Patent Publication Number: US-3874333-A

Title: Preventing edge wrap-around in one-side electrostatic coating

Description:
[ Apr. 1, 1975 1 PREVENTING EDGE WRAP-AROUND IN ONE-SIDE ELECTROSTATIC COATING [75] Inventor: James N. Baker, McKees Rocks, Pa.  
 [73] Assignee: National Steel Corporation,  
 Pittsburgh, Pa.  
  221 Filed: Sept. 28, 1972 21 App1.No.:293,088  
 [56] References Cited UNITED STATES PATENTS 2,915,039 12/1959 Wardley ..118/504 3,359,941 12/1967 3,490,115 1/1970 Owens et a1. 118/621 10/1972 Fabre et a1 118/624 1/1973 Winkless l17/17.5  
 Primary Examiner-Mervin Stein Assistant ExaminerLeo Millstein Attorney, Agent, or FirmShan1ey, ONeil and Baker [5 7] ABSTRACT In electrostatic operations for coating one side of continuous strip, a system is provided for preventing particulate coating material from wrapping around the edge of the strip and depositing on the side of the strip which is desired to be left free of coating material. Edge wrap-around is prevented by provision of barriers which are mounted along each edge of the strip in contact with the strip edges, and which are at the same electrical potential as the strip. The barriers are mounted for sidewise movement, and are yieldably urged into contact with the strip edges. Hence, the barriers move sidewise with the strip as it wanders from a centered pass line position, yet the barriers maintain continuous contact with the strip edges.  
 8 Claims, 3 Drawing Figures SHEET ATENTED APR 1 5 PREVENTING EDGE WRAP-AROUND IN ONE-SIDE ELECTROSTATIC COATING BACKGROUND OF THE INVENTION In some continuous-strip, electrostatic coating lines, it is desired to apply particulate coating material to only one side of the strip. In such lines, there is a problem in that there is a tendency for the particulate coating material to wrap around the edge of the strip and deposit on the side of the strip which is desired to be left free of coating material. This problem is overcome by the present invention.  
  Other advantages of the invention will appear from the following detailed description which, in connection with the accompanying schematic drawings, discloses a preferred embodiment of the invention for purposes of illustration only and not for definition of the limits of the invention.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view illustrating an electrostatic coating system embodying principles of the invention.  
  FIG. 2 is a cross-sectional view taken on a plane designated by line 2-2 of FIG. 1.  
 FIG. 3 is a top view of apparatus depicted in FIG. 1.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, continuous metallic (e.g., steel) strip is unwound from coil 12 which is supported by uncoiler 14. The strip is pulled in the direction indicated by arrow 16 along a pass line through coating chamber 18 by a conventional recoiler (not shown). The walls of the coating chamber are schematically indicated by phantom lines 20. The strip moves into and out of chamber 18 through entry and exit openings in the chamber walls. In coating chamber 18, the strip moves through an electrostatic coating region 22 in which a coating of metallic (e.g., chromium) powder is deposited on the top surface of the strip. The strip in conducted through the chamber by transport structure including a plurality of knife&#39;edge rollers 24 (FIGS. 2, 3). Rollers 24 are carried by shafts 26, 28 which are rotatably mounted in bearings carried by the chamber walls, and which extend transversely to the direction of travel of the strip.  
  Electrostatic coating region 22 is a zone between a carriage 30 in the coating chamber and the top surface of the strip. The planview boundaries of the coating region 22 are generally defined by the peripheral outline of carriage 30. Carriage 30 includes a pair of a spaced-apart, parallel frame members 32, 34 which support a plurality of charging wires 36 spaced above the top surface of the strip. Charging wires 36 are parallel to one another, are coplanar, and are spaced along the direction of travel of the strip. The charging wires are electrically connected to one another in parallel circuit relationship by conductor 38, and the charging wires are electrically connected by conductor 40 to one electrical side of power source 42. The other side of power source 42 is electrically connected to ground potential by conductor 44, and the circuit for the electrostatic coating apparatus is completed by connection 46 to uncoiler l4. Connection or conductor 46 electrically connects strip 10 to ground potential.  
