BACKGROUND OF THE INVENTION In the formation of paper, including tissue and paperboard, the process commences with the formation of a pulp slurry in the headbox and subsequent depositing of the pulp slurry of fibrous material through an elongate slice opening onto a forming medium such as, for example, a wire screen, fabric screen, a felt or the like. It should be understood that any further reference in the disclosure to a wire or fabric means a wire screen or a fabric screen, respectively. The headbox is physically separated from the forming wire. The pulp is deposited through a slice opening in the headbox as a jet onto the moving wire. The jet travels from the slice a dozen or so inches through air. The jet stream of pulp fibers strikes the forming wire or fabric at a 3-7° angle of impingement. Rapid drainage through the wire or fabric occurs with loss of fine and heavy particles. The jet travel in free air and impingement onto the wire or fabric is known to have an adverse effect on paper quality. Further the fact that the jet is unsupported as it leaves the headbox also restricts the speed differential between the jet speed and the wire speed to approximately 3%.

There is a need to increase the consistency of the pulp slurry during the formation process in paper making and thereby reduce the volume of water used in paper making. In the formation of the pulp slurry, the fibers are diluted to permit bleaching, cleaning and screening of the fibers and subsequent forming of the fibers of 1% or less consistency. Clearly, any increase in the consistency of the fibers at the time of forming would significantly reduce the amount of water used in paper making. In the process, this reduces the size of equipment such as pumps, cleaners, screens, piping, valves, headbox and former and therefore reduce both capital and operating costs of the paper making line.

To reduce the amount of water used in the formation of paper, the paper making industry has concentrated on increasing the consistency of the pulp slurry processed in the headbox prior to delivery onto a forming wire.

The formation of good quality web of low grammage paper occurs when the thickness and direction of the pulp stream is even and homogenous and passes through a slice opening in the range of 8 to 15 mm. However, for higher consistencies of 2 to 4%, slice openings of about 2 mm are required.

Currently, it is not possible to commercially form high consistency paper through such narrow slice openings.

For heavier grammage board in the order of 100 g/m2 weight or heavier, high consistency forming in the order of 3% consistency has been commercialized where the formed board is used as a part of a multi-ply liquid packaging board machine that forms board. This process requires further re-forming of the surface of the web to make it suitable for use in this application. During forming of this board or web, the fibers in this high consistency formed web pass through a slice opening of about 8 mm in thickness. The fibers, however, are randomly oriented in all directions rather than in the plane of the web making this forming process unsuitable for formation of paper. The random grain orientation is believed to be due to collision during drainage of the densely packed fibers. The formed web has high bulk, random fiber orientation, high porosity, grainy formation, increased z-direction strength (out of the plane of the web) and reduced in- plane strength. While this web is suitable for some board grades it is not suitable for publication papers.

Clearly formation of paper at higher consistency to reduce water usage is not possible due to the requirement of an impracticably small slice opening. Also, the higher consistency forming of board is not applicable to paper because the paper would exhibit unsuitable properties.

Thus there is a need to overcome these two restrictions in the formation of high consistency paper.

SUMMARY OF THE INVENTION The present invention relates to a high consistency extrusion forming unit for the laying of fibrous material such as a paper pulp onto a moving medium or forming fabric such as, for example, a wire screen, fabric screen, a felt or the like. The extrusion forming unit includes a headbox having an exit slice opening through which a high consistency liquid suspension of fibrous material is expelled. The basic aspect of the present invention relies on moving the forming fabric at a speed greater or faster than the speed the pulp slurry leaves the headbox slice opening. This causes the pulp slurry to be stretched and thinned out in the direction of paper travel.

The high consistency forming process of the present invention is based on a premise that it is possible to controllably stretch a jet of high consistency pulp by shearing it between a stationary surface and a fabric moving at a substantially higher speed than the pulp slurry and that in the process, the randomly oriented fibers are made to lie predominantly in the plane on the web. The forming unit of the present invention can be implemented to form low grammage paper webs at high consistencies and can also be used in the formation of board grades.

