US3496082A - Electrophoretic coating method and apparatus utilizing bath circulation to minimize impurities - Google Patents

Description

Feb. 17, 1970 D. E. OREM ETAL 3,49

ELECTROPHORETIC COATING METHOD AND APPARATUS UTILIZING BATH CIRCULATION T0 MINIMIZE IMPURITIES Filed Oct. 19, 1964 3 Sheets-Sheet 1 INVENTOR. m Donald E. Orem Norbert M. Zupon Feb. 17, 1970 o. E. OREM ETAL 3,496,082

ELECTROPHORETIC COATING METHOD AND APPARATUS UTILIZING BATH CIRCULATION TO MINIMIZE IMPURITIE S Filed Oct. 19. 1964 3 Sheets-Sheet 2 4 1 I '1 l ooooooo w \u 0 o 00 0o wm k oooooooooo INVENTOR Donald E. Orem Norbert M. Zupun Feb. 17, 1970 E. OREM Er AL 3,496,982

ELECTROPHORETIC comma METHOD AND APPARATUS UTILIZING BATH cxncumwxon '10',MINIMIZE mmmnxms Filed 001:. 19, 1964 3 Sheets-Sheet 5 was; P36

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m Donald E. Orem I Norbert M. Zupon United States Patent 3,496,082 ELECTROPHORETIC COATING METHOD AND AP- PARATUS UTILIZING BATH CIRCULATION T0 MINIMIZE IMPURITIES Donald E. Orem and Norbert M. Zupan, Indianapolis,

Ind., assignors to Ransburg Electro-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed Oct. 19, 1964, Ser. No. 404,624 Int. Cl. C23b 13/00; B01k 5/02 US. Cl. 204181 16 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved method and apparatus for electrically coating with a solid material, conductive parts which are supported from overhead into a liquid bath, and more particularly, an electrophoretic coating method and apparatus in which the liquid bath is circulated to provide coating material with a minimum of impurities but with adequate solids content, adjacent the article to be coated while stabilizing the position of a suspended article with respect to the adjacent electrodes.

Electrophoretic coating systems are well known in which resinous materials are deposited from liquid baths by passing an electric current between the article being coated and adjacent metallic electrodes while the article to be coated and the electrodes are immersed in the resinous material. To minimize the impairing effect of deposition from depleted coating material on the deposited film, agitation of the coating material in an electrophoretic coating system is necessary. By agitation the depleted material adjacent the article to be coated is continuously replaced by fresh coating material of adequate solids concentration. Agitation of the coatng material, however, may produce forces upon the article suspended in the bath that move it and increase the likelihood of direct contact between the article and the electrodes or tank. In addition, the forces due to such agitation make the maintenanc of a sliding electrical contact with the article being coated more difficult to achieve.

Impurities which may be present in the bath must be removed to prevent them from being included in the deposited film, particularly where the probability of their being adjacent the electrode is increased by agitation. Many of the impurities found in an electrophoretic bath are less dense than the liquid coating material and consequently are present adjacent the surface of the bath.

In the method of this invention the liquid coating material is agitated in a controlled manner to maintain undepleted coating material adjacent the articles, to produc counteracting fluid forces upon the suspended articles, and to move the coating material that is more likely to be contaminated in such a manner that it may be cleansed of its impurities. To accomplish this method the coating material is continuously circulated within the bath to produce on both sides of the bath equal rotational flows around generally horizontal axes. The coating material flows thus produced merge adjacent the path of the article throughout the bath insuring that fresh coating material is provided as deposition takes place and producing fluid forces on the article which counteract.

Because the fluid forces nullify themselves, the article may be suspended without swaying. Thus, with the position of the article stabilized, the possibility of direct contact with an electrode which would short circuit the power supply of the system or at least damage the deposited film is reduced. Stabilization of the suspended article permits it to be coated in a smaller bath than would otherwise be possible, reducing the cost of the materials present in the bath and allowing the maintenance of more uniform coating conditions in the bath. With the article stabilized, a sliding contact to the suspended article can be maintained more easily without interruption of the connection and resultant sparking. Since an electrophoretic coating system can produce flammable vapors or gases, any sparking above the bath can be a safety hazard. Controlled circulation thus contributes to a safe system. Likewise, turning olf the voltage prior to making the electrical connection with the article eliminates sparking at the liquid bath.