  At the entry end of the coating chamber, discharge nozzle 48 is positioned above strip 10. Nozzle 48 is cen- LII tered with respect to centerline 50 (FIG. 3) of the strip pass line. Nozzle 48 is fed by conduit 52 (FIG. 1) with a pressurized aerosol composed of metal powder and gas (e.g., air) and the nozzle emits a cloud of metal powder into coating region 22 between the top surface of the strip and the charging wires. The aerosol is formed by blower 54 to which is fed a supply of metal powder from hopper 56. For example, the powder can be minus 200 mesh in particle size.  
  The high potential (e.g., 20 Kv) from power source 42, which is applied to charging wires 36, and the ground potential of the strip, establish an electrical field between he charging wires and the strip. The particles of metal powder discharged from nozzle 48 are propelled by the electrical field to impinge and collect on the top surface of the strip. This occurs substantially throughout coating region 22 in a manner well known in the art. It will be appreciated that the details of such powder coating systems, including strip pre-treatment and post-treatment, are conventional and form no part of the present invention. Such details are shown in e.g., US. Pat. No. 3,575,138, of common assignee.  
  As indicated herein above, prior one-side electrostatic coating systems were subject to the disadvantage that the powder particles tended to wrap around the two opposite edges of the strip and deposit on the marginal edge portion of the bottom surface, where coating is unwanted. This is avoided in accordance with the present invention by the provision of barrier means in the coating region for preventing particulate coating material from bypassing the edges of the strip and depositing on the bottom surface of the strip. In the embodiment illustrated, the barrier means include barrier members in the form of rectangular plates 58, 60. Plates 58, 60 extend along the respective opposite edges of the strip, and are connected by conductors 61, 64 respectively to ground potential (the same potential as strip 10). Each plate extends essentially the entire length of the coating region. The plates are of sufficient width to accomplish their function of barring coating powder from bypassing the edges of the strip and depositing on the bottom surface, and for example can be 6inch wide.  
  Plates 58, 60 are mounted in contact with the respective strip edges by structure which supports the plates for movement tranversely to the direction of travel of the strip. The movable support structure for each plate includes a pair of bearing members 62, 64 which are respectively joined to the opposite ends of the plate in supporting relation to the plate by stub sections of structural angle. Each bearing is carried by one of the roller shafts, and is movable back and forth along the respective shaft. Strip tends to Wander sidewise from a centered pass line position in continuous processing lines, and the movable mounting structure allows the plates to move laterally with the lateral movement of the strip under the laterally directed forces applied by the strip.  
  Plates 58, 60 are yieldably urged into contact with the respective strip edges by pressure means including flexible tension members in the form of chains 66. Each chain 66 joins a pair of bearings which are on the same shaft and at opposite edges of the strip. For example, the left-side chain 66 as viewed in FIG. 1 joins the pair of bearings 62 (FIG. 2) which are axially slidably mountd on shaft 26. A freehanging weight 68 is carried by each chain 66 at a location between the joined bearings and yieldably applies to each bearing and thus to each of plates 58, 60, a force having a component directed inwardly toward the center of the pass line. With the movable mounting and yieldable urging structure described, plates 58, 60 are urged into continuous contact with the respective strip edges to prevent edge wrap-around at all times, irrespective of wandering of the strip from its centered pass line position, yet the strip is free to wander without binding between plates 58, 60.  
  Since each plate 58, 60 is generally coplanar with strip 10, each plate carries at each end a pairof respectively upwardly and downwardly projecting pegs 70, 72 which prevent the strip from overriding the top or bottom of the plate. The pegs are contiguous to the sides of the plates which are in contact with the strip edges.  
  A summary of operation of the system will be initiated with the assumption that the strip is traveling on its centered pass line position and barrier plates 58, 60 are in contact with the strip edges under the influence of the inwardly directed forces applied by the weight and chain arrangement. Coating powder is ejected in a cloud from nozzle 48 and the powder impinges and collects on the strip under the influence of the electrical field. Coating particles are prevented by the barrier plate electrodes from bypassing the edges of the strip and depositing on the strip bottom surface. Instead, powder particles which would normally bypass the strip edges are collected on the barrier plates. If the strip wanders to the left of its centered pass line position, the strip applies laterally outwardly directed forces to plate 60, and the movable mounting of the plate allows it to move to the left with the strip. The laterally inwardly directed forces applied to the plate 60 by the chain and weight arrangement allow such yielding, yet still maintain inwardly directed forces on the plate. These forces assure that plate 60 remains in continuous contact with the strip edge. At the opposite edge of the strip, the inwardly directed forces applied to plate 58 by the weight and chain arrangement cause the plate to move to the left with the leftward movement of the strip, thereby maintaining continuous contact with the strip edge to avoid formation of a gap between strip edge and plate which would allow bypass of powder particles. The movable mounting of plate 58 allows such action to take place. Should the strip wander to the right, plates 60 and 58 move to the right with the strip. In such case plate 58 yields for the rightward movement of the strip in the manner described for plate 60 in the case of leftward movement. Plate 60 follows rightward movement of the strip in the manner described for plate 58 with respect to leftward movement.  
