US2744033A - Method and apparatus for spray coating of articles - Google Patents

Description

May 1, 1956 J. w. JUVINALL METHOD AND APPARATUS FOR SPRAY comma OF ARTICLES Filed NOV. 28, 1951 INVENTOR.

JAMES W. J UVINALL United States Patent METHOD AND APPARATUS FOR SPRAY COATING OF ARTICLES James W. Juvinall, Brendonwood, Ind., assignor to Ransburg Electro-Coafing Corp., a corporation of Indiana This application relates to the spray coating of articles and more particularly to the electrostatic deposition of liquid coating material on articles.

When spaced articles are successively exposed to the spray issuing from a spraying device in an-electrostatic coating system where deposition of the coating material particles is primarily the result of electrostatic forces and where the particles are given an electric charge at the time they leave the spraying device or shortly thereafter, the commercial practicability of handling the articles on a rapidly moving conveyor has heretofore sometimes resulted in the deposition of coating on the articles in an uneven manner resulting in undersprayed or oversprayed areas.

In a majority of electrostatic coating systems a field of electrostatic force is set up which includes the article being coated and the atomized coating material particles in movement toward such article. In normal commercial practice it is desirable to mount the article on a grounded conveyor and thus to make the articles themselves (if they areof conducting material) or electrodes within or immediately behind the articles, at ground potential; and to provide another electrode at a high electrical potential with respect to ground, this electrode sometimes being the spraying device and sometimes being a separate electrode near or past which the spray passes in its movement toward the article.

Various means may be used for producing the atomized particles, such as pneumatic spray guns and the like, or the atomizing means may take the form of the extremely efficient spraying device including an electrostatic atomizing head with a circular atomizing edge of the rotary conical type such as disclosed and claimed in the copending application of E. M. Ransburg filed in the United States Patent Office on February 13, 1950, as Serial No. 143,994.

Where the articles are closely spaced on the conveyor,

the coating zone, which is the zone defined by the electrostatic field created between theelectrode carrying the high potential and the article, follows the nearest article 7 and as such article is moved away from the atomizing means the field, and hence the coating-zone, quite suddenly transfers to the next succeeding article, resulting in a relatively small change in direction of movement of the particles moving through the electrostatic field and hence without substantial interruption of the coating operation. Where the articles are widely spacedon the con- 2,744,033 Patented May 1, 1956 may under certain circumstances be suflicient to permit the leading edge or front portion of the article to pass completely or almost completely out of the coating zone before the atomized particles can reach it. Undersuch conditions the leading portion of the article is either uncoated or is insufficiently coated. V

A further difiiculty has arisen in the coating of articles which are so shaped as to'provide certain areas which are more difiicultto coat than other areas. The difliculty may arise from the particular shape of the area, such as a flange orthe like, or because one area presents more or less surface to be coated during its passage at a given linear speed through the coating zone than preceding or succeeding areas.

One example of an area'which is diflicult to coat, all such areas hereinafter being referred to as critical areas, is the leading edge of a pipe section.

it has been found that when pipe sections are fed to a conveyor which is moving at a rapid speed of upwards of 200 feet per minute it is practically impossible consistently to space the sections along the conveyor sulficiently close to each other so that the electrostatic field transfers itself substantially immediately from the trailing edge of one pipe section to the leading edge of the following pipe section andthus the leading end of the pipe receives insufiicient coating as it passes the atomizing means although the following noncritical areas of the pipe receive an adequate coating. It is believed that the failure of the critical area of the pipe to receive adequate coating is due to the fact that while the electrostatic field between the leading end of the pipe and the atomizing head is created substantially instantaneously as the pipe end nears theshead, the inertia of the atomized particles is such as to require a certain amount of time beforethey can reach the pipe 'end, during which time such pipe end moves beyond coating range so that the particles coat other areas of the pipe. The extent of the critical area is thus further dependent, at least partially, upon the ICC speed of movement of the article to be coated and it has been found that when coating pipe section, the area at the leading end of the pipe which is undersprayed may extend backwards therefrom a considerable distance, which distance is increased as the speed of movement of the pipe is increased.

