Evaluation of Optical Formation

Measurements on Printing Papers and How They Can Be Used to Predict Print

Quality on Uncoated SC Papers

R O B E R T T O L K K I

Master of Science Thesis Stockholm, Sweden 2009

Evaluation of Optical Formation

Measurements on Printing Papers and How They Can Be Used to Predict Print

Quality on Uncoated SC Papers

R O B E R T T O L K K I

Master’s Thesis in Media Technology (30 ECTS credits) at the School of Media Technology Royal Institute of Technology year 2009 Supervisor at CSC was Christer Lie Examiner was Nils Enlund TRITA-CSC-E 2009:135 ISRN-KTH/CSC/E--09/135--SE ISSN-1653-5715 Royal Institute of Technology School of Computer Science and Communication KTH CSC SE-100 44 Stockholm, Sweden URL: www.csc.kth.se

Evaluation of optical formation measurements

on printing papers and how they can be used to

predict print quality on uncoated SC papers

Abstract Different paper grades have been used to evaluate the optical formation measurements for news-

print and SC paper. Formation of the SC papers has been measured both before and after super-

calendering to see how it affects the optical measurements. The optical measurements have been

compared to β-radiation measurements and the results showed that optical measurements give a

fair estimation of the formation on unbleached newsprint and uncalendered SC paper. The

results also showed that due to differences in light-scattering, optical formation of papers with

different grammages and brightnesses cannot be compared.

After the formation was measured, both optically and by β-radiation, the SC paper sheets were

printed in a Prüfbau gravure press to investigate whether there is any correlation between for-

mation and and print quality. The print quality aspects which were taken into consideration were

print mottle, print density and missing dots. The results show that optical formation measure-

ments can be used to predict the print mottle of uncoated SC papers. A strong correlation was

found for print mottle and formation in the 1–4 mm wavelength scale which is within the wave-

length range that affects the visual perception of print quality. Similarities in correlations bet-

ween print quality and formation before respectively after supercalendering make it possible to

assume that formation measurements before supercalendering give fair estimations of formation

after supercalendering. Unlike mottle, no correlation was found between sheet formation and

missing dots, which is one of the most crucial print quality aspects in gravure printing. It is like-

ly that missing dots are caused by smaller-scale properties and can be predicted by porosity

measurements.

Utvärdering av optiska formationsmätningar

på tidnings- och journalpapper och hur de kan

användas för att förutse tryckkvaliteten på

obestrukna SC-papper

Sammanfattning Olika papperskvaliteter har använts för att utvärdera optiska formationsmätningar för tidnings-

papper och journalpapper. För journalpappren har formationen mätts både före och efter super-

kalandrering för att se hur den påverkar de optiska mätningarna. De optiska mätningarna jäm-

fördes med β-strålningsmätningar och resultaten visade att optiska mätningar gav en ganska

rättvisande uppskattning av formationen för oblekt tidningspapper och okalandrerat journalpap-

per. Resultaten visade även att på grund av skillnader i ljusspridning är optisk formation inte

jämförbar för papper med olika ytvikter och ljusheter.

Efter att formationen mättes, både optiskt och genom β-strålning, trycktes pappersarken i en

Prüfbau djuptryckspress för att undersöka om det fanns någon korrelation mellan formation och

tryckkvalitet. De aspekter av tryckkvalitet som togs i beaktande var flammighet, densitet och

missade rasterpunkter. Resultaten visade att optiska formationsmätningar kan användas for att

förutsäga flammigheten på obestruket journalpapper som har tryckts med djuptryck. En stark

korrelation kan ses mellan flammighet och formation i våglängdsområdet 1–4 mm, vilket ligger

inom det område som påverkar visuella bedömningar av tryckkvaliteten. På grund av likheter

mellan korrelationerna för tryckkvalitet och formation på okalandrerat respektive superkaland-

rerat papper kan man anta att formationsmätningar före superkalandreringen ger en skaplig upp-

skattning av formationen efter superkalandreringen. Till skillnad från flammighet fanns ingen

korrelation mellan formation och missade rasterpunkter, som är en av de viktigaste faktorerna

gällande tryckkvalitet vid djuptryck. Det är troligt att missade rasterpunkter beror på papprets

mer småskaliga egenskaper och kan förutspås genom porositetsmätningar.

Foreword This master’s project was a part of my master’s degree at CSC, KTH. It has been carried out at

Stora Enso Magazine Paper, Kvarnsveden mill in Borlänge. Supervisor at CSC was Christer Lie

and at Kvarnsveden paper mill Dr. Jan-Erik Nordström.

I would like to give my deepest gratitude to my supervisor at the paper mill, Dr. Jan-Erik Nord-

ström, who has contributed with invaluable input. I would also like to thank Hans Ersson and

Eric Nyberg for giving me access to instruments and people at the development department and

the paper laboratory. Of course I would also like to thank all persons in Kvarnsveden who have

helped me, and especially Göran Johansson who has been patient and answered all of my end-

less stupid questions. My last thanks go to Dr. Per-Åke Johansson and Margareta Lind at

Innventia for helping me with information and measurements.

Robert Tolkki

October 2009

Table of contents 1 Introduction ........................................................................................................................... 1

1.1 Background ................................................................................................................... 1

1.2 Objectives ...................................................................................................................... 1

1.3 Aim................................................................................................................................ 2

1.4 Delimitations ................................................................................................................. 2

1.5 Notation ......................................................................................................................... 2

1.6 Word list ........................................................................................................................ 2

2 Literature overview ............................................................................................................... 4

2.1 Papermaking .................................................................................................................. 4

2.1.1 Fibres ..................................................................................................................... 4

2.1.2 Optical properties .................................................................................................. 4

2.1.3 Paper grades .......................................................................................................... 5

2.1.4 Calendering ........................................................................................................... 6

2.1.5 Paper formation ..................................................................................................... 7

2.2 Print quality ................................................................................................................... 9

2.2.1 Print density .......................................................................................................... 9

2.2.2 Print gloss .............................................................................................................. 9

2.2.3 Print mottle ............................................................................................................ 9

2.2.4 Missing dots ........................................................................................................ 10

2.2.5 Print-through ....................................................................................................... 10

2.3 Formation and print quality ......................................................................................... 10

3 Analysis ............................................................................................................................... 12

4 Methods ............................................................................................................................... 13

4.1 Formation measurements ............................................................................................ 13

4.2 Paper grades ................................................................................................................ 14

4.3 Samples for formation correlations ............................................................................. 14

4.3.1 Optical formation and -radiation absorption formation .................................... 15

4.3.2 Optical formation and print quality ..................................................................... 15

4.4 Printing ........................................................................................................................ 16

4.5 Print quality measurements ......................................................................................... 17

4.5.1 Print density ........................................................................................................ 17

4.5.2 Missing dots ........................................................................................................ 17

4.5.3 Print mottle .......................................................................................................... 17

4.6 Numerical measurements ............................................................................................ 18

4.6.1 PM 8 .................................................................................................................... 18

4.6.2 PM 10 .................................................................................................................. 18

4.6.3 PM 11 .................................................................................................................. 19

4.6.4 PM 12 .................................................................................................................. 19

5 Results ................................................................................................................................. 21

5.1 Evaluation of instruments ........................................................................................... 21

5.1.1 Added formation numbers ................................................................................... 21

5.1.2 Formation divided by wavelengths ..................................................................... 22

5.2 Optical formation and print quality ............................................................................. 26

5.2.1 Formation with added wavelengths..................................................................... 26

5.2.2 Formation divided by wavelengths ..................................................................... 27

5.3 -radiation formation and print quality ....................................................................... 28

5.4 Factors affecting formation ......................................................................................... 29

5.4.1 PM 8 .................................................................................................................... 29

5.4.2 PM 10 .................................................................................................................. 30

5.4.3 PM 11 .................................................................................................................. 30

5.4.4 PM 12 .................................................................................................................. 31

6 Discussion ........................................................................................................................... 33

6.1 Evaluation of instruments ........................................................................................... 33

6.2 Optical formation and print quality ............................................................................. 34

6.3 -radiation formation and print quality ....................................................................... 35

6.4 Factors affecting formation ......................................................................................... 37

7 Recommendations ............................................................................................................... 39

7.1 Improved formation number ....................................................................................... 39

7.1.1 Formation SC ...................................................................................................... 39

7.1.2 Formation News/MF/IN ...................................................................................... 40

7.2 How to improve print quality ...................................................................................... 41

8 Conclusions ......................................................................................................................... 42

9 Literature ............................................................................................................................. 43

Introduction

1

1 Introduction

1.1 Background Paper manufacturing is a competitive industry. There are numerous manufacturers all over the

world and a major way for them to attract customers is to produce a paper of high quality. The

most important aspect of quality is to which extent the paper allows printed graphics and text to

be reproduced – often referred to as print quality. Stora Enso’s paper mill in Kvarnsveden (KP),

works continuously towards a higher print quality in their products and aims to improve the

paper with regard to print quality in order to be one of the best paper manufacturers in Europe.

To allow graphics or text to be reproduced on a paper, the ink transfer is of high importance. A

good ink transfer demands a high surface smoothness and there are several factors affecting it.

One of the most important factors is the paper formation, which describes how the fibres and

fillers are distributed within the sheet. There are different ways of measuring formation and the

different methods have different advantages and disadvantages. The actual formation is measu-

red through β-radiation absorption, but this method is slow, laborious and expensive. Another

way is to measure the optical formation, which is based on a transmission of light. This method

is faster and less expensive than β-radiation, but unfortunately, it can give misleading results.

The light transmission in optical formation is affected by fillers, which have other optical

properties than the fibres, and when the paper is calendered (Norman, 2005).

There have been some studies on how formation affects the print quality, but they have all been

on products that are not similar to the ones that are manufactured at KP. Bernié et al. (2006)

showed in a study on North American fine papers that sheet formation has an effect on one of

the most important print quality aspects – mottle. The studied papers were manufactured with a

different pulp than at KP and the grammages were ranging from 72 to 104 g/m2 which are con-

siderably higher than the products at KP which range between 45 and 60 g/m2. Ahlroos & Nis-

kanen (2000) found that formation was the base paper property that showed the strongest corre-

lation to print mottle in half tone areas, but this was on coated papers and at KP only uncoated

paper is produced. Sävborg (2000) could not find any correlation between base paper formation

and print evenness on coated paper. Since it has been shown that there is a correlation between

sheet formation and print mottle but no investigations has been made on paper similar to the

grades that are manufactured at KP, it was considered relevant to examine whether the results

are applicable for newsprint and SC paper.

1.2 Objectives The main focus of this thesis is to find a correlation between print quality and sheet formation of

the paper that is produced at KP .

Following questions are central for the thesis:

Is there a correlation between β-radiation formation and optical formation?

How does calendering affect the formation measurements?

Can print quality be predicted with help of formation measurements?

Which factors affect formation?

Introduction

2

1.3 Aim When the correlation between formation and print quality has been determined, the aim is to

provide the operators of the paper machines with a recommendation on which paper formation

specifications they should strive for during the manufacturing. This will hopefully be one source

to enhance the print quality of KP’s products.

