EP0252954A1

EP0252954A1 - Electrostatic device for powder spraying with triboelectric powder charging.

Info

Publication number: EP0252954A1
Application number: EP87900639A
Authority: EP
Inventors: Gerhard Friedrich Vohringer; Kurt Pfeifer
Original assignee: Esb Elektrostatische Spruh- und Beschichtungsanlagen Gf Vohringer GmbH; ESB VOEHRINGER
Current assignee: Esb Elektrostatische Spruh- und Beschichtungsanlagen Gf Vohringer GmbH; ESB VOEHRINGER
Priority date: 1986-01-14
Filing date: 1987-01-09
Publication date: 1988-01-20
Also published as: JPH0673646B2; EP0252954B1; DE3760194D1; US4798340A; WO1987004088A1; DE3600808A1; JPS63502408A

Abstract

Un courant de gaz porteur de particules pulvérulentes est introduit dans une conduite à poudre (1) par son extrémité d'admission, dans le sens indiqué par la flèche (3), pour recouvrir des pièces à usiner. Les matériaux dont se composent la poudre de revêtement, la conduite à poudre (1) et des corps de refoulement (21, 22) encastrés dans celle-ci ont des constantes diélectriques différentes. A une certaine distance de l'extrémité de sortie (5) est agencé un dispositif de chargement (12) pourvu d'électrodes de recharge (13) faisant saillie vers l'intérieur et connecté à une source de haute tension (17) via une résistance de protection (15). Une contre-électrode arrière (7, 26, 27) comprend un corps de décharge (7) mis à la terre (9). Le courant de particules pulvérulentes est accéléré par les corps de refoulement (21, 22). Les particules pulvérulentes touchent de façon répétée la surface intérieure (11) de la conduite à poudre (1) et les corps de refoulement (21, 22), auxquels elles transmettent, après séparation des charges, des particules à charge négative (10), alors que les particules à charge positive (4) restent collées aux particules pulvérulentes. La concentration de particules à charge négative (10) et à charge positive (4) à des extrémités opposées (2, 5) de la conduite est accrue par le champ électrostatique formé entre les électrodes de recharge (13) et la contre-électrode (7, 26, 27). La poudre de revêtement est ainsi chargée à un si haut degré qu'il n'est plus nécessaire de prévoir un champ électrostatique extérieur entre une électrode de pulvérisation et la pièce à usiner.A stream of gas carrying powdery particles is introduced into a powder pipe (1) through its inlet end, in the direction indicated by the arrow (3), to cover workpieces. The materials of which the coating powder, the powder pipe (1) and the discharge bodies (21, 22) embedded therein are made have different dielectric constants. At a certain distance from the output end (5) is arranged a charging device (12) provided with charging electrodes (13) projecting inward and connected to a high voltage source (17) via a protective resistor (15). A rear counter electrode (7, 26, 27) includes a grounded discharge body (7) (9). The stream of powder particles is accelerated by the discharge bodies (21, 22). The powder particles repeatedly touch the inner surface (11) of the powder pipe (1) and the delivery bodies (21, 22), to which they transmit, after separation of the charges, negatively charged particles (10), while the positively charged particles (4) remain stuck to the powder particles. The concentration of negatively charged (10) and positively charged (4) particles at opposite ends (2, 5) of the pipe is increased by the electrostatic field formed between the recharge electrodes (13) and the counter electrode ( 7, 26, 27). The coating powder is thus charged to such a high degree that it is no longer necessary to provide an external electrostatic field between a spray electrode and the workpiece.

Description

Electrostatic powder spraying device with triboelectric powder charging

description

The invention relates to an electrostatic powder spraying device with triboelectric powder charging by means of a powder tube enclosing a conveying channel for the spray powder and at least one displacement body arranged in the flow channel to increase the flow velocity, the powder tube being grounded on the outside and having a triboelectric material in at least on its inside the frictional electrical voltage series is arranged at a distance from the material of the powder particles to be sprayed.