The particular problem just discussed is often encountered in electrostatic coating systems in which the particles are atomized at least in part by electrostatic forces and is not normally found in systems using more conventional atomizing means. Thus, for example, where compressed air is employed to atomize the coating material, the atomized particles are carried by a current of air into the electrostatic field and toward an article to be coated and the inertia of the air stream is willcient to carry atomized particles into the path along which the article to be coated will be moved. This is true even though at a particular instant an article is not veyor, however, the next following article may be too 7 far removed to act as an electrode to draw the field to it, and thus there is created a period during which the particles are not following a well-defined field and the coating zone-is substantially nonexistent. When such a situation is coupled with rapid movement of the articles along the conveyor, the next succeeding article will, upon its approach, draw the electrostatic field instantaneously to itself. However, a finite time elapses before the particles may accommodate themselves to the new' field and be moved therethrough to the article, and such'finite time present in the path. Thus in a system where the atomized particles are carried in a current or stream-of air, the air may be turned on long prior to the arrival of the leading edge of the article at the coating zone and thus, in eifect, a stream of atomized particles may be directed at and through the path of movement of the article to await the arrival of the leading edge, and there is no time loss in the movement of particles toward and onto the leading edge. In electrostatic coating systems employing electrostatic atomization, reliance is had upon the electrostatic field existing between the atomizing means and the article to direct the particles toward the article. For this reason unless an article is present and in field creating relationship with the atomizing means the particles atomized are not directed toward the path 'of movement of the article, but on the contrary are emitted from 7 3 the atomizing means in a rather aimless pattern. Thus the use of electrostatic atomization ma 'r's'un'is difficulty in producing coating of proper thickness on the leading edge of an article moved relatively rapidly along a path adjacent the atomizing means.

The above problems and difliculties can be overcome by the use of the methods and apparatus comprising the present invention and articles possessing both critical and noncritical areas can be given coatings of the desired thickness throughout. The present invention contemplates varying the rate of relative movement between the atomizing means and the article to be coated as the critical area is near the atomizing means. The relative speed of movement obviously can either be increased or decreased depending upon the nature of the critical area as to whether it would otherwise receive an overspray or an underspray. Thus the relative speed between the article and the atomizing means may be increased or decreased at the location of the critical area or, as exemplified in the specific embodiment chosen for illustration, the atomizing means itself may be moved, which movement coupled with the movement of the article along the conveyor may e in a direction either to increase or to decrease the relative movement therebetween.

Thus the general object of this invention is to provide a method and apparatus for producing a spray coating of predetermined thickness by electrostatic means upon an article having critical and noncritical areas. This and other features and objects of the invention will be readily apparent from the following description and drawings, in which:

Fig. l is a perspective view of one form of coating apparatus embodying the invention;

Fig. 2 is a side elevation, somewhat schematic inform, showing a portion of the apparatus; and

Fig. 3 is a view of the upper right-hand portion of Fig. 1 showing a modified form of the invention.

While two dilferent forms of apparatus are illustrated in the accompanying drawings, and will be described hereafter as providing means for practising the present invention, it Will be understood that these are representative embodiments only. It will also be understood that forms of spraying devices other than the particular electrostatic atomizing devices shown may be utilized, and that other forms of apparatus may be employed; and it is to be understood that other embodiments may be utilized without departing from the contemplated scope of the present invention, and that no limitations are to be implied from such specific description as shall now be provided.

Referring now to Fig. 1, there is shown one form of apparatus embodying the inventions hereof which is illustrated as being employed in the coating of a plurality of pipe sections and 11 moving along a conveyor 12, which carries the pipe sections in a substantially linear path past a frame 13 which carries the coating apparatus.

The frame includes two pairs of insulated support columns 14- and 15 mounted'upo'n suitable bases 16 and 17 with the supports 14 and 1-5 carrying at their upper ends plates 18 and 19 respectively. Secured tothe' upper surface of the plates 18an'd 19 is a pair of tracks '20 and 21 which are parallel to each other and to the direction of movement of the pipe sections along the conveyor. The tracks provide a support for a carriage 22 movable along the tracks on wheels 25 provided thereon. Movement of the carriage along the track is accomplished through the medium of aslot '24 provided in the carriage which is engaged by a pin '25 carried by an endless chain 26 which passes'around a pair of sprockets 27 and 28 rotatably mountedon angularmembers29'and 30 mounted respectively on plates Hand 1). Rotation 'of the sprocket 27 to drive the chain is achieved through a drive shaft 31 which carries at its lower end a bevel gear 32 meshing with a second bevel gear 33'fix'ed to a shaft 34 and rotatably carried by pillow blocks 35" mounted 4 on the plate member 18. The opposite end of the shaft '34 carries a bevel gear 3'6 meshing with a bevel gear 37 carried at the upper end of an insulated drive shaft 38 connected at its lower end by means of a right angle drive 39 to an electric motor 40.

It can be seen from the foregoing that operation of the motor 40 through the shafts and gear means just described drives the chain about the sprockets 27 and 28. Such movement of the chain causes the carriage 22 to move back and forth along the tracks 20 and 21 from one end to the other of the'frame 13.

