Evaluation of Optical Formation Measurements on Printing

Transcription

Evaluation of Optical Formation
Measurements on Printing Papers and
How They Can Be Used to Predict Print
Quality on Uncoated SC Papers
ROBERT
TOLKKI
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
ROBERT
TOLKKI
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 newsprint and SC paper. Formation of the SC papers has been measured both before and after supercalendering 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 formation 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 measurements 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 wavelength range that affects the visual perception of print quality. Similarities in correlations between 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 likely 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 tidningspapper och journalpapper. För journalpappren har formationen mätts både före och efter superkalandrering för att se hur den påverkar de optiska mätningarna. De optiska mätningarna jämfördes med β-strålningsmätningar och resultaten visade att optiska mätningar gav en ganska
rättvisande uppskattning av formationen för oblekt tidningspapper och okalandrerat journalpapper. 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 superkalandrerat papper kan man anta att formationsmätningar före superkalandreringen ger en skaplig uppskattning 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 Nordström, 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 endless 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
2
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
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
4.6.1
Numerical measurements ............................................................................................ 18
PM 8 .................................................................................................................... 18
5
4.6.2
PM 10 .................................................................................................................. 18
4.6.3
PM 11 .................................................................................................................. 19
4.6.4
PM 12 .................................................................................................................. 19
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
6
7
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
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
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 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 measured 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 considerably higher than the products at KP which range between 45 and 60 g/m2. Ahlroos & Niskanen (2000) found that formation was the base paper property that showed the strongest correlation 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?
1
Introduction
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 misleading 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 indicated 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 processes 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.
2
Introduction
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.
3
Literature overview
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 explanation 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 exceptions, and they can be liberated from the wood matrix either mechanically or chemically. In chemical 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 broken fibres and they are very important for the optical properties of the mechanical pulp (Brännvall, 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 refractions 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.
4
Literature overview
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 regarding 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. Printing 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
5
Literature overview
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/m2 for standard newsprint and between 49 g/m2 and 60 g/m2 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/m2.
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 surroundings. 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 phenomenon is called blackening because the paper appears to contain black spots in certain light conditions when local areas are compressed to the extent that the collapsed pores conduct the light instead of refracting it. The effect of blackening can be seen in figure 2.3.
6
Literature overview
Figure 2.3. Effect of calendering on the relationship between light transmittance T and grammage 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 illuminated 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
7
Literature overview
and eventually breaks apart. Fibre suspensions behave in different ways in a pipe flow depending 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 advantage 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 chemical additives. Yan, Lindström and Christiernin (2006) discuss three different classes of additives 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 measuring 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. Opposing 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
8
Literature overview
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 βbeam 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 original 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 parameers. It is the optical density of the print and it is defined as the logarithm of the ratio of the reflectivity 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 depends highly on the amount of ink, and the properties of the used ink, such as pigment properties 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 illuminated 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 glossy 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 lower 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).
9
Literature overview
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 compressibility 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 affected by the print on the other side. The thinner the paper, the greater the chance that printthrough will occur. Print through is measured as the print density of the reverse side of the printed paper which is printed at a standard print density. There is a difference if the ink is transferred 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 measurement 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 analyzes 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
10
Literature overview
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 formation did not correlate to print quality.
11
Analysis
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 simultaneously, 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.
12
Methods
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
for L&W Autoline Formation.
Table 4.2. Wavelength ranges
for β-radiation measurements
Component of formation
and range of scale [mm]
Wavelength
intervals [mm]
Scale 0.6 (0.5–0.7)
0.25–0.5
Scale 0.8 (0.7–1.1)
0.5–1
Scale 1.25 (1.1–1.8)
1–2
Scale 2 (1.8–2.6)
2–4
Scale 3 (2.6–4.5)
4–8
Scale 5 (4.5–6.7)
8–16
Scale 8 (6.7–12)
16–32
Scale 14 (12–18.5)
F1 0.3–3
Scale 22 (18.5–31)
F2 3–30
Scale 37 (31–55)
Ftot 0.3–30
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 software. The formation was described as coefficient of variation and divided into the ranges as can
be seen in table 4.2.
