GB1566738A

GB1566738A - Electrostatic powdering method and installation

Info

Publication number: GB1566738A
Application number: GB48849/76A
Authority: GB
Original assignee: Air Industrie SA
Current assignee: Air Industrie SA
Priority date: 1975-11-24
Filing date: 1976-11-23
Publication date: 1980-05-08
Also published as: DE2653219B2; DE2653219A1; NL7611709A; ES453677A1; IT1072914B; JPS5265536A

Description

(54) ELECTROSTATIC POWDERING METHOD AND INSTALLATION (71) We, AIR INDUSTRIR, a French Body Corporate, of 19 Avenue Dubonnet, 92401 Courbevoje, France, 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 methods of and apparatus for coating objects by electrostatic powdering.

When it is desired to deposit enamel powder, for example by electrostatic spraying, it is in general necessary to deposit at successive coating stations at least one under-coat and one finishing coat. In the case ofenamelling, the undercoat or "body-coat", normally black or bluish in colour, is the "body enamel" and the finishing coat of white colour is the "white enamel".

In coating operations of this kind, substantial economies of installation and power are effected-when the two successive deposits are applied without intermediate baking, the two coats being then melted simultaneously by a single backing at the end of the operation, in accordance with U.S. Patent No. 3,513,012.

On the other hand, a method of this kind presents the ptoblem of pollution of the finishing coat by the granules of the body coat deposited and not baked. In fact, even if the powder-coated parts are blown on after their passage through the coating stations, it is not impossible for body granules to become detached from these parts during the course of their passage through the finishing booth. Furthermore, there always exists the risk of a bodly-suspended part coated with body powder falling in the finishing booth.

In these various cases, the finishing booth is polluted by body coat granules; in the case of enamelling for example, the recovered and recycled white enamel will contain black spots which will result in defects in the final baked coating.

Furthermore, and more generally, it will readily be understood that when coating an object by electrostatic powdering. it is highly desirable that at least the last thickness of coating of the visible parts of the objects to be coated should be effected by powder which has a perfect colour. Now, the real advantage of electrostatic powdering is to be able to recover and recycle the powder which has not been deposited on the objects, in order to avoid any loss of material. Unfortunately, the recovered and recycled powder is always more or less soiled or polluted when compared with now powder, due to impurities which are always contained in the recovery and recycling circuit, so that the colour of this powder is certainly not as perfect as that of the orginal powder.

The present invention has for an object to alleviate this disadvantage.

Accordingly from one aspect the present invention consists in apparatus for coating articles by electrostatic deposition of enamel powder comprising means for displacing an article to be coated successively through three powder coating stations, the first station having an electrostatic powder coating unit for depositing an undercoat of a first colour, the second station having an electrostatic powder coating unit for depositing an intermediate coat of a second colour, and the third station having an electrostatic powder coating unit for depositing a topcoat of the second colour, and an oven for simultaneously firing the undercoat, intermediate coat and the top coat on the article, whereby contamination of the topcoat due to undercoat powder is substantially eliminated.

From a second aspect the present invention consists in a method of coating articles by electrostatic deposition of enamel powder, comprising displacing an article to be coated successively through a first, second and third powder coating stations, electrostatically depositing an undercoat of a first colour on the article in the first coating station, electrostatically depositing an intermediate coat of a second colour on the undercoat on the article in the second coating station and electrostatically depositing a topcoat of the second colour on the intermediate coat of the article in the third coating station, and thereafter simultaneouslv firing the undercoat, intermediate coat and topcoat on the article, whereby contamination of the topcoat by undercoat powder is substantially eliminated.

A practical embodiment of apparatus according to the invention comprises, for example, an electrostatic powdering installation which has successively: - A complete booth for the application of undercoat of a certain colour, for example a body-colour powdering booth; the booth being provided with powder recovery and recycling devices; - If so required, an air-blowing booth; - A booth for applying the external finishing coat of the desired colour, comprising at the upstream end one or more powder spray devices supplied either with new powder or recycled powder, or solely with recycled powder, and at the downstream end one or more powder spray devices supplied solely with new powder.

It is then advantageous to separate these latter two families of spray-devices by a lock-chamber or a partition which extends fairly low but which nevertheless provides access to the base of the booth so as to allow recovery of powder from all the spray devices of the said booth.

