US2730988A

US2730988A - Electrostatic coating apparatus

Info

Publication number: US2730988A
Application number: US167362A
Authority: US
Inventors: William A Starkey
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1950-06-10
Filing date: 1950-06-10
Publication date: 1956-01-17
Anticipated expiration: 1973-01-17

Description

Jan. 17, 1956 w. A. STARKEY 2,730,988

ELECTROSTATIC COATING APPARATUS Filed June 10, 1950 2 Sheets-Sheet l INVENTOR. Fig. 2 WILL/AM A. STAR/(5) ATTORNEY Jan. 17, 1956 w. A. STARKEY 2,730,983

ELECTROSTATIC COATING APPARATUS Filed June 10, 1950 2 Sheets-Sheet 2 Fig.7 54

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l l 24/ ll If V 27 -,--3o V J W 1 i i i Jlli [Wlil'i iTm LHII i||H INVENTOR. L 4 WLLAMAS Ak ATTORNEY United States Patent ELECTROSTATIC CGA'I'ING APPARATUS William A. Starkey, Plymouth, Ind., assignor to Ransburg Electro-Coafing Corp., Indianapolis, Ind, a corporation of Indiana Application June 10, 1950, Serial No. 167,362

Claims. Cl. 118-51) This invention relates to a method and apparatus used in electrostatic coatings of articles with adherent liquid coating material. More particularly it relates to a support for mounting the articles prior to, during and after the coating operation and to a method of keeping this support reasonably clean of coating material. This application is a continuation-in-part of my prior application Serial No. 619,371, filed September 29, 1945, now Patent No. 2,632,716.

In the electrostatic method and apparatus for applying liquid coating mater-id to a series of articles, it is customary to mount the articles on supports attached to the conventional conveyor, have the conveyor move them into a coating zone and introduce into this zone particled coating material which is charged for precipitation upon the articles. Basically the primary function of the support in this process is to provide the mechanical means by which the article is attached to the conveyor but in many instances the support is electrically conducting and forms a portion of the electrical connection between the article itself and the conveyor by virtue of which the article is maintained at a potential to attract the coating material. A still further function which such support may fulfill is to influence the distribution of the coating material upon the article which it supports. Such support is usually constructed of metal because it is conducting and also is readily available. Since in the operation of the process, the article and its support are both at the same potential and since their polarities are opposite to that of the coating material, this material will be collected without discrimination upon either the article or the support whichever is nearest. This collection on the support is naturally undesirable since material collected on the support during successive passages through the booth will soon accumulate to proportions which prevent contact between the article and the support and the support might thus cease to serve its function of electrically connecting the article to the conveyor. Further, that material which collects upon the support naturally is not available to be collected upon the articles and, therefore, leads to a reduction of the efliciency of the coating operation.

Accordingly, it is an object of the present invention to provide a simple, inexpensive support which in addition to serving the above functions also tends to inhibit coating material from collecting upon its surface. I have found that the tendency of the support to collect coating material otherwise intended for precipitation upon the article is minimized by covering it with a layer of material, such as hard rubber or polyethylene, which is capable of accumulating and retaining a surface charge of like polarity to that of the approaching coating material particles. The overall efficiency of the coating operation is thus increased. At the same time, the support is 'kept cleaner and more free of material so that little or no ditiiculty is encountered in having successive articles remain in constant electrical contact with the conveyor.

The use of materials having such dielectric properties that a surface charge is accumulated thereon to control the deposition of particles in an electrostatic field is the basis of the present invention.

For better understanding of the invention, together with other and further objects thereof, reference may be had to the following specifications in connection with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims.

In the drawings:

Fig. 1 is a diagrammatic end view of one form of electrostatic paint spraying equipment in which the improved method and apparatus can be used;

Fig. 2 is a side section of the same arrangement taken centrally of the conveyor of Fig. 1;

Fig. 3 is a diagrammatic end view of another form of electrostatic spraying equipment utilizing electrostatic atomization and in which the improved method and apparatus can be used;

Fig. 4 is a side section of the apparatus shown in Fig. 3;

Fig. 5 is a detail side elevation, in section, showing the construction of the improved article support;

Fig. 6 is a detailed enlarged sectional view taken along the line 66 of Fig. 5;

Fig. 7 is another form of the improved support; and

Fig. 8 is still another form of the improved support.

Referring to Figs. 1 and 2 of the drawings, an open ended spray booth designated generally by the numeral 10 may be constructed of any suitable material. Located on the roof of the booth in any suitable manner are brackets 11 and 12 from which extend

insulator rods

13 and 14.

