Paper webs may be manufactured using a variety of devices, such as suction breast roll formers, twin wire formers, crescent formers, or the like. A common trait of these devices is that they utilize a headbox to provide a jetstream of stock slurry which is deposited on a traveling forming surface. While the aim of paper forming equipment is generally to produce a uniform sheet in both the machine and cross-machine directions, each type of former has particular advantages and disadvantages.

One disadvantage often associated with these devices and, in particular, suction breast roll formers is the great variability in web formation generated by changes in the location and operating parameters of the headbox. Generally speaking, suction breast roll formers operate well within a specific and relatively narrow set of conditions. Operating outside these conditions results in less than optimal sheet formation. Typical defects in a tissue sheet produced with a suction breast roll former outside its optimal operating conditions are intermittent streaks oriented in the machine direction of the sheet and flocs in the sheet.

These defects may become more significant as the paper machine speed is increased beyond its original design speed. While it may be possible to use a lower consistency stock slurry (i. e., aqueous suspension of papermaking fibers) to compensate for less than optimal conditions, supply and handling of the additional amount of water required is generally prohibitive.

Such operating variables present challenges to increasing the production output of paper machines with suction breast roll formers without spending capital to replace the equipment. Additionally, such operating limitations appear to be a dead end in the evolution of tissue machines toward flexibility so they are adapted to make a variety of products on the same machine under different operating conditions. The ability of tissue machines to operate over a wide range of process conditions is becoming economically important.

Accordingly, there is a need in the art for improved jet-type forming equipment (e. g., suction breast roll former) which maintains consistent web formation over a wider range of operating conditions and at higher speeds than is presently available. There is also a need for improved jet-type forming equipment (e. g., suction breast roll former) that can readily be constructed as a retrofit of current formers.

For example, in certain situations when suction inside the forming breast roll may be inadequate, either in terms of magnitude or coverage, the sheet formation may be unacceptable. More particularly, in a typical suction breast roll former there is a gap between the apron and the traveling fabric which creates a free jet of stock slurry as it leaves the apron tip. As a free jet, it is in an atmospheric environment from the moment it leaves the end of the apron lip to the moment it contacts the traveling fabric. The differential pressure needed to move the fiber to the forming surface (i. e., traveling fabric) and drive the water out through the fabric to the forming roll for drainage is generated by the pressure above the fabric and the suction below. If the suction is inadequate, the pressure differential is created by the pressure above the fabric which is converted from the velocity of the moving stock slurry in accordance with Bernoulli's Theorem. The resulting large uncontrolled reduction in velocity causes significant changes in the effective jet (i. e., jet of stock slurry) to forming wire velocity ratio (or jet/wire velocity difference) sufficient to place the operating conditions outside the desirable range. These changes cause formation of the paper sheet to suffer.

Summarv of the Invention The present invention addresses the needs described above by providing a bridge.

The bridge spans a gap between the apron lip of a headbox of a paper forming device and a forming surface (i. e., traveling fabric). Eliminating the gap substantially eliminates a free jet of stock slurry as it leaves the headbox apron lip. This serves to maintain pressure from the headbox to the forming zone above the forming surface thereby avoiding a substantial reduction in the velocity of the stock slurry within the forming zone.

According to the invention, the bridge improves conditions in the forming zone to generate more uniform sheet formation and enhanced sheet strength. In an aspect of the present invention, the bridge could be used to increase speed and improve web formation over a wider range of operating conditions. Desirably, the invention could allow adequate formation over a wider range of consistencies (fiber to water ratio) than could be achieved without use of the bridge.

According to another aspect of the invention, the bridge may be a flexible bridge.

