US3694242A

US3694242A - Method for electrostatically coating the surfaces of articles with pulverulent materials

Info

Publication number: US3694242A
Application number: US104138A
Authority: US
Inventors: Max Ofner
Original assignee: SFB Spezial Filterbau M Ofner KG
Current assignee: SFB Spezial Filterbau M Ofner KG
Priority date: 1966-07-29
Filing date: 1971-01-05
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26

Abstract

A METHOD FOR ELECTROSTATICALLY COATING THE SURFACES OF ARTICLES WITH PULVERULENT MATERIALS, ESPECIALLY COLORING SUBSTANCES, BY SPRAYING THE MATERIAL DISPERSED IN AN AIR CURRENT UPON THE ARTICLE AND PERMITTING THE RECOVERY OF THE EXCESSIVE MATERIAL WHICH IS DRAWN OFF BY SUCTION FROM THE SPRAY CHAMBER, TREATED, AND THEN RETURNED TO THE SUPPLY FOR IMMEDIATE REUSE IN THE SPRAYING OPERATION.

Description

Sept. 26, 1972 M. OFNER 3,694,242

METHOD FOR ELECTROSTATICALLY COATING THE SURFACES 0F ARTICLES WITH PULVERULENT MATERIALS Original Filed July 26, 1967 4 Sheets-Sheet 1 26 m II: III 2 I l ETE /n ven tor:

Max OFNER his AHorney Sept. 26, 1972 M. OFNER 3,694,242

METHOD FOR ELECTHOSTATICALLY COATING THE SURFACES 0F ARTICLES WITH PULVERULENT MATERIALS Original Filed July 26, 1967 4 Sheets-Sheet 2 Inventor: Max OFNER his Aflorney Sept. 26, OFNER METHOD FOR ELECTROSTATICALLY COATING THE SURFACES OF ARTICLES WITH PULVERULENT MATERIALS Original Filed July 26, 1967 4 Sheets-Sheet 5 /0/ z /E0 1 105 i W lnventor:

Max DFNER 1, mm 0% his Afiorney P 1972 M. OFNER 3,694,242

METHOD FOR ELECTROSTATICALLY COATING THE SURFACES OF ARTICLES WITH PULVERULENT MATERIALS Original Filed July 26, 1.96"

4 Sheets-Sheet 4 r R w w w nNn m m b m m M m M m a United States Patent 3,694,242 METHOD FOR ELECTROSTATICALLY COATING THE SURFACES 0F ARTICLES WITH PULVERU- LENT MATERIALS Max Ofner, Leonberg, near Stuttgart, Germany, assignor to SP3 Spezial-Filterbau M. Ofner K.G., Friolzheim, Wurttemberg, Germany Continuation of abandoned application Ser. No. 656,081, July 26, 1967. This application Jan. 5, 1971, Ser. No. 104,138 Claims priority, application Germany, July 29, 1966, P 16 96 368.1; May 5, 1967, P 16 46 104.4 Int. Cl. B05b 5/02; B44d 1/094 U.S. Cl. 117-17 12 Claims ABSTRACT OF THE DISCLOSURE A method for electrostatically coating the surfaces of articles with pulverulent materials, especially coloring substances, by spraying the material dispersed in an air current upon the article and permitting the recovery of the excessive material which is drawn off by suction from the spray chamber, treated, and then returned to the supply for immediate reuse in the spraying operation.

This application forms a continuation application of application Ser. No. 656,081, filed July 26, 1967, and now abandoned.

The present invention relates to a method for electrostatically coating the surfaces of articles with pulverulent materials, especially coloring substances, wherein the material which is supplied from a container and dispersed in an air current is electrically charged and sprayed upon the article to be coated and the excessive material is removed by suction. The invention further relates to an apparatus for carrying out this method.

The known methods of the above-mentioned type have two serious disadvantages, namely, that they do not permit the application of materials the particles of which have the tendency to stick to each other and to form lumps which will prevent the material from being properly sprayed and will clog the spraying device, and that the amount of material which during the spraying does not reach the article to be coated and will be sucked otf is relatively large as compared with the total amount of the sprayed material. Since the material which is removed by suction can no longer be used because of the practically unavoidable formation of lumps, this method is very uneconomical, especially when the materials used, for example, epoxide resins, are relatively expensive.