1.4 Delimitations The correlations between formation and print quality will be concentrated on SC paper. Most

SC paper is produced to be used for gravure print while the newsprint and improved newsprint

mainly are printed with offset. The mill has equipment for printing and evaluating the print on

SC paper (Prüfbau gravure) but not as good for newsprint due to a relatively small area printed.

It is possible to print newsprint in a gravure cylinder but the result would probably be mislea-

ding due to unevennes of the paper surface. SC paper is more complex than newsprint since the

formation is measured on base paper while it is printed after calendering and thus it is more

interesting to investigate whether there is any correlation.

1.5 Notation An assumption that it used throughout this thesis is that a good sheet formation improves the

print quality. For the print quality aspects that have been measured in this project this means

that a good formation results in fewer missing dots, lower print mottle and a higher density. If

the correlations that are found agree with the assumption it is expressed as a positive R² value

(between 0 and 1). In the cases when the correlations do not agree with the assumption it is indi-

cated with a negative R² value (between 0 and -1).

Another assumption that is used in this thesis is that a good optical formation corresponds to a

good β-radiation formation, i.e. a high optical formation number should result in a low value for

the coefficient of variation in the β-radiation measurements. Just as in the print correlations a

correlation that agrees to the assumption is expressed as positive R2 value, while a correlation

that is contradictory to the assumption is expressed as a negative R² value.

1.6 Word list Since this is a cross-disciplinary thesis that concerns both the areas graphical arts and paper pro-

cesses an explanation of words and abbreviations that are used might be useful for those who do

not master both disciplines.

base paper, SC paper before calendering.

blackening, when fibres are compressed to such an extent that the collapsed pores conduct light

instead of refracting it.

calender, hard or soft rolls that are used to smoothen the paper surface.

calendering, the technique to smoothen the paper surface between rolls in order to improve

gloss and surface smoothness, reduce surface porosity and compress topographic variations.

CD, cross direction, the direction in which the width of the paper machine is measured.

density, see »print density».

fillers, added to the pulp to fill the gap between the fibres.

finished paper, SC paper after calendering.

Introduction

3

grammage, the weight of the paper per a defined area. Usually expressed in g/m2.

groundwood pulp, pulp made of fibres that are scraped off logs against a rotating cylinder

made of sandstone.

headbox, where the pulp enter the wire.

jet, pulp leaving the headbox with a high pressure.

LWU, light weight uncoated paper with a D65 brightness of 78-79.

magazine paper, or SC paper, paper that is smoothened in a supercalender and is used in

magazines and catalogues.

MD, machine direction, the direction in which the paper goes through the paper machine.

MF, machine finished uncoated mechanical paper.

missing dots, the areas of paper which lack contact with the print cylinder.

mottle, see »print mottle».

parent reel, the paper is wound onto a reel when it has gone through the paper machine.

pope speed, the speed at which the paper is wound onto the reel.

PM, paper machine.

print mottle, a »cloudy» appearance of the ink when it is transferred to the paper surface.

print density, how much ink that is transferred to the paper.

pulp, fibres and other constituents, for example fillers, mixed with water.

reel, see »Parent reel».

SC-A, a paper grade with a D65 brightness of 67.

SC-A+, a paper grade with a D65 brightness of 72.

SC paper, or Magazine paper. Paper that is smoothened in a supercalender and is used in

magazines and catalogues.

SGW, stone groundwood pulp, see »groundwood pulp».

spool, the iron cylinder which the paper is wound onto when it leaves the paper machine.

supercalender, a type of calender which always is located off-line and can have up to 12 rolls.

TMP, thermomechanical pulp. Pulp made of fibres that are scraped of logs using a under a high

temperature.

wire, machine clothing which takes the paper web through the paper machine.

Literature overview

4

2 Literature overview

2.1 Papermaking Papermaking is a complex process which involves many steps and different kinds of papers

with different properties. This part will contain the theory behind the factors that are most

important in order to understand the meaning of paper formation and print quality. An expla-

nation of the products that are manufactured at KP will also be given.

2.1.1 Fibres

Something that most papers have in common regardless of their appearance is that they consist

of cellulosic fibres. The fibres are usually acquired from wood, although there are a few excep-

tions, and they can be liberated from the wood matrix either mechanically or chemically. In che-

mical pulping, chemicals are used to release the fibres from the lignin that glues them together.

This method gives flexible fibres and good strength properties of the paper. Mechanical pulping

on the other hand gives stiff fibres which results in a weaker paper that is used for example in

newsprint and it is the process that is used at KP. In mechanical pulp the fibres are released by

grinding wood or wood chips and in this process not only fibres are released from the wood but

also smaller material called fines. These consist of fragments from the fibre wall as well as bro-

ken fibres and they are very important for the optical properties of the mechanical pulp (Bränn-

vall, 2005a). Two different kinds of mechanical pulps are groundwood pulp and refiner pulp. In

groundwood pulp logs are pressed against a rotating cylinder made of sandstone and the fibres

are scraped off. In refiner pulp chips are fed into the centre of two refining discs and the fibres

are abraded off. By heating the wood until approximately 120°C under high pressure and high

level of moisture, the lignin becomes softer and a higher portion of long fibres can be extracted

which leads to a stronger paper. Such pulp is called thermomechanical pulp (TMP). The chips

can also be soaked in sodium sulphite which makes the lignin sulphonated and thus a lower

temperature is required for the lignin to soften. It is known as chemo-thermomechanical pulp

(CTMP).

2.1.2 Optical properties

When light strikes a paper many different things happen to the light due to the complex network

structure of the paper. Some light is reflected at fibre and pigment surfaces in the surface layer

and deeper down in the paper structure. The light can also hit a fibre and change direction,

which is known as refraction. Other light is absorbed, but the remaining light moves on and is

then reflected and refracted by other fibres and pigments. After several reflections and refrac-

tions some light reaches the surface of the paper again and is reflected at different directions

from the surface. The human eye does not perceive all those reflections and refractions. Instead

it perceives that the paper has a matt white surface, i.e. a diffuse surface reflection (Pauler,

2002). Another effect that takes place in the paper is diffraction. It occurs when the light meets

particles or pores that are of the same size or smaller than the wavelength of the light and these

small elements oscillate with the light oscillation and function as sites for new light sources

(Pauler, 2002). See figure 2.1.

Literature overview

5

Figure 2.1. When light strikes a paper, a number of optical phenomena occur. The human eye perceives this only as diffuse surface reflection. (Pauler, 2002)

Two processes that affect optical properties are bleaching and filler content. Unbleached pulp

has a high light absorption, but with bleaching the light absorption decreases. When a paper

contains fillers, there are several factors that affect the optical properties of the sheet. Two of

the most important are the refractive index of the pigment and the pore structure (Pauler, 2002).

Other processes that affect the optical properties are dyeing and adding fluorescent whitening

agents.

2.1.3 Paper grades

There are according to Brännvall (2005b) four major paper grades with different properties re-

garding for example strengths, absorption ability and print quality. These are:

Tissue

Printing papers

Fine paper

Board and packaging

The relevant grade for this thesis is printing papers, which is the only grade that is produced at

KP. Important properties for printing papers are fracture strength, surface strength, opacity and

smoothness. The first two properties are of highest importance in the printing press because low

fracture strength can result in web breakage, and a low surface strength leads to fibres being

torn off the paper surface. The torn off fibres may result in dusting if they end up in the air or in

linting if the fibres get stuck on the printing machinery and disturb the print result. High opacity

is an important property because it means that it is less easy to see through the paper and that

print on one side of the paper does not show through on the opposite side. Finally the surface

smoothness is important because a rough surface usually leads to an uneven print quality. Print-

ing paper can be divided into several categories, but the ones that are produced at KP are

newsprint, improved newsprint and SC paper.

Newsprint

Newsprint is an important product in Sweden. Standard newsprint has a grammage of 45 g/m2

and specified properties for strength and optical properties. (Kassberg et al., 1998). The pulp is

TMP and some newsprint is produced using a big part of recycled fibres. An advantage with

recycled fibres is that they may be already bleached, but a disadvantage is that they contain

contaminants, like ink and stickies, which need to be removed before the fibres can be used. If

Literature overview

6

they are not removed, there is a chance that they will stick in the paper machines and cause

problems. KP uses only virgin fibre pulp, i.e. no recycled pulp. The newsprint products on KP

range from 42,5 g/m2 to 48,8 g/m

2 for standard newsprint and between 49 g/m

2 and 60 g/m

2 for

improved newsprint, which besides the higher grammages is brighter than regular newsprint and

has a better surface quality.

Journal paper or SC paper

Journal paper is thin as newsprint, but is often coated to improve the print result. The paper is

often used for journals which are sent by mail because of its good printability in combination

with the thin paper and hence the name »journal paper» (Kassberg et al., 1998). At KP a type of

uncoated journal paper called SC paper or magazine paper is produced. Instead of coating, the

paper has a high content of fillers and is supercalendered, which means that it is smoothened

between several hard steel or iron rolls and compressible soft rolls. The SC grades at KP range

in grammage from 45 g/m2 to 65 g/m

2.

2.1.4 Calendering

Calendering is a way of smoothening the paper surface to improve the surface and the print

quality. The method can simply be described as compressing the web in one or several rolling

nips and it can be done in different positions in the process, for example on-line or as an off-line

operation afterwards. The moisture content of the web can be as high as 15 % before the last

dryer section and as low as 5 % in off-line calendering and these differences lead to differences

in the paper structure (Wikström, 2005). Three typical calenders are hard nip calenders, soft

calenders and supercalenders. The supercalender is used on two of the paper machines at KP

and it is always located off-line and can have up to 12 rolls. The major reasons to use a calender

are to improve gloss and surface smoothness, reduce surface porosity, compress topographic

variations, control dust and linting, as well as reduce thickness. Calendering may, however, not

only improve the quality of the paper. According to Komppa and Ebeling (1983) calendering

may cause changes in the relationship between light transmittance and grammage because the

local values of pressure will be high on heavy spots of material in comparison to the surround-

ings. See figure 2.2.

Figure 2.2. A schematic drawing of the pressure distribution and the deformation of the paper

structure when using a hard and a deformable backing roll, respectively. The shading illustrates

stress concentrations directed toward the fibre flocs. (Wikström, 2005)

Where the local grammage is high the light scattering coefficient decreases which leads to more

light being let through the part of the paper that contains more substance matter. This phenome-

non is called blackening because the paper appears to contain black spots in certain light condi-

tions when local areas are compressed to the extent that the collapsed pores conduct the light in-

stead of refracting it. The effect of blackening can be seen in figure 2.3.

Literature overview

7

Figure 2.3. Effect of calendering on the relationship between light transmittance T and gram-

mage w for uncalendered and calendered laboratory handsheets (Komppa & Ebeling, 1983).

2.1.5 Paper formation

Paper formation is defined as the local grammage distribution of a sheet and it is affected by

how the fibres and other constituents of the sheet are distributed. Figure 2.4 shows two illumi-

nated sheets and the grammage distribution is seen by variations in the grayscale. Where the

grammage is high the gray color is darker while a low grammage results in a brighter gray

color.

Figure 2.4. Illuminated pictures of two sheets. To the left a sheet with small flocs and to the

right a sheet with larger flocs.