The consistency of the pulp material formed by the former may be in the range from 2% to 5% and, is preferably 3 %. The slice opening at the headbox may be in the order of 5 to 10 mm and at the slice exit from the extruder is the final thickness of the formed web.

The high consistency extrusion forming unit of the present invention preferably has an upper slice lip extending inclined and preferably curved upwardly away from the slice opening to provide a Coanda effect drawing the fibrous material expelled from the exit slice along the surface of the upper lip. The upper slice lip surface may have an irregular surface. The irregularities in this surface may comprise anchors or ribs or other structures that introduce more disturbances in the fibrous jet pulp slurry passing over this surface. Alternatively, the upper slice lip extension surface may be smooth. The forming fabric travels below the slice opening and along a path adjacent to the upper extended slice lip extension so as to sandwich the fibrous material between the forming medium and the upper slice lip extension for a preselected length of travel. The displacement or thickness between the upper slice lip extension and the forming fabric narrows over the travel length of the fabric. The shearing effect on the pulp slurry occurs between the upper slice lip surface and the moving fabric and is believed to maintain the fibrous pulp material in a fluidized state facilitating fiber grain orientations in the direction of paper travel as the paper is stretched. Further, it is contemplated within the realm of the present invention that the slice lip extension surface has a flat surface immediately downstream of the curved surface that is spaced apart a constant distance from the forming fabric. This zone of constant distance overwhich the fibrous pulp material travels supported on two sides by the slice lip extension and the moving fabric is believed to provide a pressure normalization zone that permits any pressure abnormalities built up in the fibrous pulp material to find a pressure equilibrium or stability. The present invention is further characterized by the speed of travel of the forming medium or fabric being greater than the speed of the fibrous material expelled as a jet onto the forming medium.

This characteristic causes the forming medium to pull or stretch in an extruded manner the fibrous material in the direction of paper travel. This together with the shearing or fluidization along the upper lip results in the thinning or the reduction in thickness of the sheet of fibrous material to something less than the thickness of the slice opening.

It should be understood that the speed of travel of the forming medium relative to the speed of the fibrous material from the slice opening determines the amount of thinning or reduction in thickness of the fibrous material as well as the gap distance between the forming medium and the upper slice lip of the headbox.

In accordance with the present invention, it is believed that the moving speed of the forming fabric is in the range of 20 % to 500%, or more, faster than speed at which the fibrous material is expelled from the headbox.

It should be understood that the description of the preferred high consistency forming unit so far has been to a headbox having upper and lower slice openings. It should be understood that the orientation of the headbox can result in many different angles of the jet being emitted from the headbox relative to ground. Hence, in some embodiments of the present invention there may not be an upper and lower slice lip but rather two spaced apart slice lips with one of the slice lips including the slice lip extension.

In accordance with an aspect of the present invention there is a high consistency extrusion forming unit for laying a jet of fibrous material on a forming fabric to produce a high consistency web. The forming unit comprises a headbox having two spaced apart slice lips defining a slice opening and a jet of fibrous material being emitted from the slice opening at a predetermined jet speed. One of the slice lips has a slice lip extension surface extending in a downstream direction beyond the slice opening to support one surface of the jet. A forming fabric travels past the slice opening and adjacent the slice lip extension surface to support a second surface of the jet. The forming fabric travels at a speed which is a predetermined amount greater than the jet speed to extrude the fibrous material in the jet.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and its advantages will become more apparent to those skilled in the art by reference to the following drawings, in conjunction with the accompanying specification, in which: FIGURE 1 illustrates a former for a paper making machine in which the high consistency forming process of the present invention is utilized.

FIGURE 2 is a sectional view through the slice of a headbox showing the high consistency extrusion forming process of the present invention; FIGURE 3 is a sectional view similar to Figure 1 showing first alternative embodiment or aspect of the high consistency extrusion forming process of the present invention; FIGURE 4 is a sectional view similar to Figure 1 showing a second alternative embodiment or aspect of the high consistency extrusion forming process of the present invention; FIGURE 5 is a sectional view similar to Figure 1 showing a commercial embodiment for the high consistency forming unit of the present invention; and, FIGURE 6 to 8 illustrate graphs of relationships between jet speed, wire speed, headbox consistency, and paper consistency when the high consistency extrusion process of the present invention is employed.

