High turbulence pressure screening apparatus of the kind shown, for example, in U. S. Pat. No. 4,155,841 issued May 22,1979 to Chupka and Seifert, 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 Typically, the paper stock is fed to the interior of a vertically oriented cylindrical screen, and the rejects are withdrawn from one end of such a screen, while the accepts pass through the slots or perforations in the screen and are collected at a location outwardly of the screen.

Rotating foils or vanes are positioned usually on or adjacent the inside of the screen surface, in close relation to the surface, to reduce the plugging of the screen slots or holes.

Typically, the screen cylinder of the kind described above is formed by machining the required slots or grooves in a flat plate of metal material, such as stainless steel, rolling the flat plate into the shape of a cylinder, and welding the rolled plate at the abutting ends to form a welded seam. Thereafter, end rings, formed from bar stock, are rolled, welded at their respective ends, and then attached by welding to the cylinder plate, at its respective ends. Further, one or more intermediate reinforcing rings, also formed from rolled bar stock, are attached by welding at longitudinally spaced locations on the outer surface of the rolled screen plate. An example of such a fabricated cylinder screen is shown in U. S. Pat. No.

4,017,387 issued Apr. 12,1977 to Hatton and Lehman.

Complications arise in the manufacture of screen cylinders in the manner described above as a result of openings, in the form of holes, relieved areas, or slots, cut or formed in the plate while in the flat condition. These openings have a

tendency to become inherently distorted by the rolling process. Further, since a large number of slots are formed in closely spaced relation, each usually terminating at common positions on the plate to form rows of such slots separated by land areas, the rolling process is inherently nonuniform circumferentially of the plate due to the differences in strengths between slotted and non-slotted regions. It is accordingly believed that stress concentration points tend to be formed at the terminal ends of the slots during rolling due to differential bending at these locations.

In order to avoid these problems, manufacturers have made seamless screen cylinders with laser cut openings, such as shown in U. S. Pat. No. 5,064,537 issued Nov. 12,1991 to Chupka, Constiner, and Vitori, incorporated herein by reference as needed to complete this disclosure. An integral seamless cylinder body is formed essentially free of stress and free of welded connections, as in the'537 patent, and is thereafter slotted or cut by laser cutting to form the screening openings therethrough. The cylinder body is formed by either centrifugal casting or by cold working a seamless blank by using the roller extrusion process. The openings or slots, as the case may be, are formed by a laser cutting technique, by focusing a laser cutting beam or by directing such laser beam energy, at the outlet surface toward the inlet surface. The cylinder and/or the beam may be moved in relation to the other to make a slot.

Alternatively, it is known to make screen cylinders having a plurality of parallel bar-shaped screen elements mounted on crossbars for forming a screen area and defining screen slots therebetween. An example of one such screen cylinder is shown in U. S. Pat. No. 5,472,095 issued Dec. 5,1995 to Malm. The screen elements or bars are arranged in parallel adjoining spaced-apart relationship so as to define screen slots therebetween having a predetermined slot inlet width. End rings are welded to the axial ends of the bars. Longitudinally spaced-apart reinforcing rings are welded to the outlet side of the bars.

In order to increase the capacity and efficiency of screen cylinders of the type just described to screen shives and other impurities from a liquid suspension, the bars are machined or formed, as by drawing or extruding through a die, so as to have a desired cross-section profile. Examples of screening bars having different profiles are shown in U. S. Pat. No. 5,255,790 issued Oct. 26,1993 to Einoder and

Knodel. Such bars or wedgewires typically have a generally wedge-like shape with a wider inlet end and a narrower outlet end.

Screen cylinders manufactured in accordance with the above are susceptible to variations in screen slot spacing due to inherent movement and misalignment between screen bars. In addition, the use of a great many shims, as well as machined holding racks or mandrels, increases material handling costs and contributes to the variability of screen slot spacing.

Accordingly, there is a need for improved pressure screen designs and methods of manufacturing same that provide for ease of manufacture and help to ensure stability and dimensional accuracy of the structure both during and after manufacture.

Summary of the Invention Applicants have developed a method for making screen cylinders for the separation of shives and other impurities from a liquid suspension including paper fibers that combines the advantages of screen elements or bars having a desired cross-section profile and screen slots formed between screen bars after the screen material has been shaped to its final form. Screen cylinders in accordance with the invention are characterized by having a high open area for a given slot width and slot spacing.

The method includes the steps of machining a first continuous axial groove profile on an inlet side of a metal plate, machining a second continuous axial groove profile on an outlet or accepts side of the metal plate, rolling the plate to a desired diameter, cutting the plate to a desired circumferential length, welding reinforcing rings to the outlet side of the plate, welding end rings to the axial ends of the plate, and cutting substantially continuous slots through the plate, thereby connecting opposite inlet and outlet grooves.