One of the most contributing factors to the formation is the jet speed or rather the difference in

speed between jet and wire. If the speed difference is zero the fibre distribution in the headbox

will remain the same onto the wire and that is why it is important to make sure that the fibre

distribution and orientation is high already in the headbox (Norman, 2005). If the jet speed is

slightly higher than the wire speed an increase in fibre orientation will occur but if the headbox

contraction is high the best formation occurs at a speed difference of zero. This is because a

high contraction leads to an improved state of flocculation in the jet and it has been long known

that formation is affected by the flocculation (Mohlin, 2000). Elongated and elastic cellulose

fibres have a strong tendency to form flocs in water and it is important to break them apart

before they reach the forming wire in order for the fibres to be evenly distributed within the

sheet. Flocs occur when the fibre concentration in the pulp exceeds the sediment concentration,

which is the lowest concentration at which a connected floc can be created. A way of breaking

fibre flocs apart is to have a high turbulence in the headbox, but it is just a temporary solution as

reflocculation will take place when the turbulence decreases (Norman, 2005). A more efficient

way to break flocs apart is by stretching. In a contracting nozzle the fluid at the front end of a

floc will have a higher velocity than that at the back end which leads to that the floc stretches

Literature overview

8

and eventually breaks apart. Fibre suspensions behave in different ways in a pipe flow depen-

ding on the flow speeds. At low speeds the fibres move as a connected network and a plug flow

occurs. At higher flow speeds, the boundary layer becomes more turbulent and first transforms

into a mixed flow and eventually it becomes completely turbulent (Norman, 2005). Pipe flow

can be described by a single dimensionless parameter, the Reynolds number

Re = UD/υ

where U is the mean (or bulk) flow speed, D the pipe diameter, and υ the kinematic viscosity of

the fluid (Hof et al, 2004). For a Reynold number below approximately 2300 the fluid is laminar

and over the same number the fluid is turbulent.

After the fibres have passed through the headbox there are still ways to improve their formation.

At higher machine speeds, about 1000 m/min, a twin-wire is often used because of the advan-

tage that the dewatering capacity increases when two wires are used. In twin-wire forming it is

also possible to generate a pulsating dewatering pressure by using deflection of the wires across

a deflector blade. A pressure zone is then created that depends on the shape of the deflector, the

wire speed and the mix thickness between the wires. The pressure pulse causes acceleration in

the fibre flocs at the downstream end of the pulses which stretches the flocs and eventually

breaks them apart (Norman, 2005).

Another way to decrease flocculation and hence improve the paper formation is by using che-

mical additives. Yan, Lindström and Christiernin (2006) discuss three different classes of addi-

tives that affect the fibre dispersion. One class of additives increases the medium viscosity of

the dispersion and there are two classes of formation aids that decrease the flocculation either by

reducing the friction between the fibres or by affecting the rheological properties of the

suspension.

Measuring formation

The most accurate way of measuring formation is through β-radiation absorption. A radiograph

is generated when the sample is exposed to a β-ray source (C-14) and the transmitted radiation

is captured on an X-ray film. β-radiation has a practically constant absorption coefficient and

thus no light scattering occurs, which leads to an accurate determination of the local grammage

distribution (Norman 2005). The wavelength spectrum obtained describes occurrence of flocs of

different sizes. Flocs in the wavelength range 0.3–3 mm can be described as small-scale flocs

while flocs in the range 3–30 mm are large-scaled. A disadvantage with the β-radiation

technique is the method being slow and time-consuming because of the time it takes to develop

and scan the exposed film. More practical methods are used instead, for example the Ambertec

which uses the β-radiation technique but instead of measuring the formation of an entire area, it

samples some spots with a defined distance. The amount of samples is adjusted by the distance

between the sample points. The default value between the points is 1 mm and the default measu-

ring area is 70 x 70 mm, but both can be adjusted in the x- and y-direction.

The other common way to measure formation is through light transmission absorption. Oppo-

sing to β-radiation this technique is based on transmission of usual light. The method does not

differ very much from when β-radiation is used but with light transmission a digital camera can

be used to speed up the process. A sheet is illuminated to a predetermined level and thus fibre

flocs of different sizes appear on the image that is captured with a digital camera. The image is

analysed using image analysis software, which results in formation values for different floc

sizes, divided by wavelengths. Some wavelengths are likely to have more effect on print mottle,

seen by visual inspection, than others (Bernie et al., 2006).

A problem with comparisons between the β-radiation method and optical formation is that the

optical formation is affected by light-scattering which depends on constituents of the paper. The

results may be especially misleading when the paper contains fillers, which have other optical

properties than the fibres, and when the paper is calendered (Norman 2005). Other examples of

constituents in the paper that affect the light transmission are the amount of retention aid, shares

of kraft pulp, groundwood pulp and TMP as well as bleaching. Other variables that can be

Literature overview

9

assumed affecting the transmission are the thickness and the density of the paper, since the

amount of air between the fibres affects the light scattering abilities of the paper. The properties

of the fibres such as length, width and pore sizes are also of importance (Pauler, 2002). The β-

beam has on the contrary a practically constant absorption coefficient which means that less

scattering occurs of the β-wavelengths. Different densities between fibres (ca 0-8–1.2 g/cm³)

and fillers (ca 1.5–1.9 g/cm³) may affect the accuracy of the β-radiation measurements.

2.2 Print quality Print quality is basically a measurement of how well the paper and ink transfer allows the origi-

nal image to be reproduced. The concept of print quality is how the beholders visually perceive

the printed image (Johansson, 1999). There is no widely accepted general method for measuring

the overall print quality, but there are several factors affecting different aspects of print quality

and these can be measured separately. One method is to evaluate the print visually but the result

may vary if different beholders perceive print quality differently. More practical methods are to

use instruments which measure print quality or scanning sheets and analyse the images with

computer programs, using defined mathematical principles.

2.2.1 Print density

According to Ström, (2005) print density (D) is one of the most important print quality parame-

ers. It is the optical density of the print and it is defined as the logarithm of the ratio of the re-

flectivity of the paper (R∞) and the reflectance of the print (Rp).

D=log(R∞/Rp)

The paper is placed on top of a pad of unprinted paper sheets during the measurements so the

background is consistent and not affected by what is behind the paper. The print density de-

pends highly on the amount of ink, and the properties of the used ink, such as pigment proper-

ties and pigment concentration.

2.2.2 Print gloss

Ström (2005) also defines gloss as the ability of the surface to reflect light. The surface is illu-

minated at a certain angle and the reflected light is measured at the same angle. Oftenly used

angles are 20°, 45° and 75°. Different angles are used for different surfaces. For example a glos-

sy surface is measured at a small angle, while a matt surface like newsprint is measured at a

high angle. The reason is mainly to get a better resolution and response on the gloss appearance

in the detecting sensors. Factors that affect the print gloss are surface roughness, ink levelling

and refractive index of the ink film.

2.2.3 Print mottle

Print mottle is an effect that is caused when there are variations in the brightness of the printed

surface, which create a cloudy appearance on the print. It is mainly an unwanted feature when

the original consists of a homogeneous grey tone and the wish is normally for the appearance of

the reproduced image to be similar to the original. This error can be caused by many different

reasons. One of the most common reasons is a variation of the amount of ink due to local

variations in the paper pore structure (Ström, 2005). This leads to varying absorptivity on

different areas of the paper surface. Where there is a high absorption, there tends to be a high

amount of ink, whereas there tends to be a less amount of ink where the paper surface has a lo-

wer absorption. This is called backtrap mottle (Johansson, 1999). Another cause to print mottle

is inhomogeneities in the coating layer or in the ink film (Fahlcrantz, 2005).

Literature overview

10

The print mottle is expressed as the covariance of either print reflectance or print density.

CoVR = σR/R

CoVD = σD/D

σR = Standard deviation of print reflectance, σD = Standard deviation of print density.

R = Mean reflectance, D = mean density.

2.2.4 Missing dots

When gravure is used as a printing method, there can be small areas on the papers where no ink

is transferred from the engraved cylinder. These are often referred to as missing dots. In offset

print, the ink is transferred to the paper from a compressible blanket, but in gravure printing, the

gravure cylinder is made by metal which lacks compressibility. If the paper also lacks com-

pressibility or contains deep pits or heights, i.e. areas deviating from the mean flat surface, they

may not be in contact with the print cylinder which results in missing dots (Ström, 2005). The

missing dots look like white spots in the printed area and an easy way to measure the amount of

them, is to scan the image and use computer software for measuring how big part of the image

that is not printed. It is measured in percent missing dots of the entire measured area. Various

resolutions in the printed dots, i.e. line frequency, in the printed image may cause various levels

of missing dots.

2.2.5 Print-through

One aspect of print quality which is very important when it comes to thin paper grades such as

newsprint, is print-through. It is desirable that the print on one side of the paper will not be af-

fected by the print on the other side. The thinner the paper, the greater the chance that print-

through will occur. Print through is measured as the print density of the reverse side of the print-

ed paper which is printed at a standard print density. There is a difference if the ink is transfer-

red to the reverse side of the paper or if the print just can be seen through the paper due to a low

opacity. The latter case is known as show-through, where the ink components, as oil, decrease

the diffraction of light and thus more light passes through the paper.

2.3 Formation and print quality Bernié et al. (2006) made a determination on how the sheet formation affects the printability of

uncoated fine paper ranging in grammage from 72 to 104 g/m2. They used print mottle as a mea-

surement of printability and the light absorption method with an image analyzer based on Fast

Fourier Transformation to measure formation. The measurements showed that there were some

correlation between formation and print mottle at the scales 4 and 8 mm where the R2 values

were between 0.4 and 0.5. For scales smaller than 4 and larger than 16 mm the R2 was below

0.2 and hence the correlation was low. Bernié et al. (2006) claim that previous measurements

have shown that scales of mottle between 4 and 8 mm correlate most with results from visual

mottle evaluations. It should be noted that the wavelengths contain both flocs and voids and

thus wavelengths between 4 and 8 mm correspond to fiber flocs of sizes between 2 and 4 mm

and comparable voids.

Ahlroos and Niskanen (2000) determined the most important base paper properties affecting

print mottle in half tones of coated fine paper. The papers were single and double coated and

both calendered and uncalendered papers were used. Formation measurements were done with

the β-radiation method and coat weight variations were measured through burnouts and analy-

zes of the ash content. The papers were printed on a Heidelberg sheet-fed offset press and print

mottle was measured on a 40 % black half tone area. Variations were divided into different floc

sizes with a band pass filter. The papers were characterized according to formation, porosity,

absorption properties and surface properties and the measured base paper properties as well as

Literature overview

11

coat weight variations were correlated to print mottle in half tones. Of all the measured base

paper properties it was found that formation showed the best correlation with print mottle in

40 % half tone areas.

Sävborg (2000) used different base papers with one set single coated and another set double

coated. Many different paper properties were measured by standard methods on both the base

and coated papers and the coated papers were printed. After that print evenness was evaluated in

three different ways: pairwise comparison by four judges, comparison against a standard scale

by one judge and MDS (Proscale) with 15 judges. Sävborg’s finding was that base paper for-

mation did not correlate to print quality.