DETAILED DESCRIPTION OF EMBODIMENTS FIGURE 1 illustrates a former 10 for a paper making machine in which the high consistency forming process of the present invention is utilized. Former 10 includes a high consistency headbox 12 installed on a twin-wire former with intense reversible drainage pulsing units 16 for further conditioning the web. The web is transferred from the forming section 10 onto a pick-up felt 18 at the beginning of the press section 20 of the paper making machine.

The headbox 12 has a slice opening 24 through which a pulp slurry 26 of high consistency fibrous material (3% or higher) in the form of paper pulp 26 is emitted as a jet extending along the width of the slice opening 24. The jet of pulp 26 is expelled onto a lower forming wire 28. It should be understood that in an alternative embodiment, the pulp slurry can be projecte onto an upper wire with the slice lip extension located on the lower lip and the former can be orientated in any direction. Forming wire 28 co- operates with upper forming wire 30 to support the web as it passes over dewatering or suction boxes 32 which remove water from the pulp slurry 26.

The second, downstream suction box also transfers the web onto the bottom wire. The wires 28 and 30 are continuously driven around supporting rolls 34 and each have a driving roll 36 for driving the wires 28,30. Each wire 28,30 moves around rolls 34 in the direction respectively indicated by arrows 38,40.

Referring to Figure 2, the high consistency forming aspect of the present invention is shown in the relationship between the lower forming wire 28 and the slice opening 24 of the headbox 12. The paper pulp 26 is emitted from the slice opening 24 at a consistency of 2 to 5%. The pulp 26 is delivered onto a moving fabric or wire 28 fully supported by the top slice surface 42 which is curved along the slice exit 44 to guide the jet of pulp 26 utilizing the"Coanda"effect onto the wire 28 at small angle of impingement. The wire 28 speed Vw is considerably higher than jet speed Vj and therefore at point of impingement 46, the contacting surface of the jet is rapidly accelerated, creating a shear gradient through the thickness of the jet.

The slope and intensity of this gradient depends on pulp viscosity (consistency) and on the speed differential between jet and wire at point of contact. It is believed that an increase in pressure may occur as the jet of pulp 26 enters the wedge 48 created between the wire 28 and extended upper slice lip surface 50. As the pulp jet 26 travels along the wedge 48, some drainage may be initiated. Following the initial acceleration of the pulp jet 27 after it impinges the wire 28, the entrapped pulp slurry is continuously stretched and increasingly sheared until it reaches the required grammage as web 53 at the second slice exit 52 from the extended upper slice lip surface 50. It should be understood that the pulp slurry 26 in the headbox 12 becomes a jet 27 at 44 and later forms into web 53 as it passes along the wedge 48. The upper slice lip extension surface 50 includes a series of ridges or ribs 51 that facilitate fluidization of the fibers in the web passing along the wedge 48.

The parameters which effect the extrusion and consistency of the pulp slurry 26 exiting the headbox 12 as a jet 27 and into the wedge 48 as web 53, as described in Figure 2, are the radius r, and angle 0, the radius and duration of the Coanda approach portion, L the length over which the jet is stretched in the wedge 48, and hh, the slice opening 24. The value of hh can be any value from, for example, 5 to 10 mm, since basis weight of the paper is not controlled in the former of the present invention only by the slice opening 24 and the paper consistency leaving the slice opening 24. The surface of the Coanda surface 54 in Figure 2 and the extrusion properties at the extended slice lip 50 control the consistency and basis weight of the web 53. Under some conditions, for example for certain pulps, it may be beneficial to provide surface irregularities 51 over at least a portion of the top lip 50 to provide more effective anchors for flocs to adhere to and be sheared apart.

The slice lip extension surface 50 further has a flat surface 55 immediately downstream of the wedge curved surface 48 that is spaced apart a constant distance from the forming wire 28. The flat surface 55 provides a zone of constant distance with the wire 28 immediately prior to the slice exit 52 overwhich the fibrous pulp material travels supported on two side by the slice lip extension and the moving fabric. This zone of constant spaced apart displacement is believed to provide a pressure normalization zone that permits any pressure abnormalities built up in the fibrous pulp material to find a pressure equilibrium or stability prior to travelling through the slice exit 52.

