =7.07%

2*10*l*sin

(67.5)

= 18.48_{units =9.24%}

2*10*l*sin(90)

=_{20 units} = _10%

DoublingAngle

double0

double 22.5

double45

double 67.5

double90

Theaverageofahalfsine curveis defined

by

theequation: l/2*Tt*amplitude

DotScreen=_(2*2.07)/tc = _{1.317 : 1.317 +5.00}= 6.317 %whichrepresentsthe average

value ofthe dotscreen

CHAPTER SIX

METHODOLOGY

Theanalysis procedures utilizedinthisexperimentwere chosentofulfillthe

following

tasks:

1. Toshow whether sluris a major part_ofdirectional dot_gain

2. Tounderstand whetherthe

doubling

magnitudeisafunctionofthe

doubling

angle

3. Todeterminewhetherregistervariation onthe twodimensionsoftheweb

(circumferentially

and

laterally)

isrelatedto theproblem ofdoubling.

4. To examinewhether a_relationshipexistsbetweentheconcentric circlesandthe

50% tintanddetermine iftheconcentric circlescanbeusedtorepresenta50%tint

intermsofnon-directionaldotgain.

The First Experiment:

Inorderto_study theeffect of_slur,we can_purposelyproduceit

by

_removingthe

packing fromtheplateand_{replacing it}underneaththeblanket. Thiswill resultina change

indiameterinboth theplate andblanketcylinder. Theplate cylinderdiameterwasreduced

by

.3mmofpackingandtheblanketcylinder wasreduced

by

thesameamount. A

conditionassuch would_{theoretically}create slur. TheExperimentalRIT

Doubling

Test

Targetwas usedto_{visually verify}whetherslurhadoccurred.

The Second Experiment:

Since itwassuspectedthattheautomaticweb alignment mechanismmight cause

doubling

andregistrationproblems,twowebpressruns were carried out withthe

following

twoconditions: Conditions:

1- WebAlignerandRegisterControl wereonautomatic.

A_{web-steering deviceknownasthe tiltbox is found}

basically

attwolocationson a press_namelyattheinfeedcontrols andbeforethefolders. Thismechanismis

responsibleforthesideways placement ofthewebthroughthe_printingunitsandthe

folders.

Running

two testswherethetilt boxwas activatedanddeactivatedwouldpermit

studyingits effect on register anddoubling. Twotest targets sensitive toregister variations

were usedto_{visually detect}and measure_anyregisterfluctuations. Onewaspositioned

horizontally

andtheother_verticallyonthe testform.

Severaltest targets sensitivetodotgain wereincorporated inthelayoutofthe test

form.

They

were used as a visualreferenceforcomparisonbetweenthenewtesttarget

andthecommon onesinuse. Thecolorsequencethatwas run on thepress was

black,

cyan,magenta andyellow. Yellow

being

thecolor onthelastunit was run with ablank

platebecauseitwasfeltthat itwouldnotthrow_{any light}on

doubling

sincethelastuniton

thepress suffers theleastornodoubling.

Oncemakereadywas complete andtherequired

density

levelswereattained, 1000

consecutive sheets weretakenoffthe stackerandnumberedinproper sequence. The two

press runs were given code numbers 1 and2 signifyingtheabove mentionedconditions.

Magentaandblackwerethe twounits studiedfor

doubling

since_theyrepresenttheextreme

casesonthepress. Black

being

thefirstunitisexpectedtohavemore

doubling

than

magenta. Cyanwas not analysedbecause itwasassumed thatitsresponse willbe

intermediatebetweenblackandmagenta.

Twovisualreadings weretakenfromtheRITVisualRegistrationScale target.

One readingwasfor lateralregister andthesecondwasforcircumferentialregister. The

remainder of readings weretakenofftheRIT

Doubling

Target. Onewasavisualreading

oftheangle ofthedouble. Thedensitometricreadings weretakenwithaGRETAGD186

following

patches were read:thesolid_patch,the50%patchandthedarkandlight

segmentsintheconcentric circles. Itisimportanttonote that thesametargetsatthesame

locationonthepress sheet was usedinthecollectionofdata.

