Sappi Technical Series Folding and Scoring

7/28/2019 Sappi Technical Series Folding and Scoring

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Folding and ScoringFinishing of Coated Papers afterSheet-Fed Offset Printing


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Folding and ScoringFinishing of Coated Papers afterSheet-Fed Offset Printing

Table of Con tentsI Introduction

Description of the situation in the printingand paper industry

Description of the situation in finishing Definition of terms

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V Influences on coating cracking What should be borne in mind during

paper production? What influence does the printing shop have? Recom mendations for folding

II Types of folding Types of folding for the finishing

of printed sheets

III Folding machines Folding machines for the finishing

of printed sheets

IV Folding What happens during folding? Comparison of 7 grades

VI Scoring Practical scoring experiments in 10

letterpress printing Evaluation of the practical experiments 10 Recom mendations for scoring in 12

letterpress printing

VII Scoring on the gatherer-stitcher and th e folding machine Scoring unit on the folder feeder 13 Scoring unit on the folding machine 13

VIII Con cluding rem arks 15

Overview of severaltypes of foldRecommendations forscoring

C 4

C 5


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I IntroductionDescription of the situation in the printingand pape r industryOne of the aims of the printing industry sector is to reproducecopies in a quality as close as possible to that of the original.Achieving this demands fine screen densities and a widerange of colours corresponding to nature. The range ofcolours, which defines the number of reproducible coloursin printing, is influenced by a diversity of facto rs.In addition to the scanning and printing technique , the surfacequality of the paper plays an important role in imagereproduction in offset printing. Coated papers can reproduce a larger colour space, and therefore more colours,than uncoated paper. Fine screen densities can also bereproduced w ith a higher contrast on coated papers.The paper industry has therefore always been faced with thechallenge of producing papers with surfaces capable ofmeeting higher and higher standards. App lication of a coating, either single or multiple, fulfils the requirement for anoutstanding surface.The trend towards more coating and further optimization ofthe paper surface will also continue in future.

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Sappi Alfeld GmbH

As a global paper manufacturer Sappi holds a significantmarket share of coated papers in the Western world. SappiFine Paper Europe has 7 produ ction sites for coated papersin Europe. Using state-of-the-art technology, these sitespredominantly manufacture woodfree coa ted papers for theprinting industry. A priority for Sappi is innovation and ongoing development of both the production processes and thepapers themselves.The world's largest and most modern paper machine forwoodfree coated papers is located in the Gratkorn mill. Inthe year 1706, paper production began in the Alfeld workswith the erection of a paper mill. The mill, with integratedpulp production, has continuously made its mark on thehistory of paper technology.

"-100 0 Glossy coated paper. g - -. Matt coated paper & LWC paper- - - 0 - - Uncoated paper

Fig. FOGRA colour space for sheet-fed offset printing


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Description of the situation in finishingFinishing the printed sheets after printing is the final stage inthe long production chain leading to a finished product. Duringfolding, the printing carrier (paper/board) is subjected to considerable mechanical stresses.Coloured areas or images are frequently printed over thefold. During subsequent finishing the coating layer or thefibre structure may split ope n. This splitting open is particularly obvious in places which have been printed .Coated papers are more sensitive towards this kind ofcracking during folding. The following information and tipsshow how fold cracking can be avoided, or at leastreduced.This brochure is intended to provide anexplanation and helpwith the folding of coated papers. In addition, a simpleoverview of folding techniques is presented.

\

k. -J

L J Scoring knife

Printingcarrier

Full score

1

- Scoring knifePrintingnarrip.rJ

K^^yScoringchannel

Hollow score

We distinguish betwee n:Definition of term sFoldingFolding a printed sheet in a regular sequence in order to giveit the required shape for finishing.GroovingCutting a chip out of the material to enable it to be bent orbroken along the line of the groove.ScoringProducing a linear depression to enable the material to bebent or b roken (folded) along the line of the score.In the printing industry the term grooving is often used whenscoring is meant. The brochure deals solely with the topic"scoring".