In the apparatus relating to enamelling referred to above, the booth will comprise successively the spray device or devices fed at least partly with recycled white enamel powder, and then the spray devices or devices supplied solely with new white enamel powder, these two groups of spray devices being preferably separated by a lock-chamber or a screen. In other cases in which it is particularly desired to coat the visible parts of articles with new powder, and the non-visible parts with a powder which may be recycled, the booth will comprise one or more projectors supplied at least partially with recycled powder and directed towards the non-visible parts of the articles, and one or more spray devices supplied solely with new powder and directed towards the visible part of the said objects.

In this case it will also be advantageous to separate the zones of operation of these two groups of spray devices by a lockchamber or a partition while obviously leaving common access to the base of the booth in such manner that recovery of powder from all the spray devices in the booth may be effected; - A baking oven is arranged at the end of the chain.

From the above it will be appreciated that the present invention is particularly applicable to the case in which the powder of a first colour (dark enamel body powder for example) is deposited on an article, and then the powder of a second colour (white enamel powder for example), and there is interposed between these two depositing operations an auxiliary or intermediate depositing operation for powder of the said second colour (white enamel) during the course of which some pollution by granules of powder of the said first colour (body enamel) is admissible.

In this way, the object is coated before its passage into the final application booth of the second colour (white) with only slightly polluted powder. Thus even before the final stage there are very few granules which will become detached from the object powder. Thus in the final coating stage there is practically no probability that a black granule should appear on the surface of the object. In a manner of speaking, this method amounts to multiplying by itself a probability which is already small, thereby practically eliminating the possibility of pollution of the final coating.

In order that the present invention may be more readily understood two embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 shows diagrammatically a first embodiment of an enamelling installation for electrostatic powdering according to the present invention Fig. 2 shows diagrammatically a second embodiment of an enamelling installation for electrostatic powdering according to the present invention.

In Fig. 1, the reference 1 indicates objects to be enamelled hooked on a travelling conveyor 2 which is coupled to the electrical earth of the installation.

The objects 1 travelling along the conveyor 2 move in the direction indicated on the drawing, that is to say from left to right.

The reference 3 indicates a powder coating station in the form of a conventional electrostatic powdering booth in which the parts 1 are coated with body enamel of black or bluish colour.

The booth 3 is associated in known manner with a device for supplying new enamel powder, and also for recovering and recycling of the powder which is not deposited on the parts to be coated, com prising: - A suction and filtering device 4 connected to the booth 3, for example a conventional cyclone; - A powder supply tank 5 receiving the powder filtered by the device 4 and new powder through an inlet 6, and supplying through the outlets 7, 8, 9, 10, 11 and 12 electrostatic powder spray devices 13, 14, 15, 16, 17 and 18.

The reference 19 relates to a powdering booth for white enamel. It is divided into two coating stations by an internal partition 20 extending downwardly for a major portion of the depth of booth 19 to divide the booth into two sub-booths. The partition 20 does not extend to the base 21 of booth 19, this base 21 being connected to a suc- tion and filtering device 22.

The reference 23 indicates a supply tank which is fed with recovered and filtered powder by the device 22 and with new powder through the inlet 24. The supply tank 23 is only provided with three outlets 25, 26 and 27, supplying the first three electrostatic spray devices 28, 29 and 30, which as shown in the drawing operate in the lefthand or upstream sub-boeth of the booth 19.

The last three electrostatic projectors 31, 32 and 33, which operate in the right-hand or downstream suSbooth of the booth 19, are supplied solely with new powder from another supply tank 34 provided with an inlet 35 and three outlets 36, 37 and 38 towards the three electrostatic spray devices 31, 32 and 33 respectively.

The operation of the apparatus just described is as follows: The parts 1 to be enamelled, for example the bodies of domestic electrical apparatus, are hooked by suitable hooks or cradles on the conveyor 2 which carries them at a low speed. They receive a coating of body enamel during their passage through the first coating station provided by the booth 3, leave this booth and pass, if so required, through a blowing booth (not included in the embodiment being described) and then pass into the booth 19.

In booth 19, at a second coating station formed by the spray devices 28, 29 and 30, the parts 1 receive a certain quantity of white enamel, sufficient to cover entirely the previously deposited coat of body enamel, As previously indicated, the spray devices 28, 29 and 30 are supplied with new and recycled powder. If the booth 19 is polluted by granules of body enamel, the white powder which it contains may therefore contain black granules of body enamel.