13
Methods
4.2 Paper grades
A scope of different paper grades and grammages were used for the correlations between βradiation 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
8 SC
10
11
LWU 57 g/m2
SC-A+ 54 g/m2
IN/MF 49 g/m2
News 45 g/m2
12 base
12 SC
SC-A 52 g/m2
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.
14
Methods
175
Optical formation
170
165
160
155
150
24
0
56
0
88
0
12
00
15
20
18
40
21
60
24
80
28
00
31
20
34
40
37
60
40
80
44
00
47
20
50
40
53
60
56
80
60
00
63
20
66
40
69
60
72
80
76
00
79
20
82
40
145
Machine width [mm]
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 between 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 corresponding 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-radiation 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
15
Methods
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 calculated. 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 gravure 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 βradiation 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 measured. Figure 4.1 shows the printed motive and the black area (100 % ink) to the left of the picture 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 measurements 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.
16
Methods
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 measurements 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 measurement 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 missing 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 topography effects and a polarization filter is used to reduce gloss. The analysis of the image is done
with the program HandyMeasure_v3.
17
Methods
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 consuming 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 corresponds to approximately 2–3 months. Further, it is only possible to access values from one category, such as process data and print properties, at a time. There is, however, an add-in function 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
TMP SC
SGW
Kraft
Speed
Fillers
Grammage
Property unit
14 %
56 %
30 %
900–940 m/min
28.0–30.6 %
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/m2 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.
Property
Speed
Kraft
Fillers
Property unit
909.4–909.7 m/min
23–25 %
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 corresponding diagram instead of as a constant in the table.
18
Methods
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.
Property
Speed
Kraft
Grammage
Property unit
1500–1504 m/min
3–5 %
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 %
SGW
Kraft
3
17.6–17.8 %
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 %.
19
Methods
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 %
20
Results
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
β-rad.
Optical
formation formation
PM 10
10-10
10-2
10-3
10-4
10-8
B8-1
B8-14
B8-8
B8-11
G8-10
G8-11
G8-4
G8-3
G8-1
150.8
153.4
157.8
160.2
167.5
113.4
115.8
118.5
122.1
226.3
230.4
233.5
238.4
241.1
PM 8
base
PM 8
SC
PM
PM 11
13.2
13.4
12.9
12.9
13.1
10.9
10.2
10.1
9.8
9.4
9.6
9.7
9.6
9.8
PM 12
base
PM 12
SC
Sample Optical
formation
11-12
114.9
11-11
119.2
11-8
122.4
11-5
126.1
11-16
131.0
B2-8
103.4
B2-2
108.4
B2-17 111.1
B2-1
116.0
G2-2
207.0
G2-6
209.2
G2-21 212.3
G2-1
215.7
G2-22 218.8
β-rad.
formation
14.2
13.6
13.7
13.7
13.4
12.1
11.7
11.3
11.1
10.7
10.7
10.9
10.8
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.
PM 11
PM 10
135
y = -15,69x + 363,38
R2 = 0,2059
165
Optical formation
Optical formation
170
160
155
150
145
12,8
12,9
13
13,1
13,2
13,3
13,4
125
120
115
110
13,2
13,5
y = -16,795x + 353,12
R2 = 0,6808
130
13,4
13,6
13,8
14
14,2
14,4
β-formation (CV)
β-formation (CV)
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.
21
Results
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 diagrams 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.
PM 8 base
120
y = -6,8613x + 187,74
R2 = 0,851
120
Optical formation
Optical formation
125
PM 12 base
115
110
y = -12,51x + 254,11
R2 = 0,9422
115
110
105
100
9,6
9,8
10
10,2
10,4
10,6
10,8
11
11,2
11
11,2
β-formation (CV)
11,4
11,6
11,8
12
12,2
β-formation (CV)
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.
PM 8 SC
220
y = 36,092x - 113,48
R2 = 0,8018
240
Optical formation
Optical formation
245
PM 12 SC
235
230
225
9,3
9,4
9,5
9,6
9,7
9,8
215
210
205
10,6
9,9
y = 32,325x - 137,41
R2 = 0,7778
β-formation (CV)
10,7
10,8
10,9
11
11,1
β-formation (CV)
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.