Analysis

12

3 Analysis Choosing suitable samples for measuring formation and print quality was of high importance.

For sheets with different light-scattering properties formation can not be compared because the

light-scattering will give a misleading result. The samples must have similar optical properties

in order to be relevant, but they still need to differ in formation. Many constituents of the pulp

affect the optical properties of the finished sheet. These constituents vary rather frequently and

therefore it was decided to take samples which were produced with as little time difference as

possible. The smallest possible time difference is when the samples are produced simultaneous-

ly, i.e. they have the same position in the machine direction. The formation still varies due to for

example local pressure variations in the headbox (Norman, 2005).

When optical formation is compared to print quality, both base paper and finished paper will be

used. Since base paper is too uneven to give an acceptable print quality when printed in a

gravure press, it was not an ideal solution to print the base sheets. It was decided that it was

more suitable to use base sheet samples for formation measurements as usual but instead of

using the same samples for printing, taking the closest calendered samples in the machine

direction. A calendered cross-directional profile can be taken from the top of the reel according

to the normal procedure, but to receive the closest profile in the machine direction, that profile

needs to be taken from the bottom of the reel as can be seen in figure 3.1.

Figure 3.1. Simplified drawing of the end of the calendering process, seen from the side. Almost

all paper has been calendered and is wound onto the right reel.

Neither when the β-radiation formation is compared to print quality the exact same samples can

be both measured and printed. The β-radiation measurements requires samples of the size 124 x

84 mm because the equipment can not radiate a larger area. The ideal solution would be to print

those samples and try to find a correlation between formation and print quality, but they are too

small to fit a satisfactory reproduction of the print motive. The gravure cylinder that is used for

printing at the mill has a diameter of approximately 300 mm which means that just slightly

more than a third of the image on the cylinder would appear on the sample. Therefore it was

decided to print the remainder of the strip which was not used for β-radiation measurements. It

was longer which made it possible for a larger part of the motive on the cylinder to fit the strip

and since this strip was connected to the piece that was used for formation measurements in the

machine direction, the formation should be very similar.

Methods

13

4 Methods

4.1 Formation measurements For optical formation measurements, the L&W Autoline Formation was used. It is a camera

based image analyzer, which determines the formation by analyzing an illuminated image of a

paper sheet. It measures 256 grey levels with an aperture of 66x88 mm and with the resolution

14 µm/pixel. The image analysis method is called Paper Perfect and it breaks down formation

into the different scales of formation that can be seen in table 4.1.

Table 4.1. Wavelength ranges Table 4.2. Wavelength ranges

for L&W Autoline Formation. for β-radiation measurements

Component of formation

and range of scale [mm]

Scale 0.6 (0.5–0.7)

Scale 0.8 (0.7–1.1)

Scale 1.25 (1.1–1.8)

Scale 2 (1.8–2.6)

Scale 3 (2.6–4.5)

Scale 5 (4.5–6.7)

Scale 8 (6.7–12)

Scale 14 (12–18.5)

Scale 22 (18.5–31)

Scale 37 (31–55)

The β-radiation formation measurements were made by Innventia. The samples were cut into

124x84 mm large pieced and radiated. The film was scanned and analyzed with computer soft-

ware. The formation was described as coefficient of variation and divided into the ranges as can

be seen in table 4.2.

Wavelength

intervals [mm]

0.25–0.5

0.5–1

1–2

2–4

4–8

8–16

16–32

F1 0.3–3

F2 3–30

Ftot 0.3–30

Methods

14

4.2 Paper grades A scope of different paper grades and grammages were used for the correlations between β-

radiation formation and optical formation which can be seen in table 4.3.

Table 4.3. Paper grades and grammages for the formation measurements.

PM Grade and grammage

8 base LWU 57 g/m2

8 SC SC-A+ 54 g/m2

10 IN/MF 49 g/m2

11 News 45 g/m2

12 base SC-A 52 g/m2

12 SC SC-A 52 g/m2

For correlations between optical formation and print quality only paper from PM 12 was chosen

and the different grades and grammages can be seen in table 4.4.

Table 4.4. The used paper grades when measuring correlation between optical formation and

print quality.

Grade Grammage

SC-A 52 g/m²

SC-A 56 g/m²

SC-A 60 g/m2

SC-A+ 54 g/m²

SC-A+ 56 g/m²

4.3 Samples for formation correlations Cross-directional profiles were measured in the Autoline Formation instrument with a distance

of 160 mm between the measurement points on the top side of the paper. The received values

were used to plot a graph to determine where the optical formation was high respectively low.

The five scales of formation between 1.25 and 8 mm were added and the sums resulted in a

graph as can be seen in diagram 4.1.

Methods

15

145

150

155

160

165

170

175

240

560

880

1200

1520

1840

2160

2480

2800

3120

3440

3760

4080

4400

4720

5040

5360

5680

6000

6320

6640

6960

7280

7600

7920

8240

Machine width [mm]

Op

tical

form

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Diagram 4.1. Optical formation of a CD- profile with 160 mm between the measuring points.

Samples were chosen from sections on the profile were the optical formation differed the most.

120–160 mm wide sections were cut from the profile and the optical formation of these samples

was measured again and noted. The measurements were made in two different ways. The first

one was by using the existing method on the mill today which is to add the wavelengths betwe-

en 1.25 and 8 mm and use this formation value. The other method was to divide the formation

numbers in the different wavelength intervals that can be seen in table 4.1 and use the different

numbers for the correlations. The β-radiation method and the optical method give information

about different intervals and thus they cannot be exactly compared. Instead the most correspon-

ding intervals are compared to each other. In the cases when the optical formation is in between

two β-formation intervals the optical formation is compared to both intervals (e.g. 2 mm optical

formation is compared to both 1–2 and 2–4 mm β-formation).

The optical formation measurements were used for two different correlations; with β-radiation

formation and print quality.

4.3.1 Optical formation and -radiation absorption formation

For samples which were used to correlate optical formation and β-radiation formation, five

samples were taken from each of the two paper machines which produce newsprint. From the

two paper machines which produce SC paper five samples of finished paper were taken from

each of the machines and in addition four samples of base paper were chosen. The samples

which were chosen were the ones with highest respectively lowest formation and two or three

samples in between to get a range of formation. First the optical formation of the samples were

measured in the Autoline Formation. Then the β-radiation formation was measured by Innventia

and it was investigated whether any correlation between optical formation and β-radiation

formation could be found or not. For comparisons with added formation numbers the β-radia-

tion wavelength intervals between 1 and 8 mm were added in a similar way as for the optical

wavelength intervals.

4.3.2 Optical formation and print quality

Paper with two different brightnesses and different grammages were chosen to see whether

those factors affect the correlation (see table 4.4). For each of the four grades a strip was taken

Methods

16

from the top of the parent reel on finished paper, i.e. after the calendering. Thereafter another

strip was taken from the same parent reel on the piece of paper that was close to the spool and

never went through the calender. The base paper strip was run through the Autoline formation

device with measure points every 160 mm which results in a profile similar to the one in

diagram 4.1. To get a large range of formation values, which give more accurate correlations,

160 mm wide samples were cut from the profile in the areas where the formation values were

lowest, highest respectively a couple of values in between. The optical formation of these

samples were then measured again twice in different positions and the mean value was calcu-

lated. The position of the samples which showed highest respectively lowest formation and

some formation in between was noted and samples from the same position were cut from the

calendered paper. The formation of these samples were measured as the previous samples and

printed.

4.4 Printing The printing was made on a Prüfbau gravure printing press. It is used for laboratory purpose and

can print one sheet of the dimensions approximately 120 x 300 mm at a time by attaching the

sheet to a shuttle and feed it through the cylinders. The printing cylinder has a width of 165 mm,

a diameter of 300 mm and rotates with a speed of 2.5 m/sec, responding in 500 rpm. The gravu-

re angle is 130° and the resolution is 70 LPI. The mixture of ink consists of 320 g ink, 180 g

varnish and toluene added to a viscosity of 23 seconds with a 100 ml Gardner DIN cup, 3 mm

nozzle.

To maintain the best possible print quality the print sessions could not last longer than 15

minutes because after that period of time too much solvent evaporates from the ink which can

cause stripes on the printed surface. The samples were divided into three different sessions but

the same equipment, including the mixture of ink, was used in all three sessions. The viscosity

of the ink was thoroughly measured every time to make sure that it remained the same and in

addition to this five reference sheets were used in all three printing sessions to make sure that

the printing conditions were equal. Some of the samples were too short to cover the whole

diameter of the printing cylinder, for example when a part of the sample was used for β-

radiation measurements of the formation. In those cases the remainder of the sample was taped

together with reference sheets not to get ink on the shuttle that the samples are attached to when

fed between the printing cylinder and the impression cylinder. Too short samples are, however,

not a problem because only a part of the printed motive is relevant for the print quality

measurements. Only the areas with 100 %, 20 % and 40 % ink which can be seen in figure 4.1

are needed for the measurements and with a positioning system it was possible to get these areas

on the samples and not on the reference sheet.

After printing, the print quality of all the samples together with the reference sheets was mea-

sured. Figure 4.1 shows the printed motive and the black area (100 % ink) to the left of the pic-

ture was used to measure density, the light grey area (20 % ink) was used to measure missing

dots and the dark grey area (40 % ink) was used to measure mottling. In some cases measure-

ments could not be made because an area was not reproduces on the sample or because of an

interference that is shown especially in the 40 % ink area when low grammages are printed.

Methods

17

Figure 4.1. A printed sample which was used for measurements of three aspects of print quality.

To measure the density, ten evenly distributed measurements were made in the 100 % ink area

and the mean value was calculated and noted for each sample. An opaque pad of unprinted

paper of the same grade as the sample was used as background for all density measurements.

The same pad was used as background for the measurements of mottle. Two mottle measure-

ments were made on the 40 % ink area of every sample and the mean value was calculated. The

share of missing dots was measured only once in the 20 % ink area but the measurement area

was almost as big as the area with 20 % ink, unlike the previous measurements when the mea-

surement area was much smaller.

4.5 Print quality measurements The three following aspects of print quality were considered.

4.5.1 Print density

The density measurements were made with a Techkon Spectrodens that was connected to a

computer through the USB-slot. The density values were instantly exported from the program

SDConnect to Microsoft Excel where the mean value of the measurements was calculated.

4.5.2 Missing dots

A Prüfbau Verity IA scanner that was connected to a computer was used to measure the share of

missing dots. The scanning resolution was 2000 dpi and the image was analyzed with a program

called Prüfbau Verity IA print target. The missing dots value is expressed as the number of mis-

sing dots divided by the size of the area, i.e. the share.

4.5.3 Print mottle

The instrument for measuring mottle is a mobile measurement device called HandyMeasure. It

is a digital video camera connected to a computer through the USB-slot. The resolution of the

camera is 100 µm/pixel. The lightening consists of LEDs which are located to minimize topo-

graphy effects and a polarization filter is used to reduce gloss. The analysis of the image is done

with the program HandyMeasure_v3.

