US2888362A - Method and apparatus for electrostatically coating an article - Google Patents

Description

y 6, 1959 w. A. STARKEY 2,888,362

METHOD AND APPARATUS FOR ELECTROSTATICALLY COATING AN ARTICLE Original Filed Aug. 8, 1946 2 Sheets-Sheet 1 Fig.2

qjg w l F|g.3 INVENTOR.

William A. Starkey May 26, 1959 w. A. STARKEY METHOD AND APPARATUS FOR ELECTROSTATICALLY COATING AN ARTICLE Original Filed Aug. 8, 1946 2 Sheetg-Sheet 2 B. y Fig.8

INVENTOR.

William A Starkey Nd47 awun/ Unite States Patent METHOD AND APPARATUS FOR ELECTROSTATI- CALLY COATING AN ARTICLE William A. Starkey, Columbus, Ind., assignor to Ransburg Electro-Coating Corp, Indianapolis, Ind., a corporation of Indiana Continuation of application Serial No. 689,116, August 8, 1946. This application May 26, 1952, Serial No. 290,007

15 Claims. (Cl. 117-47) The present invention relates to electrostatic coating of articles and material.

In the electrostatic coating of articles it is often desirable to coat articles made of materials that possess electrical insulating properties.

Heretofore, it has been found difficult to coat some types of such articles by electrostatic coating means. This situation particularly arises where the insulating material is exceptionally thick.

Also in the coating of articles, it is often desirable to apply a thick, uniform coating. However, heretofore it has been difiicult to achieve such thick, uniform coating without objectionable sagging.

One object of the present invention is to provide a simple and inexpensive method and apparatus for coating, with improved results, material that normally possesses electrical insulating properties.

A further object of the present invention is to provide an improved method and apparatus whereby coating material whether miscible or immiscible with water can be applied more effectively to porous, electrically insulating materials.

A further object of the present invention is to provide means whereby an exceptionally thick, uniform coating can be applied to material without appreciable sagging.

In accordance with the present invention material to be coated possessing electrical insulating properties is treated to temporarily reduce its capability of accumulating a surface charge. Next, the treated material is subjected to an electrostatic field. Preferably the electrostatic field is created by a source of high voltage which is connected between a discharge electrode and the treated material. However, any other way of establishing an electrostatic field is suitable. For example, it is also satisfactory to create the electrostatic field between electrodes and place the treated material therebetwcen. In any event, the next step is the introduction of particles of coating material to the electrostatic field for deposition of the coating particles on the treated material.

The fact that the reduction in the electrical insulating properties of the article to be coated is only temporary can be very desirable where the restored electrical insulating properties of the coated articles are of importance in their ultimate use.

Material to be coated such as rubber, glass or ceramics may be treated by heating. If the material to be coated is receptive to impregnation by liquids it may be treated by spraying with, or dipping in, conducting liquids. One of the most common of such liquids is water.

If the coating material being used has hydrophobic characteristics, it is desirable not to have it come in contact with water. In such case, either water should not be used as the treating liquid, or if it is used, the impregnation of the material to be coated should be partial so that the surface to which the coating material is applied is not wetted by the water.

Where the material to be coated is treated by heating,

it is thereafter subjected to an electrostatic field, which serves as an extended coating zone. Here the coating material is continuously deposited on the heated material ice I as it progressively cools and a thick coating Without the usual sagging or running results. 7

The means for introducing atomized coating material into the electrostttic field may comprise one or more spray guns positioned at one or both ends of the electrostatic field. The positioning of the spray means depends upon the shape of the material being coated, the coating material being used, and the treatment the material to be coated is receiving.

As examples of electrical insulating materials capable of accumulating a surface charge in an electrostatic field, I may mention hard rubber, especially where the graphite content is below approximately twelve percent, hard'rubber coated steering wheels, Bakelite, Acoustic Tile, Masonite or hard board (compressed wood fibres), Celotex, wallboard of compressed asbestos fibres, papier mach'e, plaster of Paris, glass, ceramics, cardboard and fabrics.

