Paper Roll Quality and Roll Hardness
Paper compressibility and roll hardness
Newsprint is compressible and bulky paper compared to LWC and coated woodfree.
During winding newsprint loses the original wound-in tension very fast and the rolls are
easily too soft. High density papers and especially plastic foils are not compressed as
much and rolls are harder.
0 50 100 150 200
Radial Pressure, kPa
Coated woodfree, gloss
Every wound paper layer on the roll surface
increases radial pressure and compress the
inner portion of the roll. Compression reduces
the original roll periphery and thus
circumferential tension as long as there will be
balance between the outer tension band and the
spring-back of the inner roll body.
When the balance is reached, there is a
negative circumferential tension inside the roll.
This tries to buckle the layers, but paper
stiffness and radial pressure prevent the
buckling in a good roll.
The positive tension band of a bulky paper is
only some centimeters from the roll surface.
On glossy SC and LWC rolls this positive tension
band is much thicker and the highest pressure is
reached deeper in the roll.
after winding additional layers
Internal roll stresses in a good roll
In a good roll the radial pressure between paper
layers is highest against the core and then
constant or slightly decreasing to the roll
If radial pressure P can be measured,
circumferential tension T can be calculated:
• T = - P - dP/dr x r
• where r = roll radius/(core outside radius)
• P = internal roll pressure, kPa
• T = internal web tension, kPa
In a newsprint roll P can be 300 kPa (3 bar) and if
the curve is flat in the middle (dP/dr = 0) tension T
is also - 300 kPa (= MD compression).
If paper caliper is 0.07 mm we get a negative
tension of - 21 N/m.
Roll stresses and paper properties
The positive tension band on top of the
roll is only some centimeters thick. Inside
this band paper will be plastically strained
and under the band plastically
compressed during storage. This paper
deformation reduces all roll stresses.
In the following unwind process paper on
the roll surface has lower elongation and
dynamic stretch. This might have
influence on paper runnability and breaks
during flying splice can happen.
Paper inside the roll is more compressed
and reaches better smoothness, lower
bulk and caliper, lower porosity and better
elongation and dynamic stretch
Main part of the paper inside a roll is not
similar as a sample from the roll surface.
Can be runnability problem
Example of paperboard properties in a roll
Physical properties of paper are changed in a roll due to the stresses during winding and
on roll storage. Paperboard (here FBB) changes more than printing paper.
The picture below is an example of paper gloss, which is about 10 % higher (about 6 %-
unit) on the core compared to the roll surface.
This is from a two-drum winder. Belt supported winder gives more even result.
Distance from roll core0 750 mm
JUHO WALDÉN, 2014
Winding and ”hardness memory”
Radial pressure in a machine reel is normally decreasing
towards the periphery so that the surface is softer than
the start of the winding. One reason is that the nip widens
when the reel diameter grows. In addition, all paper reels
have low radial pressure on the surface due to less and
less compressing layers.
Paper layers on the machine reel surface are less
compressed than inside the reel (they are thicker on the
Winding is a very dynamic process. Web travel from
unwind to rewind can be some 10 m and winder speed
40 m/s. The delay time from unwind to rewind is then
0.25 s. In this short time paper is without compression
and it tries to expand to the original thickness. However,
the time is so short that the thickness inside the machine
reel is almost ”copied” to the winder roll and the
compression due to winder parameters still reduce the
The result is that the first set wound from the machine
reel surface tends to be soft at start, and hardness is
increasing towards the roll surface. The last set tends to
be more constant in hardness and especially soft starts
or roll deformations are not as big problems.
of parent reel
Roll density measurement
Online roll density measurements can be made in winding or unwinding supposing that
the original paper thickness is constant. This is possible by calculating paper caliper
inside the roll from the angular and surface speeds of the roll.
The picture below shows typical newsprint roll density distributions and how the first
set on the winder is softer than the other sets.
100 300 500 700 900
Roll diameter, mm
Second and last sets
First online density measurements
Pekka Komulainen, 1977
Paper roll is a spiral
Paper roll is not a cylinder formed of paper rings but a
continuous spiral of paper with continuous MD tension
(plus or minus).
CD differences of web tension, strain and original web
length are important due to the spiral form.
