Screen plates

Abstract: The invention relates to a screen plate (10), and method of making a screen plate, for the pressure screening of papermaking stock in pressure screening apparatus. The screen plate includes slots (16) which have been cut therethrough by a machining laser beam focused at the inlet side (20) of the plate, to provide slots with nearly straight walls (32) with minimum taper, and which have a recast layer (42) and which have a "glassy" appearance by reason of the melting and resolidification of the immediate wall surface, to provide a very hard and low friction path for the stock. Wear bars (25) are applied to the plate (10) for coaction with the rotating foils on the screening apparatus, in the form of beads of weld material of high hardness applied directly to the plate.

Description: This invention relates to screen plates and more particularly to plate-type screens which are used in pressure screening apparatus for removing contaminant particles from papermakers' stock. High turbulence pressure screening apparatus of the kind shown, for example, in U. S. Patent No. 4,155,841 issued May 22, 1979 to Chupka and Seifert, and assigned to us, is used in the preparation of papermaking stock. A suspension of liquid and paper fibers, which may in varying degrees contain undesirable rejects or contaminate particles, is supplied to the inlet of the apparatus as disclosed, for example, in the '841 patent, where it is applied to an annular cylindrically shaped screen having specifically designed slots or perforations therethrough. Typically, the paper stock is fed to the interior of a vertically oriented cylindrical screen, and the rejects are withdrawn from one end of such a screen, while the accepts pass through the slots or perforations in the screen and are collected at a location outwardly of the screen. Additionally, rotating foils or vanes are positioned either inside or outside the screen surface, in closely adjacent relation to the screen surface, to reduce the plugging of the screen slots or holes. Such screens, as used in the separating apparatus, may further be provided with a plurality of generally axially-extending, radially spaced bars on the inlet surface of the screen which, in Patent '841, is the radially inner surface of the screen. The bars cooperate with the rotating foils to reduce flow movement parallel to the surface of the screen and cause regions of turbulence in the stock adjacent the inlet side of the screen cylinder, to improve screening as disclosed for example in U.S. Patent no. 4,410,424 issued October 18, 1983 to Chupka and Seifert and assigned to us. Similar turbulence-inducing obstacles in the form of wear bars are disclosed in FR-A-1,539,846. In the design of such screen plates, it is desirable that the slots or holes present a sufficient percentage of open area as to provide adequate capacity. However, increasing the number of slots to increase the percent of open area, with conventionally formed slots, results in a substantial loss of metal of the plate and a corresponding decrease in the plate's strength. This is due to the fact that conventional metal cutting practices result in the formation of substantial relief areas on either side of the slot, often dependent upon the thickness of the plate itself. Commonly, such plates range between 6.4 and 1.3mm (0.25" and 0.50") in thickness, with 7.9mm (0.31") thickness being typical, with a size approximately 22.9cm (9") high and 61cm (24") in diameter being typical, although screen sizes may be as large as 152.4cm (60") high and 152.4cm (60") in diameter. Accordingly, the limitations of the machining or cutting step often define the extent of openness or percent of open area which can practically be achieved in a given plate. In the manufacture of such screen plates, the turbulence-inducing bars have commonly been separately formed and welded in the desired position. In order to prevent warpage, this has commonly resulted in the use of bars of reasonably heavy stock, such as 6.4mm (0.25") stock material, although smaller bars would normally be preferred to permit the foils to pass closer to the slots than is possible when using thicker or larger bars. The bars, when applied to a slotted plate, also tend to warp the slots and may have the effect of rendering the slot size so variable that the cylinder must be scrapped. Further, the act of welding such bars on the land areas induces and causes weaknesses in the cylinders. In conventional screen plates as described above, the material of the plate and material of the bar is subject to wear by friction and erosion over time. Such wear results in a loss of distinctiveness of the individual orifices or slots by reason of the rounding off of the lines of demarcation or edges between the inner or inlet surface of the plate and the orifices, as well as the edges of the bars, and result in an undesirable effective increase in orifice diameters or slot widths. There is accordingly a need for a manufacturing technique and process for making screen plates in which substantially harder materials may be economically used to extend the effective life of the plate, and to increase the percentage of open area without sacrificing strength. There further exists a need to recondition or rebuild an existing screen plate or cylinder, either where it was