It is an object of the present invention to provide a method of the type as first described in which the above mentioned disadvantages will not occur. This is attained according to the invention by permitting the larger as well as smaller particles of the excess material which are drawn otf by suction to be recovered for further use by being separated from the air current coming from the suction line and by eliminating the electric charge of the entire recovered material and screening the latter before it is sprayed again.

Since the present invention permits the recovery of all the particles of the material which during the spraying operation do not reach the article to be coated, there will be practically no loss of material. The recovery of this material has the further advantage that the coating operation may be carried out at a greater speed since the usual spray gun which is employed for the spraying operation no longer has to be pointed at all times very exactly upon the article to be coated as was necessary in the known methods which did not provide for any recovery of the Patented Sept. 26, 1972 'ice excess material in order to reduce the loss of material as much as possible. Since the electrostatic charge of the material which is drawn off by suction is removed and this material is screened and if necessary disintegrated, it will be treated so as to permit its reuse without danger that any clogging or other breakdown of the spraying apparatus may occur.

In order to prevent any interference in the proper operation of the spraying apparatus it is advisable to keep the larger and smaller particles of the recovered material which has been separated from the suction current in a mixed condition. This has also the advantage that no rough spots will be formed on the surface of the coated article. This result which by the known methods could not be attained is surprising insofar as it was previously the prevailing opinion that if the excess material was recovered it could be used again only if the fine particles were removed because of the dust which might be intermixed with them.

The larger and smaller particles of the material may be kept in a mixed condition by holding them in suspension by means of air. According to a very advantageous embodiment of the inventive method, the air which is required for this purpose is conducted into the supply container of the material by passing it through a porous layer on the bottom of this container.

When employing materials which have the tendency to form lumps, it is advisable also to subject the new material to a disintegrating and screening process before it is sprayed. Lumps or larger particles which might have been formed during storage of the material will thus be eliminated so that all possible causes of interferences in the spraying apparatus will be avoided.

According to a preferred embodiment of the invention, the screened material is conducted back into the supply container from which the material is taken for the spraying operation. Thus, a closed circuit is formed which insures a very economic operation since it is then only necessary to pass approximately the same amount of material into the circulation which is required for the actual coating process. A change from one material to another may then also be carried out in a very simple manner and will not lead to any noticeable loss of material.

For attaining the further object of the invention to provide an apparatus for carrying out the new method, this apparatus comprises a spraying device for ejecting the electrically charged particles of the material within an air current, a suction device for drawing off the excessive material, and a separating device following the suction device for segregating the excessive particles and collecting them in a container, wherein this separating device is provided with a filter for separating and recovering the fine particles which might still be contained in the air which is discharged from the separating device. The object of the invention is further attained by designing the container in which the separated particles including those which are separated by the filter are collected so that the surfaces of this container with which the particles come in contact form a discharge electrode for removing the electric charge from these particles, and by providing suitable means for disintegrating and screening the material which is collected in this container.

The separating device of the new apparatus is preferably provided in the form of a cyclone separator and the filter is preferably mounted in such a manner within the path of the air which is discharged from this separator that the particles which are separated from this air by the filter will pass into the collecting container which re ceives the particles which are separated by the cyclone separator. This arrangement insures that not only the larger particles which are separated from the air by the cyclone separator but also the smaller particles which are subsequently separated from the exhaust air by the filter will pass into the same container and will therefore remain as an intimate mixture with each other.

In an apparatus which forms a very advantageous embodiment of the invention, the filter is built into the mentioned collecting container or into the cyclone separator which is located above this container. The particles which are separated by the filter then do not need any means for conveying them into the collecting container since they will fall into the latter due to their own gravity. This construction of the apparatus has the further advantage that it has a relatively low height. If desired, it is, however, also possible to mount the filter on top of the separating device. If this apparatus should have a lower height, it is possible to provide it with two lower filters which are connected in parallel.

