Papermaking headbox having extended divider sheet

Abstract: The formation of paper webs produced by papermaking headboxes having damaged or imperfectly-formed headbox lips (6) can be improved by providing the headbox with an extended flexible divider sheet (10) which extends beyond the slice opening (4, 6) and is positioned adjacent the defective headbox lip (6). The extended divider sheet (10) so positioned substantially eliminates the adverse effects of the defective lip on the stock jet characteristics.

Description: In the manufacture of paper sheets, including creped tissue paper, a headbox is used to deposit the papermaking stock onto a forming wire, where the stock is partially dewatered to form the paper web. Oftentimes the formation of the paper web is flawed due to the presence of minor damage or imperfections in the headbox apron lip, which create jet disturbances as the stock flow leaves the headbox. Correction of these problems usually requires repair or replacement of the headbox apron lip which can be a difficult and inexact task. Therefore, there is a need for a better means for improving web formation defects caused by imperfections in the headbox lip. It has now been discovered that web formation affected by imperfections in the apron lip of the headbox can be improved by providing an extended divider at the divider sheet position closest to the damaged or otherwise imperfect apron lip. As is well known in the papermaking industry, headboxes can be provided with a multiplicity of internal divider sheets which create microturbulence in the stock flow to improve mixing and therefore formation of the resulting web as it is deposited onto the forming wire. The number of internal divider sheets is usually about four or more and varies with the headbox design. EP-A-0 147 350 discloses such a headbox where the internal divider sheets are formed with self-positionable anisotropic trailing elements, one of which can extend beyond the slice opening of the headbox, although this does not seem to be critical. It is also known to provide extended dividers to produce a layered web. GB-A-2 093 879, for instance, discloses a headbox provided with partitions positioned for separating stock flow and maintaining layer purity in the resulting web. A flexible foil is connected to the end of each partition and optionally to the headbox top lip plate. Such headbox does not include any internal divider sheet in addition to the partitions. Other known headboxes include such internal dividers, but the extended dividers are symmetrically positioned within the headbox from top to bottom and the internal divider closest to the headbox lip is not one of the extended dividers (see for instance US-A-4 141 788). The ability of an extended divider to overcome formation defects, when the extended divider is positioned close to the headbox lip, has not been heretofore appreciated. Hence in one aspect, the invention resides in an improved papermaking headbox comprising a top wall which ends at a slice lip, a bottom wall which ends at an apron lip, wherein the slice lip and the apron lip define a slice opening therebetween, and a plurality of internal divider sheets that do not extend beyond the slice opening, the headbox being characterized in that a first flexible extended divider sheet extends through and beyond the slice opening and is positioned directly adjacent to the top wall of the headbox, a second flexible extended divider sheet extends through and beyond the slice opening and is positioned directly adjacent to the bottom wall, and said plurality of internal divider sheets are positioned between the first and second flexible extended divider sheets. In the case of a layered headbox which has a divided inlet for receiving different stocks and extended layering dividers which maintain separation of the stock until after the different stocks leave the slice opening, the flexible extended divider sheet which extends through and beyond the slice opening is positioned directly adjacent to the top or bottom wall of the headbox and none of the extended layering dividers is directly adjacent to the top or bottom headbox wall. Hence this invention is applicable to layered or unlayered headboxes. Depending on the style of headbox, the extended divider sheet of this invention can be next to the apron lip (bottom of the headbox) or the slice lip (top of the headbox), or there can be two extended divider sheets of this invention wherein one is next to the slice lip and the other is next to the apron lip. A single extended divider sheet next to the apron lip is preferred. The terms "top" and "bottom" of the headbox are used as a matter of convenience to identify the two headbox sidewalls which are approximately parallel to the plane of the internal divider sheets and are intended to also apply to those headboxes which, in operation, are positioned vertically. In the drawings : Figure 1 is a schematic cross-sectional view of a headbox, illustrating an extended divider sheet positioned adjacent to the apron lip, said headbox not being an embodiment of the invention. Figure 2 is a schematic cross-sectional view of an alternative headbox illustrating an extended divider sheet positioned adjacent to the slice lip, said headbox not being an embodiment of the invention. Figure 3 is a schematic cross-sectional view of a headbox of this invention having an extended divider sheet positioned adjacent to both headbox lips. Figure 4 is a schematic cross-sectional view of an alternative headbox of this invention in which the headbox is a three-layered headbox having two extended layering dividers in addition to the extended divider sheet of this invention. Figure 5 is an actual size photograph of a paper sheet made with a headbox as illustrated in Figure 1 having a damaged apron lip, illustrating the streaking caused by poor formation resulting from the damaged apron lip. Figure 6 is an actual size photograph of a paper sheet made with the same headbox as was used to make the paper sheet shown in Figure 5, but having an extended divider sheet positioned adjacent to the damaged apron lip (extending 1.0 inch (2.54 cm) beyond the slice opening), illustrating the resulting improved sheet formation. Figure 7 is an actual size photograph of a paper sheet made with the same headbox as was used to make the paper sheet shown in Figures 5 and 6, but having the extended divider sheet positioned adjacent to the damaged apron lip and extending 3.75 inches (9.52 cm) beyond the slice opening. Referring to the drawing, the invention will be described in greater detail. For all of the Figures, like reference numerals represent like features. Figure 1 is a schematic cross-sectional view of a non-layered headbox (Converflow Concept III, Beloit Corporation, Beloit, Wisconsin) which has been modified by providing an extended divider sheet. Shown is the inlet manifold 1, the step-diffuser tube bank 2, the top wall 3, the slice lip 4, the bottom wall 5, the damaged apron lip 6, internal divider sheets 7, 8, and 9, and a flexible extended divider sheet 10. This modified headbox differs from the non-layered headbox according to the invention in that it comprises a single flexible extended divider sheet. The flexible extended divider sheet is made of any material which can withstand the headbox operating conditions and which can flex in response to fluid pressure. An example of a suitable material for extended divider sheets for this particular headbox is Lexan (Polycarbonate, General Electric, Pittsfield, Massachusetts). The thickness of the extended divider sheets can be, for example, about 0.40 inch (1.02 cm) and is preferably tapered toward the tip. The flexible extended divider sheet preferably extends beyond the slice opening a distance of about fifteen times the height of the slice opening. However, lesser degrees of extension can still provide improvements in the formation and are within the scope of this invention. For example, for the headbox used to provide the photographs of Figures 6 and 7, the single extended divider sheet extended 1.0 and 3.75 inches (2.54 and 9.52 cm), respectively, beyond the apron lip. The height of the slice opening for that headbox was 0,50 inch (1.27 cm). For most tissue making headboxes, however, extensions of about 2 inches (5.08 cm) or more, and preferably about 6 inches (15.24 cm) or more, beyond the slice opening are preferred. Figure 2 is a view similar to that of Figure 1, illustrating an alternative example of a non-layered headbox wherein the single extended divider sheet is adjacent to the top wall rather than the bottom wall of the headbox. Figure 3 illustrates an embodiment of this invention in which two extended divider sheets 10 are provided, one being adjacent to the top wall and the other being adjacent to the bottom wall of the non-layered headbox. In this embodiment, only two internal divider sheets are shown although, as with all the other embodiments, there can be more internal divider sheets depending on the design and size of the headbox. Figure 4 is a view similar to Figure 3, illustrating an embodiment of this invention in which a three-layered headbox, having two extended layering dividers 11 and 12, has one extended divider sheet 10 adjacent to the apron lip. Also indicated are partitions 13 and 14 which separate the different stocks for the three layers. Note that as is typical for layered headboxes, an internal divider sheet (7, 8 and 10) is present within each layer and hence neither extended layering divider 11 or 12 is positioned adjacent to the top or bottom wall of the headbox. This is one distinguishing characteristic between conventional extended layering dividers and the extended divider sheets of this invention. In operation, another distinguishing feature is the fact that the papermaking stock flowing on both sides of the extended divider sheets of this invention is the same, whereas for extended layering dividers the stocks on either side of the layering dividers are different. An alternative embodiment of this invention includes two extended divider sheets in conjunction with a layered headbox. This can be achieved by extending divider sheet 7 of Figure 4. Note that in all cases where two extended divider sheets are utilized, the degree to which each extends beyond the slice opening can be the same or different, depending on design requirements. Figure 5 is a photograph of an uncreped paper sheet made on a conventional headbox (Beloit Converflow, Concept III) having a damaged apron lip. The photograph was taken by passing light up through the sheet such that the light areas of the photograph indicate holes or thinner areas of the sheet. Note the two large light streaks indicating poor formation uniformity. In contrast, Figures 6 and 7 are photographs of the uncreped paper sheet made on the same headbox, but-provided with a flexible extended divider sheet (Lexan) as illustrated in Figure 1 and previously described. The extended divider sheet used for making the paper sheet of Figure 6 extended beyond the slice opening 1.0 inch (2.54 cm). The extended divider sheet used for making the sheet of Figure 7 extended beyond the slice opening 3.75 inches (9.52 cm). Note the improved uniformity and substantial reduction of the effects of the apron lip damage as the amount of the extension beyond the slice opening increases. It is believed that further extensions would further improve the formation of the sheet as well as the ability to mask the disturbances caused by any other headbox lip imperfections.