Electrode frames

15 and 16, which are shown as constructed of rods of suitable size welded together to form a rectangle, are secured to

insulator rods

13 and 14. Ionizing elements, preferably in the form of fine wires 17, are strung vertically between the top and bottom horizontal bars of the electrode frames to form an electrode grille. The electrode frames are arranged one on each side of the path of movement of the articles 18 to be sprayed and are connected to a suitable high-voltage source 19 by an insulated conductor 2% and to each other by a similar conductor 21. The other terminal of the voltage supply is grounded as at 22 as is the conveyor and booth as at 23. A field is thus established between the articles 18 on the conveyor and the electrode system 17 as has been more fully explained in U. S. Patent No. 2,425,652 issued August 12, 1947.

A conveyor, indicated generally by the numeral 24, has suspended therefrom a series of rotatable supports 25 each supporting at a suitable distance below its top an article 18 to be coated. These supports are rotated as they pass adjacent the electrodes by being affixed to pulleys 27, which contact bar 28 extending alongside the conveyor.

One or more spray guns 29, shown in this modification as the normal air operated type, are positioned on supports (not shown) to introduce atomized coating material into the space between the electrodes and the articles. This material becomes charged and therefore is repelled by the electrode system and attracted by all objects in the field which are of opposite polarity.

Referring to Figs. 3 and 4, in which is shown that type of electrostatic coating equipment which utilizes electrostatic atomization as more fully set forth in the application of myself and Edwin M. Ransburg Serial No. 556,390, filed September 29, 1944, similar articles 18 are supported from a grounded conveyor 24 by supports 25 identical to those described above. Electrostatic atomizers 30 are positioned appropriately adjacent the path of article travel and are connected to the high-voltage source 31 by a conductor 32 the other terminal of which is grounded. A field of high potential gradient is thus established about the forward edge of each atomizer and a field of lesser gradient to all parts adjacent which are at a different potential. Under the action of the high gradient field, coating material supplied to. each atomizer will be atomized from its forward edge as finely divided charged particles, which will be dispersed and attracted to objects in the vicinity which are at opposite polarity.

From the foregoing, it will be evident that in the operation of both these types of electrostatic coating apparatus the supports 25 serve the dual function of physically supporting the articles to carry them through the field or past the atomizer and of electrically conducting to the-articles a charge opposite in polarity to. that of the atomizers or ionizing electrodes 17. It is apparent, therefore, that for the apparatus to function efiiciently, a good electrical contact must be established and maintained between the support and the article prior to and during the entire periodv of transit of the article through the coating station.

' Moreover, it is of practical importance that this contact be quickly and easily established by a simple motion when the article is placed upon the support. 7

' As before stated, when an ordinary conductive support is employed, the coating material issued into the field by guns, such as 29 of Figs. 1 and 2 or the atomizer 39 of Figs. 3 and 4, is attracted to the support as well as to the article to be coated because o-f their similarity of character and polarity. Material thus precipitated upon the support is waste and greater efficiency would be effected if it were collected by the article as useful coating material. Passage ofthe support through the coating zone again and again causes rapid accumulation of coating 7 material upon the support often to such an extent that it becomes diflicult if not impossible to make satisfactory electrical contact with the article. The present invention is directed toward overcoming the aforesaid difiiculties.

Referring to Figs. 5 and 6, one modification of the improved support comprises an electrically conductive strip or core 4%. formed in the shape of the desired support. The core 4% is suitably electrically connected at its upper end to a conductive member, such as pulley 27 of the conveyor 24. The remainder of the core of the support is substantially completely sheathed with non-conducting or insulating material 41 such as glass, porcelain, polyethylene, or nonconducting rubber. Only at the small area 42, which is arranged to contact the article when it is positioned on the holder, is the core 4t} exposed.

Fig. 6 shows the manner in which this contact with the article is established. At the position 42 a sharp V or groove is cut into the insulation to expose the core over a small area. 'The article 13 placed in the groove automatically is directed into contact with the exposed portion of, the core 4% as at 42. The back non-contacting portion of the core is coated with insulation as is the rest of the hook.

With this'type of article-support used in the apparatus shown in Figs. 1, 2, 3, and 4, the quantity of coating material which'collects on the support is practically negli gible when compared with the amount collected on an unsheathed support. As the support with the article in place moves into coating position, the highly ionized atmosphere which is created about the electrode 17 or the atomizer 3'0 produces over the surface of the insulating sheathing on the support an electrical surface charge of i the same polarity as that of the charged atomized particles. 'This surface charge, which builds up on the sheathed support as the result of impingement thereon of ionized atmospheric particles moving under the influence of electrostatic forces, is retained as the article is moved through. the coating zone. Coating material particles which under the influence of the field arecharged to the same sign as the electrode or atomizers and thus seek their way toward the article will, therefore, be re pelled from the support because of their similarity in sign. The quantity of material collected on the support is therefore greatly reduced.

