IMPROVED SCREEN FOR SEPARATING AND CLASSIFYING PARTICULATE MATERIAL AND METHOD OF MAKING SAME

Abstract: A screeen (21) for separating and classifying smaller particles from larger particles of a particulate substance wherein the screen (21) is of a planar configuration and made from a slab of synthetic material. The screen (21) includes a plurality of spaced perforations (23) formed by cutting the slab with a pressurized stream of fluid, which method of forming produces perforations (23) of highly uniform configuration and dimensions, particularly when the slab is a composite comprising plural layers of different synthetic materials. The perforations (23) are configured to compensate for foreshortening of their top openings (27) and accelerated wear tendencies during use of the screen (21) in an inclined disposition.

Description: IMPROVED SCREEN FOR SEPARATING AND CLASSIFYING PARTICULATE MATERIAL AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally involves the field of technology pertaining to devices for separating and classifying particulate materials. More specifically, the invention relates to such a device in the form of a screen made of synthetic material and a method for making same. 2. Description of the Prior Art The field of prior art pertaining to screens and similar appliances for separating and classifying parti¬ culate materials according to their relative sizes has been quite active and is well developed. A specific application of such screens is in the mining and quarrying" " fields wherein particles of rather hard substances, such as coal, ores and rock, must under¬ go separation and classification for various purposes. Screens used for this purpose are necessarily subjected to severe wear conditions due to the abrasive nature of the hard particulates being processed. It is known to make these screens from either metal or synthetic materials. Metal -screens are made by simply drilling or punching a metal plate to provide the necessary perforations. Synthetic screens are made by molding plastic material wι > in a press under heat and pressure, with the perforations being formed either during the molding process or through subsequent drilling or punching of the molded body. An example of this technique wherein the perforations are formed during the molding process is disclosed by the Kramer U.S. Patent No. 3,843,476. The aforedescribed techniques for producing screens are characterized by fundamental disadvantages. The mechanical drilling or punching of perforations in a screen body is both time consuming and expensive. The simultaneous molding of perforations during molding of the screen body is less labor intensive and fairly econo¬ mical, but the resulting perforations are not uniform in configuration and dimensions since the synthetic material is inevitably displaced in the regions of the perforations, particularly when a composite screen body formed from plural layers of different synthetic materials is. being molded. In addition to the above considerations, another important factor having a bearing on the performance of a screen is the specific disposition and configuration of the perforations. It is known to provide a screen with spaced perforations disposed in either linear arrays or staggered rows, and wherein the perforations may be any of a variety of different transverse cross-sectional con igurations, including rectangular, round, hexagonal and variations of same. It has further been recognized as advantageous to provide each perforation with a substan¬ tially conical-shaped longitudinal cross-sectional con- figuration for defining relief angles to minimize the possibility of blinding the perforations since particu- late material is always passing through a progressively larger opening. However, when a screen is disposed in a conventional inclined position of use, the particles being processed travel downwardly along the incline and impact against the top surface of the screen. Depending upon the angle of impact, the top openings of the perfora¬ tions shall- appear foreshortened in varying degrees to the particles, and consequently become reduced in effec¬ tive screening size. Moreover, the relief angles defined by those wall portions of the perforations disposed downstream of the inclined screen are subjected to an accelerated rate of wear which exceeds that realized by their opposed upstream counterparts. SUMMARY OF THE INVENTION It is an object of the invention to overcome the aforementioned and other disadvantages of conven- tional screens for separating and classifying particulate materials by providing an improved screen of synthetic material and a method of making same. It is another object of the invention to pro¬ vide a method for efficiently and economically making a screen from synthetic materials. It is a further object of the invention to pro¬ vide an improved method for forming perforations of highly uniform configuration and dimensions in a cast or molded slab of synthetic material. It is still another object of the invention to provide an improved screen for separating and classifying particulate materials wherein the screen is specifically intended for use in an inclined disposition and is pro¬ vided with perforations specifically configured to enhance screening efficiency and extend the service life of the screen. These and other