Sappi presentation climate and paper

Climate and PaperThe interaction between climateand the processing of coated papersin printing and finishing

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Cover H

annoArt G

loss 250 g/m2, text H

annoArt G

loss 150 g/m2, 2004, ©

Sappi E

urope SA

, comm

[email protected]

Climate and Paper is one of Sappi’s technical brochures. Sappi brought together this paper related knowledge to inspire our customers to be the best they can be.

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Water Interference MottlingIs water an interference factorin offset printing?

Folding and CreasingFinishing of Coated Papers afterSheetfed Offset Printing

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

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The Printing ProcessSheetfed and heatset web offsetprinting technology

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Sappi Fine Paper EuropeSappi Europe SA154 Chausseé de la HulpeB-1170 BrusselsTel. + 32 2 676 97 36Fax + 32 2 676 96 65

sappiThe word for fine paper

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The Paper Making ProcessFrom wood to coated paper

Verarbeitung von MattpapierWarum verdienen Mattpapierebesondere Beachtung?

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Processing Matt PaperWhy do matt papersrequire special attention?

This one and the other technical brochures are freely available at our knowledge bank:

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Climate and Paper, the seventh technical brochure from Sappi Idea Exchange

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Sappi is committed to helping printers and graphic designers use paper in the best possible way. So we share our knowledgewith customers, providing them with samples, specifications, ideas, technical information and a complete range of brochuresthrough the Sappi Idea Exchange. Find out more on our unique web site.

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Electronic measuring equipment

Nowadays, electronic gauges with digital displays havebecome widely-used tools for measuring air humidity. Thesemodern gauges combine very fast response times with easeof use and calibration. Common humidity gauges all useone of two possible methods of measurement – conductivitymeasurement or capacitive measurement.

Conductivity measurement uses the changes in conductivityof hygroscopic electrolytes under the influence of vapourabsorption as the basic input for measurements. Capacitivehygrometers measure the capacitive changes of dielectricsubstances – non-conductors – under the influence ofvapour absorption. In both cases, these changes occur as aresult of changes in air humidity. Numerous instruments ofdifferent design are available for these measurements –sword gauges for measuring humidity in stacks of paper,surface gauges for measuring sheets and reels of paper.There are also sensors for measurements of air humiditywhich are consequently used to control moisturising andconditioning installations. Most of these instruments comewith a set of tools for easy calibration. To carry out thiscalibration process, the measuring cell at the tip of the instru-ment is sealed air-tight. Next, a saline solution is introducedin the very small space directly under the measuring cell inorder to set the air humidity. The values indicated by the toolare then compared to the values derived from the salinesolution, which must always be kept at an exact, prescribedtemperature.

Sword gauge

Surface gauge

lX Concluding remarks

The contents of this brochure are the result of practical experience and close collaboration with FOGRA, an organisation which has been very helpful in many ways.

FOGRAForschungsgesellschaft Druck e. V., Münchenwww.fogra.org

We would like to thank FOGRA for making textsavailable to us from FOGRA Praxis Report 50, "Klima, Papier und Druck" byDipl.-Ing. (FH) Karl-Adolf Falter, 1998

For the illustrations used in this brochure, we thank:

Rotronic Messgeräte GmbH, Ettlingenwww.rotronic.de

MAN Roland Druckmaschinen AG, Augsburgwww.man-roland.com

Wilh. Lambrecht GmbH, Göttingenwww.lambrecht.net

DRAABE Industrietechnik GmbH, Hamburgwww.draabe.de

17 18

Table of contents

I Introduction

� The situation in paper production 2and printing industry

� The situation in printing 3and finishing industry

II Definition of terms

� Weather, weather situation and climate, 4interior climate and surrounding climate

� Air temperature and air humidity, absolute 5moisture content, condensation pointtemperature and relative humidity

� Humidity of materials, absolute moisture 6content and humidity balance

III The influence of interiorclimate on paper flatness

� The influence of air humidity 7Wavy edges and tight edges

� The influence of interior temperature 8� The influence of humidity 8

on curling tendency� The influence of stack humidity 8

and temperature on ink drying

IV The relation between climate and technical printing problems

� Dimension variations 9� Static charge 9

V Circumstances withinthe printer’s power to control

� Circumstances in the workshop 10� Paper handling 10

VI Special issues in 11web offset printing

VII Problems in web offset printing

� Blistering 12� Breaking in the fold 13� Fluting 14� Expanding 15

VIII Measuring temperatureand humidity

� Measuring interior temperature 16and room humidity

� Measuring the moisture content of paper 16� Measuring the humidity balance of paper 16� Electronic measuring equipment 17

lX Concluding remarks 18

Climate and PaperThe interaction between climateand the processing of coated papersin printing and finishing