Since many impurities collect adjacent the surface, the surface fluid is urged to flow over a weir into a collecting trough by adding excess coating material to the bath and using the weir to control the level of the bath. The resultant flow adjacent the surface of the bath carries many impurities into the collecting trough from which the contaminated coating material is removed for filtering. The rotational flows conjoined with appropriately located weirs can create such a surface flow themselves, or they can act in combination with a horizontal flow which is uniform throughout the tank to create a pair of spiral-like flows moving toward the overflow. Further, by placing the overflow weir at the end of the bath near the point where the articles emerge, the resultant surface flow through which the articles pass as they emerge is relatively swift, and the impurities present will not cling to the article. If the article has been subjected to the coating action in the bath for a proper length of time, deposition has essentially ceased when it is withdrawn from the bath. Similarly, by introducing the filtered coating material into the bath adjacent to, and directed along, the surface of th bath near the point where the articles are immersed and coating begins, a flow of pure coating material is provided where the rate of deposition is the highest, thus reducing the likelihood of impurities being incorporated into the deposited film.

An object of this invention is, therefore, to provide an electrophoretic coating method and apparatus to produce a coating from undepleted coating material upon a movably supported article while stabilizing the article.

Another object of this invention is to remove contaminants and impurities from the coating material of an electrophoretic coating system and to reduce the possibility of their being included in the deposited film.

Still another object is to produce an electrophoretic coating method and apparatus in which a coating can be obtained without the danger of igniting volatile solvents or combustible gases that may be present because of the coating operation.

A further object of this invention is to provide an electrophoretic coating method and apparatus in which the likelihood of a contact between the article being coated and the oppositely charged electrode which may damage the deposited film or short circuit the system power supply is greatly reduced.

A still further object is to reduce forces on a movably supported article that would tend to interrupt the electrical connection between the article support and the system power supply.

Other objects and advantages of this invention will be apparent from the following description and the drawings in which:

FIGURE 1 is a perspective view of one form of our apparatus for electrophoretically depositing coating materials on articles to be coated;

FIGURE 2 is a longitudinal sectional view of the electrophoretic coating tank of FIGURE 1; showing a uniform horizontal flow throughout the cross section of the tank and a surface flow;

FIGURE 3 is a cross-sectional view of the electrophoretic coating tank showing the counter-rotating flows in each side of the tank;

FIGURE 4 is a cross-sectional view of the electrophoretic coating tank showing the perforated weir and the level overflow weir at section 4-4 from FIGURE 2;

FIGURE 5 is the longitudinal view, partially in section, of another form of electrophoretic coating tank;

FIGURE 6 is a cross-sectional view of the electrophoretic coating tank of FIGURE 5 showing the stand pipes and liquid impellers at section 66 of FIGURE 5;

FIGURE 7 is a cross-sectional view of an electrophoretic tank showing a common opening for the stand pipes on each side of the tank, and a deflecting vane for splitting the flow and directing it adjacent the bottom of the tank; and 7 FIGURE 8 is a cross-sectional view of another form of this invention;

FIGURE 9 is an elementary schematic diagram including the electrical control circuit.

An embodiment of my invention is shown in FIGURE 1, where articles 10 are being carried by grounded overhead conveyor 11 for immersion in electrophoretic coating tank 12. The articles 10 are supported from the conveyor by insulated hangers 13 which carry an article connecting means 14. As the conveyor carries the article forward it moves down the inclined portion of the conveyor and is gradually immersed into the liquid coating material. Hanger 13 trips switch 15 disconnecting power supply 16 from the article charging bus bar 17 and the grounded metallic electrophoretic tank 12 which serves as the other electrode. While the voltage is disconnected, article connecting means 14 engages the article charging bus bar 17. This connection being made, article hanger 13 trips switch 1 8 reconnecting the power supply to the article charging bus bar 17 and the electrophoretic tank 12 to cause electric current to flow between the tank and the article 10 as well as other preceding articles in the liquid toreinitiate the coating action. Electrical switches 15 and 18 may be located at the article charging bus bar or, as shown, remotely from the bus bar but positioned to be actuated in synchronism with the electrical connections being made at the bus bar.

