Oct. 1, 2001
As we advance into the new millennium,
calendering technology in North America seems to constantly
evolve through dynamic developments in composite cover materials,
equipment, and service techniques. In large part, these
developments are in response to the changing needs of paper mill
customers. North American papermakers, challenged by tight
budgets, limited manpower, and the need to be profitable, now
seek resolution of issues they may have tolerated in the past. A
competitive marketplace, along with an increasing demand for
composite covers that perform longer, improve paper quality, and
provide substantial value, are catalysts for roll cover
The proper application of composite
covers is dependent on good input and communication within mill
operations, as well as research and analysis from suppliers.
Monitoring of covers and attention to small details can eliminate
problems that mills experience every day.
This article will address new composite
cover technology and an approach to its successful application.
The article will also discuss monitoring and detecting issues
associated with composite roll cover use.
NANO-TECHNOLOGY. Cover supplier R&D
activities have steered work into the fascinating arena of
nano-particle technology. Nano-technology is being utilized by
almost every industry in the world in one respect or another.
Major companies investing in this technology include IBM,
Motorola, Dow Chemical, and Hewlett-Packard.
Examples of the application of
nano-technology include: Toyota, who has combined polypropylene
and rubber at the nanometer scale to make a bumper that is 60%
thinner, but with the same strength as a standard unit; and Zyvex
Corp., who is using this nano-scale approach to produce a 16-bit
circuit smaller than a virus.
It is in this technology realm that the
next composite cover advances will be made. The science of small
has led to some very promising developments for improving the
wear resistance, impact toughness, and surface finish of current
roll covers. A surface finish with an average roughness (Ra) of 6
µin (micro inch) is achieved by utilizing "super"-finishing
techniques. These covers continue to smoothi.e., become
smootheras they run in the calender stack and can reach a
surface finish of 3-µin Ra.
A lower Ra value indicates increased
roll cover smoothness. The replacement of large particles with
small, well defined, well-distributed particles is key in taking
composite covers to the next level of performance (Figure 1).
Figure 1. Nano-particle technology is providing new tools in roll
The North American market for
calendersincluding supercalenders, soft-nip calenders, and
gloss calenderscan be characterized in several different
ways. It can be segmented by the technology being utilized to
manufacture a product or by the size and speed of the equipment.
The mill grade structure, region of operation, or ownership is
another method that can be used. However, no matter how a
particular mill is classified in this diversified area of paper
manufacturing, most mills experience similar successes and
failures as they relate to calendering and calender covers.
The newest calendering technology
applied today comes from the major paper machine builders Voith
and Metso. With high-speed calenders running both on-machine and
off-machine, the newest technology and designs run at
temperatures as high as 200º F (93º C) and loads as
high as 2,850 pli (500 kN). These calender stacks utilize 100%
composite covers for their covered roll positions, and they are
used to manufacture a vast range of paper and paperboard
Load compensation capabilities from 80%
to 100%, where all or part of the weight of the stack of rolls
can be relieved, have provided the papermaker with the ability to
manage the nip intensity in each nip. Load compensation thus
eliminates the progressive loading from the top to the bottom of
conventional stacks. While there are a handful of these new
stacks currently running in North America, the majority of mill
superintendents are dealing with the day-to-day issues of
conventional off-machine supercalenders.
Off-machine conventional supercalenders
continue to provide the highest quality capabilities. The biggest
drawbacks to conventional supercalendering are relatively slow
speeds, off-machine operation, and dependence on the use of
filled rollsall of which limit production capacity. In
some cases, high-speed paper machines require three or four
supercalenders just to keep up with a single paper machine.
Advances in hot soft-nip calendering and
supercalendering have improved the state of calendering
technology and have overcome some of the limitations of
traditional supercalenders and machine calenders. Soft-nip
calendering has enhanced the finishing for several grades of
paper and paperboard, producing sheet quality exceeding that of
conventional machine calendering using hard nips. At the same
time, the newer systems simplify the calendering process by
reducing the number of nips needed, and they have also simplified
web runs. In some applications, on-machine soft-nip calendering
offers the capability to finish the sheet to a quality
approaching that of supercalendering, especially with respect to
OPERATIONAL AND MAINTENANCE ISSUES.
Independent of the calender type used, superintendents and those
responsible for roll cover performance have communicated the same
needs. The message is that today's paper mill superintendent
needs a cover that is wear resistant and has superior toughness
and surface finish.
Wear resistance refers to the cover's
ability to perform in the application for extended periods of
time without losing its intended roll or nip profile. Wear that
leads to roll removal is typically seen at the sheet edge.
Impact toughness is the ability of the
cover to absorb impacts to the cover surface that can lead to a
crack, delamination, and failure. Impacts are caused by "wads" of
paper, wraps, foreign objects, and build-up of materials that
pass through the nip. The impacts elevate the peak stress in a
localized area for a short period of time. Superintendents
require a cover that runs without excessive wear and withstands
the day-to-day impacts experienced in the most brutal nip at the
However, we all know that sheet quality
takes top priority. The cover's surface finish is increasingly
becoming a higher priority as mills develop specialty grades that
allow them to compete in higher revenue markets. Most covers
provided today can be ground to a finish of less than
10-µin Ra. The ability of the cover to maintain this
surface finish during its life in the stack is key. The ideal
cover enters the calender stack at 8-µin Ra and continues
to smooth during operation instead of roughening during use.
