GB1567244A - Apparatus and method for electrostatically powder sprayingparticularly of enamel - Google Patents

Description

(54) AN APPARATUS AND METHOD FOR ELECTROSTATICALLY POWDER SPRAYING, PARTICULARLY OF ENAMEL (71) We, FERRO CORPORATION, a corporation organised and existing under the laws of the State of Ohio, United States of America, of One Erieview Plaza, Cleveland, Ohio 44114, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- The present invention relates to a powder spray apparatus generally of the type comprising a booth with a reservoir in which powder is dispersed in air, a powder feed tube, a spray gun in which the powder is electrically charged and sprayed toward the workpiece, an exhaust system, a system with a fan and a filtering device to return the powder.

Electrostatically powder-spraying of organic powders is a method which has generally been applied for the last ten years.

Apparatus for this method are described in United States of America Patent Specification No. 3,918,641, French Patent Specification No. 1,337,069 and Dutch Patent Specification No. 82,716.

The method according to the above mentioned patents is to automatically return the powder, which is not deposited on the work-piece to be coated, to the reservoir, thus providing a saving in use of powder and labour expenses. In addition air pollution is reduced.

An apparatus according to the known methods consists of a hopper in which the powder is dispersed in air, a powder feed tube, a spray gun in which the powder is electrically charged and sprayed toward the work-piece, a booth in which the spray gun and the work-piece are provided, an exhaust system at the bottom side of said booth, a transport system to return the powder-air mixture to the recuperation installation, a recovery system consisting of a cyclone and a filtering device to separate the powder from the air and of a transport system to return the recuperated powder to the hopper again.

Electrostatically spraying enamel suspensions is also a method which has been applied for a long time. In this method the enamel together with clay, water, settling salt and inorganic pigments is milled and the obtained suspension is sprayed onto the work-piece. A disadvantage of this method is, however, that the material, which does not land on the work-piece is deposited on the bottom of the spray booth and in the air filtering installation. This waste material must be manually collected at the end of a working day, or during the operation to change the colour of the enamel, it has to be milled again with clay, water and settling salts and after being sieved it has to be added to a fresh milling. So, automatic recuperation is impossible with this known method.

A method, analogous to the above described method for organic powders will therefore also provide considerable savings and advantages for inorganic powders.

Electrostatically spraying dry ground enamel powders appeared, however, to be impossible. Because of the considerably lower electrical resistance, when compared with organic powders (organic powder 1 x 1016 ohm/cm enamel powder 1 x 109 ohm/cm), it was not possible to maintain the charge on the powder particles, as a result of which the powder adherence was insufficient.

Only when coating the enamel particles with an organic coating (see Dutch Patent Specification No. 75 05337) was it possible to increase the electrical resistance such that the adherence of the enamel powder was maintained within the enamel furnace.

An apparatus for this method which is known per se for the organic powder spraying operation, could only be used at a location where high air velocities are developed, as for instance in the venturies of the powder pump and in the spray gun, the usual soft materials like poly-tetrafluoro-ethylene were replaced by more wear resistant material such as tool steel or ceramic material. The latter was necessary because the considerably higher hardness of enamel particles (hardness 6 according to the Mohs-scale) caused too much wear. In practice the apparatus for powder spraying inorganic powder appeared to involve considerable disadvantages.

Because of the above mentioned considerable hardness of the enamel powder particles an unacceptable wear in the recovery installations, especially in the exhaust system, transport system, and cyclone is developed. Moreover, the powder is polluted by the abraded metal particles, causing a strong undesired change of colour of the enamel surface after being passed through the furnace.

Another disadvantage of abrading the enamel particles against the walls of the recovery installation is that the organic coating, which has a thickness of only 0.0ole is damaged, causing a decrease in the electric resistance and consequently the powder adherence is greatly reduced.

The method applied in the usual "wet" enamel techniques, according to which the enamel is milled together with the inorganic pigment, neither seems to be transferable to these installations.

When the enamel and the inorganic pigment are dry milled and then sprayed in a device suitable for organic powder spray operations it appears that a portion of the inorganic pigment disappears from the recirculated powder and that the remaining portion of the pigment is no longer homogeneously distributed. These losses and demixtures can be explained on the basis of the differences in particle sizes and specific gravity of enamel and pigment particles.

Enamel: specific gravity + 2.60 particle size 95"b between 5-70 u Pigment: dependent upon the type of pigment -- specific gravity varying from 4.0 to 7.0 particle size 100% < 517 The collection efficiency of the cyclones in the usual recovery installations for organic powders having a particle size of 95ooh between 20o0 y is not sufficient for the less coarse inorganic powder systems.

It is an aim of the present invention to provide an apparatus and a method for electrostatically powder spraying inorganic powder systems, in which all the abovementioned disadvantages, which occur during the use of the methods and apparatus for organic powder spraying, which are known up to now, are avoided.

It is another aim of the present invention to provide an apparatus and a method having the possibility of changing quickly from one colour to another without the danger of pollution and without extensive cleaning operations.

