GB2119824A - Headbox - Google Patents

Abstract

A headbox for delivering a multilayer stock jet to a forming surface (9) in a twin-wire former comprises slice chamber (15) with slice opening (17), and separator vanes (33', 33'') arranged in the slice chamber extending from headbox side wall to headbox side wall and at least up to slice opening (17). The vanes are provided with thickened upstream ends which are anchored by screws (63) in grooves in wall member (65). Each vane has a bending rigidity that is sufficient to permit the stock flow on both sides of the vane to take place at such different pressures and such different flow velocities in relation to one another that the headbox is capable of delivering a smooth and stable multilayer stock jet with a velocity difference of at least about 15 m/min between at least two adjacent layers in the stock jet. <IMAGE>

Description

SPECIFICATION Headbox The present invention relates to headboxes, for delivering a multilayer stock jet to a forming surface in a twin-wire former, of the type comprising a slice chamber and a slice opening, means for conducting at least two stocks in parallel to but separated from each other through the headbox to the slice opening, said means including at least one separator vane arranged in the slice chamber and extending from headbox side wall to headbox side wall in the slice chamber and, in addition, at least up to the slice opening, and means for anchoring the vane at its upstream end, said anchoring means being designed to produce a rigid clamping of the vane.

A headbox of this type is already known from, for example G.B. 2019 465A and has proved to be able to contribute effectively to an improvement of the layer purity in the manufacture of multi-ply paper, in that the separate layers of the multilayer stock jet discharged from the headbox could be kept separated from each other for a further distance in the direction of the forming surface of the paper machine by means of a wedge of air or other gas arranged at the downstream end of the vane. The vane is pivotably fastened at its upstream end in that the plate-shaped portion of the vane is secured by screws or attached in some other suitable way to a bar, which is pivotally restrained in a groove extending from headbox side wall to headbox side wall.Air or other gas can be supplied to the wedge from a suitable source through channels in the bar and down to the downstream end of the vane. The pivotable mounting is such that the vane is freely pivotable within a limited range. Alternatively, the vane can be rigidly connected to the headbox or possibly adjustably mounted in the same. By this means each stock slice opening can be adjusted separately by setting the top lip of the headbox and/or bottom lip and/or the position of the vane or vanes by means of a well-known, simple manually-operated control means located outside the headbox. This makes it possible to operate with slightly different velocities in the different layers in order to adjust the sheet properties in the various paper plies.

Previously, it was considered by those skilled in the art, see e.g. G.B. 1;492,323, G.B. 1,595,558 and G.B. 1,597,809, that a multilayer stock jet must be discharged with the same velocity in all layers in order to be able to produce satisfactory paper. G.B. 1,492,323 and G.B. 1,595,558 disclose that the stock separating members, which should be flexible foils, must be self positionable in response to the pressure differences across the same. A multilayer headbox according to G.B. 1,492,323 must be designed for a selected ratio between the volume flows of the separate stocks, and during operation every appreciable deviation from this selected ratio will bring about a reduction of the paper quality.

In the headbox is disclosed in G.B. 1,597,809, the ratio between the volume flows can be varied in that the upstream end of the vane or vanes is displaceable in a direction across the direction of flow. If the ratio is adjusted, there will be an automatic change in the ratio between the cross sectional areas of the separate flow channels in the slice chamber. In this way no pressure differences will arise across the vanes, which can have a rigid upstream portion, and a downstream portion that is rigid but pivotally secured to the upstream portion or is flexible. Alternatively, the whole of the vane is freely pivotable. It is stated that in this way the creation is avoided of pressure differences that would tend to force the vanes away from the positions which give equal speeds to the stock streams discharged from the headbox.These speed differences are stated to make the streams of stock mix with each other and thus break the desired structure of the web.