Level overflow weir 19 is provided to prevent the collection of contaminating material on the surface of the liquid within the tank. Additionally, weir 19 maintains a constant liquid level to insure total immersion of the part. The coating material flows over weir 19 into the trough generally designated 27. It is removed from this trough through conduit 28 by means of pump 20, forced through filter 21 to remove contaminates and back into the tank through conduit 29. This conduit enters the tank at its side and directs the flow of filtered coating material along the surface of the coating material throughout the area where the article is immersed to'minimize the contaminants in this area. To insure that undepleted coating material is provided at all times adjacent the articles to be coated, pump 22 continuously circulates the coating material in the tank by pumping the coating material through pipe 23 into a fluid dispensing pipe 31 which lies along the bottom of the electrophoretic tank in a manner that is more completely described herein.

In practicing our invention it is important that the material within the tank be circulated to prevent the pigment and resin materials in solution from settling, but more importantly, to insure that fresh material of adequate solids concentration is provided at all times adjacent all surfaces of the article to be coated. FIG- URES 2 and 3 indicate the circulation in one design of a tank in which these functions can be accomplished. The tank 12 is provided wall portions 24 which are sloped to allow the article to be coated to be immersed in the tank when supported from a sloping conveyor section 11a (FIGURE 1). At the other end of the tank, perforated flow-control weir 25 is similarly sloped to allow the articles to be removed by the ascending sloping portion of the conveyor. Flow-control weir 25 is perforated with a pattern of holes 2 6 so arranged that a uniform flow of coating material is maintained throughout the cross section of the electrophoretic tank from top to bottom. FIGURE 4 shows the arrangement of the holes 26 in the flow-control weir 25 and the attached overflow weir 19 as viewed from the line 4-4 in FIG- URE 2. The holes are arranged so that they are equally spaced horizontally but in a pattern with more holes located near the surface of the bath than near its bottom where the suction orifice is located.

Thehole pattern necessary to achieve uniform flow depends upon the cross-sectional shape of the tank and upon the head loss across the perforated weir created by the circulating pump. For example referring to FIGURE 4, with a tank 16 inches in Width and 24 inches in depth with the topmost 17 inches of each side parallel, the following pattern of /8 inch diameter holes has been found to produce uniform flow at flow rates of 20 to 40 gallons per minute; ten holes symmetrically spaced about the center line of the weir on 1 inch centers 4 inches below the edge of the weir, seven holes on 2 inch centers 8 inches below the edge of the weir with the hole farthest to the left aligned with the left-most hole of the upper row of holes, seven holes on 2 inch centers 12 inches below the edge of the weir with the hole farthest to the light aligned with the right-most hole of the upper row of holes, six holes on 2 inch centers arranged symmetrically about the center line 16 inches below the edge of the weir, three holes on 3 inch centers 20 inches below the edge of the weir arranged symmetrically about the center line of the weir, and two holes on 3 inch centers arranged symmetrically about the center line 23 inches below the edge of the weir.

The material within the tank is continuously agitated by means of pump 22. After flowing through the holes in flow-control weir 25, pump 22 draws the coating material through conduit 30 into conduit 23 where it is forced into the fluid distributing pipe 31 at the bottom of the tank. Fluid distributing pipe 31 is provided with a series of holes 32 arranged along the bottom of its periphery so that the fluid expelled through these holes impinges on the bottom of the tank creating a washing action which remixes any coating material constituents which tend to settle out of the coating material. The electrophoretic tank 12 isprovided with a rounded bottom section 12a (see drawings 1 and 3) avoiding areas sheltered from the flow so that the material expelled through the series of holes 32 can keep the entire surface freeof sediment. Circulation of the coating material within the tank is further provided by another series of holes generally designated as 33 which are arranged on both sides of the fluid distributing pipe 31 so that the material issuing therefrom impinges on each side of the tank creating counter-rotating flows about a generally horizontal axis on each side of the tank. These flows merge near the center where the articles are being coated. This flow, in addition to providing fresh coating material adjacent the article, provides stability in the position of the article by producing counteracting fluid forces upon it. This reduces the chance of intermittent contact and the resultant sparking between the article connecting means 14 and the article charging bus bar 17 due to movement of the article. The coating material is supplied to fluid distributing pipe 31 at the end of the tank into which the uncoated articles are introduced thus producing thegr'eatest circulation of coating material at the point where the coating rate is at the highest. The agitation created by the coating material expelled through the holes 32 and 33 decreases along the length of the tank as the pressure created by pump 22 within fluid distributing pipe 31 diminishes along the length of the tank.