Examples of aqueous coating materials which may be used with this invention are solutions or emulsions of casein paint such as those known in the trade as calcimine, Kemtone and Texolite. However, other coating materials of non-setting or setting character or materials adhesive in character, such as lacquers, paints or varnishes may be used.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

Referring now to the drawings: Fig. 1 is a diagrammatic side view of one embodiment of the invention,

Fig. 2 is an enlarged diagrammatic side view of a portion of the apparatus shown in Fig. 1.

Fig. 3 is a transverse sectional elevation on an enlarged scale of a portion of the conveying mechanism shown in Fig. 2,

Fig. 4 illustrates an article that may be coated,

Fig. 5 is a plan view of another embodimentof the invention,

Fig. 6 is a side view of the apparatus shown in Fig. 5, Fig. 7 is an end view of the apparatus shown in Fig. 6, and

Fig. 8 is a diagrammatic view showing stages of coating accumulation.

Referring now to Fig. 1, at one end thereof is shown a liquid treating apparatus which includes an overhead'housing 10 and a drain basin 11. A nozzle 12 is inserted in the top of the housing 10 as shown and is connected to the basin 11 by piping 13. In the piping 13 isinserted a pump 14 to move the liquid from the basin 11- to the nozzle 12 so that the drained liquid may be re-used. The conducting liquid used in the liquid treating apparatus is introduced to it preferably by a connection, not shown leading to the basin 11.

While a liquid treating apparatus adaptable to fully automatic operation is desirable my invention is not limited to such apparatus. Articles may also be treated'b'y hand dipping or any other suitable means or method.

An article to be treated, for example, article 16, which is a piece of acoustic tile, may be introduced to the liquid treating apparatus by a suitable conveyor 17 :having a grooved supporting bed 18 over which ride a number of cables 10 as more clearly shown in Fig. 3.

When it is desired a tum-over mechanism maybe provided at the exit end of the conveyor 17. This mechanism includes a supporting member 21 which :isidriven by the conveyor 17 through a gear ratio device 22 and a pinion member 23 as shown in Fig. l.

Adjacent the turn-over mechanism, is another conveyor 25 which, as shown, may be of the same construction as conveyor 17 (see Fig. 3). The conveyors .17

and 2S and the turn-over mechanism are so arranged that the treated articles are taken from the conveyor 17 by the tum-over mechanism, turned over and placed on the conveyor 25 with the untreated side of the article exposed. The conveyor 25 moves the treated articles into a spray booth which preferably is ventilated by drawing a current of air in the general direction of the spray by means of the ventilating system which includes an exhaust chamber 27. An electrode frame structure 28 is supported from the top of the spray booth 26 by insulators 29. The frame 28 supports a plurality of suitable ionizing or discharge electrodes 30, which in this embodiment are shown as fine wires and extending transversely to the path of the conveyor 25.

The frame 28 is connected to one terminal of the high voltage source by conductor 31. The other terminal of the high voltage source may be grounded (as shown) in which case the conveyor 25 is also grounded; or it may be connected directly to the conveyor 25. The high voltage source is of a type to supply a suificiently high voltage to produce an electrostatic ionizing zone between the frame 28 and the bed of the conveyor 25. At the exit end of the booth 26 are positioned one or more spray guns 33 preferably of the type which project into the electrostatic field finely divided liquid coating particles in a spray or diverging carrier air-stream. The spray guns 33 are positioned preferably so that the streams therefrom are directed into the electrostatic field, the axes of such streams being more nearly parallel than normal to the plane of the surface of the material being coated as shown in Fig. 2. The velocity of the streams from the guns 33 is adjusted so that when the particles from the streams reach the electrostatic field they are deposited on the top and sides of the articles. In some cases it is desirable to direct each gun so that its stream travels substantially parallel to the surface being coated and in still other cases it can be directed away from parallel to the material surface. In these latter cases it is found that with these gun positions the action of gravity combined with the distributing ionizing action of an extendedfield reduces the velocity of the particles and increases the uniformity of the coating obtained.

The character and quality of the coverage of a particular article is determined by the number of guns employed, their positioning, the velocity of the stream from theguns, .and the size and shape of the article being coated.