objects of the present invention are realized by providing an improved screen for separat¬ ing and classifying particulate materials wherein the screen is made by utilizing a pressurized stream of fluid to cut a plurality of spaced perforations through a slab of cast or molded synthetic material. The slab is advan¬ tageously a composite laminate comprised of plural layers of different synthetic materials, perferably a layer of curred rubber intimately bonded or vulcanized to a layer of plasticized ultrahigh molecular weight polyethylene. The forming of the perforations through the slab is accomplished by utilizing a water jet cutting tool through which a thin jet of water, under pressures of around 55,000 pounds per square inch, serves as the cutting medium. Control of the cutting system may be accomplished through the use of conventional industrial robots and com¬ puter systems. Each perforation of the screen is preferably defined by a longitudinal cross-sectional configuration in the form of an asymmetrical taper from the bottom open- ing of the perforation towards its top opening, with one side wall being perpendicular to the opposed surfaces of the screen and serving as the downstream side of the perforation. The opposed side wall is angled from the perpendicular to form a relief angle and serves as the upstream side of the perforation. The transverse cross- sectional configuration of the perforations may be round, polygonal or variations thereof, and is preferably elon¬ gated in the direction of material flow to compensate for foreshortening of the effective top opening areas of the perforations when the screen is utilized in an inclined disposition. Other objects, features and advantages of the invention shall become apparent from the following descrip¬ tion of the preferred embodiments thereof, with reference to the accompanying drawings which form a part of the specification, wherein like reference characters desig¬ nate corresponding parts of the several views. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partial sectional top view of a prior art screen used for separating and classifying smaller particles of a substance from larger particles thereof; Figure 2 is a view taken on the line 2-2 of Fig. l r particularly showing the screen in its conventional inclined position of use; - Figure 3 is a partial sectional top view of a screen according to a first embodiment of the invention; OMPI Figure 4 is a view taken along the line 4-4 of Fig. 3, particularly showing the screen in an inclined position of use; Figure 5 is a partial top view of a screen according to a second embodiment of the invention; Figure 6 is a view taken on the line 6-6 of Fig. 5; Figure 7 is a partial top view of a screen according to a third embodiment of the invention; and Figure 8 is a view taken on the line 8-8 of Fig. 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A prior art screen 1 is shown in Figs. 1 and 2 disposed in an inclined position of use, wherein parti- culate material to be separated and classified is directed onto a top surface 3 and moved downwardly thereacross in the general direction indicated by arrow A. Screen 1 is typically inclined at an angle of about 10° - 24° from the horizontal, preferably 17°, and may further be provided with a vibrating means (not shown) to facilitate the move¬ ment of particulate material across top surface 3 and through a plurality of spaced perforations 5. As shown in Fig. 2, screen 1 is made from a molded body of synthetic material, with perforations 5 being either molded in place or mechanically formed, such as through punching or drilling, after the molding of the synthetic material. The longitudinal cross-sectional configuration of each perforation 5 is substantially coni¬ cal and tapers from a bottom surface 7 towards top surface 3, thus defining a top opening 9 having the same circular configuration as its corresponding bottom opening 11 but of smaller area. The symmetrical tapering of perforations 5 in this manner from bottom surface 7 towards top surface 3 is conventionally known as providing relief angles in the walls of perforations 5 in order to prevent blinding of the latter since the particulate material being pro¬ cessed is always passing through progressively larger openings. OMPI Each perforation 5, as shown in longitudinal cross section in Fig. 2, is defined by a downstream wall 13 and an opposed upstream wall 15, with both walls 13 and 15 being disposed at relief angles of approximately 5° to 10° from the longitudinal axis of perforation 5, which axis is perpendicular to top and bottom surfaces 3 and 7. Since the taper defined by walls 13 and 15 is symmetrical, walls 13 and 15 therefore form equal relief angles. However, it has been discovered that particulate material impacting upon top surface 3 of screen 1 and traveling downstream thereacross in the direction indi¬ cated by arrow A tend to impact with greater force against downstream walls 13, with only minimum impact forces being sustained by upstream walls 15. This is of course due to the inclined disposition of screen 1, with the accelerated wear being realized by downstream walls 13 serving to reduce the service life of screen 1. When screen 1 in its inclined disposition is viewed from the top as shown in Fig. 1, top openings 9 of perforations 5 are effectively foreshortened, thereby