US2994618A

US2994618A - Method and apparatus for electrostatic coating

Info

Publication number: US2994618A
Application number: US635209A
Authority: US
Inventors: George F Landgraf
Original assignee: Trion Inc
Current assignee: Trion Inc
Priority date: 1957-01-22
Filing date: 1957-01-22
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01

Description

aan.

ORNE YS INVENTOR.

5 Sheets-Sheet l llllllnllll' George E Landgraf BY M Fig./

G. F. LANDGRAF METHOD AND APPARATUS FOR ELECTROSTATIC COATING Aug. 1, 1961 Filed Jan. 22,

Aug- 1, 1961 G. F. LANDGRAF 2,994,618

METHOD AND APPARATUS FOR ELEcTRosTATIc comme 5 Sheets-Sheet 2 Filed Jan. 22, 1957 INVENTOR. George Landgraf METHOD AND APPARATUS FOR ELECTROSTATIC COATING Filed Jan. 22, 1957 Aug. 1, 1961 G. F. LANDGRAF 5 Sheets-Sheet 3 INVENToR.

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Aug. l, 1961 G. F. LANDGRAF 2,994,518

METHOD AND APPARATUS FOR ELEcTRosTATIc COATING Filed Jan. 22, i957 5 Sheets-Sheet 4 Fig.5

INVENTOR. George E Landgraf BY MJP /Vapff MJ HIS A T TORNE YS Aug. 1, 1961 G. F. LANDGRAF 2,994,618

METHOD AND APPARATUS FOR ELEcTRosTATIc coATNG Filed Jan. 22, 1957 5 Sheets-Sheet 5 O co INVEN TOR. E Gearge E Landgraf HIS C BY I] H M yTORNE YS United States Patent C) 2,994,618l METHOD AND APPARATUS FOR ELECTRO- i STATIC 'COAI'ING George FL Landgraf, Pittsburgh, Pa., aignor to-Trion,

Inc., McKees Rocks, Pa., a corporation of Pennsylvanla Filed Ian.A 22,.1957, Ser. No. 635,209 3Claims. (.Cl. 117-93),

My inventions relate to electrostatic coating apparatus, more particularly, apparatus forcoating moving articles. It is especially designed for coatingV a continuous strip which moves at varying' speeds;

My inventions are particularly useful for applying a coating of oil Ito a continuous steel stripwhichhas been plated with tin such as is' rolled in steel' mills and commonly referred to--astin strip. Therefore, my inventions will be described with particular reference toV this product', although it is to be-imderstood that they are usefulin coatingd continuously movingstrips of otherA metals and other moving objects.

In.the steelindustry, ithas always been the practice to coat so-calledv tin plate withy a coatingofV vegetable oil, such as cottonseed'oil or one of the synthetic substitutes dibutyl sebacate4 or dioctyl sebacate, tol protect' the sur hace of the. strip from being marredv or scratched. Tin plate is made in the steel industry from long continuous coils of steel strip and; in this method. of manufacture, applyingj oil to the finished platedstrip is diihcult because the strip moves at varying rates of speed.

The problem has been aggravatedV by increasing emphasis on close control of the quantity of oil or'coating materialv applied to the surface of .the strip. Close control is emphasized to provide sufficient protection for the tin plated surface without excessive oil which would interfere with subsequent processing-operations, such as soldering or lithographing. control has become-so close that present specifications oall for as little asv 0.05 gnam of oil per base box of,tin plate. The closeness of thiscontrol can be -appreciated when it is realized that a base box of tin plate is 2l S'square feet' of tin plate whichmust be uniformly coated onv both sides;

Since the advent oflthe present processv of; making tin plate in long continuous strips, itV has been the practiceto apply the oil 'to the moving stripl by electrostatic deposition. That is, a high electrostatic field has been created on both sides of the continuously moving strip by electrodes spaced awayfrom and on opposite sides ofthey strip and by spraying the coating material in the form of a mist between the strip and Ithe electrodes. The mist is electrostatically charged by the electrodes and the charged mist particles are attracted to and deposited onto the strip by the electrostatic field created between the electrodes and the strip and thereby the eiciency of the coatingV process is greatly improved. This coating method has not, however, solvedv the problem of controlling the amount of coating material applied to .the strip because the strip moves at frequently fvaryingrates of speed. For example, the strip in a modern itin mill may move at rates varying from 200 feet to 2400 feet a minute and the line occasionally may be stopped.

