US2728689A

US2728689A - Spray coating of articles

Info

Publication number: US2728689A
Application number: US257874A
Authority: US
Inventors: Edwin M Ransburg
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1951-11-23
Filing date: 1951-11-23
Publication date: 1955-12-27
Anticipated expiration: 1972-12-27

Description

Dec. 27, 1955 Filed Nov. 23.

E. M. RANSBURG 2,728,689

SPRAY COATING OF ARTICLES 2 Sheets-Sheet l IN VEN TOR.

EDWIN M. RANSBURG fiwuw A Home y Dec. 27, 1955 E. M. RANSBURG 2,728,689

SPRAY COATING OF ARTICLES Filed Nov. 25, 1951 2 Sheets-Sheet 2 u "I, VIIIA INVENTOR.

EDWIN v} M. RANSBURG BYM, Mm,

MW-M

A fro/nay United States Patent SPRAY COATING 0F ARTICLES Edwin M. Ransburg, Indianapolis, Ind., assignor to Ransburg Electro-Coating Corp., a corporation of Indiana Application November 23, 1951, Serial No. 257,874

Claims. (Cl. 117-93) This application relates to the spray coating of articles, and more particularly to the electrostatic deposition of liquid coating material on articles.

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 article itself (if it is of conducting material) or an electrode within or immediately behind the article 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.

In the form of electrostatic coating apparatus where the spraying device is the electrode, it has been found that the coating material can be atomized from an edge which is maintained at high potential with respect to ground, the atomization taking place from the edge from a multiplicity of cusps which are formed thereon by virtue of the electrostatic forces present. lectrostatic coating apparatus employing such an atomizing edge are shown in the copending applications of E. M. Ransburg and W. A. Starkey, filed September 29, 1944, as Serial No. 556,390, now Patent 2,685,536; W. W. Crouse, filed on March 5, 1948, as Serial No. 13,174; R. H. Turner, filed March 10, 1949, as Serial No. 80,753, now Patent 2,651,287; and the following applications of E. M. Ransburg: Serial No. 57,260, filed October 29, 1948, now Patent 2,658,472; Serial No. 107,687, filed July 30, 1949, now Patent 2,684,656; Serial No. 143,994, filed February 13, 1950, and other applications owned by the same assignee of these applications.

When an article is exposed to the spray issuing from an atomizing edge in an electrostatic coating system,-deposition of the coating material particles is primarily the result of electrostatic forces and the particles as they are atomized from the edge carry an electric charge relatively opposite to the charge of the article and thus are deposited thereon with a minimum of waste. One of the problems encountered in an electrostatic coating system where atomization is achieved from an atomizing zone such as an atomizing edge of the type referred to above is the feed of coating material to the zone for atomization therefrom. In the said copending application of E. M. Ransburg, filed on February 13, 1950, as Serial. No. 143,994, feed of coating material to the atomizing edge is achieved by employing a rotating bell-shaped atomizing head, with the coating being fed through an orifice at the center of the head for flow across the interior surface of the head to the edge by the centrifugal force created. However, when it is desired to feed coating material to an elongated atomizing edge which is not a figure of revolution, such as an arcuate or linear edge, the method of feeding coating material to such'an edge by centrifugal force is impracticable.

"ice

According to this invention, feed to an elongated edge is achieved through the use of a jet stream of coating material which originates from an oscillatable source and is directed by oscillation of the source across a surface for flow from such surface to the atomizing edge. Inasmuch as oscillation of the source causes the jet stream to traverse the surface just mentioned, means are provided for assuring a uniform flow of coating material to the entire length of the atomizing edge and thus either the rate of oscillation of the source is varied so as to produce a traverse of the surface by the jet stream at a substantially constant linear rate or, where the source is oscillated at a constant angular rate producing inconstant rate of linear traverse, the flow of coating material to the source is varied in direct proportion to the speed of traverse so as to deliver a substantially uniform quantity of coating material to the surface for flow to the atomizing edge.