1

l Introduction

The situation in paper productionand printing industry

Discussions about the effect of climatic influences during storageand transport and in the process of printing and finishing, are asold as the industrial production of paper itself. Generations ofexperts have had to deal with the more pronounced negativeeffects and there are numerous publications in the field ofspecialist literature on measuring temperature and humidity. Soit is not as if experience is lacking on the subject. Still, many ofthe relations have not yet been sufficiently explained. And insome cases, cause and effect remain completely obscure. Inthe practice of paper processing, many delusions and misunder-standings still exist, particularly with respect to the interactionbetween climate, paper and printing.

In modern production facilities, paper intended for sheet offsetprinting is prepared at a relative humidity of 50%, with a deviationtolerance of 5%. Papers intended for web offset printing havesimilar or slightly lower humidity values, depending on quality.These humidity properties are continuously monitored in all stagesof the production process. And in order to keep the paper in

optimal condition for the subsequent printing process, it iswrapped in special packaging material that protects it fromclimate changes in the environment. How the paper – or, morespecifically, the fibre – will react to climate circumstances atthe printer’s or binder’s facilities or at the location of the endconsumer, however, is a matter beyond the control of thepaper manufacturer.

As a global supplier of quality papers, Sappi has a market shareof 25% in coated woodfree papers for the Western world.Sappi Fine Paper Europe manufactures coated papers inseven different mills across Europe, using state-of-the-arttechnology for the production of, mostly, woodfree coatedpapers for the printing industry. Innovation and continuousdevelopment are among our key priorities.

Gratkorn mill houses the world's largest and most advancedpaper machine for woodfree coated paper. All seven of ourproduction facilities can look back on a long tradition of papermanufacturing and several of our paper mills with integratedpulp production have led the way in new technologies for thepaper industry.

Sappi Ehingen

2

3

The situation in printingand finishing industry

Normally speaking, there are few climate related problems inprinting and finishing. As a result, climate only becomes anissue when printing problems, such as dot doubling, mis-registering, creasing or curling, do occur. Obviously, the likeli-hood of this happening, is more pronounced in seasonswith extreme weather conditions – hot summers and coldwinters. In these periods, improper handling – prematurelyunwrapping the paper, use of paper that is too cold – orunfavourable conditions in the printer’s or binder’s facilities,can have disastrous consequences for the flatness andrunnability of the paper.

This publication is intended to help clarify some of the moreserious negative effects of climate circumstances. It offersguidelines for correct handling of coated papers, one of thepre-conditions for optimal control during the actual processof printing.

ll Definition of terms

When we use the term weather, what we actually mean arethe atmospheric conditions at a certain location at a specificpoint in time. When we refer to the weather during a longerperiod of time, we talk of the weather situation.

The concept climate is slightly more complex. It refers to thelong-term weather conditions or weather situation in a certainregion in terms of temperature, air humidity, air pressure,precipitation, wind direction and wind-force, cloudiness andsun hours.

Interior climate is a term used for the air condition in roomspartially or completely shutting off people and equipmentfrom the influence of outside climate conditions. The interiorclimate, in other words, the climate condition of the imme-diately surrounding air, is decisive not only for human comfort,but also for the course of production processes and for thecondition of stored goods sensitive to temperature and humidity.

In this respect, there is obviously a big difference betweenconditioned and non-conditioned locations. A non-conditioned location is a room or workshop where climateconditions are not artificially controlled. In a conditionedlocation, climate conditions are controlled by means ofheating, humidifying and re-moisturing. In the case of non-conditioned locations, the influence from outside conditionsis strong.

In conditioned locations, heated during the winter months,but not air-conditioned, relative air humidity is the reverse ofoutside air humidity. When the heating is on, during winter,inside air humidity is at a minimum. During summer, it reachesmaximum levels.

Finally, there are the climate conditions in the immediate vicinityof an object, in our case a reel of paper or a stack of sheets.Here, the term surrounding climate is sometimes used.

4

Data logger HygroLog-D

Data logger, an instrument for measuring temperature and air humidity in adjustable intervalsof 15 seconds to 120 minutes for the maximumduration of one year.

5

Air temperature

Air temperature is a unit for measuring the warmth of the air,or, technically speaking, a unit for measuring the energy ofgas molecules – nitrogen and oxygen.

When air takes on heat energy, the air temperature rises.The molecules accelerate and the air volume expands.