In an embodiment, the present invention relates to an improved suction breast roll former. The suction breast roll former includes a breast roll and a headbox. The breast roll has a support surface defining apertures therein that may or may not be operatively connected to a vacuum source. The headbox is positioned in close proximity to the suction breast roll and is adapted to supply an aqueous suspension of papermaking fibers. The headbox includes a slice roof and an apron that together define a slice opening of the headbox. A forming roof which is generally rigid (but may be flexible) extends from the slice roof beyond the slice opening and, together with the forming fabric, defines therebetween the forming zone of the suction breast roll former. In a typical machine, a gap exists between the tip of the apron and the forming fabric. A feature of the present invention is that a bridge extends between the apron and the traveling fabric. This bridge substantially eliminates the gap and the free jet of stock slurry present during operation without the bridge.

For example, in a typical suction breast roll former there is a gap between the apron and the traveling fabric which generates or results in a free jet of stock slurry during normal operation. As a free jet, it is in a generally atmospheric environment from the moment it leaves the end of the apron lip to the moment it contacts the traveling fabric.

The differential pressure needed to move the fiber to the forming surface (i. e., traveling fabric) and drive the water out through the fabric to the forming roll for drainage is generated by the pressure above the fabric and the suction below. If the suction is inadequate, the pressure differential is created by the pressure above the fabric which is converted from the velocity of the moving stock slurry in accordance with Bernoulli's Theorem. The resulting large uncontrolled reduction in velocity causes significant changes in the effective jet (i. e., stock slurry) to forming wire velocity ratio (or jetlwire velocity difference) sufficient to place the operating conditions outside the desirable range.

Eliminating the gap and the free jet serves to transfer pressure from the upstream channel to the forming zone which helps avoid a substantial reduction in the velocity of the stock slurry (i. e., aqueous suspension of papermaking fibers) within the forming zone. Even if vacuum boxes were designed to extend through the length of the forming zone and/or the vacuum level is adequate for proper drainage, it could be desirable to have the bridge at least for the reasons stated above. In addition, large vacuum levels may adversely affect operation and reliability and may still create a velocity difference.

Furthermore, it is believed that air is irregularly introduced into the forming zone through the gap by jet entrainment and by the traveling fabric. This can disturb formation of the sheet. Eliminating the gap is thought to prevent air from being introduced into the forming zone Desirably, the bridge is mechanically attached to the apron lip. The attachment of the bridge by mechanical means protruding into the flow can be used to create controlled turbulence in the jet in scale, intensity and depth, to promote improved sheet formation.

The invention also encompasses providing texture or roughness (e. g., hummocks and dimples) on the surface of the bridge and/or apron and roof to generate such controlled turbulence to promote improved sheet formation.

As used herein, the terms"sheet formation"or"formation"refers to a property which is determined by the degree of uniformity of distribution of the solid components of the sheet with special reference to the fibers. It is usually judged by the visual appearance of the sheet when viewed by transmitted light and may be determined utilizing equipment such as, for example, a Paprican Microscanner Model LAD94 formation analyzer, available from Optest Equipment, Inc., Hawkesbury, Ontario, Canada. Sheet formation is an important property, not only because it influences the appearance of the sheet but because it influences the values and uniformity of values of many other sheet properties.

The breast roll includes apertures, such as holes, slots or other open areas, to hold the water away from the wet web until it can be thrown off into a saveall, for recycling process water. The breast roll can be operatively connected to one or more vacuum sources so that vacuum is supplied to the apertures, and correspondingly to the forming zone. The zones right before and after the top lip are provided with vacuum to eliminate free water and therefore throw-off of water after the tip. The pressure differential required can come from pressure only or a combination of pressure and vacuum from a different source in the forming zone itself.

In an aspect of the present invention, the bridge may be used in conjunction with existing suction breast roll former equipment. As a result, current suction breast roll formers can be relatively easily and inexpensively retrofit to improve conditions in the forming zone thereby generating more uniform sheet formation and enhanced sheet strength as well as extending the range of water handling capability, increasing speed and improving web formation over a wider range of operating conditions.

In the embodiment illustrated in FIG. 1, the bridge is mechanically attached to the apron lip. In an embodiment of the invention, the bridge could be flexible such that there is controlled pressure against the fabric when there is a jet of stock slurry exiting the slice.