Methods

18

4.6 Numerical measurements Many variables regarding the manufacturing process and quality of the paper are accessible

through a program called ReportFlex. Several values are measured online, but more time consu-

ming measurements, e.g. formation, and measurements that affect the paper, e.g. tensile

strength, are made in a laboratory and are reported to the software from there. Values for each

reel since 2003 can be accessed from the three paper machines that existed then. The program

can present the values as numerical or as graphs and they can also be plotted as trends over a

time interval. A limitation is that the program only can handle 1000 reels at a time, which cor-

responds to approximately 2–3 months. Further, it is only possible to access values from one ca-

tegory, such as process data and print properties, at a time. There is, however, an add-in func-

tion in Excel which makes it possible to work with more than 1000 reels at the same time and

values from different categories can be combined using basic Excel commands. In order to find

out the important parameters for sheet formation on the different paper machines, data from

ReportFlex was used.

Numerical measurements have been made on paper from all four paper machines to investigate

which properties in the manufacturing process affects the formation.

4.6.1 PM 8

Between 26/02/2009 and 28/02/2009 PM 8 produced SC-A-+ 45 g/m2 with the same proportion

of pulp and a fairly consistent share of fillers as can be seen in table 4.5

Table 4.5. Paper and pulp parameters in the numerical measurements on PM 8.

Property Property unit

TMP SC 14 %

56 %

30 %

SGW

Kraft

Speed 900–940 m/min

Fillers 28.0–30.6 %

Grammage 45.2–46.5 g/m2

In a similar manner as in the previous measurement, it was examined if the pulp has any effect

on the optical formation. Between 19/01/2009 and 23/01/2009, PM 8 produced SC-A 51 g/m2,

LWU 51 g/m2 and LWU 57 g/m

2 with a fairly constant speed and share of fillers, with changing

shares of TMP SC and SGW pulp. The constant values are presented in table 4.6.

Table 4.6. Paper and pulp parameters in the numerical measurements on PM 8.

Property Property unit

Speed 909.4–909.7 m/min

Kraft 23–25 %

Fillers 30.17–33.17 %

4.6.2 PM 10

Properties for the numerical measurements on PM 10 are presented in table 4.7. The paper grade

was improved news 52 g/m2 produced between 07/05/2009 and 12/05/2009. When correlation

for a property is examined it acts as a variable which can be seen on the y-scale in the corre-

sponding diagram instead of as a constant in the table.

Methods

19

Table 4.7. Paper and pulp parameters in the numerical measurements on PM 10.

Property Property unit

Speed 1100–1120 m/min

Broke 10–20 %

Bleached TMP 80 %

4.6.3 PM 11

The data from PM 11 comes from standard newsprint produced from 1/12/2008 to 17/12/2008.

Constant parameters are presented in table 4.8. When correlation for a property is examined it

acts as a variable which can be seen on the y-scale in the corresponding diagram instead of as

the constant in the table.

Table 4.8. Paper and pulp parameters in the numerical measurements on PM 11.

Property Property unit

Speed 1500–1504 m/min

Kraft 3–5 %

Grammage 44.61–44.86 g/m2

4.6.4 PM 12

The data regarding pulp and retention aid from PM 12 can be seen in table 4.9. The paper grade

was SC-A 52 g/m2 produced between 04/05/2009 and 05/05/2009.

Table 4.9. Paper and pulp parameters when measuring correlation between optical formation

and other properties.

Property Property unit

CaCO3 3.6–6.4 %

Clay 30.8–34 %

Tot. amount of fillers 36.9–37.8 %

Speed 1737–1775 m/min

Ret. aid 1 478–530 g/tonne

TMP SC 2 70.4–71.2 %

SGW3 17.6–17.8 %

Kraft 11–15 %

The constant variables when correlating pope speed and formation can be seen in table 4.10.

The paper grade was SC-A 56 g/m2 produced between 28/04/2009 and 12/05/2009.

1 When the correlation for retention aid and formation is investigated, the amount of retention aid is not

consistent but varies between 478 and 655 g/tonne.

2 When the correlation for pulp and formation is investigated, the share of TMP SC is not consistent but

varies between 68 and 86 % and the share of SGW between 4.25 and 17.8 %.

3 When the correlation for pulp and formation is investigated, the share of SGW is not consistent but

varies between 4.25 and 17.8 %.

Methods

20

Table 4.10. Paper and pulp parameters when measuring correlation between optical formation

and pope speed.

Property Property unit

CaCO3 3.6–6 %

Clay 31–33.7 %

Tot. amount of fillers 35.8–38.3 %

Speed 1744–1757 m/min

Ret. aid 350–500 g/tonne

TMP SC 73–77 %

SGW 12.9–13.5 %

Kraft 11–14 %

Results

21

PM 10

y = -15,69x + 363,38

R2 = 0,2059

145

150

155

160

165

170

12,8 12,9 13 13,1 13,2 13,3 13,4 13,5

β-formation (CV)

Op

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PM 11

y = -16,795x + 353,12

R2 = 0,6808

110

115

120

125

130

135

13,2 13,4 13,6 13,8 14 14,2 14,4

β-formation (CV)

Op

tical

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5 Results In this section results from the formation and print quality measurements are presented together

with results from the data analysis regarding which factors that affect the optical formation.

5.1 Evaluation of instruments

5.1.1 Added formation numbers

The results of the two different formation measurements can be seen in table 5.1.

Table 5.1. Optical formation values for the different samples.

PM Sample Optical

formation

β-rad.

formation PM Sample Optical

formation

β-rad.

formation

PM 10 10-10 150.8 13.2 PM 11 11-12 114.9 14.2

10-2 153.4 13.4 11-11 119.2 13.6

10-3 157.8 12.9 11-8 122.4 13.7

10-4 160.2 12.9 11-5 126.1 13.7

10-8 167.5 13.1 11-16 131.0 13.4

PM 8 B8-1 113.4 10.9 PM 12 B2-8 103.4 12.1

base B8-14 115.8 10.2 base B2-2 108.4 11.7

B8-8 118.5 10.1 B2-17 111.1 11.3

B8-11 122.1 9.8 B2-1 116.0 11.1

PM 8 G8-10 226.3 9.4 PM 12 G2-2 207.0 10.7

SC G8-11 230.4 9.6 SC G2-6 209.2 10.7

G8-4 233.5 9.7 G2-21 212.3 10.9

G8-3 238.4 9.6 G2-1 215.7 10.8

G8-1 241.1 9.8 G2-22 218.8 11.0

As can be seen in diagram 5.1 the R2 value for improved newsprint on PM 10 is only 0.21. A

higher optical formation number results in a lower coefficient of variation for the β-formation.

The correlation between optical formation and β-radiation formation is rather high on regular

newsprint from PM 11 which can be seen in diagram 5.2. The R2 value for added wavelengths

between 1 and 8 mm is 0.68.

Diagram 5.1 and 5.2. Correlation between the optical formation number and the coefficient of

variation for β-formation on newsprint from PM 10 and 11.

Results

22

PM 8 base

y = -6,8613x + 187,74

R2 = 0,851

110

115

120

125

9,6 9,8 10 10,2 10,4 10,6 10,8 11 11,2

β-formation (CV)

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PM 12 base

y = -12,51x + 254,11

R2 = 0,9422

100

105

110

115

120

11 11,2 11,4 11,6 11,8 12 12,2

β-formation (CV)

Op

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PM 8 SC

y = 36,092x - 113,48

R2 = 0,8018

225

230

235

240

245

9,3 9,4 9,5 9,6 9,7 9,8 9,9

β-formation (CV)

Op

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PM 12 SC

y = 32,325x - 137,41

R2 = 0,7778

205

210

215

220

10,6 10,7 10,8 10,9 11 11,1

β-formation (CV)

Op

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When the wavelength scales between 1 and 8 mm are added, the R2 values for the correlations

on base paper from PM 8 and PM 12 are 0.85 and 0.94 respectively which can be seen in dia-

grams 5.3 and 5.4. A higher optical formation number results in a lower coefficient of variation

for the β-formation also in these cases.

Diagram 5.3 and 5.4. Correlation between the optical formation number and the coefficient of variation for β-formation on base paper from PM 8 and 12.

The R2 values of calendered paper from PM 8 and 12 are 0.80 and 0.78, but the slope of the line

is positive as can be seen in diagrams 5.5 and 5.6. This means that a higher optical formation

number results in a higher coefficient of variation for the β-formation.

Diagram 5.5 and 5.6. Correlation between the optical formation number and the coefficient of variation for β-formation on SC paper from PM 8 and 12.

5.1.2 Formation divided by wavelengths

When the correlation is divided by wavelengths the only R2 values for paper from PM 10 that

are bigger than 0.5 is in the range 4–8 mm, which can be seen in diagram 5.7. For the rest of the

wavelength spectra the R2 values are smaller than 0.3.

Results

23

Correlation divided by wavelength PM 10

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

0.6 0.8 1.25 2 2 3 5 8 8 14 22 37

0,5–1 mm 1–2 mm 2–4 mm 4–8 mm 8–16 mm 16–32 mm

Wavelength (optical / β)

Co

rrela

tio

n (

R²)

Diagram 5.7. Correlation between the optical formation number and the coefficient of variation for β-formation divided by wavelengths. Newsprint from PM 10.

In the correlation measurements for PM 11, on the other hand, a much stronger correlation is

shown, especially in the range 2–4 mm. Most of the scales between 1 and 16 mm have R² values

bigger or at least close to 0.5 which can be seen in diagram 5.8.

Correlation divided by wavelength PM 11

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

0.6 0.8 1.25 2 2 3 5 8 8 14 22 37

0,5–1 mm 1–2 mm 2–4 mm 4–8 mm 8–16 mm 16–32 mm

Wavelength (optical / β)

Co

rrela

tio

n (

R²)

Diagram 5.8. Correlation between the optical formation number and the coefficient of variation for β-formation divided by wavelength. Newsprint from PM 11.

The base paper on PM 8 shows a very good correlation in almost all ranges and especially in the

range 2–8 mm with R2 values larger than 0.7. The exceptions are for the smallest and the largest

wavelength ranges and when the optical formation scale 8 mm is compared to the β-radiation

wavelengths 8–16 mm. In the latter case a good optical formation agrees with a poor β-radiation

formation, but the R² value is smaller than -0.3. The results can be seen in diagram 5.9.

Results

24

Correlation divided by wavelength PM8 Base

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

0.6 0.8 1.25 2 2 3 5 8 8 14 22 37

0,5–1 mm 1–2 mm 2–4 mm 4–8 mm 8–16 mm 16–32 mm

Wavelength (optical / β)

Co

rrela

tio

n (

R²)

Diagram 5.9. Correlation between the optical formation number and the coefficient of variation

for β-formation divided by wavelength. Base paper from PM 8.

Measurements show that there is no correlation between optical formation and β-radiation for-

mation on supercalendered paper from PM 8. Eight out of twelve R² values are smaller than 0.1.

In three wavelength ranges a high optical formation value results in a low β-radiation formation

value which is indicated by negative R2 values smaller than -0.5 in diagram 5.10. Only the

optical formation scale 37 mm has a positive R2 value larger than 0.5, but it is compared to the

closest β-radiation wavelength range of 16–32 mm and can not really be compared.

Correlation divided by wavelength PM8 SC

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0.6 0.8 1.25 2 2 3 5 8 8 14 22 37

0,5–1 mm 1–2 mm 2–4 mm 4–8 mm 8–16 mm 16–32 mm

Wavelength (optical / β)

Co

rrela

tio

n (

R²)

Diagram 5.10. Correlation between the optical formation number and the coefficient of varia-tion for β-formation divided by wavelength. Supercalendered paper from PM 8.