Various expcdients for regulating the amount of` coating material applied to the strip in accordance'with the speed of the strip have been attempted, for example, by regulating the air pressure to a spray gun which sprays the coating material into the electrostatic iield' in accordance with the rate ofl speed of the strip; None of these attempts, however, has been successful in that they have not provided sufficiently close control of the' amount of oil applied to meet present requirements.

In accordance with my invention, I create a line mist of coating material in fog chambers using conventional ice aspirators. The mist is .then carried into a charging chamber having an ionizing electrode from which it passes to the coating zone for the strip, i.e., a zone in which an electrostatic iield is created whichl forces the particles in the mist towards the continuously moving strip. I thus employ two separate electrostatic stages; inone, the mist particles are ionized or charged and, in the-second, the particles yare deposited by -the electrostatic fieldv on the strip without further ionization.

As noted, I create a very dense line mist of coating material in fog chambers using multiple conventional aspirators. Compressed air supplied to the Aaspirators displaces some mist from the fog chambers, carrying it into theV ionizing or charging chambers at relatively low velocity. The mist moving slowly through the charging chamber under these conditions is not only charged but has time [to be completelyV precipitated on the walls of the charging chambers and drains back into the oil reservoirs in the bottom ofthe fog chambers. Thus, under the above conditions-when only the air supplied to theaspirators is introduced into the fog chambers, no mist ever gets past lthe charging chambers to be deposited. In order to'get some charged mist through the-ionizer to be deposited on the Itinstrip in 'the separate electrostatic eld of the coating zone, it is necessary to increase-the velocity of .the mist through the ionizer. The faster the mist travelsL through the ionizer, the less time is allowed for precipitation on the walls of the ionizing chamber and, therefore, the more charged mist forced into the coating zone to be deposited on the ftirr strip. To elect this increased velocity, secondary air is introduced into the fog chambers, thus displacingythe mist more rapidly. I have found experimentally thatv under the above described conditions. the amountV of charged mist deliveredY to Ithe coating zone and deposited on the tin strip bears essentially a linear relation to the rate of flow'ofV secondary air: Therefore, by controlling the rate of ow o f secondary air in accordance with the speed of the strip being coated,` a uniform weight of oil per unit area of strip will be deposited.

In the accompanying drawings, I have illustrated certain presently preferred embodiments ofv my inventions, in which:

FIGURE l is a partial side view ofA my apparatus;

FIGURE 2 is an end view of the apparatus of FIG- URE 1;

FIGURE 3 is a partial vertical section on `the lines III- III of FIGURE 2 but to a larger scale;

FIGURE 4 is a view similar to FIGURE 3 but showing only one-half of the apparatus and on a still larger scale;

FIGURE 5. is a section, along the lines V-V of FIG- URE 4; Y

FIGURE 6 is a diagrammatic showing of the apparatus which I employ for regulating the flow ofsecondary air in accordance Withy the speed of the strip.

Referring to FIGURES 1 and 2,` the strip 7 to be coated is led under a guide roll 8 between two fog chambers 9 and into a coating chamber y10. After coating, the strip passes out through the top of the coating chamber.

FIGURE 3 shows the construction of the two fog chambers 9, which are placed one on each side of the pathof movement of the strip. Each -fog chamber 9 is in the formV of a tank generally rectangular in section and having a length approximating, the width of the widest strip toV be coated. Each chamber is parallel to the surface of the strip but extends lengthwise at right angles to the-path of movement of the strip.

Each chamber has an outside vertically extending wall 11", a bottom 12, and an inner wall. 13 which slopes outwardly from the inner' edge of the. bottomV wall towards they path of` travel of the. strip, then vertically for a short distance parallel to the path of travel of the strip. Each chamber has a top wall 14 which extends horizontally in from the top of the outer side wall 11 and then slopes inwardly and downwardly towards the top of the inner side wall 13 to form a base 15 for a charging chamber, designated generally by the reference numeral 16, from which mists of ionized particles of coating material pass into the coating zone 17 which is within the coating chamber 10.

The bottom and the lower portions of the side walls of each fog chamber form reservoirs for the liquid coating material and each fog chamber has conventional aspirators 18 which draw liquid coating material up from a reservoir, atomize it, and spray it in tine particles against the inner walls 13 of the fog chambers. The atomizers are supplied with compressed air through pipes 19 from a common header 20 connected to a source of compressed air which is not shown.V As shown in FIG- URE l, each fog chamber has several aspirators spaced from each other along the length of the chamber.