Differences in web length and strain also have effect on
roll hardness in a spiral.
Often an accuracy of 0.1% is enough in papermaking. If
we have this difference in original paper length, it means
that after winding only 1000 m we would have the longer
part of the web one meter ahead of the shorter part. We
need web tension to even out these differences.
When crepe wrinkles inside the roll are formed, it is not
only one layer slipping but sometimes two or more layers.
This causes a longer total movement and more severe
damages between the slipping layers.
Due to the slipping, telescoping can be an additional
Example of the spiral web effect
If you measure drum speed difference in
a two-drum winder you will find, that the
front drum has slower speed in spite of
The explanation is that there is all the
time one web layer less on the front
drum compared to the rear drum.
Theoretical curve can be calculated. The
speed difference is higher in the
beginning and depends on the paper
Some winders have speed difference
control instead of torque control.
However, if there are several paper
grades it will be very difficult to find
correct speed difference curve for each
100 400 700 1000
Roll diameter, mm
Winding parameters for two-drum winder
1 Web tension, T
2a Rider roll load, N
2b Roll weight, N
3 Torque (Winding force), T
Roll hardness in radial direction
Ideally the hardness should be slightly decreasing from the core to the periphery.
However, this is only possible when the winder has center drive and support.
More common than center drive winder is a two-drum winder, where roll’s own weight
increases nip force and normally some other optimization must be made.
Picture: R. Duane Smith
Typical two-drum winder hardness distribution
It is quite easy to get a tight and hard start to
the roll by using high torque.
When the roll diameter increases the roll nip
force decreases due to the geometry. In
addition, effect of torque decreases very fast
in the beginning.
The only way to keep roll hardness constant
would be fast increased rider roll load. For
most papers the rider roll load of large roll
diameters should be so high that there would
be wrinkles and bursts in the roll.
When the roll grows, its own weight causes
very high nip load. To compensate this
hardening effect, web tension should be
reduced. However, too small web tension
together with high nip load can cause
wrinkles and bursts.
Winding and CD profile properties
It would be easier to make directly sheeted paper without
winding like in a pulp dryer.
Total thickness of a paper pile under pressure is related to
original caliper and compression properties of the paper.
The diameter of a paper roll is related not only to the
caliper and compressibility but also to the original web
length and MD strain properties of the paper web.
To make good rolls we should have online measurements
of all the four variables. Sometimes we measure and
control only paper caliper profile without any information of
the three other profiles.
If there are CD profile related problems after winding,
papermakers usually argue that grammage, moisture and
caliper profiles are straight.
One should first think if these measurements are correct
and relevant and then look, how much worse the other
profiles become by controlling the basic profiles straight.
Pile of paper
Pile thickness depends on
- paper caliper
- paper compressibility
Roll diameter depends on
- caliper & compressibility
- web length & strain
Paper elongation and roll hardness
The picture below shows how sheet length or elasticity effect on roll hardness and
If web caliper is constant, longer part of the web forms softer roll or even bagginess
Web caliper, length and reel hardness
To make even reel hardness in cross-machine direction it is important to have correct web
length related to the roll diameter in every CD position.
For heavy calendered, high density papers constant caliper is more important (paper is not
any more compressible to even out reel diameter differences). For bulky webs, length and
caliper differences are both important.
Thin Average Thick
Short Good Hard
Average Soft Good Hard
When = π = 3,14 it means that roll hardness is even
Paper roll quality testing
Cross Direction Testing
Backtender’s Billy club (wooden
ACA Systems RoQ
On-line backtender’s friend
Roll diameter profile
Cameron gap test
Improved needles or plastic strips
Core torque wrench test
Wit-Wot Roll Analyzer
On-line density measurement
Principle of general hardness testers
General hardness testers are of two types: static or dynamic.
Hardness with static testers can be defined as the resistance of a
material to permanent, plastic deformation. The traditional hardness
tests used for metals and concrete are based on well-defined
physical indentation tests.
The original dynamic scleroscope test consists of dropping a
hammer, which falls inside a glass tube under the force of its own
weight from a fixed height onto the test specimen. The height of the
rebound travel of the hammer is measured. In original use for hard
materials the scleroscope test does not mark the material under test.