A comparable powder spraying device is known from DE-A-29 38 8θβ, according to which the powder is charged in a tubular nozzle body, the flow channel of which is narrowed by a guide body in order to increase the speed and thereby generate a flow vortex. In addition, the outside of the nozzle is grounded or connected to high voltage in order to separate the opposite charges of particles on contact with the wall of the nozzle lined with triboelectric material or a displacement body. This is intended to improve the charging of the particles, but a separate electric field is used between the nozzle and the workpiece to guide the charged particles to the material. However, since the electrostatic field peaks, Edges and other protrusions of the workpiece form neutral zones on the workpiece. The coating powder can therefore only penetrate into corners and cracks of the workpiece to an insufficient extent. The coating therefore becomes uneven, which is a major reason why surface damage such as rust stains or the like occurs early on the coated workpiece.

The invention is based on the electrostatic powder spraying device defined at the outset and pursues the task of designing this device in the simplest possible manner in such a way that the precipitation of the coating powder is evened out on workpieces of complex design and the precipitation efficiency is increased.

According to the invention, a reloading device for the triboelectrically charged powder particles with the same polarity adopted in the frictional charging is used to achieve this object.

In this way, the charging voltage of the powder particles can be increased to such an extent that these particles find their way to the workpiece without an electrostatic guiding field established between the spraying device and the workpiece and largely independently of other directional forces such as gas pressure flow forces and the like. As a result, the powder particles can penetrate to the bottom of even complicated recesses on the workpiece, even the coating over the entire surface and improve protection against damage or destruction from the outside.

It is therefore essential that the friction charging is operated as intensively as possible or with the necessary flow rate by selecting the suitable triboelectric substances and the spray powder and by guiding the powder flow to the walls fixed to the device, and therefore Care is taken to discharge the charge that forms on the device side.

The recharging should primarily be carried out by electrodes fed from an electrical high-voltage line, such as radially inwardly protruding wire electrodes. However, it may be appropriate to carry out a further recharge shortly before the powder emerges from the nozzle, which under certain circumstances can again take place triboelectrically.

Also of particular importance is the design and arrangement of multiple displacement bodies, which can be provided in the flow channel, but also in an upstream delivery channel, just before the nozzle outlet.

Further features and advantages of the invention are set out in the subclaims and explained in more detail in the following description of exemplary embodiments of the invention. Show it

1 shows a partial longitudinal section through an electrostatic powder spraying device according to the invention, which is shown schematically in the rest,

2 shows a cross section along the line II-II in Fig.l,

3 shows a partially sectioned side view of a powder spray gun designed according to the invention,

4 shows a first modification of this gun partially in longitudinal section and

5 shows a further development of the embodiment.

The supporting part of the device shown in FIGS. 1 and 2 is a powder tube (1), to which a flow conveying gas with a large number of powder particles evenly distributed therein is supplied from the supply end (2) according to arrow (3), which according to FIG .l are shown as positive charge carriers (4) and the powder tube (1) at its outlet end Leave (5) again in order to be guided to a workpiece that is connected to earth potential, in particular by means of electrostatic * charge forces.

A grounded lead body (7) in the form of a cylindrical bushing, which is flush with the outer surface (6) of the powder tube, is embedded in the outer surface (6) of the powder tube as part of a counter electrode. This discharge body (7) is connected to a ground connection (9) by a line (ö).

While the discharge body (7) expediently consists of highly conductive metal such as copper or brass, the powder tube (1) is expediently formed by an insulator, in particular an insulating plastic. Polytetrafluoroethylene (PTFE) is preferred. A particular advantage of this material is the great difference between its electricity constants and those of the most commonly used coating powders, especially epoxy resin. When passing through the flow channel (14) formed by the powder tube (1), the individual powder particles come into contact with its inner surface (11) several times, a charge sharing occurring at the initially neutral particles. The negative charge carriers (10) adhere to the inner surface (11) of the powder tube, while the positive charge carriers (4) move on with the powder particles.

