EP0808663A2 - Electrostatic spraying apparatus - Google Patents

Abstract

The spray coating device has a rotatable spray jet body (32) with a jet opening (34) at its front end and a HV electrode (18) for charging the fluid shortly in front of the jet opening. The jet body is rotated relative to the main body (2) of the device about a rotation axis (4) extending in the main spray direction of the jet opening via a turbine wheel drive (20,26,28) with pressurised air jets (20) coupled to a pressurised air line.

Description

The invention relates to an electrostatic spraying device for spraying coating fluid in the form of powder or liquid coating article according to the preamble of claim 1.

Spray devices of this type are generally referred to as an electrostatic spray coating gun. They can be designed as hand-held devices and have a handle for them, or they can be designed as automatic pistols in such a way that they can be carried by a stationary device or a lifting stand or a robot. All of these possibilities also exist for the present invention.

An electrostatic spray gun according to the preamble of claim 1 is known for example from US-A-4 196 465. It is used to spray coating powder onto an object to be coated. The pistol has a pistol barrel to which a handle can be attached or which can be attached to a carrier machine. Through the gun barrel, a powder channel extends from a rear barrel end to a spray nozzle at the front barrel end. One or more high-voltage electrodes are arranged in the flow path of the powder near the nozzle and are electrically connected to the high-voltage side of a high-voltage generator. The high voltage generator is housed in the gun barrel and contains, for example, a direct current / alternating current converter or oscillator, a transformer and a cascade circuit, which are connected to one another in this order. The The low voltage side of the high voltage generator can be connected to an external low voltage source by means of an electrical cable. According to another embodiment, the high-voltage generator could be arranged externally from the spray gun and connected to the high-voltage electrodes via a high-voltage cable. The coating powder is conveyed pneumatically by a stream of compressed air and sprayed at the spray nozzle to form a powder cloud which flows onto the object to be coated. The powder is sprayed or atomized in the spray nozzle by the nozzle effect and / or by the diffuser effect. The spray nozzle forms a stationary body together with the gun barrel. The high voltage electrical on the high voltage electrode or electrodes can have a value between 1 kilovolt and 170 kilovolt and is usually in the lower half between these two values.

An electrostatic spray gun for spraying liquid coating material onto an object to be coated is known from EP-A-0 513 626, in which the spraying or atomization of the coating liquid at the spray nozzle is supported by additional atomizing air. Coating liquid can be atomized in the same way as coating powder on the spray nozzle, for which purpose it is necessary that the coating liquid is supplied to the nozzle at a relatively high pressure. So that the delivery pressure of the Coating liquid can be reduced, additional atomizing air is used in the manner mentioned. The atomizing air can be "high-pressure air with a low flow volume" or "low-pressure air with a high flow volume" or a variant in between. The latter is also known as high-volume-low-pressure or HVLP air. Furthermore, in order to form a flat coating liquid atomizing jet, it is known from this EP to "compress" the atomized coating liquid jet into a flat jet by shaping air acting laterally on it. The coating liquid spray guns also use one or more high-voltage electrodes for electrostatically charging the coating material because this leads to better coating quality and better efficiency.

Both when using coating powder and with coating liquid, a flat spray jet can be generated in that the nozzle opening has a slit-shaped, flat cross-sectional shape.

Instead of a stationary spray nozzle of the aforementioned type, a rotating atomizing bell can be used for spraying the coating powder or atomizing the coating liquid, as is known, for example, from US Pat. No. 5,353,995 and EP-A-0 410 717.

When using a rotating atomizing bell for atomizing coating powder, it has been shown that a main advantage of such rotating bells lies in the fact that they can be used to produce particularly uniform coating thicknesses on flat surfaces. The reason for this uniformity of the coating thickness is the special powder cloud shape. The rotary movement of the rotary bell creates a powder cloud with a relatively large diameter of, for example, 0.5 m with a homogeneous powder particle distribution from a powder stream with a small cross section.

The powder particles of the powder cloud, however, penetrate the cavities of the object to be coated very poorly, which is due to the large cloud shape. The kinetic energy of the powder particles in the powder channel of the spray device is distributed over the much larger cross-sectional area of the powder cloud. This gives the powder particles less "forward moment of motion" so that the powder particles can better follow the electric field lines that run from the high voltage electrical electrode to the grounded electrical conductor closest to the electrode. However, this in turn has the consequence that the electrical field lines and thus also the powder particles cannot penetrate into cavities of the object to be coated, or only insufficiently. The objects to be coated are mostly made of electrically conductive material and are made by one made of metal transported, which is grounded.