The scleroscope measures hardness in terms of the elasticity of the
material and the hardness number depends on the height to which
the hammer rebounds, the harder the material, the higher the
rebound. Advantages of this method are portability and non-marking
of the test surface.
Dynamic Leeb hardness
According to the dynamic Leeb principle (invented
by Leeb and Brandestini, 1975), hardness value is
derived from the energy loss of a defined impact
body after impacting on a sample, similar to the
The Leeb quotient (vr /vi) is taken as a measure of
the energy loss by plastic deformation: the impact
body rebounds faster from harder test samples
than it does from softer ones, resulting in a
greater value of 1000 × vr /vi.
While in the traditional static tests the force is
applied uniformly with increasing magnitude,
dynamic testing methods apply an instantaneous
load. A test takes a mere 2 seconds and, using
the standard probe D, leaves an indentation of
~0.5 mm in diameter on steel or steel casting with
a Leeb hardness of 600 HLD.
Proseq Equotip and PaperSchmidt are examples
of Leeb principle.
Needs of paper roll hardness testing
The main general need of hardness testing is the level of the hardness. For paper
roll the main need is to measure the cross-direction stable variation of the
hardness, not the universal level of the hardness. Hardness curves as a function of
CD position are important, not only one variation number.
Because paper roll hardness tester should be nondestructive, portable and fast,
static testers commonly used for metals and concrete are not suitable for paper roll.
Paper roll is a layered structure of paper sheets and air between. With porous
papers there is air also inside the sheet. Without air paper roll could be totally
elastic and very hard. The more there is air, the softer the roll is.
One can understand that on the top of the roll there are much thicker air layers than
inside and close to the core. Actually hardness test measures more air content in
the roll than material itself. Air is the viscous element causing deformation. Energy
is used to this plastic deformation or air removal (inside fibers, between fibers, from
surface roughness and finally between paper layers).
Hardness test and roll structure
Paper roll structure is normally understood to
be pressure distribution between paper
layers. In a good roll pressure increases from
roll surface to the core. This increase is very
fast on top of the roll but much slower inside.
It is very interesting question how deep
hardness testers measure. This should be
If the hardness measurement depth is less
than 20 mm, the result can vary very much
depending on the ratio of paper elastic moduli
Positive tension band on roll surface is thicker
with dense papers (LWC) and narrower with
bulky papers (Newsprint).
Conventional paper roll hardness testers
The basic and conventional methods are Beloit Rho meter, Schmidt hammer and Paro
tester. All these measure with a quite strong impact to the roll.
There are no studies how deep hardness testers measure. Schmidt hammer measures
concrete to about 10 cm depth. Paper roll is softer and contains layers and air. Hence
the measuring depth must be much lower.
Picture: DIPESH DILIP MISTRY
PaperSchmidt technical information
Proceq has published more information than
the other suppliers.
For example Tapio tells only: “Hitting velocity
and the shape of the hammer's tip can be
selected to suit each customers preferences.
The goal is to mimimize the possible
damage inflicted to the tested material
(paper, plastic rolls or aluminium foil rolls)
and to provide accurate measurements.”
PaperSchmidt roll hardness tessting
Fishbone diagram of hardness testing
Tapio RQP and ACA Systems RoQ
The newest type of testers such as Tapio RQP and ACA Systems RoQ measure 30-50
times per second. The measuring time of one point for the conventional testers is
several seconds. It is supposed that the impact of the newest testers must be much
lower than with the older testers. With low impact the measurement depth is also low.
This is not studied, but the measurement depth should be calculated in number of paper
layers, not in centimeters.
The lower the measurement depth is, the more the test measures paper properties such
as porosity, roughness, stiffness, friction and elastic moduli of paper.
TAPIO RQP v2.0 and hammer penetration
Tapio Technologies states that the new RQP v2.0 tester
is based on the position measurement instead of the
acceleration measurement used earlier.
