US2334648A - Method of spray-coating articles - Google Patents

Description

Nov. 16, 1943. H. P. RANSBURG ETAL 2,334,648

METHOD OF SPRAY COATING ARTICLES Original Filed June 29, 1939 2 Sheets-Sheet 1 Nov. 16, 1943. H. P. RANSBURG ETAL 2,334,648

METHOD OF SPRAY COATING ARTICLES Original Filed June 29, 1959 2 SheetsSheet 2 Patented Nov. 16, 1943 LIETHOD OF SPRAY-CDATING ARTICLES Harold P. Ransburg and Harry 3. Green, Indianapoiis, Ind, assignors to Harper J. Ransburg, doing business as Harper J. Ransburg Com- Original application June 29, 1939, Serial No.

281,766. Divided and this application December i, 1939, Serial No. 308,024

2 Claims. (Cl. 117-17) This invention relates to a method of spray coating articles. The present application is a division of application Serial No. 281,766, filed June 29, 1939.

In spray coating methods involving the application of a siccative coat such as paint, varnish, lacquer, etc., to an article or surface to be treated, in which an electrostatic field is used it has heretofore been the practice to provide a collecting electrode and a discharge electrode between which an electrostatic field is set up. Usually the article to be coated is the collecting electrode and the discharge electrode is in the form of a spray gun or other device used to direct a spray of coating material into the electrostatic field. In some cases the discharge electrode has been an object placed near the spray gun or in the path of the spray from the gun rather than the gun itself. The whole has been enclosed in a metal cabinet maintained at the potential of the discharge electrode. The theory of operation of such apparatus has been that the spray particles in touching or passing near a positively charged discharge electrode, for example, pick up initial positive charges which cause, them to be attracted to the negatively charged collecting electrode and to be repelled from the positively charged walls of the cabinet.

We have found, however, that spray particles do not behave in the manner to be expected from the application of this theory. Even though the gun itself is the discharge electrode and all particles of the spray may then be expected to be electrically charged, still the spray particles apparently move toward a positively charged electrode as readily as toward a negative one, other things being equal. The walls of the cabinet, even when charged at the same potential as the gun, have substantially the same attractive effect as when charged at the opposite potential. In cases where the discharge electrode is placed in the path of the spray said electrode apparently acts as a collector of spray particles.

One object of the present invention is generally to increase the efficiency of spray coating methods of the type described and particularly to reduce the amount of coating material lost by deposition on the walls of the cabinet and on the discharge electrode. This object is attained in part, first, by substituting a cabinet formed completely of insulating material for the metallic cabinet heretofore .used; second, by placing the discharge electrode as completely as possible out third, by suitably proportioning the sizes of the discharge and collecting electrodes.

The substitution of an insulated cabinet for a metallic one in any given installation materially reduces the amount of spray material collected thereon. In some installations the deposition of spray material on the cabinet has been practically eliminated Whereas with a metal cabinet a heavy coating-of such materiai quickly appeared on the cabinet. Apparently only such particles as receive an initial velocity and direction from the gun such that they cannot be deflected by the electrostatic attraction of the electrodes can reach the walls of the insulated cabinet, whereas, with a metal cabinet, a large number of such particles are actually attracted to the cabinet irrespective of the potential at which the cabinet is maintained.

The behavoir of the particles indicates that it is unnecessary to give them an initial charge by contact or proximity to an electrode charged in the opposite potential to the collecting electrode. Apparently particles discharged into an electrostatic field are attracted to either electrode. Other things being equal, the attractive effect of a positively charged electrode is considerably greater than that of a negatively'charged electrode but the attractive eflect of any electrode increases immensely with an increase in the size of electrode. These facts have made it possible to increase the efll'ciency of the apparatus by placing the discharge electrode out of the path of spray particles and by making the discharge electrode relatively small and the collecting electrode relatively large. For the coating of small objects the discharge electrode may be in the form of a point, while for a succession of articles moving on a conveyor the discharge electrode is preferably in the form of a fine .wire or a series of points arranged parallel to the direction of travel.

Another object of the invention is to provide efficient means for coating articles having a surface area too small to comprise an efficient collecting electrode. This object is attained by grouping a plurality of such articles in such way as to cause all members of the group to be presented simultaneously to the discharge electrode in such manner that their aggregate area is nc'iluch greater than that of the discharge elecro e.