As is well known, flat jet nozzles have a different characteristic: the layer thickness distribution on the object to be coated is mediocre, but the penetration capacity of the powder particles into cavities of the object to be coated is very good.

The object of the invention is to be able to use both advantages in spray coating with coating fluid in the form of coating liquid, but especially in the form of coating powder, namely the generation of uniform coating thicknesses over large surfaces and good penetration of the coating fluid particles into cavities of an object to be coated in order to thereby coat interior surfaces of such cavities with good quality and good efficiency.

The object of the invention is also to achieve a more uniform particle distribution in the sprayed fluid jet.

These objects are achieved according to the invention by the features of claim 1.

Further features of the invention are contained in the subclaims.

The invention produces a better homogenization of the spray jet.

According to a preferred embodiment of the invention, a flat jet nozzle is used for spraying coating powder and this flat jet nozzle is rotated at a relatively low speed around an axis of rotation which is the central longitudinal axis of the flat jet nozzle. The low speed of the flat jet nozzle, or in other applications, round jet nozzles or other shaped nozzles, means that the coating fluid cloud does not expand too much and therefore does not lose too much forward moment of motion, but the uniform layer thickness distribution on the object to be coated is essential is better than with non-rotating spray nozzles because the coating fluid cloud is homogenized by the rotating movement.

Another advantage of the invention is the good application efficiency with which the coating fluid is applied to the object to be coated.

The speed of the spray nozzle can be regulated in a simple manner, although in many cases no speed control is required. A further advantage is that stationary spray nozzles can be exchanged for rotary spray nozzles in spray devices already on the market.

The invention is described below with reference to a spray device for spray coating with coating powder, but the invention can also be used for liquid coating fluid.

All of the features and modes of operation described above with reference to the prior art, in particular flat jet nozzles, omnidirectional nozzles, atomizing air, powder cloud shaping air, electrical high voltage etc. can also be used for the subject matter of the invention.

Fig. 1

schematisch einen Axialschnitt durch eine elektrostatische Sprühvorrichtung nach der Erfindung zum Sprühbeschichten von Gegenständen mit Beschichtungsflüssigkeit, insbesondere jedoch mit Beschichtungspulver,

Fig. 2

eine vordere Stirnansicht in Richtung von Pfeilen II-II von Fig. 1, welche eine Rotations-Zerstäuberdüse mit schlitzförmiger Düsenöffnung zeigt,

Fig. 3

eine der Fig. 2 ähnliche vordere Stirnansicht einer Zerstäuberdüse mit im Querschnitt kreisrunder Zerstäuberdüsenöffnung,

Fig. 4A

einen hinteren Abschnitt im Axialschnitt einer weiteren Ausführungsform einer elektrostatischen Sprühbeschichtungsvorrichtung nach der Erfindung,

Fig. 4B

einen vorderen Abschnitt der weiteren Ausführungsform nach Fig. 4A im Axialschnitt gesehen, wobei in den Fig. 4A und 4B die Querschnitts-Trennebene IV-IV angegeben ist und zur besseren Erkennbarkeit der Zugehörigkeit der beiden Figuren einzelne Teile sowohl in Fig. 4A als auch in Fig. 4B dargestellt sind.

The invention is described below with reference to the drawings using preferred embodiments as examples. Show in the drawings

Fig. 1

schematically shows an axial section through an electrostatic spray device according to the invention for spray coating objects with coating liquid, but in particular with coating powder,

Fig. 2

2 shows a front end view in the direction of arrows II-II of FIG. 1, which shows a rotary atomizer nozzle with a slot-shaped nozzle opening,

Fig. 3

2 shows a front end view similar to FIG. 2 of an atomizer nozzle with an atomizer nozzle opening that is circular in cross section,

Figure 4A

a rear section in axial section of a further embodiment of an electrostatic spray coating device according to the invention,

Figure 4B

a front section of the further embodiment according to FIG. 4A seen in axial section, the cross-sectional plane IV-IV being indicated in FIGS. 4A and 4B and individual parts both in FIG. 4A and in FIG. 4 for better recognition of the affiliation of the two figures 4B are shown.