The penetration of the hammer is also measured
providing new interesting information about the measured
What could this interesting information be? It is well
known that paper moisture and temperature have effect
on paper plasticity and static hardness testers measure
plasticity. Very often when there is moisture variation in
the sheet, moist areas are cool before calendaring. After
calendering paper is cooled down and dry areas cool
faster than moist areas i.e. moist areas are hot and very
If penetration is something else e.g. more correlated to
paper moisture than to hardness number, it would help in
understanding CMD profiles better. When test methods
develop, more studies are needed!
Plastic and elastic deformation
Elastic and plastic deformation of multiple
sheets. The maximum force used in these
experiments is 100 N.
These blue curves are the experimental
force-deformation curves, the red points
show the positions of the biggest
deformations, the green points show the
plastic (residual) deformations.
The sheet numbers are 16, 32, 48 and 72.
It can be seen that higher pressure force
gives higher plastic deformation.
Modern hardness testers use very small
impact and the deformation is mostly
Dissertation Jian Chen
Paper plasticity has effect on calendering and paper caliper. In addition, plasticity has
effect on roll hardness directly. It is important to remember that moisture and
temperature change, when the roll travels from paper machine to winder and finally to
Need of CD test sampling frequency
The main need of roll hardness test is to get so
accurate CD profile that decisions of rejected
paper can be made. Online scanning of paper
never can separate MD and CD variations.
The best real CD measurement to compare with
hardness test is thermographic IR camera. It will
mainly react to CD moisture streaks and they
can be very narrow as in the picture on the right.
It depends on the process what is the correlation
between the measured profiles. Very often the
root cause is dry basis weight linked to moisture
of IR camera → caliper → paper length → roll
Higher density papers react mainly through
caliper and low density papers through paper
Anyway, hardness test must be fast to get at
least some points per second.
Moisture streaks on machine reel
The wide temperature variations seen on the left picture, caused by evaporative
cooling, correspond to variations in moisture.
On the right a reel having severe moisture streakiness is presented. This variation
probably is so narrow that it is not shown in online scanning measurement.
Photo: Albany International
Tapio RQP measurement speed
New testers can measure with a continuous high speed. An example below shows that 1
m/s (lowest CD profile) is too high speed and 0.1 m/s is too low and accurate (red).
Suitable speed can be about 0.5 m/s (blue). If reel width is 5 m, profiling time is 10 s.
Picture: Sami Uhlbäck, 2008
Overall hardness and CD profile
Newest roll hardness testers are good and fast in CD profile measurement but probably
not as good in overall roll hardness as the conventional methods. For overall hardness
calculated roll density is still a good method. Operators often compare web length to roll
diameter and then notice if something happens.
In addition, it must be remembered that all instruments require maintenance. The
following picture is from Proceq and tells that all testers have some wear, which has
influence on the hardness measurement level (not on CD profile form).
It is important to compare available testers to get the best for a specific purpose.
Important point is to study measurement variation. This picture is from Proceq showing
measurement of one roll and corresponding variation.
One important point is usability – how fast the test is and how much the procedure
requires learning. Additionally spare parts and maintenance are always important
Today the prices of testers are so low compared to the benefits that return on investment
is always guaranteed.
Human errors in hardness testing
The following publication is one of the best concerning winding and CD profiles:
WINDING EXPERIMENTS ON NONUNIFORM THICKNESS WEBS, By JARED
Jared Gale noticed in his study: “There were some slight inconsistencies within the
hardness test. This can be attributed to human error due to the difficulty in insuring
that the trigger was pulled at same rate and pressure for each test. It was also
difficult maintaining the Rho meter in a tangent position to the roll.”
There are basic differences between conventional point-by-point hardness testers
and the newest higher speed continuous testers. For comparison it is very important
that several persons make tests of the same roll and the results are compared.
Hand-held moving testers can have different speed, angles to CMD/roll tangent and
pressure against roll depending on the individual person. In addition, top layer can
be more or less loose.
Theory of Mill Assist new RHO meter
There is also a new
version of conventional
There is very little
about the theory. ACA
and Tapio tell impact
frequency, but very little
about theory and what
important roll properties
the tester really
Something can be
found about BEL-2000
GEN II, which is copied
to the right.