The atomizing means herein may, as previously noted. be of the type shown in the said copending Ransburg application and includes an electrostatic atomizing head 41 having an atomizingedge 42 and rotatably mounted in a body portion 43. The atomizing head is suspended from the carriage by a plurality of support members 43a. Means in the form of a flexible supply line 44 is provided for supplying liquid coating material to a hollow shaft extending through the body portion and opening at its inner end to an orifice located substantially at the center of the atomizing head. Rotation of the head 41 to flow the coating material outwardly to the atomizing edge is achieved through gear means which includes a pair of right angle spiral gears 45 and 46. The gear 45 is splined to a shaft 47 rotatably mounted in the frame and driven, through the gear means 48, insulated drive shaft 49 and right angle drive 50, by a second electric motor 51.

A-high voltage source 52 is connected by the means 53 to the atomizing head and by means of connection 54 to a suitableground. Since the supports 14 and 15 and the shafts 38 and 49 are of electrically insulating materials, the entire assembly supported on the plates 18 and 19 can be maintained at a high potential with respect to ground.

In the embodiment illustrated in Fig. 1, let it be assumed that the pipe sections are carried by the conveyor from left to right past the frame 13. As previously pointed out, the critical areas on such pipe sections are the leading marginal edge, for example the edge 10a. As the space between the edge 10:: and the trailing edge 11b of the preceding pipe is normally of such an extent as to prevent the electrostatic field from the edge 42 from snapping from the edge 11b to the edge 10a, movement of the pipe section 10 rapidly past the head 41 causes underspraying of the edge which in some cases may be accompanied by underspraying of the area immediately adjacent the leading edge. To prevent such underspraying, the atomizing head is mounted in the manner just described for movement in a path substantially parallel to the path .of movement of the pipe sections and means are provided for initiating movement of the atomizing head as the critical area, in this case the leading edge 10a, is near the'head.

For this purpose there is provided a microswitch 55 adapted to be operated by a lever arm 56 which is spring urged into the path of movement of the pipe section to be contacted by the leading edge 10a. Operation of the'switch 55 through-suitable wiring connection operates the motor 40 to move the carriage along the track and hence-to move the atomizing head from a first position at the'left-handend of the frame to a second position at the right-hand end. The motor 40 is designed to move the atomizing head at approximately the speed of movement of the pipe-section so as to maintain the head and the leading edge of the pipe at approximately their position of closest approach during the period required tomove the head from its first to its second position. When the head reaches the second position, the carriage contacts the plunger 57 of a switch 58 wired to stop the motor uponsuch contact. As the remaining noncritical areas of the pipe pass the atomizing head it remains in its second position to coat such areas. To restart the motor 40 to return the head to its first posiswitch 59 is so constructed that release of the arm 60 as the trailing edge of the pipe passes the switch actuates the motor 40 to move the'carriage toward the opposite end of the track. As the carriage reaches its initial starting position it contacts a plunger 61 to operate a switch 62 again to stop the motor. The following pipe section initiates a similar cycle of operations.

It is clear that as the leading edge, such as the edge a, of a pipe section enters the coating 'zone at a high speed of movement relative to the atomizing head, the atomizing head is moved parallel to and in the same direction as movement of the edge 10a to permit the atomized particles to travel through the electrostatic field to be deposited on the leading edge. In the particular embodiment of the invention just described, the head moves at substantially the same speed as the leading edge so that the relative movement is approximately zero. When the carriage strikes the plunger 57, further motion of the carriage along the track is discontinued although the spraying continues along the entire length of the pipe until the trailing edge operates the switch 59 to return the head to its first position.

In the modified form of the invention shown in Fig. 3, the switch 59 and its attendant lever arm 60 are omitted and the switch 58 is designed to reduce the speed of the motor 40, for example to one-fourth speed (or some other fraction or multiple of the speed on its initial movement) so that the head is returned toward its first position while spraying the noncritical areaof the article. The speed of the motor upon the return of the head is so chosen that the remaining areas of the article are substantially completely coated before the carriage strikes the plunger 61 to halt its motion.