Another object of the invention is to coordinate the velocity and direction of the spray with the strength of the electric field and the position of of the path of the spray from the gun; and the article to be coated in such manner that as little as possible of the spray fails to reach the articles. In the ordinary spray gun used for paint spraying, particles of varying sizes are created and these particles lose their initial velocities at various distances from the gun, depending upon their size. In prior practice in the painting of small articles particularly, many of the larger particles reach the region of the articles to be coated with such a high velocity remaining that they cannot be pulled in by electrostatic attraction and are carried past the article. Small particles, on the other hand, lose their velocities before they can approach close enough to the articles to be attracted electrostatically. In the Present invention, in one of its preferred forms, a series of articles to be coated is moved in a. predetermined path adjacent to the spray gun. The gun is directed roughly parallel to the path or inclined at an acute angle to the path so that the articles successively pass close to the points at which the initial velocities of the particles of varying sizes are lost. Thus the particles of all sizes find themselves in the region of one of the articles at the time that their initial velocities have been sufiiciently reduced to permit the same to be electrostatically attracted to the article. By this means a large percentage of the paint ordinarily lost because of the varying size and velocity of the spray particles actually reaches the articles.

Another object of the invention is to minimize the hazard of explosion in electrostatic spray painting with materials having inflammable solvents.

Another object of the invention is to provide an efficient method for electrostatic spray painting of articles formed of non-conducting materials.

Other objects and features of the invention will be understood from the accompanying drawings showing a preferred form of apparatus by means of which the invention may be practiced and from the following description and claims:

Fig. 1 is a plan view of a spray painting apparatus constructed in accordance with a preferred form of the invention and having the cabinet shown in section. Fig. 2 is a vertical sectional view taken on the line 2-2 of Fig. 1. Fig. 3 is a plan view of conveyor apparatus used with the invention and is taken substantially on the line 3-3 of Fig. 2. Fig. 4 is an elevational sectional view taken on the line 4--4 of Fig. 3. Fig. 5 is a plan view of a detail of the apparatus of Figs. 3 and 4. Fig. 6'is an elevational view of the same partly in section. Fig. '7 is an end elevational view showing an adaptation of the invention to the coating of a different type of object.

In the preferred form of the apparatus shown in the drawings there is provided a cabinet III consisting of side walls I l top 12 and fioor plates l3, all supported by vertical posts M. The cabinet, including the vertical posts I4, is preferably constructed entirely of electrically non-conducting materials such as wood or Bakelite or other suitable plastics. Attention is called particularly to the use of non-conducting material for the supporting posts l4. Even though an insulated cabinet is used, if the supporting posts are of metal, an appreciable deposit of coating material is found on the inner wall of the cabinet in the region of the supporting posts. As will be apparent from the drawings, the cabinet is completely open to the atmosphere at both ends. It has been found that when the combined areas of the open ends of the cabinet measured in square feet is at least equal to one-third of the total volume of the cabinet measured in cubic feet, there is no danger of explosion with electrostatic deposition of paints or enamels using ordinary commercial solvents. If the area of the open ends is less than this figure, the interior of the cabinet approaches too closely to a confined space in which an explosive mixture may be accumulated. In the cabinet illustrated in the drawings, the ratio of end area to volume is approximately one square foot to three cubic feet.

Beneath the floor of the cabinet there is provided a conveyor chain I5 supported on a trackway formed by an angle iron IS, an angle iron I1 and a plate l8 (Fig. 4). Said trackway is suitably supported by legs IS. The conveyor chain is trained about sprockets 20 and may be continuously driven by a motor 2| operating one of said sprockets through a reduction gear unit 22, a belt drive 23 and beveled gears 24. The path of the conveyor chain is best seen in Fig. 1 and leads from beneath the cabinet past an unloading station 25 and a loading station 26 and then again beneath the cabinet. At suitable intervals along the conveyor chain there are provided upwardly extending pins 21 each adapted to enter a mating opening in the lower end of a carrier unit 28. Each of said carrier units, as best seen in Fig. 4, includes a sleeve 29 preferably formed of insulating material and having in its lower end a plug 30 provided with an opening to receive the pin 21. A sprocket 3| is mounted near the lower end of the sleeve 29. A plug 32 is secured in the upper end of the sleeve 29 and is provided with a recess adapted to receive an upwardly extending rod 33. A collar 34 tightly secured about the upper end of the sleeve 29 prevents undesired displacement of the plug 32. The rod 33 extends upwardly through an opening between the two bottom plates l3 of the cabinet and carries at its upper end an electrode 35 herein shown as of frusto-conical shape formed of sheet metal and serving as a support for an article 36 to be coated. In the example shown herein the article is in the form of a flowerpot and may be of non-conducting material, such as ceramic ware.

