Patent Application
20050092454
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The present invention relates to a deflocculating means for use in a paper making machine. In particular it relates to a means which can be used in conjunction with a forming board or foil blade to reduce flocculation and produce a controlled agitation of the pulp stock.
In a paper making machine, pulp stock, which comprises a thin aqueous suspension of fibers and fillers is ejected from a headbox on to the surface of a moving endless screen belt called a forming fabric, which is made of woven metal wires or plastic filaments. The forming fabric passes over various fabric support devices to withdraw the water to leave a thin formation of matted fibers that is lifted off the fabric and subsequently treated, ultimately to produce a finished sheet of paper.
The quality of the finished paper depends to a large extent on the uniformity of the fibre suspension in the pulp stock being maintained during the dewatering process which occurs on the forming fabric. Thus, means are conventionally employed to create micro-turbulence in the pulp stock on the forming fabric to prevent the fibers in the pulp from agglomerating and forming flocs.
Typical deflocculating means comprise forming boards or foil blades which are located beneath the forming fabric to interrupt the drainage of water and cause the development of pressure pulses in the pulp stock. Such a forming board is described in British patent GB 2190932, which describes a board with an upper surface comprising one or more transverse slots. The depth of the or each slot can be varied by the location in the slot of a slat. A plurality of slats of varying thickness is provided so that the depth of the slot in the board can be adjusted according to the thickness of the slat used. In this way, the forming board can be readily adapted to produce the requisite degree of turbulence required.
The object of the present invention is to provide a deflocculating means for use in a paper making machine which provides an improvement in the tear strength of the resulting paper over the aforementioned conventional means described above.
According to the present invention there is provided a deflocculating means for use in a paper making machine comprising a support member for location beneath a forming fabric belt, the support member having an upper surface defining at least one groove that runs in a cross-machine direction, the groove being provided with retention means for holding an insert in a sliding engagement in the groove in order to induce micro-turbulence in pulp stock being carried by the forming fabric belt, and characterized in that the groove has a longitudinal axis which is angled at between 0.38° and 0.77° to a transverse axis in the cross-machine direction.
It has been found that a slight angling of the groove and thereby of the insert to the transverse axis across the direction of travel of the forming fabric induces sideways moving pressure pulses in the pulp stock which results in a more controlled agitation of the pulp stock than inserts which are simply mounted parallel to the transverse axis in a conventional fashion. In the latter case, the pulses produced are more chaotic because a multiplicity of waves are formed which tend to interfere with one another, thus nullifying their effect in regions of the pulp stock. In the present invention, however, the pressure pulse commences in the pulp stock from a single source at one side of the forming fabric as it passes over the leading edge of the insert and then travels across the fabric along the line of the insert as a single wave. The agitation thus produced is therefore more uniform in nature and affects the whole of the pulp stock equally.
Also, in a conventional arrangement, repeated agitation of the pulp stock tends to make the fibers therein align with the direction of movement of the forming fabric rather than retaining a random alignment, which is preferably if the resulting paper is to have a high tear strength. The present invention, however, tends to realign the fibers so that they are no longer parallel to their direction of travel on the forming fabric.
Preferably, the upper surface of the support member defines a plurality of grooves running in a cross-machine direction.
The grooves may run parallel to one another across the surface but alternatively they can be arranged so that leading ends of the grooves with respect to the direction of travel of the belt are located on opposite sides of the support surface.
Preferably also, the insert changes the depth of the groove. Alternatively, the insert may project above the level of the support surface so that if a plurality of grooves are arranged adjacent one another across the support surface, the inserts define a series of cross-machine channels therebetween.
Preferably also, the upper surface of the insert is flat.
Preferably also, the upper surface of the insert has a downstream portion which diverges backwards at an angle between 0° and 5°.
Preferably also, the trailing side walls of the grooves are angled so that they slope in the direction of travel of the belt at an angle which diverges by up to 45° from the vertical.
Preferably also, the leading edge of the support member diverges downwardly from the support surface at an angle of 30°.
Preferably also, the retaining means is provided by the groove defining a T-shaped or dovetail-shaped transverse cross-sectional profile into which a complementary shaped insert can be slidingly engaged from one end of the groove.
The support member may comprise a forming board. Alternatively, it may comprise a foil blade.
