Coating Blade Model for Paperboard Coating

   Examining the coating blade model for paper coating reveals that the inclined surface of the blade can cover two large clusters of densely packed flocculates (irregularities on the paper surface). This may hold true for the coating of thin paper with a relatively smooth surface. However, for poorly formed paperboard, the height difference between the peaks of high-density flocculates and the troughs of low-density flocculates is several times greater than the height of the blade’s inclined surface. 

Figure 1 illustrates the coating model of a straight doctor blade applied to cardboard. At high unit coating pressure, the doctor blade is pressed against the cardboard surface, and the distance between the blade and the cardboard equals the sum of the deformation of the cardboard and the backing roll. Fibers on the base paper surface are scraped up by the “tip” and “heel” of the doctor blade’s bevel, resulting in high-density flocs being stripped away or completely covering the paper surface without being coated. For coating with a slightly curved doctor blade, the load is concentrated at the blade tip. 

The blade bends, causing the blade surface to become nearly parallel to the coated paper surface. See the model shown in Figure 2. This increases the contact area between the blade surface and the coating, thereby reducing the unit coating pressure. Consequently, more coating is retained on the high-density flocculent, ensuring complete coating of the paper surface.