Another feature of the invention is the provision of a second surface positioned above and angularly inclined to the first surface, together with means for supplying coating material to the second surface for gravity flow therefrom to the edge supporting surface and thence to the atomizing edge. The second surface may be in the form of a bafile and the coating material may be supplied thereto by a jet stream from an oscillatable source of the type previously mentioned. By the use of the methods and apparatus disclosed herein, it is possible to supply coating material to a feed member which feeds the coating to the atomizing edge, with the quantity of coating supplied to the feed member being substantially uniform along the length of the feed member and supplied from a source operable to deliver varying quantities of material thereto in accordance with requirements.

The above and other features of the invention will be readily apparent from the following description and drawings, in which:

Fig. 1 is a perspective view partially broken away for clarity of illustration of an apparatus embodying the invention;

Fig. 2 is a vertical section through the coating head shown in Fig. 1; and

Fig. 3 is a top plan view of a modified form of coating apparatus.

While two different forms of apparatus are illustrated in the accompanying drawings and will be described hereafter as providing means for practicing 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 device 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 contemplatedscope of the present invention, and that no limitations are to be implied from such specific description as shall now be provided.

Referring to Fig. 1 of the drawings, there is shown a coating head 10 including an elongated wedge-shaped member 11 having an edge 12 thereon from which edge liquid coating material is atomized by electrostatic forces. The wedge member 11 has a horizontally arranged flat top surface 13 which is substantially coextensive with the edge 12 and over which coating material is adapted to flow to the edge for atomization. Positioned just above the surface 13 is a bafile surface 14. As will be noted from Fig. 2, the bafile surface 14 is slightly spaced above the surface 13 and is substantially vertically arranged. A cover member 15 is formed integral with the bafile surface 14 to prevent loss of coating material during opera tion of the device.

The edge carrying member 11 and the bafile housing are supported upon bracket arms 16 which in turn support a pan 17 having a drain opening 18 connected to a drain pipe 19 so that any excess of coating material which is delivered to the coating head and which is not atomized from the edge 12 may be collected and returned for disposal or for future use. The pan also prevents contamination of the surrounding area by coating material drippings.

An oscillatable head 20 is rotatably mounted on the top of a supply pipe 21 made of suitable insulating material and is provided with a nozzle 22 which is directed toward the baffle member 14. Suitable packed bearings are provided between the head 20 and the insulated pipe 21 so as to permit the former to oscillate relative to the latter without loss of coating material due to leakage. Fixedly secured to the head 20 is an arm 23 having a slot 24 therein which is engaged by a pin 25 carried on the end of an arm 26 secured to a cross head 27. The cross head 27 is slidably mounted on a pair of rods 28 fixed in vertically spaced parallel arrangement to a supporting frame member (not shown) which also carries a pair of

sprockets

29 and 30 over which a chain 31 may travel. Secured to the chain is a pin 32 engageable in a slot 33 in the cross head 27 so that rotation of the sprocket 30, and hence movement of the chain 31, causes the cross head 27 to reciprocate on the guide rods 28. Suitable insulating means, such as the rubber belt 34, may be provided to drive the sprocket 30, the belt passing around a pulley 35 mounted on a shaft 36 connected to a speed reducing mechanism 37 and driven by a drive shaft 38 connected to an electric driving motor 39.

Liquid coating material is delivered to the nozzle 22 under pressure by means of a pump 40 connected through a variable speed transmission 41 to the drive shaft 38 by the worm gear mechanism 42 as shown. The pump is connected to a source of coating material (not shown) by means of a supply pi e 43 and connects to the pipe 41 by a delivery pipe 44.

Means are provided for maintaining the assembly, including the wedge member 11 and the bafile housing, at high potential with respect to ground, and for this purpose there is provided a high voltage supply 46 having one terminal connected by the line 47 to the coating head and having its other terminal connected by the line 48 to ground.