Claims: Papermaking headbox comprising a top wall (3) which ends at a slice lip (4), a bottom wall (5) which ends at an apron lip (6), the slice lip (4) and the apron lip (6) defining a slice opening therebetween, and a plurality of internal divider sheets (7,8,9) that do not extend beyond the slice opening, characterized in that a first flexible extended divider sheet extends through and beyond the slice opening and is positioned directly adjacent to the top wall (3), a second flexible extended divider sheet (10) extends through and beyond the slice opening and is positioned directly adjacent to the bottom wall (5), and said plurality of internal divider sheets (7,8,9) are positioned between the first and second flexible extended divider sheets. Papermaking headbox comprising a top wall (3) which ends at a slice lip (4), a bottom wall (5) which ends at an apron lip (6), the slice lip (4) and the apron lip (6) defining a slice opening therebetween, and a plurality of internal divider sheets (7,8,9) that do not extend beyond the slice opening, and having a divided inlet (1) for receiving different stocks and extended layering dividers (11,12) which maintain separation of the stock until after the different stocks leave the slice opening, none of the extended layering dividers (11,12) being directly adjacent to the top (3) or bottom (5) headbox walls, characterized in that at least one flexible extended divider sheet (10) extends through and beyond the slice opening and is positioned directly adjacent to the top (3) or bottom (5) wall of the headbox. Headbox of Claim 1 or 2, characterized in that the flexible extended divider sheet(s) (10) extend(s) beyond the slice opening at least about 2 inches (5.08 cm). Headbox of Claim 1 or 2, characterized in that the flexible extended divider sheet(s) (10) extend(s) beyond the slice opening at least about 6 inches (15.24 cm).

Inventors: ALLEN PETER JOHN, ALLEN, PETER JOHN
Applicants: KIMBERLY CLARK CO, KIMBERLY-CLARK CORPORATION
Application year: 1992
Main classification: D21F1
IPC codes: D21F1