The present invention will now be described by way of example with reference to the accompanying drawings, in which:
With reference to the drawings, a forming board 1 comprises substantially flat, rectangular upper support surface 3 and a lower surface 3. Each of the surfaces 2 and 3 defines a series of grooves 4 and 5 respectively. The board 1 is also provided with a leading edge 6 and a trailing edge 7, with respect of the direction of travel B of a forming fabric belt beneath which it is to be located in a paper making machine. As is conventional, the leading edge 6 of the board 1 diverges downwardly from the upper surface 2 at an angle a of around 30°.
Both sets of grooves 4 and 5 are open ended and run across the board 1 in a cross-machine direction with regard to the direction of travel B of the forming fabric. The cross-sectional profiles of the grooves 5 in the lower surface are T-shaped uniformly along the length of the groove to enable the board 1 to be attached in a conventional fashion to T-shaped cross-rails mounted on a frame located beneath the forming fabric belt.
The grooves 4 in the upper surface 2 of the board 1 also have a uniform T-shaped transverse cross-sectional profile to provide a retaining means for an insert 8 in the form of a complementarily shaped slat that can be slidingly engaged in the groove 4 from one end thereof. However, the grooves 4 need not have a T-shaped profile, other profiles also being suitable to provide the retaining means, for example a dovetail-shaped profile.
Whereas the grooves 5 in the lower surface 3 of the board run 1 transversely across the board normal to the direction of travel B of the forming fabric belt, the grooves 4 in the upper surface 2 have a longitudinal axis which is angled at between 0.38° and 0.77° to a transverse axis in the cross-machine direction. In practice, forming boards 1 have a width which varies between 2000 mm and 11000 mm, this means that there will be an approximate variation of between 13 mm and 148 mm between the distance of the leading end 9 of the leading edge 10 of the groove 4 from the leading edge 6 of the board 1 and the distance of the trailing end 11 of the leading edge 10 of the groove 4 from the leading edge 6 of the board 1. This variation is indicated in
Although in the drawings three grooves 4 are shown in the forming board 1 which are all arranged parallel to one another, the grooves 4 need not be arranged in this fashion. Also, the numbers of grooves 4 in the board 1 can be varied as required. In some applications of the invention it will be advantageous to arrange the grooves 4 so that the leading ends 9 of the grooves 4 with respect to the direction of travel B of the belt are located on opposite sides of the support surface 3. In this case, therefore, the grooves 4 will lie at oblique angles to one another across support surface 2 of the board 1. In particular, the adjacent grooves 4 of a series can be arranged to run obliquely to one another.
The inserts 8 provided for sliding engagement in the grooves 4 may either change the depth of the groove 4 by being of a smaller depth than the depth of the groove 4 or may project above the level of the support surface 2. In the latter case, if a plurality of parallel grooves 4 are arranged adjacent one another across the support surface, the inserts 8 will define a series of cross-machine channels therebetween. It will be appreciated that in essence this will produce the same effect as the use of the inserts 8 in the former case where the grooves 4 themselves define the cross-machine channels.
In addition to the aforementioned inserts, inserts 8 may also be provided which are the same depth as the depth of the grooves 4, effectively to blank out any grooves 4 in the board 1 which may not be required for a particular application.
The upper surface of the inserts 8 is preferably smooth and flat. However, some inserts 8, such as the insert shown in
In use, as previously mentioned, the forming board 1 is mounted conventionally beneath a forming fabric belt in a paper making machine. As the belt passes over the board 1, the cross-machine channels defined by the board 1 by the combination of the grooves 4 and inserts 8 induce pressure pulses in the pulp stock. This occurs principally by forcing water which has draining from the stock through the forming fabric back through the fabric into the pulp. In the present invention, the cross-machine channels will be encountered first at their leading ends 9 at one side of the fabric. These will set up a pressure pulse in the pulp stock that will move sideways across the fabric as fabric passes over the grooves 4. Hence, a single wave is formed which travels across the fabric along the line of the insert 8 and this produces a controlled agitation of the pulp stock. Also, the sideways traveling wave will agitate the fibers and induce them to move out of alignment with the direction of travel B on the forming fabric, which would otherwise tend to be their preferred direction of alignment.
A forming board 1 as described above may be made from any suitable material, which is easy to machine and will withstand wear. Particularly suitable for the purpose are machinable plastics materials, such as high density polyethylene, as they will not be corroded by the water draining out from the pulp stock.
As previously stated, both forming boards and foil blades can be adapted in accordance with the present invention. Whilst a forming board has been used in the example given above to demonstrate the invention, a man skilled in the art will appreciate that a foil blade in accordance with the invention could be readily manufactured.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0129900.7 | Dec 2001 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB02/01068 | 3/8/2002 | WO |
