Adjustable activity drainage box

Paper making and fiber liberation – Apparatus – Running or indefinite length product forming and/or treating...

Reexamination Certificate

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Details

C162S355000, C162S356000

Reexamination Certificate

active

06780286

ABSTRACT:

SUMMARY OF THE INVENTION
The present invention relates to a process, and an apparatus for improving sheet properties in the forming section of a paper making machine, which section includes a forming fabric moving in the machine direction and a head box having a head box slice which delivers a stock jet onto the moving forming fabric. It is particularly concerned with a process and an apparatus in which the scale and intensity of the activity within the stock is adjusted by altering the position of some of the fabric support elements beneath the forming fabric adjacent the head box slice, so that they either contact, or do not contact, the machine side surface of the forming fabric and observing the effect, if any, that the adjustment of the position of the selected support element or elements has on the quality of the paper product being made on the paper making machine. These fabric support elements are located on at least one drainage box. This invention is thus of use in both a conventional papermaking machine forming, section having a single open surface forming fabric, in the initial open surface section of a so-called hybrid gap former having two superposed forming fabrics, and in twin fabric papermaking machines equipped with a curved forming shoe.
BACKGROUND OF THE INVENTION
In the manufacture of paper and board products, a highly aqueous stock consisting of about 98-99.8% water and from 0.2-2% papermaking fibers and other solids is ejected at high speed from a headbox slice onto a moving forming fabric. Adjacent the head box slice, the forming fabric passes in sliding contact over a plurality of static fabric support elements which serve to support the forming fabric, and to define a reference surface over which the forming fabric moves. Depending on the surface profile chosen for the fabric support elements, they may also assist in draining water from and generating turbulence in the stock on the forming fabric. The fabric support surfaces usually include a lead blade located more or less underneath the point at which the stock jet impinges the forming fabric, followed downstream by at least two additional surfaces, each of which may be flat, or profiled to act as foils (e.g. as disclosed by Wrist in U.S. Pat. No. 2,928,465) or as agitators (e.g. as disclosed by Johnson, in U.S. Pat. No. 3,874,998), although stock agitation is not typically initiated at this very early point in a forming section. In current practice, the individual fabric contacting elements adjacent the head box slice are not vertically adjustable after installation (see for example Rulis et al., U.S. Pat. No. 6,274,002). These elements are normally mounted onto the supporting structure using either a dovetail, or a T-bar, and in some cases are solidly mounted. The mountings are also arranged on the supporting structure so that all of the elements are in permanent non-adjustable contact with the forming fabric. Stock activity can only be adjusted by changing, or physically removing, the fabric contact elements by sliding them out of their dovetail or T-bar mount. It is both very difficult and time consuming to carry out such a change in the configuration of the forming section fabric supporting elements while the paper making machine is running, bearing in mind that the forming fabric can be moving at up to 100 kph.
Good sheet properties result from the injection of kinetic energy into the stock, which causes the papermaking fibers to become agitated and thus relatively more uniformly dispersed. This is particularly important in the early part of the forming section adjacent the head box slice, where the paper making fibers are still relatively mobile.
Paper makers are currently seeking means to accommodate changes in the basis weight of the paper product being manufactured by the same machine. For example, a papermaking machine producing liner or board grades at a relatively low basis weight may have to shift to a far higher basis weight product which requires a much slower machine speed. This change in speed reduces the beneficial stock turbulence and adversely affects product quality. It is thus apparent that a given papermaking machine, without some means of adjustment, is best at making one grade of paper product and cannot readily be altered to make a significantly different one. It is not uncommon today for a papermaking machine to have to accommodate a doubling or tripling of the basis weight of the product being manufactured. Such changes are frequently difficult to manage without compromising sheet quality, and therefore alternative means of achieving a required level of activity in the stock are necessary to accommodate changes in paper grade with minimal disruption to production.
This invention seeks to provide a method and an apparatus by means of which the paper maker can alter the number of fabric support elements in contact with the forming fabric in the area immediately after the stock impinges onto the forming fabric adjacent to the head box slice so as either to enhance, to maintain or to diminish stock activity and thereby optimize agitation in accordance with papermaking conditions to provide a product of acceptable quality. It also offers the possibility that a contact element with a given profile can be vertically adjusted to be out of contact with the forming fabric and a support element with a different profile vertically adjusted to be in contact with the forming fabric. It further offers the opportunity to change the support element so that one element profile can be changed for another while the element is not in contact with the forming fabric. Since the effect, if any, of adjusting a particular support element can only be determined after the paper making conditions have stabilized and product made under the new conditions retrieved from the paper making machine and examined, there inevitably is a time lag between a support element being adjusted and the consequences of that adjustment being known. This time lag generally will be at least several minutes, but can be up to an hour.
DISCUSSION OF THE PRIOR ART
Smith et al. in U.S. Pat. No. 5,080,760 disclose a sealed pressurized forming board in which the rate of drainage of fluid from the stock is slowed by applying a small positive pressure to the machine side of the forming fabric, which also allows stock activity to be initiated without increasing drainage. The level of stock activity can only be controlled by physically removing the fabric supporting elements and replacing them with others having differing support surface profiles.
Johnson in U.S. Pat. No. 4,140,573 discloses a sealed cover for a low vacuum suction box in which all of the fabric contacting elements shown have an essentially flat surface in contact with the fabric and some of the fabric contacting elements are fixed in a position that is a small amount below the others. This unit requires an applied vacuum which is sufficient to cause the forming fabric to contact all of the element surfaces so that the forming fabric follows an undulating path, thereby inducing agitation in the stock. Johnson discloses that the distance below the normal plane of the fabric of some of the elements can be adjusted by means of pins and set screws, or by movement of an element having a sloping T-recess relative to sloping a T-bar. This box is not generally used adjacent the headbox slice, since vacuum assisted drainage is usually not desirable at that early point in the forming section.
Miller in U.S. Pat. No. 5,421,961 discloses a forming board whose cross-machine direction orientation relative to the headbox is adjustable and controlled so as to maintain it in parallel relation to the slice. There is no effort or intention to control stock activity.
Thorp in U.S. Pat. No. 4,443,298 discloses an adjustable blade support member whereby, in one embodiment, the blade-to-blade spacing in the machine direction is adjusted by means of a screw. However, the elements are only moved laterally, not vertically.
Ibrahim in U.S. Pat. No. 4,684,441 and U.S. Pat. No. 4,718,983 discl

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