US3392044A - Electrostatic coating apparatus and method for applying a hammertone finish to an article - Google Patents

Description

y 9, 1968 R. o. NORCROSS ET AL 3,39 ,0

ELECTROSTATIC COATING APPARATUS AND METHOD FOR APPLYING A HAMMERSTONE FINISH TO AN ARTICLE Filed Sept. 23, 1964 2 Sheets-Sheet 1 ii"? fl w 5 76 .5 o9? E A i 6/; 614/ I cfi r Aea POWER 25 suPPLY 65 23- UB/ m ''1 PP'T java/71%;: 5&4? 0.]61fic7/va4 a 'lupewdae July 9, 1968 R. o. NORCROSS ET AL 3,392,044

ELECTROSTATIC COATING APPARATUS AND METHOD FOR APPLYING A HAMMERSTONE FINISH TO AN ARTICLE Filed Sept. 23, 1964 2 Sheets-Sheet 2 United States Patent 3,392,044 ELECTROSTATIC CGATING APPARATUS AND METHOD FOR APPLYlNG A HAMMERTONE FENISH T0 AN ARTKCLE Ralph 0. Nor-cross, Indianapoiis, and Frank L. Perdue,

Brownsburg, IntL, assignors to Ransburg Electra-Coating Corporation, a corporation of Indiana Filed Sept. 23, 1964, Ser. No. 401,286 20 Claims. (Cl. 11737) ABSTRACT OF THE DISCLOSURE An electrostatic coating apparatus and method for applying a hammertone finish to an article. The article is carried on a loop conveyor through a coating zone around a rotating disc atomizer which is reciprocated vertically to make several applications of coating material to each article as it passes through the coating zone. An operating condition is changed in synchronism with the reciprocation of the atomizer to vary the particle size during succeeding applications.

This invention is concerned with an apparatus and method for applying a hammer finish to articles, utilizing electrostatic coating techniques.

The hammer finish is one in which the coating, as a paint, is patterned, having an appearance similar to that of a surface tapped with the rounded end of a ballpeen hammer. A finish of this type aids in hiding surface defects of the article, yet is smooth and may readily be cleaned by Wiping, in contrast with a wrinkle finish which traps dirt in small cracks.

In the past, hammer finishes have been applied by either of two methods. In the first, utilizing a two-coat application, the article is first sprayed with the desired paint in which there has been dispersed a small percentage of leafing or non-leafing metallic pigment. Then, when the paint is partially set, it is sprayed with droplets of paint thinner which produce localized wet areas in which metallic pigments can orient to produce the desired pattern. The second method produces a hammer pattern with a one-coat hammer material. A one-coat hammer coating material has a component which is incompatible with other components of the coating material so that areas will be formed on the deposited film where this incompatibility shows itself as the film dries to form a hammer pattern. Great care is required in adjusting the solvent content of these one-coat paints to insure that the resulting deposited film has the correct degree of wetness to produce a large hammer pattern while achieving a satisfactory film thickness without sagging of the film. This solvent adjustment has made the application of one coat hammer finishes difiicult to control. To aid in overcoming this diificulty, one-coat hammer finishes normally are applied with an air spray device where the degree of atomization and the rate of film build-up can readily be controlled.

In the electrostatic application of hammer finishes, this sensitivity has been more difficult to overcome, Because of the fine atomization which is obtained with electrostatic spraying apparatus under normal operating conditions, the paint arrives at the article to be coated in a relatively dry condition so that the areas of incompatibility cannot develop within the coating material and the pigment orientation in these areas does not become apparent to produce marked hammer patterns. The addition of high boiling point solvents does not completely solve the problem because of the difficulty of obtaining a satisfactory film thickness without the film sagging. Attempting to spray electrostatically under conditions of poor atomization results in poor transfer efficiency, poor wrap around deposition, and a poor over-all quality of finish. Use of the two-coat method has the obvious disadvantage when compared to a one-coat operation of requiring a duplication of equipment and labor.

This invention is concerned with the utilization of electrostatic coating apparatus and methods in the application of a hammer finish. An improvement in the uniformity of the hammer design is achieved in addition to the high quality finish and coating material savings associated with electrostatic coating operations.