Further, a multilayer headbox for a former of the Fourdrinier type or breast roll type is disclosed in G.B. 1,229,741. This headbox delivers the stock to the forming wire successively layer by layer, and each layer is partially dewatered before the next layer is applied. With this combination of headbox and former, it is stated to be possible to deliver the different stocks to the forming wire at mutually different speeds in order to affect the quality of the paper, i.e. its strength properties. As an example, it is stated that the first stock can be applied to the wire at a speed less than the speed of the wire, so that the fibres are aligned in the machine direction, which will increase the strength of the paper in the machine direction, but reduce the strength in the cross machine direction.The next layer could be applied at a speed faster than the speed of the forming wire, which it is stated would give the delivered stock a slight rolling action, thereby mixing the fibres in all directions. The third stock can be applied in the same way as the first, so that its fibres would be aligned in the machine direction. This technique cannot be applied to a multilayer headbox according to G.B. 1,597,809, as the speed differences would cause a detrimental turbulence in the boundary layer between the separate stock jets, which would produce considerable to complete intermixing of the separate stock jets, so that the finished paper web would have substantially the same fibre mixture throughout and thus would not be a layered product. Neither can this technique be made use of for multilayer headboxes according to G.B. 1,492,323 and G.B.

1,595,558, as the flexible foils used as stock separating members will automatically position themselves in such a way that the pressure differences necessary to produce speed differences are avoided.

The object of the present invention is to provide a headbox which is capable of delivering to a forming device such as a twin-wire former a multilayer stock jet with properties such that the paper web produced in the twin-wire former will have improved layer purity and formation.

In accordance with the present invention there is provided a headbox for delivering a multilayer stock jet to a forming surface in a twin-wire former, said headbox comprising a slice chamber with a slice opening means for conducting at least two stocks in parallel to but separated from each other through the headbox to the slice opening, said means including at least one separator vane arranged in the slice chamber and extending from headbox side wall to headbox side wall in the slice chamber and, in addition, at least up to the sliced opening, and means for anchoring the vane, at its upstream end, said anchoring means including a thickened upstream end of the vane, said thickened end being bar-shaped, having a noncircular cross sectional shape and having a plurality of seating faces, a substantially complementarily shaped groove in which the thickened upstream end of the vane is inserted, said groove having abutment faces for said seating faces, and means for pressing the seating faces against the abutment faces so that the vane becomes rigidly clamped in the groove, and wherein the vane is comparatively rigid, at least from its clamping point up to the slice opening, and has a bending rigidity that is sufficient to permit the stock on both sides of the vane to flow at such different pressures in relation to one another and such different flow velocities in relation to one another that the headbox is capable of delivering a smooth and stable multilayer stock jet with a velocity difference of at least about 1 5 metres per minute between at least two adjacent layers in the stock jet.

Assuming an operational case with this headbox in which there is no pressure difference across the comparatively rigid vane anywhere along its length in the direction of flow then there will not be any difference in velocity either and the stock layers will be discharged at equal speeds in relation to one another. If the pump feeding the stock into a first channel on one side of the comparatively rigid vane is now operated to deliver, for example, a greater flow, then the pressure in the first channel will increase and there will arise across the comparatively rigid vane a pressure difference that tends to bend the rigidly clamped and comparatively rigid vane away from the first channel and into the second channel on the other side of the vane.This deflection will throttle the flow in the second channel, thus producing a counter-pressure in this which is greatest at a downstream portion of the vane and tends to bend the vane back, so that the bending line of the vane will curve back somewhat. The increase in flow in the first channel increases the velocity of the jets from both channels, but the velocity of the jet from the first channel will increase more than that from the second channel. Obviously, the reverse will apply at a reduction of the flow in the first channel. A change in the velocities of the separate jets from the channels can also be produced by altering the geometry of the channel, for example, in the manner indicated in G.B. 1,597,809.

The vane must be comparatively rigid and have the bending rigidity stated above. Surprisingly, it has been proved that when stock jets are being delivered from a multilayer headbox, if the velocity of the stock jet closest to a plain forming roll in a twin-wire former of roll type is maintained one or a few percent higher, though at least about 1 5 metres/min., than the speed of an adjacent stock jet, multi-ply paper of superior layer purity and formation can be produced.