As noted, the aspirators 18 are directed against the inner walls 13 of the fog chambers, with the result that large droplets of coating material impinge upon and adhere to the walls 13 and drop back into the reservoir of coating material at the bottom of the fog chambers. Therefore, only very fine air-borne mist passes from the fog chambers into the charging chambers 16. To further insure the removal of large droplets and to create a turbulence in the fog chambers, I provide in each a baie 21 which extends outwardly and downwardly from a point adjacent the top of the side Wall 13, leaving only a relatively small passageway 22 between the outer edge of the baille and the outside side wall 11.

j The construction of the chambers 16 will be later described. Itshould be noted here that each carries tine ionizing wires 23 (see FIGURE 5) which are supported on supports 24 at each end of the chamber and which extend the full length of the chambers. It should also lbe noted that at the bottom of each chamber there is a narrow passageway 25 which also extends the length of the chamber and through which mist from a fog chamber passes into the chambers 16.

A high potential unidirectional current is connected to the ionizing Wires Within each chamber 16. These wires then react with the sides of the chamber, causing ions to flow from the wires to the side walls whereby the particles of mist of coating material are ionized before they reach the coating zone 17.

Within each charging chamber, there exists a turbulent electrostatic field surrounding the ionizing wires which tends to precipitate the oil mist onto the walls of the chamber, from whence the material refluxes back into a fog chamber. When the velocity of the mist in the charging chamber is created solely by the air supplied to the aspirators 18, practically all of the mist will be precipitated in the charging chambers and returned to the fog chambers and thence to the reservoirs of fluid at the bottom of the chambers. To drive the mist of coating material out of the chambers 16, I provide an additional stream of gas (preferably air) to the fog chambers through conduits 26. The velocity of the air supplied through the conduits 26 is adjusted `so that it will move the mist particles with suflicient speed through the chambers 16 that a portion of them will not be precipitated on the side walls of the conduit. They will, therefore, pass into the coating zone 17 Y Referring to FIGURE l, it will be seen that air to the conduits 26 is supplied by a conventional blower 27. Uniform distribution of air supplied through the conduits 26 to the fog chamber is obtained by adjustable plate valves or shutters 28 which may be adjusted to vary the size of the opening 29 between the conduits 26 and the side walls 11 of the fog chambers. As has been noted, the volume of air through the conduits 26 is adjusted in accordance with the speedcf the strip. This is accomplished by adjusting a butterfly valve 30 positioned in the conduits 26 adjacent the blower 27. The control of the valve 30 will be later described.

When the mist of coating material passes from the charging chamber 16 into the coating zone 17, the mist particles pass into a high potential electrostatic eld created between the strip l7 and two plate electrodes 31 equally spaced from the strip on opposite sides of the strip and extending parallel to the path of travel of the strip in the coating chamber 10. The plates 31 are mounted within the coating chamber 10 in a conventional manner by insulators 32. The plates 31 have the same polarity asthe ionizing wires in the chambers 16 and maintain a high potential with respect to the strip. Oil mist passing into the coating zone om the charging chambers 16 is, therefore, attracted to and deposited on the moving tin strip.

FIGURE 4 shows one of the charging chambers 16. Each chamber is formed from two elongated rectangular `shaped plates 33 whose length is slightly greater than the width of the maximum width of strip to be coated in the apparatus. The plates extend across the strip and at right angles to the path of movement of the strip. The lower portions 34 of the plates are bent inwardly towards each other to form a narrow rectangular shaped passageway 25 through which mist passes into the chambers 16. The lower ends of the inclined portions '34 of the plates are bent inwardly to form anges 35 extending at right angles to the plane of the major portion of the plates. The anges 35 rest on plates 36 which extend the full length of the plates 33 and which extend across the plates 33 at each end to support the plates 33 in proper position and also to support insulators 37 for posts 24 for ionizing wires which, as noted, extend lengthwise within the chambers 16.

The charging chambers 16 are supported below the ycoating chamber 10 by plates 38 which extend irst inwardly from the sides of the coating chamber and then outwardly and downwardly to the top plates 15 of the fog chambers 9. The charging chambers are also snpported -by flat plates 39 which are mounted on the top plates 15 at right angles thereto. Angles 40 having a flange extending inwardly from the

plates

38 and 39 are spaced a short distance above the top plates 15 so as to form narrow slots 41 in which the outer edges of the base plates 36 may slide. The charging chambers 16 can be quickly and easily removed by sliding them lengthwise out of an end wall of the coating chamber because of this sliding support arrangement.