The disintegrating and screening device is preferably provided in the form of a sifter of a conventional construction in which the material is conveyed by a rotating worm which throws it at the same time against a surrounding screen of a cylindrical shape.

The collecting container is preferably mounted directly above the inlet opening of the disintegrating and screening device so that no additional conveying means will be required to pass the material from one of these units to the other. In addition it is advisable to provide the apparatus between the collecting container and the disintegrating and screening device with a dosing device, for example, a rotatable gate wheel with a plurality of compartments, the operation of which is adapted to be controlled so as to permit the amount of material to be varied which is passed to the disintegrating and screening device.

Another advantageous feature of the invention consists in providing the outlet channel of the collecting container to the gate wheel with a plurality of peripherally spaced nozzles which are adapted to blow blasts of compressed air at predetermined intervals into this channel so as to prevent the material from becoming condensed within the outlet of the collecting container and from clogging this outlet. These nozzles may be arranged so as to blow the air upwardly and at an oblique angle inwardly into the cutlet channel of the collecting container.

In place of such nozzles, it is another feature of the invention to provide the collecting container with a porous layer at least on the inner side of the bottom of this container adjacent to its outlet to the gate wheel, and with a line for passing compressed air through this layer into the collecting container so as to produce therein a suspension of air and the particles of the material which insures that all particles will be uniformly mixed with each other and also that a formation of lumps or a clogging of the outlet of the collecting container will be prevented. The apparatus is further preferably designed so that not only the recovered material is passed into this collecting container so as to insure that an intimate mixture between the recovered and the new material will be formed.

In connection with such a porous layer it is another fea-v ture of the invention to cover at least the inner side of this layer with an electrically conductive coating. This has the advantage that also the part of the collecting or supply container which is covered by the porous layer will serve as a discharge electrode for removing the electric charge from the particles of the material.

The spray chamber which is preferably provided with a plurality of suction inlet openings of the suction device may either be mounted at a place separate from the apparatus itself or it may be combined therewith by being attached to one side thereof.

The various features and advantages of the present invention will become more clearly apparent from the detailed description thereof which is to be read with reference to the accompanying drawings, in which- FIG. 1 shows a front view of an apparatus according to a first embodiment of the invention;

FIG. 2 shows a side view of the same apparatus;

FIG. 3 shows a top view of the apparatus according to FIGS. 1 and 2;

FIG. 4 shows a front view of an apparatus according to a second embodiment of the invention;

FIG. 5 shows a cross section which is taken along the line V-V of FIG. 4;

FIG. 6 shows a cross section of the bottom of the collecting container of the apparatus according to FIGS. 4 and 5; while FIG. 7 shows a front view of an apparatus according to a third embodiment of the invention.

Referring first to FIGS. 1 to 3 of the drawings, the apparatus according to the invention comprises a frame consisting of a supporting plate 1 and feet 2 on which a collecting container 3 is mounted which is composed of an upper cylindrical vessel 4 and a lower funnellike supply container 5 which is directly connected to this vessel 4. The collecting container 3 is symmetrical about its vertical axis and mounted on the supporting plate 1 in such a manner that the vessel 4 is located above this plate 1 and the lower end of the funnellike supply container 5 which is provided with an outlet channel 6 is located between plate 1 and the floor on which the feet 2 are resting. The conical lower container part 5 has an angle of approximately Near its point of connection to the outlet channel 6, the lower container part is provided with a plurality of peripherally spaced inclined nozzles 7 which are connected to each other by an annular line and are adapted to pass blasts of compressed air at predetermined intervals into the outlet channel 6.

As indicated in FIG. 2 an outlet tube 11 of a blower 12 which is mounted on the supporting plate 1 extends tangentially and at a slight downward inclination into the vessel 4. This blower 12 which is driven by an electric motor 13 forms together with the vessel 4 a cyclone separator. A suction line 14 leads from the blower 12 to a suction channel 15 which is provided with openings 15 and is located adjacent to the frame 1, 2 on the bottom of a spray chamber 17 and extends across the entire width of the frame. As shown in FIG. 1, a second suction line 18 which has a considerably smaller diameter than the suction line 14 extends to the area underneath the supporting plate 1 and its end may be inserted into or connected to a container in which the new material to be sprayed is held for conveying this material into vessel 4.

