Chatter can lead to a loss of sheet integrity, increased
waste and damage to the yankee. What happens to produce chatter?
And how can we reduce it?
By S Archer, G Furman,
V Grigoriev, L Bonday,
and W Su
In light dry creping (LDC), some recent process
changes have led to the evolution of 'chatter', resulting in CD
sheet defects, marks in the yankee coating and even grooves in
the metallic surface of the yankee dryer (Figure 1).
Chatter in tissue making is caused by out-of-plane
movement related to vibration of the tip of the creping doctor
blade. This vibration can be mechanically or process-induced.
Traditionally, studies have focused on mechanically-induced, harmonic
or sympathetic vibration caused by external stimulation. Although
many occurrences of chatter have been directly related to excessive
mechanically induced vibration (Corboy 2003), a growing number
have been related to specific process changes (Archer 2008), including:
creping at lower moisture; higher adhesion creping; and use of
functional chemical additives.
THE SYSTEM
Figure 2 defines system elements around the doctor
blade tip. The tip of the doctor blade normally resides in a softer
layer of coating, below the sheet, and is supported by harder
layers of coating next to the dryer surface. Effective tip movement
is caused by the rotation of the yankee dryer. Tangential resistance
(Equation 1) opposes doctor tip movement and is a function of
a number of coating characteristics as discussed below. The sum
of all supporting and opposing forces determines the exact position
and travel of the doctor blade tip in the coating.
Where:FN
= normal force of the doctor blade tip, (μk = coefficient
of kinetic friction, dependent on surface characteristics, m =
mass of coating material, mc, and sheet, ms, on the dryer. The
mass associated with the sheet is a function of basis weight and
a sheet adhesion factor, A, (0 to 1), m = (mc+msA), v = surface
velocity of yankee moving the coating and sheet into contact with
the tip of the creping doctor blade, G' = elastic behavior of
the coating developed on the yankee at the current velocity, Cx
= tendency of the coating adhesive to become hard or react with
other chemistries in the process, and P = pocket angle, P=(90-θ+α)
In a stable creping
process, the tip of the doctor blade rides in the yankee coating
in a relatively steady manner. If process conditions change, a
significant out-of-plane movement of the doctor tip can occur
that can lead to generation of chatter. The following mechanistic
model can be very useful in understanding how process conditions
lead to chatter occurrences.
In a stable creping process, the tip of the doctor
blade rides in the yankee coating in a relatively steady manner.
If process conditions change, a significant out-of-plane movement
of the doctor tip can occur that can lead to generation of chatter.
The following mechanistic model can be very useful in understanding
how process conditions lead to chatter occurrences.
Process conditions develop that cause the yankee
coating to become harder. Total tangential resistance increases
as coating modulus and friction increase. As total resistance
increases, a point is reached where a phenomenon known as 'stick-slip'
begins (McMillian 1997).The blade tip no longer traverses evenly
through the coating and velocity in the machine direction (MD),
drops toward zero. At this point the blade is becoming 'stuck'.
The force imposed by the rotating yankee reaches
a point that exceeds the static 'stuck' condition. The tip of
the doctor blade lifts out of the coating plane.
The doctor blade tip reaches its maximum out-of-plane
elevation and accelerates toward and penetrates through the yankee
coating. The total impact force, FI, at the doctor blade tip is
approximated by Equation 2.
Equation 2: FI = m dv/dt
Where: m = mass of the doctor system, dv = change
of velocity at the doctor blade tip as it comes in contact with
the yankee dryer, and dt = time of a single contact event.
The increased downward speed at the tip of the doctor
blade and the short impact time cause the coating to appear very
hard and brittle. The molecular network of the coating does not
have sufficient time to absorb and distribute the energy. The
coating fractures and chips away from the yankee surface.
As the blade tip penetrates deeper into the coating,
tangential resistance increases dramatically, leading to stick-slip-induced
cyclic movement or vibration. If this induced vibration frequency
and the natural vibration frequency of the system are synchronous,
a high-amplitude, resonant vibration can develop. The increased
amplitude at the doctor blade tip will result in increased cyclic
impact forces on the yankee surface and eventually chatter. Occasionally,
the impact force will be sufficient to penetrate through the protective
yankee coating, leading to metallic chatter marks.
SOFT TISSUE PROCESS CONDITIONS THAT LEAD TO CHATTER
Lower-moisture, high-adhesion creping is practised
throughout the industry to increase bulk and surface softness
attributes in tissue. Shifting from normal creping moistures of
4-6% to low-moisture creping conditions of 1.5-3% will result
in elevated yankee surface and coating temperatures. Components
of the coating react differently to low moisture / high temperature
conditions. Thermosetting yankee coatings and cross-linking functional
process additives react chemically with resident materials producing
larger macro-structures that display harder film characteristics.
Non-cross-linking coating materials and natural cellulosic materials
dehydrate and become harder. With less moisture present, these
materials will have higher glass transition temperatures, Tg,
(Furman 1993) and higher storage moduli, G' (Figure 3). This harder
coating results in higher tangential resistance at the doctor
blade tip, eventually leading to the stickslip phenomena and the
formation of chatter.
High adhesion creping to improve sheet softness
can also be attained through higher adhesive to release (and/or
modifier) ratios. Such conditions will also tend to increase coating
hardness and propensity to chatter.