While two conveyors are shown in Fig. 1, this is not a limitation to the invention here disclosed. Any suitable apparatus for presenting the materials or articles to be coated to their various treatments may be used.

In the modification shown in Fig. 4, a hollow manikin head 41 is supported on a grounding support comprising a disc or spider 42 and a rod 43. The head 41 is composed of plaster of Paris, papier mache or similar porous material. Water or other electrically conducting liqiud may be sprayed or poured into the interior of the head and thus the material of the head may be wetted to any desired degree. The penetration of the liquid in the material depends on the character of the liquids, length of time the liquid remains in contact with the material and the character of the material of which the head is composed. When the desired degree of penetration is obtained the head is removed from contact with the liquid source, and the head 41 may be coated by a suitable electrostatic coating apparatus. During electrostatic coating the head is grounded and made as one terminal of the field by the internal contact as shown.

When water is used to treat such article as head 41 or article 16 as shown in Fig. 1, it is not necessary to prevent full penetration if a coating material readily miscible with water is used. But care should be exercised to avoid excessive penetration, if a coating material immiscible with water is used. Complete penetration in 4 the latter case may result in blisters or poor adhesion of the coating material.

By hydrophobic material when used herein I mean a material which is immiscible with water. By hydrophylic material when used herein I mean a material which is miscible with water.

In Figs. 5, 6, and 7 another embodiment of my invention is shown. The conveyor 51 is shown diagrammatically. At one end of the conveyor 51 there is provided a heat treating chamber 52 also shown diagrammatically.

The conveyor 51 is adapted to move articles 53, shown in the drawings as automobile steering wheels through the chamber 52 for heating the articles to an elevated temperature.

Where a uniform coating is desired a rotating apparatus should be used for rotating the articles to be coated as they are moved through the spray booth 54. Such rotating apparatus used in conjunction with a conveyor is shown and described in Patent No. 2,247,963 of Harold P. Ransburg et al., issued on July 1, 1941.

From the heat treating chamber 52 the conveyor 51 passes through a spray booth 54. In this particular arrangement the conveyor makes a degree turn after entering the booth 54. However, this is not a necessary limitation to my invention. It is equally satisfactory if the conveyor 51 is continued through both the chamber 52 and the booth 54 in substantially a straight line or in any other suitable manner. At one end of the booth 54 there is located an exhaust chamber 55 connected by a duct with a suitable fan not shown. Ports 56 are located between plates 57 to permit uniformly distributed withdrawal of air from the spray booth. Toward the end of the booth opposite the exhaust chamber an electrode frame 58 is supported by insulators 59 depending fromthe top of the booth. The frame 58 is in theform of an inverted trough-shaped electrode extending along the conveyor and through which the articles 53 pass as they proceed through the booth. The frame 58 carries a plurality of spaced ionizing or discharge electrode elements 60 preferably in the form of fine wires or points which are connected through a conductor 61 to a suitable high voltage source to create an electrostatic field between all parts of the electrode and the article being coated and which field on all sides is directed toward the article. The frame 58 is of a length sufficient to embrace a plurality of the articles arranged in spaced relation on the conveyor and the ionizing elements are arranged in spaced relation along the length of said frame. The purpose of such arrangement will become evident as the description proceeds.

At the end of the chamber 54 there is provided a plurality of spray guns 62 for introducing atomized coating material to the field. Although any number of guns may be employed, in this particular embodiment only two are shown. These guns are preferably positioned on opposite sides and slightly above and below the upper and lower faces of the articles 53 in such a manner that the spray paths issuing from them are directed at an acute angle to the path of articles as they are progressively advanced by the conveyor 51. By this arrangement each gun acts as a complement to the other. The spray pattern from the upper gun projects spray toward the upper surface of the article and stray particles passing downwardly through openings in the article or around articles, are reionized by the opposed portion of the field and are thereby deposited on the underside of the articles. Conversely, the spray pattern from the lower gun is projected upwardly and deposits a coating on the under surface of the object and particles passing upwardly through openings in the article or around articles are reionized and electrostatically deposited on the upper surface of the articles. Material issuing from guns so positioned will either be deposited on the object directly by the combined mechanical and electrical action of the air-blast and the field respectively, or will pass through the openings in the arti- .ments on the opposite sides of the articles.

