Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Patent
1995-05-17
1997-09-16
Hastings, Karen M.
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
162213, 162301, D21F 100
Patent
active
056676409
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a double-wire former for a paper machine fashioned as a gap former and having a first and second wire jointly forming a double wire, a headbox which injects a suspension directly between the double wire and a first dewatering zone which, viewed in the machine direction, consists of a curved, nonrotating perforated surface, for example a forming shoe or a surface formed of several slats.
Double-wire formers for paper machines, notably so-called gap formers, are known in many styles. Reference is made to the following publications: gap former and having a first and second wire jointly forming a double wire, a headbox which injects a suspension directly between the double wire and a first dewatering zone which, viewed in the machine direction, consists of a curved, nonrotating perforated surface, for example a forming shoe or a surface formed of several slats. Concerned here is a double-wire former fashioned as a gap former, where among others also an initial dewatering is being shown over a fixed forming box, albeit with a relatively slight curvature. The forming box has several dewatering slots, bounded by slats, across which pass the wires with the paper substance contained in between. Shown as an alternative is also an initial dewatering via a rotating forming roll which, e.g., may be designed as a suction roll. attempt is made at improving the congested conditions between headbox and wire gap to the effect that the top wire and the bottom wire are passed to the first dewatering zone, each across a fixed deflection element with a slight radius of curvature, attempting to reduce the high friction that occurs on the fixed deflection elements, between wire and deflection element, by way of an additional water spray introduction before the deflection element.
A disadvantage of the double-wire former illustrated in (1) is that one needs to choose either a double-wire former with a stationary forming box featuring several slats arranged successively in the direction of wire travel, said slats forming an initial dewatering zone with a very large radius of curvature, or a double-wire former with a rotating forming element situated in the initial dewatering zone and possessing a relatively small radius of curvature. If the choice is a fixed forming box, a paper with a good nonflaky formation is ultimately obtained, owing to the many slats, but with poorer basis weight profiles, because the wires tend with large radii of curvature to a wave formation transverse to the machine direction. When choosing a first forming element with a small radius of curvature, that is, a forming roll, a paper with improved basis weight profile is obtained, but deductions in terms of the formation of the paper are the trade-off.
Unfavorable with the double-wire former illustrated in (2) is its very high wire wear and particularly the necessity of a separate water spray introduction, in order to assure a lubrication between wire and fixed deflection elements. The forming box following the deflection elements has again a very large radius of curvature, with the result of a poor basis weight profile.
SUMMARY OF THE INVENTION
The problem underlying the invention is to propose a double-wire former that meets two requirements simultaneously. Namely, a paper is meant to be generated which has a very good, that is, maximally nonflaky formation, along with a maximally uniform basis weight profile. The latter should match the basis weight profile of a paper produced with the use of a forming roll as first dewatering element.
This problem is solved by providing at the start of the first dewatering zone, beginning directly at the point of jet impingement, a dewatering element of convex curvature which, when viewed in the machine direction, possesses at least two radii of curvature R.sub.i in contact with the wire, each radius of curvature R.sub.i being smaller than the radius of curvature R.sub.i+1 which follows radius R.sub.i in the machine direction.
The inventor has recognized that the advantages of a double-wire forme
REFERENCES:
patent: 4557802 (1985-12-01), Waris
patent: 4623429 (1986-11-01), Tissari
patent: 4734164 (1988-03-01), Irwin et al.
patent: 5074966 (1991-12-01), Koivuranta
patent: 5203967 (1993-04-01), Bando et al.
Hastings Karen M.
J.M. Voith & GmbH
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