Operation of the motor 39 delivers coating material under pressure to the nozzle 22 for ejection therefrom as a jet stream 4-5 against the battle member 14. Simultaneously, the motor causes reciprocation of the cross head 27 and hence oscillation of the head 20 to cause the jet stream to traverse the bafile plate.

The pin and

slot connection

24 and 25 produces oscillation of the head 21) at a variable rate, the oscillation being fastest as the jet stream traverses the center portion of the bafile 14 and slowest as it traverses the end portions. The apparatus is so arranged as to produce a substantially constant linear speed of traverse of the jet stream across the bafile to deliver a substantially uniform quantity of coating material along the extent of the baffie. Coating material supplied to the baflle flows by gravity to the surface 13 and thence to the edge 12 for atomization therefrom. c

The modification shown in Fig. 3 is similar to the apparatus shown in Figs. 1 and 2 except for means for oscillating the head 20' and the means for supplying coating material under pressure to the nozzle 22 carried thereby. For this reason only those portions of the device shown in Fig. 3 which diifer from the corresponding portions of the device of Fig. 1 will be described in detail.

Secured to the oscillatable head 24) is a gear segment 50 provided with teeth meshing with a second rotatable gear segment 51, which latter segment is connected by an arm 52 to a drive member in the form of a rotatable gear 53. The gear 53 is driven by a suitable worm 54 connected by a shaft 55 to the drive motor and thus rotation of the shaft causes rotation of the drive member 53 and hence oscillation of the head 28. With this arrangement, however, the rate of angular motion of the head 20 is more nearly constant so that the .lineal speed of the jet stream 45 against the bafile 14 is inconstant, it being most rapid near the extremities and slowest at the center of the bafile. Thus if the supply of coating material to the nozzle 22 were constant, an unequal quantity of coating material would be supplied across the bafile 14'. An unequal supply of coating material to the bafiic would result in an unequal feed to the atomizing edge. Such inequality in feed results in uneven coating application on an article being coated. To avoid uneven coating due to unequal feed or other factors in the coating operation, means are provided for varying the flow of coating material through the nozzle 22. For this purpose a variable displacement pump 56 is provided for supplying coating material to the nozzle 22', together with means for varying the displacement of the pump during the coating operation to produce a flow of coating material to the nozzle 22' which varies directly proportionally to the linear speed of the jet stream across the bafile member 14.

To this end the pump 56 includes a cylinder 57 having a piston 58 reciprocable therein with the piston being fixed to a connecting rod mechanism including a piston rod 59 and a connecting rod 60 pivotally connected at one end to the rod 59 and at the other end to the drive member 53 at the same point of the connection of the arm 52 to the drive member. Obviously the connecting arm 60 could be connected to the driv ing member at a point removed 180 from the illustrated point of connection.

The cylinder 57 is connected to the source of supply through a pair of

supply channels

61 and 62, each opening into the cylinder and each provided with a flap valve 63 and 6 Delivery to the nozzle 22 is through a fiexible supply line 65 made of insulating material connected to a passage 66 connecting in turn to a pair of fiuid passages 67 and 68, each opening into the interior of the cylinder 57 and each provided with

flap valves

69 and 70. During upward movement of the piston 58 the lessened pressure beneath the piston closes the flap valve 69 While opening the flap valve 63 to permit coating material to flow into the cylinder beneath the piston. Simultaneously, the increased fluid pressure above the piston closes the flap valve 64 while opening the flap valve 70 to permit coating material under pressure to pass through the passages 68 and 66 and into the delivery line 65 for ejection from the nozzle 22'. Because of the connecting

rod arrangement

59 and 60, the speed of travel of the piston 58 is variable, being at its maximum at the point illustrated in the drawings at which time the linear speed of the jet stream 45 across the baffle is also at its maximum. The piston 58 travels at its minimum speed when its connecting rod mechanism is at approximately dead center at which time, however, the jet stream is traveling at itsminimum linear speed inasmuch as it is then traversing the central portion of the bathe plate. Thus the delivery of the pump is varied in order to meet the requirements of proper feed to the atomizing edge.