The spray device shown schematically in FIGS. 1 and 2 is described below for spraying coating powder onto an object to be coated. Since the spraying device is shown schematically, however, it could also be described using liquid coating material. The spraying device has a non-rotating main body 2 and a rotating body 6 rotating relative thereto about an axis of rotation 4. The non-rotating main body 2 contains a housing 8 which is annular in cross section to the axis of rotation 4 and a tube 10 connected to the housing 8 in a rotationally fixed manner, which extends coaxially to the axis of rotation 4 through the housing 8 and has a tube section 12 projecting outward from the housing 8. The housing 8 contains a high voltage generator 14, the low voltage input side of which is electrically connected to an electrical low voltage cable 16 and the high voltage side of which is connected to high voltage electrodes 18 for electrostatically charging the coating powder. The direct voltage high voltage of the high voltage electrodes 18 can be in the range between 1 KV and 170 KV, preferably in the range between approximately 20 KV and 100 KV. The housing 8 contains turbine compressed air nozzles 20 which are connected to a compressed air line 22 and direct compressed air 24 against turbine blades 26 of a turbine wheel 28 in such a way that the turbine wheel 28 is rotated about the axis of rotation 4 relative to the housing 8.

The turbine wheel 28 is screwed by a thread 30 to a nozzle body 32, the downstream end of which is designed as a nozzle element 33 which has a nozzle opening or spray opening 34 arranged axially to the axis of rotation 4. The spray opening 34 has the slot shape shown in FIG. 2, so that a powder stream 36 flowing through the tube 10 through the spray opening 34 is atomized into a powder cloud 38 with a flat cross section, which flows onto the object to be coated. The turbine wheel 28 and the nozzle body 32 together form the rotary body 6 and are on the forward protruding pipe section 12 is rotatably mounted about the axis of rotation 4. As a result, the nozzle body 32 and its spray opening 34 rotate together with the turbine wheel 28 and the particles of the axial powder stream are rotated in a vortex-like manner by the nozzle body 32 and its spray opening 34 about the axis of rotation 4. As a result, the powder particles in the powder cloud 38 are driven radially outward to the axis of rotation 4 and distributed more uniformly within the powder cloud 38 than is possible without rotation of the nozzle body 32. The atomization of the coating powder is not caused by the rotation, but by the effect of the nozzle. Instead of a nozzle effect, the spray opening 34 could also be designed such that it atomizes the coating powder by means of a diffuser effect. The rotation can support atomization.

The high voltage electrodes 18 are located in the nozzle body 32 in or next to the flow path of the coating powder 36 in or close to the spray opening 34 in such a way that they can electrostatically charge the coating powder. As is known, separately supplied electrode air flows around the high-voltage electrodes 18 so that no powder particles can adhere to them and thus the ions of the high-voltage electrodes 18 are driven into the powder stream 36. Instead of two electrodes 18, only one or more electrodes can be provided. Instead of the arranged according to FIG. 1 on the inner circumference of the nozzle body 32 Electrodes 18 or additionally, a high-voltage electrode can be arranged in the radial center in the axis of rotation 4.

The high-voltage path from the high-voltage generator 14 to the high-voltage electrodes 18 consists of a non-rotating stationary electrical conductor 40 in the housing 8 and an electrical conductor 42 rotating relative to the axis of rotation 4, which extends through the turbine wheel 28 and the nozzle body 32. The adjacent ends of the two conductors 40 and 42 are separated from one another by such a small air gap 44 between the housing 8 and the rotating body 6 that the high electrical voltage can jump from the non-rotating conductor 40 to the rotating conductor 42 through the air gap 44 . As a result, a contactless electrical connection is formed between the two conductors 40 and 42.

In the non-rotating housing 8 or, as shown in FIG. 1, in the rotating body 6, an electrical resistor 46 can be arranged in the electrical conduction path 40, 42, which limits the maximum electrical current of the high-voltage electrodes 18 in the event of a short circuit.

3 shows a front view of another embodiment of a nozzle body 32 which has a nozzle opening or spray opening 34 which is circular in cross section.

In the embodiment according to FIG. 1, a slide bearing 48 is formed between the tube piece 12 of the base body 2 projecting forward and the rotary body 6.

In the further embodiment according to FIGS. 4A and 4B, parts that correspond to FIG. 1 in terms of function are provided with the same reference numbers. The principle is the same as in the embodiment of FIGS. 1 and 2. For this reason, only the differences are described below.

In the embodiment according to FIGS. 4A and 4B, the housing 8 has a sleeve part 50 which extends from the rear to the front over the turbine wheel 28 and the adjoining rear section of the nozzle body 32 and between them and these parts 28, 32 an annular gap 52 forms. As a result, at least a part 53 of the turbine exhaust air can flow forward through this annular gap 52 over the nozzle body 32 and blow powder particles away from it. Another part 55 of the turbine exhaust air can escape through bores 54 which are formed in the sleeve part 50 around the turbine blades 26. Throttles 56, which have throttle openings of different sizes, can be screwed interchangeably into the bores 54. By using different flow restrictors 56 or flow restrictors with a controllable opening cross section, the parts 53 and 55 of the turbine exhaust air can be adjusted in quantity, and the flow velocity of the turbine exhaust air and thus can the speed of the turbine wheel 28 can also be controlled to a limited extent.

