Abstract:
A method for the production of a screen, particularly a rigid screen, for the treatment of fibrous suspensions suitable for the production of paper, the method including the steps of: providing at least two screen layers; introducing a plurality of openings in the at least two screen layers, at least a portion of the openings being shaped and arranged such that the openings run through each of the at least two screen layers; and joining the at least two screen layers flat to one another thereby forming a flat joint, wherein the plurality of openings are introduced into the at least two screen layers before joining the at least two screen layers.

Description:
[0001]    This is a continuation of PCT application No. PCT/EP2008/050128, entitled “METHOD FOR THE PRODUCTION OF A SCREEN, PARTICULARLY A RIGID SCREEN, FOR THE TREATMENT OF FIBROUS MATERIAL SUSPENSIONS SUITABLE FOR THE PRODUCTION OF PAPER”, filed Jan. 8, 2008, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for the production of a screen for the treatment of fibrous suspensions suitable for the production of paper. 
         [0004]    2. Description of the Related Art 
         [0005]    Screens which are produced in a method of the type of the present invention are preferably used for the wet screening of fibrous suspensions in order to remove contraries present therein. They are generally rigid and, as a result, differ from flexible endless screens which are used in screen presses and papermaking machines. The characteristics of such a screen result substantially from the size, shape and number of openings found therein. As a rule, these are kept smaller than the substances to be screened out. Such screens are advantageously used, for example, in pulpers, secondary pulpers and sorters for the preparation of paper fibrous suspensions, having the task of holding back contraries. For applications which result in particular in the paper and pulp industry, such screens are intended to have a screening characteristic which can be achieved, for example, by means of round openings between 1 and 30 mm, depending on the coarseness of the stocks. Of course, the aim is also to reach the highest possible throughput through screens of this type, which is to say the largest possible quantity of non-rejected stocks are intended to pass through the openings. This may be promoted by there being as many openings as possible. Expressed overall, trouble is taken to have the largest possible free screen area, based on the total surface of the screen element. 
         [0006]    An additional requirement is relatively high resistance to hydraulic pressure. Such screen elements are used in a production operation in which faults occur, which leads to different and considerable pressure loading of the screen elements. Since a blockage cannot always be ruled out, it is entirely possible for high pressures and, with the correspondingly large areas, also for high forces to act on the surface of such screen elements. These increased forces have to be able to be absorbed by the screen without any damage occurring. 
         [0007]    Screen elements of the type herein described above have to be fabricated from high-quality material and with a great deal of effort, so that it is a continuous aim to reduce the fabrication costs by means of appropriately good production methods. In DE 195 47 585 A1 a fine screen is described which includes two layers, a supporting layer and a screening layer, in which the problem of strength has been solved by a relatively thick supporting layer. Although this leads to success, it is often not optimal in technological terms. 
         [0008]    There are other screen element production methods in which a fine screen fabric is shrunk from outside onto a cylindrical supporting body provided with relatively large holes. Even in the case of such screen elements, blockage-free operation is difficult and the screening characteristic is not sufficiently good for many applications. 
         [0009]    What is needed in the art is a method for the production of screen elements which, measured by the utility value of the screen element, is relatively inexpensive and ensures an optimum screening characteristic, in particular when screening fibrous suspensions. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides an efficient method for the production of a screen, particularly a rigid screen, (suitable for the production of paper) for the treatment of fibrous material suspensions. Screens which are intended to be used for the treatment of fibrous suspensions must have an adequate wall thickness for strength reasons. The method according to the present invention, in which the screens are produced from a plurality of layers, offers a fundamental economic advantage in production. As a rule, it requires less effort to introduce holes or other screen openings into thinner metal layers than into thick ones. In particular, in the range of sizes typical here for the screen openings, which, for example, lies between 1 and 20 millimeters, methods such as punching or laser cutting can be employed. This generally gives rise to lower costs than those which arise in the case of drilling. Further methods for the production of the openings in the screen layers are milling, water jet cutting, etching, erosion, electrochemical boring, extrusion hole forming or broaching. By using the method according to the present invention, screen openings of which the cross sections are not rotationally symmetrical can easily be produced, for example slots, rectangles, diamonds, hexagons or other polygons having round corners. This can offer particular advantages in the wet screening of the fibrous suspensions. 
