US2733171A

US2733171A - ransburg

Info

Publication number: US2733171A
Authority: US
Publication date: 1956-01-31
Anticipated expiration: 1973-01-31

Description

Jan. 31, 1956 E. M. RA-NSBURG ,733, 7

APPARATUS AND METHOD FOR ELECTROSTATIC COATING Filed March 22, 1952 2 Sheets-Sheet 1 INVENTOR.

EDWIN M. RANSBURG Fig.l M QM Alla/nay Jan. 31, 1956 E. M. RANSBURG APPARATUS AND METHOD FOR ELECTROSTATIC COATING Filed March 22. 1952 2 Sheets-Sheet 2 Fig.

INVENTOR.

EDWIN M. RA NSBURG United States Patent APPARATUS AND METHOD non ELECTROSTATIC COATING Edwin M. Ransbnrg, Indianapolis, Ind., assignor to Ranshnrg Electro-Coatlng Corp., Indianapolis, Ind., a corpontlon of Indiana Application March 22, 1952, Serial No. 277,967

3 Claims. (Cl. 111-93 This invention relates to the spray coating of articles. More specifically, the invention relates to means for producing a more uniform coating on the article being coated by controlled movement of the points at which atomization of the coating material takes place.

In spray coating in an electrostatic field, the coating material is atomized from a plurality of spaced emission points on a discharge member. This efiect obtains not only where specific emission points are provided, as by spaced tips to which the coating material is supplied for discharge therefrom, but also where the discharge member is a smooth blade or wire all portions of which receive coating material uniformly distributed therealong. Even though the emission points are closely spaced and each point becomes the source of an individual widely dispersing spray jet, these separate jets do not intermingle or combine into a composite, uniform spray pattern. This is due to the spray particles being similarly charged and thus each jet exhibits a mutual repulsion for its adjacent jets. Such point-emission tends to cause undesirable striping or ribbing of the deposited coating material. With certain colors and with certain types of coating material, such undesirable striping may be pronounced and may be commercially objectionable. ',It is only by changing or moving the position of one spray jet that its adjacent jet can satisfactorily overlap the location formerly occupied by the first jet so as to produce a uniform deposition of coating material.

It is an object of this invention to achieve a greater uniformity in the, deposition of coating material on articles.

It is another object of my invention to provide a movable discharge source for atomized material which will be, in effect, that of a continuous line, each point of which is equally contributive as a material-emission source. These and other objects will be apparent from the following description of my invention.

In carrying out my invention in one of its preferred forms, I use the basic features of an atomizing head such as is more fully described in the co-pending application of Harry J. Green and Robert H. Turner, Serial No. 170,204, filed June 24, 1950. Liquid coating material is pumped into a discharge header, from which header opens a series of discharge nozzles each presented in spaced relation to the article to be coated. The coating material is maintained at a great electrical potential difference from the articles so that an electrostatic field causes the liquid to be atomized and discharged from these nozzles and electrostatically deposited as a coating on the article. An oscillator is used to reciprocate the header along its axis so that the resulting movement of the nozzles carried on the header will give a constantly changing discharge-source, which in integrated effect is a continuous-line source, thus avoiding the unevenness of coating which results from spaced, fixed-point material discharge. To effect this axial reciprocation, I connect the header, through a connecting rod, eccentrically to a rotary power source.

PatentedJarl. 31,1956

In another form of my invention, I employ a liquid discharging head and a liquid supply means substantially as is more fully described in the co-pending applicationof Edwin M. Ransburg and Emery P. Miller, Serial No.