The core 40 of this new type of support is preferably special care in mounting the articles.

made of stock on the order of diameter, and the thickness of the sheath or envelope surrounding the strip is preferably at least equal in thickness to the thickness of the core overall surfaces of the latter. It is noted, however, that the thickness of the sheath with respect to the diameter of the core may be varied in accordance with the dielectric constant possessed by the particular type of insulating material used. As the dielectric constant of the insulating material increases, the thickness requirement decreases, although the purposes of the invention will be served so long as the thickness is never less than that required consistent with the objective of obtaining a surface charge. That is, the support is so constructed that the combined thickness and dielectric constant of the insulating material employed will, in the presence of an electrostatic field, accept and retain a surface charge of such magnitude as to cause repulsion of coating particles which would otherwise be normally attracted to the support. it will be understood, of course, that in constructing the support'the insulating material may be first fashioned into the desired formand be of a sufiicient diameter and be of asubstance of sufiicient tensile strength to perform completely, per se, the physical fmctionof supporting the article and that a bore of proper diameter may be provided therein to receive the conductive strip 4% which need only be of such diameter as to function of physically supporting the article or they may be combinatively employed for this purpose. 7

Referring to Figs. 5 and6, we see that to contact the article a notch or opening 42 is provided in the insulating 7 material at the curved portion of the core 40. The opening 42 is'preferably of a width and length just sufficient to permit a conductive part of the article 18 to come into electrical contact with the core when the article is placed upon the support. Preferably, the. opposite sides 44 'of the opening are beveled slightly inwardly toward the conductor, so as to cam or guide the article 18 into home or contact position with respect to the core 49 as an incident to such member approximating an aligned relationship with the opening 42. This slant formation of the sides 44 relieves, the operator from having to exercise ticle. 18 is brought into an approximate relationship with the internal surface of the curved portion it sockets securely into the home position shown in Fig. 6.

Since the opening 42 can be made so that it is substantially filled by the part of the article making contact with' the core, the relatively small exposed contact area of the core can be physically shielded against deposition of material thereon. Even when a support so constructed is run through the field several times without an article mounted. thereon, this relativelysmall exposed contact area. of the core is effectively electrically shielded against deposition of. material by the relatively large charged surface area surrounding it. The surface charge markedly inhibits the collection of coating material upon the support by repelling the similarly charged. coating material particles therefrom. This .conduces toward extending and rendering less frequent the periods for cleaning the supports'and makes it possible to mount the articles in rapid succession upon the supports without requiring special.

care and, attention on the part of the operator. By the use of the present inventioma proper electrical contact is established merely by mounting the article upon the support, and assurance of maintenance of the contact isafexample, as. the type having-floor supported spindle type As soon as the ar-.

supports in place of the pendant rotators shown in Fig. 1.

Another improved support to be used with an overhead conveyor is illustrated in Fig. 7. This support for a bucket 52 is designed not only to contact the article electrically and carry it through the spray zone but also serves to keep the material from depositing on the inside surface of the bucket and from depositing it in too heavy a coating on the exposed upper ring of the bucket. In this modification of the invention there is provided a rotatable hanger 50 arranged to be supported from an overhead conveyor (not shown) which hanger includes an open ended cylinder or sleeve 53 of conducting material having radial hanger rods 54 welded or otherwise secured therein, and spring tongues 55 extending below its bottom portion. The radial hanger rods 54 converge to attach to a central support rod that connects with the conveyor. To reduce the deposition of coating material on the sleeve 53 while still permitting it to serve its function of shielding the inside surface and upper edge of the container 52, a sheath of insulating material 56 which is capable of accumulating and retaining a surface charge is applied over its outer surface.

In use, the bucket 52 to be supported is sprung into place so that the extending tongues 55 recess into the groove 57 of the bucket. The lower edge of the sleeve 53 is thus held in aligned spaced relation above the beaded edge 58 of the bucket to be coated. This relationship of sleeve edge to bucket top inhibits the deposition of material on the inside of the bucket and the presence of the surface charge on the covering 56 reduces, by electrostatic repulsion, the material collected at the exposed top rim of the bucket and support.

If it is desirable to do so, the entire conducting core consisting of cylinder 53, radial rods 54 and central rod 50 extending to the conveyor may be coated externally with a suitable insulating coating 77 (Fig. 8) after the manner described above in relation to the support of Fig. 5. Only the point of attachment to the conveyor and the spring tongues 55 and the extreme lower edge of cyl inder 53 are left exposed or uncoated.