The base paper from PM 12 shows a good correlation in many ranges, especially between 1 and

8 mm where the R2 values are larger than 0.6. Above this range there is no correlation at all ac-

cording to the results that are presented in diagram 5.11.

Results

25

Correlation divided by wavelength PM12 base

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

0.6 0.8 1.25 2 2 3 5 8 8 14 22 37

0,5–1 mm 1–2 mm 2–4 mm 4–8 mm 8–16 mm 16–32 mm

Wavelength (optical / β)

Co

rrela

tio

n (

R²)

Diagram 5.11. Correlation between the optical formation number and the coefficient of varia-tion for β-formation divided by wavelength. Base paper from PM 12.

The supercalendered paper from PM 12 does not either show any correlation for optical forma-

tion and β-radiation formation. As can be seen in diagram 5.12 all significant bars are below the

x-axis which means that if there is any correlation at all, an improved optical formation agrees

with a decreased β-radiation formation and vice versa. Especially the optical wavelength scales

1.25, 5 and 8 mm correlate to the closest corresponding β-radiation wavelength ranges with R²

values of -0.7 and smaller. Between 1.25 and 5 mm no correlation can be seen at all and the

same is valid for scales larger than 8 mm.

Correlation divided by wavelength PM12 SC

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0.6 0.8 1.25 2 2 3 5 8 8 14 22 37

0,5–1 mm 1–2 mm 2–4 mm 4–8 mm 8–16 mm 16–32 mm

Wavelength (optical / β)

Co

rrela

tio

n (

R²)

Diagram 5.12. Correlation between the optical formation number and the coefficient of varia-

tion for β-formation divided by wavelength. Supercalendered paper from PM 12.

Results

26

5.2 Optical formation and print quality

5.2.1 Formation with added wavelengths

None of the print quality aspects show a particularly strong correlation to the optical formation

except for a few exceptions. For base paper the tendency is a low correlation between optical

formation and mottle. Three out of the five grades showed R² values between 0.3 and 0.5 for a

high formation number and high mottle while the R² values for the other two grades were close

to 0.

After supercalendering a good optical formation seems to result in high print mottle, but only

for two out of the five measured grades where the R² values are between -0.6 and -0.8. For the

other three grades the R² values are lower than ±0.2. Missing dots and optical formation show

some correlation for a few grades both before and after supercalendering but for the rest of the

grades it does not seem to be any correlation at all. Regarding density the R² values for all gra-

des are lower than 0.4 both before and after supercalendering.

Diagrams 5.13 and 5.14 below show that although the different measurements varied between

the grades and grammages, the bars show similarities before and after supercalendering.

Base

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

SC-A+ 54

g/m²

SC-A+ 56

g/m²

SC-A 52 g/m² SC-A 56 g/m² SC-A 60 g/m²

Grade

Co

rrela

tio

n (

R²)

....

Missing dots

Mottle

Density

Diagram 5.13. Correlation between optical formation and print quality on SC paper before su-percalendering.

Results

27

SC

-0,8

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

SC-A+ 54

g/m²

SC-A+ 56

g/m²

SC-A 52 g/m² SC-A 56 g/m² SC-A 60 g/m²

Grade

Co

rrela

tio

n (

R²)

....

Missing dots

Mottle

Density

Diagram 5.14. Correlation between optical formation and print quality on SC paper after su-

percalendering.

5.2.2 Formation divided by wavelengths

Neither when the optical formation is divided by wavelengths, there seems to be any strong

correlation to print quality. All R² values are lower than 0.4 both before and after supercalen-

dering except for two exceptions where the R² value is slightly larger than 0.4 on the negative

side, which can be seen in diagrams 5.15 and 5.16. The curves show similarities before and after

supercalendering and a dip occurs before supercalendering at the 5 mm scale and a peak after

the same scale. The correlations between optical formation and missing dots respectively den-

sity show similarities, especially after the supercalendering. The density correlations are mainly

below the x-axis both before and after calendering for all scales which indicate that the hypothe-

sis regarding density is not correct, but the correlations are low with only one R² value larger

than 0.4 and four larger than 0.3 on the negative side.

Results

28

Base

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0.6 0.8 1.25 2 3 5 8 14 22 37

Wavelength (mm)

Co

rrela

tio

n (

R²)

Missing dots Mottle Density

Diagram 5.15. Correlation between optical formation and print properties divided by wave-

lengths for uncalendered SC-A 60 g/m².

Calendered

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

0.6 0.8 1.25 2 3 5 8 14 22 37

Wavelength (mm)

Co

rrela

tio

n (

R²)

Missing dots Mottle Density

Diagram 5.16. Correlation between optical formation and print properties divided by wave-

lengths for supercalendered SC-A 60 g/m².

5.3 -radiation formation and print quality When the formation is divided by wavelengths it correlates well to print mottle with all R² va-

lues of 0.68 or larger up to the range of 2–4 mm as seen in diagram 5.17. At wavelengths of 8

mm and larger the correlation is drastically lover and a good formation in the largest scales

seems to result in higher print mottle. The correlations between the formation wavelength ran-

ges and missing dots are very low. The R² values for all wavelength ranges are around 0.2 or

smaller. Print density shows the lowest correlation to β-radiation formation and the trend is very

similar to the one for print mottle.

Results

29

Print quality and β-radiation formation

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

0.25 - 0.5 0.5 - 1 1 - 2 2 - 4 4 - 8 8 - 16 16 - 32

Wavelength (mm)

Co

rrela

tio

n (

R²)

MD

Mottle

Dens

Diagram 5.17. Correlation between optical formation and print properties divided by wave-

lengths for supercalendered paper grades from PM 8 and PM 12.

5.4 Factors affecting formation Many different parameters affect the formation and by changing one parameter while the others

remain constant a correlation can be found between the variable and optical formation.

5.4.1 PM 8

Diagram 5.18 shows the correlations between optical formation and machine speed respectively

share of TMP SC. It can be seen that speed and optical formation correlate to some extent in the

wavelength range 0.8–1.25 mm where the R² values are slightly larger than 0.4. For larger wa-

velength scales the correlation to optical formation decreases until 8 mm where it increases

again and shows a little peak at 22 mm. For the largest scale 37 mm the correlation decreases.

The correlation indicates that the formation improves when the paper machine is run with a

higher speed, at least up to 940 m/min which was the highest speed in the investigation.

The correlation between pulp and optical formation is even lower than for machine speed and

optical formation. For most wavelength scales the R² value are lower than 0.2 with exceptions

for 8 and 14 mm where the R² values are 0.21 and 0.3. If there is any correlation, it indicates

that a higher share of TMP SC results in a lower formation for the wavelength scales where the

R² values are larger than 0.1.

Results

30

PM 8

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

0,6 0,8 1,25 2 3 5 8 14 22 37

Wavelength

R²

Speed TMP SC

Diagram 5.18. Correlations between optical formation and machine speed respectively share of

TMP SC for PM 8.

5.4.2 PM 10

Diagram 5.19 shows clearly that there is no correlation between any of the investigated parame-

ters and optical formation at any wavelength scales with all R² values smaller than 0.15.

PM 10

-0,2

0

0,2

0,4

0,6

0,8

1

0,6 0,8 1,25 2 3 5 8 14 22 37

Wavelength

R²

Speed Broke Bleached TMP

Diagram 5.19. Correlations between optical formation and machine speed respectively share of

broke and TMP 2 for PM 10.

5.4.3 PM 11

Diagram 5.20 shows that there is a low correlation between the investigated parameters and

optical formation in all wavelength scales. The property which showed highest correlation was

the share of kraft pulp in the wavelength range 1.25–5 mm with R² values varying between 0.30

and 0.36 but it is not very strong. The tendency is that a higher share of kraft pulp improves the

formation. The correlation between machine speed and optical formation showed an R² value of

Results

31

0.39 in the 0.6 mm scale but the correlation decreases for larger wavelength scales. Increased

machine speed results in decreased formation in the 0.6 mm scale. A higher amount of starch

seems to decrease the formation in the 8–14 mm scales, but the correlation is very weak with R²

values of 0.26–0.28. The share of TMP1 does not seem to affect the optical formation at all with

R² values lower than 0.1 in all wavelength scales.

PM 11

0

0,2

0,4

0,6

0,8

1

0,6 0,8 1,25 2 3 5 8 14 22 37

Wavelength

R²

Speed TMP 1 Sulphate Starch

Diagram 5.20. Correlations between optical formation and machine speed respectively share of

TMP 1, kraft pulp and starch for PM 11.

5.4.4 PM 12

Diagram 5.21 shows a strong correlation between optical formation and share of TMP SC in the

lower wavelength ranges. The R2 values are smaller than -0.5 between the 0.8 mm and 2 mm

scales which means that a higher share of TMP SC decreases the optical formation. The reten-

tion aid also seems to affect the small scale formation with R2 values lower than -0.5 for the 0.6

mm and 0.8 mm scale. A larger amount of retention aid decreases the formation in the lowest

scales but for formation scales larger than 2 mm the retention aid increases the formation. The

correlations are, however, very low with R² values around 0.2–0.3. The correlation between

machine speed and optical formation is strong for the most scales except for 8 mm and larger.

For the smallest scales 0.6 and 0.8 mm the correlation indicates that a higher machine speed

decreases the formation while the formation in the scales between 1.25 and 5 mm is improved

by an increased machine speed. Formation in the largest scales seems to be decreased by an

increased machine speed, just as in the smallest scales.

Results

32

PM 12

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

0.6 0.8 1.25 2 3 5 8 14 22 37

Wavelength

R²

Speed

Ret. aid

TMP SC

Diagram 5.21. Correlations between optical formation and machine speed respectively share of

TMP SC and amount of retention aid for PM 12.

Discussion

33

6 Discussion

6.1 Evaluation of instruments The correlations between optical formation and β-radiation formation showed some diverging

results. In the measurements where the wavelengths between 1 and 8 mm were added it was

apparent that a good optical formation correlated with a good β-formation before supercalen-

dering on PM 8 and PM 12 where the R2 values were bigger than 0.85. The measurements on

SC paper from the same paper machines also showed a high correlation with R2 values of 0.78

and 0.80, but with the difference that a good optical formation correlated with a bad β-radiation

formation and vice versa. The results are, however, not very surprising. Kajanto, Komppa and

Ritala (1989) wrote that optical formation depends on light scattering and light absorption of the

sheet. Many variables in the furnish composition such as fibre type, fillers, color and dye affect

the accuracy of the optical measurement. Variables in the process such as beating and calende-

ring also affect the accuracy. They continue writing that optical measurements are often the only

practical choice because it is the fastest technique available, but it should under no circumstan-

ces be used for papers that have been heavy calendered. Supercalendering strongly increases the

local variation of light scattering which decreases the accuracy of the optical measurements. A

fibre floc before calendering absorbs light, but after calendering the floc is heavily compressed

which leads to blackening and where the light earlier was absorbed it is easier transmitted after

calendering (Komppa & Ebeling, 1989).

For paper from PM 10 and PM 11 the optical formation and the β-radiation formation correlated

rather well for unbleached newsprint with a R2 value of 0.68, but the improved, bleached news-

print had a very low correlation with just a R2 value of 0.20. This can probably be explained by

the bleaching of the pulp which affects the light scattering to a high extent.