The charging chambers 16 each have mechanism for adjusting the eiective width of the slot 25 at the base of the charging chambers so as to vary the width of the path of mist and air which ilows through the chambers 16 in accordance with the width of the strip being coated. Immediately below each slot 25 in the bases of the charging chambers, there is an elongated slot 42 in the top plate 15 of each `fog chamber. U-shaped channels 43 extend along both edges of the slots 42 for the full length of the slots. Two

strips

44 and 45 having the same width as the bottom flanges of the channels 43 are secured to the channels in line with the bottom tianges, as shown in FIGURE 4. The strips 45 have notches 46 cut out in their inner corners adjacent the strips 45 to yform opposed narrow slots in which shutters 47 extending inwardly from each end of the lfog chambers may slide. The shutters 47 each have racks 48 which mesh with pinions 49. The pinions 49 are turned =by means of a hand wheel 50 which is connected to the pinions 49 by gearing 51 and shafts 52 (see FIGURES l and 2).

Turning the hand wheel 50, therefore, ywill move the shutters 47 inwardly and outwardly in the slots formed by the notches `46. As the shutters move outwardly, they extend out through the ends of the fog chambers into rectangular shaped casings 53.

It was mentioned that the chambers11'6 can be removed from the apparatusl by sliding them lengthwise in thefslot between the angles 40 and the-top plates 15 off the fog chambers 9' (see FIGURE At).v 'I'hey are thus removed out of the end of the apparatus which appears ini FIG- URE 2.- The oppositeend of the apparatus carrie'smeans for electrically connecting the ionizingl-wires in the charging chambers 16. This is shown in FIGURE 5.

The end of the casing opposite to the end from which the charging chambers are withdrawn has openings above the fog chambers which are `in line with the charging chambers when they are in operating position. These openings are closed by cover plates 54 and bolting plates 55. The connections -between the end of the casing 10, the cover plates 54, and the bolting plates 55 are sealed by a gasket 56. A junction box 57 is secured to the outside of the

plates

54 and 55. It carries a hollow conically shaped insulator 58 which is held to the cover plate 54 by a collar 59 bolted to the

plates

54 and 55. The wide end of the insulator is next to the cover plate 54. The small end of the insulator carries a jack plug socket 60 which, in turn, has a bolt 61 to which a high voltage wire can be connected.

Referring to FIGURE 5, the post support 24 for the ionizing wires 23 has soldered to it at its base an elongated metal rod 62 which extends at right angles to the post 24 and parallel to the center line of the charging chamber 16. The other end of the rod 62 forms a jack 63 adapted to tit into the socket 60. The post 62 except for the jack 63 is surrounded by a glass tube -64 which is spaced from the rod 62 by rubber washers 65.

It will be noted from FIGURE that, when a chamber 16 is slid along its support above a fog chamber, the post 62 slides into the socket 60 in the junction box and thus a connection for the ionizing wires is automatically made at the same time the chamber is slid into operative position.

Holes

66 and 67 are formed in the

plates

54 and 55, respectively, so as to allow the rod 62 to pass into the insulator 58 in the socket 60.

As has been pointed out, an important feature of my invention lies in the concept of regulating the amount of coating material supplied to the strip by adjusting according to the speed of the strip the volume of air supplied to the fog chambers independently of the air supplied to the -aspirators FIGURE 6 shows the apparatus for thus regulating the ow of air to the fog chambers. The gure is diagrammatic because all of the components used in the control are conventional. A tachometer generator 68 is placed where it can be actuated by a rotating part which rotates in accordance with the speed of the strip, for example, the axle 69 of the roll 8 which guides the strip into the coating apparatus. The tachometer generator sends an electric signal which is proportional to the speed of the strip to an electropneumatic link receiver 70.

rIhe receiver 70 adjusts the pressure of air in the line 71 which is supplied by a line 72 connected to a source of compressed air, not shown. The line 71 leads to a ratio relay 73 which has a beam mounted on a movable fulcrum. One end of the beam is actuated by the air pressure in the line 71. 'Ihe other end of the beam sends a signal, i.e., air pressure, through a line 74 to a sending head 75. 'Ihe fulcrum in the ratio relay 73 is manually adjusted by the operator to set the device for a predetermined ratio of strip speed to volume of air and the relay sends a signal to the sending head 75, which signal corresponds to the amount of air which should be supplied for a particular speed of the strip in accordance At' duit 26 just-beyondthevalve 30. Ilines1.7.9"f an'd80 connect the sendingzhead acrossY the orice plate 78.