It is suitable that the bending rigidity of a vane with a ratio of wall length in the direction of flow to wall thickness of about 75, is at least about 3 kN2m per metre width across the direction of flow The partition can then consist of, for example, glass-fibre-reinforced epoxy plastic with a modulus of elasticity of about 30x109 N/m2.

It is suitable that the anchoring means include a perforated wall member, transverse in relation to the direction of stock flow and in which said groove is located, that the groove has a depth direction that is parallel to the direction of stock flow through the perforations in the wall member, and a bottom, transverse in relation thereto, that the thickened upstream end of the vane is formed by a bar attached to a plate-shaped portion of the vane, and that the pressing means keep the bar pressed against the bottom of the groove, whereby a further simplification of erection and possible replacement of the vane is attained.

In order to facilitate manufacture and reduce costs, it is suitable that the groove has a substantially rectangular cross section.

A further simplification of erection and possible replacement of the vane will be obtained if the pressing means comprise a plurality of headed screws, each extending through its own clearance hole arranged in the perforated wall member into its own tapped hole arranged in the bar, so that the head of each screw bears against a shoulder around the clearance hole.

Conveniently, the means for conducting the stocks through the headbox include a bank of tubes parallel to each other and internally in contact with stock, the downstream ends of the tubes extending through and being fixed in the perforated wall member, the screws being parallel to the tubes, and flow channels for a gaseous medium extending through the perforated wall member and continuing through the bar and the plate-shaped portion of the vane to the downstream end of the vane in order to permit the formation of a gaseous wedge, which keeps apart two adjacent layers of stock in the multilayer stock jet for a further distance downstream of the downstream end of the vane.

In order to take advantage of the possibilities that this design offers, it is suitable that the upstream ends of the tubes extend through and are fixed in a second perforated wall member, that the head of each screw is substantially extended in the lengthwise direction of the screw and extends into a through hole arranged in the second perforated wall member, so that the screw can be turned by means of a suitable tool, and that additional wall members are arranged so as to form together with the two perforated wall members a closed box through which the bank of tubes extends, and that means are provided for connection of the interior of the box to a source for the gaseous medium.

The invention will now be described further, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a cross sectional view of the most important portion of a headbox constructed in accordance with a preferred embodiment of the invention; Figure 2 is a detail enlargement from Figure 1; and Figure 3 is a cross sectional view showing one of several alternative embodiments of the anchoring means; The headbox shown schemetically in Figure 1 is arranged to deliver a multilayer stock jet to a forming surface in a twin-wire former.The schematically indicated twin-wire former is of roll type and comprises an inner wire 1 and an outer wire 3 in the form of endless loops, a rotatable forming roll 5, which is located inside the inner wire loop and in the embodiment shown has a plain shell surface, and a rotatable breast roll 7 for the outer wire loop. From the breast roll 7 the outer wire 3 runs to the inner wire 1 supported by the forming roll 5 and follows this for a distance along the periphery of the forming roll until they run off together from the forming roll 5. The outer wire portion, which is located between the points of run-off of the outer wire 3 on the breast roll 7 and the forming roll 5, comprises said forming surface, which is denoted by 9.Naturally, it is within the scope of the invention that the shell surface of the forming roll 5 instead of being smooth can be e.g. blind-drilled, or grooved in the circumferential direction and/or axial direction and this roll can even be a suction roll. Further, the twin-wire former does not need to be of the roll type, i.e. that it has a forming roll, and of the wires 1 and 3, if so desired, at least the inner can be a felt or of other water-transmitting material.

The principal components of the headbox shown include a bottom member 11, a top member 13 and two side walls not shown. These main components enclose between them a slice chamber 1 5 with a slice opening 17, from which a multilayer stock jet will be delivered to the forming surface 9. The slice chamber 1 5 converges in a direction towards the slice opening 17, and the gap width of this can be adjusted by turning the top member 13 about an axis 19 relative to the bottom member 11 by means of a jack, not shown, which actuates a linkage system 21, which is partially shown. The principal components also include a mixing chamber member 23 connected to the bottom member 11 and through which the stocks are conducted into the headbox.