Thus as shown in the tank of FIGURES l, 2, 3 and 4 with a length of 50 inches between the bottoms of the sloping end portion and perforated weir, a width of 16 inches between the parallel sides and an overall depth of 24 inches, the perforated fluid dispensing pipe 31 can be made of 2 inch, schedule 40 pipe. It is closed at the end adjacent the perforated weir and is drilled with three rows of 7 inch holes along its length. The holes in each row are on 3 /2 inch centers and the rows of holes are arranged about the periphery of the pipe so that one row of holes 32 is adjacent the bottom of the tank and the rows of holes 33 are angularly disposed about the pipe circumference at 120 spacing from the bottom row of holes 32. With this fluid dispensing pipe and the previously described perforated weir, satisfactory results are obtained if the coating material is circulated by a horsepower motor driving a 50 gallon per minute, centrifugal pump which is designed to operate against a head of feet of water. Other configurations of apparatus can, of course, be devised to practice this invention.

Another form of our invention is shown in FIGURES 5 and 6. By constructing the electrophoretic tank with a series of stand pipes 34 along each side of the tank with openings 35 in the sides of the tank below the surface of the coating material and openings 36 at the bottom of the tank, it is possible to create a circulation in the tank which will converge adjacent the article path and replenish the depleted material adjacent the article being coated while producing counteracting fluid forces upon the article. Electric motors 37 rotates shafts 39 and impellers 40 through rotating seals 38 located at the top of the stand pipes where they are exposed to a minimum of pressure. The impellers 40 can be rotated to create a flow through the draft tubes in either direction. By adjusting the flow produced by each impeller, equal forces acting upon the article to be coated can be produced. In order to keep the shafts 39 short and to prevent vortexing, vanes 46 are provided in each stand pipe above and below the impeller 40. The surface flow of coating material necessary to keep the surface free of buoyant contaminants is created by the flow of excess material over weirs 42 into troughs 43 along each side of the tank above openings 35. This material is urged from the trough by means of a centrifugal pump and forced through a filter for reintroduction through conduit 41 into the coating material bath. Conduit 41 can be so located to direct the coating material to the point where the deposition on the article cornmencecs as previously described. Flow rates of 4 gallons per minute are satisfactory.

Another modification of the invention can be seen in FIGURE 7. The electrophoretic tank in this modification is so constructed that both stand pipes expel the coating material through a common opening 44. A deflecting vane 45 is located adjacent the common opening 44 to split the stream of coating material as it enters the tank and to direct it along the bottom of the tank. This arrangement will produce a counteracting flow in each side of the tank and, additionally, will wash the bottom of the tank remixing any heavier constituents which may tend to settle from the coating material solution. No perforated weir is used with this form of our invention; adequate circulation can be obtained with the flow from the series of stand pipes.

With a tank 14 inches long, 36 inches deep, 42 inches Wide with a stand pipe of 4 inch, schedule 40 pipe arranged on both sides as shown in FIGURE 7, good agitation and mixing can be obtained by driving a three-bladed marine propeller as an axial flow pump within each stand pipe at 1200 r.p.m. The use of a /8 horsepower, adjustable speed motor with speeds of 01700 r.p.m. has been found to allow a latitude of adjustment in the agitation rate for this size apparatus.

The electrophoretic tank and all its constituent parts may be constructed of a metallic material such as steel to provide the strength necessary to support a quantity of coating material and to serve as one electrode in the coating system. In this regard, the interior surface of the tank should be uncoated to insure good electrical contact between the coating material and the metallic surface. It is not necessary that the coating material tank be used as an electrode. The interior surface of the tank may be insulated, or the tank itself may be made of insulating material, and additional electrodes 47, as shown in FIG- URE 8, can be used to produce the coating action.