         [0011]    In many applications of the screens produced in accordance with the method of the present invention, it is advantageous if the screen openings widen in the flow direction of the liquid to be screened. By using the method according to the present invention, such an embodiment can be produced virtually without additional costs by the openings in the downstream screen layers (viewed in the flow direction) being larger than in the respective upstream ones. This can be implemented in the case of cylindrical holes, or else in the case of cross sections deviating therefrom. A specific possibility results for the punching technique, in which a hole shape differing from the cylindrical shape arises in any case for production reasons. This means that the inner walls of the openings are not parallel to one another but, rather, are at an acute angle to one another which, lies between 1 and 3°. Given an arrangement of the individual screen layers which is expedient in this regard, it is, therefore, possible to choose this flow cross section widening in the direction of the intended flow of the liquid to be processed. 
         [0012]    Moreover, there is a series of advantageous embodiments of the method of the present invention which, with little effort, permit the production of such screen openings, the course of which in the screen is specifically configured. This is because there is the possibility, by means of asymmetry and/or a lateral offset of the openings in the various screen layers, of achieving a skewed position of the screen openings, which is to say that their main direction is not perpendicular to the plane of the screen. This skewed position can be coordinated with the inflow direction of the liquid during operation of the screen. Even in the cases in which no offset is provided, different shapes of the openings in different screen layers can be used for a configuration of the flow cross sections in the screen openings which is very rich in variants. For instance, round and angled regions can follow one another. It is also possible for abrupt widenings to be formed to better keep the screen openings free. 
         [0013]    In particular, the specific configuration of the screen openings, viewed over their course in the screen, can be carried out very well given the choice of a relatively large number of screen layers, that is to say considerably more than 3, for example, 6 to 8 screen layers. Then, the individual screen layers consist of thin metal sheets (e.g. 2 mm), which are provided with openings by punching, laser cutting or other methods. 
         [0014]    It can, therefore, be assumed that, by means of finer subdivision of the screen into individual screen layers, the possibilities of producing more complicated shapes of the screen openings become better and better. 
         [0015]    The screen layers are produced, for example, from metal, plastic, rigid foam or natural structural materials. In this case, because of the different stressing of individual screen layers, a combination of different materials is advantageous, for example, abrasion-resistant material in the screen layer on the feed side (top layer) and, for example, ductile material in the central layers and/or the bottom layer. 
         [0016]    In specific embodiments, it may also be desired for the screen produced in accordance with the method of the present invention to be provided with blind holes, which of course can be achieved without great effort by the new production method by there being no opening at specific points of the appropriate screen layer. 
         [0017]    Since—as already mentioned—the screen produced by the method of the present invention is generally subjected to high forces, the individual screen layers can be joined by a method in which correspondingly high forces can be absorbed. Such methods are, for example, brazing methods, which is to say soft brazing, brazing or high-temperature brazing, since these produce a high-strength flat joint and are thermally relatively unproblematic. Of course, such a joint can be produced by welding if appropriate measures are taken to prevent distortion of the screen. A further possibility consists in technical adhesive bonding methods. As is known, it has been possible to join metals or plastics by using those adhesive bonding methods which ensure high strength. Since such screens are generally used at temperatures considerably below 100° C., the thermal loading of such adhesive bonds is low during operation of the screen. 
         [0018]    In other cases, it is possible to configure the joint between the screen layers to be detachable in order to have the possibility of replacing worn parts simply. Detachable joints are formed, for example, by screwing, riveting, clamping or a form fit, to mention only these. Given more than three screen layers, these can be partly detachably and partly non-detachably joined to one another, which offers the possibility of providing only the layer on the feed side (top layer) which is particularly subjected to wear to be replaced. 