dis¬ torting their otherwise circular configurations into sub¬ stantially elliptical configurations. This foreshortening effect becomes progressively more severe when top openings 9 are viewed from directions forming decreasing angles with respect to top surface 3. It is therefore apparent that particulate material impacting upon top surface 3 of screen 1 shall in effect encounter varying degrees of fore¬ shortening of top openings 9, depending upon the angle at which such material impacts against top surface 3. This foreshortening effect tends to hinder effective processing of particulate material by screen 1. An improved screen 21 according to a first embodi¬ ment of the invention shall now be described with reference to Figs. 3 and 4. As seen in Fig. 3, screen 21 includes a plurality of spaced perforations 23 disposed in linear arrays. Screen 21 includes a top surface 25 within which a top opening 27 of each perforation 23 shall appear circu¬ lar in configuration, notwithstanding the inclination of screen 21 at an angle of about 17° from the horizontal. As also shown in Fig. 3, a downstream wall 29 of each perforation 23 is also visible from this direction of view. This is possible because each top opening, when viewed from a perpendicular direction as indicated by arrow B in Fig. 4, is actually elliptical in configuration, with the longitudinal axis of the ellipse being disposed parallel to the direction of material flow, as indicated by arrow A. Accordingly, particulate material impacting against top surface 25 of screen 21 from the direction of view in Fig. 3 shall encounter top openings 27 of the desired circular configuration, rather than in the foreshortened configura¬ tion previously described with reference to prior art screen 1. As seen in Fig. 4, each downstream wall 29 is disposed substantially parallel to top surface 25 and a bottom surface 31, thereby defining a zero relief angle. Each downstream wall 29 has an opposed upstream wall 33 which is disposed in a relief angle of about 5° to 10° from the longitudinal axis of each perforation 23. In this configuration, walls 29 and 33 still converge towards top surface 25, but in the form of an asymmetrical taper. Since downstream walls 29 do not define relief angles, they present surfaces that are more wear resistant to the impact of particulate material being processed, thereby providing enhanced perforation control as downstream walls 29 recede from wear. This situation greatly extends the overall service life of screen 21. It is understood that each top opening 27 of each perforation 23 has an opposed bottom opening 35 of sub¬ stantially the same configuration, in this case elliptical, with the area of top opening 27 being smaller than that of bottom opening 35 because of the relief angle defined by upstream side 33. Therefore, the transverse cross- sectional configuration of each perforation 23 shall be elliptical and increase in area from top opening 27 down to bottom opening 35. Openings 27 of perforations 23 have been described as being elliptical for the purpose of compensating for the foreshortening effect so that openings 27 shall ultimately appear circular when screen 21 is used in an inclined disposition of about 10° - 24° from the horizontal, though preferably 17°. Nevertheless, top openings 27 may assume other geometric configurations including hexagonal, rectangular or the like, which are elongated up to about 10% when viewed in the direction " of arrow B but assume substantially their standard geometric proportions when viewed in the general directions of par- ticulate material flow against top surface 25 of screen 21. A second embodiment of the invention shall now be described with reference to Figs. 5 and 6. A screen 41 according to this embodiment is provided with a plurality of spaced perforations 43 disposed in a staggered array with each perforation 43 including a hexagonal-shaped top opening 45 and a corresponding hexagonal-shaped bottom opening 47. A downstream wall 49 of each perforation 43 is disposed perpendicular to a top surface 51 and a bottom surface 53 of screen 41. Each perforation 43 further includes an upstream wall 55 disposed at an angle of about 5° to 10° from the longitudinal axis of perforation 43 to define a relief angle. Accordingly, downstream and upstream walls 49 and 55 form an asymmetrical taper which converges towards top surface 51. Though the transverse cross-sectional configuration of each perforation 43 is that of a regular hexagon, it is preferred that such con¬ figuration be elongated approximately 10% to compensate for the foreshortening effect when screen 41 is utilized in an inclined disposition. With specific reference to Fig. 6, screen 41 is made from a composite of layers of different synthetic materials, including a first layer 57 and a second layer 59 bonded to each other through vulcanizing or chemical bonding, such as gluing. A preferred method of bonding may be vulcanizing as taught by the aforementioned Kramer U.S. Patent No. 3,843,476, the entire disclosure of which is incorporated herein. Layer 57 is preferably thicker than layer 59, with the upper surface of the former layer defining top surface 51 of screen 41. It is further preferred that layer 57 be of elastomeric material, such as rubber or the like. Layer 59 is preferably of a syn¬ thetic polymer, such