US2730461A

US2730461A - Electrostatic coating method

Info

Publication number: US2730461A
Application number: US337853A
Authority: US
Inventors: John E Vawter
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1953-02-19
Filing date: 1953-02-19
Publication date: 1956-01-10
Anticipated expiration: 1973-01-10

Description

Jan. 10, 1956 J. E. VAWTER ELECTROSTATIC COATING METHOD I Filed Feb. 19, 1953 INVENTOR.

JOHN E. VAWTER BY A; f %@-urma/ Ar o/news 2,730,461 ELECTROSTATIC COATING METHOD John E. Vawter, Cleveland Heights, Ohio, assignor to Ransburg Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana Application February 19, 1953, Serial No. 337,853 1 Claim. (Cl. 117-93) This invention has to do with the atomizing and electrostatic charging of particles of liquid coating material and relates particularly to improvements in apparatus for controlling the path of movement of such charged particles.

The electrostatic coating of articles of manufacture, utilizing conventional compressed air spray guns and also utilizing other methods of atomization wherein charged spray particles move in quiescent air, is widely used in industry. One very efiicient commercial type of coating with atomized, charged particles is described in the copending application of E. M. Ransburg, Serial No. 143,994, filed February 13, 1950, and involves the use of a rotating bell-shaped atomizing head and a conveyor which moves a plurality of articles to be coated along a predetermined path spaced from the atomizing head. Liquid coating material is fed to the interior of the rotat-' ing head which is connected to a source of high voltage to establish an electrostatic field between the head and the articles which are grounded through the conveyor. Finely divided particles of coating material are atomized from the edge of the rotating head without turbulent air and atent '0 are attracted to and deposited on the articles under the influence of the electrostatic field.

in electrostatic spraying the finely divided coating material particles are deposited on the various surfaces of the article generally in direct proportion to the number of electrical lines of force projecting from the discharge electrode (such as the atomizing head) and terminating on various portions of the article. Those surfaces which face and lie most closely adjacent the discharge electrode will have the greatest concentration of lines of force and hence of coating material particles. However, it has been found that certain other factors such as the mutual repulsion of the charged particles, the momentum of the particles due to other than electrostatic forces, etc., as Well as the existence of lines of force cause the particles to be deposited widely over the surfaces of the article. deposit on surfaces which do not face toward the discharge electrode is technically termed wrap-around.

In certain cases, particularly in the coating of panels, the coating material may often be distributed over the various surfaces of the article in a manner or to thicknesses other than that which is desired. For example, in

' coating a plurality of fiat-surfaced three-dimensional panels racked one above the other by moving a rack of panels on a conveyor generally edgewise past an atomizing head, the coating material will be deposited generally uniformly over the large surface of the panel which faces the discharge electrode as well as over the four relatively narrow edges of the panels which lie normal to the path of conveyor movement, the edges being coated by Wrap around. However, there appears to be a tendency to produce an extremely heavy coating, or in extreme cases no coating at all, at the corner portions of the large surface of each panel. Such heavy deposition of coating material on these corner portions may cause the coating material to sag or run and in any event The may result in a commercially unacceptable coating. Likewise, in the electrostatic spray coating of articles other than three-dimensional panels which have corners or like projecting portions constituting part of the boundaries of the article a similar problem of non-uniform coating of the corner surfaces may be encountered.

In order to eliminate the heavy coating on corners of three-dimensional panels racked one above the other as previously described, various types of electrodes spaced from the corners have been tried. While some reduction in the heavy coating applied to corners was noted with electrodes spaced from the corners of the panels, the presence of such electrodes in certain instances inhibited the wrap around coating of the narrow edges of the panels extending generally normal to the path of conveyor movement.

I have discovered that uniform distribution of coating material at the corners of panels and like projectingportions of articles can be achieved by providing a member in contact with the article substantially at its corner or projection. Apparently the presence of such a member, which may have an extended surface portion immediately adjacent the corner, modifies the arrangement of the electrical lines of force which would otherwise concentrate at the corner portion of the article. In certain instances I have found it convenient to combine this member with a support for the articles as a matter of convenience and economy.

It is, therefore, an object of my invention to provide improvements in the electrostatic spray coating of articles moving on a conveyor through a coating zone.

Another object of my invention is to provide means for rearrangement of the lines of electrical force existing in an electrostatic coating arrangement on the corners and like projecting portions particularly three-dimensional articles.

A further object is to provide a field-modifying member, which member may have an extended surface portion, in contact with a corner of an article for improving the coating deposition about such corner.

Still another object is to provide an improved article supporting structure for use in the electrostatic spray coating of articles being carried on a moving conveyor.

Apparatus for accomplishing the above stated and related objects of my invention is hereinafter fully described with reference to the accompanying drawing wherein:

Fig. l is an isometric view,- somewhat diagrammatic, of a preferred embodiment of my invention;

Fig. 2 is a detailed view showing in detail the field modifying member used in the apparatus shown in Fig. l; and

Fig. 3 is a detailed view of alternative forms of fieldmodifying members.