One feature of the invention is the provision of an apparatus for applying a hammer finish to an article including a source of coating material connected with an atomizing device together with a source of high voltage which establishes an electrostatic field between the atomizing device and articles being coated, and a means for varying the gradient of the electrostatic field to vary the size of the coating material particles. More particularly, a rotating disc atomizer is utilized and in a preferred embodirnent of the invention the high voltage is cyclically varied, varying the electrostatic field gradient between the disc and article, to cause variations in the coating characteristics of the apparatus and thus produce the hammer finish. A degree of hammer effect may be secured by varying the speed of rotation of the disc to produce non-uniformities in the rate of flow of coating materials to all, or a portion of, the disc edge or by varying rate of flow of coating material to the disc through control of the coating material pump. At the present time, however, these characteristics are more difiicult to control because of the inertia involved in the system, and do not provide the Wide range of control of the coating characteristics achievable by varying the field gradient. Any one or a combination of the characteristics may be varied, however, depending on the nature of the pattern desired.

Another feature of the invention is that the article being coated passes through an extended coating zone and the disc distributes coating material in such a manner that several applications of the material are made to the article. The field gradient or other coating characteristic is cycled in synchronism with the several applications of coating material so that the design is uniform over the entire extent of the article.

A further feature of the invention is that the average electrostatic field gradient is varied from a maximum of the order of about 5 kilovolts per inch to a minimum of the order of about 1.5 kilovolts per inch.

Still another feature of the invention is the method of applying a hammer coating to an article including the steps of atomizing coating material, establishing an electrostatic field which affects the transfer of atomized particles of coating material to the article being coated, and cyclically varying an operating condition of the atomizing step to vary the size of the particles of coating material produced.

Further features and advantages of the invention will readily be apparent from the following specification and the drawings, in which:

FIGURE 1 is a diagrammatic plan view of an apparatus embodying the invention;

FIGURE 2 is an elevation of the apparatus of FIG- URE l; and

FIGURE 3 is the schematic diagram of a control circuit for the apparatus.

The advantages of electrostatic coating in terms of quality of the finish and efficiency of coating material deposition are well known and will not be discussed in detail here. It is sufiicient to note that the present invention provides an apparatus and method for achieving an improved hammer finish while at the same time enjoying 6 many of the other benefits of electrostatic coating procedures.

Turning now to the drawings, FIGURES 1 and 2 illustrate an electrostatic coating'system utilizing a rotating disc atomizer of the type illustrated in Simmons Re. 24,- 602, which may be utilized in the practice of the present invention. In FIGURE 1, a rotating disc 19 is shown digrammatically, with the articles to be coated, as panels 11, carried by a conveyor in a generally annular path 13 around the disc. Coating material is discharged from the rotating disc around the periphery thereof providing an extended coating zone through which the panels 11 move. As will appear, the disc 10 is maintained at a high DC potential with respect to the articles which are grounded establishing an electrostatic field therebetween, which charges the particles atomized from the edge of the rotating disc and effects deposition thereof on the articles to be coated.

The DC potential which establishes the electrostatic field may be, but is not necessarily, well filtered. The specific potential figures given represent the average value of the unidirectional potential without regard to the peak value of an alternating component which may be present. I

In order to coat a large article completely, it may be necessary to provide for relative movement between the disc and article, as by vertically reciprocating the disc while articles pass through the coating zone. FIGURE 2 illustrates such an apparatus. The articles 11 are supported by hangers 14 from a conveyor 15 in the generally circular path around atomizing disc 16. Disc 10 is carried on a shaft 17 which extends from disc drive motor 18. Drive shaft 17 extends through ametal tube 20 which depends from an insulating support 21 and passes through a metal sleeve 22 carried by a fixed insulating support 23. A high voltage DC power supply 24 is mounted on insulating member 23 and has one terminal connected through a high voltage cable 25 with sleeve 22 and the other terminal connected to a conductive frame member 26, providing a reference or ground for the high voltage.

Coating material is supplied to the upper surface of disc 10 by a pump 28, from a paint supply 29, through a flexible tube which discharges the paint at a point near the center of the disc.

