Spraying method and a spray system for coating liquids

Abstract

The invention relates to a spray system and a spraying method for coating liquids. An accessory liquid (10) wets the front, external surface's terminal zone (8) of the spray system (2) discharging the spray jet (6) of coating liquid, for instance the front terminal zone of the external periphery of a rotating atomizing bell (4), in order to prevent drying and layering of coating liquid (10) precipitated on said external surface.

Description

[0001] The invention relates to a method for spraying and to a system for spraying a coating liquid as defined in the preamble of claim 1 and claim 8.

[0002] Spray systems comprising a rotary atomizing element in the form of a so-called bell to atomize and spray coating liquids onto an object to be coated are known from the U.S. Pat. Nos. 4,275,383 and 4,505,430; German patent documents 30 00 002 A1 and 35 09 874 A1. They disclose applying a high electrical potential, which may be positive or negative, to the rotary atomizers and/or to the spray coating liquid. Typically the high voltage is in the range of 4 kv to 140 kv. A high-voltage spray system fitted with an irrotational spray nozzle is known from U.S. Pat. No. 3,731,145.

[0003] Some spray particles of the sprayed coating material tend to migrate rearward from the spray jet and to precipitate on the front terminal outer surface zone of the spray system where they will cure and constitute a hard coating.

[0004] The objective of the invention is to prevent or at least reduce adhesion and curing/hardening and the ensuing coating, i.e. layering of the coating material on the front terminal outer surface zone of the spray system.

[0005] This goal is attained in the invention by means of the features of claims 1 and 8 resp.

[0006] Accordingly the invention concerns a method for spraying coating liquids wherein such a coating liquid is sprayed from a liquid atomizer in the form of an irrotational nozzle or preferably in the form of a rotating, rotary atomizing element onto an object to be coated, and said method is characterized in that the terminal zone of an outer surface of the spray system delivering the spray jet of coating liquid is wetted with an accessory liquid in order to prevent or reduce layering of coating liquid precipitated on said surface.

[0007] The invention also concerns a coating-liquid spray system containing a liquid atomizer in the form of an irrotational nozzle or preferably in the form of a rotating rotary atomizing element to spray coating liquid onto an object to be coated, characterized in that it comprises a wetting unit to wet a front, terminal zone of the external surface of the spray system during spraying with an accessory liquid in order to prevent or delay drying and layering of coating liquid precipitated on said surface.

[0008] The invention offers the advantage that the accessory liquid prevents or strongly delays drying and layering of coating liquid on the spray system.

[0009] The wetting unit to wet the outer surface's front terminal zone with said accessory liquid preferably contains one or more boreholes or nozzles through which the accessory liquid—which illustratively is in the form of liquid drops or of an aerosol—is deposited in metered manner on the pertinent system component, for instance on rotating liquid atomizers.

[0010] In a further feature, the invention contains a cooling unit to cool at least part of the spraying system. Because of lowered temperature, drying or layering of the coating liquid on the outer surface's front terminal zone shall be prevented or much delayed. Accordingly cooling the outer surface's terminal zone enhances the effectiveness of the accessory liquid, namely to prevent or reduce drying and layering of coating liquid on said surface.

[0011] Preferably a cooling unit to cool the coolant shall be integrated into the spraying system. Preferably a compressed gas, for instance compressed air, shall be used as the coolant and it is cooled by the cooling unit and blown onto the cooling portion of the spraying system.

[0012] The voltage applied to the spraying system may be higher than ground or another predetermined reference potential on part or all of said system. The higher voltage may be constant or variable and it may be positive or negative relative to the reference voltage.

[0013] The invention is illustratively elucidated below by means of a preferred embodiment and the attached drawings.

[0014] FIG. 1 is a schematic sideview, also shown partly in longitudinal section, of a spray system of the invention, and

[0015] FIG. 2 is a front elevation of the left, front side of the spray system of FIG. 1.

[0016] The coating-liquid spray system 2 shown in FIGS. 1 and 2 contains a liquid atomizer in the form of an atomizing bell 4 driven by an omitted drive—preferably a compressed air turbine—at an angular speed high enough to spray the coating liquid in the form of a spray jet 6 onto an object to be coated. The coating liquid flows through the front side 5 of the atomizing bell 4.

[0017] The external surface's peripheral front terminal zone 8 of the atomizing bell 4 is wetted during spray coating with an accessory liquid 10. The accessory liquid 10 prevents or delays drying and layering of coating liquid that migrated back from the sprayjet 6 and precipitated on the external surface's front, peripheral zone 8. The accessory liquid 10 is metered through one or more discharge apertures 18 of a wetting unit 16 and is deposited on the external surface's front, peripheral terminal zone 8, for instance being applied drop-wise or in the form of an aerosol, for instance being applied as a spray. A liquid feed line 20 of the discharge apertures 18 is connected 20 through an external supply line 22 containing a valve 24 to an accessory-liquid container 26. From- a compressed air source 30, compressed air is fed in controlled manner through a valve system 28 to the container 26 where it drives the accessory liquid out of said container 26 toward the spray system 2.

[0018] Preferably the accessory liquid shall be water when the coating liquid is water soluble, or it shall be a solvent when the coating liquid contains solvents.

[0019] As shown by FIG. 2, several, for instance three accessory liquid nozzles are configured as discharge apertures 18 across the external surface's front, terminal zone 8 in an air hood 32.