In an actual installation and operationofmy inventions, I have found that' the arnount'` of oilV or coating; material supplied to the stripis directly proportional to thevolu'me of air supplied to" the fog' chambers. separate rom that supplied through the aspirators: By the controls whichl have just described, it is possible to regulate the volume of air in accordance with the speed of the strip. The result is that the volume of oil supplied to the strip is closely regulated in accordance with the speed of the strip. I have found that my inventions provide a much closer control for the amount of oil supplied to the strip in a conventional continuous tin line than has heretofore been possible.

The structure of the charging chambers for delivering oil into the coating zone whereby they can be quickly removed and replaced in the event of breakage of the ionizing wires is also a yfeature of some importance. 'Ihe ionizing wires have to be replaced at intervals and it is important that this be done quickly because of the great cost loss involved in stopping a continuous tin strip line. For example, it has been estimated that stopping a tin line in a modern strip mill costs a minute. 'Ihe importance of being able to replace the ionizing Wires quickly can, therefore, be appreciated.

While I have described certain presently preferred embodiments of my inventions, it is to be understood that they may be otherwise embodied Within the scope of the appended claims.

I claim:

l. Electrosatic coating apparatus comprising a coating chamber, means ffor moving the article through said chamber, at least one electrode spaced from the path of travel of the article through the coating chamber, means for creating a unidirectional electrostatic eld between the article and said electrode as the article passes through said chamber, at least one fog chamber, means in said fog chamber for creating a mist of coating material, a charging chamber leading from said fog chamber .to the coating chamber, means in the charging chamber for creating an electrostatic field through which mist of the coating material passes from the fog chamber to the coating charnber, a high voltage source for said electrostatic -eld yin the changing chamber, means independent of said mist generating means for supplying a flowing gas to the fog chamber to carry the of coating material from the fog chamber through the charging chamber to the coating zone, and control means yfor regulating only said supply of flowing vgas to the -fog chamber in proportion to the rate of movement of the article to be coated through the coating chamber.

2. Electrostatic coating apparatus as described in claim l in which the charging chamber has an entry opening connected to the yfog chamber, an exit opening leading to the coating chamber, at least one electrode spaced from the walls of the chamber and between said openings for creating With said Walls, a high-potential electrostatic iield, and a source of high potential for creating said eld.

3. A method of coating an article comprising moving the article through an electrostatic tield extending substantially at right angles to the path of movement of the article, forming a mist of particles of a coating material and maintaining the mist out of contact with the article, supplying a flow of gas separate from the mist forming means to move the mist from the position where it is formed to a position adjacent the article in the rst mentioned electrostatic eld, electrostatically charging all of the mist particles in an electrostatic eld removed from the path of movement of the article, as they move towards the article, and controlling the rate at which the mist moves to the article by regulating only the rate of flow of said gas in accordance with the speed of 'movement of the article.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Macau etal Aug. 12, 1930 Cerwin Mar. 9, 1937 Waite Aug. 8, 1939 Kaufman Mar. 9, 1948 8 Rausburg June 28, 1948 Pegg Aug. 24, 1948 Brunner June 14, 1955 Fleck Sept. 25, 1956 Miller Nov. 13, 1956 Rendel June 25, 1957

Claims

3. A METHOD OF COATING AN ARTICLE COMPRISING MOVING THE ARTICLE THROUGH AN ELECTROSTATIC FIELD EXTENDING SUBSTANTIALLY AT RIGHT ANGLES TO THE PATH OF MOVEMENT OF THE ARTICLE, FORMING A MIST OF PARTICLES OF A COATING MATERIAL AND MAINTAINING THE MIST OUT OF CONTACT WITH THE ARTICLE, SUPPLYING A FLOW OF GAS SEPARATE FROM THE MIST FORMING MEANS TO MOVE THE MIST FROM THE POSITION WHERE IT IS FORMED TO A POSITION ADJACENT THE ARTICLE IN THE FIRST MENTIONED ELECTROSTATIC FIELD, ELECTROSTICALLY CHARGING ALL OF THE MIST PARTICLES IN AN ELECTROSTATIC FIELD REMOVED FROM THE PATH OF MOVEMENT OF THE ARTICLE, AS THEY MOVE TOWARDS THE ARTICLE, AND CONTROLLING THE RATE AT WHICH THE MIST MOVES TO THE ARTICLE BY REGULATING ONLY THE RATE OF FLOW OF SAID GAS IN ACCORDANCE WITH THE SPEED OF MOVEMENT OF THE ARTICLE.