The liquid conduits may be made from any standard pipe material. The size and number of conduits which are required depends on the size of the tank which is required.

The electrical control apparatus for practicing our invention is shown in the schematic diagram of FIGURE 9. In this apparatus, the pumps 20 and 22 (FIGURE 1) are turned on by normally open push button PB-3 which actuates relay R-2 and which in turn energizes pump motors PM-20 and PM-22 respectively. The conveyor is energized by normally open push button PB-S which actuates relay R-3 and which in turn energizes the conveyor motor CM. The power supply is energized by normally open push button PB1 which actuates relay R1 energizing the power supply primary. In operation relay R-4- is not energized as the connection with each article is made, therefore the normally open contact at R-4 which is in series with the article charging bus bar 17 is open and the article charging bus bar is not charged. As the articles to be coated come down the conveyor line, their supports trip switch 15 which deenergizes relay R-4 immediately before the article connecting means 14 of each article contacts the article charging bus bar 17. While a brush-like contact is shown as article connecting means 14 this electrical connection can be made with any sliding electrical contact, such as that incorporating a rolling wheel-like contact. Shortly thereafter the article support trips switch SW-18 (also identified as 18 in FIG. 1) energizing relay R-4 and reconnecting the power supply to the article charging bus bar.

One coating material formulation for electrophoretic deposition consists of a styrene-allyl alcohol copolymer mixed with linseed fatty acid, the mixture being esterified in a conventional manner. This mixture is then intimately blended with red-oxide pigment and combined with melamine formaldehyde. An emulsifying agent consisting of ammonium hydroxide, or an organic amine, is then added to produce a stable emulsion. This composition is reduced with demineralized water for use in the electrophoretic bath. In using this invention the article is immersed into a bath of such a coating material while the coating material is being circulated as previously described, and an electric field of to 400 volts is established between the article and an electrode in spaced relation with the article by electrically connecting the article with an adjustable voltage power supply whose output voltage is adjusted to obtain the best deposition. The resulting current flow will deposit a film of coating material on the article. The magnitude of current flow depends primarily upon the voltage, the surface area of the article, and the conductivity of the coating material and can reach in many cases several amperes per square foot of surface area being coated.

Throughout this application whenever a reference is made to electrophoretic deposition or apparatus, it is to be understood that the term is used in a broad sense and includes deposition of resins from solution as well as deposition of suspensions and apparatus for such deposition.

While I have described certain embodiments of my invention, it is to be understood that my invention is capable of many modifications without departing from the spirit and scope of the invention as disclosed in the appended claims.

We claim:

1. A method of electrophoretically depositing coating material on an article to be coated comprising: immersing the article to be coated into a tank of electrophoretic coating material intermediate its sides, moving the article through the electrophoretic coating material along a path intermediate the sides of the tank, creating essentially equal circulation of coating material on each side of the article by means near the bottom of the tank for urging movement of coating material within the tank at equal rates on each side of the path of movement, and making a sliding electrical connection to the article to establish an electric field between the article and the tank walls for current flow therebetween to deposit the coating material on the article while maintaining said circulation of coating material.

2. The method of claim 1 wherein the voltage of the power source is removed from the connection as the sliding electrical connection is made.

3. The method set forth in claim 1 wherein there is a. flow of coating material at the surface of the bath and coating material is withdrawn from the surface, filtered, and directed into the bath adjacent and parallel to the surface.

4. The method set forth in claim 3 wherein the flow of coating material carries contaminants to the surface for removal in the surface flow.

5. A method of electrophoretically depositing coating material upon articles to be coated comprising, movably supporting a plurality of articles from an overhead conveyor, maintaining an electrophoretic coating material bath within a tank at a predetermined level, transporting the articles along a conveyor path which immerses said articles in the bath intermediate the sides of the tank, establishing an electric field between the article and an electrode for current flow therebetween to deposit the coating material upon the surface of the articles, transporting the articles along a path to emerge them from the bath, and directing a flow of coating material in the direction of article movement at the surface of the bath and withdrawing coating material and contaminants at the end of the bath where the articles emerge.

6. The method of claim 3 including introducing coating material into the bath adjacent the article path at the point of immersion by directing said coating material adjacent to and parallel with the liquid surface.