22
Results
Correlation divided by wavelength PM 10
1,0
0,8
Correlation (R²)
0,6
0,4
0,2
0,0
-0,2
-0,4
-0,6
0.6
0.8
0,5–1 mm
1.25
2
1–2 mm
2
3
2–4 mm
5
8
4–8 mm
8
14
8–16 mm
22
37
16–32 mm
Wavelength (optical / β)
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
1,0
0,9
Correlation (R²)
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0.6
0.8
0,5–1 mm
1.25
2
1–2 mm
2
3
2–4 mm
5
8
4–8 mm
8
14
8–16 mm
22
37
16–32 mm
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.
23
Results
Correlation divided by wavelength PM8 Base
1,0
Correlation (R²)
0,8
0,6
0,4
0,2
0,0
-0,2
-0,4
0.6
0.8
1.25
0,5–1 mm
2
1–2 mm
2
3
2–4 mm
5
8
4–8 mm
8
14
8–16 mm
22
37
16–32 mm
for β-formation divided by wavelength. Base paper from PM 8.
Measurements show that there is no correlation between optical formation and β-radiation formation 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
0,6
Correlation (R²)
0,4
0,2
0,0
-0,2
-0,4
-0,6
-0,8
0.6
0.8
0,5–1 mm
1.25
2
1–2 mm
2
3
2–4 mm
5
8
4–8 mm
8
14
8–16 mm
22
37
16–32 mm
Diagram 5.10. Correlation between the optical formation number and the coefficient of variation 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 according to the results that are presented in diagram 5.11.
24
Results
Correlation divided by wavelength PM12 base
1,0
Correlation (R²)
0,8
0,6
0,4
0,2
0,0
-0,2
-0,4
0.6
0.8
0,5–1 mm
1.25
2
1–2 mm
2
3
2–4 mm
5
8
4–8 mm
8
14
8–16 mm
22
37
16–32 mm
Diagram 5.11. Correlation between the optical formation number and the coefficient of variation 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 formation 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
0,2
Correlation (R²)
0,0
-0,2
-0,4
-0,6
-0,8
-1,0
0.6
0.8
0,5–1 mm
1.25
2
1–2 mm
2
3
2–4 mm
5
8
4–8 mm
8
14
8–16 mm
22
37
16–32 mm
Diagram 5.12. Correlation between the optical formation number and the coefficient of variation for β-formation divided by wavelength. Supercalendered paper from PM 12.
25
Results
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 grades 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
0,8
Correlation (R²)....
0,6
0,4
Missing dots
0,2
Mottle
0
Density
-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
Diagram 5.13. Correlation between optical formation and print quality on SC paper before supercalendering.
26
Results
SC
0,6
Correlation (R²)....
0,4
0,2
Missing dots
0
Mottle
-0,2
Density
-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
Diagram 5.14. Correlation between optical formation and print quality on SC paper after supercalendering.
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 supercalendering 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 density 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 hypothesis 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.
27
Results
Base
0,2
Correlation (R²)
0,1
0
-0,1
-0,2
-0,3
-0,4
-0,5
0.6
0.8
1.25
2
3
5
8
14
22
37
Wavelength (mm)
Missing dots
Mottle
Density
Diagram 5.15. Correlation between optical formation and print properties divided by wavelengths for uncalendered SC-A 60 g/m².
Calendered
0,4
0,3
Correlation (R²)
0,2
0,1
0
-0,1
-0,2
-0,3
-0,4
-0,5
0.6
0.8
1.25
2
3
5
8
14
22
37
Wavelength (mm)
Missing dots
Mottle
Density
Diagram 5.16. Correlation between optical formation and print properties divided by wavelengths 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 values 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 ranges 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.
28
Results
Print quality and β-radiation formation
1,0
0,8
Correlation (R²)
0,6
0,4
0,2
MD
0,0
Mottle
-0,2
Dens
-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)
Diagram 5.17. Correlation between optical formation and print properties divided by wavelengths 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 wavelength 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.