MeasurementofVerticaland HorizontalRegister

Thetermsverticalandhorizontalregister_signifycircumferential andlateralregister

variations onthe twodimensionsontheweb. Sideways movementoftheweb caused

by

theweb alignment andtheautomatic registersystem can resultinregister variations. Two

RIT Visual Registration Scaletargetswereincorporated intothedesignofthetestformto

relatethese twovariablesmagnitude ofdoubling. Onetargetwaspositionedinthe

directionof webtravelinorderto_{indicate any}circumferential variationthatmight_occur,

andtheother was positioned perpendiculartowebtravelto show_{any lateral}orsideways

changeinregister. Anotheradditional variable wascalculatedfromthevaluesobtained

fromcircumferential andlateralmeasurement andwasindicatedasMagnitudeof

Misregistration. Thisvariableis defined

by

theequation:

Magnitudeof Misregister=

V

(Vreg

Vave)2+

(Hreg

- Have)2

Vrfl: VerticalorCircumferentialRegisterVariation

I_eg

Vave: Averageor mean value ofVerticalorCircumferentialRegisterVariation

H.^:HorizontalorLateral Register Variation

Have: Averageor mean value ofHorizontalorLateralRegisterVariation

Hypothesis2was anattemptat_relatingthemagnitude of

Doubling

to the

corresponding

doubling

anglesimultaneouslytakenfromthe sametarget.

However,

the

processof_tryingtorelatethesetwovariablesdidnotend atthispoint. Itwasfeltthat

relatingmagnitude of

doubling

toverticalandhorizontalregisterchangeswouldprovide

anglederived fromthe two_coordinates,vertical andhorizontalregistercan also bestudied

inrelationtomagnitudeofmisregister. Thevaluefor magnitude of misregister canbe

calculatedfromthe twovariables vertical andhoriontalregister variation. Theregister

angle and magnitudeofmisregister woulddefinethe twocoordinateson a polar system of

coordinates.

RIT Visual Registration

Scalepositi onet

horizontally

(lateral_effect)

Operator Side

Lead Edge RIT VisualRegistration Scale

positioned_vertically

(circumferential effect)

[image:39.540.53.451.189.596.2]

DirectionofPaperTravel

Figure 1 1. LayoutofTest Formused onWeb-PressRun

Circumferential Register

MagnitudeofMisregister

? _{LateralRegister}

AngleofMisregister

Tangentofthe angle = x_=lateralregister

y circumferentialregister

[image:39.540.56.442.201.396.2]

Determinationof

doubling

angle

Adegreeof_{uncertaintywas notedwhen}

tryingtoreadthevaluefor

doubling

angle offthe

doubling

target:

1. Itwasfeltthat

readingthe

doubling

angleoffthe targetdidnot needtoexceed an

accuracyof+₅ 10

degrees.

2. Becausetherearetwo_segments,thereisa_symmetryof180ratherthan

360. Thereforeanangleof0degreescorrespondsoristhe same as an angle of

180.

3. Inaddition, iftherewasnomoire or_specificallynodirectional dotgainthat was

evidentonthe_{target, it}wasimpossibletoassign a

doubling

angle. Wechosein

sucha situationtogiveita value of zero.

Analysis:

Aspecial programwrittenin BASIC languagewas usedtocollectthenumericdata that

wastaken_visuallyanddensitometrically. The Statistical MethodofAnalysis usedtotest

Hypotheses 2 and3were:

1)

Regression andCorrelation Analysis inorderto

develop

alinearequation_relatingthe

two variables,namely

Doubling

and

Doubling

Angleand50% Dot Tintand50% Line Tint

(concentriccircle). Itis importanttopoint out thatthismethod ofanalysisdoesnot

imply

theestablishment of a cause-and-effect_{relationship between}the twovariables.

2)

ThePearson ProductMomentCorrelationCoefficientr was usedtoinvestigatethe

nature ofthe_{relationship between}the twovariables. Values forr weredefined between- 1

and +1. Avalue of+1 indicatesthatx andyareperfectlyrelatedinapositivelinearsense

meaningthatallthepointsinthescatterdiagramlieona straightlinewith a positive slope.

Avalueof- 1 indicatesthat the twovariables areperfectlyrelatedina negative sense. Anr

value closeto zeroindicatesthatx and_yarenot

linearly

related.