7. The method of claim 6 wherein the coating material is withdrawn by forcing it to flow over a weir adjacent where the articles emerge, the withdrawn coating material is filtered and introduced by directing it adjacent to and generally parallel with the liquid surface, and the coating material level is controlled by said weir.

8. An electrophoretic coating apparatus comprising: a conductive tank adapted to contain electrophoretic coating material with a predetermined surface level; a conveyor with descending and ascending portions positioned with respect to said tank to lower articles into, move them through and raise them out of the coating material in said tank; an article charging bus bar adjacent the portion of said conveyor over said tank; article support means insulating the supported article from said conveyor and contact means connected with said articles and adapted for sliding electrical connection with said article charging bus bar during movement through the coating material; power supply means connected with said tank and said article charging bus bar for establishing an electric field between the articles and the tank and in the coating material for coating the articles; and distributing means near the bottom of the tank for directing the flow of coating material within the tank to create counteracting flows relative to the articles being coated to stabilize the position of the articles during their movement through the tank and maintain the sliding electrical connection.

9. The apparatus of claim 8 wherein said distributing means for creating counteracting flows includes, a fluid distributing pipe lying longitudinally of said tank adjacent its bottom and being closed at one end; a fluid exit in the tank; a pump; a conduit connecting the fluid exit, the pump, and the open end of the fluid distributing pipe, said fluid distributing pipe having rows of holes disposed to direct coating material against the bottom of the tank and each side wall when the pump is running to wash the tank bottom and create said flows.

10. The apparatus of claim 8 wherein said distributing means for creating counteracting flows includes a series of stand pipes arranged along both sides of the tank, each stand pipe being connected with the tank through an opening near the top of the tank and near the bottom of the tank, and axial flow pump means within each stand pipe to produce said flows of coating material.

11. The apparatus of claim 8 wherein distributing means for creating counteracting flows includes opposing stand pipes arranged to meet at the bottom of the tank for entry through a common opening at the center of the tank and deflector vanes positioned adjacent said common opening to split the flow of coating material and to direct it along the bottom and each side of the tank to wash the tank bottom and to create said flows.

12. The apparatus of claim 11 including, level weirs at each side of said tank and containers to collect contaminated material falling over each weir for subsequent removal from the bath, said level weirs thereby controlling the level of the coating material in the rank.

13. An electrophoretic coating apparatus comprising: a tank adapted to contain electrophoretic coating material with a predetermined surface level; a conveyor with descending and ascending portions positioned with respect to the tank to lower articles into, move them through and raise them from the tank; means to force electrical current through coating material in the tank to the article to coat the article; withdrawal means to withdraw coating material and contaminants at the end of the tank that is adjacent the ascending portion of the conveyor; and flow control means to create a flow of coating material toward the withdrawal means at the liquid surface.

14. The apparatus of claim 13 wherein said withdrawal means includes a weir at the end of the tank where the articles emerge, and a container adjacent said weir to collect the contaminants in the coating material falling over said level weir for subsequent removal from the bath.

15. The apparatus of claim 13 wherein said withdrawal means includes a weir; and a container adjacent the weir to collect coating material falling over the weir and said flow control means includes means to direct a flow of coating material adjacent to and generally parallel with the coating material surface; a pump; and conduit means connecting said container, said pump and said coating material directing means, said pump for removing coating material from said container and forcing it into the tank through said coating material directing means.

16. Apparatus for electrophoretic deposition including; a tank adapted to contain coating material; a weir at one end of the tank; a container adjacent the weir to collect coating material passing over the weir; a pump; a filter; conduit means connecting said container, said pump, and said filter with the tank so that coating material collected in said container is filtered and put back into the tank; a series of stand pipes arranged on each side of the tank, each having an opening into the tank at a level below the level of the weir and an opening into the tank near the bottom; impellers within the stand pipes; motors to drive the impellers to create a flow of coating material through the stand pipes and within the tank and over the weir.

(References on following page) References Cited UNITED STATES PATENTS Wallace 204-237 Roehl et a1 204-238 Graham 204-481 5 Gelfand et a1. 204238 Burnside et a1. 204--181 10 FOREIGN PATENTS 11/1935 Switzerland.

US. Cl. X.R.

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