The vessel 4 carries on its upper side two

filters

8 and 9 which are connected in parallel and through which the exhaust air of the cyclone separator is conducted. These

filters

8 and 9 are designed so that the particles of the material which are separated by them from the exhaust air of the cyclone separator will fall into the supply container 5. In order to permit the inside of the container 3 to be cleaned when necessary, the upper wall of vessel 4 1s provided with a manhole 10 which may be closed by a cover.

The outlet channel 6 is connected by a gate wheel 19 with a plurality of separate compartments to the inlet of a conventional sifter 21. Gate wheel 19 which is driven by an electric motor 20 is adapted at every revolution to pass a certain quantity of the material from the collecting container 5 into the sifter 21. This sifter is provided with a rotating feed worm for conveying the material and for throwing it against a resilient screen surrounding it. As illustrated in FIG. 2, the sifter 21 is provided with two

separate outlets

22 and 23 for the treated material and for any waste which has been screened out and is to be discarded.

The outlet 22 for the treated material terminates into a supply container 24 of a pneumatic conveying device which conducts the material which is dispersed within an air current through a connecting line 25 to a spray gun 26 of a conventional construction which is provided with an atomizer nozzle and an electrode for electrically charging the particles of the material which emerge from this nozzle.

The mode of operation of the method and apparatus according to the invention which may be employed, for example, for treating and spraying epoxide resins is as follows:

The amount of material which is required for the particular coating operation is conveyed through the second suction line 18 into the container 3. Thereupon the sifter 21 and thereafter the gate wheel 19 are started. Gate wheel 19 conveys the material in measured quantities into the sifter 21. The material is supplied from the container to the gate wheel 19 automatically by its own gravity, while the nozzle 7 prevents the material from being packed in the outlet channel and from clogging the same.

After entering the sifter 21, the material is disintegrated therein insofar as it may be necessary and screened. After this treatment, the material emerges from the outlet 22 and passes into the storage container 24 of the pneumatic conveying device. The operation of the gate wheel 19 may be controlled so that the quantity of material which is contained in this storage container will always remain between a certain maximum and minimum amount.

After entering the container 24 of the pneumatic conveying device, the material to be sprayed is dispersed therein within an air current and conducted through the connecting line 25 to the spray gun 26 in which the particles are electrically charged before being ejected. During the spraying operation, the spray gun 26 is directed upon the article to be coated and held at a suitable distance therefrom.

Those parts of the sprayed material which did not settle on and adhere to the article to be coated are drawn off by the suction of the blower 12 through the openings 15 of the suction channel 16 and are blown into vessel 4. Due to the downward inclination of the outlet tube 11, the particles which are blown from this tube receive a motion component in the downward direction which facilitates their precipitation from the air current. Those parts of the drawn-off material which are not as yet sep arated from the air current are then separated in the

filters

8 and 9 and fall into the connecting container 3.

The particles of the material which are separated from the exhaust air come in contact with the wall of the vessel 4 and partly also with the wall of the supply container 5 and thereby lose their electric charge. The large surfaces of the container 3 facilitate the release of the electric charge of the particles which is then conducted to the ground. If any charge remains in the particles, it will be passed off within the gate wheel 19 and in the sifter 21. Thereafter, the particles of the material may again pass through the circuit as above described.

Of course, the outlet 22 of the sifter 21 does not need to be connected directly to the supply container 24 of the pneumatic conveying device, but the treated material coming out of the sifter 21 may be conveyed to the pneumatic conveying device in any other suitable manner.

While it is advisable, as illustrated in FIGS. 1 and 3, to feed the new material through the line 18 to the container 3 so as to mix it therein thoroughly with the recovered material coming from the spray chamber 17 and also to treat it together with the recovered material, it is also possible to omit the treatment of the new material by feeding it to a point between the container 5 and the gate wheel 19 or to the sifter 21 or directly to the container 24 of the pneumatic conveying device in which the new material is then thoroughly mixed with the recovered material before the mixture is electrically charged and sprayed by the spray gun 26.