Modern hardness testing and real life
Modern hardness testing
Very dynamic and fast
Low adjustable impact
Narrow local pressure
Deformation mainly elastic
Knowledge of hardness testing
No good theories or studies
Not clear what is measured
In laboratory studies paper pile is
under pressure, but not under MD
tension like in reality
Real roll life
Long time from paper machine to
Reel-wide pressure and tension
Several impacts during handling
Deformation viscoelastic and plastic
Creep and relaxation have effect
Varying humidity and temperature
Amy Thuer has published the picture below in a study of plastic film rolls. This might also
be quite valid for paper rolls. However, it must be remembered that paper is different
compared to plastic foil. High density papers (D>1000 kg/m3) behave more like plastic,
but other papers have voids inside the material and then the differences in caliper are
not as dominant. Paper can compress and even out the hardest lanes of the roll.
Picture: Amy Thuer, WHEN
ROLL HARDNESS CAN
AND WHEN IT CAN’T
Roll density vs. roll hardness
Roll density and hardness were measured from three different kraftliner grades.
Correlation is very good. Density was calculated from roll weight and compared to
average measured hardness.
Thickness scanner vs. iRoll CD profile (Valmet)
Hardness profile is a basic roll quality measurement. It cannot be replaced by
online caliper measurement. Valmet iRoll is a good alternative, but very
expensive. The following is what Valmet comments on caliper measurement:
There are four main issues with thickness scanner performance. These are:
1. Resolution is too low, in practice 1 micrometer or worse. The needed
resolution should be at least ten times better. Quality Control System (QCS)
suppliers sometimes state that they have better resolution but that is not true
2. Requires constant care. There is a constant need for cleaning, calibration
and other maintenance and tuning. Without this the whole measurement
concept fails to work. In practice, it's not possible to provide this constant
service in a mill environment.
3. Caliper is not the right thing to measure. To assure good reel buildup and
runnability, the primary parameter is the hardness/diameter profile.
Thickness is only a secondary parameter that effects hardness and diameter.
4. Web breaks are caused by contacting caliper measurement scanners.
Continuous hardness measurement on winder
Due to the shrinkage on paper drying the edges are normally soft and loose under the
rider roll on winder. On a two-drum winder this is a demanding situation.
It is dangerous to press edges more than the center under rider roll or otherwise wrinkles
will arise (loose web and high nip load at the same time). To avoid this, roll hardness
profile is normally slightly lower at edges or rider rolls are installed slightly higher at edges.
Selected Roll Hardness Profiles
Example TAPIO RQP Plastic Foil Roll
Machine reel hardness profiles
The shape of the CD hardness profile in a machine reel is quite constant.
This picture shows relative hardness profiles of four sets measured on the
unwind roll surface from the winder.
Picture: Sami Hyötynen
Metso Paper Oy
Parent reel and roll profile forms
The shape of the curves from parent reel compared to customer rolls is very similar.
The average hardness of customer rolls is higher.
Soft spots in parent reel are about 20 units softer than in customer rolls.
D.M.S. WANIGARATNE et al.
Parent reel and roll hardness profiles
Customer rolls are harder than parent reel.
First set made from the parent reel surface is softer than the second set from the inner
part of the parent reel.
First set from parent reel surface
Second set from parent reel surface
Customer roll hardness profiles
Parent reel hardness profile
Caliper and reel hardness
Reel hardness of 205 g/m2 packaging board correlates very well with caliper
D.M.S. WANIGARATNE et al..
Effect of supercalender on hardness profile
After coating the hardness profile of LWC paper is quite even (blue).
After supercalendering there is more short and long range variation (red with hard center).
Picture: Ilari Ikonen, 2010
CD caliper profile and reel hardness
Caliper as such is not very important. However, papermakers try to control the process
so that the shape of the CD caliper profile is straight (why?).
This does not guarantee a straight hardness profile and it is common, that edge rolls
have too soft ends after winder.
The second roll from the right shows a too large local hardness difference leading to
problems such as bagginess, bursts or corrugations.
Four rolls in a set, edge
rolls are soft at edges.
Roll hardness and paper caliper profiles
PAROtester2 CD hardness of uncoated
magazine paper. The profiles are from
the top: grammage, caliper and roll
It can be seen that caliper follows
grammage and hardness caliper.
Final problem is the marked red range
of corrugation, where all profiles reach
Hardness is an accurate measurement
compared to the other measurements.