3)

At testwas usedto testforasignificant_relationshipbetweenthedependentvariable and

theindependentvariables. Thecritical valuefora two-tailedtdistribution fora sample size

of500 andaconfidencelevelof99%equals2.58.

Any

t-valueintheexperiment above

4)

Graphical

Representation

wascarriedout_usingthe Macintosh Cricket Graphsoftware.

Allcalculationsandcomputationsweredone usingtheMinitab Statistical

Computing

System.

Testfor Significance:

In thestatistical analysisofthis study,the regression equation_relatinganytwo

variables thatwere studiedwasobtained. Theregression equationis

E(y)

=80+ BjX where

E(y)

=_{mean value of}

y fora given value ofx;

B0

=y-intercept

oftheregressionlineand

Bj

= _{slopeoftheregressionline.}

Theestimatedregression equation was obtained anditisrepresented

by

the

following

equation: _y =

b0

+bjx

Ifa_relationshipexistsbetweenxandy,thecoefficientBjX shoulddiffer fromzero.

Aconclusion_regarding thesignificance oftheregression_relationship canbe tested

usingthe

following

hypothesis:

HQ:

Bi

= 0

Hj.B^O

Oneofthepropertiesforthe_samplingdistributionfor

b:

isthat

Mean:

E(b1)

= B11

Thet test regarding

Bj

=bi Bi

/ sbl

hasatdistributionwith n-1 degrees offreedom

(n_representingthesample size and n-1 representingthedegreesoffreedom).

By looking

underatdistribution table, a valuefort_{corresponding}toalevel of

confidenceof99%and

(n-1)

degreesoffreedomis obtained. Ifthis tvalueis less

thanthecalculated value

B,

=

bx

Bx

/ sbl

, then thenullhypothesiswillberejected

and one can concludethat thereisasignificantrelationshipbetweenthetwovariables.

FOOTNOTES

CHAPTER SIX

iDavid

R.

Anderson,

Dennis J.

Sweeney,

Thomas A. Williams. Statistics Concepts

andApplications. Second Edition. West

Publishing

Company: St.

Paul,

CHAPTERSEVEN

RESULTS

First

Hypothesis

Visualexaminationofthe targetrevealedthatnoslur or

doubling

hadoccurred.

Therewas nomoireindicativeofthe_phenomenon,howevertherewastheexpected change

intheprintlengthoftheimageasaresult oftheshift_{in packing from}theplatetothe

blanket (Refertochapter2underslur). The imageprintedlonger becauseofthedecrease

inplatediameter. What isofimportance ina situationlikethis isthat achangeinprint

length isexpectedtohappen butnotthatofslur. The Null Hypothesiswasaccepteddueto

theobtained results.

Second Hypothesis

Therewas no_strongcorrelationbetweenmagnitudeof

doubling

and

doubling

angle. This wasobviousfromthelowvaluesforthecorrelation coefficientfor both black

and magenta. Thevalues obtainedfromthettest howeversignifiedthat therewasa

significant_{relationship between}the twovariables. Thissignificant_{relationship is}an

indicationthatweare99% surethat thevariablesinvolved forthetestforthecorrelation

didnotoccur

by

chance.

Table2. Tableofr andtvaluesforMagnitude of

Doubling

versus

Doubling

Angle

Magnitudeof _Doubling

Black 1 Magenta 1 Black2 Magenta2

r

Doubling

Angle .38 .46 .43 .49

t

Doubling

Angle 9.17 11.65 10.59 12.58

Whatwas

interesting

tonoteinthegraphical_analysis, wasthata_relationshipcan

beshowntoexistbetweenthe twovariables magnitude of

doubling

and

doubling

angle

by

simple observationofthepolar plots. Oneofthefeaturesofthescatter andpolarplotsas

showninfigures(13 and

14)

and

(15)

wastheorientation ofthepoints. Thereappearedto

bea preferred anglefor bothcolorsblackand magenta. Inthecaseofblack forconditions

1 and

2,

thepreferred angle waswithintherangeof120- 160 degrees. Magenta for

conditions 1 and2 hadapreferred anglebetween 60and 120 degrees. Magenta hada

distinct feature inthat thepreferred

doubling

anglewasintheorientation of papertravel.