29
Results
PM 8
1,0
0,8
R²
0,6
0,4
0,2
0,0
-0,2
0,6
0,8
1,25
2
3
5
8
14
22
37
Wavelength
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 parameters and optical formation at any wavelength scales with all R² values smaller than 0.15.
PM 10
1
0,8
R²
0,6
0,4
0,2
0
-0,2
0,6
0,8
1,25
2
3
5
8
14
22
37
Wavelength
Speed
Broke
Bleached TMP
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
30
Results
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
1
R²
0,8
0,6
0,4
0,2
0
0,6
0,8
1,25
2
3
5
8
14
22
37
Wavelength
Speed
TMP 1
Sulphate
Starch
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 retention 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.
31
Results
R²
PM 12
1,0
0,8
0,6
0,4
0,2
0,0
-0,2
-0,4
-0,6
-0,8
-1,0
Speed
Ret. aid
0.6 0.8 1.25
2
3
5
8
14
22
37
TMP SC
Wavelength
TMP SC and amount of retention aid for PM 12.
32
Discussion
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 supercalendering 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 calendering 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 circumstances 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 newsprint 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 formation correlate well between 1 and 8 mm for base paper from PM 8 and PM 12 and the correlations 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 connection 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 formation 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
33
Discussion
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.
Autoline formation number
135
130
y = -6,8613x + 187,74
125
R2 = 0,851
y = -16,795x + 353,12
R2 = 0,6808
120
SC-A
LWU
115
News
110
y = -12,51x + 254,11
R2 = 0,9422
105
100
9
10
11
12
13
14
15
β-radiation form ation [CV]
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 grades and grammages show stronger correlations than others, for example SC-A 56 g/m² with correlations 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 results. 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
34
Discussion
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 grammage 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 throFigure 6.1. Local grammage distribution for
ugh the supercalender rolls they may not
SC-A paper in the cross-direction. The scanonly be compressed but can also be moved
ning distance is 1 mm.
in the machine-direction of the paper and
even out some of the topographical variations. Patterns in the rolls may also cause a change in the surface of the paper compared to before 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 connected 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 correlation is strong. The strongest correlation can be found between print density and optical formation 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 aspects 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 according 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 formation 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 (Johansson, 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 contact areas are periodic and it is possible that the print mottle software cannot separate for example two areas and instead see them as one area of the size 1.8 mm (assumed that the noncontact areas are of the same size as the contact areas).
35
Discussion
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, contradictory 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 between 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 values, 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 made 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 measured sheets. With these numbers in consideration it can be established that the amounts of missing dots in the measurements vary much more than both the mottle and formation values. In
36
Discussion
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].
0.25–
3–
Sample
0.5 0.5–1 1–2 2–4 4–8 8–16 16–32 0.3–3 30
1
3.3
4.3
3.7
3.1
2.8
2.4
1.9
6.9
4.6
2
3.4
4.4
3.8
3.2
2.8
2.4
1.9
7.0
4.6
0.3–
30
8.3
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 suspected 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 correlation 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 larger, 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 between 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 increased 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-brightness of 72 for SC-A+ and 78–79 for LWU. The brighter the paper, the worse the optical formation 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
37
Discussion
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.
38
Recommendations
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 solution 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 aspects 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 number 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 formation and β-radiation formation on improved newsprint differs a lot from the correlation on standard newsprint which can be seen in diagram 5.7 and 5.8. Even if it was possible to change templates according to which paper machine the paper comes from a problem would occur. Newsprint is for example sometimes produced on PM 10 and sometimes on PM 11 and it is most likely 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 scales. 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 correlations 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.
39
Recommendations
Table 7.1. R² values for β-radiation formation-print mottle, β-radiation formation-optical formation 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 distribution weighs the scale 1.25 half of the original value, 2 and 5 mm slightly more, 3 mm is weighed 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. However, 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
40
Recommendations
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 wavelength 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 formation 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.
41
Conclusions
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 different 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 formation 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 situation 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 βradiation 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.
42
Literature
9 Literature
AHLROOS, J. & NISKANEN, T. (2000). ‘Base paper properties effecting print mottle of wood free
coated paper’. Report no 00/2027 RCF. Falun: Stora Enso Research Center.