According to the present invention there is provided a powder spray apparatus comprising a spray booth with entrance and exit openings to permit the passage of the articles to be coated, and a reservoir in which powder is dispersed in air to form a fluidized bed, a powder supply pump connected by tubes to the reservoir and at least one spray gun in which the powder is electrically charged and sprayed toward a work piece to be coated, an exhaust system with a fan and a filtering device to collect powder which has not reached the work piece or has not immediately fallen back into the reservoir, wherein the booth, supply pump, spray guns arranged on at least one guide, the exhaust system and connecting tubes are united in a powder spray module in which inorganic powder is recirculated.

According to the present invention there is also provided a method for powder spraying using the apparatus according to the preceding paragraph in which the workpiece or a plurality of workpieces to be sprayed are moved past the spray gun or guns by a conveyor.

The filtering devices and the reservoir are preferably simply exchangeable as the cleaning of these parts takes more time than the cleaning of the remaining parts of the module. So, this feature of the invention reduces the number of necessary modules.

The exhaust air is blown back through the filtering devices via air seals substantially such that an inwardly directed jet is developed. The advantage hereof is that the air condition in the module as for instance the air moisture content is kept constant. The relative humidity of the air influences the depositing velocity and the adherence of the enamel powder on the workpiece.

Another advantage of such an apparatus is that an air treatment installation for the ambient air is not necessary and can be left out.

When, as is preferably the case, the guide of the spray guns is arranged in the module itself, only narrow slot openings are necessary to guide the work-pieces through the module.

Such a module can be applied most efficiently in a method which is characterised in that the articles to be sprayed are moved along the spray guns by means of a conveyor.

An embodiment of the apparatus, according to the invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic plan of the interior of a module according to the invention, and Figure 2 shows a cross section taken along the line II--II of Figure 1.

With reference now to the drawings in detail, a powder spray booth 1 has located in its bottom a reservoir 13 in which the powder is dispersed with air, powder is fed via powder supply tubes 7 to spray guns 2, in which the powder is electro-statically charged, and sprayed towards a plurality of work-pieces 3 by a motion means 12. The work-pieces 3 are moved past the spray guns 2 by a conveyor. The powder, which is not deposited on the work-pieces, will, for the greater part, fall back into the reservoir 13 under the influence of gravity. The powder in the reservoir passes through the reservoir bottom 10 and fed via supply connection 11 to the powder supply tubes 7.

In order to ensure that the powder, which does not fall back directly to the reservoir, cannot leave the module via the entrance and exit openings 6, a slight under-pressure is provided in the module by exhausting air via filtering elements 4 using fans 5. The power collected by the filter is periodically blown in the opposite direction of the filters by means of pressure blow.

The air sucked by the fans 5 is partially blown back into the module via slots at both sides of entrance and exit openings 6, by which an inwardly directed air jet is developed to ensure that no powder particles, which float in the module itself can escape via the openings.

A guide 8 is provided along which the spray guns 2 can be moved up and down, is arranged within the module, so that no openings for the spray guns 2 and powder supply tubes 7, which cannot be easilv sealed, are created, and the escape of powder along the guide 8 is prevented.

Using the apparatus as described above, the quantity of air which has to be exhausted via the filtering elements 4 is so small that in comparison with the known apparatus both the number of filters and the charge per filter can be considerably reduced.

For organic powders a rather high air displacement must be maintained to reduce the danger of explosion.

Because of the low charge per filter the alternately periodically cleaning by means of a blow of pressurised air and returning the air via the slots, the recuperation efficiency is also very high for very fine powders as is usual in inorganic enamel pigment systems.

The apparatus according to the invention has a number of advantages over the above described prior art apparatus for the organic powder spraying operation.

As the enamel and pigment particles can no longer contact the metal walls of the transport and recuperation systems at a high velocity, the mechanical wear of the recuperation system is prevented. The reduction in quality of the enamel powder by pollution and the loss of powder adherence is avoided.

Due to the simple and cheap construction it is possible to change the colour, by quickly changing the module by simply uncoupling fans, the motion mechanism and the pressurised air conduits. A module for use with another colour is driven in and connected, after which the production is immediately continued without the time consuming cleaning of the module and the recuperation system.

The above described modules seem to be most suitable for production increase and for the application of "2-coat-one fire" process by placing the modules one behind the other.

WHAT WE CLAIM IS: 1.A powder spray apparatus comprising a spray booth with entrance and exit openings to permit the passage of the articles to be coated, and a reservoir in which powder is dispersed in air to form a fluidized bed, a powder supply pump connected by tubes to the reservoir and at least one spray gun in which the powder is electrically charged and sprayed toward a work piece to be coated, an exhaust system with a fan and a filtering device to collect powder which has not reached the work piece or has not immediately fallen back into the reservoir, wherein the booth, supply pump, spray guns arranged on at least one guide, the exhaust system and connecting tubes are united in a powder spray module in which inorganic powder is recirculated.

