This invention relates to a method, and to apparatus for carrying out the method, for facilitating the transfer of a fibrous web in a paper-making machine from a first foraminous belt to a second foraminous belt. In conventional Fourdrinier paper-making machines, the fibrous web formed on a wire is usually transferred from the wire to a felt by any of a number of known pickup devices. A common form of pickup system that is used in instances in which both the web and the felt have suitable moisture contents, relative to each other, the felt is merely brought into engagement with the web, such as by leading it around a plain roll, and the web will naturally stick to the felt and will thereafter cling to the underside of the felt, even when the felt runs horizontally. In many cases, for example, in the manufacture of thin fibrous webs with low basis weights and high water and air permeability, relatively high speed paper-making machines are used. Several machines for making low basis weight paper, such as tissue and similar light grades, involve the use of two wires or a wire and a felt that are brought together to form a convergent paper-forming zone, sometimes a curved paper-forming zone, where the water is extracted from the stock through one or both wires. One form of such a machine is described and shown in U.S. Pat. No. 3,326,745. In the machine described in that patent, the forming zone is constituted by a converging arcuate space formed between a wire and a felt that are led around part of the perimeter of the forming roll. In such machines, the web and felt both have relatively high moisture contents after formation of the web and the web naturally follows the felt when the felt and wire separate.
It is difficult to separate lightweight fibrous webs, say those with a basis weight of 25 g/m.sup.2 or less, from a forming wire and transfer it to a relatively dry felt when the paper-making machine runs at a speed greater than about 1300 m/min (4000 ft./min). Among the problems that occur at separation is the clinging of pieces of fibers or small pieces of web to the wire when the web is separated from the wire. With the lightweight grades of sheet, the number of long fibers that lend strength to the sheet per unit of sheet area is relatively small, and the area of contact between the fibers in the sheet is not significantly greater than the area of contact between the fibers and the wire.
In machines in which the stock is drained under significant pressure through the wire during the web forming and consolidating process, such as in the case of machines having two wires or a wire and felt trained to define a converging pressure nip constituting the forming zone, the ends of many of the fibers are forced through the strands of the wire, and capillary forces develop between the wire and the fiber ends on the side of the wire opposite from the side on which the web is carried. Such capillary forces hold the ends of the fibers against the outside of the wire and result in a relatively strong adhesion of the web to the wire. Inasmuch as the web is of relatively low strength, complete fibers or pieces of fiber and small parts of the web frequently break away from the web when the web is separated from the wire, thereby tending to make the wire dirtier, clog the wire, cause increased wire wear and result in a substantial loss of controlled forming conditions.
USPTO Backgrounds
USPTO Backgrounds