Air humidity

Air always contains a certain amount of humidity in the form ofvapour. There are two types of air humidity: absolute moisturecontent and relative humidity. Here are the definitions:

Absolute moisture content

The mass of vapour in a given volume of air, in other words,the amount of moisture, measured in grams, in a cubicmetre of air. In terms of printing practice, absolute moisturecontent is of minor significance, since it does not take intoaccount one vitally important climate component – tempe-rature.

Condensation point temperature

When humid air cools down to a certain point, the moistureit contains starts to condense. This temperature is referredto as the condensation point. It is one of the variables usedin measuring relative air humidity.

Relative air humidity

At a given temperature, air can contain only a specific amountof moisture in the form of vapour. The higher the temperature,the more moisture it can absorb. Air is called saturatedwhen it has absorbed the maximum amount of moisture itcan contain at a specific temperature. Relative humidity,then, is the proportion of absolute moisture content in relationto the highest possible moisture content at a given tempe-rature:

Since maximum moisture content is temperature dependent,temperature is one of the elements that determine relativehumidity. As we have seen, this is not the case with absolutemoisture content. The figure on page 6 shows the relationsinvolved. Using these relations, relative air humidity can becalculated on the basis of room temperature and absolutemoisture content. Reversely, absolute moisture content canbe calculated on the basis of relative humidity.

Thermo-hygrograph in action

There are two fixed points in temperature:

0 °C = the temperature at which ice melts

and

100 °C = the temperature at which water boils (at sea level).

relative =absolute moisture content

x 100 (%)humidity maximum absolute moisture content

Thermo-hygrograph, an instrument for measuringtemperature and humidity over a period of 24 hours or 7 days.

6

Humidity of materials

Porous materials like paper contain moisture – in the form ofvapour in the larger pores and in liquid form in the minutecapillaries of the paper structure. As with air, the humidity ofmaterials can be defined in two different ways:

Absolute moisture content

Humidity measured in percentages is the proportion ofmoisture inside the paper, in relation to the mass of thematerial. In paper production, absolute moisture content iscommonly used as a unit for measurements and control, butin printing and finishing, it hardly ever enters into the equation.

Humidity balance

Porous materials like paper aspire to an equilibrium – abalance – between their own humidity and the humidity ofthe surrounding air. This accounts for the balance in humiditythat will always exist between the humidity of the air separatingindividual sheets of paper in a stack and the humidity of thepaper itself. Humidity balance, then, is the relation betweenthe humidity of a material and the humidity of the surroundingair. As long as both values are balanced, the paper will notabsorb moisture, nor will it exude moisture. But when thereis a difference in humidity levels, the paper will adapt itself tothe humidity of the surrounding air by either absorbing orexuding moisture.

Relation between air humidity and temperature

30

25

20

15

10

5

0 10 20 30

10%

100%

90%

80%

70%

60%

50%

40%

30%

20%

Vapo

ur p

ress

ure

in m

m H

g

gH2O

/m3

air

40

30

20

10

Air temperature in °C

Rel

ativ

e ai

r hu

mid

ity

Vapour saturation pres

su

re

7

lll The influence of interiorclimate on paper flatness

The influence of air humidity

Particularly nasty problems occur in offset printing when thepaper used has certain deformations, either in the form ofwavy edges or tight edges. The reason why these phenomenacause so much trouble, is the full contact between blanketcylinder and impression cylinder in the printing zone, wherethese deformations can lead to dot doubling, misregisteringand creasing.

Wavy edges occur when the humidity of the sheets of paperin the stack is below that of the surrounding air, in otherwords, when excessively dry paper is subjected to average,but inevitably higher air humidity, or when normally humidpaper is subjected to extremely high air humidity. This willpredominantly be the case during the hot and humid monthsof summer in non-conditioned warehouses and printingshops, or when dampproof wrapping is not used duringtransport or storage in humid conditions. On the other hand,

if, during winter, too cold and already unpacked paper is introduced into the warm air of the printing shop, thesurrounding air temperature will sharply drop, thus causinga sudden rise in air humidity. In both cases, the edges of thesheets will absorb moisture, making them swell in relation tothe centre of the sheets. The result is wavy edges.

Tight edges occur when sheets of normally humid paperare subjected to exceedingly dry air humidity. In this case,moisture is absorbed from the edges of the sheets, which,as a result, shrink in relation to the centre. This will mainlyoccur during winter, when the relative air humidity in heated,non-conditioned or non-humidified working spaces candrop to levels as low as 20 % of the normal values. Normally,dampproof wrapping provides efficient protection againsthumidity influences. Obviously, to be able to offer suchprotection, the wrapping must be completely intact.