The formation measurements were also compared for each wavelength interval and the results

emphasize the measurements with the added intervals. The different methods of measuring for-

mation correlate well between 1 and 8 mm for base paper from PM 8 and PM 12 and the cor-

relations are slightly worse for unbleached newsprint from PM 11. For bleached newsprint from

PM 10 it can be said that there is a low correlation in all wavelength intervals except for the 4–8

mm β-formation wavelengths. The optical formation at the optical scale 5 mm gives an R2 value

of 0.82 and the 8 mm interval gives an R2 value of 0.60. The supercalendered SC paper show a

high correlation for only some intervals which might indicate that the added formation numbers

are a bit misleading. On the supercalendered sheets from PM 12 a good optical formation

correlates well with a bad β-radiation formation in the interval 4–8 mm, but the same con-

nection can not be found on the other base papers from PM 8. The only interval where both

supercalendered papers from PM 8 and PM 12 shows a high correlation is the optical formation

interval 1.25 mm compared to the β-formation interval 1–2 mm with R2 values of 0.67 and 0.97

respectively.

When the correlation graphs for base paper from PM 8 and PM 12 as well as newsprint from

PM 11 are merged into the same diagram (6.1) it can be seen that the linear trends have different

slopes. As can be seen in the equations the β-formation values result in different optical forma-

tion values for the different paper grades. This means that formation cannot be compared for

different paper grades with the light transmission method. The reason for this is probably once

again the different brightnesses of the grades and the different grammages which make the light

behave differently both on the paper surface and inside the fibre structure. From diagram 6.1 the

conclusion can be drawn that optical formation measurements do not work well for either too

bright or to dark papers. For brighter paper the slope of the trend seems to be more horizontal

and for darker paper the slope seems to be more vertical. A horizontal line means that different

β-formation values result in the same optical formation value and a vertical line means that the

Discussion

34

same β-formation value may result in different optical formation values. Even if the lines are

not completely horizontal or vertical the differences between the optical values will be small for

a close to horizontal line and big for a close to vertical line.

y = -6,8613x + 187,74

R2 = 0,851

y = -12,51x + 254,11

R2 = 0,9422

y = -16,795x + 353,12

R2 = 0,6808

100

105

110

115

120

125

130

135

9 10 11 12 13 14 15

β-radiation formation [CV]

Au

tolin

e f

orm

ati

on

nu

mb

er

SC-A

LWU

News

Diagram 6.1. Correlations between β-radiation formation and optical formation for different

paper grades.

The optical formation measurements have indicated that the deviation between the values is

much bigger for the larger scales. This probably means that the optical method is less accurate

for larger formation scales. The smallest flocs that are being detected in the 0.6 mm scale are

approximately 0.3 mm in diameter. The aperture of the camera is 66 mm x 88 mm, which

means that there are many flocs of the smallest size in the measured area. The largest flocs that

are detected are, however, approximately 19 mm diameter and thus less flocs of those sizes will

fit in the measured area. If there are many flocs in the measurement, the result will be more

statistical significant for smaller flocs.

6.2 Optical formation and print quality Overall there does not seem to be any vast correlation between optical formation and any of the

measured print quality properties when the added formation numbers are used. Some paper gra-

des and grammages show stronger correlations than others, for example SC-A 56 g/m² with cor-

relations larger than 0.3 for missing dots and density on both base paper and supercalendered

paper. SC-A+ shows a correlation larger than 0.3 between optical formation and missing dots on

base paper and even larger than 0.5 on supercalendered paper. These correlations can, however,

probably be regarded as coincidences because the other grammages, neither lower and higher,

do not show any similar correlations and the grammages should not have any effect on the re-

sults. Noticeable is that most of the mottle correlations show a negative correlation, which

means that they oppose to the hypothesis that an improved formation decreases the mottle. The

negative correlation can be seen on both base paper and supercalendered paper which means

that it can not be explained with blackening due to calendering.

A notable finding is that the optical formation for the investigated paper grades correlates rather

similar to print quality both before and after the supercalendering. It is most visible for the

correlation to print density in diagram 5.13 and 5.14 where the bars are negative for all grades

except for one – SC-A 56 g/m² – where the bars are positive both before and after calendering.

Though the optical formation and β-radiation formation do not correlate for supercalendered

paper their print properties are similar anyway. It probably means that the formation is similar

Discussion

35

Figure 6.1. Local grammage distribution for

SC-A paper in the cross-direction. The scan-

ning distance is 1 mm.

both before and after supercalendering, but that it is not detectable with the optical formation

measurements and once again due to optical phenomena. The bars being just similar and not

identical before and after supercalendering can be explained in many ways. Figure 6.1 shows a

profile for the local grammage distribution for uncalendered SC-A paper with a mean gramma-

ge of 57 g/m². The local grammage varies

between 55 and 59 g/m² and the standard

deviation is 0.68 g/m². The profile shows

high peaks but when the paper is run thro-

ugh the supercalender rolls they may not

only be compressed but can also be moved

in the machine-direction of the paper and

even out some of the topographical varia-

tions. Patterns in the rolls may also cause a change in the surface of the paper compared to be-

fore the calendering. Another reason that there are differences in the measurements before and

after calendering is that it is impossible to measure the formation on the exact same position on

the sheet both times. In fact, the distance between the measured base paper and the measured

finished paper is rather big. Though the samples are taken from the bottom of the reel before

supercalendering and from the top of the reel after supercalendering, they are far from connec-

ted to each other. To receive a sample from real supercalendered paper more than a kilometer of

paper from the top of the reel needs to be removed before the sample is taken.

Neither when the divided wavelengths are compared to the print quality properties the correla-

tion is strong. The strongest correlation can be found between print density and optical forma-

tion on the supercalendered paper with several R² values larger than 0.3 and one larger than 0.4.

This correlation means, however, that an improved optical formation results in a lower print

density. In diagrams 5.15 and 5.16 it can be seen that the curves for the three print quality as-

pects have similar characteristics which can be explained with that they are connected. A high

density often means that more ink is transferred to the paper and this usually results in a smaller

amount of missing dots. More ink can also lead to more print mottle.

6.3 -radiation formation and print quality When the correlation between formation and print quality is divided by wavelengths it is accor-

ding to diagram 5.17 rather apparent that the formation wavelength range 0.25 to 8 mm has a

larger impact on the print quality than the range between 8 and 32 mm. The correlations for for-

mation and missing dots respectively density are very low with the largest R² values around 0.2

but the correlations for formation and mottle are much higher. In the range 0.25–4 mm the R²

values are between 0.7 and 0.8 and in the range 4–8 mm the R² value is larger than 0.4. The fact

that no correlation can be seen for wavelengths larger than 8 mm can probably be explained

with that print mottle is only measured in the range 1–8 mm. This range has been shown to be

useful in most cases because it correlates well with perceptive assessments of print mottle (Jo-

hansson, 1999). It can be noted that formation in the range 0.25–1 mm also correlates well with

print mottle though it is outside the measured wavelength range. A possible explanation for this

is the periodic patterns that are caused by the forming and drying fabrics for example. As can be

seen in figure 6.2 the contact area between paper and dryer fabric can be 0.6 mm, but the con-

tact areas are periodic and it is possible that the print mottle software cannot separate for ex-

ample two areas and instead see them as one area of the size 1.8 mm (assumed that the non-

contact areas are of the same size as the contact areas).

Discussion

36

Figure 6.2. Dimensions of machine clothing (Voith paper).

A notable conclusion from diagram 5.17 is that formation in the range 16–32 mm correlates

very well with print mottle with an R² value of -0.8. Since it is indicated with a negative number

in the diagram it means that a good formation in this range results in a high print mottle, con-

tradictory to the smaller ranges. What makes the strong correlation even more notable is that

print mottle is not even measured for wavelengths larger than 8 mm. A possible source of error

is that the formation-print quality correlations were made with two different paper grades, SC-A

and LWU. The LWU papers had better formation for wavelength ranges up to 4 mm while the

difference was close to zero for larger ranges. The mottle value was higher for the LWU papers

and when the difference in formation between the grades decreases for larger wavelength ranges

while the mottle measurements does not take the larger wavelength scales into account, it gives

the effect that large-scale formation has a negative impact on the print mottle. Because print

mottle was measured only between 1 and 8 mm the correlations are only reliable for the same

range.

It was assumed that formation should affect the missing dots but the measurements have clearly

shown that such a correlation cannot be found. There is, however, a significant difference bet-

ween the measurements of mottle which showed a high correlation to formation and missing

dots which showed low correlation to formation. The mottle values were ranging from 3.5 to

4.54 while the missing dots values were ranging between 0.401 and 2.02 %.

Table 6.1. Calculation of coefficient of variation for missing dots and print mottle.

Missing dots Mottle

Average 1.05 3.99

Standard deviation 0.52 0.40

Coefficient of variation 0.49 0.10

In table 6.1 it can be seen that the standard deviations for missing dots and mottle do not differ

much, but since the mottle values are approximately four times bigger than the missing dots va-

lues, it is more relevant to divide the standard deviations with the mean values and compare the

coefficients of variation. Then it can be seen that the coefficients of variation are approximately

five times bigger for missing dots than for mottle. When the corresponding calculations are ma-

de for the five reference sheets which were printed together with the other sheets, the coefficient

of variation is 0.02 for mottle and 0.26 for print mottle. As a comparison are the coefficients of

variation for formation between 0.03 and 0.08 in the different wavelength ranges for all measu-

red sheets. With these numbers in consideration it can be established that the amounts of mis-

sing dots in the measurements vary much more than both the mottle and formation values. In

Discussion

37

table 6.2 are the coefficients of variation of formation for two LWU sheets. The coefficient of

variation does not vary more than 0.1 for any of the ranges, but yet the amount of missing dots

is 0.401 for the first sample and 0.901 for the second – a difference of more than 100 %.

Table 6.2. Formation in the different wavelength scales for two LWU sheets [CV].

Sample

0.25–

0.5 0.5–1 1–2 2–4 4–8 8–16 16–32 0.3–3

3–

30

0.3–

30

1 3.3 4.3 3.7 3.1 2.8 2.4 1.9 6.9 4.6 8.3

2 3.4 4.4 3.8 3.2 2.8 2.4 1.9 7.0 4.6 8.3

A new question is that if formation does not affect the transfer of ink to the sheet – then what

does? A possible explanation is the porosity of the sheet. The porosity is a more small-scale

property than the formation and it is affected for instance by the refining process. It can be sus-

pected that if the paper is porous, i.e. the fibres are not strongly bond together, the network of

air passages in the paper cause a lack of contact between paper and print cylinder in gravure

printing.