To increase the positive charge state of the powder particles, a recharging device (42), which is arranged close to the outlet end (5), is used, for example, with two recharging electrodes (13) designed as wire electrodes, which protrude approximately radially into the conveying channel defined by the inner surface (11) and through a high-impedance protective resistor (15) is connected to a high-voltage supply (17). The friction charge is reinforced by two displacement bodies (21) and (22), which, like the powder tube (1), are made of PTFE and bear against the inside (11) of the powder tube with slight pressure.

Both displacement bodies each have a core (23) with a square cross-section, which carries guide ribs (24) arranged on the outside in a cross shape, which run parallel to the pipe axis (16) with the same circumferential pitch and are provided with wedge-shaped bevels (25) at the ends of the displacement bodies. The edges (28) of the guide ribs (24) can also be sharpened in a cutting shape on the inflow side (arrow 3) «

Both displacement bodies (21, 22) are rotated by 45 ° to one another as shown in FIG. 2. Each displacement body reduces the free flow cross-section in the flow channel (14) to less than half the available cross-sectional area. In addition, the powder grains can not flow linearly, but are deflected in the circumferential direction and thereby swirled, which brings additional contacts with the boundary surfaces of the displacement body and the powder tube (1). This also takes place at about twice the speed, which considerably increases the effect of the frictional charge.

In order to keep the charging effect on the boundary surfaces of the displacement bodies approximately the same as on the inner surface (11) of the powder tube, an axial hollow of the core (23) of the displacement body (21) has a cylindrical metal rod (26) which is connected to the respective displacement body , the powder tube (1) and its grounding body (7) is connected by a radially guided pin (27). This pin serves both to ground the displacement body and its positional security. It also serves to form a counter electrode, which includes the grounding body (7) and the metal rod (26). During operation, the coating powder distributed as evenly as possible in its conveying gas is blown into the powder tube (1) in the direction of arrow (3) at the feed end (2). Due to the contacts with the inner surface (11) and the displacement bodies (21) and (22), more and more charge particles release their negative charge carrier (10) to the powder tube (1) and the metal body (26) after a division process. The powder is positively charged triboelectrically.

This charge is increased towards the outlet end (5) by the reloading device (12). This forms between the recharge electrodes (13) and through the part

«

(7,26,27) formed an electrostatic field, which causes a charge equalization with the negative charge carriers (10) according to the field lines (18) and at the same time brings the already positively charged charge carriers (4) to a higher charge potential. The endeavor of the positively charged charge carriers (4) to balance the negative charge carriers (10) leads to the entire electrostatic field moving into the flow channel (10). This prevents the field from emerging from the nozzle opening (31) and thus the formation of electrically neutral zones (Faraday cages) on the workpiece. Due to the improved charging of the coating powder, higher powder throughputs can be achieved. Since the artificial high-voltage field extends far into the interior (14) of the powder tube (1) against the feed direction of the powder, the previously used external high-voltage field between the electr and the spray part can be avoided. Since no neutral zones can form on the workpiece, a complete coating is also possible in the area of deep corners and inner edges.

The operating voltage on the high-voltage supply (17) can change the individual operating relationship

be adapted accordingly, for example, to the materials used for the powder tube (1), the coating powder and also the type and number of displacement bodies used. It can also be automatically adjusted according to operating data, such as the distance to the workpiece, the percentage of powder deposited, and also according to the current intensity.

Mach Fig. Is the powder tube (1) in the manner known from DE-A-25 59 472 installed in the shaft (40) of a spray gun (50) within a shaft tube (50) which is rotatably between a head piece (52) and a foot piece (47) is held on the front of the gun housing (51).

On the front of the head piece (52), a tubular nozzle bush (62) is held inside by means of a nozzle sleeve (69), in which the flow channel (14) is widened from a rear cylinder section (30) to an annular nozzle opening (31). An inner collar (32) of the head piece (52) fits smoothly between the powder tube and spray nozzle into the wall of the flow channel (14). In the front end of the nozzle bushing (62) ^'"" in this is supported by axially extending radial webs (65) a nozzle bar (39) of the longitudinally adjustable supports a baffle (67).