cles or the spaces around the articles. The by-passing material will come under the influence of ionizing ele- In general such free material on all sides of the articles will form a cloud 64 about the path of the articles as shown in Fig. ,6. This cloud extends along the conveyor under the combined confining action of the field and the extending action of the air currents. This material is thus exposed to the successive ionizing elements 60 along the frame 58 and if not thus deposited by the action of the field in its first, second or third repulsion will for the most part certainly be deposited before it escapes from the field. The positioning of the guns may be altered or changed in accordance with the requirements of the particular articles being coated and the coating material being used and the positioning above specified is not meant to be limiting in any way.

The heat treatment applied to the articles 53 as they pass through chamber 52 effects a material reduction in their inherent tendency to accumulate a surface charge and by this means they are rendered more receptive to the particled coating as it is applied in the electrostatic field. By means of the forced draft created by blowers 65, the

exhaust chamber 55 and heat exchanger 66 cold or hot air may be introduced into the spray zone to aid in controlling the temperature of articles and the path of the s ay- While in Fig. the heating chamber 52 is shown as being spaced from the spray booth 54, it is to be understood that such chamber may be mounted directly adjacent to the booth or that heat radiating fins such as suggested by the dotted lines 67, may be mounted on the chamber in such manner that the fins project toward the booth to maintain the articles at a desired temperature.

Where articles to be coated are already heated to the proper temperature due to methods of manufacture or any other'reasons they may be directly introduced to the spray booth 54 without first passing through the heat chamber 52.

In addition to the fact that the preliminary heating reduces the tendency of electrical insulating material to accumulate surface charge, it has a marked benefit in the electrostatic spray coating of all types of materials. In practice, where the preliminary heat treatment of material is used, in conjunction with electrostatic spraying it is possible to apply coating films which are markedly heavier than can be obtained by ordinary methods. In electrostatic spraying the extended region over which the coating material is applied results in the coating being gradually and continually built up to its ultimate thickness. The nature of this gradual film growth will be understood by reference to Fig. 8 which shows various sections taken at various stages of the coating process. The sections are labeled A to E and are illustrative of samples removed from the process at points A to E in the field as shown in Fig. 2.

It will be seen that as the article first enters the field a light mist coat is applied at A. This coat is gradually increased through B, C, and D until at E a full uniform coat is obtained.

One of the properties of this film accretion is the fact that it allows the various portions of the fihn to be exposed continually and hence setting will occur more rapidly. This more rapid setting of the applied material has the advantage that more material and hence more film thickness can be applied to the surface of the articles than has been possible by other methods and still not have the film so wet and so thick that it sags. These advantageous features of electrostatic spraying and of heating are accentuated when these processes are used in combination to finish an article. The heating of the article hastens the setting process and thus in combination with electrostatic spraying a thicker coating can be obtained with their combined action than it is possible to obtain with both processes used separately.

assssee This invention has been used to apply a protective and decorative coating to rubber automobile steering wheels. The heavy coating obtainable by this process is desirable because of the condition of extreme abrasion to which such wheels are subjected during their use. In' this application the wheels were run on an apparatus similar to that shown in Figs. 5, 6, and 7. The Wheels were preheated to a temperature of 250 F. and then introduced into the spray zone. The discharge electrode frame which extended for four feet along the conveyor in the spray Zone was in the form of an inverted trough and was spaced from the wheels at a distance of twelve inches. The ionizing or discharge wires on the electrode frame were circumferentially located about the trough and were placed every eight inches along its length. A static voltage of 100,000 volts was applied to the discharge electrode to create the field between it and the heated wheels. Atomized coating material was introduced into the field about the wheels from guns positioned approximately as shown and described. The initial velocity of the atomized coating material as it entered the field was approximately 400 feet per minute and the general air velocity through the booth as created by blowers 65 and exhaust 55 was approximately feet per minute and uniformly distributed. The wheels were rotated appproximately 3 revolutions for each foot of linear travel. Under these conditions the desired heavy over all coating was obtained on the wheels in one continuous coating operation when the linear travel of the conveyor was 22 feet per minute and the spacing of the wheels on the conveyor was on 25 inch centers.

