Multi-layered Fibrous Web Forming System Employing A Suction Roll Positioned Adjacent The Web Side Of The Forming Wire And Around Which The Forming Wire Is Wrapped

Abstract

A multi-layer paper web forming system which includes a first endless forming screen upon which multiple slurries are deposited. The first slurry is delivered onto the moving wire opposite a suction forming roll and the second slurry is deposited upon the first slurry downstream of the forming roll. A second suction roll is positioned downstream of the second slurry discharge device adjacent the paper web side of the screen around which the endless screen is wrapped in an arcuate path. The second suction roll may be wrapped in part by a second endless forming screen which may then travel away from the second suction roll in conjunction with the first endless forming screen.

Description

BACKGROUND OF THE INVENTION

This invention relates to forming fibrous webs on moving screens from slurries of fibers in liquid suspension, and is particularly applicable to manufacture of paper and non-woven textiles.

SUMMARY OF THE INVENTION

The invention makes possible formation of improved (e.g., stronger heavier, more uniform) webs by providing for rapid liquid removal (even with fibers offering high resistance to liquid flow) with excellent control over fiber orientation and solids retention, with a system featuring low cost and complexity, simplified control, minimum screen wear, applicability to a wide variety of web weights and compositions, and ability to produce multi-layer webs with excellent interlayer bonding. Existing installations can easily be modified to embody the invention.

In general the invention features partial web formation on a moving screen which carries the partially formed web through a reverse concave arcuate path around a suction roll to produce liquid removal through the web surface opposite that contacting the screen. In some preferred embodiments at least one second slurry is deposited on the partially formed web upstream of the suction roll of the invention, forming a second web layer from which liquid is removed by the suction roll in a direction away from the first layer. In other preferred embodiments a second moving screen runs in contact with a sector of the suction roll of the invention to facilitate web separation from that suction roll. Web separation can also be facilitated by a zone of positive pressure following a vacuum zone in the suction roll, the removed liquid being retained momentarily in the roll shell perforations by inertial forces at the time of web separation. Preferably, the web remains on a single, continuous, moving screen through its formation.

BRIEF DESCRIPTION OF THE DRAWING

The DRAWING is a diagrammatic side elevation of a web forming machine embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, a moving, endless forming screen 10 is looped around a suction forming roll shell 12 and other forming, water removal and screen driving and carrying rolls 11 as illustrated. The shell 12 of the forming roll is perforated, the perforations communicating with one or more fixed boxes 14, 16 on the inner side of the shell, boxes 14, 16 being connected in conventional fashion as desired to sources of sub- and super-atmospheric pressure 15, 17, respectively. A slurry supply device 18, of design well known in the art, delivers a uniform layer of suitable dispersed slurry to the outer surface of screen 10 at a point where screen 10 is rigidly supported on roll shell 12. The top lip 20 of the nozzle of slurry delivery means 18 is hinged at 21 to permit adjustment of opening 22 to control the velocity and depth of the slurry stream discharging onto screen 10. A lip extension 24 covers part or all of screen 10 while in contact with roll shell 12. Lip extension 24 is hinged at opening 22, permitting control of gap 26 at the point where the forming wire initially enters the atmosphere.

All or a part of the liquid in the slurry stream deposited on screen 10 is withdrawn through screen 10 into roll shell 12 under the influence of pressure maintained in boxes 14, 16 and the positive pressure of the slurry stream under lip 24. solids are retained on screen 10 as liquid is removed to form the desired web. A wide range of factors determining web properties, such as fiber orientation, solids retention, and the proportion of total liquid drained from the slurry may be positively and predictably controlled by regulation of opening 22 and gap 26, the positive pressure in the slurry stream, and the pressures in boxes 14, 16.

Liquid withdrawn into shell 12 can be later discharged from the reaches of screen 10 by centrifugal throwout into pan 13 or by withdrawal into boxes 14, 16.

The partially formed web and any remaining liquid are carried away from shell 12 by screen 10, which is trained reversely concavely about a sector of a second perforated suction roll 30, containing one or more internal boxes 32, 34, 36 which communicate with the perforations of the shell of roll 30 and are connected as desired to sources of sub- and super-atmospheric pressure 15, 17, respectively. A second moving, endless screen 38 is looped around shell 30 and driving and support rolls 40 and 42.