The connecting rod 52 may be of insulating material and the entire assembly supported upon insulated supports so that the atomizing edge, its wedge-shaped support and the bafile housing may be maintained at a high potential with respect to ground and for this latter purpose there is provided a high voltage supply 71 having one terminal connected by the line 72 to the edge supporting member and having its other terminal grounded as indicated at '73.

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 defined in the appended claims.

I claim:

1. Electrostatic coating apparatus of the character described comprising a member having an extended surface terminating in an elongated edge, a bafile angularly inclined to and spaced vertically above and imme diately adjacent to said surface, means for traversing the bathe with a jet stream or" coating material for gravity flow from the battle to said surface, and means for atomizing coating material from said edge and for dispersing and depositing the atomized particles on an article including means for creating an electrostatic charge differential between the atomized particles and the article.

2. Electrostatic coating apparatus of the character described comprising a member having an extended substantially horizontal upper surface and terminating in an elongated atomizing edge, a substantially vertical baiile vertically above and extending along said surface in closed spaced relation thereto, an oscillatable nozzle positioned adjacent but spaced from the baffie, means for supplying coating material under pressure to said nozzle for ejection therefrom as a jet stream against the baffle, means for oscillating the nozzle about an axis substantially normal to the major extent of the baffle to cause traverse of the bafiie by the jet stream, a power supply having one terminal connected to said edge to make said edge one electrode of an electrostatic field of sufficient strength to electrostatically atomize coating material from said edge and to disperse the atomized particles and deposit them on an article.

3. Electrostatic coating apparatus of the character described comprising a member having an extended surface and therebeneath a second surface substantially coextensive therewith and terminating in an elongated atomizing edge, a nozzle oscillatable about a fixed axis and positioned adjacent but spaced from said extended surface, means for supplying coating material under pressure to said nozzle for ejecting therefrom as a jet stream against said extended surface for gravity flow over said surfaces to said edge, means for oscillating the nozzle to cause traverse of the surface by the jet stream,

a power supply having one terminal connected to said edge to make said edge one electrode of an electrostatic field of suiiicient strength to electrostatically atomize coating material from said edge and to disperse the atomized particles and deposit them on an article.

4. Electrostatic coating apparatus of the character described comprising a member having an extended substantially horizontal upper surface terminating in an elongated atomizing edge, a substantially vertical bafiie vertically above and extending along said surface in closely spaced relation thereto, an oscillatable nozzle positioned adjacent but spaced from the bafiie, means for supplying coating material under pressure at a substantially constant rate to said nozzle for ejection therefrom as a jet stream against thebafile, and means for oscillating the nozzle about extent substantially normal to the major axis of the baffle including an oscillating drive member, a. slotted arm connected to the nozzle, a pin carried by the drive member and engaging the slot whereby to oscillate the nozzle at a varying rate to cause the jet stream to traverse the baffle at a substantially constant linear rate, a power supply having one terminal connected to said edge to make said edge one electrode of an electrostatic field of sutficient strength to electrostatically atomize coating material from said edge and to disperse the atomized particles and deposit them on an article.

5. The method of coating an article which comprises providing a member terminating in an elongated edge, ejecting a jet stream of coating material from a source, providing a surface vertically spaced above and substantially co-extensive with the edge, oscillating the source to cause the jet stream to traverse said surface for gravity flow therefrom to said edge, atomizing coating material from said edge, electrostatically charging the atomized particles and electrostatically dispersing said particles and depositing them on the article, the spacing between said edge and an article being sufficient to permit substantial dispersion of the particles as they move toward the article under the influence of said electrostatic charge.

No references cited.