In dealing with articles other than pipe sections which are provided with critical areas either at their leading edge or in other portions of the article, the carriage may be moved at a speed equal to or at speeds other than a speed equal to the speed of movement of the article along the conveyor. Thus the atomizing head may be moved, for example at one-half speed, if the critical area is extended in length. Such movement of the carriage, and hence of the head, is initiated as the critical area, wherever it may be located on the article, nears the atomizing head.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

I claim:

1. The method of coating an elongated article having a critical area along a marginal edge by means of electrostatic deposition of atomized coating material particles by an electrostatic field existing between the article and an electrode spaced from the article which comprises moving the article, with the marginal edge leading, in a path passing adjacent the electrode, moving the electrode generally parallel to said path and in the direction of movement of the article as the critical area becomes the terminus of said field to coat said area and then continuing the movement of the article without so moving the electrode to coat immediately following areas of the article.

2. The method of coating an article having a length any times its width by means of deposition of atomized coating material particles by an electrostatic field existing between the article and an electrode spaced from the article which comprises moving the article longitudinally of its major axis in a P Passing adiawnt the f"- trode, moving the electrode generally parallel to said s path in the direction of movement of the article and at approximately the speed thereof as the leading end of vthe article becomes the terminus of said field to coat said end, and then continuing the movement of the article without so moving the electrode to coat the remainder of the article.

3. The method of coating a pipe section by means of electrostatic deposition of electrostaticaily charged atomizing coating material particles issued from an atomizing means which comprises moving the pipe longitudinaily of its axis in a path passing adjacent the atomizing means, moving the atomizing means generally parallel to said path as the leading end of the pipe nears the atomizing means to maintain the atomizing means at a substantially constant predetermined distance from said end during a portion of the movement of the pipe, and then discontinuing such movement of the atomizing means while continuing the movement of the pipe to coat the remainder of the pipe.

4. The method of coating a pipe section by means of electrostatic deposition of electrostatically charged atomizing coating material particles issued from an atomizing means which comprises moving the pipe longitudinally of its axis in a path passing adjacent the atomizing means, moving the atomizing means generally parallel to said path from a first position at approximately the closest approach of the leading end of the pipe to maintain the atomizing means at a substantially constant predetermined distance from said end during a portion of the movement of the pipe, halting the movement of the atomizing means at a second position While continuing the movement of the pipe to coat the remainder of the pipe, and then returning the atomizing means to said first position.

5. The method of coating a pipe section by means of electrostatic deposition of electrostatically charged atomizing coating material particles issued from an atomizing means which comprises moving the pipe longitudinally of its axis in a path passing adjacent the atomizing means, moving the atomizing means generally parallel to said path from a first position at approximately the closest approach of the leading end of the pipe to maintain the atomizing means at a substantially constant predetermined distance from said end during a portion of the movement of the pipe, halting the movement of the atomizing means at a second position, and then returning the atomizing means to said first position while continuing the movement of the pipe to coat the remainder of the pipe.

6. Apparatus for electrostatically coating an article having a length many times its width comprising a conveyor for moving the article generally longitudinally of its major axis in a linear path, atomizing means positioned adjacent said path, means for creating an electrostatic charge difierential between the atomized coating material particles and the article, a carriage for carrying the atomizing means along a second path spaced from and parallel to said linear path, means for moving the carriage a predetermined distance along said second path, and means actuated upon the close approach of the leading end of the article to the atomizing means for operating the carriage moving means to move the carriage with said end along said second path.

7. Apparatus for electrostatically coating an article having a length many times its width comprising a conveyor for moving the article generally longitudinally of its major axis in a linear path, atomizing means positioned adjacent said path, a carriage for carrying the atomizing means along a second path spaced from and parallel to said linear path, a motor for moving the carriage a predetermined distance along said second path, control means positioned along said linear path opposite one end of the second path and adapted to be operated by the leading end of the article to actuate the motor to move the carriage parallel to said end along said second path, and a second control means for reversing the motor to re themarriage to the fi-r'st mentioned end ofthe second pa n 8. Apparatus for electrostatically coating a' pipe section comprising a conveyorfor moving the pipe generally longitudinally of its axis in a linear path, electrostatic atomizing means positioned adjacent said path, a carriage for carrying the atomizing means along a second path spaced from and parallel to said linear path, an electric motor for moving the carriage a predetermined distance along said second path, a switch positioned along said linear path opposite one end of the second path and adapted to be operated by the leading end of a pipe to energize the motor to move the carriage parallel to said end along said second path, a second switch positioned at the other end of the second path and adapted to be operated by the carriage as it is moved to said other end to stop the motor, and a third switch positioned along said linear path opposite said other end of the second path and adapted to be operated by the trailing end of the pipe to reverse the motor to return the carriage to the first mentioned end of the second path 9. Apparatus for electrostatically coating an article comprising a conveyor for moving the article in a linear path, a frame adjacent the path, a track on the frame generally paralleling the path, a carriage movible along the track, atomizing means including a body portion and a rotatable atomizing head mounted on the carriage, a spline shaft rotatably mounted in the frame and extending parallel to and in spaced relationship to the track, gear means in said body portion including a gear meshing with the spline shaft for rotating the head with rotation of the shaft, means for rotating the shaft, an electric motor for moving the carriage along said track, a switch positioned along said linear path opposite one end of the track and adapted to be operated by the leading end of an article to energize the motor to move the carriag'e parallel to said end along said track, and a second switch positioned at the other end of the track and adapted to be operated by the carriage as it is moved to said other end to reduce the speed of the motor to return the carriage at a reduced speed to the first mentioned end of the track.