A common problem that occurs during low moisture,
high adhesion creping is the appearance of hard deckle edge deposits
and bands (Figure 1-c). These deposits can result in chatter in
the sheet, coating, and on the yankee dryer surface. Higher temperatures
at the deckle edge of the sheet force coatings to thermo-set and/or
dehydrate faster. Over time, these deposits build in size, both
in cross direction and out of plane. Out-ofplane deposits can
become very thick and hard, leading to stick slip behavior and
chatter. As the deposit continues to build, the doctor blade tip
will ride further away from the yankee surface. This lifting of
the blade effectively reduces doctor loading in adjacent areas
leading to excessive doctor tip vibration and chatter. Coating
chatter in these deposits can act as a template resulting in sympathetic
vibration of the doctor blade across the width of the dryer.
Achieving functional properties in bath tissue products
sometimes requires the use of temporary wet strength (TWS) or
glyoxylated polyacrylamide (GPAM) resins. TWS added to the stock
system will attach to long fibres as intended, but a portion will
also become associated with fines, colloids and dissolved contaminants.
During the drying process, some of the TWS resin-laden fines and
colloidal materials migrate with water from the sheet and become
a part of the coating (Furman 2007). TWS materials that migrate
to the surface of the dryer can chemically react with polyaminoamide
epichlorohydrin (PAE) adhesive materials that were applied through
the spray boom. This reaction leads to formation of a harder and
less adhesive coating. Figures 4 and 5 illustrate the impact of
a typical TWS additive on commercial yankee PAE coating adhesives.
In Figure 5 the adhesive peel force decreased while G' increased
significantly. Both results are consistent with a harder coating.
As a result of these reactions tangential resistance at the tip
of the blade can increase leading to the formation of chatter.
MINIMIZING CHATTER
Process conditions and variation can result in chatter
on the yankee dryer. Unfortunately, there is no single actionable
solution that will prevent its appearance. There are, however,
a number of directional moves that will minimize the occurrence
of chatter and yankee damage. These moves can be implemented alone
or in combination, but will require rigorous monitoring to evaluate
the impact on the process.
General action plans: Reduce variation throughout
the tissue making
process; conduct
a full machine audit to both identify and eliminate mechanical
and process variation that could result in doctor blade vibration;
closely monitor the yankee dryer and doctor blades to document
the occurrence of coating or yankee chatter; characterize creping
system mechanics.
Process-driven action plans: Optimize creping doctor
set-up and operation; stiffen the creping doctor blade; reduce
the doctor blade stickout- to-thickness ratio to stiffen the doctor
blade (increasing the doctor blade loading effectively stiffens
the system. This should only be attempted if it is believed there
is sufficient 'acceptable' coating on the dryer. Coating add-on
strategies should be reviewed and adjusted to fit this operating
condition); consider an adjustment or replacement of components
to increase the stiffness of the creping doctor assemblies; increase
the setup angle (θ in Figure 2).
Optimize yankee coating characteristics: Soften
the yankee coating; use modification technologies. (Furman 2004);
change to a softer nonthermo- setting adhesive yankee coating
platform. (Archer 2005). Eliminate edge deposits: Consider use
of appropriate edge deposit control technologies. (Archer 2006).
CONCLUSION
The pursuit of increased production and improved
quality will predictably continue. Softness and productivity improvements
can be achieved with changes to existing processes. However, some
of these changes can be associated with a stick-slip phenomenon
and vibration at the tip of the doctor blade. Excessive vibration
can lead to appearance of chatter on the yankee surface. Operational
changes can be implemented to moderate the appearance of chatter
and its impact on machine operations and stability.
References
S Archer,
G Furman,
C Llanos, "Coating space -a 3D view of creping
cylinder coatings," Associacao Brasileira Tecnica de Celulose
e Papel Conference,
ABTCP, Brazil,
October 2005.
S Archer, G Furman, "Method for targeted application
of performance enhancing materials to a creping cylinder",
US 7,048,826 B2, 23 May 2006.
S Archer, V Grigoriev, G Furman, L Bonday, W Su, "Chatter
and soft tissue production - process driven mechanisms," Tissue
World Americas Conference, Miami, Florida, 11 March 2008.
W Corboy, "Vibration-induced yankee surface wear - an
overview of chatter," Tissue World Conference, Nice France,
27 March 2003. G Furman, W Su, "A review of chemical and
physical factors influencing yankee dryer coatings," Nordic
Pulp and Paper J., Vol. 8 No. 1, 217- 222, 1993.
G Furman, V Grigoriev, W Su, C Kaley, "Effects of modifying
agents on adhesive film properties - findings toward improved
yankee coatings," Tissue World Americas Conference, Miami,
Florida, September 2004. G Furman, R Phillips, S Archer, "Wet-end
impacts on creping - part 1 - the effect of retention aids on
yankee coating," Tissue World, Nice France, March 2007.
A McMillian, "A non-linear friction model for self-excited
vibrations," Journal of Sound and Vibration, 205(3), 323-335,
1997. Messrs Archer, Furman, Grigoriev, Bonday and Su are with
Nalco Company, 1601 West Diehl Road, Naperville, Il 60563-1198.
The authors would like to thank Chris Kaley (Nalco) for his
contribution to this work.
Copyright: 2009 Nalco Company |
