Patent Publication Number: US-6216877-B1

Title: Screen panel and method of its manufacture

Description:
This invention relates to a screen device for screening fiber material and to a method of manufacturing this screen device. 
     The screening of fiber material, such as fiber suspensions of cellulosic material has the object to separate coarse particles in the form of shives, fiber bundles or other impurities from the fiber material. The screening normally is carried out in screen devices comprising a screen cylinder, which is stationary or rotary. The screen cylinder is provided with holes or slots, through which the fibers, but not the impurities, can pass. 
     Screen cylinders of this kind normally are manufactured by drilling holes or milling slots in a metal sheet which then is formed to a cylinder. For obtaining an open area as large as possible, i.e. the portion of the screen cylinder surface, which is perforated, the holes or slots must be arranged in dense relationship while at the same time the strength of the cylinder must be maintained. This requires great precision work. 
     Especially at the manufacture of slotted cylinders, which in certain cases are more advantageous than cylinders with drilled holes, difficulties arise concerning the strength, because the material remaining between the slots will be thin. The slots, therefore, must be milled short and at a certain distance from each other, which implies a restriction of the open area. 
     One way of solving this problem is to form the cylinder of a great number of bars, which are kept in place by ring-shaped ledges. This is, however, a complicated and expensive way of manufacturing a screen cylinder. It is also difficult to achieve a uniform slot width across the entire cylinder. 
     This problem is solved according to the invention by forming the screen device of a number of screen panels, which together form a screen cylinder. The screen panels are manufactured individually by forming them in one piece, whereafter the screen apertures are formed in the panels. A plurality of panels are thereafter assembled to a screen cylinder. The characterizing features of the invention are apparent from the attached claims. 
     The forming of the panels can be carried out by casting, extrusion, compression moulding or the like of a material, which consists of a suitable metal alloy or ceramic material, for example aluminium oxide. The material should be wear-resisting and un-weldable. After their forming, the panels are provided in a suitable way with screen apertures in the form of holes or slots. This can be brought about for example, by water-, laser- or electron-cutting. For improving the surfaces of the screen apertures still more, they can be polished, for example by wet abrasive polishing. 
    
    
     The method is described in greater detail in the following, with reference to the accompanying Figures illustrating an embodiment of the invention. 
     FIG. 1 shows a screen panel according to the invention. 
     FIG. 2 is a cross-section through the screen panel according to II—II in FIG.  1 . 
    
    
     The screen panel shown in FIG. 1 is a portion of the jacket of a screen cylinder. By assembling a plurality of screen panels, a screen cylinder is formed. Each panel  1  preferably is formed by casting according to the shell moulding method. The casting mould has the form desired of the final panel. In FIG. 2 the profile of the screen surface is shown, comprising deep axial channels  2  on one side and grooves  3  on the other side. 
     After the casting, the panels are provided with screen apertures in the form of holes or slots  4  in the channels  2 . The screen apertures are cut with high precision by water-, laser- or electron-cutting, whereby a holed diameter or slot width of 0,1-1 mm can be achieved. The surface of the screen apertures can be improved by polishing, for example by wet abrasive polishing. By forcing a suspension consisting of more or less wearing particles, for example olivine sand, through the screen apertures, a burring and polishing effect is obtained. In order to increase this effect, the suspension preferably is flushed repeatedly and in changing direction through the apertures. It can thereby even be possible to modify the size of the screen apertures. 
     The invention implies that a slotted screen cylinder can be manufactured with high dimensional accuracy and with a large open area without deteriorating the strength. The screen panels are assembled in a suitable way to a cylinder, for example by welding, screw joint or the like by means of support rings holding the panels together. 
     A screen cylinder manufactured of screen panels according to the invention can be given high precision and great strength. It is easy to mount and can be formed of very wear-resisting material yielding a long service life. Due to the high precision of the apertures, the screening result can be improved. 
     The invention, of course, is not restricted to the embodiment described above, but can be varied within the scope of the invention idea.