Full scoringThe reduction of flexural stiffness through compressing theprinting carrier.Hollow scoringThe reduction of flexural stiffness through forming of theprinting carrier.When processing coated papers, a hollow score with thecorresponding scoring channel produces better results incomparison with a full score. This brochure therefore onlycovers finishing w ith the hollow sco re.Coating crackingCracks and signs of breakage through localized over-stretching of the paper texture along the folding seam.Fold crackingBursting open and tearing apart of the paper texture at thefold seam through loss of strength.Fold breaksFold breaks indicate the num ber of folds in a folded sheet.In sheet-fed offset printing the main problem is coatingcracking. In rotary offset printing and digital printing, on theother ha nd, the main problem is fold cracking as a result ofheavy losses of m oisture from the paper.


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II Types of foldingTypes of folding for th e finishing ofprinted sheetsThere are several different types of fold that can be producedmechanically. We differentiate between parallel folds andcross folds.Parallel foldThe parallel folding techniques, e.g. spiral fold, zig-zag orLeporello fold, window fold or parallel centre fold, are mainlyused for processing leaflets and p rinted advertising materialsfor a wide diversity of requirements. A parallel fold is alwaysmade parallel to the preceding fold.

Folding knife

Printing carrier TFolding rollers

Knife folder

Front stop

Folding pocket

Folding rollers*

Printing carrier

MBuckle folder

Cross foldCross folds are primarily used in book w ork. A cross fold ismade at right angles to the previously made fold. In this way,for example, a 3-break cross fold generates 16pages.Combined folding methodsThe two kinds of fold can be combined to meet special requirements. This is referred to as combination folding.

An overview of the types of fold is presented on the 4th sideof the cover

III Folding machinesFolding machines for the finishing ofprinted sheetsDepending on the folding principle, a distinction is made b etween three different folding machine systems for processing single sheets:

Knife folderA folding knife presses the aligned and flat lying sheet between two rotating folding rollers. The folding rollers grip thesheet, fold it and carry it away. Higher basis weights can alsobe folded with a folding knife. In modern knife folders thearriving sheet triggers a pulse for the knife movem ent.Buckle folderA folding unit based on the buckle folding principle consistsof three folding rollers and a folding pocket with an adjustablefront stop. The sheet is fed into the folding pocket. When itmeets the front stop, set ac cording to the folding format, it isprevented from moving further. Since it cannot move anyfurther inside the pocke t it must do so dow nwa rds. There, itis gripped by the folding rollers, folded an d carried away.


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Combination folder

The buckle folder achieves a greater capacity comparedwith a knife folder since it is not subject to clock-pu lse-control. It is very easy to adjust and can also process severalcopies produ ced from one sheet at the same time.Combination folderThe combination folder is a combination of buckle folderand knife folder, which enables the advantages of bothtypes of folding to be utilized in a single machine. C ombinedfolding machines are very flexible and require little spac e.Modern folding machines are equipped with electronicsetting up, production monitoring and production control.Different sheet feeders and delivery systems facilitate handling of the printed sheets and the folded sheets.

Comparison of 7 gradesThe folding characteristics of the 7 most important silkpapers with basis weights of 170 g/rm2 were examined in afolding test. The sheets were printed with a blue area andwere then folded at right angles and parallel to the grain direction.Result: All the papers show coating cracking. The basepaper is visible at the burst open positions. If the fold is printedover, this becomes more obvious with inscoring contrastbetween the layer of ink and the white base paper. In thiscase the visual impression produc ed by the printed materialis impaired and the customer complains about fold breaksor coating cracking.

IV FoldingWh at happen s during folding?The paper is subjected to high mechanical stress duringfolding. Excessive stretching or compression of the fibrescan lead to cracks or bursting open of the outer coat (coating layer) of the paper texture. The coating consists predominantly of chalk and kaolin and is bound by a binder. Thecoating layer is not as strong as the base paper. The thickerthe coating layer, the greater is the danger of coating cracking.