Still in the same booth 19, but at a third coating station formed by the last series of spray devices 31, 32 and 33, the parts 1 then receive an additional quantity of white enamel. However, as the spray devices of this third coating station are only supplied with new powder, the final surface coating which they deposit is absolutely white, even if the booth 10 is polluted.

It will in general be preferred to separate the second and third coating stations by the screen 20 referred to above. In fact, in practice the jets of powder sprayed are not directional but are rather in the form of clouds mixing with each other. If the jets emitted by the second and third stations of spray devices are mixed, it is quite obvious that the desired effect will not be obtained.

After their passage through the booth 19, the parts 1 pass into an oven (not shown), in which the deposited coats of body and white enamel are subjected to a common baking operation.

It will be observed that the inlet 24 for new powder into the tank 23 may be closed after a certain period of operation, the spray devices 28, 29 and 30 then being only supplied with recycled powder coming from all the spray devices 28, 29, 30, 31, 32 and 33.

In Fig. 2, the reference No. 39 indicates a carrier conveyor supporting articles to be enamelled. These latter are carried, as is known per se by hooks and/or swinging cradles (not shown in the drawings). In Figure 2, there can be seen: a a booth 40 for the application of body colour enamel powder; - a booth 41 for the application of white enamel powder; - a baking oven 42 for enamelled parts; and a a blowing booth 43 for the hooks and/or swinging cradles.

The booth 40 forms part of a complete unit for the deposition of body enamel powder comprising, as known per se: - A supply tank 47 for powder, provided with an inlet 44 for new powder and an inlet 45 for recycled powder and an outlet 46 for powder to be sprayed; - electrostatic powder spray devices 48, 49 mounted on a moving support 50 which gives them a vertical to-and-fro movement, and supplied with powder through the conduit 46; - a recovery and recycling device 51 for powder, provided with an outlet for suction air 52, an inlet 53 for recovered powder coming from the booth 40, an inlet 54 for powder recovered (after coating and un- hooking the articles) from the hooks and/or swings, and finally an outlet 45 for recovered powder, connected to the supply tank 47, In an optical but advantageous manner, the booth 40 is provided at its downstream extremity with a blowing inlet 55 which generates a slight airstream blowing over articles covered with body powder in order to remove any grains of powder which have not adhered sufficiently to the parts.

The booth 41, where white enamel powder is deposited on the articles, consists of two definitely separated white enamel coating stations.

For this purpose, the booth 41 is divided into two sub-booths 56, 57 which are made completely independent by means of a screen 58. Each of the sub-booths 56, 57 contains in a manner similar to booth 40: - a tank 59 and 60 respectively for powder supply, provided with an inlet 61 and 62 for new powder, an inlet 63 and 64 for recycled powder and an outlet 65 and 66 for powder to be projected: - electrostatic powder spray devices 67, 68 and 69, 70 respectively mounted on movable supports 71 and 72, and fed with powder respectively from the conduits 65 and 66; - a device 73 and 74 respectively for the recovery and recycling of powder, which device is provided with an outlet 75 and 76 for suction air, an inlet 77 and 78 for recovered powder coming from the corresponding portion of the sub-booths 56 and 57 respectively, and an outlet 63 and 64 forrecovered powder, connected to the corresponding supply tank 59, 60.

The apparatus also comprises a manual unhooking station 79 for the powdered but unbaked articles and for transferring the said articles onto a baking conveyor 80 passing through an oven 42. The baked parts are removed from the conveyor 80 at 81.

The hooks and/or cradles continue their travel on the conveyor 39 and pass into a booth 43 in which they are subjected to a powerful blowing action so as to remove the powder, both white and black, with which they are covered. This powder is collected in the bottom of the booth 43 and is re-injected into the recovery device 51 associated with the body colour deposit booth 40, through the intermediary of a venturi extractor connected to the conduit 54.

The operation of the apparatus just described is as follows: The articles to be enamelled first pass into the body enamel depositing booth 40.