The recharge electrodes (13) are located at a distance from the nozzle opening (31) in the rear end of the nozzle bushing (62). They are connected in the manner known from the aforementioned DE-A-25 59 472 via a high-impedance protective resistor (75) to the high-voltage feed (17), which is led to the outside in the form of a high-voltage cable at the lower end of the pistol grip (33).

A conveyor channel (34) is provided in the pistol grip (33) parallel to the routing of this high-voltage cable just as the powder tube (1) itself is connected to a manifold-shaped body (35) in the gun housing (51). In the deflection area (36), an injector nozzle (37) opens into the rear extension of the powder tube (1), to which compressed air is fed through a hose (38).

This creates a negative pressure in the deflection area (36), through which the powder is sucked up in the conveying channel (34) and mixed again with the conveying air in the pressure jet of the nozzle (37).

The wall of the conveyor channel (34) usually consists of the same insulating material as the parts (1, 21 and 22) so that the frictional charging of the powder begins here. «Therefore, a displacement body (19) is attached there, which essentially the same effects developed as the displacement body (21 and 22).

4, a baffle (671) with its central bore (72) sits on the front end of a tubular nozzle rod (b6l), which is led out of the back of the gun housing (51) and connected to a compressed gas supply line ( 73) is connected. This nozzle rod can be moved in the direction of the double arrow (74) and the distance of the impact body (67D from the nozzle opening (31) can be changed. While the largely axially ejected powder particles strike the front (77) of the impact body and are deflected radially outwards at an angle , The compressed air from the bore (72) on the rear side (78) of the ^{~ "} impact body is also radially deflected by a nozzle device (79) and takes the flow impinging on the front side (77) with it on the ring edge (80) Prevent powder particles from sticking to the back of the impact body. According to Figure 5, the nozzle bushing (621) is smooth cylindrical and shortened compared to the above-described embodiment. It is held in the head piece (52) by a nozzle sleeve (691) overlapping this head piece, on which in turn a nozzle cap

(70) sits.

The interior formed between the aforementioned parts

(71) widens from the flow channel (14) via a first frustoconical surface (41) formed on the end faces of the nozzle bushing (621) and the nozzle sleeve to a cylindrical middle space (42) and narrows down to the annular nozzle opening (311) along a frustoconical surface ( 43). A guide body (44), which carries the nozzle rod (662) with the impact body (67), is non-rotatably seated in the cylindrical middle space (42) of the interior (71).

The guide body (44) is provided on its circumferential side with helically arranged or formed guide elements (45), which can be designed as grooves or as ribs, so that in each case helical narrow channels are formed through which the powder flow is passed, the Powder is additionally connected to the guide body (44) and the wall of the nozzle cap (70). The guide body, like the nozzle cap (70), is expediently made of triboelectric material such as PTFE in order to open up a further possibility of reloading. A tapered guide insert (46) attached to the end of the guide body (44) serves to introduce it into the screw channels.

While in the embodiments of FIGS. 1 to 4, charging is first carried out exclusively by means of friction contacts and then increased by means of a powder charging device (12) from the outside, a third charging process by means of triboelectric charging is provided here External charging achieved state of charge can hardly be increased significantly. However, it is ensured that the state of charge is maintained until it emerges from the nozzle opening (311) during the process of swirling the emerging flow. The coating powder thus guided to the workpiece by an electrostatic force without an externally applied electrostatic guiding field can therefore penetrate much more easily into electrically neutral zones of the workpiece and thereby improve the loading efficiency. In addition to the electrical advantages, the guide body (44) has the task of converting the high linear flow velocity into a rotating powder outlet. This process results in a better powder distribution with a reduced outlet speed _. , which additionally leads to an increase in the degree of coating efficiency.

Are the materials in the. Frictional electrical series opposite to the example above, that is, that the charge carriers on the powder tube are positive and negative on the powder particles, a negative voltage must also be applied to the recharging electrodes (13).