Fold comparison ofmultiple coated papers available on the market


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V Influence on coating crackingWh at should be considered during paper production?Coating cracking is influenced by many factors. One importantinfluence is the quality of the fibre composition for the basepaper. High-strength fibres and a good bond between thebase paper and the coating layers are decisive in preventingfold breaks.The strength of the individual fibres is determined by themethod used to produ ce the fibre material. During chemicalfibre material production using the sulphite or sulphateprocess, digestion of the fibres must be optimally tuned tothe types of wood used. The subsequent bleaching stageshould be as gentle as possible in order to preserve thestability of the fibres.

During mechanical production of fibre materials the fibresshould suffer as little dam age as possible.The use of mechanically treated fibre materials impairs thefolding characteristics in comparison to chemically digested,woodfree pulps.In the case of impregna tion, care should be taken to minimiseembrittlement of the paper surface.With careful selection of short and long fibres, combinedwith optimal refining of these fibres, the paper maker caninfluence the fibre binding and thereby the folding characteristics. This is monitored during production by constantlymeasuring the strength in the laboratory.

Sappi aboratory


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V Influence on coating crackingWh at should be considered during paper production?Coating cracking is influenced by many factors. One importantinfluence is the quality of the fibre composition for the basepaper. High-strength fibres and a good bond between thebase paper and the coating layers are decisive in preventingfold breaks.The strength of the individual fibres is determined by themetho d used to produce the fibre material. During chemicalfibre material production using the sulphite or sulphateprocess, digestion of the fibres must be optimally tuned tothe types of wood used. The subsequent bleaching stageshould be as gentle as possible in order to preserve thestability of the fibres.

During mechanical production of fibre materials the fibresshould suffer as little dam age as possible.The use of mechanically treated fibre materials impairs thefolding characteristics in compa rison to chemically digested,woodfree pulps.In the case of impregnation, care should be taken to minimiseembrittlement of the paper surface.With careful selection of short and long fibres, combinedwith optimal refining of these fibres, the paper maker caninfluence the fibre binding and thereby the folding characteristics. This is monitored during production by constantlymeasuring the strength in the laboratory.

Sappi laboratory


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What influence does the pressroom have?Other influences arise during finishing in the pressroom orbookbinder/.Grain directionThe grain of the paper exerts an important influence on thequality of the fold. The final fold should always be m ade parallel to the grain. This ensures that the larger part of the fibresin the base paper are not over-stretched.If the fibre texture does burst op en, the breakage points areconsiderably smaller than for a fold at right angles to thegrain.Printing acro ss the foldIn a folding test we have established that after printing thepaper across the fold, the fold quality deteriorates and co ating cracking becomes worse as a result. In an extensivelaboratory investigation the strength of the paper was me asured before and after printing. The laboratory results revealed that the paper becomes less flexible. The increase instiffness depends on the binder in the printing ink. Thegreater the am ount of binder w hich penetrates into the texture of the paper and makes it brittle, the greater the increase in coating cracking.UV inks and UV varnishes are especially liable to cause increased brittleness of the paper, with considerable coatingcracking as a result.

Macro enlargement, unprinted

Macro enlargement, printed

Residual moisture I climatic conditionsA further influence is the residual moisture in the paper. Thepaper fibres try to ada pt to the prevailing climatic co nditions.The paper is delivered with a residual moisture content ofapproximately 50 %. In a humid climate the paper absorbsmoisture; in a dry climate it emits it. The moisture content ofthe paper is also redu ced as a result of heat, e.g. through anIR drying unit in the printing machine.

Folding test in the laboratory


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This showed that the climatic conditions have an importantinfluence on the quality of the fold. Printed sheets wereplaced in a climatic cabinet and acc limatized to different relative humidities from 55 % to 25 % at 21 C. The sheetswere then folded parallel and at right angles to the grain.Result: The folded sheets were evaluated in terms of coatingcracking. In this respect, folding quality deteriorated withdescoring relative humidity. The break positions becamelarger and the coating cracking co uld be seen m ore clearly.It may be conclude d from these results that constant climatic conditions have an influence on fold quality in paper p rocessing.As a paper manufacturer, we notice that more complaintsare made about coating cracking during the winter months.This period of the year is especially prone to very dry climaticconditions. For this reason, many printers have installed airhumidifiers both in the printing shop and in the boo kbinderyin order to ensure constant climatic conditions.