They pass out of this booth, which acts as a first coating station, coated with a layer of black powder deposited but not yet firmly fixed. The articles then pass into a second coating station formed by the sub-booth 56 where they are coated with white enamel powder which is necessarily more or less polluted by grains of body colour which have become detached from the said articles. The articles thus coated with an intermediate coat of polluted white enamel, then pass into a third coating station formed by subbooth 57, which, as has previously been shown, is to all intents and purposes not pollutable by body-colour granules. The articles are then unhooked and then transferrred at 79 to the baking conveyor 81, while the hooks and/or swinging cradles continue their way towards the blowing and re-injection unit 43, 54.

With reference to Fig. 2, there has been described apparatus equipyed with conventional electrostatic powdering booths. It will be understood that the various units in question 40, 56 and 57 could very well be replaced, in a manner known per se, by electrified fluidized beds, as described for example in United States Patent No.

3,248,253.

The invention finds its application in the industry concerned with the coating of objects by electrostatic powdering. It has an especially advantageous application in enamelling processes.

WHAT WE CLAIM IS:- 1. Apparatus for coating articles by electrostatic deposition of enamel powder comprising means for displacing an article to be coated successively through three powder coating stations, the first station having an electrostatic powder coating unit for depositing an undercoat of a first colour, the second station having an electrostatic powder coating unit for depositing an intermediate coat of a second colour, and a third station having an electrostaic powder coating unit for depositing a topcoat of the second colour, and an oven for simultaneously firing the undercoat, intermediate coat and the topcoat on the article, whereby contamination of the topcoat due to undercoat powder is- substantially eliminated.

2. Apparatus as claimed in claim 1, wherein the first coating- station comprises a booth. and the second and third coating stations comprise a booth divided by a partition into sub-booths, each sub-booth forming one coating station.

3. Apparatus according to either of claims 1 or 2, in which the first coating station further comprises powder recovery means for recovering undeposited and/or detached enamel powder and recycling it back to the coating unit of the first spray station.

4. Apparatus according to any one of the preceding claims, in which the second coating station further comprises powder recovery means for recovering undeposited and/or detached enamel powder and recycling it back to the coating unit of the second spray station.

5. Apparatus according to any one of the preceding claims, in which said third coating station comprises means for recovering undeposited and/or detached enamel powder and recycling it back to the coating unit of said second coating station.

6. Apparatus according to any of claims 1 to 4, in which the third coating station comprises means for recovering undeposited and/or detached enamel powder and recycling it back to the coating unit of the third coating station.

7. Apparatus according to any of the preceding claims wherein the second and

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. completely independent by means of a screen 58. Each of the sub-booths 56, 57 contains in a manner similar to booth 40: - a tank 59 and 60 respectively for powder supply, provided with an inlet 61 and 62 for new powder, an inlet 63 and 64 for recycled powder and an outlet 65 and 66 for powder to be projected: - electrostatic powder spray devices 67, 68 and 69, 70 respectively mounted on movable supports 71 and 72, and fed with powder respectively from the conduits 65 and 66; - a device 73 and 74 respectively for the recovery and recycling of powder, which device is provided with an outlet 75 and 76 for suction air, an inlet 77 and 78 for recovered powder coming from the corresponding portion of the sub-booths 56 and 57 respectively, and an outlet 63 and 64 forrecovered powder, connected to the corresponding supply tank 59, 60. The apparatus also comprises a manual unhooking station 79 for the powdered but unbaked articles and for transferring the said articles onto a baking conveyor 80 passing through an oven 42. The baked parts are removed from the conveyor 80 at 81. The hooks and/or cradles continue their travel on the conveyor 39 and pass into a booth 43 in which they are subjected to a powerful blowing action so as to remove the powder, both white and black, with which they are covered. This powder is collected in the bottom of the booth 43 and is re-injected into the recovery device 51 associated with the body colour deposit booth 40, through the intermediary of a venturi extractor connected to the conduit 54. The operation of the apparatus just described is as follows: The articles to be enamelled first pass into the body enamel depositing booth 40. They pass out of this booth, which acts as a first coating station, coated with a layer of black powder deposited but not yet firmly fixed. The articles then pass into a second coating station formed by the sub-booth 56 where they are coated with white enamel powder which is necessarily more or less polluted by grains of body colour which have become detached from the said articles. The articles thus coated with an intermediate coat of polluted white enamel, then pass into a third coating station formed by subbooth 57, which, as has previously been shown, is to all intents and purposes not pollutable by body-colour granules. The articles are then unhooked and then transferrred at 79 to the baking conveyor 81, while the hooks and/or swinging cradles continue their way towards the blowing and re-injection unit 43, 54. With reference to Fig. 2, there has been described apparatus equipyed with conventional electrostatic powdering booths. It will be understood that the various units in question 40, 56 and 57 could very well be replaced, in a manner known per se, by electrified fluidized beds, as described for example in United States Patent No. 3,248,253. The invention finds its application in the industry concerned with the coating of objects by electrostatic powdering. It has an especially advantageous application in enamelling processes. WHAT WE CLAIM IS:-