This probably indicatesthatsidewayslateralregister variation(sideways lateralpaper

movement) doesnoteffecttheproblem of

doubling

asmuchasthevertical or

circumferential register variation onthe web. Aprobable explanation canbetheaffect of

webtension,thatcan_ultimately affect paper performance_{therebyexaggerating}the

doubling

problem. The t testalsoprovidedfurtherinformationthat99%ofthevariables

involved forthe testofcorrelationdidnotoccur

by

chance. Basedonthegraphical

representation andtheresults ofthet_test, onecan arriveattheconclusionthatthereexists

a_{relationship between}magnitude of

doubling

and

doubling

angle. Theresults allow usin

turntorejectthenullhypothesis.

Third Hypothesis

Therewasa_strongcorrelationbetweendot gain measured onthe50% tintmade with a150

line_{screen-ruling}anddotgain measured ontheconcentric circles. Thiswasindicated

by

thevaluesforthecorrelation coefficient rforconditions

(1)

and

(2)

onthepressfor both

colorsblackandmagenta(refertofigures (16 and 17). Thevalues were.84and.79 for

blackand magentaincondition

(1)

and.91 and.88forconditon (2).

Furthermore,

the t

testindicatedthatasignificant_relationshipexistsbetweenthe twovaluesfor dotgainin

that99%ofthevariablesinvolvedforthe testforthecorrelationdidnot occur

by

chance.

y=3.4437_{+0.016x R}=_{0.38 Black 1} c a 3 O Q O o 3 ?^ C D) J s 90

Doubling

Angle 180 c 3 o Q *-O V D 3 'c (0

y==3.339+0.0201X R==0.43 Black2

10

-0

-i : '

. . : 1

: '

"

; .

! > i _: . .

i

!

i :

'

| : , .

: ; . i

.i

1

90

DoublingAngle

[image:45.540.102.354.94.318.2]

180

Figure 13. Scatter DiagramandStraight Line Approximation between

C

3 O Q * o ffi o 3

'E

o> ra

y= 1

.9453 +0.0256x R= 0.46 Magenta 1

180

Doubling Angle

y= 1.892 +0.0248x R=0.49 Magenta 2

3 O Q

(9 (J

O Q

c o

180

[image:46.540.103.353.84.305.2]

DoublingAngle

Figure 14. ScatterDiagramandStraightLineApproximationbetween

150

180

150

0 180

OSI.

Black 1

OS I

150

180

150

0 180

OSt OSL

The distance fromthecenterindicatesthemagnitudeof

doubling

in % dotarea. [image:47.541.33.443.81.564.2]

Directionofpapertravelisat90 degrees.

y= _{-19.8185+1.181x}

R=0.84 Black₁

o w c o

cs O

o

D

65 75 85 95

AverageDot GainonConcentricCircle

y=

-12.6855+1.0999X R=0.91 Black2

5?

s

c o

ca U

o

a

65 75 85 95

[image:48.540.93.342.91.298.2]

Average Dot GainofConcentric Circle

Figure 16. ScatterDiagramandStraightLineApproximation between Dot Gainon50% TintandAverage DotGainon

y= _{-30.9446+ 1.31 05x}

R=0.79 Magenta ₁

o lO c o

(3

o Q

65 75 85 95

Average DotGainofConcentricCircle

y= -8.7546+ 1.0568x R=0.88 Magenta2

ss o IT)

C o c

'<5

(3 +*

o Q

65 75

[image:49.540.101.349.91.296.2]

Average Dot GainonConcentric Circle

Figure 17. Scatter DiagramandStraight Line Approximationbetween

Table 3. Tableof r andtvaluesfor Dot Gainon 50% tint

versus AverageDot GainonConcentric Circles

Dot Gainon50% Tint

Black1 Magenta1 Black 2

r Ave. Dot Gainon Concentric .84 .79 .91

Circles

t Ave. Dot Gainon Concentric 34.28 28.83 47.48 Circles

significance at alphalevelof .99 .99 .99

Further

Observations

Magenta 2

.88

40.78

.99

The Blackunithadahighervaluefor

doubling

than Magentaunderboth

conditions

(1)

and(2). Black

being

thefirstunitisexpectedtodoublemorethanMagenta.

Magenta

being

thelastunit printed onhadalowervaluefor doubling. Thiswas observed

fromthemean valuesfor MagentaandBlackintheanalysis.