BERNIÉ, J-P., PANDE, H., GRATTON, R. (2006). ‘An instrumental determination of the effect of
sheet formation on the printability of uncoated fine paper’. Tappi Journal October 2006. TAPPI
Press, Atlanta, p. 28.
BRÄNNVALL, E. (2005a). ‘Overview of pulp and paper processes’. The Ljungberg Textbook, Paper Processes 2005. Stockholm: Fibre and Polymer Technology, KTH. Trita-FPT-Report
2005:31. ISSN 1652-2443.
BRÄNNVALL, E. (2005b). ‘Pulp characterisation’. The Ljungberg Textbook, Paper Processes
2005. Stockholm: Fibre and Polymer Technology, KTH. Trita-FPT-Report 2005:31. ISSN
1652-2443.
FAHLCRANTZ, C-M. (2005). On the Evaluation of Print Mottle. ISBN 91-7178-205-2.
HOF, B., VAN DOORNE, C.W.H., WESTERWEEL, J., NIEUWSTADT, F.T.M., FAISST, H., ECKHARDT, B. ‘Experimental Observation of Nonlinear Traveling Waves in Turbulent Pipe Flow’.
Science Magazine. 10 September 2004, Vol 305, Pages 1594-1598.
JOHANSSON, P-Å. (1999). Optical Homogeneity of Prints. TRITA-PMT Report 1999:16. ISSN
1104-7003.
KAJANTO, I. M., KOMPPA, A., RITALA, R. K. (1989). ‘How formation should be measured and
characterized’. Nordic Pulp and Paper Research Journal no. 3/1989.
KASSBERG, M., ERLANDSSON, M., GAVELIN, G. (1998). Massa och papper – en grundbok.
Skogsindustrins Utbildning i Markaryd AB. ISBN 91-7322-233-X.
KOMPPA, A., EBELING K. (1983). ‘Correlation between the areal mass and the optical densities
in paper’. The Role of Fundamental Research in Papermaking Symposium. Vol 2. pp 603-633.
London: Mechanical Engineering Publications Ltd.
MOHLIN, U-B. (2000). ‘Fibredimension – Formation and strength’. Progress in Paper Physics
Seminar 2000, Grenoble, France.
NORMAN, B. (2005). ‘Web forming’. The Ljungberg Textbook, Paper Processes 2005.
Stockholm: Fibre and Polymer Technology, KTH. Trita-FPT-Report 2005:31. ISSN 1652-2443.
PAULER, N. (2002). Paper Optics. Kista: AB Lorentzen & Wettre. ISBN 91-971 765-6-7.
SHALLHORN, P.M., HEINTZE, H.U. (1996). ‘Offset printing and the formation of uncoated fine
papers’. Proceedings, TAPPI 1996 Product Quality Conference. Atlanta, GA, USA: TAPPI
Press, 1996. ISBN 0-89852-671-X.
STRÖM, G. (2005). ‘Paper and Printing’. The Ljungberg Textbook, Paper Processes 2005.
Stockholm: Fibre and Polymer Technology, KTH. Trita-FPT-Report 2005:31. ISSN 1652-2443.
SÄVBORG, Ö. (2000) ‘Coating Coverage. Effect of base paper on Print Evenness’. Report no
00/2202 RCF. Falun: Stora Enso Research Center.
WIKSTRÖM, M. (2005) ‘Calendering’. The Ljungberg Textbook, Paper Processes 2005. Stockholm: Fibre and Polymer Technology, KTH. Trita-FPT-Report 2005:31. ISSN 1652-2443.
43
Literature
YAN, H., LINDSTRÖM, T., CHRISTIERNIN, M. (2006). ‘Some ways to decrease fibre suspension
flocculation and improve sheet formation’. Nordic Pulp and Paper Research Journal. Vol 21
no. 1/2006.
44
TRITA-CSC-E 2009: 135
ISRN-KTH/CSC/E--09/135--SE
ISSN-1653-5715
www.kth.se

Evaluation of Optical Formation Measurements on Printing

Transcription

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