Further, the headbox includes means for conducting at least two stocks in parallel to but separated from each other through the headbox to the slice opening 17, and these means comprise at least one separator vane arranged in the slice chamber 1 5 and extending from headbox side wall to headbox side wall in the slice chamber 1 5 and in addition at least up to the slice opening 1 7. In the preferred embodiment shown, the mixing chamber portion 23 includes three rows of pipe stubs 25', 25" and 25"', in the cross machine direction, to which the stocks are supplied in hoses, not shown, from conically tapering cross distributors, also not shown. Each row of pipe stubs can be supplied from its own distributor or also two of the rows, as a rule the two outer, can be supplied from a common cross distributor.Through the pipe stubs 25', 25", 25"', the stocks enter a mixing chamber space, which is divided into three mixing chambers 29', 29" and 29"', one for each row of stubs, by means of two partitions 27' and 27" in the cross machine direction. A plurality of rows of tubular channels 31 parallel to each other extend through the bottom member 11 and connect each of the mixing chambers 29', 29" and 29"' with the slice chamber 1 5.In this chamber two separator vanes 33' and 33" are arranged in the cross machine direction and have their upstream end attached to the bottom member 11, extend from headbox side wall to headbox side wall in the slice chamber 1 5 and out through the slice opening 17, so that the slice chamber is divided into three slice channels 1 5', 1 5" and 15"' converging towards the slice opening 1 7 and which correspond to the mixing chambers 29', 29" and 29"', respectively. The vanes 33' and 33" are attached at their upstream end to the bottom member 11 by anchoring means, which are denoted in general by 35' and 35". These anchoring means 35' and 35" are designed to produce a rigid clamping of the vanes 33' and 33".

Each vane is comparatively rigid, at least from its clamping point up to the slice opening 17, and has a bending rigidity that is sufficient to permit the stock flow on both sides of the vane 33' and 33" to be able to take place at such different pressures in relation to one another and such different velocities in relation to one another that the headbox is capable of delivering a smooth and stable multilayer stock jet with a velocity difference of at least about 1 5 metres/minbetween at least two adjacent layers in the stock jet. Preferably, the bending rigidity of a vane with a ratio of wall length in the direction of flow to wall thickness of about 75, is at least about 3 kNm2 per metre width across the direction of flow.In the preferred embodiment shown the vanes 33' and 33" have a thickness of 12 mm and a length of about 0.9 m and 0.75 m, respectively and consist of fibre-class-reinforced epoxy plastic with a modulus of elasticity of 30x109 N/m2.

The anchoring means 35' and 35" comprise as is best shown in Figure 2, a thickened upstream end of the vane 33' and 33", the thickened end 37' being bar-shaped, having a non-circular cross sectional shape and having a plurality of seating faces 39' to 45', means 47' to 51' that define a substantially complementarily shaped groove 53', in which the thickened upstream end of the vane 33t is inserted, said groove 53' having abutment faces 55' to 61' for said sealing faces 39'45', and means 63' for pressing the seating faces 39'-45' against the abutment faces 55'-61', so that the vane 33' becomes rigidly clamped in the groove 53'. Naturally, the anchoring means 35" for the vane 33" are designed in a corresponding way, although they are not shown complete.Similar to what is shown in Figure 2, the anchoring means 35' suitably comprise a perforated wall member 65, transverse in relation to the direction of stock flow and in which said groove 53' is located, the groove 53' having a depth direction that is parallel to the direction of stock flow through the perforations in the wall member 65, and a bottom 51' transverse in relation to this, the thickened upstream end of the vane 33 being formed by a bar 37' attached to a plate-shaped portion 67' of the vane 33', and said pressing means 631 keeping the bar 37' pressed against the bottom 511 of the groove 53'.The groove 53' has a substantially rectangular cross section, and the pressing means 63' comprises a plurality of headed screws each extending into its own clearance hole 69' arranged in the perforated wall member 65 and into its own tapped hole 71 arranged in the bar 37', so that the head 731 of each screw 63 bears against a shoulder 75' around the clearance hole 69'. Further, the tubular channels mentioned in connection with the description of the bottom member 11 are a bank of tubes 31 31" and 31 stl parallel to each other and internally in contact with stock, the downstream ends of which extend through and are fixed in the perforated wall member 65.The screws 63' are parallel to the tubes 31, and flow channels 77" for a gaseous medium extend through the perforated wall member 65 and continue through the bar 37" and the plateshaped portion 67" of the vane 33" to the downstream end of the vane 33" in order to permit the formation of a gaseous wedge, not shown, that keeps apart two adjacent stock layers in the multilayer stock jet for a further distance downstream of the downstream end of the vane 33". The upstream ends of the tubes 311 31" and 31"' extend through and are fixed in a second perforated wall member 79.The head 73' of each screw 63' is substantially extended in the lengthwise direction of the screw 63' and extends into a through hole 81 arranged in the second perforated wall member 79, so that the screw 63' can be turned by means of a suitable tool, e.g. a socket screw hexagon wrench, not shown. In addition, further wall members 83 to 91 (see Figure 1) are arranged together with the two perforated wall members 65 and 79 to form a closed box, through which the bank of tubes extends, and means 93' and 93" are arranged for connection of the interior of the box to a source of the gaseous medium.