The high consistency forming or extrusion process of the present invention as shown in Figure 2 had three contributing factors that significantly reduce the energy of the forming process. First, the shear rate increases during the extrusion process from impingement at 46 and reaches a maximum at exit 52 where the volume of pulp in the web 53 is very small.

This location 48 thus effectively becomes the fluidizer in the entire system, so that it is not necessary to fluidize large volumes of pulp. Secondly, headbox flow speeds are low relative to the wire speed Vw. In particular the jet speed, Vj may be as much as 5 times lower than wire speed Vw which results in a low velocity and therefore low pressure loss across the headbox.

Lastly, the fluidizing energy is provided by a driven wire 28, rather than by a hydraulic pump. Given the large differences in volumes to be fluidized in the two systems and differences in the fluidizing processes, the process of the present invention requires far less power than the hydraulic fluidizing systems previously employed. An incidental, but an important benefit, is that the fluidizer exhibits the largest local pressure drop in the forming process and, therefore, becomes a control point in the stock preparation headbox loop.

Referring to Figure 3 there is shown an alternative embodiment for the high consistency former of the present invention shown in Figure 2. The former 10 of Figure 3 differs from Figure 2 in that the projecte or extended slice surface 50 is curved, rather than substantially flat as shown in Figure 2.

The curved surface 50 of Figure 3 facilitates creation of hydraulic pressure and therefore drainage within the pulp slurry during its extrusion. The pressure, p, at any given point below surface 50 and wire 28 is defined by the radius r, which the wire assumes at that point and by wire tension T, by the relationship p = T/r. Wire tension is thus an additional control factor in the forming process. The new curved surface 50 can have the same radius as the Coanda portion (rl) or a different radius (r2), as shown in Figure 3.

Figure 4 illustrates still a further alternative embodiment for the high consistency extrusion forming aspect of the present invention. In this aspect the portion of the web 53 that contacts the extended upper slice surface 50 is subjected to a controlled stream of liquid 56 such as process white water.

The liquid or water stream 56 contacts the web's upper surface 58 ahead of or during the web's extrusion zone 48. The liquid stream 56 can be continuous or in separate controllable narrow zones across the width of the machine to perform the dual function of aiding in the extrusion process and at the same time, in controlling the cross direction basis weight profile by dilution profiling. Figure 4 shows the liquid stream 56 to be introduced at wire speed Vw. It should be understood that the liquid fluid speed may be at speeds other than the wire speed.

Depending on the requirement, this liquid stream is utilized to perform different functions. The speed of the stream is set to be higher than the jet speed to assist in the extrusion of the jet. The speed differential depends on jet thickness, pulp type and consistency and does not have to be restricted to the speed differential value between jet 27 and wire 28. The liquid stream facilitates dispersal of formed flocs of fibers during the extrusion formation of the web and contributes to a more uniform formation of a paper web. The liquid stream may also provide a basis weight control by consistency profiling by adding the fluid in narrow bands across the former width and by controlling the consistency of each band.

Referring to Figure 5 there is shown a high consistency former incorporating the features of the formers shown in Figures 2 to 4. The consistency and basis weight of the paper formed by this former are controlled by the slice opening hh, wire/jet speed differential (Vw-Vj), headbox consistency Ch and drainage Qww, controlled by wire tension T.

With the introduction of a new control parameter (Vw-Vj), the properties of the web 53 at the exit slice 52 can be controlled in a new manner. For example, basis weight can be controlled by headbox consistency, slice opening, speed differential between jet and wire, or individually by any one of the three. This is because the jet is fully supported and is subjected to a much higher shear and stretching without being destroyed. In existing paper former, jet speed is only marginally faster or slower than wire speed, typically + 3%, beyond which, the operation becomes unstable and web quality deteriorates rapidly. In the present invention the wire speed Vw is as much as five times higher than jet speed (+ 500%). The speed differential is selected, in combination with headbox consistency and slice opening, to produce optimum paper quality. The new operating parameters, i. e. supported jet with high wire/jet speed differential, widens the operating window and reduces the compromises revalent in conventional forming where one property is often optimized at the expense of another.