Fold at 50 % relative air humidity

Fold at 25 % relative air humidity

Setting up the folding machineModern folding machines are very fast and quite accurate.A digitally controlled folding machine is set up through anautomatic pre-set configuration provided by a foldingprogram. Nonetheless, the exact adjustment of the foldingrollers continues to exert an important influence on the foldquality. A sharp-edg ed fold has a greater tendency tow ardscoating cracking. This is caused by too much pressurebeing exerted on the paper. The quality of the folding rollersis also important. Elastic rollers reduce the pressing forceand improve the result of folding.Today's soft PU rollers require the lowest pressing force andconsequently produce the best result in terms of coatingcracking. W ith the knife folding principle, the folded sheet isdrawn into the folding m echanism by only 2 rollers. As a result, the stress on the folded sheet is somew hat lower thanthat generated through the buckle folding method with its3 - 5 rollers. With higher basis weights , the knife folder canachieve a marginally better result than the buckle folder inrespect of coating cracking.

Recommendations for Folding

How can the pressroom avoid coating cracking? Do not print dark areas or images over the fold. Always make the final fold parallel to th e grain. Set the pressing force of the folding rollers exactly

to the thickness of the folded product. The relative air hum idity in the finishing a nd

storage rooms should be about 50% . Avoid using UV varnish in the folding area if possible

since this can m ake the paper m ore brittle. Thorough scoring should take place before printing

across the fold in basis weights of 100# text a nd over.


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Evaluation of 250 g/m 2 HannoArt Gloss, Silk und Matt

3 p , ; K ^2.5 U~

2 j .co'" 1.5 l-mm -.- -l lIB lo ! : H L _ H1 2

Evaluation 25 0g /m 2

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3 4 5 6 7 8 9Differences in scoring tool, run direction and orientati on of the b ulge

10 11

9B|

|@ Gloss SilkD Matt

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The evaluation shows us that the results of scoring (and thecorresponding fold quality) for the different paper surfacesEvaluation of all basis weights using HannoArt Silkas an example

such as m att, silk or gloss, are comparable between the individual basis weights.

170 g/m 2 on the inside; 170 g/m2 on the outside;

; 200 g/m2 on the inside 200 g/m2 on the outside 250 g/m2 on the inside:H 2 5 0 g/m2 on the outside! 300 g/m 2 on the inside j

j B 300 g/m2 on the outside II 350 g/m2 on the insideI 350 g/m2 on the outside

S c o r i n g k n i f e

HannoArt Silk

1.5 E v a l u a t i o n

|jr L---^ P * T 200 g/m2 on the jnside o u s i e**^ 170 g/m2 on the outside

170 g/m2 on the inside w e i g h tSon the outside B a s i s w e yinside

Sappi Ev aluationThe Sappi evaluation scaleis interpreted as follows:0-0.75 Top1 -1.25 Upper standard1.5-1.75 Standard2- 2. 25 Lower standard2.5-2.75 Critical3 - 4 Unacceptable

All grades and basis weights produce d a better result if thescoring bulge of the folded produc t was on the inside. Thismeans that the printed sheet is pressed into the scoring

channel by the scoring knife from the outside. A scoringknife with a thickness of 0.7 mm (2 pt) produced a goodresult for all basis we ights.

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Optimal scoring with hinging effect: scoring bulge on the inside, good line ofthe score

Ba d scoring: scoring bulge on the outside, wrong line of the score

Further evaluation of the practical testIt can also be s tated that scoring against the grain producesbetter results in the large majority of cases, especially forbasis weights of 100# text -120# cover.In many cases the grain direction is chosen depending onthe later use of the product. At high basis weights > 120#cover scoring should take place parallel to the grain since,otherwise, the tendency of the folded product to open up becomes too great.However, the influence of the grain direction on the result offolding after scoring is lower than had been assumed at thestart of the experiment.