Deviations in humidity balance of up to 5% in either directiondo not lead to wavy edge or tight edge effects. At a differencein relative humidity of 8 to 10%, however, the situationquickly becomes critical.

Exchange of humidity in a stack of paper

The mutual influences of relative air humidityand stack humidity.

The arrows show the direction in which moisture is absorbed by or exuded from the stack of paper.

Tight edges

Wavy edges

A sword gauge for measuring relative humidity

8

The influence of interior temperature

Temperature has only minor effects on stack humidity. Never-theless, temperature remains an issue to reckon with, sinceit is one of the elements determining relative air humidity.This means that, in case of an observed difference in stacktemperature and room temperature, the paper should remainwrapped in its dampproof packaging until this difference intemperature has been balanced out. The time this takes, willvary in individual cases, dependent on the extent of thetemperature difference and the size of the stack. The figureto the right contains general guidelines.

One thing to keep in mind is that different types of paperhave different properties of heat conductivity. Hence, tem-perature balancing times can also vary with different papertypes.

The influence of humidityon curling tendency

The tendency to curl is closely connected to fluctuations inhumidity. Curling is caused by the paper fibres expandingand shrinking in the cross direction (see figure below). When

paper is moistened on one side, the fibres expand in onedirection, causing the paper to curl toward the dry side. Assoon as a balance in humidity within the paper structure hasbeen restored, the effect is cancelled out – unless this is prevented by an uneven fibre distribution.

The influence of stack humidityand temperature on ink drying

Exceedingly high humidity balance of the paper stack canlead to significant extension of ink drying times. Experienceshows that stack humidities of up to 60% do not cause dryingtimes to significantly go up. Above 60%, however, the effectis pronounced indeed, in some cases leading to drying periodsthree times as long as normal.

Extended drying times can also occur when the stack ofprinted paper is too cold. When printed paper is temporarilystored in a cold room (temperature dropping from 25 to5°C), the ink will take 10 to 15 hours longer to dry.

Expanding behaviour of paper fibres

Relation of temperature balancing time and difference in temperature andstack size

0 10 20 30 40

0.15

0.35

0.70

2.5

m3

1.5

Tim

e re

quire

d fo

r tem

pera

ture

bal

anci

ng (h

ours

)

Difference in temperature (°C)

20

0

40

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80

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120

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180

– 1 day

– 2 days

– 3 days

– 4 days

– 5 days

– 6 days

– 7 days

Orientation

Direction of expansion

9

lV The relation between climateand technical printing problems

Vegetable fibres are the primary raw material for paper, andthese fibres are sensitive to moisture. Depending on the humidity of the surrounding air, they either absorb or exudemoisture.

The extent to which paper contains moisture, is largely theresult of the raw materials used, but the way these rawmaterials have been prepared in the pulping process alsohas an effect. If the fibres have been intensively beaten, theirsurface size will have increased, and this, in turn, increasestheir capacity to absorb moisture.

Mineral fillers, such as calcium carbonate and kaolin, arenot actively involved in any processes of moisture exchange.Therefore, papers with a large proportion of fillers containless moisture than papers with low quantities of fillers or nofillers at all. Sizing (the application of a glue layer) has nosignificant effect on moisture content.

Depending on paper type, the level of moisture content caninfluence the general properties of a paper. For instance interms of its tensile strength, folding resistance and surfacesmoothness. In general, however, the issues mentioned onlylead to processing problems under exceptionally adverseconditions. This is very different in the case of two othercommon phenomena that do cause serious problems: staticcharge and dimension variations. Both can have a negativeimpact on the runnability of the paper, thus leading to mis-registering and other disturbances of the printing process.

Dimension variations

Depending on relative air humidity, the fibres contained inthe paper either absorb or exude moisture, causing them toswell or to shrink. In other words, the shape of the fibrechanges, significantly so in the cross direction, much less inthe machine direction. On top of this, during the process ofpaper production, the fibres orient themselves in the machinedirection (the run direction of the wire). The combined effectof these two phenomena inherent to the production ofpaper, is that dimension variations are far more pronouncedin the cross direction of the paper than in the machinedirection.