The reversely arcuate concave contact of screen 10 with roll 30 will exert centrifugal force on liquid remaining in the web, causing further removal of liquid through the web and screen 10, the liquid discharging into pan 44.

Additional liquid is drawn from the side of the formed web which is remote from the screen 10 by sub-atmospheric pressure in box 32, thus allowing formation of heavier webs or webs of high flow resistance, as it is not necessary to withdraw all liquid through the formed web. By application of a sufficient degree of sub-atmospheric pressure in box 32, the discharge of liquid through the web into pan 44 can be controlled over a wide range or eliminated entirely.

Box 34, following box 32, may be held at sub- or super-atmospheric pressure as required to prevent holding the formed web on screen 38, retaining the web on screen 10. The separation of screen 38 from shell 30 also facilitates web separation from the shell. With easily separated webs, screen 38 can be arranged simply to surround the entirety of roll 30, eliminating rolls 40 and 42. In any event, as web and shell separate, removed liquid is momentarily held in the perforations of the shell by inertia. Box 36, following box 34, may be held at sub-atmospheric pressure to retain liquid removed from the top of the web in shell 30 and prevent return of liquid to the web. The liquid retained may be discharged into suction collector 46 and pan 48 or discharged into the internal boxes. Showers 50 and 52 remove fibers and maintain roll 30 and screen 38 in clean and operable condition.

The formed web, with sufficient liquid removed for subsequent processing, continues its travel on screen 10. The web may be further dewatered and compacted by devices well known in the art, such as fixed suction box 54, suction rolls 56 containing boxes 59, 57, and presser roll 58. The formed, dewatered, and compacted web may be removed continuously from screen 10, being pulled freely from the screen or removed by suction roll 60 and felt 62. The endless screen 10 is returned to roll shell 12 over the driving and carrying rolls 11.

Advantageously, a second slurry source 70, similar to primary slurry source 18, may be provided as shown, introducing additional slurry onto the top of the web initially formed. The secondary slurry may be similar to or significantly different from the primary slurry. For example, a primary web may be formed of low quality fibers supplied at slurry source 18. This primary web may then be covered with a secondary web of higher quality fiber, to impart special properties to the web surface with a minimum of costly material. The removal of the greater part of liquid in the secondary web into roll 30 rather than through the flow resistant primary web, allows the formation of heavy secondary webs on heavy primary webs. However, the ability to allow a controlled flow of liquid through the primary web under centrifugal force results in fiber entanglement at the interface of the webs, providing excellent bonding of the webs and eliminating subsequent delamination. By the addition of one or more additional suction rolls and slurry sources downstream of the roll 30 multi-layer webs of three or more layers can be formed.

Conventional web forming machinery is generally compatible with the invention. A substantial advantage of the invention is the adaptability of existing machinery to the practice of the invention without extensive replacement or relocation of existing components.

A further advantage of the invention is the absence of components fixed relative to the travelling screens. Such components consume substantial power and cause severe wear of the screens due to rubbing contact.

Other embodiments are within the following claims.

Claims

1. In apparatus for forming fibrous webs, comprising an endless travelling forming screen, a device for depositing slurry on the screen, and a first suction roll wrapped in part by said screen and located on the side of said screen opposite said slurry depositing device, that improvement comprising

a second suction roll located downstream of said first suction roll with respect to the direction of screen travel and adjacent the web-carrying side of said screen,

means forming a chamber disposed within said second roll adjacent and in effective communication with said web-carrying side of said screen, and

means connected to said chamber for creating sub-atmospheric pressure therein,

said screen being trained to wrap along a reversely concave arcuate path about said second roll to permit liquid removal radially into said second roll through the web surface opposite that contacting said screen, and

means for depositing a second slurry on the partially formed web carried on

2. The improvement of claim 1 further comprising a second endless travelling screen surrounding and in contact with the portion of said second roll through which said chamber is in said effective communication

3. The improvement of claim 2 wherein said second screen is spaced from said second roll beyond said portion of said second roll.