Claims

1. Electrostatic powder spraying device with triboelectric powder charging by means of a powder tube (1) enclosing a conveying channel (14) for the spray powder and at least one displacement body (21, 22) arranged in the flow channel to increase the flow velocity, the powder tube (1) on its outside (6 ) is grounded and has at least on its inside (11) a triboelectric material which is arranged in the frictional electrical voltage series at a distance from the material of the powder particles to be sprayed, characterized by a Mach loading device (12) for the triboelectrically charged powder particles with the same during friction charging adopted polarity.

2. Device according to claim 1, characterized in that the triboelectric material is formed by an insulating plastic, in particular polytetrafluoroethylene.

3. Apparatus according to claim 1 or 2, characterized in that the displacement body (21,22) is provided at least on the outside with preferably the same triboelectric material as the powder tube (1) inside.

4. The device according to claim 3, characterized in that the powder tube (1) and displacement body (21, 22) are each predominantly formed in one piece from triboelectric material.

5. Device according to one of claims 1 to 4, characterized by a feed device (37, 38) for accelerating air which serves to further increase the flow rate.

6. The device according to claim 1, characterized in that the ground connection of the powder tube has an attached on its outside, in particular tubular earthed lead-off body (7) made of electrically highly conductive material.

7. Device according to one of claims 1 to 6, characterized in that the reloading device (12) within the powder tube (1) arranged reloading electrodes (13).

8. The device according to claim 7, characterized in that the recharging electrodes (13) are formed by wire electrodes, which in particular protrude radially inwards from the wall of the powder tube (1).

9. Apparatus according to claim 7 or 8, characterized in that the recharging electrodes (13) are mounted at a distance from the outlet end (5) of the powder tube (1) and a counter electrode (7, 26, 27), in particular grounded, is assigned to them.

10. Apparatus according to claim 7, 8 or 9, characterized in that the recharging electrodes (13) are connected to a high-voltage supply (17) via a high-resistance protective resistor (15).

11. The device according to one of claims 1 to 10, characterized in that at least one displacement body (21,

^* 22) has a plurality of guide ribs (24) inclined to one another and axially arranged in the powder tube (1).

12. The apparatus according to claim 11, characterized in that a plurality of displacement bodies (21, 22) arranged one behind the other in the flow direction are arranged rotated relative to one another about the axis (16) of the powder tubes (1).

13. The apparatus according to claim 11 or 12, characterized in that at least one displacement body (21,22 is tapered towards at least one end, in particular di guide ribs (24) tapering wedge-shaped.

14. Device according to one of claims 1 to 13, characterized in that at least one displacement body (21, 22 is held in a rotationally fixed manner under line contact on the inner surface (11) of the powder tube (1).

15. The apparatus according to claim 6 and optionally one of claims 11 to 14, characterized by a metallic connection (27) serving in particular as a rotation lock between an earthed discharge body of the powder tube (1) and a preferably metallic connecting part (26) of a displacement body (21, 22nd) ).

16. The device according to one of claims 1 to 15 with an in front of the outlet end (5) of the powder tube (1) arranged impact body (671) which sits on a through the powder tube (1) rod, characterized in that the rod as a hollow rod (66l) is formed and has an air supply for ventilating the impact body (671).

17. The device according to one of --_ claims 7 to 10, characterized in that the powder tube (1) between the recharging electrodes (13) and the outlet end (5) is expanded (30,31).

18. The apparatus according to claim 17, characterized "characterized in that a guide body (44) with guide surfaces (45) is used in the extension of the flow channel (14,31,32,).

19. The apparatus according to claim 18, characterized in that the outlet end (5) of the powder tube (1) from the extension (71,42) is narrowed again (43).

20. The apparatus according to claim 18, characterized in that the guide body (44) consists of tetrafluoroethylene.

21. The apparatus according to claim 20, characterized in that the guide body (44) is designed as an impact body receptacle.

22. The apparatus according to claim 20, characterized in that the guide body (44) is arranged for the frontal ventilation of the impact body (67).