6.4 Factors affecting formation The correlation between pope speed and optical formation has been investigated on all paper

machines. The measurements show that the speed has a bigger effect on formation in the paper

machines that produce SC paper than in the paper machines which produce newsprint where

almost no correlation occurs. The correlations for magazine paper on PM 8 are not very strong

but some R² values in the lower ranges are larger than 0.4 while the correlations for PM 12 are

stronger with R² values in several wavelength ranges larger than 0.8. In the measurements there

was, however, a big difference in the correlations between PM 8 and PM 12. The slight corre-

lation that could be seen on PM 8 indicated an improved formation with a higher speed whereas

the strong correlation on PM 12 indicated that only the formation scales between 1.25 mm and 5

mm were improved with a higher speed. The formation in the other scales, both smaller and lar-

ger, seemed to be poorer instead. When the correlation curves for PM 8 and PM 12 are studied

it can be seen that they show almost opposite characteristics. The formation in the middle of the

range is mostly affected by the speed on PM 12 while the same range on PM 8 is least affected

by the machine speed. These differences may depend on the fact that the speed differences bet-

ween the machines are vast. The speeds for the investigations on PM 8 were 900–940 m/min

while the speeds for PM 12 were 1737–1775 m/min. In percentage the speed differences were

4 % for PM 8 and 2 % for PM 12. The speed differences leads to different contractions in the

headboxes which affects the flocculation in mix and the formation in the forming process

according to Norman (2005).

For the other properties that were investigated on the different paper machines only the shares

of TMP SC and groundwood pulp on PM 12 showed a significant correlation but only at the

small scales between 0.8 and 2 mm where the R² values varied between 0.5 and 0.8. An in-

creased share of TMP SC results in a poorer formation. As can be seen in diagrams 5.19 and

5.20 the correlations for PM 10 and PM 11 are very low, which probably depends on the fact

that the papers have been calendered and thus the optical formation measurements are

unreliable. With β-radiation absorption the correlations may have been stronger.

A reason why the correlations are much stronger on SC paper from PM 12 than from PM 8 is

that PM 12 produces SC-A paper, which often has been used in the investigations. SC-A paper

has a D65-brightness of 67 compared to SC-A+ and LWU from PM 8 which have D65-bright-

ness of 72 for SC-A+ and 78–79 for LWU. The brighter the paper, the worse the optical for-

mation measurements works due to optical phenomena such as light scattering. According to

this logic the newsprint should show the highest correlation because it has the lowest brightness

but this is clearly not the case. This may depend on that all newsprint is calendered online and

Discussion

38

thus all correlations on newsprint are made on calendered paper. The trends on SC paper show

that the calendering has a devastating effect on the optical formation so the fact that newsprint

correlates at all to the β-radiation formation is probably because of the low brightness.

Recommendations

39

7 Recommendations

7.1 Improved formation number To improve the formation number, the print mottle measurements and the β-radiation absorption

measurements on paper from KP have been used together with literature on print mottle.

Since the optical formation measurements correlate differently with the β-radiation formation

for different paper grades the ideal solution would be to adjust the formation number after each

grade. Now the wavelength scales between 1.25 and 8 mm are added for all grades but as can be

seen in table 6.11 the optical formation for MF/IN paper only correlates to β-radiation formation

in the 5 and 8 mm scale. For this grade it would be more relevant to consider only these scales

since the other scales do not say any thing about the actual formation of the sheets. Such a solu-

tion would, however, demand a major change of the software that is used to process the values

of the formation measurements and other paper properties. It would also be necessary to make a

more thorough investigation with many more paper grades and grammages to see how these as-

pects affect the correlations to β-radiation formation in the different wavelength ranges. Today

it is possible to adjust the formation formula to only SC paper or newsprint because these two

templates are used. With these two templates it is at least possible to divide the formation num-

ber in Formation SC and Formation news/MF/IN instead of using the same formation number

for all grades, but these numbers will still lack precision. The correlation between optical forma-

tion and β-radiation formation on improved newsprint differs a lot from the correlation on stan-

dard newsprint which can be seen in diagram 5.7 and 5.8. Even if it was possible to change tem-

plates according to which paper machine the paper comes from a problem would occur. News-

print is for example sometimes produced on PM 10 and sometimes on PM 11 and it is most li-

kely the optical differences of the paper grades that lead to differences in the optical formation

measurements and not which paper machine the paper comes from.

7.1.1 Formation SC

In table 5.21 it can clearly be seen that print mottle correlates well to formation up to the 4 mm

scale and to some extent in the 4–8 mm scale while no formation can be seen for the larger sca-

les. Since data collected in short wavelength ranges give quite misleading results concerning

print quality, the scales 0.6 and 0.8 mm have been disregarded together with the 14, 22 and 37

mm scales which do not correlate to print mottle according to the measurements (Johansson,

1999). The scales which are left to determine the print quality are hence 1.25, 2, 3, 5 and 8 mm.

To weight them properly the R² values in the correlation to print mottle were used. The corre-

lations between optical formation and β-radiation formation on SC paper were also used to

receive a value that tells something about the actual formation. The R² values were rounded off

and can be seen in table 7.1.

Recommendations

40

Table 7.1. R² values for β-radiation formation-print mottle, β-radiation formation-optical for-

mation on PM 8 and 12 and how the scales are suggested to be weighed in the new formula.

Scale Mottle PM 8 PM 12 Product Weight

1.25 0.7 0.4 0.6 0.17 11 %

2 0.7 0.6 0.85 0.36 23 %

3 0.8 0.9 0.9 0.65 42 %

5 0.5 0.9 0.8 0.36 23 %

8 0.2 0.3 0.4 0.02 1 %

To receive a formation number that is of the same size as the existing one the shares in decimal

form are multiplied by 5 which gives the new multipliers

F1.25: 0.55

F2: 1.15

F3: 2.1

F5: 1.15

F8: 0.05

and the new formula for the formation number on SC paper is

FSC = 0.55 F1.25 + 1.15 F2 + 2.1 F3 + 1.15 F5 + 0.05 F8.

In the existing method all five wavelength scales are weighed equally. The suggested new distri-

bution weighs the scale 1.25 half of the original value, 2 and 5 mm slightly more, 3 mm is weig-

hed double, while 8 mm is weighed much less.

7.1.2 Formation News/MF/IN

In a similar manner as for SC paper the multipliers for newsprint and MF were calculated. How-

ever, since these grades have not been printed the print quality correlations were not used but it

was supposed that wavelengths larger than 8 mm were not suitable because of the results on SC

paper. The R² values were rounded off and can be seen in table 7.2.

Table 7.2. R² values for β-radiation formation-optical formation on PM 10 and 11 and how the

scales are suggested to be weighed in the new formula.

Scale PM 10 PM 11 Product Weight

1.25 0 0.2 0 0

2 0.05 0.6 0.03 4 %

3 0.05 0.7 0.04 6 %

5 0.8 0.5 0.4 60 %

8 0.4 0.5 0.2 30 %

To receive a formation number that is of the same size as the existing one the shares in decimal

form are multiplied by 5 which gives the new multipliers

Recommendations

41

F1.25: 0

F2: 0.2

F3: 0.3

F5: 3

F8: 1.5

and the new formula for the formation number on newsprint and MF is

FNews/MF = 0.2 F2 + 0.3 F3 + 3 F5 +1.5 F8.

The wavelength scale of 1.25 is not taken in consideration in the new formula, 2 and 3 mm is

weighed much less, 5 mm is weighed three times as much and 8 mm is weighed 50 % more in

the new formula.

7.2 How to improve print quality As the measurements have shown, it is possible to improve the print mottle by improving the

formation mainly in the 1–8 mm range. PM 12 was the only paper machine that showed any

strong correlation between formation and pulp or production parameters but it is likely that the

same conclusions can be drawn for the rest of the paper machines.

The strongest correlation that was found in PM 12 was between formation and machine speed.

The formation was increased in the wavelength range 2–5 mm with a higher speed while the

formation in the wavelength range 0.6–0.8 became worse with a higher speed. The other wave-

length ranges did not show any R² values larger than 0.5. A higher amount of retention aid has

shown to reduce the formation in the scales 0.6–0.8 mm, but leave the larger scales unaffected.

The range 0.6–0.8 mm is, however, too small to affect the visual perception of print mottle. A

higher share of TMP SC and a smaller share of groundwood pulp seem to decrease the forma-

tion in the 0.6–2 mm range and leave the larger scales unaffected. Since the 1.25 mm and 2 mm

scales are within the wavelength range of 1–8 mm where mottle is detectable for the human eye,

the pulp also affects the print quality.

Conclusions

42

8 Conclusions The main purpose of this thesis was to find out how the sheet formation affects the print quality

on SC paper from KP. By measuring sheet formation, print the sheets and then measure diffe-

rent print quality properties it has been established that formation affects print mottle to a high

extent, which agrees with earlier findings with in the area. Shallhorn and Heintze (1996) found

that the formation on the base papers had a strong influence on the uniformity of the offset

prints. Bernie et al. (2006) made a similar investigation with optical formation measurements

and found a correlation to print mottle in the 5–8 mm scale. Unlike the earlier investigations,

this investigation made using SC papers and gravure print instead of offset papers and offset

print. An important print quality property in gravure print is missing dots which occur when the

paper is not in contact with the print cylinder due to lack of compressibility and/or pits on the

paper surface. No significant correlation could be found between formation and missing dots,

which probably means that missing dots are caused by smaller-scale properties such as porosity,

individual crossings or pits between fibres.

In addition, an evaluation of the optical method of measuring formation was made. The forma-

tion of sheets was measured with both an optical method and a β-radiation absorption method.

The measurements showed that optical formation gives a fair result for uncalendered SC paper

and standard newsprint. For supercalendered SC paper and improved newsprint there was a

slight or none correlation. This can probably be explained with that supercalendering of SC

paper causes optical changes in the paper structure, such as blackening, and the bleaching of

improved newspaper also causes the light to behave differently. The conclusion was that the

optical method can be used to predict the formation but only for uncalendered SC paper and

unbleached newsprint. The measurements also showed that different brightness of the paper

affects the optical measurements differently and therefore it is not possible to compare the

optical formation of sheets with different brightnesses. Different grammages may have the same

effect on the optical formation as different brightnesses because the light is absorbed by more or

less fibres.

An attempt was also made to improve the optical way of measuring formation. The current si-

tuation at the mill is to use a formation number that consist of five added wavelength scales

between 1.25 and 8 mm. This number is not of any use for the operators of the paper machines

when it comes to improving or maintaining the quality of the paper. One reason is that it is not

clarified how the formation affects the end product. The measurements have shown that there is

a strong correlation between formation measured with the β-radiation absorption in the 2–4 mm

scale and print mottle with an R² value of 0.77. The optical method of measuring formation that

is used on the mill today did, however, not show any correlation to print mottle. Measurements

have also shown that there is a strong correlation between light transmission absorption and β-

radiation absorption on paper before calendering. Under the assumption that calendering only

compress the paper and not affect the formation, which is not really accurate due to for example

patterns in the calender rolls, the formation is comparable before and after calendering. Then

optical formation on base paper can be used to predict the print mottle in the end products.

The purpose of the last part of the thesis was to clarify which factors affect the formation on

newsprint and SC paper and what can be done to improve it. The measurements showed that the

strongest correlation was found between machine speed and optical formation. A higher speed

results in an improved formation in the wavelength scales where print mottle is detectable for

the human eye. The share of TMP also has some effect on the formation in the same scale.

Higher machine speed, a bigger share of TMP and less groundwood pulp should result in an

improvement of the formation in the 1–8 mm range and thus also the print quality according to

the measurements which have been carried out.

Literature

43

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