  Electrostatic coating systems in accordance with the invention are highly advantageous. Edge wraparound of powder particles is avoided while the movable mounting of the barriers and the yieldable pressures applied to the barriers assure that prevention of edge wrap-around is effected notwithstanding wandering of the strip from a centered pass line position. And, the strip is free to wander without interference from the barriers. The rectilinear side edges of the plates are in continuous contact with the moving strip edges at all times and the connection of the plates to the same electrical potential as the strip assures that the strip will have no greater attraction for the powder particles than the plates, to further promote the prevention of edge bypass of the powder particles.  
  While the invention has been described in connection with a preferred embodiment, many modifications can be made as will be readily apparent to those skilled in the art. For example, strips of materials other than steel can be used. The coating material need not be chromium; other powder materials, metallic (e.g., aluminum) and non-metallic, can be used. And, other yieldable pressure means, e.g., springs or hydraulic systems, can be used. Many other modifications can be employed. Accordingly, reference will be made to the appended claims for definition of the scope of the invention.  
 I claim 1. Continuous-strip electrostatic coating apparatus, comprising means defining an electrostatic coating region,  
 transport means for conducting a continuous strip having opposite surfaces and having opposite edges along a direction of travel through the coating region for deposition of particulate coating material on one of the surfaces of the strip,  
 means for connecting the strip to a source of electrical potential, and  
 barrier means in the coating region for preventing coating material from bypassing the edges of the strip and depositing on the other surface of the strip,  
 the barrier means including a barrier member extending along each edge of the strip, mounting means for mounting the barrier members in contact with the respective strip edges, and means for connecting the barrier members to the source of electrical potential of the strip, the mounting means including movable means supporting the barrier member for movement transversely to the direction of travel of the strip to accommodate movement of the strip transversely to the direction of travel, and pressure means for yieldably urging the barrier members into continuous contact with the strip edges.  
 2. Apparatus as defined in claim 1,  
 each barrier member being a plate member generally coplanar with the strip.  
 3. Apparatus as defined in claim 1,  
 each barrier member including override-prevention means projecting transversely relative to the plane of the strip for preventing the strip from overriding the barrier member.  
 4. Apparatus as defined in claim 3,  
 each barrier member being a plate member generally coplanar with the strip,  
 the override-prevention means including peg members projecting in opposite directions from the plate member.  
 5. Apparatus as defined in claim 3,  
 the transport means including a plurality of roller shafts extending transversely to the direction of the travel of the strip,  
 the movable means including barrier means carried by the shafts and movable along the shafts,  
 the barrier members being supported by the bearing means.  
 6. Apparatus as defined in claim 5 the pressure means including flexible tension means joining at least two bearing means which are located at opposite edges of the strip, and  
 means acting on the flexible tension means to place tension on the flexible tension means for urging the two bearing means toward each other.  
 7. Apparatus as defined in claim 5,  
 the pressure means including flexible tension means joining at least two bearing means which are located at opposite edges of the strip, and  
 weight means carried by the flexible tension means at a location between the joined bearing means.  
 8. Continuous-strip electrostatic coating apparatus,  
 comprising means defining an electrostatic coating region,  
 means for conducting a continuous strip having opposite surfaces and having opposite edges along a direction of travel through the coating region for deposition of particulate coating material on one of the surfaces of the strip,  
 means for connecting the strip to a source of electrical potential, and  
 barrier means in the coating region for barring coating material from bypassing the edges of the strip and depositing on the other surface of the strip,  
 the barrier means including a barrier member extending along each edge of the strip,  
 mounting means for mounting the barrier members in contact with the respective strip edges,  
 the mounting means including movable means supporting the barrier members for movement transversely to the direction of travel of the strip to accommodate movement of the strip transversely to the direction of travel, and  
 means for yieldably urging the barrier members into continuous contact with the strip edges.