1. Apparatus for coating articles by electrostatic deposition of enamel powder comprising means for displacing an article to be coated successively through three powder coating stations, the first station having an electrostatic powder coating unit for depositing an undercoat of a first colour, the second station having an electrostatic powder coating unit for depositing an intermediate coat of a second colour, and a third station having an electrostaic powder coating unit for depositing a topcoat of the second colour, and an oven for simultaneously firing the undercoat, intermediate coat and the topcoat on the article, whereby contamination of the topcoat due to undercoat powder is- substantially eliminated.

7. Apparatus according to any of the preceding claims wherein the second and

third coating stations are sub-booths formed from a single booth divided by a partition.

8. Apparatus according to any one of the preceding claims, in which the coating unit in the third coating station deposits fresh or new enamel powder only.

9. Apparatus according to any one of the preceding claims, wherein in operation the first colour enamel powder deposited by the coating unit of the first coating station is black or blue and the second colour enamel powder deposited by the coating units in said second and third coating spray stations is white.

10. Apparatus according to any one of the preceding claims, further comprising means at the first coating station for blowing off enamel powder poorly adhering to the article after deposition thereon, but before the article proceeds to the second coating station.

11. Apparatus according to any one of the preceding claims, further comprising hooks or cradles for carrying articles on said means for displacement, means for blowing off enamel powder adhering to said hooks or cradles, and means for recycling powder blown off the hooks or cradles to the spray unit of the first spray station or second booth or sub-booth.

12. Apparatus as claimed in any one of the preceding claims, wherein the coating stations each comprise electrostatic powder spraying devices.

13. Apparatus for coating articles by electrostatic deposition substantially as herein described with reference to or as illustrated in Figure 1 or Figure 2 of the accompanying drawings.

14. A method of coating articles by electrostatic deposition of enamel powder, comprising displacing an article to be coated successively through a first, second and third powder coating stations, electrostatically depositing an undercoat of a first colour on the articles in the first coating station, electrostatically depositing an intermediate coat of a second colour on the undercoat on the article in the second coating station and electrostatically depositing a topcoat of the second colour on the intermediate coat of the article in the third coating station, and thereafter simultaneously firing the undercoat, intermediate coat and topcoat on the article, whereby contamination of the topcoat by undercoat powder is substantially eliminated

15. A method according to claim 14, in which undeposited and/or detached enamel powder recovered from the first coating station is recycled for spraying again in the first coating station.

16. A method according to either of claims 14 or 15, in which undeposited and/ or detached enamel powder recovered in the second coating station is recycled for deposition again in the second coating station.

17. A method according to any of claims 14 to 16, in which undeposited and/or detached enamel powder recovered from the third coating station is recycled for deposition in the third coating station.

18. A method according to any of claims 14 to 17, in which undeposited and/ or detached enamel powder recovered from both the second and the third coating stations is recycled for deposition in the second coating station only.

19. A method according to any one of claims 14 to 18 in which the colour of enamel powder deposited in the first coating station is black or blue and that of the enamel powder deposited in the second and third coating stations is white.

20. A method according to any one of claims 14 to 19, further comprising blowing off enamel powder poorly adhering to an article after deposition in the first coating station before the article proceeds to the second coating station.

21. A method according to any one of claims 14 to 20, further comprising blowing off enamel powder adhering to hooks or cradles for carrying articles for displacement through the booths and/or sub-booths and recovering powder blown off the hooks or cradles for deposition on the articles prior to the deposition of the topcoat.

22. A method according to any one of claims 14 to 22, wherein the powder is deposited at each of the coating stations by electrostatic powder spraying devices.

23. A method substantially as herein described with reference to or as illustrated in either Figure 1 or Figure 2 of the accompanying drawings. ~~~