An additional feature is that the bridge may have sufficient flexibility so it would bend up during the passage of contraries or if the fabric overlaps. The bridge can be made as an integral part of the apron lip (e. g., as an extension of the apron lip). Alternatively, the bridge may be joined to the apron lip in a variety of configurations. For example, the bridge may be butt jointed to the apron lip or it may be lap jointed with the apron lip.

Suitable materials for forming the flexible bridge include Lexan (Polycarbonate, General Electric, Pittsfield, Massachusetts), glass-or carbon-fiber reinforced epoxy resins, other polycarbonate materials, fiberglass or other composites, stainless steel or the like, where machining or fiber orientation can achieve the correct flexibility. Fiber reinforced resins may be built to preferentially bend in the machine direction, while maintaining stiffness in the cross-machine direction.

Cross-machine direction flexibility of the flexible bridge is generally undesirable, and thus the flexible bridge may include a plurality of cross-machine direction stiffening elements. These stiffening elements, which are desirably disposed on the surface of the flexible bridge facing away from the forming zone, may be formed of stainless steel, carbon fiber, fiberglass or other composites, plastic compounds or the like. An alternative method is to produce a composite where the stiffness in the cross-machine direction is significantly higher than the stiffness in the machine direction. The bridge may be prestressed in the cross machine direction to eliminate any compressive conditions at the tip of the bridge.

In further embodiments of the invention, a surface of different material may attached at the tip of the flexible bridge on the side of the fabric. The purpose of the different surface is to offer reduced wear of the bridge and the fabric. It can be of a harder material such as a ceramic. It can be mechanically attached, glued or molded into the bridge material.

In yet other embodiments of the invention, a shower may be utilized to apply a lubricant to the traveling fabric upstream of the apron lip. The lubrication shower may be of conventional construction and is intended to reduce wear and improve the seal between the bridge and the traveling fabric.

It should be understood that the bridge device is utilized with paper making equipment to make a paper web. The wet paper web is formed on a forming fabric that

travels through the forming zone between the suction breast roll and a rigid roof. The terms"forming fabric"and"traveling fabric"refer to an endless mesh belt adapted to drain water away from the papermaking fibers and provide support as the web is being formed.

Suitable forming fabrics include synthetic fabrics and are available from fabric suppliers such as Lindsay Wire and Albany International.

An aspect of the invention concerns a method of forming a paper web having improved formation characteristics utilizing a paper making apparatus. In an embodiment, the method includes the steps of: 1) providing a paper making apparatus having a forming zone created between a forming surface and a headbox, the forming surface having a support surface that can be operatively connected to a vacuum source and the headbox having a slice roof and an apron that define therebetween a slice opening, the headbox further including a bridge extending from the apron to a traveling fabric supported by the forming surface; 2) configuring the bridge to substantially eliminate the gap between the apron and the forming surface; 3) supplying an aqueous suspension of papermaking fibers to the headbox under conditions to form a jet; and 4) depositing the aqueous suspension of papermaking fibers onto the traveling fabric under conditions that substantially eliminate formation of a free jet and substantially maintains pressure from the headbox to the forming zone above the forming surface thereby avoid a substantial reduction in the velocity of the aqueous suspension of papermaking fibers within the forming zone.

In an aspect of the invention, the step of depositing the aqueous suspension of papermaking fibers on to the forming surface may further include introducing controlled turbulence to the jet which may be, for example, micro-turbulence.

An embodiment of the invention concerns a method of forming a paper web having improved formation characteristics utilizing a suction breast roll former. In an embodiment, the method includes the steps of: 1) providing a suction breast roll former having a forming zone created between a breast roll and a headbox, the breast roll having a support surface defining apertures therein that can be operatively connected to a vacuum source and the headbox having a slice roof and an apron that define therebetween a slice opening, the headbox further including a flexible bridge extending from the apron to a traveling fabric supported by the breast roll ; 2) configuring the flexible bridge to substantially eliminate the gap between the apron and the breast roll ; 3) supplying an aqueous suspension of papermaking fibers to the headbox under conditions to form a jet; and 4) depositing the aqueous suspension of papermaking fibers onto the traveling fabric

under conditions that substantially eliminate formation of a free jet substantially maintains pressure from the headbox to the forming zone above the forming surface thereby avoiding a substantial reduction in the velocity of the aqueous suspension of papermaking fibers (e. g., stock slurry) within the forming zone.