Table 4. Mean ValueandStandardDeviationfor

Mag

jnitude

of

Doubling

and50% Tint

Doubling

50% Tint

Mean St.Dev Mean St.Dev. Min. Max. DotGain

Black 1 4.93 2.68 77.27 2.82 61.7 92.9 27.3

Magenta 1 3.86 1.70 81.08 2.13 76.9 90.1 31.1

Black 2 5.13 2.95 76.39 2.39 71.5 84.4 26.4

Thetestrun underbothconditionswherethewebalignmentand automatic register

systemwas activated anddeactivateddidresultina_{statistically}significantdifference in

magnitude of

doubling

andthemeanvaluefor dotgain onthe50% tint for bothconditions.

Themeanvalue ofthemagnitudeof

doubling

for blackwas_{slightly higher}undercondition

2thanitwas forcondition

1,

whilethereverse wastrueforthemean value ofdotgain for

the50% tint. Inthe case of_{magenta, the}mean valuesobtainedformagnitude of

doubling

anddotgain were _{slightly less}undercondition2thanunder condition 1. Eventhough

thesedifferencesare _{statisticallysignificant,}a0.2% areadifference dueto

doubling

has

simplyno practical significance.

Alltests showed averagedotgainwithintheSWOPspecifications of24+4%

exceptMagenta2whichhadanaveragedotgain of31%.

Table5. Tableofrandtvaluesbetween Magnitudeof

Doubling

and

Lateral-Circumferential Register

Lateral Register

Circumferential Register

Lateral Register

Circumferential Register

significanceatalphalevelof

Magnitudeof Doubling

Black 1 Magenta 1 Black 2 Magenta 2

.04 -.02

.19

.16

.04

-.14

-.03

.17

.89 -.45

4.54

3.71

.88 -3.10

-.73

3.95

no no

.99

.99

no no

no

CHAPTER

EIGHT

SUMMARY,

CONCLUSIONS

ANDOBSERVATIONS

Twomainhypotheses weretestedin thisexperiment. The first hypothesiswasif

doubling

occurs,doesit_necessarilyoccurinapreferred_{direction. After analyzing}thedata

to_try andindicatewhether a

relationshipexistsbetweenthe two variables, theresults

obtainedfromthe_{correlation coefficientr showedthat}

doubling

angle showedno correlationto themagnitudeofdoubling. Thismeansthat_generally, smaller values of

doubling

are not associatedwithdifferentanglesthanlargervalues ofdoubling. However

thescatter andpolarplotsindicatedthatfor both blackand_{magenta, there}wasapreferred

rangeof

doubling

angles.

Reading

the

doubling

angleoffthe targetwas not anaccurate

procedure andanelementof_uncertaintywasevidentinthe readings.

The secondhypothesis wasatestofthe target'susefulness andtofurther

determinewhetherthe targetiscapableof_representinga50% tint intermsofdotgain. The

originalintent inthedesignoftheconcentric circleswasthatitshouldindicate

non-directionaldotgain similartowhathappensona50% dotpattern. Thetargetpermits

separatemeasurement ofdirectionalandnon-directionaldotgain. _{The reading is important}

inthatitthrowslightonwhattotalportion oftotaldotgainis duetothe directionalpartof

theoveralldotgain value. Oneofthedrawbacksofthismethod andofthe targetin

general,isthatone can_onlytakemeasurements_manuallyandnot with a_scanning

densitometer. One hasto_visuallydeterminethelightanddarksegmentondie concentric

circlesbefore_attemptingto takeactualreadings with thedensitometer. Asmart

densitometercouldovercomethisproblemif itwas programmedto takereadingsat

specifiedlocationsontheconcentric circle. The_resultingvaluescanbecalculatedto

providethe numeric valuesfordirectionalandnon-directionaldotgain. Targets similarin

designto the Kodak Customized Colortargetmakeiteasiertousea_scanningdensitometer

Theconcentric circlesweremadewiththeintentof_simulatinga50% tintof150

line screen-rulingasfaras non-directionaldotgainis concerned. Thevaluesfor dotgain

ontheconcentric circles andthe50% tint didnothowevermatch. A strong andsignificant

correlationwasindicatedinthevalues obtainedforthecorrelationcoefficientr. However

thevaluefor dotgainon theconcentric circleswashigherthanthatofthe tint. Further

studies are still neededto accountforthis

discrepancy

_{especially if}theerrorcannotbe

attributedtoscreen-ruling.*&

Otherobservationsdrawn fromthis experiment:

The Experimental RIT

Doubling

Targetwasfoundtobea sensitivetargetin its

configuration anddesigntodirectional dotgain. Thecomponentsofthe targetlend

themselves tovisualandnumericalevaluationsofdotgain.