The former of Figure 5 requires no dampening devises to dampen the machine direction pulsations emanating from the stock as in the case with present commercial high consistency experience. Cross machine direction basis weight profile can be controlled either conventionally by means of a flexible slice lip 60 installed at the extended lip exit 52, or by the dilution control method, whereby the slice opening is kept constant while stock consistency is varied in narrow bands 64 across the machine width. The slice lip extension surface 50 is smooth. In conventional headboxes, dilution normally occurs at the back of the headbox. In the former of the present invention, dilution also occurs close to the slice wedge 48. Dilution close to the slice wedge 48 is preferred since this is the where shear rate is highest.

This high shear rate promotes excellent mixing between high consistency stock and low consistency white water dilutant resulting in little, if any, lateral spreading of the diluted stock at slice exit 52. This substantially improves the control resolution.

The following relationship illustrates the scope in control strategies possible in controlling basis weight and consistency of paper formed in the high consistency extrusion former of the present invention. In the relationship the following terms represent: Wp = Basis weight paper-glm~ Vw = Wire speed-m/min Vj = Jetspeed-m/min Mh = Fiber flow at headbox-g/min/cm Qh = Stock flow at headbox-cc/min/cm Ch = Headboxconsistency-%<BR> <BR> <BR> <BR> <BR> <BR> hh = Slice opening-mm R = First pass retention-% M, = White water fiber flow-cc/min/cm Qww = White water drainage - Z% of Qh C ww= White water consistency-% Mp = Fiber flow in web @ exit-g/min/cm <BR> <BR> <BR> <BR> Qp = Total flow paper-cc/min/cm<BR> <BR> <BR> <BR> <BR> <BR> Cp = Paperconsistency-% hp = Web thickness @ exit - mm The mass and flow balance yields that the speed of the pulp or jet leaving the head box slice Vj is a function of Mh, Qh, Ch, and hh. The speed of the wire Vw is a function of Mp, Qp, Cp, and hp. The drain through the wire Z% is a function of Mww, Qww, and Cww. Another variable Another variable defined variable (Ch-CN,) lCh. From the mass flow balance the following simplified equation defines the general relationship between basis weight, wire speed, jet speed, headbox consistency, drainage and retention: Vi vlywp Equation (1) 10hhCh [1- (Z-. 01R)] Figures 6 to 9 show graphs of wire speed Vw vs jet speed Vj for different conditions in accordance with Equation (1). Each of the graphs is explained briefly as follows: Figure 6 is plot of wire speed vs jet speed for a paper having a basis weight of 80 glu2, the headbox having a slice opening 24 of 4 mm. Each line or curve 100 plotted represents a different headbox consistency of the paper pulp exiting the headbox from 2 to 5%. The wire tension remains constant. Drainage and retentions are also assumed constant for all pulp consistencies. Altering slice opening or wire tension (drainage pressure) changes these relationships and in the process, changes paper quality. By optimizing the settings it is therefore possible to optimize paper quality.

Figure 6 shows that it is theoretically possible to operate with very high wire/jet speed differentials-i. e. the maximum in this case is 2.5: 1 for headbox consistency of 5% for a web of 80 glu2. This means low headbox speeds, low pressure drops and low power requirements.

In Figure 7, the curves 110 are similar to Figure 6 but are for a slice opening 24 of 6 mm. The speed differential for a headbox consistency of 5% is raised to 5: 1. Speed differentials as high as 6: 1 are achieved for some of the curves 110.

In Figure 8, a curves 120 are plotted for headbox consistency vs jet speed for a paper having a basis weight of 60 glm2 with a wire speed of 1500 m/min and with drainage and retention which varies with headbox consistency i. e. the higher the consistency, the lower the retention and the higher the drainage. An increase in headbox consistency reduces jet speed which requires a greater speed differential setting to obtain paper of desired grammage.