A plate of material, typically stainless steel, having a desired thickness, is first provided. Substantially parallel grooves having a first desired profile are machined on an inlet side of the plate by conventional means. The grooves may extend substantially the entire length of the plate. The plate is turned over and substantially parallel grooves having a second desired profile are machined

on an outlet side of the plate. The outlet side grooves are machined so as to be in substantial registration with the inlet side grooves on the opposite side of the plate.

The inlet and outlet profile grooves may be formed by means well known to those with ordinary skill in the art such as, for example, by milling or stamping.

To form a cylindrical pressure screen of the type wherein the flow of paper stock suspension is from the interior of the basket shaped screen to the exterior of the screen, the grooved plate is rolled or bent to a desired diameter with the inlet surface of the plate directed radially inwardly. The grooved plate may be formed about a mandrel if desired. Rings possibly formed from rolled stainless steel bar or other stock are affixed, as by welding, to the axial ends of the rolled plate. Support rings, also made of rolled stainless steel bar stock, are affixed at longitudinally spaced-apart locations, as by welding, to the outlet side of the rolled plate.

Longitudinally, axially extending screen slots are cut through the plate so as to extend through the plate from the outlet side of the rolled plate to the inlet side of the rolled plate. The screen slots define fluid flow passages through the rolled plate joining the grooves of the inlet side with the corresponding oppositely disposed grooves of the outlet side of the plate. The screen slots extend substantially the entire length of the rolled plate. Uncut or interrupted portions may be provided as one proceeds from one axial length end of the slot to the other end. These uncut portions serve as bridges or spacers connecting the solid material extending between the slots to improve overall structural integrity of the screen cylinder and to accurately maintain slot width dimensions.

As used throughout the specification and claims, the grooves and slots are said to extend substantially along the entire longitudinal dimension of the plate, which dimension corresponds to the axial direction when the plate is ultimately formed as a cylinder. The term"substantial"in this connection means that the grooves and slots extend either all the way along the axial direction (i. e., 100% of the axial length) or that the slots and grooves extend almost the entirety of that direction.

The screen slots are preferably formed by laser cutting, but may be formed by any suitable means such as water jet, electron beam, electro-discharge machining, or other machining or cutting methods.

Although the pressure screen cylinder design and manufacturing method as described may be used to create baskets for paper pulp screening, such cylinder screens are not limited solely to this use.

Accordingly, it is an object of the invention to provide a method for making a screen cylinder, and a screen as made by the method.

It is another object of the invention to provide a method of making a screen cylinder having a maximum open area for a given circumferential slot spacing.

A particular object of the invention is to provide a screen cylinder which is formed with individual screening elements that have the screening advantages of a wedge wire type screen (i. e., large open area) cylinder but one in which the slot dimensions may be more accurately and easily maintained and which has higher mechanical integrity.

A further object of the invention is to provide a method of making a screen cylinder that results in dimensionally precise screen slots.

Another object is to provide an improved pulp slurry screen, by fitting it within a cylinder according to the invention.

Another object is to provide a screen cylinder wherein the slots are cut late in the manufacturing method.

Another object is to provide an open area for accepts slurry to pass through the slot, even in those areas which are obstructed by the reinforcing ring.

Accepted slurry would simply flow axially along the grooves of the accepts side of the plate, into the accepts chamber.

Other objects and advantages of the invention will be apparent from the following description, and the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a perspective view of a screen cylinder made in accordance with the invention; Fig. 2 is a broken away top plan view of a plate that may be used in forming the screen cylinder of the invention;

Fig. 3 is a broken away plan view of the plate shown in Fig. 2, during an initial step of the manufacturing method by which the screen cylinder is made, showing what will ultimately become the accepts side of the screen cylinder; Fig. 4 is a broken away top plan view of the plate shown in Fig. 3, showing what will ultimately become the inlet side of the screen cylinder; Fig. 5 is an enlarged transverse cross sectional view of the plate shown in Fig. 4; Fig. 6 is an enlarged transverse cross sectional view of a portion of the screen cylinder shown in Fig. 1, during one of the final assembly steps; Fig. 7 is a simplified perspective view of the screen cylinder during the assembly stage shown in Fig. 6, with only certain of the axial extending screen bars and the transversely oriented support rings shown for increased clarity; Fig. 8 is a simplified cross-sectional view taken along the plane shown by lines and arrows 8-8 of Fig. 6; and Fig. 9 is an enlarged elevational view in partial cross section showing the inlet side of another embodiment of the screen cylinder.