Recommendations for scoringBased on the results of the evaluation we can make the following recommendations for our papers when scoringin letterpress printing and at the scoring table.

Scoringknifein mm / pt0.5/1.50.7/21.0/3

Scoring groove width in inches100# 60-80# 80-100# 100-120#Text Cover Cover Cover0.031 0.031-0.039 0.039 0.039

0.031-0.039 0.039 0.039 0.039-0.0510.039-0.051 0.039-0.051 0.051

>120#Cover-

0.0510.051-0.059

After selecting the s coring knife and scoring groove suitable for the material, the following instructions shouldalso be followed: Far superior results are obtained if the scoring

bulge faces inwards. For basis weights of 100# text-120# cover scoring

across the grain leads to much better results. For basis weights over 120# cover the scoring

should run parallel to the grain . The relative air humidity in the finishing and

storage rooms should be about 50% . The scoring rule must be exactly centred on

the scoring channel. The scores must be formed sufficiently

strongly. Sharp-edged scoring tools must not be used.

A second copy of these recommendations is on Page 5inside the cover. You can take out this copy and, forexample, hang it in the printing shop or in the finishingdepartment.

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VII Scoring at the gatherer-stitcherand the folding machineIn order to avoid an additional working step, scoring is oftenperformed inline at the folding machine, the folder-feeder of the gatherer-stitcher or the cover feeder of theadhesive-binder.We have also carried out practical tests on a HeidelbergStichmaster ST 100gatherer-stitcher and on the HeidelbergStahlfolder KD 66 Topline folding machine.

Folder feeder in a gatherer-stitcher

Scoring unit on the folder feederOn the folder feeder all the basis weights from 170 g/cm2 to350 g/cm 2 were scored at right angles to the grain andparallel to th e grain. The setting there can also be change dsuch that the scoring bulge faces outwards or inwards afterfolding. The pressure exerted by the scoring tool can be setvery precisely. Different scoring channel widths are alsoavailable for sco ring.In this test we established that the information obtainedduring the scoring and folding tests in letterpress printingcan also be applied in many respects to scoring on thefolder feeder. An inwardly facing scoring bulge also produced better results here in most cases. This means that thescoring wheel presses the folded sheet into the scoringchannel from the outside. It was not possible to achieve asatisfactory result for the basis weights greater than 300 g/m2.Before finishing at the gatherer-stitcher, these should bescored in the letterpress printing.

Scoring devices on the folding machineWe distinguish between the following scoring devices at thefolding machine:Conven tional scoringIn conventional scoring a steel scoring knife is pressedagainst two counterknives with a sharp or rounded edge.The depth of the score is set by the separation betweenscoring knife and counterknives. Reducing the separationproduces a stronger score. A score which is too strong andtoo sharp is associated with the risk of the paper being cutthrough. Fine adjustment is possible through repositioningthe knife shafts.Scoring against PU (polyurethane)This scoring device consists of a steel scoring knife and arubber scoring channel (polyurethane). A scoring knife witha small diameter produces less of a depression than ascoring knife with a large diameter. The diameter of thescoring knife must therefore be selected according to theweight of the paper. Here too, fine adjustment is possiblethrough positioning the knife shafts. The risk of cuttingthrough also exists if the pressure exerted is too great or thescoring is too strong."Channel score" scoring from Heidelberg finishingIn this recently developed scoring device for Heidelbergfolding machines a score is produced by pressing a softscoring knife of natural rubber against a steel scoringchannel.