FIGS. 4 to 6 illustrate another embodiment of the apparatus according to the invention in which a cyclone separator consisting of a vertical cylindrical vessel 104 and a blower 112 is mounted in the same manner as previously described on a frame 101 to which a spray chamber 117 is attached. Of course, this spray chamber 117 may also be mounted separately from the frame 101. The pressure outlet 111 of the blower 112 terminates tangentially into the vessel 104 and the suction inlet of blower 112 is connected by a connecting line 114 to the inside of the spray chamber 117.

Vessel 104 is directly connected underneath to a supply container which together with the vessel 104 forms the main collecting container 103. The supply container 105 consists of two coaxial cylindrical sections 105a and 10Sb and an intermediate conical section 1050. Sections 105a which is directly connected to the vessel 104 has a diameter considerably larger than that of the section 1051) but equal to the diameter of the vessel 104.

The supply container 105 is closed at its lower side by a bottom which has a central outlet channel 106. As shown in FIG. 6, the container bottom 130 is provided on the inside with a porous layer 132 which is spaced from the outer wall 131 and secured thereto by spacing elements 133. The space between the porous layer 132 and the outer wall 131 forms a chamber which communicates with a compressed-air supply line 134 which terminates into this chamber within the level of the outlet channel 106.

On its inner side, the porous layer 132 which consists of a synthetic material is provided with an elec trically conductive coating 135 which does not affect the porosity of the layer 132 and participates in completely removing the electric charge from the recovered particles.

The upper section 105a of the supply container 105 contains an air filter 108 from which the discharged air emerges from the container 105 to the outside through an outlet 136.

The powder to be treated may be passed into the supply container 105 through an opening (not shown) in the cover 136' of the vessel 104 or by means of a pneumatic conveying device similar to the conveying device 18 which is connected in parallel to the suction line 114.

In all other respects, the apparatus according to FIGS. 4 to 6 is designed substantially in the same manner as the apparatus according to FIGS. 1 to 3. The coinciding features therefore do not need to be described again.

Since the compressed air which is supplied through the supply line 134 is forced through the porous layer 132 into the supply container 105, the powder which is located in this container forms a kind of a suspension in this air which results in an excellent mixture of the particles of the material and also prevents the powder from forming lumps which otherwise it has a tendency to do and which might result in the danger that the outlet channel 106 might become clogged.

The powder is removed from the supply container 105 in the same manner as described with reference to the apparatus according to FIGS. 1 to 3 by means of a gate wheel 120 and passed through a sifter 121 to the spraying device. The particles of the material which are drawn by the blower 112 out of the spray chamber 117 are blown tangentially into the vessel 104 of the cyclone separator in which the larger particles are separated from the air current in a conventional manner. Due their gravity, these larger particles then drop into the supply container 105, while the fine particles are separated from the air current in a conventional manner. Due to their gravity downwardly into the supply container 105. Since a suspensionlike mixture of air and material is thus formed, the recovered particles will then remain in the supply container in an intimate mixture with the other particles.

FIG. 7 illustrates an apparatus according to a further embodiment of the invention which differs from the apparatus according to FIGS. 4 to 6 only by having a larger frame 201 and by being provided with two equal separating units 2.03 and 203' which are supported by this frame. These two separating .units are connected by separate connecting lines 214 and 214' to the inside of the spray chamber, and each of these units is followed by a sifter 221 and 221' respectively. The mode of operation of these two recovery units is the same as described with reference to FIGS. 4 to 6.

The twin apparatus according to FIG. 7 has the advantage that the change from the treatment of the material contained in one supply container to the treatment of the material in the other supply container can be carried out very quickly since it is only necessary to exchange the spray gun and to switch off the blower of one apparatus and to switch on the blower of the other apparatus.

Although my invention has been illustrated and described with reference to the preferred embodiments thereof, I wish to have it understood that it is in no way limited to the details of such embodiments but is capable of numerous modifications within the scope of the appended claims.