The entire motor and disc assembly are mounted for vertical reciprocation, the tube 20 sliding in the sleeve 22. An arm 32, pivoted at 33, has a pin 34 at the free end thereof which extends into slot 35 of bracket 36, which is a part of the movable disc and motor assembly. Pneumatic piston and cylinder device 38 is pivotably mounted to frame member 39 and the piston rod 40 thereof is connected at an intermediate point of arm 32. Compressed air, from a source 42, is connected through conduits 43 and 44 with the piston and cylinder device 38 to effect reciprocation of the arm 32 and atomizing disc assembly 10. A suitable valve and control are shown in the aforementioned Simmons Patent Re. 24,602, to which reference may be had for further information.

The relative speeds of the articles 11 along conveyor 15 and of the reciprocation of atomizing disc 10 are such thatthe disc will undergo several complete cycles of reciprocation during the time required for articles to pass along the circular path 13 through the coating zone.

In accordance with the invention, it is desired to vary or modify an operating condition of the coating apparatus during the period of coating to cause a variation in the size of the particles of the coating material being discharged from the atomizer to improve the hammer finish. By spraying with fine particles during a portion of the coating cycle under normal operating conditions, a uniform coating can be obtained with the proper dryness to allow satisfactory film thickness while maintaining a high quality finish (without sagging) and the deposition of substantially all the paint particles on the articles to be coated. Alteration of the operating conditions to increase the paint particle size results in a splattering effect upon the uniform coating because of the increased size of the particles and their increased wetness. This approximates the advantage of a two-coat method with one-coat materials and without the duplication of equipment. The increased wetness of the larger paint particles as they are depositedis due to the reduced evaporationof solvents from the larger particles because of their increased volume to surface ratio. This invention renders the-uniformity of the resulting hammer pattern subject to control by selecting the size to which thepaint particles are increased. A decrease in atomization quality and an increase in particle size-can be caused by control ofany one or a combination of several operating conditions. For example, increasing the amount of material delivered to the atomizer as by increasing the speed of the pump 28 will result in larger paint particles, or reducing the speed of rotation of the disc 10 at least in some circumstances will result in non-uniformities in the rate of flow of paint to all or different portions of the disc periphery and, thus, larger paint particles. The most effective way of accomplishing this result, however, is by varying the level of the high voltage which establishes the electrostatic field between the atomizer and the articles to be coated. Furthermore, as the atomizer reciprocates the voltage variations can readily be synchronized with the reciprocation to achieve a uniform hammer tone finish.

An extension 45 of arm 32 carries a rod 46 at the lower end of which is mounted a housing 47 containing two switches 75 and 76 which are actuated in accordance with reciprocation of the disc 10. The points of actuation of the switches are determined by adjustable dogs 48 and 49, mounted on a rod 50 fixed to the frame of the machine. As will appear, the switches in housing 47 control the level of the high voltage applied to the disc 10. The switches in housing 47 may also be connected to control the actuation of piston and cylinder device 38, as explained more fully in the Simmons patent.

Turning now to FIGURE 3, an electrical circuit for the apparatus is shown. The lines 55 and 56 are connected with a suitable electrical source, as volts, 6O cycle AC. Housing 47 contains two normally open, single-pole, single- throw switches 75 and 76 so arranged that they are alternately actuated at the upper and lower limits of each reciprocator stroke by adjustable dogs 48 and 49 respectively. Connected through switches 75 and 76 respectively'and across the power line are the two coils 59 and '60 of a mechanically latching relay 61. The coils 59 and 60 are alternately energized at the upper and lower limits of travel of the disc. Associated with latching relay 61 are normally open contacts 61a and normally closed contacts 61b in a single-pole, double-throw arrangement.

When relay 61 is latched at one extreme of the disc movement as by energizing coil 59, contacts 61a close while 61b open. At the other limit point of disc movement, coil 60 is energized, unlatching the relay and returning the contacts to the condition shown in the drawing. Direct current power supply 24 is selectively connected through contacts 61a or 61b with two different sources of alternating current, as adjustable tapped transformers 64 and 65. The output voltage of the power supply is a function of the alternating input voltage which may be adjusted by setting the taps of the two transformers. Thus, as the disc is reciprocated, the high voltage applied thereto is cyclically varied in synchronism with the disc movement.