The operating parameters of the suction breast roll and headbox such as the throat opening, vacuum levels, and headbox orientation and other operating parameters such as the fan pump speed can be adjusted as will be recognized by those skilled in the art in order to achieve the desired sheet properties. The fiber type, fiber consistency, and other factors will need to be taken into consideration but are less critical to operation with improved formation due to incorporation of the bridge.

Many fiber types may be used for the present invention including hardwood or softwoods, straw, flax, milkweed seed floss fibers, abaca, hemp, kenaf, bagasse, cotton, reed, and the like. All known papermaking fibers may be used, including bleached and unbleached fibers, fibers of natural origin (including wood fiber and other cellulosic fibers, cellulose derivatives, and chemically stiffened or crossiinked fibers) or synthetic fibers (synthetic papermaking fibers include certain forms of fibers made from polypropylene, acrylic, aramids, acetates, and the like), virgin and recovered or recycled fibers, hardwood and softwood, and fibers that have been mechanically pulped (e. g., groundwood), chemically pulped (including but not limited to the kraft and sulfite pulping processes), thermomechanically pulped, chemithermomechanically pulped, and the like. Mixtures of any subset of the above mentioned or related fiber classes may be used.

The fibers can be prepared in a multiplicity of ways known to be advantageous in the art. Useful methods of preparing fibers include dispersion to impart curl and improved drying properties, such as disclosed in U. S. Patents 5,348,620 issued September 20, 1994 and 5,501,768 issued March 26,1996, both to M. A. Hermans et al. and U. S. Patent 5,656,132 issued August 12,1997 to Farrington, Jr. et al. ; which are incorporated herein by reference.

Once formed, the paper web can be processed using a wide variety of papermaking operations. For example, the paper web may be formed into an uncreped throughdried tissue as disclosed in U. S. Patent 5,667,636 issued September 16,1997 to S. A. Engel et al. ; and U. S. Patent 5,607,551 issued March 4,1997 to T. E. Farrington, Jr. et al. ; which are incorporated herein by reference. The term"paper"is used herein to broadly include writing, printing, wrapping, sanitary, and industrial papers, newsprint, linerboard, tissue, napkins, wipers, towels, or the like.

Description of the Drawing The invention will now be described in greater detail with reference to FIG. 1, which depicts a simplified cross-sectional schematic view of a suction breast roll former with an exemplary bridge device. Conventional papermaking apparatuses and operations (not shown) can be used with respect to the stock preparation, forming fabrics, web transfers, creping, drying and the like.

FIG. 1 depicts a suction breast roll former 10 for making a paper web from an aqueous suspension of papermaking fibers. The suction breast roll former includes a papermaking headbox 12 placed in operable relation to a suction breast roll 14. The headbox 12 deposits the aqueous suspension of papermaking fibers onto the surface of a forming fabric 16 in a forming zone 18 of the suction breast roll former. The forming fabric 16 forms an endless loop traveling between the headbox 12 and the breast roll 14 in the direction of arrow 100. The forming process allows partial dewatering of a newly-formed paper web. For example, a newly-formed paper web may be partially dewatered to a consistency ranging from between about 7 and 15 percent. The aqueous suspension of papermaking fibers travels in the direction of arrow 200.