Having thus fully disclosed my invention, what I claim is:

1. A method of electrostatically coating the surfaces of a articles with pulverulent materials for producing a uniformly smooth surface thereon, said pulverulent materials having particles of different sizes, comprising the steps of dispersing said material in a compressed-air current and conveying it by said air current to a spraying device, electrically charging said material and spraying said material from said spraying device upon an article to be coated, drawing off by suction at a suction station substantially all of the excess spraying material not adhering to said article, recovering said excess sprayed material by separating the larger and smaller particles of the recovered material from the air current coming from said suction station, thoroughly mixing all of the larger and smaller particles of the recovered excess material after they have been separated from the air current coming from said suction station, substantially eliminating the electric charge from said recovered particles, dispersing said thus-treated recovered particles in a compressed-air current and conveying them by said air current to said spraying device, and then electrically charging said recovered particles in said spraying device and spraying the same therefrom upon an article to be coated.

2. A method as defined in claim 1, wherein said larger and smaller particles after being separated from the air current coming from said suction station are thoroughly mixed with each other by maintaining them in a fluidized state by a second current of compressed air.

3. A method as defined in claim 2, wherein said second current of compressed air is passed into and mixed with said recovered particles by being conducted from the outside through a porous layer covering at least a part of the inner wall of a container in which said particles are collected.

4. A method as defined in claim 1, comprising removing in separate operations the larger and smaller particles of the recovered excess sprayed material from the air current coming from said suction station.

5. A method as defined in claim 4, wherein the separate operations of removing the larger and smaller particles from the air current are carried out at zones in generally vertical alignment, whereby the particles removed at one such zone fall into at least a portion of 8. A method as defined in claim 4 wherein the larger particles are removed in a cyclone separator, and the smaller particles are removed by a porous filter.

9. A method as defined in claim 8, wherein the porous filter is disposed in alignment with and above at least a portion of the cyclone separator whereby the removed smaller particles fall into the lower portion of the cyclone separator.

10. A method as defined in claim 1, comprising supplying new material to the recovered material, and said recovered material before being electrically charged and sprayed.

11. A method as defined in claim 10, wherein the suction for drawing off the excess material in said section station is also employed for supplying the new material and for passing said recovered excess and new material together into a common cyclone separator for separating the larger particles of the mixture, While the remainder is passed throu-gh a filter disposed above at least a part of the cyclone separator for also separating the smaller particles which then fall into the lower portion of the cyclone separator, then collecting the larger and smaller particles from the bottom of the cyclone separator by gravity and dispersing them in and mixing them by a second compressed air current and removing from the mixture the electric charge from the preceding spraying operation, and then screening the mixture and conveying predetermined amounts thereof to said spraying device from which the mixture after being newly-charged is sprayed.

12. A method of electrostatically coating the surfaces of articles with pulverulent materials, especially coloring substances, comprising the steps of dispersing said material in a compressed-air current and conveying it by said air current to a spraying device, electrically charging said material and spraying said material from said spraying device upon an article to be coated, drawing off by suction the excessive material not adhering to said article in a suction station, and recovering said excessive material by separating the larger and smaller particles of the recovered material from the air current coming from said suction station, substantially eliminating the electric charge from said recovering particles and screening the same, dispersing said recovered particles in a compressedair current and conveying them by said air current to said spraying device and then electrically charging said recovered particles in said spraying device and spraying the same therefrom upon an article to be coated.

References Cited UNITED STATES PATENTS- 3,377,183 4/1968 Hurt et al. 117-19 2,173,032 9/1959 Wintermute 118-312 3,236,565 2/1966 Kester et a1 302-17 727,030 5/1903 Tilghman 51-12 2,770,924 11/1956 Mead et a1. 51-8 3,299,853 1/1967 Whitacre 118-312 2,757,635 8/1956 Lipsius 118-312 2,992,126 7/1961 Roberts et a1. 117-17 3,073,070 1/1963 Mead 51-12 3,167,442 1/1965 Brooks 117-19 3,291,630 12/1966 Deyle et al 117-19 3,598,626 8/1971 Probst et al. 117-17 3,307,296 3 1967 Ashworth 51-8 WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R.