Different types of paper can show swelling levels of 0.1% to0.3 % in machine direction, as opposed to 0.3% to 0.7% incross direction. These are values that in the practice of printingwill never be reached, but they can be measured in tests ofmoisture-induced expanding according to DIN / ISO 8226-1.These tests show that a change in relative air humidity of10% causes the paper to “grow” in a proportion of 0.1% to0.2% across the width. This means that a paper of 100centimetres across, will expand 1 to 2 millimetres – a changein dimension that will definitely lead to printing problemssuch as misregistering. Fortunately, most printers are awareof the issue, and take these dimension variations intoaccount during pre-press and actual print run. Apart fromthat, the problem of misregistering caused by absorption ofmoisture has to a great extent been solved by technicalinnovations – such as moisturising installations, “low-fount”offset plates, the addition of alcohol to the fountain solutionand, last but not least, increased printing speeds signi-ficantly reducing the “dwell time” of the paper in the printingpress.

Static charge

Another problem that occurs from time to time, is sheets ofpaper “sticking” together. In most cases, this is due to staticcharges, primarily produced by friction, direct contact withother materials and sudden separation. Static charges mostcommonly occur when exceedingly dry paper is processedin conditions of low air humidity. A level of 40% to 32%humidity appears to be the critical bottom limit, both for thepaper itself and for the relative humidity of the air in theworkshop.

Static charges, causing sheets to stick together, can resultin multiple sheets being fed into the press at the same time.Static charges can also make the cushion of air, separatingtwo sheets in the delivery, dissolve too quickly, thus causingink from the printed side of one sheet to set off onto theunprinted side of the next one.

V Circumstances withinthe printer’s power to control

Circumstances in the workshop

Today, moisturising installations are used in practically everypaper processing environment. Most of these installationsare fully automatic or semi-automatic and require little or nomaintenance. Particularly during winter, when relative airhumidity can drop to very low levels, moisturising installationshelp to create optimal conditions in storage rooms andworkshops. The basic design is the same in all cases: a seriesof spray nozzles, operating on compressed air and spreadinga thin mist of water. The installation automatically maintainsthe required air humidity according to a set range of humidityvalues.

Paper handling

Especially in periods of critical climate circumstances,printers are advised to follow these guidelines:

� Paper is not an efficient heat conductor. Therefore,allow for sufficient time to let the paper adapt itselfto the temperature in the workshop.

� Do not open the paper wrapping until printing isabout to begin. The wrapping protects the paperfrom fluctuations in temperature and humidity.

� Infrared-drying, which drastically reduces therelative humidity of the paper, should be usedsparingly.

� During drying, the paper should not beexposed to extremely low temperatures, asthis would significantly extend drying times.

� Avoid damaging the paper wrapping and carefullyre-wrap remaining pallets.

10

Air humidifying installation EuroFog

11

Vl Special issues inweb offset printing

The technique of heatset web offset printing, with its specialdrying process, has its own, unique requirements. In multi-colour web offset printing on coated papers, the paper webis printed on both sides and thermally dried after it leaves thelast printing unit. Drying takes place at this point because anunsettled layer of ink would rub off on the turning bars, theguide bars and the former fold, causing the print to smearand preventing successful processing in the folding unit.

Heatset inks settle (or “set”) when the thin-liquid bindingagents evaporate. To make this happen, the printed paperweb is heated in a multisection drying oven, with differenttemperatures in each of the different sections. Usually, thefirst section has the highest temperature, which is thengradually reduced in the following sections. Overall, however,very high temperatures are used, because processing takesplace at high speed and the paper does not remain in thedrying oven for very long. When it leaves the oven, the paperweb is usually at a temperature of 100 to 130°C, dependingon paper quality, substance and ink covering.

Outline of a web offset printing press

Two-sided printing in a printing unit

12

Vll Problems inweb offset printing

Blistering

When the ink dries, so does the paper. In intensively printedareas, if the drying temperature is too high or if the paper issensitive to such problems, this can lead to blistering. Thesudden, extreme rise in temperature produces a build-up ofwater vapour in the internal structure of the paper. Andbecause the paper is not only coated, but also printed onboth sides – at certain places covered in thick layers of ink –this vapour has nowhere to go. This leads to tearing in theinternal structure of the paper and blistering in the printedareas (see figures).

From a technical point of view, blistering is a direct result ofink layer thickness and high temperatures in the drying oven.Thick layers of ink reduce the air (or vapour) permeability ofthe paper surface and high temperatures increase the amount– and the pressure – of vapour building up in the internalstructure.

The easiest and most efficient way to prevent blistering, isreducing the oven temperature. This means that printingspeed will have to be reduced as well, in order to achievesufficient settling of the ink at a lower drying temperature.

Since blistering only occurs in areas with intensive ink coveringon both sides, reducing the thickness of the ink layer – bymeans of UCR (UnderColour Removal), for instance – canalso have a positive effect. Apart from this, there are certainpaper properties that affect blistering, such as the type of

binding agents and coating pigments used, the amount ofbinding agents contained in the paper and the amount ofcoating used, and the degree to which the surface has been“closed” as a result of calendering. Obviously, the humidityof the paper is a very important factor as well. Usually, papersintended for web offset printing, particularly the woodfreetypes, have lower humidity levels than papers for sheet offsetprinting.