Reverting to Figure 2, the vanes 33' and 33" in the preferred embodiment shown consist of a glued construction in which two 4-mm thick sheets of glass-fibre-reinforced epoxy plastic are joined together across a plurality of parallel, 4mm thick and 20-mm wide flat strips of the same material arranged with 20-mm gaps, so that each air channel 77 in the vanes has a cross sectional area of 4x20 mm. On condition that the glassfibre-reinforced epoxy plastic has a modulus of elasticity of 30x 109 N/m2, the vane will have a bending rigidity of more than 4 kNm2 per metre width across the machine direction.

It can be shown by calculations that with a multilayer headbox of substantially the type shown in Figures 1 and 2, but constructed for two layers and thus fitted with only one vane containing air channels and of the design described in the previous paragraph, having an initial gap width of 2x6 mm at the slice opening and having an initially unloaded vane and identical converging angles for the two slice channels, a flow of 0.16 m3/second-per metre width across the machine direction-in one of the slice channels and 0.20 m3/second in the other, will provide a jet velocity of about 1800 metres/min. with a difference in velocity of 26 metres/min. between the two jets.The 0.75 metre long vane with its upstream end rigidly clamped is bent very slightly first in one and then in the opposite direction, and the main deflection is 1.1 mm and occurs after about two-thirds of the wall length measured from the clamping point. The free downstream end of the vane is pressed in about 0.6 mm into the slice channel with the lesser flow rate, so that the two gap widths are changed from 6 mm to about 6.6 mm and about 5.4 mm.

Desirably, the downstream ends of the vanes 33' and 33" projecting from the slice opening 17 are provided with comparatively thin, flexible foils 95' and 95" (Figure 1) of suitable material, e.g.

plastic, similar to that which is disclosed in G.B.

201 9465A. The foils 95' and 95" can be the same width across the machine direction as the vanes 33' and 33" and suitably extend far enough in the machine direction to keep the separate stock jets separated for a further short distance after these have met at the downstream end of the air wedge. These foils will eliminate any velocity components perpendicular to the plane of the jets and thereby contribute to an improvement of layer purity and formation.

In the illustrated, preferred embodiment the ceiling surface of the slice chamber 1 5 and the vanes 33' and 33" deflect the jet flows slightly less than 90 when the stocks flow out of the tube bank. Due to this large angle, the distance between the clamping points of the vanes 33' and 33" can be kept large, so that there will be space for a plurality of rows of tubes 31", despite the fact that the maximum height of the slice channel 1 5 is kept at a low value. The equivalent applies to the rows of tubes 31 ' and 31 "' and the slice channels 1 5' and 1 5"'. By this means improved flow conditions are obtained and contribute to improved layer purity and formation.Two rows of tubes 31"' are shown in Figure 2, whilst the positions for a further two rows are indicated by chain-dotted centre lines for the tubes, so that a total of four rows of tubes 31 " deliver stock to the slice chamber 1 5. The tubes 31 shown are telescoping tubes of the kind described in US 4,225,386 (Edblom et al.).

