US3486483A - Electrostatic spray coating apparatus - Google Patents

Description

Dec. 30, 1969 R, mm ET AL 3,486,483

ELECTROSTATIC SPRAY COATING APPARATUS Original Filed April 12, 1967 3 Sheets-Sheet 1 R. TILNEY ET AL ELECTROSTATIC SPRAY'COATING APPARATUS Dec. 30, 1969,

3 Sheets-Sheet 2 Original Filed April 12, 1967 E QE @ AZV/ N fl////////%/ Q .Q A m U E r\\/\ \m N W S R. TILNEY ET AL ELECTROSTATIC SPRAY COATING APPARATUS Original Filed April 12, 1967 Dec. 30,1969

3 Sheets-Sheet 5 N wE United States Patent Int. Cl. Bb 5/02 a v US. Cl. 118-626 i i Claims ABSTRACT OF ,THEDISCLOSURE A safe electrostatic coatingapparatuscapableof producing a spray pattern having an adjustable Width achieved by a laminated resistive blade assembly. received in a hollow body, and forming with the body a shallow recess. The hollow body forms a coating material reservoir and has a series of coating material openings spaced from the blade. In operation the blade is charged through a resistive connection and coating material is forced from the reservoir through the opening to flow over theouter surface of the body into the recess and to'form a thin film on the surface of the blade from which it is atomized by the electrostatic field to the article and deposited upon the article. Adjustment of spray width is achieved by obstructing a portion of the openings in the body.

This application is a continuationof application Ser. No. 630,236, filed Apr. 12, 1967, now abandoned.

This invention relates to electrostatic spray coating apparatus of the kind in which a liquid coating material 'to be atomized is fed to the edge of an elongated member, between which and an article to be coated, a highvoltage electrostatic field is maintained to cause atomization of the coating material and to promote its deposition upon the article by electrostatic attraction.

Features and advantages of the invention'will appear from the following description of an embodimentthereof; given by way of example, in conjunction with the a'c'com Panying drawings, in which: FIGURE 1A is a sectional plan-viewof a'liquid'coating atomizing and charging apparatus; I

FIGURE 1B is a sectional view of a body member cooperatively associated withthe apparatus of FIGURE 1Ai and r FIGURE 2 taken on the line'22 of FIGURE 1;

The apparatus which is shown in these drawings is suitable for coating an article or succession'of articles presenting a surface to be coated which is elongated and of roughly constant width. Thus, the apparatus can be used for coating webs of material, or-for coating a shecession of tray-like'articles 'on a conveyor. In either case,

and coating apparatus.

The apparatus comprises a main body "member of hollow cylindrical form from which extends in an ap-' proximately radial direction a blade assembly 11. Coating material is fed to the interior of the body' at 12 and is exuded from the body through a series of parallel spacedradial openings 13; the exuded material falls by gravity over the outersurface of the body and-onto the surface of the blade assembly, forming a thin film along the knifelike edge of the assembly. A high voltage is caused to appear at this edge with respect to the article or articles being coated and by this means the liquid coating material is atomized and deposited. I v

Thebody member 10 is formed of insulating material and at one end, which is the inlet end for liquid coating the article or articles is or are moved past the atomizing ice material, is closed by aninlet plug 14, having a shoulder 15 which abuts against the end of the body and is held by a plug fixing cap 16, threaded onto the end of the body. The body is provided with drain plug openings closed by drain plugs 17 and 18. In FIGURE 1, the drain plugs are shown angularly displaced, for purposes of illustration.

, The other end of thebody member 10 is closed by an end tube assembly. The end of the body is counterbored at 20 to receive the end of an end plug 21, which is stepped in diameter to fit within the counterbored end of the body, and is there secured by adhesive such as an epoxy resin. The surface of the end plug is cylindrical, and fitting on this is anend tube 22, the end of this tube being reduced at 23 to fit Within a furthercounterbored portion 24 of the main body. Theend tube 22 is held on the end plug 21 by means of alocknut 25, engaging the threaded end of plug 21 with an interposedwasher 26. Together theend tube 22 and themain body member 10 present an approximately cylindrical external surface. With the end of thebody member 10 closed by plug 21 in the way described, and with the insertedinlet plug 14,

' there is formed a reservoir within the body member for coating material, and the material can be fed to this reservoir through afeed inlet passage 27, to which connection can be made at 28 to an external supply of coating material. This coating material is fed by means of a metering pump so as accurately to maintain the desired rate of feed of coating material to the blade assembly.

