FIELD OF THE INVENTION
The present invention relates to an apparatus for improving the fiber alignment at the opposing edges of the headbox outlet of a papermaking machine and for controlling the qualities of the edges of the rolls of paper or paperboard being manufactured. More specifically, the present invention relates to an improved nozzle section design and an improved tube bank design of a headbox which results in improved fiber alignment at the edges of the nozzle outlet and also provides a consistent basis weight across the entire roll of paper or paperboard, including the edges.
In the manufacture of paper and paperboard, it is important to achieve a flow of stock suspension out of the nozzle section of the headbox having a uniform machine direction velocity profile. In other words, the speed and direction of the flow of stock in the middle of the nozzle should be the same or as close to the same as possible as the speed and direction of the stock flowing at or near the edges of the nozzle. When the machine direction velocity profile of the stock changes across the width of the nozzle, the basis weight or grammage and the fiber alignment of the resulting product will vary across the width or roll of product. As a result, printers or purchasers of paper and paperboard rolls often avoid the purchase of "edge rolls" because they differ in basis weight or fiber alignment. Further, if the paper fiber alignment varies from the machine direction, the misalignment can result in breaks in the paper during production. As a result, the paper production must be interrupted. Still further, when the paper fiber alignment varies from the machine direction, paper is produced with dimensional stability problems. Specifically, sheets of paper cut from the roll will not have consistent dimensions and therefore, as a result, a stack of sheets from the roll may not lay flat with square corners but may exhibit a curled or waviness appearance when laid flat on a table. This problem is particularly evident for some specialized computer papers that are folded in an accordion-like fashion. Such paper with dimensional stability problems may not form a square stack which clearly signifies to the consumer that the paper is of an inferior quality.
The specific problem associated with the headbox or, more specifically, the nozzle section of the headbox, is illustrated in FIGS. 1 and 2. Turning first to FIG. 1, a horizontal cross sectional view of a headbox 10 is illustrated which includes a header 11 connected to a tube bank 12 which is disposed between the header 11 and a nozzle section 13. The input flow of the stock suspension shown at 14 and a small output flow of stock suspension is shown at 15. However, the majority of the stock suspension is delivered to the nozzle section 13 through the plurality of parallel tubes shown generally at 16. The suspension then flows through the nozzle section 13 and outward to a forming section (not shown) in the direction of the arrows shown generally at 17.
FIG. 1 illustrates an idealized solution whereby the stock flows outward from the nozzle section 13 with a uniform velocity profile as illustrated by the arrows 17. In other words, in an ideal solution, the velocity profile across the width of the nozzle section 13 is uniform. However, those skilled in the art have long recognized that a uniform velocity profile exiting the slice opening 18 is not uniform but varies at the outer edges 21, 22 of the slice opening 18 as illustrated schematically in FIG. 2. More specifically, it has been found that friction between the flow of stock against the pondsides 23, 24 (or sidewalls) of the nozzle section 13 results in velocity profiles 17a, 17b at the opposing edges 21, 22 of the slice opening which are not in alignment with the machine direction (see the arrows 17) but which are dispersed outwardly from the machine direction indicated by the arrows shown at 17. The outward, non-aligned profiles shown at 17a, 17b adversely affect the edge portions of a roll of product being manufactured. Specifically, the edge portions of the roll will not have the same basis weight or fiber alignment as the center of the roll and, accordingly, many consumers of the roll product do not like to purchase material formed at the edges of the roll and the edge portion might be sold at a discounted price or even recycled.
Accordingly, there is a need for an improved headbox configuration which will avoid the velocity profile illustrated in FIG. 2 and more closely approximate the velocity profile illustrated in the idealized situation shown in FIG. 1.