Paper Coating Machines
Coating machines are separate machines in which a previously produced dry base paper is unwound, coated, dried, and then reeled again (Fig. 7.24). In contrast to coating applicators placed on-line in a paper machine (Section 6.8), they are usu¬ally called “off-line coaters” or “OMC” (off-machine coaters).
In a paper mill, one off-line coater is usually associated with one paper machine. Sometimes, however, one OMC receives paper from different paper machines. In most cases, the off-line coater has the same width as the corresponding paper machine (except for the edge trim). Some coaters with half width are in operation, where the jumbo reels from the paper machine are split, and the two halves are coated separately.
Off-line coating has the advantage that the paper machine can continue produc¬tion during machine stops on the off-line coater. These stops can be for instance due to coating color changes, which is especially important for machines with a very broad product range. Since off-line coaters operate faster than the paper ma¬chine to which they belong, the time efficiency of the entire line is higher than for on-line concepts.
The actual coating process usually comprises two steps: (i) the application of the coating medium onto the paper or board and (ii) the metering of the coating medium to the desired quantity (or coat weight). If the metering is done directly on the paper after the application, the process is called direct coating or blade coating. If, however, metering is done before the transfer of the coating medium to the paper or board, the process is called indirect coating or film coating.
Each type of paper coaters as well as coater dryer systems are separately explained Here .