At each end of the cabinet close to the floor there are mounted a pair of rods 31 suitably supported in fittings 38 and each having mounted thereon one or more slidable fittings 39. Said fittings 39 may be secured at any desired place along the rods 31 by means of thumb screws 40 and each of said fittings is adapted to receive a vertical rod 4|. Fittings 42 are carried by the vertical rods 4| and are similar to the fittings 39. Said fittings 42 carry horizontal rods 43 adapted to support spray guns 44. Said spray guns may be of a well known commercial form and are connected by tubing 45 to a suitable source of a coating material of siccative nature such as paint, lacquer or enamel and a source of compressed air for atomization. By means of this construction the spray guns may be positioned as desired adjacent either end of the cabinet. In the arrangement shown in the drawing two such guns are used at one end of the cabinet and are located to direct a spray 46 of coating material in the general direction of the opposite end of the cabinet. One of said guns is positioned on each side of the path of movement of the articles 36 through the cabinet. Preferably all or a part of the supporting structure for the guns is formed of insulating material and the tubing 45 is also of non-conducting material. The guns are, therefore, isolated from the electrical apparatus andizdd n t 121 par anyrelectrlcal charge to the face of the article. It is not necessary in all sprayparticlesanolzltend to attract stray particles cases that the collecting electrodes be carried by of the coatingzmaterial. the conveyor and act as supports for the articles. On reach; {lathe side-malls Ii of the cabinet Where only one side of an article is to be coated there iSNIHQHHtBdS 1.;pair of: tubular insulating 5 it may be advantageous in some cases to mount membe'rs t'l extending inwardlystoward the path a collecting electrode in a stationary position on of traveharticles,dfiarmnrelectrode 48 in the side of the conveyor opposite to the discharge theaformi. fia fiIlB'iWj-IEI .strungi between the electrode. Such an arrangement is illustrated inner;endsgofaieaclrzofstheinsulators 41. and exin Fig. '7 in which an object I36 to be coated on tendslgeneraillye pa'rallelrto-zthe; pathliofrtravel of one side only is in the form of a thin plate of theiartiqlescrsEachgofrsaiihelectrodesazextendsioutnon-conducting material. Said object is carried ward-lyfithroughtoneg n'ibotlnof theiinsulaitorsfil by a support I33 which may be mounted in the ands onnecte'd by: conductorz lii toboneitermiec. carrier 28 in the same manner as the rod 33 previously shown. The discharge electrode may be in the form of a metallic point I48 mounted on the end of one of the insulators 41. The collecting electrode is shown as a plate I35 carried on a similar support on the opposite side of the cabin'e land zareiisupported 0115813131113 .clip path of travel of the article I36. The spray carriediabyrithescabinetiifloom 'Ihe'yviitesrfi at gun discharges the paint spray into the space adapted to engage the ;ver.t-icalsrods'. 3:; ..their between the discharge electrode and the object. travel ltl'roughcitheeicabinetiiand lthUSrTfS8IeV61t0 n appa s of th s nd it is adv us to makm electriaahtconngfitiqn;metweenffthe'jgen; move the conveyor intermittently, stopping each crating apnaratnsififlzandra-eaoh,otitheneleotrolie 0f the Objects I36 between the electrodes for a retemm a sufficient length of time to receive a thorough coating of paint.

For objects not placed too closely together on the, conveyor, the electrical attraction is sufficient to give a relatively uniform coating on all sides of the articles, even though the articles are not turned during their movement while subjected to the spray. For articles of this type and for other articles where the spray is to be applied I1 tonne side only, there is provided a construction as shown in Fig. 6 of the drawings. Each of the sprockets 3| is provided with a suitable opening in which there is inserted a pin 60, the lower endggof which is received in an opening in the ent link of the chain I5. The pin, therefore, prevents the articles from turning during their travel through the cabinet.