         [0019]    In particular embodiments of the method of the present invention, the screen layer on the feed side (top layer) which is critical for the screen characteristic can be kept relatively thin, a screen of high strength nevertheless being produced by means of an appropriately ruggedly designed further layer or further layers. The possibility of using a thin top layer also has the advantage of a lower pressure loss during operation since, in the finished screen, only short flow paths are produced within the openings of the top layer determining the screen character. The length of this flow path corresponds to the thickness of the top layer. Even if a conical widening of the screen openings in the intended flow direction is required, production is nevertheless simpler than in the case of single-part screens. 
         [0020]    Typically, the method of the present invention is used to produce flat screens. Such a screen can have the shape of a circular ring, which is possibly assembled from circular ring segments. If, in specific cases, a screen is produced as a cylindrical screen basket, in which the screen layer on the feed side is radially on the inside and further layers are on the outside, the layer on the feed side can be provided with the openings when in the flat state. If it is then bent, the openings, viewed radially, widen from the inside to the outside as a result of the bending. Thus, as a result of special fabrication methods, conical openings are produced in the screening layer, in that the flow cross sections widen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0022]      FIG. 1  shows a part of a screen produced by the method according to the present invention, illustrated sectioned; 
           [0023]      FIGS. 2-6  shows various variants using the example of different screens, illustrated sectioned; 
           [0024]      FIG. 7-8  each show a screen part produced by the method according to the present invention in a view on the outflow side; 
           [0025]      FIG. 9  shows a screen produced by the method according to the present invention having rings of different material. 
       
    
    
       [0026]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrates embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring now to the drawings, and more particularly to  FIG. 1 , there is shown by using a first example, screen  1  produced by the method of the present invention. This includes three screen layers  2 ,  3  and  4  joined flat to one another and screen openings  7  and  8  which, in this example, pass through all screen layers. The screen layers are joined flat to one another at contact areas  17 , for example, by high-temperature brazing. In the case of screen  1  illustrated here, screen layer  2  located at the top forms the feed side, which is to say the liquid to be screened flows through openings  7  and  8  from top to bottom. In this case, seen in this flow direction of the screen, the downstream opening is larger than the respective upstream opening. This means that, in the flow direction of the screen, the cross sections at specific points become larger, which results in particularly good freedom of blockage of the screen. While screen openings  8  drawn on the right are substantially cylindrical, screen openings  7  drawn on the left are composed of a combination of two cylinders and in a conical opening (screen layer  3 ) lying between. Even though openings  7  and  8  are drawn in the same screen here, this should rather be seen as an exception. Normally, the screen openings are the same as one another. The same is also true of the other figures. 
         [0028]    In the typical applications for the screen produced in accordance with the method according to the present invention, the smallest openings are located on the feed side of screen  1  (which here means in topmost screen layer  2 ), as compared with those in screen layers following downstream. In this case, these smallest openings determine the separation characteristic, that is to say which parts pass through the screen or are rejected before it. By means of the method of the present invention, it is easily possible not only to adjust the size but also the shape of the smallest opening to the requirements. Such shapes can be, for example, circles, ovals, slots, rectangles or polygons, to name only these. By means of an appropriately beneficially selected shape and arrangement (“screen pattern”), it is possible to produce a large free area, based on the total screen area. 
         [0029]    The four-layer screen shown in  FIG. 2 , having screen openings  9 , has, for example, in each case has oblique openings in central layers  3  and  4  and cylindrical openings in screen layer  5  drawn at the bottom, which have a lateral offset with respect to the upstream openings. Such a form of the screen openings can be implemented relatively easily with the aid of the method of the present invention and has the advantage that, given oblique inflow of the liquid to be screened, a better flow is possible. As a rule, the oblique inflow is based on the fact that, with the aid of screen cleaner  14  (cleaning rotor) moved past the screen (in the direction of the arrow), an oblique velocity direction is achieved, which is therefore not perpendicular to the plane of the screen. In addition, the flow can impinge obliquely because of the shape of the casing. 