It is preferable that the speed of travel of the articles along the conveyor 15 be so related to the reciprocation of the atomizing disc 10 that several complete cycles or reciprocation are effected during the time of travel of the articles through the coating zone. This not only insures a complete and uniform coating on the article, but as the voltage is varied on each reciprocation of the atomizing disc, a high quality, uniform hammer design is achieved.

In a specific example, four and one-half foot panels were carried on a conveyor traveling in a'five foot loop about a disc 20 inches in diameter. The conveyor speed was six feet per minute. The disc was reciprocated through a 60 inch stroke, from a point above to a point below the panels. The speed of reciprocation was such that at least 15 passes, or 7 A complete cycles of reciprocation, were effected during the travel of a panel through the coating zone. The disc was rotated at 900 revolutions per minute and coating material was delivered thereto at a rate of 170 cubic centimeters per minute. The voltage applied to the atomizer was varied between 90 kilovolts and about 30 kilovolts.

The precise voltages and voltage ratios are not critical, and satisfactory patterns are obtained with lower limit voltages of 25 to 40 kilovolts depending upon the hammer pattern which is desired. In general, however, it is desirable to vary cyclically the average voltage gradient of the field extending from the atomizing disc to the articles being coated, between a maximum value of the order of 5 kilovolts per inch and a minimum value of the order of 1 /2 kilovolts per inch. The lower the voltage used, the larger the hammer pattern, and the poorer the transfer efficiency of the system. Accordingly, it is desirable to use as high a minimum voltage as is consistent with the pattern desired.

Variation of other conditions of the system will also produce an improved hammer pattern. For example, varying the rate of feed of coating material to the atomizing disc has an effect on the particle size as does variation in the speed of rotation of the disc.

Both the rate of coating material flow and the disc speed may be varied as indicated in the portion of the circuit of FIGURE 3 below broken line 67. Contacts 61a and 61b correspond with contacts 61a and 61b and alternately connect disc motor 18 and pump 28 either directly across lines 55 and 56 or through voltage dropping impedance 68, which causes operation at a lower speed. The range and precise timing of the operating conditions of the pump 28 will depend on the response time of the fiuid delivery system to changes in pump motor voltage. Likewise the range and timing of the operating conditions of the disc motor 18 will be determined by the inertia of the disc motor rotor and the disc itself since these inertias will determine the magnitude of disc speed change in a iven period of time. The three factors, the high voltage or field gradient, disc speed, .and the coating material delivery rate, may be varied singly or in any desired combination to produce the desired hammer pattern.

We claim:

1. Apparatus for applying to an article a coating with a hammer finish, comprising: a source of coating material; an atomizing device connected with said source for discharging finely divided particles of coating material; a source of direct high voltage; circuit means connected with said source of high voltage establishin an electrostatic field between said atomizing device and articles to be coated, for effecting movement of particles toward said articles; means for effecting cyclical relative movement between the atomizing device and article to make plural applications of coating material to the article; and means for cyclically varying an operating condition of said apparatus, in synchronism with said cyclical move ment, to vary the size of said coating material particles.

2. The apparatus of claim 1 in which the last means cyclically varies the electrostatic field gradient.

3. The apparatus of claim 1 in which the atomizing device is a rotating member and the last means cyclically varies the speed of rotation thereof.

4. The apparatus of claim 1 in which the last means cyclically varies the rate of delivery of coating material to the atomizing device.

5. The apparatus of claim 2 in which the ratio of maximum field gradient to minimum field gradient is of the order of at least 2 to 1.

6. The apparatus of claim 5 wherein the maximum and minimum field voltages are such that the average voltage gradient of said electrostatic field varies between a maximum of the order of 5 kilovolts per inch and a minimum of the order of 1.5 kilovolts per inch.