Blistering

Blistering

13

Breaking in the fold

Breaking in the fold is a common problem in web offsetprinting, particularly when mechanical papers are used.Broken or severely weakened folds can cause press stopsand can make the end product unusable (see figure to theright). The main sources of breaking in the fold are the extremetemperatures the paper is subjected to in the drying ovenand the pressure applied in the folding unit. The single mostimportant thing here is to find a compromise that will allowfor sufficient ink drying without causing the paper to dry out.

In the folder, the pressure applied by the folding rolls mustbe carefully adapted to the thickness of the paper used.

Breaking in the fold

Paper substance > 72 g/m2

Critical range< 10 N/15 mm(Breaking in the fold as a result of paper properties)

Middle range10 N/15 mm to 15 N/15 mm(Breaking in the fold as a result of paper propertiesor caused by processing issues)

Neutral range> 15 N/15 mm(Breaking in the fold unrelated to paper properties)

Paper substance < 72 g/m2

Critical range< 10 N/15 mm(Breaking in the fold as a result of paper properties)

Middle range10 N/15 mm to 12,5 N/15 mm(Breaking in the fold as a result of paper propertiesor caused by processing issues)

Neutral range> 12,5 N/15 mm(Breaking in the fold unrelated to paper properties)

These values apply to both machine and cross direction of the paper.

Paper should have a certain residual strength, for which the FOGRA method suggests the following values:

Fluting

Even today, more or less pronounced forms of “fluting” areamong the typical problems in web offset printing on coatedpapers. The waves run parallel to the printing direction –which, in the case of web offset, is automatically the machi-ne direction. One of the main paper characteristics that af-fect the severity of fluting, is the MC/CD (machine direction /cross direction) strength ratio of the paper. Strong longitudi-nal fibre orientation makes a paper more sensitive to fluting.Interestingly, the problem occurs most with papers of thelower substances. But the decisive factor is the printingform itself. Pages that contain intensively printed areas nextto areas with hardly any or no ink at all, are particularly sensi-tive to severe fluting. When this happens, there is nothingthat can be done in the printing process to prevent it.

Waviness observed before the paper is actually fed into theweb offset press, however, is a very different issue. One me-thod of partially preventing these so-called “tensile waves”,is to reduce web tension. The problem can never be totallyeliminated, since a certain amount of web tension will al-ways be necessary to prevent creasing or misregistering.

Humidity measurements of printed paper show that, in thedrying oven, practically all moisture is extracted from thepaper. Humidity balance values of 10 % are common.

14

Fluting

Fluting

15

Expanding

The problem of expanding or “growing” of printed paper, asseen when pages produced in web offset are combined withcovers produced in sheet offset, is caused by the intensivedrying of papers after they leave the drying oven. Here, somuch moisture is extracted from the paper, that it inevitablyshrinks to some degree, up to 0.3 to 0.7%. After the signa-tures have been gathered, stitched and cut, the paperbegins to adapt to the surrounding humidity once again,and starts to “grow”, causing the inside pages to extendbeyond the size of the cover.

The best method for counteracting or totally eliminating thisproblem, is sufficient remoisturing. Remoisturing installationsserve to evenly rehumidify the entire paper web after it leavesthe drying oven. This also improves paper flatness and totallyeliminates the risk of static charges. Remoisturing equip-ment can be easily installed on most existing printing presses.

Remoisturing installations use digital web sensors to controlhumidity and temperature. The installation is placed directlyabove, but not touching the paper web, thus creating a limitedspace with a carefully regulated and measurable artificialclimate in which outside influences can easily be compen-sated for. This way, paper quality is continuously monitored.Apart from that, these measurements supply large amountsof data that can be used as a basis for process improve-ments.

Remoisturing

Web sensor

Digital web sensor for measuring humidity balance and temperature on themoving web

Hygroflex measurement converter

Analogue output

Web sensorBFC-Dio

Web sensorBFC-Dio

Vlll Measuring temperature and humidity

Measuring interior temperatureand room humidity

The usability of values in a diagram, describing, for instance,development of relative humidity with changing temperatures,is completely dependent on the accuracy of the measure-ments taken. The curve in such a diagram shows that tempe-rature measurement in particular has to be very exact. Thismeans that thermometers should be used which indicatehalf degrees and (approximations of) tenths of degrees.Exact temperature measurements are relatively easy andeffortless, but the same cannot be said for measurements ofabsolute moisture content or relative humidity.