         [0030]    In  FIG. 3 , a screen having a total of five layers ( 2 , 3 , 4 , 5 , 6 ) is illustrated, its openings  10  being designed to be slightly conical in many cases. Here, the conicity can come from the punching operation, which is to say the introduction of the openings into the screen layers, which, as is known, leads to the side walls not being parallel, but being at an acute angle to one another. This conicity is shown exaggerated in  FIG. 3 . 
         [0031]    The method according to the present invention also makes it very simple to provide a screen with non-continuous depressions  11 , as shown in  FIG. 4 . Such depression  11  is produced by the relevant opening not running through all the screen layers but, for example, only through screen layer  2  on the feed side. It can be round or groove-like, for example. Furthermore,  FIG. 4  shows a strip-like protrusion, which is formed by the inclusion of profiled rod  12 . The latter is inserted into a suitable recess which runs through one or more screen layers (here, for example, screen layers  2  and  3 ), for example, brazed in. Depressions or protrusions on the feed side of the screen are suitable to influence the flow of the liquid to be screened in such a way that the tendency to blocking of the adjacent screen openings diminishes. 
         [0032]    It is also readily possible for an embodiment to be produced in which inserts  13  are used which, in their center, have continuous screen openings  15  and are inserted into a plurality or all of screen layers  2 ,  3 ,  4 . Such inserts  13  can be designed to be particularly wear-resistant and are possibly replaceable (see  FIG. 5 ). 
         [0033]    Further variants in the use of inserts  13  having suitable screen openings  15  are illustrated in  FIG. 6 . In specific uses of the screens produced in accordance with the method of the present invention, inserts  13  may protrude by a certain amount m on the feed side and/or on the outlet side, in order as a result, for example, to improve the flow in this area. This amount m is expediently between 0.5 and 5 mm. Insert  13 ′ shown in the center of  FIG. 6  does not extend through all the screen layers, but only through upper screen layers  2  and  3 . In this way, the parts of the screen that are at risk of wear, which is to say inserts  13 ′, can be kept smaller. As already mentioned, the wear is particularly high in the area where screen cleaner  14  is used. The form illustrated in  FIG. 6  showing insert  13 ′, extending over two screen layers, can be replaced particularly easily if care is taken that two screen layers  2  and  3  can be separated without being destroyed. This also applies to inserts without any projection. 
         [0034]    A view of the outflow side of a screen produced in accordance with the method of the present invention is shown by  FIG. 7 . This is a partial view which, overall, includes three screen layers  2 ,  3 ,  4 , between which contact areas  17  are located. Screen openings  8  illustrated on the left in each case have a cylindrical cross section, the diameter of which is different in the various screen layers. In this case, the openings in the individual screen layers are offset with respect to one another, so that the result is an oblique flow direction. Screen opening  16  also has a similar offset, its flow cross section having the shape of a slot. 
         [0035]    Critical for the screening characteristic of the screen produced in accordance with the method of the present invention are the smallest openings, which are normally present on the feed side, which is to say in screen layer  2  in examples illustrated in the figures. The further screen layers, which is to say for example 3 and 4 in  FIGS. 1 and 4  to  6 , substantially have the task of providing the screen produced in this way with the necessary strength, which has already been discussed. It is, therefore, readily possible, as illustrated in  FIG. 8 , to make openings in lower screen layer  4  which are so large that a plurality of openings from another screen layer open into these. In this case, it is generally expedient if such a larger opening covers about 3 to 15 smaller openings. This measure can also be taken in other screen layers, with the exception of upper screen layer  2 . 
         [0036]    A further advantage of the production method according to the present invention resides in the possibility that at least one screen layer, in particular screen layer  2  on the feed side, is divided up into a plurality of preferably concentric circular rings  18 ,  19 , and that circular rings  18 ,  19  are produced from different material ( FIG. 9 ). As a result, it is possible to take account of the fact that, because of the higher peripheral speed of screen cleaner  14  in radially outer regions, there is also a higher risk of wear there. Radially outer screen ring  19  can then be composed of a particularly high-quality wear-resistant material, for example, while screen ring  18  located radially further in can be designed correspondingly more inexpensively, with the advantage that the wear behavior of overall screen  1  does not become poorer, despite reduced production costs. 
         [0037]    While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.