as polyethylene, and of a thickness equal to about 1/3 to 1/2 the total thickness of screen 41. Screen 41 may be made by disposing a first layer of ultrahigh molecular weight polyethylene powder into the bottom of a platen press mold cavity. A sheet of uncured rubber is then laid over the polymer powder. Subsequent application of heat and pressure to the mold serves to cure the rubber and plasticize the polymer, thereby resulting in an intimate bonding of the two resulting layers. A third embodiment of the invention shall now be described with reference to Figs. 7 and 8. In this embodi¬ ment, a screen 61 includes a plurality of spaced perfora¬ tions 63 disposed in a staggered array. Each perforation 63 has a substantially rectangular transverse cross- sectional configuration and includes a top opening 65 and a corresponding bottom opening 67. Screen 61 is also preferably formed of different synthetic materials and includes a first layer 69 of cured rubber bonded to a second layer 71 of plasticized polyethylene. Each perfora¬ tion 63 is provided with a downstream wall 73 and an up- stream wall 75. Both walls 73 and 75 are disposed in relief angles of approximately 5° to 10° from the longi¬ tudinal axis of each perforation 63. Accordingly, walls 73 and 75 converge towards a top surface 77 in a symmetri¬ cal taper. It is further understood that the transverse cross-sectional configuration of each perforation 63 may also be elongated up to about 10% in the direction of material flow when screen 61 is utilized in an inclined disposition. The screens according to the above embodiments of the invention have been described as being made from synthetic materials, preferably plural layers or different layers of synthetic materials bonded together to define a composite laminate. The formation of the required spaced perforations in a screen made of synthetic material has heretofore been accomplished by either directly molding perforations during molding of the synthetic material or by mechanical drilling or punching the perforations after the screen body has been molded. As previously indicated, the latter approach is both expensive and labor intensive. The formation of the perforations during the molding of the screen body does not produce perforations of uniform configuration and dimensions because the material being molded has the tendency to be displaced in the regions of the perforations. This problem is particularly acute when plural layers of different synthetic materials are simultaneously molded to form a composite screen body. In such case, the respective thicknesses of the indivi- dual layers would not be uniform and produces variations in cross-sectional appearance. These variations have been identified with accelerated degradation of the screen during use. An important aspect of the present invention resides in an improved method for making a screen from synthetic materials. More specifically, this method permits the efficient and economical forming of the required perforations in a screen body formed from one or more layers of synthetic material. Moreover, the method produces perforations of highly uniform and con¬ trolled configuration and dimensions, and especially without the displacement of the synthetic material in the regions of the perforations. The method of this invention is accomplished by cutting perforations in a body of synthetic material by the use of a pressurized stream of fluid, such as water. The use of pressurized fluids for cutting purposes is known and generally involves utilizing a thin jet of water subjected to pressures of about 55,000 pounds per square inch as the cutting tool. The use of such water jets for cutting the perforations in a slab of synthetic material for making a screen in the practice of the present invention results in a screen having perforations OMPI of extremely uniform configuration and dimensions, charac¬ terized by sharp edges, and without displacement of dif¬ ferent layers of synthetic materials in the regions of the perforations. Through the use of such pressurized water jets, precise control can be exercized over the actual cutting of the perforations, thereby producing consistent and predictable results. Water jet cutting tools used in the practice of the invention may be any known in the art and deemed suitable for cutting the synthetic materials forming the screen body. Fluid jet cutting systems of this type are disclosed by the Leslie et al U.S. Patent No. 3,978,748 and Pearl U.S. Patent No. 4,312,254, the entire disclo¬ sures of these patents being herein incorporated by reference. Moreover, suitable pressurized fluid cutting tools and control systems therefor may be those manufac¬ tured by Flow Systems, Inc., 21414 68th Avenue, South Kent, Washington 98031 under the trademarks Waternife TM, Instajet ™, and Jetdrill ™. While the invention has been described and illustrated with reference to certain preferred embodi¬ ments thereof, it shall be understood and appreciated that various modifications, changes, additions, omissions and substitutions may be resorted to by those skilled in the art and considered to be within the spirit and scope of the invention and the appended claims. OMPI

