| Rotary screen coating and printing are accomplished with a thin metal cylinder perforated by an array of holes to form a screen that may be patterned. This is so mounted and rotated in a coating apparatus that liquid fed into its interior is forced through the holes by a fixed flexible blade. Outside the screen the liquid issuing from the holes forms a coating bead between screen and substrate carried on a backing roll; at its downstream meniscus the bead delivers the coating (direct rotary screen). Alternatively, the bead is between the screen and a transfer roll that receives the coating and delivers it to the substrate in another roll nip (indirect rotary screen).; Experiments were performed to measure the average coating thickness delivered, the length of the coating bead, and the nature of the film splitting at its downstream meniscus. The thickness was most strongly affected by the tip-to-nip offset, i.e. the circumferential distance between the blade tip inside the screen and the nip, or minimum clearance, between screen and backing roll. Contrastingly, the blade loading, i.e. how strongly the blade was pressed against the inner screen surface, only weakly affected coating thickness. But loading did influence the coating bead's length—and thereby the film-split and the uniformity of the coating delivered by the split.; The process was modeled by splitting it into two parts: blade coating-like flow inside the screen and the roll coating-like flow outside. One-dimensional lubrication and two-dimensional Navier-Stokes flow models were developed for both flow regions. Liquid flux through the screen between them was represented by Darcy's Law. The models predicted fairly well the measurements and elucidated the main features though not the details of the film-splitting.; In addition, deformation of and flow in permeable, deformable substrates were described theoretically by adapting Biot's (1972) poroelastic model. The equations were solved for the wetpressing process, wherein a saturated porous and deformable sheet is pressed between two rollers to extrude much of the liquid. Interestingly, in regions of highest interstitial pressure within the pores, the material is predicted to swell even though the adjacent material is predicted to be compressed. |