"Channel score" creasig device on the folding machine

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By means of a scoring and folding test we have comparedthe quality of the scoring devices in respect of coatingcracking. With higher basis weights the results of conventional scoring were not satisfactory.The folding result cou ld be im proved by s coring against PU.In this case c oating cracking was visible for the high basisweights.Using the recently developed "channel score" scoring device we achieved an optimal result in the sc oring and foldingtest, even with high basis weights such as a 350 g/m2HannoArt Silk. The quality of this scoring is comparable withthat produced in letterpress printing and retrofitting to thefolding machine incurs relatively low costs.For every scoring device it was found that our papers produce a better folding result if the scoring bulge faces inwards. The scoring knife should therefore be applied fromthe outside towards the inside. Adjusting the knife shaftoptimally and applying the minimum pressure to the foldedsheet had a positive effect on the result of folding.For other folding m achine manufacturers the descriptions ofthe folding device differ to some extent from the scoringdevices presented in this brochure. Most of the foldingmachines normally used in the printing industry are eitheralready fitted with scoring devices, or can be retrofitted.

Comparison of "conventionalscoring"

... and "channel score" scoring.

Folding and scoring -120# Cover HannoArt Silk

Comparison: Q Without scoring Conventional scoring Scoring against rubber/PU Q Channel score scoring Q Scoring in letterpress printing

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VIII Concluding remarksThe contents of this brochure result from extensive researchwork closely linked with practice, and with practical experience.

We wou ld like to take this opportunity to thank the followingpeople for their supp ort and assistance:Druckerei Bakeberg und Lohner, Hildesheim,(Scoring and folding test in letterpress p rinting)Druckerei Dobler, Alfeld,(Scoring test at the gatherer-stitcher and diversefolding tests)FOGRA, Munich,(Figure "Colour space" and information)Heidelberg Finishing GmbH, Ludwigsburg,(Folding and scoring tests and extensive information)All statements and information are correct to the best or ourknowledge. All liability for losses associated with suggestions given in this brochure is excluded, regardless of thelegal basis. The right is reserved to make technical changesto our grades in the course of further deve lopment.

2nd revised edition 2006All rights reserved 2001 by Sappi Europe SAImprintText, editing and typography: Kristof MertensFigures: FOGRA Munich, Heidelberg FinishingSappi Fine Paper Europe: Hans Soemers, Kristof MertensArchive Sappi A lfeld

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Folding and Scoring is on e In a series of S appi's technical b rochures. Through them, we share our paper knowledge with ourcustomers so that they can be the best they can be.

Adhesive TechniquesDevelopments in theprinting and paper making industriesand their effect on adhesive techniquesin the bookbinding trade

sappi Folding and Scoring and the other technicalbrochures are freely available at our kno wledge bank.The word for f ine paper www.sappi.com/KnowledgeBank
http://www.sappi.com/KnowledgeBankhttp://www.sappi.com/KnowledgeBank


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Overview: Types of FoldingCross folds

Cross fold: 2-break4 Sheets8 Pages




Parallel foldsSpiral fold: 2-break

3 Sheets6 Pages

Zig-zag fold: 2-break3 Sheets6 Pages

Spiral fold: 3-break4 Sheets8 Pages

Zig-zag fold: 3-break4 Sheets8 Pages

Combined zig-zag 4-breakand spiral fold: 5 Sheets

10 Pages

Window fold: 2-break

Parallel cen trefold : 2-break4 Sheets8 Pages

Double gate fold: 3-break

sappi


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Recommendations for Scoring

We recomm end the following combination ofscoring knife and scoring groove whenscoring our grades in letterpress printing orat the scoring table.

120#Cover-

0.0510.051-0.059

After selecting the sco ring knife and scoring groove suitable for the material, the followinginstructions should also be followed: Far superior results are obtained if the scoring bulge faces inwards. For basis weigh ts of 100# text-120# cover scoring across the grain leadsto m uch better results. For basis weigh ts over 120# cover the scoring should run parallel to the g rain. The relative air hum idity in the finishing and storage rooms should be about 50 % . The scoring rule must be exactly centred on the scoring channe l. The scores must be form ed sufficiently strongly. Sha rp-edg ed scoring tools must not be used.

saoDi


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Sappi Technical Series Folding and Scoring

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