calendaring

Calendaring

Calendering is done by pressing the paper web in one or more “rolling” nips
formed by rolls with special properties

Modern Calendaring.jpg

Objective of calendering

The objective of calendering is to modify the surface characteristics of paper with
regard to its further use,
These are mainly
• gloss
• smoothness/roughness
• density
• blackening
• brightness
• opacity.

Various theoretical basis of calendering

smoothness and gloss result from slipping of thepaper in the nips.
flattening process where the smooth surface of the hard rolls is replicated on the side of the web that contacts the hard roll.
shearing action in the nip which causes gloss and smoothness by “aligning” the surface particles of the web
influence of heat: thermal energy transferred to the web softens the cellulose fibers (glass transition point) and thus enhances the development of gloss and smoothness.
Smoothing the surface and increasing gloss are accompanied by reduction in caliper, strength properties, brightness and opacity to a certain degree.

Factors

The main factors in calendering – apart from furnish and paper properties are:
• nip pressure/load
• nip dwell time
• roll elasticity
• roll surface temperature and smoothness.

Developments:

Crown rolls
Conventional rolls deflect under the influence of the load and their dead weight,
which would result in a nonuniform distribution of linear load in the press nips.
To avoid this, the rolls had to be crowned, i. e. ground with a camber to
apply uniform pressure distribution. The selected
crown does, only apply to a certain load. If it were desired to alter the
load, the rolls had to be re-crowned.

Swimming Roll

This roll consists of a fixed shaft with a shell rotating around it.
Between the shaft and the shell is an oil-filled chamber. By adjusting the oil pressure
in this chamber the shape of the roll shell can be changed .
The swimming roll allows only control of the linear load distribution across the width
only in a given overall shape

Nipco rolls

the load distribution can be controlled locally zonewise. On this
roll the rotating roll shell is carried by a large number of hydrostatic supporting
elements, which in turn are supported on a fixed shaft. In all, there are six to eight
effective hydraulic zones.They can be controlled individually, the linear load can
be specifically adjusted across the width of the calender. It is therefore possible not
only to uniformly distribute the linear load across the roll width but also to increase
or reduce it locally.

Nipco method.jpg

Machine calenders and supercalenders with width about 5000 mm and above
are today equipped with zone-controlled rolls as standard

Latest Development:

The latest development is a calender (Voith’s Janus MK 2), on which the
roll stack is no longer arranged vertically but at an angle of 45°.
Janus Concept calender
of Voith Paper) was built with all resilient rolls covered with special synthetic
materials.It has reduced number of nips,
less energy input and – most noteworthy – suitability for on-line operation.

The Different Calender Types

Machine Calendars:

Machine calenders consist of two or more hard rolls and are practically always
installed on-line. They are primarily used for paper that only requires moderate
finishing or for pre-calendering grades to obtain higher gloss and/or smoothness
Machine calenders are equipped
with overall deflection or zone-controlled deflection rolls

Supercalenders

Supercalenders are off-machine multiroll-calenders consisting of an unwind
stand, a roller stack and a rewind stand.The rolls are hard and elastic.The hard rolls are steel or cast chilled iron rolls and are often heated
The elastic rolls are filled rolls. The filling consists of a multitude of specialty paper sheets slid
onto a steel shaft, compressed to the required hardness and then locked by nuts.

Softcalenders

Its main components are the soft covered deflection control roll and the heating roll.
surface temperature of the heated roll can be up to 230 °C.In
contrast to supercalenders, softcalenders can also be installed on line because the
soft covers can withstand line loads, load cycles and temperatures that are much
higher

Modern Multinip-calenders

Modern multinip-calenders are similar in function to supercalenders. The main
difference is that the filled rolls are replaced by polymer covered rolls. As a result,
modern multinip-calenders can be installed on-line and can be run more than
twice as fast as supercalenders and with much higher surface temperatures and
line loads.
Today, there are three calender designs which make use of the new technology
(Janus MK 2 calender of Voith, ProSoft calender of Küsters, and the OptiLoad
calender of Metso).

the stack being no longer arranged vertically but inclined at a 45° angle offering operational and technological
advantages

Extended Nip Calenders
This technology is about to yield to
extended nip calendering which provides two advantages, namely a speed that is
much higher than that possible with the Yankee cylinder and a better relationship
of smoothness versus bulk than can be obtained on a soft calender.

Embossing Calenders
The objective of embossing is to give the paper a three-dimensional pattern
This is achieved by means of a single nip calender.
There are three different embossing methods, namely “matrix” embossing, “flatback”
embossing and “union” embossing.

Friction Calenders
The purpose of friction calendering is to impart glazing to the paper. Friction
calenders are single or double nip machines in which all rolls are driven separately
at speeds that differ by 10–30 %. They are mainly used for glazing playing cards.

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