10. Apparatus for electrostatically coating an article comprising a conveyor for moving the article in a linear path, a frame adjacent the path, a track on the frame generally paralleling the path, a carriage movable along the track, atomizing means including a body portion and a rotatable atomizing head mounted on the carriage, a spline shaft rotatably mounted in the frame and extending parallel to and in spaced relationship to the track, gear means in said body portion including a gear meshing with the spline shaft for rotating the head with rotation of the shaft, means for rotating the shaft, an electric motor for moving the carriage along said track, a switch positioned along said linear path opposite one end of the track and adapted to be operated by the leading end of an article to energize the motor to move the carriage parallel to said end along said track, a second switch positioned at the 8 otherend ofi thetrack and adapted to be operated by the carriage as ismoved to said other end to stop the motor, and a third :switch positioned along said linear path opposite said other end of'the track and adapted to be operated by the trailing end of the article to reverse the motor to return the carriage to the first mentioned end of the track.

11. In a method of electrostatically coating an article wherein atomized coating material is issued at a substantially constant rate from an atomizing means and the atomized particles are deposited by an electrostatic field maintained by a charging electrode having an electrical potential constantly applied thereto and wherein the article is rnoved along a predetermined path having a portion adjacent the atomizing means and the charging electrode, said article having a critical area causing a change in the field strength in the article path as said area becomes the terminus of said field and wherein such change in field strength causes a change in the concentration of atomized particles deposited on such area, the step which comprises automatically varying the rate of relative movement between the article and said atomizing means directly with said changes in concentration of coating material particles.

12. The method of claim 11 in which said relative movement is generally parallel to said path and along with an article moving therealong.

13. The method of claim 11 in which said critical area is the leading edge of an article and in which said relative movement is generally parallel to said path as the leading edge of an article moving along the path approaches the atomizing means to maintain the atomizing meansat a substantially constant predetermined distance from said edge during a portion of the movement of the article.

14. In a method of electrostatically coating an article wherein atomized coating material is issued at a substantially constant rate from an atomizing means and the atomized particles are deposited by an electrostatic field maintained between an article and the atomizing means by constantly applying an electrical potential to said atomizing means, and wherein the article is moved along a predetermined path having a portion adjacent the atomizing means, said article having a critical area causing a change in the field strength of said field, and wherein such change in field strength causes a change in the concentration of atomized particles deposited on such area, the step which comprises automatically varying the rate of relative movement between the article and said atomizing means directly with said changes in concentration of coating material particles.

References Cited in the file of this patent UNITED STATES PATENTS 2,083,633 Brackett June 15, 1937 2,421,787 Helmuth June 10, 1947 2,428,991 Ransburg Oct. 14, 1947 2,559,225 Ransburg July 3, 1951 2,568,611 Crouse Sept. 18, 195l

Claims (1)

1. THE METHOD OF COATING AN ELONGATED ARTICLE HAVING A CRITICAL AREA ALONG A MARGINAL EDGE BY MEANS OF ELECTROSTATIC DEPOSITION OF ATOMIZED COATING MATERIAL PARTICLES BY AN ELECTROSTATIC FIELD EXISTING BETWEEN THE ARTICLE AND AN ELECTRODE SPACED FROM THE ARTICLE WHICH COMPRISES MOVING THE ARTICLE, WITH THE MARGINAL EDGE LEADING, IN A PATH PASSING ADJACENT THE ELECTRODE, MOVING THE ELECTRODE GENERALLY PARALLEL TO SAID PATH AND IN THE DIRECTION OF MOVEMENT OF THE ARTICLE AS THE CRITICAL AREA BECOMES THE TERMINUS OF SAID FIELD TO COAT SAID AREA AND THEN CONTINUING THE MOVEMENT OF THE ARTICLE WITHOUT SO MOVING THE ELECTRODE TO COAT IMMEDIATELY FOLLOWING AREAS OF THE ARTICLE.

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