not originally made with wear bars and to rebuild original wear bars which have become worn. Conventional screen plates are further characterized by openings, such as slots or holes, which are normal to the surface, that is, the openings or slots extend in the direction of the shortest distance between the inlet and outlet surfaces. Further, in the case of slots, such slots are commonly either circumferential or axial to the cylinder, and are not arranged in distinctive or unique patterns. According to the invention there is provided a metal screen plate adapted for screening of papermaking stock in pressure screening apparatus in which a rotating foil moves in close proximity to an inlet surface of the screen plate, and in which the screen openings in the form of holes or slots are arranged in closely spaced relation and define land regions on an inlet surface of the screen plate and wear bars are positioned on the land regions in non-intersecting relation to the holes or slots, characterised by the fact that the wear bars are formed by the application of a hard weld bead directly to the land region. The invention also provides a method of making a metal screen plate for use in the pressure screening of papermaking stock for removing contaminate particles therefrom in pressure screening apparatus, in which said screen plate has an outlet surface, and inlet surface, and has a plurality of openings in the form of holes or slots leading from said inlet surface to said outlet surface, the openings being arranged in closely spaced relation and defining land regions on the inlet surface, and in which wear bars are applied on the land regions in non-intersecting relation to the openings characterised by applying hard weld wear beads directly to the land regions so as to form said wear bars on the land regions. Thus, the method of this invention includes the formation of the wear bars on the inlet surface by the application of extremely hard weld material directly to the adjacent surface of the plate and to each other, to a desired thickness. The hard material is applied by welding one or more layers of such material, one on top of each other directly to the screen plate, at the inlet surface thereof for coaction with the rotating foils. Additionally, where the openings are formed by laser cutting which is relatively insensitive to the hardness of the material being cut, materials such as extreme hardness steel alloys can be used for the plates and economically machined to reduce the rate of wear, and thereby extend the effective life of the plates. Similarly, the wear bars themselves may be formed of highly wear resistant or hard materials, such as "Stellite," by the weldment of such material directly to the inlet surface of the plate. The wear bars are applied by welding, in the reconditioning of existing screening cylinders or screen plates, to restore the same, or to enhance the operation of such plates where the plate had not been provided with bars in the first instance. In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which:- Fig. 1 is a perspective view of a preferred embodiment of a screen plate made according to this invention for use in paper stock screening apparatus; Fig. 2 is a flat developed view, partially broken away, of the plate of Fig. 1; Fig. 3 is an enlarged sectional view thereof taken generally along the line 3--3 of Fig. 2; and Fig. 4 is an enlarged fragmentary transverse section through one of the screen or slot rows taken generally along the line 4--4 of Fig. 3. Referring to the drawings, which illustrate preferred embodiments of the invention, a vertically cylindrical screen plate is illustrated generally at 10 in Fig. 1. The screen plate as illustrated is designed for use, for example, as the operating screen plate in high turbulence paper stock screening apparatus of the general type shown in Seifert, U. S. Patent No. 3,849,302 issued November 19, 1974, and the above referred to U. S. Patent No. 4,155,841 of Chupka and Seifert as identified by the reference numeral 20 in the latter patent. The screen plate 10 of this invention includes a body 12 formed of sheet material which is cut flat and subsequently rolled into a cylinder, as shown in Fig. 1. Thereafter, end rings 13, 14 are welded in place for supporting the screen within the screening apparatus as disclosed and described in the above-identified patents. The plate 10 is shown in Figs. 1 and 2 as having two circumferential rows 15 of generally vertically oriented, angularly spaced openings in the form of slots 16. The slots 16 in each of the rows 15 are formed into individual slot groups 18 with a land area 19 therebetween. The individual slots 16 extend through the body 12 of the plate 10 from an inlet surface 20 to an outlet surface 22, in generally normal relation to the inlet and outlet surfaces. The intermediate land areas 19 between the groups 18 of slots provide a space for the wear bars or ribs 25 on the inlet surface 20. The bars 25 run the full height of the plate 10 in parallel, radially spaced relation, one for each land area 19, and thus run through both of the rows 15 of slots 16. While parallel, axially-extending wear bars 25 are shown, it will be understood that the technique of applying wear bars by the application of weld material directly to the inlet surface of a screen