In direct coating, the web is supported by a backing roll and the coating medium is fed in excess onto the web. After a certain distance, defining the “dwell time”, the coat layer is metered down to the final thickness, being the desired coat weight.
In the “roll applicator” or “LDTA” (long dwell time applicator), feeding is done with a roll, which draws the coating medium from a pan onto the web (Fig. 7.25). The applicator roll and the backing roll form a nip. The nip pressure and, conse¬quently, the nip gap are adjustable, determining the amount of coating medium fed to the web.
The nip load also yields an external penetration pressure which, together with the capillary pressure, causes penetration of some coating medium or components thereof into the paper or board web. This is to a certain extent desirable, because it anchors the coating to the base paper and gives surface strength. Excessive penetration, however, would reduce the gain in surface proper¬ties, such as smoothness or gloss.
Penetration of the coating into the base paper can be controlled with the “water retention” of the coating medium. At elevated machine speeds, e. g. above approx¬imately 1500 m min–1, the splitting of the coating color between applicator roll and paper at the nip exit shows irregularities, which negatively influence the homoge¬neity of the coat layer.
Also, the relatively short nip and the considerable penetra¬tion caused by the nip load cause runnability and quality problems. To overcome these limitations, the application of the coat medium to the paper can be per¬formed with a free jet. In a “free jet applicator” (Fig. 7.26), the jet usually has a thickness of less than one millimeter and a length of a few centimeters.
The result¬ing coat layer (still before final metering) is much more homogeneous than that of a roll applicator, and, since the external pressure at the impingement point is considerably less than in the roll nip, the penetration of the coating medium into the base paper is reduced. This improves the so-called “coating holdout”, which results in a better surface quality of the coated paper.
The final metering of the applied coating is often performed with a blade. With this blade, most of the coating medium is removed and returns to the working tank. The coat layer remaining on the paper or board is evenly distributed.
The blade pressure ensures that the surface voids of the web are filled with coating. Due to the blade geometry, the coated web has a good smoothness. The blade usually has a thickness of less than one millimeter, typically approximately
0.4 mm. The stick-out length (or unsupported length) is usually a few centimeters.
The operating angle between blade and paper web is usually 20–35°. Under these conditions, the blade is considered a “stiff blade”, because its bending is negligible with respect to the operating principle. If, however, a blade geometry is chosen where the bending of the blade results in an operating angle close to 0°, the mode is called “bent blade”.
The bent blade gives – in comparison to the stiff blade – an even smoother surface but it is more prone to surface defects, such as blade scratches, and is more difficult to operate. Since the web has a considerably re¬duced roughness after coating, the coat layer itself must be nonuniform in caliper to even out the base paper roughness.
On certain grades, especially board, this nonuniform layer thickness can be seen as inhomogeneous coverage or mottling. If coverage is desired rather than smoothness, metering can be done with a rod rather than with a blade. Rod metering is typical for the pre-coat of board and for specialty papers where the base web has to be evenly covered with a specialty coat.
In most cases, the metering blade (or rod) is placed against the same backing roll as the coat applicator. The angle between point of application and point of meter¬ing is then approximately 60°. This results in a dwell length of 400–600 mm, de¬pending on the backing roll diameter. The resulting “dwell time” depends strongly on the machine speed. It is in the range of 10–200 ms.
A “short dwell time applicator” (or SDTA) was tried in order to reduce the dwell time to much shorter values. This SDTA consists of a closed chamber which incor¬porates the coating feed and the metering blade.
However, vortices in the chamber caused a streaky appearance of the coat layer at elevated machine speeds. There¬fore, the SDTA has not received much attention outside North America.