Thus it is seen that the electrostatic coating method and apparatus described coats material having electrically insulating properties simply, efficiently and satisfactorily and also by its use a thicker, even, uniform coating on any material may be obtained with the same amount of coating material than has been possible heretofore with available methods and apparatus.

Whereas the articles to be coated are shown and described herein as rubber coated steering wheels, acoustic tile and manikin heads for illustrative purposes, they may be of any character comprising either conducting or nonconducting material.

This application is a continuation of my prior application Serial No. 689,115, filed August 8, 1946, now forfeited.

I claim as my invention:

1. The method of coating an article comprising the steps of heating the article, creating an extended electrostatic field in a coating zone, introducing into said field and longitudinally thereof in finely divided condition a settable liquid coating material, passing said article after it has been heated longitudinally through said field generally in an opposed direction to the movement of said coating material, forcing a gaseous cooling medium longitudinally through said field generally in the direction of movement of said coating material, and depositing said coating material onto said article by said field as the temperature of said article decreases and as said article passes through said coating zone.

2. The method of coating an article comprising the steps of heating the article, creating an extended electrostatic field in a coating zone, introducing into said field in finely divided condition a settable liquid coating material, passing said article after it has been heated longitudinally through said field, forcing a gaseous cooling medium longitudinally through said field in an opposed direction to the movement of said article, and depositing the coating material onto said article by said field as it is cooled by said medium and as it passes through said coating zone.

3. The method of coating an article comprising the steps of heating the article, creating an extended e1ectrostatic field in a coating zone, introducing intosaid field- 7 in finely divided condition a settable liquid coating material, passing the article after it has been heated longitudinally through said field, progressively cooling said article as it is passing through said field, and depositing the coating material onto said article by said field as it passes through the coating zone.

4. Apparatus for electrostatically coating an article comprising a cabinet having an entrance and an exit, conveyor means for moving the article through said cabinet from said entrance to said exit, means for heating the article prior to its entry into said cabinet, an extended discharge electrode in said cabinet in spaced relation to said conveyor means, a voltage source electrically associated with said electrode and the article for providing an extended field of high potential diflerence therebetween, means adjacent said exit for directing a gaseous cooling medium into said cabinet longitudinally along said conveyor means for reducing the temperature of the article as it moves past said extended electrode, and means for introducing a spray of finely divided, settable coating material into said cabinet between said electrode and the article and generally in the direction of movement of said cooling medium for electrostatic deposition on the article.

5. Apparatus for electrostatically coating an article comprising a booth having an entrance and an exit, conveyor means for moving the article through said booth from said entrance to said exit, a discharge electrode in said booth in spaced relation thereto and extending along the path of article movement, a voltage source electrically associated with said electrode and the article for providing a field of high potential difierence therebetween, means for introducing a spray of finely divided coating material into said booth generally parallel to the path of article movement for electrostatic deposition on the article, means adjacent said exit and independent of said material introducing means for impelling a gas stream through said booth generally parallel to the path of article movement, and additional gas controlling means adjacent said entrance for aiding the movement of said gas stream through said booth generally parallel to the path of article movement.

6. Apparatus for electrostatically coating an article comprising a booth having an entrance and an exit, a conveyor means for moving the article through said booth from said entrance to said exit, a discharge electrode in said booth in spaced relation thereto and extending along the path of article movement, a voltage source electrically associated with said electrode and the article for providing a field of high potential difference therebetween, means for introducing a spray of finely divided settable coating material into said booth generally parallel to the path of article movement for electrostatic deposition on the article, means adjacent said exit and independent of said material introducing means for forcing a gas stream through said booth generally parallel to the path of article movement, means for controlling the temperature of the gas stream introduced by the last mentioned means, and additional gas controlling means adjacent said entrance for aiding the movement of said temperature controlled gas stream through said booth generally parallel to the path of article movement.