7. Apparatus for applying to an article a coating with a hammer finish, comprising: a source of coating material; an atomizing device connected with said source for discharging finely divided particles of coating material therefrom; means for causing relative movement between said atomizing device and an article through an extended coating zone adjacent said atomizing device; circuit means connected with said source of high direct voltage establishing an electrostatic field between said atomizing device and the article to be coated, for effecting movement of coating material particles toward said article; and means for cyclically varying the voltage establishing said field in synchronism with the movement between the atomizing device and the article and through a plurality of cycles during the period in which an article passes through said coating zone.

8. The coating apparatus of claim 7 including means for reciprocating said atomizing device during the passage of each article through the coating zone and means responsive to the disc reciprocating means for varying the voltage in synchronism with reciprocation of the atomizing device.

9. Apparatus for applying to an article a coating with a hammer finish, comprising: a source of coating material; a rotating disc atomizing device connected with said source for discharging finely divided coating material particles therefrom; means for transporting articles to be coated through an extended coating zone, in a generally horizontal plane, and a generally circular path around said disc; circuit means connected with said source of high direct voltage establishing an electrostatic field between said atomizing device and articles to be coated, for effecting movement of coating material particles toward said article; means for reciprocating said disc vertically through a plurality of reciprocations during the period an article passes through the coating zone; switch means actuated by reciprocation of said disc; and circuit means, actuated by said switch means for varying said high voltage in synchronism with the reciprocation of the disc, the average voltage gradient between the disc and the articles having a maximum value of the order of 5 kilovolts per inch and a minimum value of the order of 1.5 kilovolts per inch.

10. Apparatus for applying to an article a coating with a hammer finish, comprising: a source of coating material; a rotating disc atomizing device connected with said source for discharging finely divided coating material particles therefrom; means for transporting articles to be coated through an extended coating zone in a generally horizontal plane and a generally circular path around said disc; circuit means connected with said source of high voltage establishing an electrostatic field between said atomizing device and articles to be coated, for effecting movement of coating material particles toward said article; means for reciprocating said disc vertically through a plurality or reciprocations during the period an article passes through the coating zone; switch means actuated in synchronism with reciprocation of said disc; and means operated by said switch means in synchronism with reciprocation of said disc for varying the coating material particle size.

11. The apparatus of claim 10 wherein said last mentioned means effects a variation of said high voltage.

12. The apparatus of claim 10 wherein said last mentioned means effects a variation in the speed of rotation of said disc.

13. The apparatus of claim 10 wherein said last mentioned means effects a variation'in the'rate of delivery of coating material from said source to said disc.

14. The method of applying a hammer finish to an article, which comprises: directing a flow of coating material to an atomizing means; atomizing coating material from said means; establishing an electrostatic field between said atomizing means and said article; effecting relative movement between said atomizing means and said article; and varying particle size by cyclically varying an operating condition of said atomizing means in synchronism with the relative movement, with at least one full cycle of said condition during the period for coating each article.

15. The coating method of claim 14 wherein said coating material is atomized from a rotating disc, and the operation of the disc is periodically controlled to distribute coating material to different areas of said article, and the operating condition is varied in synchronism with the operation of the disc.

16. The method of claim 15 wherein the gradient of said field is varied.

17. The method of claim 15 wherein the rate of delivery of coating material to the disc is varied.

18. The method of claim 15 wherein the speed of rotation of the disc is varied.

19. The method of applying a hammer finish to an article, which comprises: directing a flow of coating material to a rotating disc atomizer; atomizing coating material from said rotating disc; elfecting relative movement between said atomizing disc and articles to be coated, the articles passing through an extending coating zone adjacent said atomizing disc; effecting reciprocation of said disc in .a direction generally at right angles to the plane of movement of said articles; establishing an electrostatic field between said atomizing disc and said articles; and varying the average gradient of said field between a maximum of the order of 5 kilovolts per inch and a minimum of 1.5 kilovolts per inch in synchronism' References Cited UNITED STATES PATENTS 2/1959 Simmons 239- 15 XR 2/1957 Juvinall 117- 93.44 XR OTHER REFERENCES Beck: Organic Finishing, August 1957, page 8.

ALFRED L. LEAVITT, Primary Examiner.

J. H. NEWSOME, Assistant Examiner.

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