plate, permits wear bars to be applied in curved regular or irregular patterns on the plate surface. Further, the technique and process of applying wear bars as herein described is useful in the repair and rebuilding or reconditioning of existing screen plates in which the conventional wear bars have been worn, by the replacement of such conventional wear bars by wear bars made in accordance with the teaching of this invention. The process of this invention, in the application of weld material to the land surface between holes or slots in the reconditioning of existing plates, permits the enhancement of the efficiency of screen plates which originally had not been provided with such bars. In the manufacture of the screen plate 10, the wear bars 25 are preferably applied to the inside surface 20 before the slots are cut, and before the body 12 of the plate 10 is rolled into a cylinder. The wear bars 25 are uniquely different from bars heretofore employed and consist of weld beads applied directly to the surface 19 such as by a TIG welder i.e. a tungsten-inert gas welder in which a plasma from a non-consumable tungsten electrode creates a weld puddle in a protective atmosphere provided by a flow of inert shield gas. The wear bars may consist of an extremely hard cobalt-chromium-tungsten alloy sold by Union Carbide and Chemical Corporation under the trade name "Stellite", applied in three or four passes. Typically, the composite bars 25 may be 3.2mm (1/8") wide and 3.2mm (1/8") high. Wear bars formed exclusively of very hard weld materials, and are formed as a single bead or as a plurality of beads applied on top of each other to the desired thickness. The bars 25 may be substantially lower in profile than heretofore possible since discrete bars of the prior art required sufficient thickness to resist warpage of bars during the welding process. Preferably, the wear bars 25 are applied to the land regions 19 while the body 12 is flat, that is, prior to rolling to form the cylinder, although they may be applied after rolling. The individual slots 16 are formed by laser energy beam cutting, preferably through the outlet surface 22 toward and into the inlet surface 20, and after the body 12 is rolled into a cylinder. By cutting the slots after rolling, the expansion or widening of the slots which would otherwise accompany the rolling process, is avoided. However, it is within the scope of this invention to cut the openings or slots in the flat stock, prior to rolling. While a C0 2 laser may be used, it is preferred to use a pulsed YAG laser, in which the beam is focused through the outlet surface 22 and at a region adjacent and immediately inwardly of, the inlet surface 20, with respect to the depth or thickness of the body 12. In this manner, since the beam enters the plate from the outlet side and exits the plate at the inlet side, and since the beam itself is focused at the inlet side, the opening formed by the beam at the region of breakthrough at the inlet side is narrower than the opening formed at the outlet side, with the extent of relief or taper being defined essentially by the convergence of the beam, with the result that the wall of the opening has a minimum of taper. The plate itself may be made of hard stainless steel, such as Armco Steel Corporation's "Nitronic 33" stainless steel, a low-nickel austenitic stainless steel having high resistance to wear and galling, with a Rockwell B hardness of around 95, or "Inconl" or other suitable materials.

Claims: A metal screen plate (10) adapted for screening of papermaking stock in pressure screening apparatus in which a rotating foil moves in close proximity to an inlet surface of the screen plate, and in which the screen openings (16) in the form of holes or slots are arranged in closely spaced relation and define land regions on an inlet surface (20) of the screen plate and wear bars are positioned on the land regions in non-intersecting relation to the holes or slots, characterised by the fact that the wear bars are formed by the application of a hard weld bead directly to the land region. A method of making a metal screen plate (10) adapted for pressure screening of papermaking stock for removing contaminate particles therefrom in pressure screening apparatus, in which said screen plate has an outlet surface (22), and inlet surface (20), and has a plurality of openings in the form of holes or slots (16) leading from said inlet surface to said outlet surface, the openings being arranged in closely spaced relation and defining land regions on the inlet surface, and in which wear bars are applied on the land regions in non-intersecting relation to the openings characterised by applying hard weld beads directly to the land regions so as to form said wear bars (25) on the land regions. A method as claimed in claim 2, including applying hard weld beads to the land regions at specific transversely spaced locations (19) to form wear bars composed entirely of weld material. A method as claimed in claim 2 or 3, in which said wear bars have a plurality of applications of weld material at the same location.

Inventors: CHUPKA DAVID E, LANDEGGER CARL C, CHUPKA, DAVID E., LANDEGGER, CARL C.
Applicants: BLACK CLAWSON CO, THE BLACK CLAWSON COMPANY
Application year: 1988
Main classification: D21D5
IPC codes: B01D29, B23K26, D21D5, B23K31