Papers of high surface quality receive a pigment coating. For graphic papers, the application of this coating was traditionally performed off-line in a separate coating machine. The coating process, including the different types of applicators and the coating media, is discussed in more detail in Chapter 7.
In a paper machine with on-line coating, the same applicators as in off-line coaters can be used. Refer to Chapter 7 for a more detailed description of these applicators.
On-line coating has the advantage of a considerably reduced total machine size compared with off-line coating, because a winder and an unwind station as well as a re-reeler can be omitted. This reduced size has a direct impact on investment and personnel costs. Paper losses due to the additional winding and unwinding proc¬ess in an off-line process are also avoided.
However, the coating process is susceptible to web breaks due to the forces acting on the paper or board. If a web break occurs in an off-line coater, the paper machine can continue production and the off-line coater will keep up with a higher machine speed. With an on-line coater, however, the entire line has to discontinue production during a web break at a coater station. Therefore, the time efficiency of an on-line machine is lower than that of a paper machine with a separate off-line coater.
On the other hand, off-line operation gives additional material losses due to the rewinding processes. Each unwinding, splicing and winding is associated with certain losses. These can add up to 1 % of the entire production.
It depends strongly on the paper grade and on the coating concept, whether on¬line or off-line coating is more economical.
Board has, because of its high basis weight, a relatively high absolute strength. Furthermore it is produced at much lower speeds than packaging or graphic pa¬pers. Therefore, it is much less sensitive to web breaks than the fast machines producing lower basis weights. Hence, the coating of board is mostly done on¬line.
Figure 6.64 shows the coating section of a typical board machine. It comprises four blade coater stations, two per side, following the pre-drying section. The bot¬tom side is pre-coated first and then the top side is pre-coated. The covering layer (called the top-coat) is applied in the same sequence. The web run for the bottom-side coating is relatively straight in the machine direction. In order to coat the top side, however, the web has to pass the blade coater against the machine direction, because the coat application is always from the bottom. Therefore, the web has first to be guided beneath the coater. After coat application, the web is turned into the machine direction again, with infrared dryers and air dryers placed above the coater followed by a short after dryer section.
Coating of graphic papers is more often performed off-line. Some machines with on-line blade coaters are in operation, but usually at moderate machine speeds.
In the 1990s, the film press was introduced as an on-line applicator for graphic single-coated papers at elevated speeds (Fig. 6.65). In film presses, the forces act¬ing on the paper are considerably lower than in a blade coater, therefore the risk of web breaks is reduced. The benefit of higher material efficiency as well as lower investment and operating costs more than compensates for the reduced time effi¬ciency. The roughness of a film-coated paper is greater than that of a blade-coated paper, therefore it is more difficult to reach the same gloss level. Nevertheless, the economic advantages together with a more uniform print image make the on-line film-coated papers very competitive.
Film coating is more suitable for an on-line process than blade coating due to the lower risk of web breaks. Therefore, it is also used for the pre-coat of multiple-coated products. There are in operation some machines – especially those operat¬ing at low speeds – incorporating pre- and top-coat on-line but in most production lines for multiple-coated graphic papers an off-line coater is included. A film pre-coat is then very often part of that off-line coater. However, there exist also on-line/ off-line combinations where pre-coating is done in the paper machine. All further coats are applied off-line.