Primarily,

itwasdesignedto

testforalltheaspectsof mechanicaldotgain, directionaland non-directional. Numerical

valuesforthelatterandtheformerarepossiblethrough the use ofdensitometricreadings

fromthedarkandlightsegments oftheconcentric circles. Theoriginalintentof

having

a

3%tintwasforvisualidentificationoftheparticularkindofdirectional dotgain,be itslur

ordoubling. The 3% highlight dotwas_probablynotthebestsizeto_visually

identify

directionaldotgain. This is

due,

inpart, becausethesesmall sizedotstendto getlostin

theroughness ofthepaper. Ittakesmore than a

loop

oflOxmagnificationtoobserve it.

During

thepress run ofthe secondexperiment,itwas evident on someofthepress sheets

thatadouble dotexists. Animprovementonthetarget thatcould aidin_visuallizing

directionaldotgain more

distinctively

wouldbe

having

several highlight dotsmade

preferablywithinarange of2% to6% inorderto_{visually distinguish} adouble dotwhen it

occurs.

Anobservationwas notedinthevalue obtainedformagnitude of

doubling

onthe

presssheets. Asmentioned earlierthemagnitudeof

doubling

was calculatedas a

with theuseofthe

Murray-Davies

_{Equation. Thevaluefor}

doubling

on_everysecond revolutionoftheplateforthe

impression

ofthe

blanket,

wasfoundto_{be significantly}

higherthan thevalue obtained onthefirstrevolutionoftheplate. Thisvariationis possibly

dueto the slackthatpaper undergoes atthesecondimpressionoftheplate andtheblanket

becauseofthe_plate-gapblanketnip. Theblanketcould experience a slackness asitgoes

through the_gaparea ofthe_plate,whichmight resultina_{momentary decrease in}web

tension. Thisslacknessinpaper

tensioning

_potentiallycould causeregistervariation,or

doubling.

The RIT

Doubling

Targetcan beusedas a visualand quantitativemeasure ofthe

twoaspectsof_{Mechanical Dot Gain namely Directional}andNon-DirectionalDotGain.

Applicationsofthe targetcan be directedtowardspresschecksbefore_theyare placedin

operationorroutine checks on olderpresses. Thisappliestoproduction sheet-fed presses

and web-presses. Itcan alsobeusedto_{detect any} slur or

doubling

on proofsheets, before

theactual production sheets are run.

Inconclusion, eventhoughthereare someweaknessesto the_target,it isa useful

CHAPTERNINE

RECOMMENDATIONS

FOR FURTHER STUDY

The

following

is alistof_{recommendations thatwould proveusefulin}

understandingthe target's_{capabilities,} thatwere notcoveredinthisstudy.

1. Thetestrunsthat_{were carried out}in

this_studyweredoneonpressesthatdidnot

necessarilyrepresenttheconditionsofpresses outin industry.

Doubling

was

studiedinrelationtopaper and register effect. Itneeds tobeaddedthatequipment

wearis also a major contributorto theproblem ofdoubling. Thepressesthatwere

used arein good condition andtherefore tended tominimizetheproblem.

Implementing

the slur and

doubling

experimenton

industry

presses totestwhether

equipmentwearisacontributor could provetobeausefulstudy.

2. The data for Cyan

(being

thesecond uniton thepress) couldalso bestudiedin

orderto testitsrelevance totheproblem ofdoubling.

3. Thewebpressrun_{actually included}twootherconditionsinadditionto the two

outlinedinthisstudy. Thethirdandfourthconditionsweresituations where the

web alignerwas on manual whiletheregistercontrolsystemwas onautomatic and

thefourth condition wastheopposite. _{A study}oftheseconditions andtheireffect

on

doubling

couldthrowadditionallightontheproblem ofdoubling.

4. Thesecondexperimentthatwas carriedoutinthis_studyshou