The suction breast roll 14 includes a support surface 20 that defines a plurality of apertures 22. The apertures 22 are open to one or more vacuum boxes 24 located within the roll 14 in fixed radial locations about the circumference of the roll and operatively connected to a vacuum source (not shown). The specific size and/or placement of these apertures is conventional. While only a single vacuum box 24 is shown, one or more vacuum boxes 24 provide a controlled amount of vacuum in the forming zone. These vacuum boxes may be located under only a portion of or the entire forming zone as well as extending beyond the forming zone. The level of vacuum is conventional for the paper making process described. The illustrated vacuum box may include section dividers (not shown) that divide the vacuum box into multiple sections.

The headbox 12 receives a suspension of papermaking fibers through a flow spreader (not shown) operatively connected to a conventional stock preparation source (not shown). The papermaking fibers travel in the direction of arrow 200.

The headbox incorporates a slice roof 40 and an apron 42 that define therebetween a slice opening 44. The apron 42 may be mounted on a flow spreader and the headbox 12 may be positioned so that a downstream end of the apron 42 is positioned in close proximity to the fabric 16 to define a gap 66 therebetween.

A forming roof 60 is attached by a pivot mount 62 to the downstream edge of the slice roof 40. The distal edge 64 of the forming roof 60 and the forming fabric 16 form a top lip clearance 46. The slice opening 44 and the top lip clearance 46 are ajustable through mechanisms not shown.

As shown in FIG. 1, a bridge 70 is mechanically attached to the apron 42 using small fasteners 72. Other fastening or mechanical attachment means may be used. For example, attachment means may be rivets, screws, clamps, clips pins, hooks, loops, welds, glues, adhesives and combinations thereof.

In an embodiment of the invention, the bridge 70 may be flexible and may touch the forming fabric 16 during operation. According to an aspect of the invention, the small fasteners 72 may be strategically designed and positioned to generate controlled turbulence. Alternatively and/or additionally, turbulence generating features such as, for example, dimples, hummocks, bars, bumps, waves, grooves or the like may be used. The size, shape, spacing and number of turbulence generated will depend on the range of operating conditions required for the particular application of this invention. The invention also encompasses providing texture or roughness (e. g., hummocks and dimples) on one or more surfaces of the apron, slice roof and/or forming roof to generate such controlled turbulence to promote improved sheet formation.

Controlled turbulence is turbulence introduced in the headbox of a paper making apparatus (e. g., suction breast roll former). Such turbulence is desirably sustained during the operation of the papermaking apparatus and may have a range of wavelengths that may vary under different operating conditions. Desirably, the controlled turbulence is micro-turbulence. As used herein, the term"micro-turbulence"refers to turbulence having a wavelength that is relatively small in relation to the dimensions of a jet of an aqueous suspension of papermaking fibers and is sufficient to prevent undesirable flocculation of the papermaking fibers. For example, the wavelength may be less than about 24 millimeters and is desirably about 6 millimeters or less.

One or more vacuum boxes 24 provide a controlled amount of vacuum generally under the forming zone, and more particularly, in the zone before and after the distal edge 64 of the forming roof 60. The level of vacuum require may be adjusted as will be recognized by those of skill in the art.

The foregoing detailed description has been for the purpose of illustration. Thus, a number of modifications and changes may be made without departing from the spirit and scope of the present invention. For instance, alternative or optional features described as

part of one embodiment can be used to yield another embodiment. Additionally, two named components could represent portions of the same structure. Further, various alternative process and equipment arrangements may be employed, particularly with respect to the stock preparation and forming fabrics. Therefore, the invention should not be limited by the specific embodiments described, but only by the claims and all equivalents thereto.

For example, the present invention is generally intended for use in high speed paper making processes. That is, paper making processes operated where the traveling fabric moves at rates of greater than 1000 feet per minute. It is contemplated that the present invention may be utilized at operating rates that are greater by a factor of three to six or even more.

While the present invention is shown in a conventional suction breast roll former, its application is not limited to such devices. The bridge device may be used with other jet forming paper making equipment including, but not limited to, twin-wire type formers, crescent formers, and the like. In such configurations (e. g., twin-wire formers or crescent formers) more than one bridge device may be used. For example, two bridge devices may be used.