Whenthe articles on the conveyor are spaced th 1 relatively close together it is desirable that they A s m fi be s owly rotated in, their movement through the i g% gg g the cabinet to insure uniform coating. For thi .p rpath of the"alrtiles'iifi it suli that both small pos'e'nlechamsm shown Part1cu1ar1Y in Flgsand large spray particles lose their initial velociand 1s provlqed- In 'thls constructlon th 1s ties at points-adjacent to the path of travel of provided a pair of brackets 6| secured the the arm C! e. Thus there is always an electrically plate l8, one of which brackets serves as a bearcharged article reasonably close to exert an to lug for a vertlcal stub shaft 62 and the other tractive eflect on each slowly moving particle. K Stub Shaft 53- A Sprocket cham The efiiciency of the paint collection is, there- 64 1S 'P about a of sprockets and fore, immensely increased over that of any a 66 earned by sa d stub shafts. The chain 64 rangement in which the spray is directed sub- 15 of proper pltch meshwlth the Sprockets stantially at right angles to the path of t 1 3| carried by the carrier units 28. A plate 61 of the articles The velocity of spray is normal rests on brackets 68 and a bracket 69 secured to l ad'usted so that none of the spray particles the Plate The edges of d plate 61 engage Sass beyond the confines of the cabinet. It is t mnel' and Outer uns f the chain 64 which immaterial in which end of the cabinet the spray is Preferably a o e chain adapted to run guns are mounted Iniact'one may be mounted smoothly on said edges. By movement of the in one end and the other in the opposite end. Plate toward away from the conv y l5 Such an arrangement is beneficial in certain inthe e 54 may be brought mto and out of stallations. Where only one side of each article mesh with the p c ets 3|. is to be coated only one gun and one discharge The plate 61 has secured thereto a vertical pin electrode may be required. In other cases, such 05 extending downwardly through a ed openas for larger objects, more than one gun may b ing II in the bracket 89 and having secured required on each side of the conveyor. thereto a -fitli g A stud 13 is threaded in It is to be noted particularly that the articles e outlet of a dT-flttmg and is connected to a to be coated need not be electrical conductors. rod 74 y a pipe Union 15. Said union is loosely When said articles are so placed as to mask the 70 adjusted o that the r 1 may e rotated withcollecting electrode so that the electrostatic field out rotating t d 13 and the threaded conmust pass through the article, the efilciency of n ion I6 therein is soldered to prevent the the collecting electrode is not materially imon becoming unscrewedinits movement. The paired. Paint which is attracted by the collectouter end of the rod 14 is guided in a bracket ing electrode is then deposited on the outer sur- 751'! secured to the bracket 69. A washer '18 is extendailongltlldina-llyebeneat the noorior-th n t e carried by the rod 14 and is held in place thereon by a pin 19. A compression spring 88 is interposed between said washer and the bracket 11 and normally urges the rod 14 and associated parts to the left in Fig. 4 to keep the chain 84 in engagement with sprockets 3|. The yielding pressure of the spring permits the chain to give slightly if the teeth of the sprockets do not initially engage the chain at the proper point.

The outer end of the rod 14 is provided with a T-fitting 8|, in one run of which there is secured a downwardly extending handle 82 and the other of which carries a stud 83. A detent 84 is secured to a portion of the cabinet in position to engage the stud 83. A guide pin 85 secured to the bracket 69 operates in a slotted opening 88 in the plate 81 and serves to guide the movement of the same. By means of this construction, the spring 88 normally presses the plate 6'! against the inner run of the chain 84 and holds said chain in engagement with the sprockets 3 I. By means of the handle 82 and the rod 14 may be rotated so that the stud 83 clears the detent 84, the rod may then be drawn to the right in Fig. 4 and the stud 83 returned to its vertical position outside of the detent 84. In this movement the plate 61 is moved to the right in Fig. 4 and disengages the chain 64 from the sprockets 3| and the detent 84 engages the stud 83 to retain the parts in this position.

A variable speed motor 81 is connected through a reduction gear unit 88, a pulley 89 and a belt 98 to a pulley mounted on the stub shaft 82 and thus serves to move the chain 64. When the motor 81 is run at the proper speed to operate the chain 84 at the same speed as the conveyor chain I5, no rotation of the articles on the conveyor is produced. By increasing the speed of the motor 81, rotation of the articles is elfected in one direction, and by reducing the speed, the direction of rotation may be reversed. Any suitable speed of rotation of the articles may thus be produced by slight variations in the speed of motor 81. Of course when rotation of the articles is to be produced the pins 60 are withdrawn.

The foregoing specification describes the invention in one of its preferred forms. It is apparent however that variations in one or more of the elements of the invention are necessary for the coating of articles of different sizes and shapes. Articles of conducting material serve as their own collecting electrodes and do not require additional collecting electrodes of large size, such as shown herein. With the high voltages used in the method, certain articles not ordinarily classed as conductors actually possess suflicient conducting power to be eflicient electrodes in themselves. For example, small panels of wood have been successfully painted without the use of additional electrodes. Some articles are of such shape that they cannot be efliciently sprayed with the spray guns located as shown herein. With such articles it may even be necessary to direct the spray at right angles to their path of travel. However. even with such an arrangement of the guns other features of the invention specifi d in the claims are applicable.

The inventio v claimed is:

1. A method of coating articles, comprising the creation of an electrostatic field of definite length and breadth, continuously passing a series of the articles to be coated in spaced relation along the longitudinal center line of said field, and projecting a spray of siccative coating material from one end and exteriorly of said field into the latter roughly parallel and at an acute angle to the said longitudinal center line and with a force insuflicient to project the spray beyond the opposite end of the field.

2. Amethod of coating articles, comprising the creation of an electrostatic field of definite length and breadth, continuously passing a series of the articles to be coated in spaced relation along the longitudinal center line of said field, and projecting a spray of siccative coating material from one end and exteriorly of said field into the latter roughly parallel and at an acute angle to the said longitudinal center line, the length of the field being greater than the projection of the spray.

HAROLD P. RANSBURG. HARRY J. GREEN.

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