In the embodiment according to FIGS. 4A and 4B, the rotating body 6 is rotatably supported on a tubular hub 57 by two roller bearings 58 and 60 arranged at an axial distance from one another. Instead of rolling bearings 58 and 60, for example ball bearings or roller bearings, plain bearings can also be used in this embodiment. The hub 57 is a non-rotatable component of the housing 8 and thus also non-rotatably connected to the tube 10, so that these parts are non-rotatable or stationary and can be held by hand or a machine, for example a lifting stand or a robot arm. The tubular hub 57 is arranged concentrically with the axis of rotation 4.

The rotary body 6 contains at its rear end a hollow hub part 62 which surrounds the stationary hub 57 and is connected to it by the roller bearings 58 and 60. On this hub part 62 there is a nozzle body 32 with an exchangeable nozzle element 33, in which the slit-shaped spray opening 34 is formed. One or more electrical resistors 46 are arranged in the rotating nozzle body 32 and a high-voltage electrode 18 is arranged axially in the axis of rotation 4 upstream just before the slit-shaped spray opening 34. The high voltage electrode 18 extends through an electrode air duct 68 of an electrode holder 70, which is in the form of a thin plate. The plate-shaped electrode holder 70 lies in the axis of rotation 4 in the powder flow path, so that powder can flow past it on both sides.

In all embodiments, the rotating body 6 is separable from the non-rotatable main body 2 and the elements of the main body 2 and the rotating body 6 can also be disassembled, so that they can be disassembled for cleaning purposes or can be exchanged for corresponding other parts to achieve different spray characteristics.

The electrical resistor or resistors 46 may be housed in the non-rotating main body 2 instead of the rotating spray nozzle body 32.

The speeds of the spray nozzle body 32 are in the range between 120 U / min and 6000 U / min and are therefore significantly lower than the speeds of the known atomizer bells, at which the coating fluid is not sprayed through a nozzle, but only by its rotation, so that the atomizer bells have speeds in the range between 2000 rpm and 12000 rpm.

The nozzle element 33 can be braked and positioned during the coating, so that the spray jet can penetrate into any folds or cavities in the object to be coated.

This results in a sprayer that can be used for both surface coating and profile coating. For braking, the turbine air flow can be reversed or a braking device used, e.g. a hose 70 expandable with compressed air, which can be clamped by expansion between a non-rotating part of the main body 2 and a part rotating relative to it together with the nozzle element 33. 4A shows such a hose 70, which is arranged in a ring between the hub 57 and the sleeve part 50.

The powder is guided in a spiral around the electrode 18 by the rotation of the nozzle element 33 and its spray opening 34. This spiral increases the distance that the powder has to travel in the charging zone of the electrode 18 compared to a stationary nozzle. As a result, the powder is charged much better electrically and the first-order efficiency increases.

Claims (6)

Electrostatic spraying device for spraying coating fluid in the form of coating powder or coating liquid onto an object to be coated, with the following Features: a main body (2); a nozzle member (33) connected to the main body (2) and having a spray opening (34) for spraying the coating fluid; at least one high voltage electrode (18) for electrostatically charging the coating fluid; a coating fluid channel (10) which extends through the main body (2) and the nozzle element (33) to the spray opening (34);
characterized in that the nozzle element (33) with the spray opening (34) is rotatably arranged relative to the main body (2); that the axis of rotation (4) extends in the main spray direction (38) of the spray opening (34); and that drive means (20, 26, 28) are provided for rotating the nozzle element (33) with the spray opening (34). Spray device according to claim 1,
characterized in that the high voltage electrode (18) is rotatably disposed together with the nozzle member (33), that a high voltage path (40, 42, 44) extends from the main body (2) to the high voltage electrode (18), and that the high voltage path does not one - Rotating high-voltage path (40) in the main body (2) and a relatively high-voltage path (42) rotatable in a part (32) which is rotatable together with the nozzle element (33). Spray device according to claim 2,
characterized in that a narrow air gap (44) is formed as a high-voltage connecting path between the two high-voltage paths (40, 42), through the air of which the two high-voltage paths (40, 42) are electrically connected to one another. Spray device according to one of the preceding claims,
characterized in that the drive means (20, 26, 28) have a compressed air turbine. Spray device according to one of the preceding claims,
characterized in that the spray orifice (34) is a flat jet orifice which creates a coating fluid cloud the width of which is different than its height. Spray device according to one of the preceding claims,
characterized by braking means (70) for braking the nozzle element (33) relative to the main body (2).

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