155,802, filed April 14, 1950, now Patent No. 2,706,964. The discharge head shown there includes an elongated discharge member which is upwardly movable along, and

shielded by, an atomizer body. Coating material from the body interior is uniformly distributed along the entire length of the liquid-discharging portion of the moving discharge member by controlled flow through a valve system in the body. Upon coming onto the discharge member, the coating material comes under the influence of an electrostatic field caused by a high voltage differential applied between the head and the article to be coated. The presence of this electrostatic field causes the coating material to form along the discharge member into a series of spaced cusps, from the tips of each of which cusps the material is atomized and discharged toward the article and deposited thereon. Each of the cusps is a point-emission discharge source. To obtain the desired incremental reciprocating movementof those liquid cusps on the discharge member, I use an intermittent movement mechanism, such as a starwheel and a one-tooth gear, interposed between the power source and the wiredriving pulley. This mechanism causes the wire to be advanced with step-by-step movement. During the dwell periods of the wire and starwheel, gravity causes the liquid cusps to descend. Upon the starwheel becoming operative, it causes the discharge portion of the wire to move upwardly, thus raising the cusps to complete their reciprocation.

The detailed method of accomplishing the foregoing will be fully understood from the following description, with reference to the accompanying drawings wherein:

Fig. 1 is an elevation, in partial section, showing one embodiment of my invention; and

Fig. 2 is an isometric view, somewhat diagrammatic, showing another embodiment of my invention.

The atomizing head shown in Fig. l is an improvement of the head shown in the above-mentioned application of Harry J. Green and Robert H. Turner, Serial No. 170,204, filedlune 24, 1950. The atomizing head 10 comprises a head frame 12 and bearings '14 which are adapted to support a header assembly 16 and accommodate oscillation thereof. This assembly 16 includes a hollow header 18 into which coating material is pumped motion of that control member is impartedto the header.-

The hollow header 18 is closed at its bottom by a plug 26, and is provided with a series of hollow projections 28 each directed toward the article 30 to be coated. The

article 30 is moved past these projections by a conveyor 32, which conveyor also serves to maintain the article at ground potential, as by grounding the conveyor at 33.

To create a high electrostatic field between the atomizing head 10 and thearticle 30 I apply a high voltage to the head. Conveniently, this is done by connecting head frame 12, through wire 34, to one terminal of a highvoltage source 36, the other terminal of which is grounded. It should be pointed out, however, that the terminals of the high-voltage source may be reversed without departing from the scope of my invention. This electrostatic field causes the coating material to be emitted in a finely divided or atomized state from each of the tips of the projections 28, and be electrostatically deposited on the article 30 as a coating.

To overcome objectionable striping or ribbing of the coating material deposited on the article caused by the concurrent conditions of smooth article travel and material emission from fixed, spaced discharge points, I reciprocate or oscillate the header assembly 16, with the discharge projections 28 carried thereon, transverse to the path of article movement. The desired magnitude of reciprocation will vary with certain operating conditions such as field intensity, coating-liquid characteristics (including viscosity, surface tension, and smoothout tendency), distance from the nozzle tips to the article, spacing of the nozzles, reciprocation frequency, speed of the conveyor, and harmonic effects of the particular reciprocator employed. However, the reciprocation is substantially less than the distance from the nozzle tips to the article to be coated. To effect the reciprocation, I provide at the lower end of the header control member 22 the secondary element of a swivel joint 40 of any wellknown type, whose primary element is fixed to the upper end of a motion transmitter such as the connecting rod 42 shown. To. provide the necessary throw, rod 42 is eccentrically pivoted to a rotary power shaft 44 by a crank member and pin, shown at 46 and 48. The shaft 44 is driven by a motor 50 through speed reducer 52, all mounted on a main frame support 58.

Desirably, the main drive elements and frame are not at the high voltage of the atomizing head. For this reason, the atomizing-head frame 12 is insulatedly mounted on frame 56, which is supported on insulated columns 54. Also, the electrical conductivity of the moving parts is broken, as by making connecting rod 42 of some suitable insulating material.