The slice channels 1 5', 1 5" and 15"' have a diverging portion at their upstream end where the rows of tubes 31 discharge, but they change over as soon as possible thereafter to converge towards the slice opening 1 7. Preferably, the rigid clamping of the vanes 33' and 33" in the bottom member 11 is designed in such a way that when the vanes are not loaded and not subjected to any torque, the gap widths at the outlets of the slice channels 1 5', 1 5", and 15"' are larger than the largest gap widths that are intended to be used during operation.Compared with a headbox with pivotably attached and/or flexible vanes, but with unchanged conditions in other respects, a headbox like this when in operation, when the slice outlets have been reduced by setting the top member 13 relative to the bottom member 11, will deliver a multilayer stock jet in which the velocity of the jet from the slice channel 1 5' nearest the top member 1 3 is greater than the velocity of the jet from the central slice channel 15", which in its turn is greater than the velocity of the jet from the slice channel 1 5"' nearest the bottom member 11. For example, a headbox for producing three-ply tissue can be constructed for a gap width of 3x8 mm with unloaded vanes, but when operating will run with a gap width of approximately 3x4 mm.A further increase of the velocity differences can be produced, for example, by increasing the flow of stock through the slice channel 1 5' and/or reducing the flow of stock through the slice channel 15", but then the stock concentrations must also be adjusted if the paper layer basic weights are to be maintained unchanged.

The bars 37' and 37" have slanting, countersunk upper sides wihich receive the upstream ends of the plate-shaped portions 67' of the vanes 33' and 33". Each of these slanting, countersunk upper sides has a downstream edge that is slightly rounded to reduce the stresses in the vanes 33' and 33" when these are subjected to a force tending to bend them over said edges.

A row of screws, not shown, have their heads countersunk into a flat bar 97' and 97" bearing against the side of the upstream end of the plateshaped portion 67' and 67" of each vane 33' and 33". The flat bars 97' and 97" together with the slanting, countersunk upper sides of the bars 97' and 974" form a pocket for the upstream end of the plate-shaped portion of each vane. The screws, which can be arranged on a pitch distance of e.g. 40 mm, extend through the plate-shaped portion 67' and 67" and into tapped holes, not shown, in the bars 37' and 37", so that the plateshaped portions 57' and 67" are rigidly attached to the respective bar 37' and 37".

The seating faces 39' to 45' and 39" to 45" on respective bars consist of lands which are raised above the surrounding area to permit machining to exact flatness and dimension. Two of the seating faces 39' and 45', 39" and 45" locate against the sides 47' and 49', 47" and 49", respectively, of the groove, which is substantially rectangular in cross section, at the top edge of the groove 531 and 53", respectively, whilst the remaining two seating faces (43' and 45', 43" and 45") bear against the groove bottom 511 and 51", respectively, near the groove sides 47' and 49', 47n and 49", respectively.A sealing ring 99' and 99" located in a groove runs substantially from one headbox side wall to the other parallel to and between the two seating faces bearing on the groove bottom 511 and 51". These sealing rings prevent the stock from entering into the air system. Tightening of the bars 37' and 37" against the groove bottoms 511 and 51 it, respectively, by means of the screws 63 (corresponding screws for bar 37" in groove 53" are not shown), which can be arranged with a pitch distance of e.g. 0.6 metre, results in a rigid clamping of the upstream ends of the vanes 33 and 33" in the bottom member 11.Compared with a possible embodiment, not shown and not preferred, in which the groove 53 has an inverted tee-shaped or parallel-trapezoidal cross section and the bar 37 is not tightened against the groove bottom 51, but instead is rigidly clamped by being pressed in the opposite direction by means of e.g.

the screws 63, the embodiment according to Figures 1 and 2 offers the advantage that the pressure of the stocks in the slice chamber 1 5 will press the bar 37 into still harder contact with the groove bottom 51, so that the clamping force increases.