The biggest relative difference is in
hardness profile (over 10%).
Picture: Juha Turkki
Example of hardness and corrugation
The picture below shows clearly that quite a
narrow resolution in CD direction is needed to
predict corrugations in uncoated SC paper.
The corrugated roll was the first from the left
(blue). Roll width was only 60 cm and there
were 3 clear peaks in hardness (20 cm
interval). TAPIO RQP measurement.
Roll width, cmPicture: Juha Turkki
Coated paper hardness profiles
The picture below shows machine reel hardness profiles of woodfree coated paper.
Profiles have been measured from winder unwind before winding each set of rolls.
There were totally five sets.
It is typical that the first set is always softer than the other four sets.
Picture: Teemu Pasi
Example of several hardness profiles
Example of 17 successive SC paper roll hardness profiles from the same
CD position. The stability of the curve form is quite good.
SC paper runnability in gravure printing
The upper curve caused web breaks in printing (too much roll hardness variation).
The lower curve run without problems.
Picture: Sami Uhlbäck, 2008
Local faults in hardness profile
Slack areas increase web breaks and paper waste.
In this picture a 20 mm wide slack area is found 100 mm from the roll edge.
This requires high resolution fast measurement to be found.
Picture: Sami Uhlbäck, 2008
Caliper and hardness control
Caliper control with calender is directly seen on hardness profile.
CD shrinkage profile in paper drying
Web edges normally have different hardness in the winder rolls. This is due to the
higher shrinkage at the edges.
This shrinkage profile is from a modern newsprint machine with single felted dryers.
One meter from the edges has 3-8% shrinkage while it is in the middle less than 2%.
CD hardness profile and bagginess
This picture from ACA Systems shows very accurate correlation of bag formation
and hard spots on the roll. This is typical for high density papers and plastic foils.
For bulky and porous papers this correlation is not that simple.
Aluminium foil and bagginess
The picture on the right shows web tension
CD profiles at different radial positions in
the reel, for the case where aluminium foil
was wound throughout the reel.
Close to the core higher web grammage
and thickness give more tension (is stiff).
When more layers are wound there will be
more plastic MD strain in the middle and
this compensates the original tightness.
Finally on the surface, the plastic strain has
higher effect on the web tightness and the
web will be slack in the middle.
The correlation between web thickness, roll
hardness and web bagginess depends on
the radial position in the final roll and how
many windings there are after first winding.
Loose surface will be close to the core in
Typical Roll Defects
More info from Roisum: www.youtube.com/watch?v=DnIAMfJYUUY
Main problems of machine reels
The material is from 2004. The mills were printing paper mills. The most common
problems seem to be wrinkles, TNT control and CD profiles/bursts.
The main problems are connected together and could be reduced with new type of reels.
Collected from several newsprint and
magazine paper mills
Pressure from spool to paper
The problem: Wrinkles and bursts on reel bottom. Coated board: glossy
spots close to the spool. Bottom broke up to 2% = 2000 m of 100 km.
There is more problems at edges when the spool is thin, reel is wide, diameter is
large, basis weight is low and paper density is high.
Thin groundwood papers are more sensitive. CD profiles are important.
Reel hardness, hardness distribution, air between layers and machine speed have
Center drive, larger diameter reel spools and new type of reel changes can solve
Main problems on printing paper winders
The most common problems seem to be dishing or uneven roll edge, CD profiles,
crepe wrinkles and loose cores or bad start.
0 2 4 6 8 10 12 14
Loose cores/bad start
Collected from several newsprint and
magazine paper mills
Product quality control by hardness profiler
The end product of a paper mill is the customer roll. Even
if paper is sheeted, it must first form a good roll.
Papermakers normally measure grammage, moisture
and caliper online. However, the correlation of roll quality
with online measurements is often very poor and the
evaluation of roll quality must be made by hand and
Roll hardness measurement reflects roll quality much
better than any other profiler from paper itself.
If the hardness measurement is easy, fast and accurate,
it could even replace in many cases online CD profile
Mill information system can draw complete CD profiles
from individual roll measurements and send the
information to the laboratory and operators.