By providing such an insulating outer coating on the core of the support a surface charge can be maintained on the support and coating material will thus be inhibited from collecting upon it. The entire coating operation will thus be improved. It will be noted that the essential characteristic of the invention lies not in the type of book or support used nor in the nature of the conveyor or electrode system with which it is used, but rather in encasing the support within an insulating material whose dielectric strength and thickness is such as to permit a surface charge to be established and maintained.

I am aware that it is common practice in the electroplating art to use coated article supports to prevent the metallic ions from collecting on the supports used to hold the articles in the plating bath. Under these circumstances the conducting hook is covered only with a coating which is adequate to prevent the charged ions in the solution from contacting the conducting part of the hook and thus yielding their electrical charge to the hook to become deposited thereon as metallic atoms. In the case of electrostatic coating, supports cannot be kept clean by the simple expedient of coating them with a covering that prevents the charged coating material particles from contacting the conducting portion of the support. The coating on the support, if it is to serve its intended function of keeping coating material particles from collecting on its surface, must not only keep the coating particles from contacting the conducting part of the support but further must actually be such as to repel these particles from its surface. The novelty of the present invention resides in providing an article support which in use with an appropriate electrostatic field actually has this necessary repelling character.

It is to be understood that the invention is not limited to use with the coating apparatus shown, but will be applicable with any coating operation wherein charged particles of adherent material are distributed in an ionizing electrostatic field adjacent a surface to be electrically attracted thereto to form a protective, decorative or functional coating.

What I claim is:

1. In an electrostatic coating apparatus having a conveyor for moving articles to be coated through a coating zone, said conveyor comprising a plurality of electroconductive hook-like article supports each having an articleengaging portion, means including an atmosphere ionizing electrode and a high-voltage source having its opposite terminals connected respectively to the ionizing electrode and conveyor, and means for creating in the coating zone a cloud of adherent coating-material particles charged oppositely to the article supports and the articles supported thereby, each of said hooks being provided with a permanent sheath of insulating material sufiiciently non-conductive to support a substantial surface charge for the duration of the passage of the article support through the coating zone, said sheath encasing all portions of the articles support exposed to the coating zone except that support-portion which is engaged by the article whereby to intercept charged particles attracted toward the sheathed support and thus acquire a surface charge repellent of the coating material particles.

2. Electrostatic coating apparatus as set forth in claim 1 with the addition that said sheath is of glass.

3. Electrostatic coating apparatus as set forth in claim 1 with the addition that said sheath is of porcelain.

4. Electrostatic coating apparatus as set forth in claim 1 with the addition that said sheath is of polyethylene.

5. Electrostatic coating apparatus as set forth in claim 1 with the addition that said sheath is of non-conductive rubber.

References Cited in the file of this patent UNITED STATES PATENTS 1,855,869 Pugh Apr. 26, 1932 2,097,233 Meston Oct. 26, 1937 2,231,324 Crompton Feb. 11, 1941 2,247,963 Ransburg July 1, 1941 2,604,870 Blood et al July 29, 1952 2,658,009 Ransburg Nov. 5, 1953

Claims

1. IN AN ELECTROSTATIC COATING APPARATUS HAVING A CONVEYOR FOR MOVING ARTICLES TO BE COATED THROUGH A COATING ZONE, SAID CONVEYOR COMPRISING A PLURALITY OF ELECTROCONDUCTIVE HOOK-LIKE ARTICLE SUPPORTS EACH HAVING AN ARTICLEENGAGING PORTION, MEANS INCLUDING AN ATMOSPHERE IONIZING ELECTRODE AND A HIGH-VOLTAGE SOURCE HAVING ITS OPPOSITE TERMINALS CONNECTED RESPECTIVELY TO THE IONIZING ELECTRODE AND CONVEYOR, AND MEANS FOR CREATING IN THE COATING ZONE A CLOUD OF ADHARENT COATING-MATERIAL PARTICLES CHARGED OPPOSITELY TO THE ARTICLE SUPPORTS AND THE ARTICLES SUPPORTED THEREBY, EACH OF SAID HOOKS BEING PROVIDED WITH A PERMANENT SHEATH OF INSULATING MATERIAL SUFFICIENTLY NON-CONDUCTIVE TO SUPPORT A SUBSTANTIAL SURFACE CHARGE FOR THE DURATIN OF THE PASSAGE OF THE ARTICLE SUPPORT THROUGH THE COATING ZONE, SAID SHEATH ENCASING ALL PORTIONS OF THE ARTICLES SUPPORT EXPOSED TO THE COATING ONE EXCEPT THAT SUPPORT-PORTIN WHICH IS ENGAGED BY THE ARTICLE WHEREBY TO INTERCEPT CHARGED PARTICLES ATTRACTED TOWARD THE SHEATHED SUPPORT AND THUS ACQUIRE A SURFACE TOWARD REPELLENT OF THE COATING MATERIAL PARTICLES.