The mixing chamber member 23 has a circumferential flange 101 and is attached to the bottom member 11 by means of screws extending through the flange and not shown. The partitions 271 and 27" are sealed at their end faces against the bottom member 11 and against the mixing chamber member 23 by means of sealing rings 103' and 105', 103" and 105", respectively, located in grooves of similar design to the sealing rings 99' and 99". In order to overcome the sealing problems at the high pressure that can exist in the mixing chambers 29', 29" and 29"', the mixing chamber member 23 is also attached to the bottom member 11 by means of screws, not shown, which extend through clearance holes, not shown, in the partitions 27' and 27" into tapped holes, not shown, in the bottom member 11. The partitions 27' and 27" are located in line with the through holes 81 for the heads 73 of the screws 63. As pointed out above, the screws for bar 37" are not shown, nor are the holes for the heads of these screws. Through holes 107, which are coaxial with the holes 81, extend through the mixing chamber 23 and the partitions 27' and 27" to make connection with the holes 81,so that a tool e.g. a socket screw hexagon key can be inserted through the holes to fit into, for example, a hexagon socket in the screw heads for tightening and undoing the screws.The mouth of the hole 107 can be closed in a suitable way, e.g. by means of a screw 109 which, in the case when the interior of the box enclosing the tube bank is supplied with pressurized air, for example, from iow pressure fans, not shown, connected to the air connection means 93' and 93", prevents air leaking out through the holes 81 and 107.

Another possibility is to arrange a seal, not shown, between the screw head 73 and the enclosing wail of the hole 81. In cases where the air wedges do not require a supply of overpressure air, but can draw in requisite air from the environment through the interior of the box and the air channels 77, no form of plug or seal will be required in the holes 81 and 107. To permit separate control of the air supply to the air wedges at the downstream ends of the two vanes 33' and 33", the interior of the box is divided into two separate compartments by means of a plate 111 extending from one of the perforated wall members 65 to the other 79 and from one side wall of the box to the other. The plate 111 can suitably be slightly bent to allow it to be deformed by thermal stresses without damage.

Figure 3 shows an alternative embodiment for the rigid clamping of a vane. However, as this embodiment has much in common with that shown in Figures 1 and 2 and described above, the corresponding items in Figure 3 have been given reference numerals in the 300 series. Thus the vane which is denoted by 33 in Figures 1 and 2 is designated 333 in Figure 3.

In the embodiment shown in Figure 3 the vane 333 has a one-sided step-shaped enlarged upstream end with two seating faces 341 and 343 each located on its own step and one seating face 339 located on and level with the opposite side of the vane at the upstream end. The vane 333 is shown inserted in a substantially complementarily shaped groove 353, in which the groove sides 347 and 349 extend substantially parallel to the plane of the vane 333.

The bottom 351 of the groove 353 is located at a depth such that the vane 333 when pressed in a direction towards the bottom 351, can be displaced in the lengthwise direction of the groove 353 without the seating faces 339, 341 and 343 coming into contact with corresponding abutment faces 355,357 and 359 in the groove 353. When the vane 333 has been pushed into a correct position in an axial direction in the groove 353, a hose 363 or other suitable pressing means fitted between the upstream end face of vane 333 and the groove bottom 351 can be expanded with the aid of a pressure medium, so that the vane 333 is displaced in a direction towards its downstream end until it is stopped in a position where it is rigidly clamped in the wall member 365.

The hose 363 also prevents stock from being pressed into the air system round the upstream end of the vane 333. A second expandable sealing hose 399 prevents stock from creeping along the opposite side of the vane 333 into the air channel 377.

The invention is not restricted to the embodiments shown on the drawings and described above, but can be freely varied within the scope of the appended claims. For example, the slice chamber can extend in a direction which substantially coincides with the direction of flow through the bottom member instead of, as described, deflecting nearly 90 in relation to this direction. Further, the tube bank and the two wall members in which this is fastened can be replaced by a block with drilled or cast and, if so desired, stepwise expanding channels. Then the air supply, if such is necessary, can take place axially through the bars 37 or some equivalent member.Incidentally, the attainment of velocity differences between the separate jets in a multilayer stock jet never requires that the air wedge technique be utilized, even if this facilitates the production of paper with the desired properties. For velocity differences to be attained, it is required that the vanes have sufficient bending rigidity and extend at least up to, preferably out through, the slice opening.