[0020] The feed system feeding coating liquid in the spray system to the atomizing bell 4 is omitted from the Figures because being known in the state of the art, for instance that of U.S. Pat. Nos. 4,275,838 and 4,505,430. Preferably a high voltage is applied to the atomizing bell 4, both the high voltage source and its connections being omitted form the Figures because being widely known.

[0021] To preclude sparkover from the spray system 2 onto the container 26 of the accessory liquid 10, both said container and the atomizing bell 4 shall be kept at the same high voltage and are configured on a bench 38 which is electrically insulated from the ground potential 34 by electric insulators 36.

[0022] Preferably the preferred embodiment also shall contain a cooling unit 40 to cool the front parts of the spray system 2, in particular to cool the front side 5 over which flows the coating liquid and to cool the front terminal zone 8 of the external surface of the atomizing bell 4. The cooling unit 40 feeds a coolant 52, preferably a cooled cooling gas, in particular compressed air, through a coolant line 46 to a coolant discharge 48 which points at the external peripheral surface 42 of the atomizing bell 4 behind latter's terminal zone 8 of the external surface, and thus this external peripheral surface 42 shall be cooled. The cold passes through the atomizing bell 4 to its front side 5. As a result, the temperature is lowered to ambient both at the external peripheral surface 42 of the atomizing bell 4 and on its front side 5, in order to prevent or at least reduce or delay a layer of coating liquid being formed on these surfaces.

[0023] A cooling unit 50 to cool the coolant 52 preferably is mounted directly on the spray system 2 or integrated into it. Consequently the cooled coolant only moves along short paths. Preferably the coolant is a compressed gas, such as compressed air from a compressed air source 54, and is fed through a coolant feed line 56 in metered manner to the cooling unit 50 and then is blown through the coolant discharge 48, illustratively in the form of one or more nozzles, onto the atomizing bell 4.

[0024] The cooling unit 50 may contain a so-called cooling cartridge to cool the coolant 52.

Claims

1. A method for spraying coating liquids, whereby a liquid atomizer in the form of an irrotational nozzle or preferably in the form of a rotating rotary atomizing element (4) the coating liquid is sprayed onto an object to be coated, characterized in that a front, external surface's terminal zone (8) of the spray system (2) discharging the spray jet of coating liquid is wetted by an accessory liquid (10) in order to prevent or delay drying and layering of precipitated coating liquid at the front, external surface's terminal zone (8) of the spray system (2).

2. Spraying method as claimed in claim 1, characterized in that the accessory liquid (10) is dripped onto the front, external surface's terminal zone (8).

3. Spraying method as claimed in claim 1, characterized in that the accessory liquid (10) is deposited in the form of an aerosol onto the front, external surface's terminal zone (8).

4. Spraying method as claimed in one of the above claims, characterized in that the accessory liquid (10) is deposited on the front terminal zone (8) of the external peripheral surface of the rotary atomizing element (4).

5. Spraying method as claimed in one of the above claims, characterized in that water is used as the accessory liquid (10) when the coating liquid is water soluble and in that a solvent is used as the accessory liquid when the coating liquid contains solvents.

6. Spraying method as claimed in one of the above claims, characterized in that at least one component (4) of the spray system (2), where said component is exposed to ambient air and is streamed over by, or wetted with, coating liquid, shall be cooled during spray coating by a cooled medium (52).

7. Spraying method as claimed in claim 6, characterized in that the cooling medium used is a compressed gas (52), preferably compressed air, which shall be blown on the pertinent component.

8. A spray system for coating liquids, containing a liquid atomizer in the form of an irrotational nozzle or preferably in the form of a rotating rotary atomizing element (4) to spray the coating liquid onto an object to be coated, characterized in that it includes a wetting unit (18, 20) to wet by means of an accessory liquid (10) a front external surface's terminal zone (8) of the spray system (2) during spray coating in order to prevent or delay drying and layering of coating liquid precipitated on said external surface.

9. Spray system as claimed in claim 8, characterized in that the wetting unit (18, 20) is designed to drip the accessory liquid onto the front external surface's terminal zone (8).

10. Spray system as claimed in claim 8, characterized in that the wetting unit (18, 20) is designed to deposit the accessory liquid in the form of an aerosol.

11. Spray system as claimed in one of claims 8 through 10, characterized in that the wetting unit (18, 20) is designed to wet the front, external surface's terminal zone (8) of the rotary atomizing element (4).

12. Spray system as claimed in one of claims 8 through 11, characterized in that water is used as the accessory liquid (10) if the coating liquid is water soluble or that a solvent is used as accessory liquid if the coating liquid contains solvents.

13. Spray system as claimed in one of claims 8 through 12, characterized by a cooling unit (40) to cool at least a component (4) of the spray system (2) using a cooled fluid medium during spray coating for the purpose of preventing or delaying curing of coating material at this component (4), said coolant being fed to said component (4).

14. Spray coating as claimed in claim 13, characterized in that the cooled component is the rotary atomizing element (4).

15. Spray system as claimed in either of claims 13 and 14, characterized in that the cooling unit (40) comprises a coolant line (46) fitted with at least one cooling discharge (48) for cooled compressed gas, said discharge pointing at the component (4) to be cooled in order to blow the cooled compressed gas onto said component.

16. Spray system as claimed in claim 15, characterized by being fitted with a cooling unit (50) to cool the compressed gas.