Detailed Description of Preferred Embodiments Turning first to the drawings and particularly to Fig. 1, there is shown screen cylinder 10, made in accordance with the invention. The cylinder includes top and bottom end rings 20,22 respectively which serve as axial end reinforcement for a multiplicity of spaced axial extending screen bars 25. Between adjacent bars 25, openings 27 are provided and serve to permit ingress and egress of fluid therebetween while performing a screening or filtering function.

Reinforcing rings 14,16, and 18 are provided at axially spaced locations along the periphery of the bars 25 to lend support and stability to the screen cylinder.

As shown, the screen cylinder is of the type wherein the pulp suspension is to be admitted into the interior of the cylinder, as shown by the arrow 29, with accepts flowing through the opening between the bars as shown by arrows 31. Rejects, or matter that does not pass through the openings exits the cylinder at an egress point (not shown) in communication with the interior cylinder space.

As is conventional in the art, one or more impellers will be mounted on the inside of the cylinder to rotate the pulp suspension and exert shear forces thereon as it is swept along the inlet face of the screen bars 25. Operation of one type of commercially successful rotary screening apparatus is shown in U. S. Patent 4,267,035. The entire disclosure of this patent is incorporated herein by reference so that the principles of operation need not be repeated herein.

Turning now to Fig. 2, there is shown a plate 40 which is composed of a rigid, durable material such as metal including stainless steel, aluminum, titanium and alloy forms thereof. At present, it is desired to use a stainless steel plate having a <BR> <BR> <BR> <BR> thickness of about 1/4-3/4", most preferably about 3/877. The length (i. e., axial direction with respect to the formed cylinder) and width of the plate are chosen so that a screen cylinder having the requisite circumference and axial length may be formed from the plate.

The plate has a top surface 42 that ultimately will be formed and used as the inlet surface of the screen cylinder, with the outlet surface of the screen cylinder corresponding to the bottom or underside 44 of the plate 40.

The plate is used to form the series of axially extending screen bars 25 and intervening openings 27 in the following manner. As can be seen diagrammatically in Fig. 4, a multiplicity of lengthwise (or axially extending) grooves 46 are formed in the top surface 42 of the plate. These grooves may be provided in any of a myriad of possible configurations; all designed to form the desired contour for the inlet surface of the screen cylinder.

One such configuration is shown both in Figs. 4 and 5 wherein the grooves are generally designated as reference number 46. Grooves 46 comprise, more specifically an inclined surface 48, a relatively flat portion 50 and a downwardly sloped surface 52. Connecting or bridging adjacent pairs of the grooves is a land portion 54 which will subsequently be treated as stated hereinafter.

The skilled artisan will appreciate that a variety of different profiles may be provided for the surface 42 and, in fact, a flat contour surface may even be provided although at present, it is desired to provide a series of grooves 46 similar to those shown in Fig. 5.

The grooves 46 may be formed in the plate surface by conventional techniques such as by milling, plasma, wet, or other etching methods, laser cutting, stamping, extruding or casting. The grooves 46 are cut or formed only partially through the thickness of the plate from the top side 42 toward the bottom 44.

Turning now to Figs. 3 and 5, the bottom surface 44 is also provided with a plurality of grooves 60 that extend along the lengthwise (axial) direction and, upon completion of other manufacturing steps, will communicate with the grooves 46 provided on the top side of the plate. Grooves 60 may be provided in the bottom surface by the same convectional means as those mentioned above in connection with the formation of the grooves in the top 42 of the plate. The grooves 60 do not extend through the entire thickness of the plate, and are separated from each other by solid sections 61. Land sections 54 block communication between the grooves 60 and 46.

The grooves 46,60, taken together, will define the intended surface contours for the inlet and outlet surfaces of the screen cylinder. Once the desired contours have been provided, the plate is then rolled into the desired cylindrical shape, corresponding to the desired shape of the finished cylindrical screen. The width wise borders of the plate may be affixed together to thereby define the cylinder such as by brazing or welding. Possibly, the widthwise ends could be affixed to an axially extending stabilizer bar or the like.

Once the thus contoured plate is rolled into the desired cylindrical shape, it can, if desired, be placed in inventory for further use upon customer demand or it can be treated such as by surface treatment to improve hardness or other properties of the metal. For example, the thus formed cylinder can be hardened such as by precipitation, flame, induction or other conventional hardening treatments.

After formation of the cylindrical form from the thus contoured plate, outer, reinforcing rings such as those shown in Fig. 1, and top and bottom support rings 20,22 may be fixedly secured to the assembly to lend structural support to the cylinder such as by brazing or welding. Preferably, at this point it is desired to laser cut through the land portions 54 of the inlet surface to effect communication with the grooves 60 on the outer or accepts surface. This can be best understood in conjunction with Fig. 5 wherein slots 62 are shown in phantom. The slots 62 are

provided in the lands portions 54 of the inlet surface. Preferably, a laser is used to cut the slots 62.