Downstream of the slice opening the vanes do not need to have any bending rigidity, as the pressure in the stock jets there is zero. If one refrains from utilizing the air wedge technique, the vane can consequently taper to a thin and foil-like or pointed downstream end located downstream of the slice opening without therefore deviating from the invention as it is claimed. Further, it is naturally possible to feed the same stock through all the stock channels in the headbox in order to obtain better control of the formation and the fibre distribution in the single-layer paper web that is being produced.

Claims (8)

Claims

1. A headbox for delivering a multilayer stock jet to a forming surface in a twin-wire former, said headbox comprising a slice chamber with a slice opening, means for conducting at least two stocks in parallel to but separated from each other through the headbox to the slice opening, said means including at least one separator vane arranged in the slice chamber and extending from headbox side wall to headbox side wall in the slice chamber and, in addition, at least up to the slice opening, and means for anchoring the vane at its upstream end, said anchoring means including a thickened upstream end of the vane, said thickened end being bar-shaped, having a noncircular cross sectional shape and having a plurality of seating faces, a substantially complementarily shaped groove in which the thickened upstream end of the vane is inserted, said groove having abutment faces for said seating faces, and means for pressing the seating faces against the abutment faces so that the vane becomes rigidly clamped in the groove, and wherein the vane is comparatively rigid, at least from its clamping point up to the slice opening, and has a bending rigidity that is sufficient to permit the stock on both sides of the vane to flow at such different pressures in relation to one another and such different flow velocities in relation to one another that the headbox is capable of delivering a smooth and stable multilayer stock jet with a velocity difference of at least about 1 5 metres per minute between at least two adjacent layers in the stock jet.

2. A headbox as claimed in claim 1, wherein the bending rigidity of said vane, with a ratio of wall length in the direction of flow to wall thickness of about 75, is at least about 3 kilonewtons square metre per metre width across the direction of flow.

3. A headbox as claimed in claim 1 or claim 2, wherein the anchoring means include a perforated wall member, transversely disposed in relation to the direction of stock flow and in which said groove is located, said groove having a depth direction that is parallel to the direction of stock flow through the perforations in the wall member, and a bottom transverse in relation thereto, the thickened upstream end of the vane comprising a bar attached to a plate-shaped portion of the vane, and said pressing means keeping the bar pressed against the bottom of the groove.

4. A headbox as claimed in claim 3, wherein the groove has a substantially rectangular cross section.

5. A headbox as claimed in claim 3 or claim 4, wherein the pressing means comprise a plurality of headed screws, each extending through its own clearance hole provided in the perforated wall member into its own tapped hole provided in the bar, so that the head of each screw bears against a shoulder around the clearance hole.

6. A headbox as claimed in claim 5, wherein the means for conducting the stocks through the headbox include a bank of tubes parallel to each other and internally in contact with stock, the downstream ends of these tubes extending through and being fixed in the perforated wall member, said screws being parallel to the tubes, and flow channels for a gaseous medium extending through the perforated wall member and continuing through the bar and the plateshaped portion of the vane to the downstream end of the vane to permit the formation of a gaseous wedge which keeps apart two adjacent layers of stock in the multilayer stock jet for a further distance downstream of the downstream end of the vane.

7. A headbox as claimed in claim 6, wherein the upstream ends of the tubes extend through and are fixed in a second perforated wall member, the head of each screw being substantially extended in the lengthwise direction of the screw and extending into a through hole arranged in the second perforated wall member, so that the screw can be turned by a suitable tool, and additional wall members being arranged so as to form together with said two perforated wall members, a closed box, through which the tube bank extends, and means being provided for connection of the interior of the box to a source of the gaseous medium.

8. A headbox for delivering a multilayer stock jet to a forming surface substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2, or in Figs. 1 and 2 as modified by Fig. 3 of the accompanying drawings