Objective and General Description of the Calendering Process
The objective of calendering is to modify the surface characteristics of paper with regard to its further use, e. g. printing. Depending on the individual grades, the focus is put on different technological properties. These are mainly
. • gloss
. • smoothness/roughness
. • density
. • blackening
. • brightness • opacity.
Printed gloss and printing smoothness are the major prerequisites for a good print quality. Both are generally dependent on the gloss of the paper and its smooth-ness/roughness as well as its levelness and compressibility. High printed gloss gives the printed product the desired shiny appearance, while high (printed) smoothness is decisive for the evenness of print and print density, e. g. reduced number of missing dots.
As to the theoretical basis of calendering, a series of explanations exist. Some scientists hold the view that smoothness and gloss result from slipping of the paper in the nips. Others maintain that calendering is a flattening process where the smooth surface of the hard rolls is replicated on the side of the web that contacts the hard roll. Still others argue that it is the shearing action in the nip which causes gloss and smoothness by “aligning” the surface particles of the web. Not in dispute is the influence of heat: thermal energy transferred to the web softens the cellulose fibers (glass transition point) and thus enhances the develop¬ment 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.
The strength properties of the paper are peremptory for the runnability of the web in the printing machine. Brightness and opacity have a distinct impact on the print quality. Blackening is found when parts of fibers have collapsed under pres¬sure. Under transmitted light the respective areas appear glassy, whereas under incident light they appear as darkened areas. This is still intensified by the printing process, i. e. the light full tone areas turn murky gray.
Calendering is done by pressing the paper web in one or more “rolling” nips formed by rolls with special properties.
The main factors in calendering – apart from furnish and paper properties such as moisture, temperature and coating – that influence the above-mentioned tech¬nological result are:
. • nip pressure/load
. • nip dwell time
. • roll elasticity
. • roll surface temperature and smoothness.
Details concerning the rolls and roll covers, the roll configuration and other im¬portant components of the calenders will be treated below.