2. A powder spray apparatus according to claim 1 in which at least one filtering device and the reservoir are exchangeable.

3. A powder spray apparatus according to claim 1 or 2, in which the air exhausted by at least the one filtering device is blown back via air seals at the entrance and exit openings of the module such that an inwardly directed air jet is developed.

4. A powder spray apparatus according to any of the claims 1 to 3, in which the complete module is simply exchangeable by uncoupling the air duct to and from the fans and the motion mechanism of the spray guns.

5. A method for powder spraying using an apparatus according to any of the preceding claims in which the work-piece or a plurality of work-pieces to be sprayed are moved past

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic plan of the interior of a module according to the invention, and Figure 2 shows a cross section taken along the line II--II of Figure 1. With reference now to the drawings in detail, a powder spray booth 1 has located in its bottom a reservoir 13 in which the powder is dispersed with air, powder is fed via powder supply tubes 7 to spray guns 2, in which the powder is electro-statically charged, and sprayed towards a plurality of work-pieces 3 by a motion means 12. The work-pieces 3 are moved past the spray guns 2 by a conveyor. The powder, which is not deposited on the work-pieces, will, for the greater part, fall back into the reservoir 13 under the influence of gravity. The powder in the reservoir passes through the reservoir bottom 10 and fed via supply connection 11 to the powder supply tubes 7. In order to ensure that the powder, which does not fall back directly to the reservoir, cannot leave the module via the entrance and exit openings 6, a slight under-pressure is provided in the module by exhausting air via filtering elements 4 using fans 5. The power collected by the filter is periodically blown in the opposite direction of the filters by means of pressure blow. The air sucked by the fans 5 is partially blown back into the module via slots at both sides of entrance and exit openings 6, by which an inwardly directed air jet is developed to ensure that no powder particles, which float in the module itself can escape via the openings. A guide 8 is provided along which the spray guns 2 can be moved up and down, is arranged within the module, so that no openings for the spray guns 2 and powder supply tubes 7, which cannot be easilv sealed, are created, and the escape of powder along the guide 8 is prevented. Using the apparatus as described above, the quantity of air which has to be exhausted via the filtering elements 4 is so small that in comparison with the known apparatus both the number of filters and the charge per filter can be considerably reduced. For organic powders a rather high air displacement must be maintained to reduce the danger of explosion. Because of the low charge per filter the alternately periodically cleaning by means of a blow of pressurised air and returning the air via the slots, the recuperation efficiency is also very high for very fine powders as is usual in inorganic enamel pigment systems. The apparatus according to the invention has a number of advantages over the above described prior art apparatus for the organic powder spraying operation. As the enamel and pigment particles can no longer contact the metal walls of the transport and recuperation systems at a high velocity, the mechanical wear of the recuperation system is prevented. The reduction in quality of the enamel powder by pollution and the loss of powder adherence is avoided. Due to the simple and cheap construction it is possible to change the colour, by quickly changing the module by simply uncoupling fans, the motion mechanism and the pressurised air conduits. A module for use with another colour is driven in and connected, after which the production is immediately continued without the time consuming cleaning of the module and the recuperation system. The above described modules seem to be most suitable for production increase and for the application of "2-coat-one fire" process by placing the modules one behind the other. WHAT WE CLAIM IS:

1.A powder spray apparatus comprising a spray booth with entrance and exit openings to permit the passage of the articles to be coated, and a reservoir in which powder is dispersed in air to form a fluidized bed, a powder supply pump connected by tubes to the reservoir and at least one spray gun in which the powder is electrically charged and sprayed toward a work piece to be coated, an exhaust system with a fan and a filtering device to collect powder which has not reached the work piece or has not immediately fallen back into the reservoir, wherein the booth, supply pump, spray guns arranged on at least one guide, the exhaust system and connecting tubes are united in a powder spray module in which inorganic powder is recirculated.

2. A powder spray apparatus according to claim 1 in which at least one filtering device and the reservoir are exchangeable.

3. A powder spray apparatus according to claim 1 or 2, in which the air exhausted by at least the one filtering device is blown back via air seals at the entrance and exit openings of the module such that an inwardly directed air jet is developed.

4. A powder spray apparatus according to any of the claims 1 to 3, in which the complete module is simply exchangeable by uncoupling the air duct to and from the fans and the motion mechanism of the spray guns.

5. A method for powder spraying using an apparatus according to any of the preceding claims in which the work-piece or a plurality of work-pieces to be sprayed are moved past

the spray gun or guns by a conveyor.

6. A method according to claim 5, in which the filtering devices are periodically cleaned by pressurized air blown in the opposite direction to the flow of media to be filtered.

7. A method according to any of the claims 5 or 6 in which a plurality of apparatuses are arranged along the conveyor.

8. A method according to any one of the claims 5 to 7 in which it is used for electrostatically spraying enamel powder of two different colours.

9. A powder spray apparatus, substantially as herein described with reference to and as shown in the accompanying drawings.

10. A method for powder spraying according to claim 5, substantially as herein described with reference to the accompanying drawings.