Normally the need to adjust CD profiles is only when
some changes are made and that would suit very well for
roll hardness measurements to control paper quality.
Severe roll hardness and
diameter variation of wrapped
Picture: Pekka Komulainen
Corrugations or rope marks
Roll hardness/roll diameter is different on
each side of rope mark.
Rope mark propensity can be found with
hardness measurement before it can be
Root causes for corrugations are:
• Differences in paper cross-direction profiles
of caliper, compressibility, tension and
• Differences of nip load in cross-direction or
web travel from unwind to rewind
• Too hard winding
• Most problematic with high density papers
Look deeper Roisum:
Bagginess can be hidden inside the roll and will show up when paper is unwound in
converting. Roll hardness and web tension profiles can easily show this kind of
bagginess propensity before it can be seen.
Typical for bagginess is:
Out-of-plane buckling (at low tension)
Bagginess is caused by CD profile variation (grammage, moisture, caliper etc.) together
with hard winding and low elasticity of paper
Typical for thin, hard calendered papers with coating or high filler content
Can be seen only inside the roll or on top of the roll
Look closer Roisum: https://www.youtube.com/watch?v=L1_9F58N9_s
Cross machine direction burst
This burst is identified by a break in the sheet across
the roll face. This burst is generally found near the
core or in the outer few centimeters of the roll that is
wound too tight (hard).
The burst can be across the full face of the roll or a
partial burst just enough to break down the web
If rider roll is pressing too much, this can be in the
middle of the roll radius.
• Too hard roll
• Tight web together with high caliper
• High MD tension and nip pressure
• Low MD strength and stretch (low TEA)
• Very dry paper
Crepe wrinkles and their reasons
Combination of low web tension and high nip load
Hard area on top of soft area or increasing hardness to the roll periphery
Low web caliper/grammage and low MD stiffness
Low coefficient of friction, slippery inside the roll (DIP is today well washed and COF is
quite high, the first DIP plants produced very slippery paper)
Hardness variation in CD profile, loose and thick web at edges
Uneven pressure from rider roll
Cross sections of crepe wrinkles
(David Mcdonald, 2014)
Crepe wrinkles start from the roll edge
J-line and crepe wrinkles
Internal slipping of paper layers under the roll surface
(decreasing tension) is measured with J-line
Shooting or marking a roll radius and then winding
layers on top of that will move the marked layers
towards the winding direction (= loosening the original
If the original tension is low and slippage high there will
be a negative tension in the machine direction resulting
in buckle of the layers (= crepe wrinkle).
Very bad combination is a high nip load, a low web
tension and a low COF. This is most common at the
machine reel edge, where caliper can be high and web
length long (=loose edges). In addition edges can be
dry and thus COF is low.
It is best to have curved CD profile = slightly lower
caliper at edges to avoid crepe wrinkles and other
Read more: Finishingnet.com
Sensitivity to crepe wrinkles
There will be slippage and possibility to get crepe wrinkles, when
- F > µ • p • A + S
- F = buckling force from nip action = f(N and nip width)
- A = area of possible sliding
- S = paper stiffness force
To avoid crepe wrinkles paper static COF must be high as well as roll hardness,
but nip load should be low.
Low paper stiffness and caliper increase possibilities to get crepe wrinkles.
With a soft drum cover the buckling force and J-lines are smaller than with a hard
winder drum. Possibility to get crepe wrinkles is very small.
When measuring J-line all possible safety
measures should be considered. A safe
method is to stop the winder, draw a radial
line to the end, and then start winding again.
After winding the J-line can be measured,
photographed and analyzed.
This method, however, is not correct, because
stopping and starting have effect on winding
forces and slippage of the layers.
The old, but not so safe method is to shoot a
chalked special string against the rotating roll
end and then look the line after winding (right
Picture: J.K. Good
Process effects on crepe wrinkle formation
Crepe wrinkle is a common defect at web edges. All parameters from raw materials to
the winder have effect on crepe wrinkle formation – especially at edges.
In unwinding it is easy to feel the wrinkles by hand. If there are wrinkles already in the
machine reel you can feel them on unwind reel at winder.
Main winder wrinkles must be checked at salvage winder.