The practical problem with these measurements is that verysmall quantities of moisture have to be measured with veryhigh accuracy. The diagram on page 6 shows that, at a tem-perature of around 20° C, an increase in moisture content ofno more than 2 g/m3 produces a rise in air humidity of no lessthan 10 %.

Measuring the moisture content of paper

Moisture content measurements are highly uncommon inthe printing and finishing industries.

Measuring the humidity balance of paper

In contrast to measurements of the “absolute” moisturecontent of printing papers, determining humidity balance iscommon practice in the printing and finishing industry.Humidity balance is a unit indicating the extent of equilibriumbetween relative humidity of paper and surrounding air. Aslong as these two levels of humidity are balanced, the paperwill not absorb or exude moisture, which means that, in thisstate of humidity balance, no changes occur in the moisturedetermined (for instance, dimensional) properties of thepaper. To determine the relative humidity or humidity balanceof paper, changes are measured in the behaviour of objectsor materials which – measurably – react to moisture. Forinstance, the length changes of animal hairs, changes in theconductivity of electrolytes or changes in the resistance ofsemiconductors. In the practice of paper production andprocessing, these methods are widely applied in measure-ment and control systems. The very accurate methods forcalibrating such equipment are all based on measurementsof condensation point temperature and determination ofpsychrometric differences or vaporisation coldness. FOGRAreport 50 describes the working of condensation point gaugesand psychrometers.

16

Here, we will only list the various measuring methodsused in practice. More information can be found inFOGRA Praxis Report 50.

� The warmth-chamber method� The infrared-drying scale� Measurement of moisture content

on the basis of microwave drying� Measurement of moisture content

on the basis of microwave absorption� The Karl-Fischer method for measuring moisture content � Other methods

Climate and Paper, the seventh technical brochure from Sappi Idea Exchange

ideaexchange

Sappi is committed to helping printers and graphic designers use paper in the best possible way. So we share our knowledgewith customers, providing them with samples, specifications, ideas, technical information and a complete range of brochuresthrough the Sappi Idea Exchange. Find out more on our unique web site.

www.ideaexchange.sappi.com

sappi

Electronic measuring equipment

Nowadays, electronic gauges with digital displays havebecome widely-used tools for measuring air humidity. Thesemodern gauges combine very fast response times with easeof use and calibration. Common humidity gauges all useone of two possible methods of measurement – conductivitymeasurement or capacitive measurement.

Conductivity measurement uses the changes in conductivityof hygroscopic electrolytes under the influence of vapourabsorption as the basic input for measurements. Capacitivehygrometers measure the capacitive changes of dielectricsubstances – non-conductors – under the influence ofvapour absorption. In both cases, these changes occur as aresult of changes in air humidity. Numerous instruments ofdifferent design are available for these measurements –sword gauges for measuring humidity in stacks of paper,surface gauges for measuring sheets and reels of paper.There are also sensors for measurements of air humiditywhich are consequently used to control moisturising andconditioning installations. Most of these instruments comewith a set of tools for easy calibration. To carry out thiscalibration process, the measuring cell at the tip of the instru-ment is sealed air-tight. Next, a saline solution is introducedin the very small space directly under the measuring cell inorder to set the air humidity. The values indicated by the toolare then compared to the values derived from the salinesolution, which must always be kept at an exact, prescribedtemperature.

Sword gauge

Surface gauge

lX Concluding remarks

The contents of this brochure are the result of practical experience and close collaboration with FOGRA, an organisation which has been very helpful in many ways.

FOGRAForschungsgesellschaft Druck e. V., Münchenwww.fogra.org

We would like to thank FOGRA for making textsavailable to us from FOGRA Praxis Report 50, "Klima, Papier und Druck" byDipl.-Ing. (FH) Karl-Adolf Falter, 1998

For the illustrations used in this brochure, we thank:

Rotronic Messgeräte GmbH, Ettlingenwww.rotronic.de

MAN Roland Druckmaschinen AG, Augsburgwww.man-roland.com

Wilh. Lambrecht GmbH, Göttingenwww.lambrecht.net

DRAABE Industrietechnik GmbH, Hamburgwww.draabe.de

17 18

Climate and Paper, the seventh technical brochure from Sappi Idea Exchange

ideaexchange

Sappi is committed to helping printers and graphic designers use paper in the best possible way. So we share our knowledgewith customers, providing them with samples, specifications, ideas, technical information and a complete range of brochuresthrough the Sappi Idea Exchange. Find out more on our unique web site.

www.ideaexchange.sappi.com

sappi

Electronic measuring equipment

Nowadays, electronic gauges with digital displays havebecome widely-used tools for measuring air humidity. Thesemodern gauges combine very fast response times with easeof use and calibration. Common humidity gauges all useone of two possible methods of measurement – conductivitymeasurement or capacitive measurement.