To elfect electrostatic atomization, high voltage which may be in the range of to 100 kv., must be impressed upon the atomizing device in such a way as to establish high voltage at the edge of the blade assembly. It is desirable that the arrangement should be such that the danger of shock or of fire hazard due to excessive or disruptive discharges from the edge of the blade assembly should be avoided. With this object, the blade assembly is made in a way which presents a low effective capacity, in the sense described in US. Patent 3,048,498, and the voltage is established at the edge of the blade through a circuit which has efiectively a high resistance.

The means for this purpose includes a voltage entry assembly, indicated generally at 30 in FIGURE 1, ex-

tending from the main body assembly substantially at right angles. Voltage from a suitable high voltage source is fed through ahigh voltage cable 31 which passes into theassembly 30, thecentral conductor 32 being attached to the end of ahigh value resistor 33. The cable and resistor are mounted in a termination unit which includes ahousing tube 34 into one end of which thecable 31 is inserted and secured; the tube has a restrictedportion 35 through whichconductor 32 extends, the conductor being attached to a spring 36in turn contacting the end of rethreaded to engage theend tube housing 44.

The high voltage connection from thefinal contact spring 39 to the edge of the blade is made in the following way. Thespring 39 enters arecess 47 formed in theend tube 22; the recess is relatively narrow and extends along the length of theend tube 22 as far as a peripheral recess at 46. In the base ofrecess 45 and around theperipheral recess 46 is laid a coating of material which has a high electrical resistance, such as a carbon or graphite loaded synthetic resin material. Once this material has been placed in position and allowed to harden, the

major part of theelongated recess 45 can be closed and sealed bymeans 48, which can be an insert or merely a filling of suitable resin.

To receive the blade assembly 11, the main body member is formed with an elongatedlongitudinal slot 50, of a width to receive the blade assembly in the manner shown in FIGURE 2. The end of this recess breaks into the inner surface of the counterbored end portion of the main body member, so that at the end of theslot 50 theperipheral recess 46 in theend tube 22 is exposed. The blade assembly 11 is formed of twoflat strip members 51 and 52 which have sandwiched between them alayer 53 of resistive material, which can be, for example, a fibre glass cloth impregnated with carbon or graphite. The twostrip members 51 and 52 are securely fixed together by an epoxy resin adhesive at 54 and 55 and also at intermediate localized areas such as 56, where the resin may extend through holes formed in the fibre glass material. Thestrips 51 and 52 are thereby securely fastened together and the exposed edges of the strips are shaped to a knife edge as shown more clearly in FIG-URE 2, on the surface of which the edge of theintermediate layer 53 will be exposed. The blade assembly is fastened inrecess 50 by means of a carbon loaded resin at 57 so that there will be a high resistance electrical circuit on the exposed edge ofstrip 53, throughresin 57, the resin inperipheral recess 46 and thus through therecess 45 back to thecontact spring 39, and thence throughresistor 33 to the conductor ofcable 31.

In operation, liquid coating material is supplied by the metering pump to thereservoir 12 and the liquid will emerge slowly through theopenings 13. Size, spacing and rate of flow of liquid are so adjusted that the liquid, moving by gravity over the outer cylindrical surface of the body assembly, will establish at the forward knife edge of the assembly a substantially uniform, thin film of the coating liquid and on this thin film there will appear a high voltage from the supply connected tocable 31. The liquid is finely atomized and is drawn off by the electrostatic field in the customary manner of electrostatic deposition. By reason of the inclusion of the relatively high resistance material in thelayer 53 and the high resistance connections extending back toresistor 33, the system will be safe inasmuch as it will be substantially free of danger of fire or shock hazard.

The apparatus described can be varied in a number of ways. For example, depending upon the nature and especially the resistivity, of the coating material, it may be arranged that the current path over which high voltage is established at the atomizing edge is provided largely or wholly by the body of the material. One way of doing this is to mount a wire within, and preferably concentrically within, the reservoir provided byspace 12. Also, to supplement or replace this arrangement, or that described with reference to the drawings, the interior walls of thespace 12 can be coated with a resistive coating material.

In some circumstances it may be preferable to construct the blade so that a single thickness of insulating material is coated on one side only with resistive material, but the sandwich construction is usually more advantageous; it can, for example, easily be sharpened, to give the knife edge required for atomization, if the edge is damaged charging can also be effected by a fine gauge copper wire 59 embedded in the blade near the junction of the blade and the body.