To improve the situation all the papermaking process should be improved – not only
winder parameters. At the edges the situation can be described as follows:
Strain on reel
Effect of splicing on roll structure
Rolls having winder splices are never as good as
If the splice is made because of web break in the
machine reel, the winder must be decelerated
before the splice, which increases web tension.
During the splicing the web can be loose, which
decreases roll hardness.
After the splice winder is accelerated, which
reduces roll hardness.
On top of this loose part nip effect is higher and
roll hardness is increased again.
On a rewinder (salvage winder) dynamic forces
are smaller and spliced rolls can be better.
Picture: Pekka Komulainen
Transport and roll storage
Sometimes rolls are stored
horizontally on each other.
If rolls are too soft they will form
On a two-drum winder soft rolls
always have harder surface, and
they easily get buckled and show
starring on the end.
Internally soft rolls are formed in
the positions where rider roll is
not contacting the rolls.
Picture: Pekka Komulainen
Roll ridges and hardness
Rolls with hardness variation can look quite nice on the winder, but after storage
and transport there can be severe bagginess on the hard ridge areas.
This kind of bagginess is higher on the roll surface, where the nip load and
diameter differences have been highest.
Storage time, humidity and temperature of the environment/paper have effect on
the final hardness profiles and bagginess formation.
Pictures: David Roisum
Look closer: https://www.youtube.com/watch?v=fdYOvlnUhB4
Starred roll and hardness
Stars or paper buckling in the machine
direction is caused by negative MD
tension inside the roll.
The basic reason is increasing roll
hardness towards the roll surface.
Starring could occur later when the roll
gets external forces and impacts in
handling, transport and storage.
It is very important that rolls are not
stored like in the picture.
Dished roll or telescoping
Reasons can be:
Bad spreading (D-bar), misalignment of
Misalignment of winder rolls
Unsymmetrical diameter and hardness
of paper roll (CD caliper or tension
Soft cores in CD direction
Core chuck pressure is too low
Air entrainment with high speed
Low paper-to-paper COF (Coefficient Of
Picture: David Roisum et al.
Why rewinder (salvage winder) makes hard rolls?
If only one roll is pressed with rider rolls there
is good contact. On the main winder edge
rolls may be smaller in diameter and not
pressed at all at the end of winding.
Bulky paper like newsprint is compressed
plastically and deformed after main winder.
Thinner and denser paper makes harder rolls.
Especially roll surface is made on rewinder
from the hardest part of the unwinding roll.
Hardness is easily increased towards the roll
surface at the rewinder.
Sometimes web tension control has same
total force for wide and narrow rolls. Then the
narrowest rolls can have very high tension (=
Drum radius of a two-drum rewinder is normally smaller than that on the main winder
(e.g. 550 mm vs. 850 mm). Narrow nip – harder roll from own weight.
Winder speed of a rewinder is about half or the main winder speed (e.g. 1200 vs. 2400
m/min). Higher speed – more air into the roll, higher centrifugal force and faster nip
impact all reduce roll hardness on the main winder.
Optimum roll hardness
Too much hardness
Good roll quality
Soft roll starring
If the roll is too soft, especially in the center, there will be handling damages such as
starring and out-of-roundness. Additionally, there can be crepe wrinkles or telescoping.
If the roll is too hard there can be bagginess and bursts.
Too soft or too hard rolls tolerate less roll hardness variation than rolls with optimum
hardness. There is always some CD profile variation in the hardness and it is important
to find the optimum average hardness level to avoid roll broke.
Roll hardness and roll deformation
It is clear that harder rolls keep roundness better in handling than soft rolls.
Permanent deformation grows very fast when the roll is softer.
However, it is evident that the roll should be hard inside but softer on the top.
Unfortunately two-drum winder tends to make the opposite.
Picture: D.MCDONALD, J. HAMEL AND A.MÉNARD, 2005
The following four things are most important to be remembered about roll quality:
The end product of papermaking process is a high quality paper roll and its quality must be controlled.
The highest cost broke of the papermaking process is at the end.
All faults should be corrected where they first appear (in the process phase and position). It is never
possible to totally compensate earlier fault with later correction.
Winding cannot make paper better, but in many cases product quality can be much lower after