Roll suppliers are meeting these
demanding requirements through use of various cover materials and
manufacturing methods. Covers are currently available that are
composed of aramid reinforced non-mineral filled resin systems,
aramid reinforced mineral-filled resin systems, pre-cast aramid
reinforced resin sleeve systems with fillers, and thermoplastic
Each of these covers offers specific
attributes that must be carefully selected for a given
application. The roll size, load, speed, and temperature of the
application, along with paper grade requirements, are items that
must be considered. Use of a finite element analysis program
developed especially for roll applications is a standard
A finite element analysis program takes
into account the roll dimensions associated in a particular nip
or nips, the temperature of the application, sheet speed, and
cover material. The algorithm then calculates the stresses
throughout the cover in the z-direction (thickness) and the peak
stress at the cover surface. These peak stresses can then be
compared to the maximum allowable stress of the cover, so that
the correct cover application can be chosen.
The impact of the chosen cover on a
particular grade must also be considered. Different types of
reinforcement patterns in the composite cover can be transferred
from the cover to the sheet via surface replication.
Lightweight-coated papers (LWC) and premium writing grades are
prime examples of where this is important.
The algorithm in the finite element
analysis program also provides nip width information that gives
the papermaker or the coater operator vital information relating
to dwell time and nip intensity. Given the low hysterisis of
composite covers, the temperature generated by the cover is low,
so this information is also available for analysis. Utilization
of higher temperatures in many mating roll applications has
become one of the toughest obstacles to manage, and proper roll
dubbing with respect to sheet width and roll face is critical.
The composite roll cover must be closely mated to the heated roll
surface to eliminate exposing the composite cover.
The cross-direction (CD) temperature
profile and CD load profile of the application must also be
considered. Recently, supercalender superintendents at Mead's
Escanaba, Mich., mill and International Paper's Courtland, Ala.,
mill have found that the introduction of glycol into the
composite covered roll core produces remarkable results with
respect to the CD roll temperature profile. It also has had a
direct impact on sheet quality. Load profiles in many older
stacks appear in a "W" or "M" shape. Resolution of these types of
issues is critical in successfully running composite calender
covers, as well as producing a quality sheet of paper.
THERMAL IMAGING PREVENTS FAILURE. To
detect, monitor, and address these types of cover issues, certain
systems, monitoring equipment, and processes are now being
recommended and utilized. Necessity is truly the mother of
invention, because superintendents, who have often suffered
through sleepless nights caused by cover failures, have devised
creative ways to help monitor rolls and prevent failures.
One fairly simple mode of monitoring
roll and cover status is for the operators to "shoot" the surface
of the roll cover with an infrared hand-held gun and record the
readings on a regular basis (Figure 2). Special note should be
taken if an area of high temperature differential arises, because
rapid response is necessary to prevent failure in this case.
Having operators document significant events (wraps, wads,
wrinkles, etc.) on a daily log sheet that records the location on
the roll face is a good practice. This will provide vital
information in the event of a cover failure.
Figure 2. Thermographic imaging, such as this supercalender hot
spot, can pinpoint problems and prevent failures.
Another way to monitor rolls and to
gather baseline information is to take multiple thermographic
camera shots on rolls at startup and throughout their life. This
often uncovers wear patterns in the roll cover and reveals any
potential life-threatening issues at startup and during a
prolonged run life.
Edge detection via dry-end scanners and
photocells is also used to eliminate exposure of the composite
cover to a heated roll surface due to a fluctuation in the sheet
width caused by change in draw or total head. These detection
devices are often interlocked to unload the stack in the event of
prolonged exposure. Often, these unload sequences are
"quick-drop" designs that allow for the stack to unload very
rapidly, preventing sheet wraps and wad damage.
IMBEDDED ROLL SENSORS. In a related
monitoring and prevention process for roll covers, a smart roll
has been developed, and the system is running on both rubber and
polyurethane covers. The Smart Roll technology (Figure 3)
incorporates a series of sensors within the roll cover that
provide a real-time nip impression as the roll runs on the
machine. These sensors relay performance information to a
transmitter located on the roll head. This transmitter sends the
information to an external receiver connected to a personal
computer. Information can be stored and analyzed for trends, and
the sample periods can be modified to meet specific requirements.
The sensors allow the papermaker to evaluate roll condition, make
changes to the system to compensate for load issues, and
determine the need for roll changes.
Figure 3. Incorporating sensors in the roll provides real-time
roll condition data for operators.
These systems are currently in
development for composite cover applications. Soft-nip
applications should benefit the most, along with use on
on-machine supercalenders, due to the ability to predict roll
cover issues and enable removal on a scheduled basis. Utilization
of these systems in conjunction with controlled crown rolls,
swimming rolls, and other anti-deflection rolls, will prove most
beneficial by allowing the operator to respond to specific areas
in the nip using zone control capabilities. The capability of the
system to work in conjunction with other systems in a closed loop
is also a consideration.
The North American calendering market is
very diversified. In the future, mill superintendents and
managers will team with suppliers of rolls and covers to improve
sheet quality and develop new high-revenue grades. Composite
cover research and development is extremely dynamic at this time,
and the next three to five years will prove to be both
challenging and rewarding.