The coating and casting layers of liquid compositions on a substrate are practiced in many industries. For instance, photographic films and papers are made by coating liquid photographic compositions on a moving support web to form wide, long rolls which are subsequently cut into the desired roll or sheet formats. A product may contain as many as fifteen to twenty discrete layers which must be of uniform thickness, both widthwise and lengthwise.
Photographic coating of aqueous compositions commonly uses a slide hopper to coat multiple layers of photographic liquids on a moving web of plastic or paper. Slide hoppers are useful for either bead coating or curtain coating, as disclosed, e.g., in U.S. Pat. No. 4,287,240 to O'Connor, which is hereby incorporated by reference. The slide hopper comprises an elongated body which is positioned transverse to the direction of coating. The body has a smooth, inclined, upper surface, or "slide", down which the coating liquid flows to the moving support web. The interior of the body contains one or more elongated transverse flow distribution conduits or "cavities". Coating liquid is supplied to the hopper through an inlet conduit in the body which terminates in an inlet port in the cavity and which feeds liquid to the middle or to one end of the cavity. Coating liquid passes from this cavity to the slide surface through an elongated outlet slot in the body which communicates with both the cavity and the slide surface over substantially the entire length of the cavity.
The opening of the elongated outlet slot onto the slide surface is rectangular with a uniform height on the order of ten thousandths of an inch and with a length of fifty inches or more. The slot must be supplied with liquid at a uniform rate over its entire length in order to form coated layers of uniform thickness. Conventionally, the cavity shape is correlated with the slot dimensions to provide a hopper which is satisfactory for given rheological conditions including viscosity and density of the coating liquid, and volumetric flow rate. However, when used under different conditions, as in the coating of different liquid compositions, this cavity and slot design cannot provide the same high uniformity.
In a distribution cavity which has a constant cross-sectional area, the drop in pressure with distance from a central feed inlet is accompanied by a drop in flow rate transversely along the length of the cavity. This results in non-uniform delivery of liquid to the outlet slot and consequently, a non-uniform thickness in the layer coated on the support. It also results in regional stagnation of flow along the wall of the cavity, a serious problem in the photographic industry. Since many photographic coating liquids contain gelatin they will solidify if allowed to stagnate. This gives rise to irregularities in the cavity walls and can cause transverse thickness variation streaks in the coating. Stagnant regions can also allow clots of partially-solidified coating liquid to form which an break loose as slugs in the coating. In addition, coating liquid which escapes the stagnation regions may have undergone a change in photosensitivity because of extended residence time in the hopper. This can cause photographic non-uniformity.
It is known to provide multiple inlet conduits for coating liquid as disclosed, for example, in U.S. Pat. No. 4,623,501 to Ishizaki. This patent discloses "auxiliary supply pipes" at both ends of the cavity, but fails to recognize that this can create areas of stagnation along the cavity wall between the auxiliary pipes and the central pipe and non-uniform distribution of flow into the slot, which could result in detects or non-uniform thickness in mid portions of the coating.