Claims: CLAIMS: 1. A screen for separating and classifying smaller particles of a substance from larger particles thereof, wherein the screen is of a substantially planar configuration including a top surface, a bottom surface and a plurality of spaced perforations of substantially uniform configuration and dimensions therethrough for passing the smaller particles through the screen, charac¬ terized in that substantially all of the perforations each includes a longitudinal cross-sectional configura¬ tion defined by: a) a straight wall disposed substantially perpendicular to the top and bottom surfaces; b) an inclined wall disposed opposite the straight wall for defining a relief angle; and c) wherein both walls form an asymmetri- - cal taper converging towards the top surface. 2. The screen of Claim 1 characterized in that the screen is of synthetic material. 3. The screen of Claim 2 characterized in that the screen is formed of plural layers of different synthetic materials. 4. The screen of Claim 2 characterized in that the screen includes a composite laminate defined by a layer of polymer material and a layer of elasto- meric material. 5. The screen of Claim 4 characterized in that the polymer material includes plasticized ultra- high molecular weight polyethylene and the elastomeric material includes cured rubber. OMPI 6. The screen of Claim 1 characterized in that substantially all the perforations are formed in the screen by means of a pressurized stream of fluid. 7. The screen of Claim 1 characterized in that the transverse cross-sectional configuration of each perforation is substantially circular. 8. The screen of Claim 1 characterized in that the transverse cross-sectional configuration of each perforation is substantially rectangular. 9. The screen of Claim 1 characterized in that the transverse cross-sectional configuration of each perforation is substantially elliptical. 10. The screen of Claim 1 characterized in that the transverse cross-sectional configuration of each perforation is substantially hexagonal. 11. The screen of Claim 1 characterized in that the transverse cross-sectional configuration of each perforation is substantially that of an elongated polygon. 12. The screen of Claim 1 characterized in that the spaced perforations are disposed in linear arrays. 13. The screen of Claim 1 characterized in that the spaced perforations are disposed in a staggered array. 14. A method of making a screen from synthetic material for use in separating and classifying smaller particles of a substance from larger particles thereof, chacterized by: a) providing a substantially planar- shaped slab of synthetic material; and b) cutting a plurality of spaced perfora¬ tions through the slab of synthetic material with a pres¬ surized stream of fluid. 15. The method of Claim 14 chacterized in that the slab of synthetic material includes plural layers of different synthetic materials. 16. The method of Claim 14 characterized in that the cutting step is carried out to form each per¬ foration so that at least a portion of its interior wall defines a relief angle for preventing blinding of the perforations by the particles being passed therethrough. 17. The method according to Claim 16 charac¬ terized by forming the slab of synthetic material by: a) disposing a first layer of polymer powder in a platen press mold; b) disposing a second layer of uncured elastomeric material onto the first layer of polymer powder; and c) applying heat and pressure to cure the elastomeric material and plasticize the polymer powder to produce an intimate bond between the elasto¬ meric and polymer materials, to thereby produce a com¬ posite elastomeric-polymer slab. 18. The method of Claim 17 characterized in that the polymer powder includes ultrahigh molecular weight polyethylene and the elastomeric material in¬ cludes uncured rubber. 19. The method of Claim 14 characterized in that: a) the slab of synthetic material in¬ cludes a top surface and a bottom surface; and b) the longitudinal cross-sectional con¬ figuration of each perforation includes: i. a straight wall disposed substan¬ tially perpendicular to the top and bottom surfaces, and ii. an inclined wall disposed oppo¬ site the straight wall for defining a relief angle, with both walls forming an asymmetrical taper converging towards the top surface. 20. The method of Claim 14 characterized in that the transverse cross-sectional configuration of each perforation is substantially circular. 21. The method of Claim 14 characterized in that the transverse cross-sectional configuration of each perforation is substantially rectangular. 22. The method of Claim 14 characterized in that the transverse cross-sectional configuration of each perforation is substantially hexagonal. 23. The method of Claim 14 characterized in that the transverse cross-sectional configuration of each perforation is substantially elliptical. OMPI 24. The method of Claim 14 characterized in that the transverse cross-sectional configuration of each perforation is substantially that of an elongated polygon. 25. The method of Claim 14 characterized in that the spaced perforations are disposed in plural linear arrays. 26. The method of Claim 14 characterized in that the spaced perforations are staggered. furor OMPI

Inventors: CARLSON JAMES D, COLUCCI STEPHEN L, CARLSON, JAMES, D., COLUCCI, STEPHEN, L.
Applicants: NAT STANDARD CO, NATIONAL-STANDARD COMPANY
Application year: 1984
Main classification: B32B3
IPC codes: B07B1, B26F3, B32B3