The atomizing head shown in Fig. 2 is an improvement of the head shown in the above-mentioned co-pending application of Edwin M. Ransburg and Emery P. Miller, Serial No. 155,802, filed April 14, 1950. The atomizing head 110 includes a body member 112 which is provided with an internal valve system including a rotary, multiported valve for uniform distribution of coating material to the material-discharge portion of the body. The header for this valve system is a manifold or header port 113 which is in the form of a longitudinally extending hole in the body 112. Into this port is pumped liquid coating material from pumps 114 through lead-in conduits 115. For communicating the header port 113 with the material-discharge portion of the body 112, the body is provided with another longitudinally extending hole 116 of accurate and true form, located so that its perimeter intersects that of the port 113. Into this true hole 116 is fitted a rotary valve rod 118, which is in the form of a cylindrical plug having a series of vertically spaced, diametrically extensive holes 121 and which is revolved by external means through a valve rod extension 119. The holes 121 are angularly staggered about the rod axis, with the ends of each hole lying on a pair of complementary spirals, so that as the rod revolves each hole 121 successively communicates with the port 113 and becomes a valve passage leading from that port.

Each half-wrap of the spirals forms a section of the valve rod. Desirably, each section is supplied from an individual manifold or port which is supplied with material from an individual pump means. To that end, the header port 113 is divided longitudinally into a plurality of non-communicating sections. This desirable port partitioning is conveniently made by inserting body screws 122 of sufficiently large root diameter transversely through the port 113 prior to the drilling of the valve-rod hole 116.

A series of radial body-outlet holes 124, vertically spaced in accordance with the vertical spacing of the valve-rod holes 121, complete the fiow passage of coating material to successive portions of the material-discharge region 125 of the body. These outlet holes 124 exit onto an exposed surface 126 in that discharge region. Extending across this surface and sloping generally downwardly are a series of flow-directing elements 128. Each 4 of the body-outlet holes 124 terminates on the surface 126 between two such elements.

The extreme forward edge of the exposed body member 126 is grooved to accept and shield the discharge member 130, which is an endless wire encircling the body 112. The discharge member 130 is moved by a driving wheel 131, over idlers 132 and through wire-cleaners 134, so that its movement in the material-discharging region 125 is upward and intermittent. To this end, power to the driver 131 is through an intermittent-motion mechanism, such as the one-toothed gear 136 and starwheel 138 shown.

The articles 140 to be coated are moved along past the atomizing head by a conveyor 142, grounded as at 144. The articles 140 are maintained at ground potential by conductive supports 146. To create an electrostatic field between the atomizer and the articles the atomizing head 112 is connected by wire to one terminal of a high-voltage source 152 the other terminal of which is grounded.

As the liquid finds itself distributed along the discharge member 130, the electrostatic field causes it to be drawn into a series of spaced cusps 147, from the tip of each of which cusps the liquid is atomized and discharged by the electrostatic field and thence electrostatically deposited on the article.

To overcome the striping effects of that spaced-point material-emission, in this improved astomizing head each cusp is caused to reciprocate by a combination of the action of the above-described movement of the discharge member 130 and the gravitational flow of coating material along the member. During the dwell periods of the discharge member, gravity causes the cusps to descend. When the starwheel becomes operative, it causes the discharge member and the cusps carried thereon to ascend. The magnitude of the reciprocation thus produced is substantially less than the distance from the discharge member to the article to be coated.

In operation of my invention, it is the emission points themselves which are moved as a group transversely relative to the article to be coated to effectuate a continuousline source of coating material discharge. Where the emission points are located at definite points on the discharge member, as in the embodiment shown in Fig. 1, such emissiompoint oscillation is by a reciprocation of the entire discharge head. But where the emission point: are not located at definite points on the discharge member, but rather are cusps formed and spaced thereon by the phenomenon of electrostatic atomization, which cusps descend due to gravity, the discharge member does not need to be reciprocated in order to efiect oscillation of the emission points. Thus in the embodiment of Fig. 2, emission-point oscillation is obtained by the intermittent upward mechanical movement of the discharge member so that during the dwell periods the cusps descend due to gravity.

An example of a set-up for conveniently practicing my invention by use of apparatus as shown in Fig. 1 was obtained wherein the following conditions were maintained:

Diameter of emission points 28.. .010 inch.

Speed of conveyor 32 10 ft. per minute.