8 exit opening, conveyor means for moving the article through said booth from said entrance opening to said exit opening, a discharge electrode in said booth in spaced relation to and extending along the path of article movement, a voltage source electrically associated with said electrode and the article for providing a field of high potential difference therebetween, means for atomi zing and directing coating material into said field toward the path of article travel for electrostatic deposition on the article, means independent of said material atomizing means for impelling a gas stream into said field from one end thereof, and gas withdrawing means for Withdrawing a gas stream from the other end of the field,

9. In a method of coating with a solvent containing paint an article having a substantially electrically nonconductive elastomer covering, the steps of heating the article to a temperature suflicient to materially increase the electrical conductivity of the covering, passing the heated article into an electrostatic field wherein the article is an electrode, and simultaneously spraying a solvent containing paint on the article whereby a uniform, strongly adhering coating is obtained.

10. A method as claimed in claim 9 wherein the nonconductive elastomer covering includes a small proportion of a conductive material dispersed therethrough 11. In a method of coating with a solvent containing paint an article having a substantially electrically nonconductive rubber covering, the steps of heating the article to a temperature sufficient to materially increase the electrical conductivity of the covering, passing the heated article into an electrostatic field wherein the article is an electrode, and simultaneously spraying a solvent containing paint on the article whereby a uniform, strongly adhering coating is obtained.

12. In a method of coating with a solvent containing paint an article having a substantially electrically nonconductive rubber covering, the steps of heating the article to a temperature of 250 F., passing the heated article into an electrostatic field wherein the article is an electrode, and simultaneously spraying a solvent containing paint on the article whereby a uniform, strongly adhering coating is obtained.

13. In a method of coating with a solvent containing paint an article having a substantially electrically nonconductive elastomer covering, in which method the article is made an electrode in an electrostatic field and the solvent containing paint is sprayed into the field for electrostatic deposition on the article, the steps of preheating the article prior to its exposure in said field to a temperature sufiicient to materially increase the electrical conductivity of the covering, and exposing the article to the field while so preheated.

14. In the method of coating a substantially electrically non-conductive article made from rubber, which article includes a metal insert therein, with a solvent containing paint, the steps of: heating the article to a temperature of 250 F., passing the heated article into an electrostatic field of defined character, and simultaneously spraying a solvent containing paint on the article whereby a uniform, strongly adhering coating is obtained.

15. In the method of coating a substantially electrically non-conductive article made from an elastomer, which article includes a metal insert therein, with a solvent containing paint, the steps of: heating the article to a temperature in the neighborhood of 250 F., passing the heated article into an electrostatic field of defined character, and simultaneously spraying a solvent containing paint on the article whereby a uniform, strongly adhering coating is obtained.

References Cited in the file of this patent UNITED STATES PATENTS 2,555,519 Tolle et al. June 5, 1951

Claims (1)

1. THE METHOD OF COATING AN ARTICLE COMPRISING THE STEPS OF HEATING THE ARTICLE, CREATING AN EXTENDED ELECTROSTATIC FIELD IN A COATING ZONE, INTRODUCING INTO SAID FIELD AND LONGITUDINALLY THEREOF IN FINELY DIVIDED CONDITION A SETTABLE LIQUID COATING MATERIAL, PASSING SAID ARTICLE AFTER IT HAS BEEN HEATED LONGITUDINALLY THROUGH SAID FIELD GENERALLY IN AN OPPOSED DIRECTION TO THE MOVEMENT OF SAID COATING MATERIAL, FORCING A GASEOUS COOLING MEDIUM LONGITUDINALLY THROUGH SAID FIELD GENERALLY IN THE DIRECTION OF MOVEMENT OF SAID COATING MATERIAL, AND DEPOSITING SAID COATING MATERIAL ONTO SAID ARTICLE BY SAID FIELD AS THE TEMPERATURE OF SAID ARTICLE DECREASES AND AS SAID ARTICLE PASSES THROUGH SAID COATING ZONE.

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