Conductivity measurement uses the changes in conductivityof hygroscopic electrolytes under the influence of vapourabsorption as the basic input for measurements. Capacitivehygrometers measure the capacitive changes of dielectricsubstances – non-conductors – under the influence ofvapour absorption. In both cases, these changes occur as aresult of changes in air humidity. Numerous instruments ofdifferent design are available for these measurements –sword gauges for measuring humidity in stacks of paper,surface gauges for measuring sheets and reels of paper.There are also sensors for measurements of air humiditywhich are consequently used to control moisturising andconditioning installations. Most of these instruments comewith a set of tools for easy calibration. To carry out thiscalibration process, the measuring cell at the tip of the instru-ment is sealed air-tight. Next, a saline solution is introducedin the very small space directly under the measuring cell inorder to set the air humidity. The values indicated by the toolare then compared to the values derived from the salinesolution, which must always be kept at an exact, prescribedtemperature.

Sword gauge

Surface gauge

lX Concluding remarks

The contents of this brochure are the result of practical experience and close collaboration with FOGRA, an organisation which has been very helpful in many ways.

FOGRAForschungsgesellschaft Druck e. V., Münchenwww.fogra.org

We would like to thank FOGRA for making textsavailable to us from FOGRA Praxis Report 50, "Klima, Papier und Druck" byDipl.-Ing. (FH) Karl-Adolf Falter, 1998

For the illustrations used in this brochure, we thank:

Rotronic Messgeräte GmbH, Ettlingenwww.rotronic.de

MAN Roland Druckmaschinen AG, Augsburgwww.man-roland.com

Wilh. Lambrecht GmbH, Göttingenwww.lambrecht.net

DRAABE Industrietechnik GmbH, Hamburgwww.draabe.de

17 18

Climate and PaperThe interaction between climateand the processing of coated papersin printing and finishing

sappi

Cover H

annoArt G

loss 250 g/m2, text H

annoArt G

loss 150 g/m2, 2004, ©

Sappi E

urope SA

, comm

[email protected]

Climate and Paper is one of Sappi’s technical brochures. Sappi brought together this paper related knowledge to inspire our customers to be the best they can be.

sappi

Water Interference MottlingIs water an interference factorin offset printing?

Folding and CreasingFinishing of Coated Papers afterSheetfed Offset Printing

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

sappi

The Printing ProcessSheetfed and heatset web offsetprinting technology

www.sappi.com

Sappi Fine Paper EuropeSappi Europe SA154 Chausseé de la HulpeB-1170 BrusselsTel. + 32 2 676 97 36Fax + 32 2 676 96 65

sappiThe word for fine paper

sappi

The Paper Making ProcessFrom wood to coated paper

Verarbeitung von MattpapierWarum verdienen Mattpapierebesondere Beachtung?

sappi

Processing Matt PaperWhy do matt papersrequire special attention?

This one and the other technical brochures are freely available at our knowledge bank:

www.ideaexchange.sappi.com/knowledgebank

ideaexchangesappi

Climate and PaperThe interaction between climateand the processing of coated papersin printing and finishing

sappi

Cover H

annoArt G

loss 250 g/m2, text H

annoArt G

loss 150 g/m2, 2004, ©

Sappi E

urope SA

, comm

[email protected]

Climate and Paper is one of Sappi’s technical brochures. Sappi brought together this paper related knowledge to inspire our customers to be the best they can be.

sappi

Water Interference MottlingIs water an interference factorin offset printing?

Folding and CreasingFinishing of Coated Papers afterSheetfed Offset Printing

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

sappi

The Printing ProcessSheetfed and heatset web offsetprinting technology

www.sappi.com

Sappi Fine Paper EuropeSappi Europe SA154 Chausseé de la HulpeB-1170 BrusselsTel. + 32 2 676 97 36Fax + 32 2 676 96 65

sappiThe word for fine paper

sappi

The Paper Making ProcessFrom wood to coated paper

Verarbeitung von MattpapierWarum verdienen Mattpapierebesondere Beachtung?

sappi

Processing Matt PaperWhy do matt papersrequire special attention?

This one and the other technical brochures are freely available at our knowledge bank:

www.ideaexchange.sappi.com/knowledgebank

ideaexchangesappi