Type of coating material White synthetic enamel,

modified urea formaldehyde, 20 sec. on Zahn No. 2 Cup at Although in its preferred form reciprocation of the points of atomization will be substantially less than the distance'to the article and slightly more than the distance between adjacent points of atomization, under certain conditions it is satisfactory to reciprocate the points of atomization of the order of one-half the distance maintained between the discharge member and the article. It is to be understood that the spacing between points of atomization with the apparatus shown in Fig. 2 may be about one-tenth of the spacing between points of atomization maintained in the apparatus shown in Fig. 1.

Two embodiments of my invention have been fully described above, but many modifications thereof will be apparent to those skilled in the coating art without departing from the spirit of my invention which is limited only by the scope of the following claims.

I claim:

l. A coating apparatus comprising an atomizing head having a movable discharge member; means for supplying liquid coating material to said head for flow along said member in a predetermined direction; conveyor means for presenting articles to be coated in spaced relation to said discharge member; a high voltage source for maintaining the discharge member at sufliciently high electrical potential in relation to the articles to be coated to discharge liquid coating material from a plurality of emission points spaced along the discharge member and to deposit the coating material on the articles, the distance between adjacent emission points being substantially less than the dis tance from the emission points to the articles to be coated; and means for intermittently moving said discharge member counter to the direction of flow of the coating material along the discharge member. I

2. A coating apparatus comprising means for moving the article over a predetermined path in a coating zone; atomizing means for producing sprays' from a plurality of closely spaced points spaced from the path of article move ment in the coating zone, said atomizing means including an extended discharge member for supporting said points along a line substantially transverse to the path of article movement; means forsupplying liquid coating material to said points; means including a high voltage source for establishing between said points and the article moving past them an electrostatic field of sufiicient strength for atomizing the liquid from said points and depositing the atomized material on the article; and means for moving said atomizing means including anintermittent-motion mechanism for moving said discharge member supporting said points in a succession. of alternate dwell periods and periods of upward movement toobtain oscillation of said points in the direction of said line transverse to the path of article movement, the magnitude of said movement being such asto oscillate said points a distance which is materially less than the distance from said points to the article moving past them.

3. The method of coating an article comprising the steps of: moving an article over a predetermined path; supplying liquid coating material to a site for gravity flow therealong spaced from said path; establishing between the article and the coating ma'wrial at said site an electrostatic field for forming at said site a plurality of material emission points, said field being of sufficient strength to atomize and disperse finely divided liquid particles from theemission points and to electrostatically deposit the particles so atomized upon the surface of the article for coating the same; and oscillating the emission points by intermittently moving said points in a direction opposed to gravity and transverse to the path of article movement, the magnitude of such oscillation being substantially less than the distance from said site to the article.

References Cited in the file of this patent UNITED STATE PATENTS 2,685,536 Starkey Aug. 1954

Claims

3. THE METHOD OF COATING AN ARTICLE COMPRISING THE STEPS OF: MOVING AN ARTICLE OVER A PREDETERMINED PATH; SUPPLYING LIQUID COATING MATERIAL TO A SITE FOR GRAVITY FLOW THEREALONG SPACED FROM SAID PATH; ESTABLISHING BETWEEN THE ARTICLE AND THE COATING MATERIAL AT SAID SITE AN ELECTROSTATIC FIELD FOR FORMING AT SAID SITE A PLURALITY OF MATERIAL EMISSION POINTS, SAID FIELD BEING OF SUFFICIENT STRENGTH OF ATOMIZE AND DISPERSE FINELY DIVIDED LIQUID PARTICLES FROM THE EMISSION POINTS AND TO ELECTROSTATICALLY DEPOSIT THE PARTICLES SO ATOMIZED UPON THE SURFACE OF THE ARTICLE FOR COATING THE SAME; AND OSCILLATING THE EMISSION POINTS BY INTERMITTENTLY MOVING SAID POINTS IN A DIRECTION OPPOSED TO GRAVITY AND TRANSVERSE TO THE PATH OF ARTICLE MOVEMENT, THE MAGNITUDE OF SUCH OSCILLATION BEING SUBSTANTIALLY LESS THAN THE DISTANCE FROM SAID SITE TO THE ARTICLE.