Calender section of a paper machine
Calendering is a process that happens towards the end of the paper machine and it is intended to give the paper a better finish and appearance. A calender can be part of the paper machine where it is calendered as part of the manufacturing process but it can also be a completely separate unit, where the paper is passed through after it has come off the paper machine and has been wound into large rolls.
The paper is passed through a number of calender rolls, the points where the calender rolls touch, and the pressure is applied, is called the nip.
The surface of paper can be, matt, smooth or rough and by calendering paper it can be made glossy and smooth. Calendering paper can improve its printability. The kind of paper used for magazines (LWC) is often calendered.
When paper is passed through a calender, the porous uneven surface of the paper is smoothed out and flattened. The thickness of the paper becomes more uniform.
A calender on a paper machine can consist of just two rolls, or can be a tower of 12 rolls. With each type of calender giving slightly different results. A paper machine making newsprint would only have to have a two roll calender. This is because newsprint is a low quality printing paper. The higher the printing quality required the more the paper must be calendered. Magazine paper would go through a calender with multiple rolls and therefore multiple nips.
Calenders can have several rollers and each of these rollers within the calender can have different properties or be made from different materials. Some calender rolls can me steel while others have specialist coatings. The calender rolls can be heated or chilled. All these combinations combine to give the desired effect to the paper.
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