Figs. 1 and 2 illustrate a typical electrostatic coating arrangement incorporating my invention. A series of flatsurfaced panels 10 to be spray coated are racked one beneath the other for movement along a conveyor track ii and through a coating zone. A number of field-modifying, supporting members 12 designed in accordance with my invention are used to hang the topmost articles of each rack of panels from the conveyor and another form of supports 13, likewise designed in accordance with my invention, are used to suspend subsequent panels one beneath another. It may be desirable in order to obtain proper coating of the topmost corners of the upper panel to provide support 12 with an extended lower extremity 14 of electrically conducting material having substantially the same width as the panel being coated.

Supports 13 may likewise be made of electrically conducting material with two panel-engaging horizontal arms 15 designed to fit into slots in the panel. Each support 13 provides an extended vertical portion 16 substantially flush with the lower corners of the upper panel and the upper corners of the lower panel. The vertical portion 16 of support 13 is substantially parallel to the vertical sides of adjacentupper and lower panels 10 and thus forms a physical extension ofthe verticalside of each panel. The corners formed by the juncture of the vertical and horizontal edges of the panel are in efiect eliminated by physically extending the vertical edge by means of the support, particularly insofar as the electric characteristics of the corners are concerned. As many panels as may conveniently be passed through the spray coating zone may be racked by use of supports 13. h

The panels 10, supports 12 and 13, and conveyor 11 are all maintained at ground potential While the atomizing head 18 is connected to a source of high voltage 19 in order to create the desired particle-depositing field between atomizing head 18 and grounded panels 1t? and supports 12 and 13. Liquid coating material from a source not shown is fed through supply line 20 to the inner surface of the bell-shaped rotating atomiz'in'g head 21 of atomizing device 18 to create a spray of charged coating material particles in accordance with the teaching of the aforementioned patent application of E. M. Ran's'burg". Such method of particle formation, however, constitutes no part of my invention which is useful in many other electrostatic arrangements.

Panels 10, grouped and supported as shown, are moved along conveyor 11 through the coating zone so that the charged spray particles are attracted toward and deposited on surfaces of the grounded members. it is apparent that if panels 10 were moved through the coating zone without the use of supports such as 12 and 13 that in accordance with established principles of electrostatics there would tend to be a heavy concentration of lines of electrical force terminating on the sharp corner portions of the article. The distribution of lines of force largely determines the deposition of the spray particles and thus a concentration of such lines as on a sharp corner or projecting portion of an article may result in an undesirable non-uniform deposition of coating material particles about such corner portions. It is to be noted that some electrostatic coating processes employ conventional compressed air guns to atomize the coating material. Such processes often result in turbulent air movement in the vicinity of the article to be coated and such air movement may negative the deposition of coating material particles strictly in accordance with the concentration of lines of electrical force. However, in the use of coating material atomization processes not requiring compressed r air as taught in the aforementioned patent application of M. Ransburg the problem of excessive coating of corners and projecting portions of articles is accentuated. Although my invention is useful in many electrostatic arrangements, it is of particular importance in electrostatic coating arrangements having a quiescent atmosphere throughout the coating zone.

An alternative arrangement to prevent non-uniform coating of corners is shown in Fig. 3. Rather than using electrically conductive members having an extended surface portion immediately adjacent the corners of the articles to be coated, there is provided a number of supportingand field-modifying members 24 for suspending articles immediately at their corners from a conveyor, Members 24 have a Y-shaped lower portion designed to provide a segment of the member substantially in immediate contact with the upper corners of panel 1%). There is also provided a number of U-shaped inter-article support members 25 to be used in pairs for racking articles. Both supporting and field-modifying

members

24 and 25 are made of electrically conductive material which is rounded to avoid any sharp corners or edges with a diameter of at least A3 of an inch in order to prevent the formation of an electrically ionizing area adiacent any of the field-modifying members. in certain coating arrangements the use of such rounded members without extended surface portions may prove to be of advantage.

While in the embodiments shown the field-modifying members have been made integral with a supporting member, such is not essential. The field-modifying members may be made to perform no other function than to modify the field. My invention is not to be limited by the foregoing detailed description and appended drawings but its scope is shown only by the following claim.

I claim:

The method of clectrostatically coating an article having a substantially fiat surface bounded by edges meeting to form sharp corners which comprises hanging the article on a grounded support with said surface lying in a substantially vertical plane, contacting each of said corners with an electrical conductor presenting a small area in said plane relative to said surface, maintaining said conductor at ground potential in a position extending away from said corner, passing the supported article through an electrostatic field with said surface substantially normal to the lines of force thereof and introducing a spray of liquid coating material particles into said field electrostatically to deposit the particles on the article.

References Cited inthe file of this patent UNITED STATES PATENTS