As an alternative to direct coating, a film press can be used for the application of coating media. The film press was derived from the size press, which is used for the application of starch or size solutions.
Today, a wide variety of coating media are applied, including pigment dispersions with high solids content. In a film press, a film of the coating medium is formed and metered on a large diameter roll. This roll forms a nip with another roll. The paper or board web passes this nip and picks up a certain portion of the film. Application can be – but does not have to be – simultaneous on both sides. A schematic drawing of a film press is shown in Section 6.7.
Metering is performed using metering rods. These can be either smooth or profiled. Profiled rods provide a certain volume of coating medium due to the open cross section in the profile. A deeper or coarser profile gives a higher coat weight than a fine profile. The application weight is mainly adjusted by choosing an ade¬quate profile.
Fine tuning of the application weight can be done by modifying the rod pressure. Profiled metering rods are mainly used for low viscosities (i. e. low solids contents of the coating medium) and low machine speeds. With increasing viscosity and speed, the rod loses contact with the roll due to the hydrodynamic forces of the coating medium, comparable to aquaplaning. Then, the application weight depends less on the profile and more and more on the hydrodynamic conditions, such as rod pressure, rod diameter, viscosity and speed. Consequently, a smooth metering rod is used.
Typical rod diameters are 14–38 mm for smooth rods. Larger diameters yield higher application weights. Higher viscosities and machine speeds require smaller rod diameters for the same application weight than lower ones. The coating medium is not transferred completely to the web.
A certain amount remains on the roll and returns to the application unit where it is mixed with fresh coating medium. The transfer ratio depends on the acceptance behavior of the web, on the properties of the coating medium and – to a limited degree – on the surface properties of the roll. For starch, where the absorptivity of the web is high, the transfer ratio can be more than 90 %. For pigment coats, it can be as low as 50 %.
The pre-metered films usually have thicknesses of 7–20 mm (or ml m–2). Lower values would require very high rod pressures and also coverage of the web would be insufficient. Higher values are not meaningful since the web has a limitation with respect to coating acceptance. If the amount of pre-metered coating medium is too high, the surface of the coated web appears uneven, with an “orange peel” character.
At elevated machine speeds, the film split at the nip exit can create a fine mist of coating medium. This mist deposits on machine parts or even on the paper or board web. Since this misting increases with film thickness, it is the major limitation for the application weight at high speed. For typical applications, mist¬ing becomes a limitation above 1500 m min–1. At 1800 m min–1, for example, coat weights above 7 g m–2 are difficult to achieve without disturbing misting.