To vary the rate of flow and distribution of coating material, other than the line of holes described can be used. Slots, disposed along the length of the cylindrical body can be used and the slots can be parallel to, or slightly inclined to the length of the body. With the same object of ensuring good flow control of the coating material, along recess 60 can be cut in the external cylindrioal surface of the body, and coating material into the recess fed from the interior reservoir through holes leading into the bottom of the recess. The recess will serve to stabilize the flow. This stabilizing effect of a trough or groove can be otherwise used; a shallow groove orrecess 58 can be provided, extending generally across the flow path of the material, between the delivery holes or recess and the atomizing edge, a convenient position for such a groove is at the junction of the blade and the body.

For uniformity of coating thickness, it is desirable that the material should be atomized from a length of blade which is about the same as the width of the article .or articles being coated. For a semi-permanent installation the blade length will about equal the width of the articles, but to adapt a blade for use with narrow articles the feed can be reduced, and limited to part only of the total length of the blade. One method of limiting the feed in this way is by the use of blocks which fit or slide over the exterior of the body and obstruct the exit openings for the coating material. Conveniently, the blocks surround the body but for the blade; if coating material is fed from a groove or recess, the blocks may lodge in the recess. The block can be of the same or a similar insulating material as that used for the body; polyethylene is suitable.

The blade arrangement can be different. The central body can support another blade, the two blades emerging from the body on opposite sides and with their upper surfaces at an angle that gives a suitable gravity flow. The central closed reservoir can be replaced by an open trough with an over-spill feed to the blade or blades. The two blade system obviously gives an increased rate of coating.

It has been assumed that the article being coated is approximately flat, but if articles of other cross-sectional shapes are being coated better deposition may be obtained by changing the shape of the atomizing edge. For example, in coating material which is longitudinally, considered in the direction of movement of the blade with respect to the article, channelled or corrugated the blade may be given an interrupted edge portion of which correspond approximately to the corrugations on the like discrete separated lengths of blade can be used with this arrangement. It is important that in use sufiicient field intensity is maintained at the edge, to give adequate atomizing and deposition.

Depending to a large extent on the nature of the coating material, there may be advantage in oscillating or vibrating the .blade assembly. Low speed oscillation may give improved lateral uniformity of coating in some application and faster speeds up to vibrations speeds of the order of alternating current supply mains frequency will assert distribution and flow of the coating material over the blade. Vibration may extend the use of the blade to powdered material. The blade can be heated, to improve the flow characteristics of the material, especially liquid material, or to assist polymerization of resin-type coating materials.

We claim:

1. An electrostatic spray coating apparatus for atomizing and depositing liquid coating material upon an article to be coated comprising a hollow body member of insulating material sealed at both ends to form a reservoir for coating material with a coating material inlet, a blade assembly comprising a strip of insulating material and resistive material received in the body and forming at its junction with the body a shallow recess, a series of radial openings in the body spaced from the blade assembly, a high voltage entry housing of insulating material extending from the body member and enclosing a high voltage conductor, means carried by the body connecting one end of the high voltage conductor to the resistive material of the blade assembly, and liquid supply means connected with the coating material inlet to exude coating material from the openings to flow over the surface of the body into the recess to form a thin film on the blade assembly.

2. The apparatus of claim 1 wherein the body is sealed at the end with an end tube assembly including a plug and an end tube having a recess, and means carried by the body connecting one end of the high voltage'conductor to the resistive material of the blade assembly includes resistive material carried by the recess in the end tube.

3. The apparatus of claim 1 wherein the body is cylindrical, the blade assembly includes two strips of insulating material with the resistive material between the two strips, and a fine wire is embedded in the blade near the junction of the blade and the body.

4. The apparatus of claim 1 wherein a recess extends over the exterior surface of the body and over the radial openings.

5. An electrostatic spray coating apparatus for atomizing and depositing liquid coating material upon an article to be coated comprising a hollow body member of insulating material sealed at both ends to form a reservoir for coating material with a coating material inlet, a blade assembly comprising a strip of insulating material and resistive material received in the body, means to permit coating material to exude from said reservoir, means to form the coating material into a thin film, a high voltage entry housing of insulating material extending from one end of the body and enclosing a resistance, electrical connection means carried by said body connecting said resistance with said resistive material of said blade ris sembly, plug means of insulating material sealing said one end of the reservoir and including a portion of said eletrical connection means and liquid supply means con- .nected with the coating material inlet to exude coating UNITED STATES PATENTS 2,651,287 9/ 1953 Turner. 2,695,002 11/ 1954 Miller. 2,748,018 5/ 1956 Miller. 3,048,498 8/1962 Juvinall et al. 3,246,844 4/ 1966 Lehman et al. 3,380,845 4/1968 Shapiro et al.

PETER FELDMAN, Primary Examiner US. or. X.R. 239 -15

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