Both CO2 and pulsed YAG lasers may be mentioned as exemplary laser cutting tools to be used to cut the slots 62. However, other methods such as E- beam, electro-discharge machining (EDM), water jet, acid etching, or other etching methods etc. may be used. The important criterion is that communication between the inlet face and outlet surface contours is effected by the slots 62.

Turning now to Fig. 6, a plurality of the thus formed screen bars 25 are shown wherein the grooves 46 on the inlet side of the cylinder communicate with the accepts side grooves 60 via the slots 62. As shown, the accepts side faces of the screen bars are fixedly secured to reinforcing ring 16 at the solid sections 61. The inlet contour comprises inclined surface 48, flat 50, and downwardly (sloped toward the accepts side) sloped surface 52. The accepts side includes large cut out portions defined by grooves 60.

Turning to Fig. 8, it can be seen that short bridges 80 can be provided along the axial length of certain ones of the slots 62. These bridges are uncut portions of the slots thus providing remaining small solid connection between neighboring bars to help insure slot dimensional accuracy and to stabilize the cylindrically shaped assembly.

Turning now to Fig. 7, there is shown a simplified version of a completed basket assembly, with only one support ring shown, and only a plurality of the screen bars shown so as to facilitate understanding. The screen cylinder has a plurality of axial extending screen bars 25 with the inner face or inlet surface contour 42 communicating with the outer face or accepts contour 44 through the slots 62.

An alternative embodiment is shown in Fig. 9 wherein the inlet surface 42 of the screen bars is shown. Here, the slots 62 have been formed so as to provide a varying width-wise dimension. As shown, slots 62 have a first width-wise dimension 63 at the axial top of the surface, with a second, wider dimension 65 provided at the bottom. Varying the width of the slots as shown may be beneficial in those cases wherein, for example, the increased concentration of dirt particles and the like in the pulp suspension will occur toward the top portion of the screen cylinder.

Narrowing of the slots at this level or cylinder height will decrease the dirt which might pass into the accepts flow.

It is to be noted that one advantage of the methods and resulting structure shown herein is that the slots can be provided along substantially the entire length of the precursor metal plate.

Presently, the plate material preferred for use is a hard stainless steel, such as Armco's"Nitronic 33"stainless steel. This is a low-nickel austenitic stainless steel having high resistance to wear and galling, with a Rockwell B hardness of around 95."Inconel"and precipitation hardened stainless steels may also be mentioned as exemplary. An advantage of this invention is that very hard, otherwise difficult-to machine materials may be used as the plate.

The method of this invention has the advantage of permitting the formation of slots over a substantially wide range of slot widths, such as 0.002" or less up to 0.035" or more. In one example, the width of the laser cut slots was about 0.010". The plate thickness was 0.31"and the accepts side grooves 60 (such as those shown in Fig. 5) were on the order of about 0.025". In one embodiment the slots 62 are spaced at a density of about six to a linear inch.

It will therefore be seen that by use of the methods as described above, a cylinder type screen can be formed which has screening elements that resemble wedge wires and therefore the screen itself closely approximates a wedge wire type screen but without certain of the disadvantages of such a screen including the difficulty of maintaining the wedge wires in a precisely aligned condition during manufacture and thereafter. Thus, the advantages of this screen product include the provision of continuous axial slots, precision slot width, and precision machine profiles on the inlet and outlet sides as well as high mechanical strength to permit operation at low speeds such as ten meters per second up to higher speeds such as 25 meters per second or more, depending upon consistency requirements. The fact that the material of the plate is held by welds at the end rings and reinforcing rings, means that the material removed by laser cutting or the like results in the formation of substantially independent axial bars resembling individual wedge wires that are held in spaced relation by welds between these bars and the reinforcing rings.

Also, although the invention has been described in conjunction with the cutting of the slots 62 in the plate after the plate has been formed into the desired cylindrical shape, the slots 62 can be formed in the flat plates (such as those shown in Figs. 3 and 4). As a practical matter, slot formation after imparting the cylindrical shape to the plate is preferred since the integrity of the plate will be better preserved if it is not slotted prior to rolling. Additionally, slot cutting after reinforcement of the cylinder via the rings 14,16 and 18 and even the top and bottom supports 20,22 greatly enhances the overall structural stability of the assembly and hence the dimensional accuracy of the slots.

While the processes and products herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise processes and products, and that changes may be made therein without departing from the scope of the invention.

What is claimed is: