NO132135B - - Google Patents

Description

The invention relates to a device for electrostatic coating

of objects with powdered coating material, consisting of a rudder of electrically insulating material, one end of which carries an atomizer for the coating material and the other end of which is connected to a feeding device for supplying a propellant-coating material stream, and a high-voltage generator with connected electrodes for electrical charging of the coating material and maintaining a high voltage field between the device and the object to be coated.

For atomization of liquid or powdered coating material and electrical charging of the particles, various devices are known which according to the resp. procedures can be divided into a few groups. At one group, the coating material is first sputtered, and the sputtered particles are then electrically charged. For charging, electrodes connected to a high-voltage source are used, which electrodes in the form of tips, needles, wires, rings, etc. are arranged in the particle beam or outside it, and preferably in the case of hand-held devices are attached to the atomization device.

In another group, the coating material in the form of a film is supplied to electrodes which are connected to high voltage and are designed with stationary or rotating edges or tips and are atomized by these into electrically charged droplets. The devices in these two groups generally have a high capacity, i.e. a relatively large amount of material is atomized per unit of time and deposited on the object to be covered. However, due to the detached electrodes connected to the high-voltage generator, they have the disadvantage that, under certain conditions, sparks can occur, leading to an explosion, and touching of the electrodes

must be avoided.

in

Various measures are known to eliminate these disadvantages. In terms of connection technology and manufacturing, the simplest is a current limiting resistor which is connected in the supply lines to the electrodes. However, this becomes ineffective when the capacity in the electrodes and the high-voltage cable lying in front of the resistor is high, as unwanted strong spark discharges can still occur. In the case of devices with spray edges which themselves have a relatively high capacity, a very high-resistance resistor is therefore connected directly to the spray electrode so that the capacity of the high-voltage cable in the event of a spark discharge becomes negligible. A device by which, as already proposed, e.g. a rudder high-voltage generator is combined with the atomization device in a hand-held device, can likewise deliver energy-poor spark discharges when the electrode device is designed for low capacity and the high-voltage generator is designed accordingly, as a high-voltage cable is no longer necessary for feeding the electrode device. Furthermore, special switches have been developed which, at the start of a spark discharge, switch off the high-voltage generator and also stop the supply of coating material to the pre-stator.

All these measures do not prevent sparking, but only cause spark discharges to have less energy.

With a third device group, the coating material is first electrically charged and then vaporized. This method is of particular interest for powder atomizers. The powdered coating material is blown through a tube of electrically insulating material and thereby electrically charged by friction and then atomized against e.g. a deboying body. For better charging of the powder, a high-voltage connected electrode is arranged in the rudder.

Atomizers are also known for liquids, in which a wire-shaped, high-voltage connected electrode is axially arranged within a rudder made of electrically insulating material with a nozzle in the outlet opening, the tip of which lies in the direction of flow of the coating material in front of the outlet opening. In this and similarly designed apparatus, the electrode connected to the high-voltage generator is arranged inside a tube of electrically insulating material, so that it cannot be touched from the outside. If the distance between the electrode and the discharge opening is sufficiently large, spark discharge is practically excluded. These devices are therefore safe and can also be used for atomizing easily flammable substances, but they have the disadvantage that their capacity is unsatisfactory.

The purpose of the invention is to provide a device for electrostatic coating, which device is touch-proof and can be used for atomization of easily flammable coating materials, whereby any spark discharge is avoided, and whose capacity is, however, significantly higher than known devices of this kind.

The device according to the invention is characterized by the fact that several electrodes are arranged on the inner wall of the insulation tube,

which for the provision of corona discharges are connected to correspondingly different high potentials and that the atomizer which is arranged on the insulating pipe and in a known manner forms a constructive unit together with this, optionally carries a further electrode which is connected to a step in a voltage divider - or voltage multiplier - coupling device.

Thereby, a significantly better charging of the coating material is achieved, while the electrodes become touch-proof and the occurrence of harmful sparks is prevented. The electrodes themselves can have any shape in and of themselves, but are, however, preferably designed as protruding tips, particularly of tungsten, from the inner wall of the insulating tube.

The charging electrodes can be arranged somewhere in the insulation pipe, in one extreme case deep inside it, in another extreme case close to the outlet opening. In the case of electrodes located far inside the tube, the electrode tips are preferably arranged in the direction of flow of the propellant-coating material mixture one after the other, e.g. at least on a screw line, and each electrode connected to a step in the voltage divider or voltage multiplier switching device in such a way that lde on successive electrodes relative to a reference potential, e.g. earth potential, shows increasing potential and that there is a voltage between two neighboring electrodes which makes up a fraction of the total available high voltage. With this electrode device, special advantages are achieved by the device and the design of the step connection elements in the voltage divider and voltage multiplier connection device. These connecting elements can be arranged on the insulating tube, as the resistors in the voltage divider connecting device can be ring-shaped and pushed onto the insulating tube. As voltage dividers can

however, a tubular resistance is also used from which individual points are connected to the electrodes. If a voltage multiplier cascade consisting of capacitors and diodes is used, these coupling elements are also appropriately arranged on the insulating tube.

In many difficult cases, the maintained high-voltage field, the transport field, between the electrodes located deep in the insulating tube and the grounded object to be coated, may be too weak for a satisfactory coating. In order to achieve a • stronger transport field, the prestove arranged on the insulating tube can carry a further electrode which is connected to a step in the voltage divider or voltage multiplier switching device. This additional electrode, preferably arranged near the outlet opening, can be completely embedded in the insulating material or be enveloped with insulating material except for a small area of the surface.

Coating devices with a particularly large capacity are achieved according to the invention by the electrodes being arranged close to the outlet opening at a distance from this which is not greater than the diameter of the opening. The electrodes are hereby connected electrically preferably in two groups of which the first electrode group is connected to the high-voltage generator output and the second electrode group is connected to a lower potential. The second electrode group may be connected to earth across a resistor, or each of the two electrode groups may be connected to a step in a voltage divider or voltage multiplier connection device. Preferably, the electrodes are arranged in a plane perpendicular to the row axis and electrically connected to each other in such a way that one electrode group comprises the electrodes with equal numbers and the other group comprises the electrodes with different numbers. In this way, it is achieved that the corona discharges that take place between the electrodes fill the entire root cross-section and that the coating material when passing through this area is particularly well charged. If a voltage multiplier cascade is used to supply the electrodes with high voltage, in which cascade the capacitors in two columns connected one after the other are interconnected with diodes in a manner known per se, each of the two electrode groups is connected to a column of capacitors, each column of capacitors according to the invention contains a damping resistor which, preferably connected between the last and second-to-last capacitor in the column, forms an RC link with associated capacitor, through which link the oscillations occurring at the beginning of spark discharges are strongly damped so that no dangerous overshoots occur.

If the device is to be used as a hand-held spray gun, a

the housing arranged on the insulating tube for receiving the voltage converter or the oscillator in a high-voltage generator must be designed as a gun butt, the gun butt being equipped with a switch for switching on and off. This hand spray gun is connected to the power supply source only with a low voltage cable. If only smaller amounts of material are to be pulverized, the insulation pipe can be connected to a feeding device with a material supply container. Only the aforementioned low-voltage cable and a propellant gas line lead from the hand spray gun to corresponding aggregates that can be built so easily that they can be carried by the user.

The invention shall be described in more detail below by means of an embodiment with reference to the drawings on which

Fig. 1 shows a section through a hand-held spray gun in which the atomizer consists of an axially displaceable flexible body,

fig. 2 shows a section A-A through the gun according to fig. 1,

fig. 3 schematically shows the switching arrangement for a multiplier cascade with a transistorized voltage converter,

fig. 4" shows corresponding connection points of the electrodes of a voltage divider/

fig. 5 schematically shows a hand spray gun with an attached storage container for the coating material,

in

fig.6 shows in section another embodiment of the device according to the invention in the form of a hand spray gun,

in

; fig. 7 shows a cross-section II-II through the front part of the gun-shaped device according to fig. 6,

fig. 8 shows a cross section III-III through the middle part of the device according to fig. 6.

fig. 9 schematically shows the connection device in a multiplier cascade with connected electrodes and

FIG. 10 schematically shows another connection device for the voltage supply to the electrodes.

In a rudder 7 (fig.1 and fig.2) made of electrically insulating material, preferably plastic, two rows of tip-shaped electrodes 16 and la 6a are arranged in such a way that their tips point in a direction radially towards the rudder axis and protrude somewhat from the inner wall. The electrodes in each row are arranged on a helical line along the rudder sheath. Each electrode is connected to a step in a voltage multiplier cascade consisting of diodes 8 and capacitors 9. The diodes and capacitors in the cascade are arranged on the outside of the insulating tube. The multiplier cascade with the electrodes is schematically shown in fig. 3. The first electrode 1 in one row is connected via a connecting line lo to a transformer 11 which is ring-shaped and arranged coaxially at one end of the insulating tube. A hose 33 can be connected to this rudder end for the supply of a propellant coating material stream. The insulating tube 7 with the diodes 8 and the capacitors 9 is stopped into a gun-shaped body 2o with electrically insulating plastic so that no voltage-carrying part can be touched from the outside. On the gun body 2o is attached an electrically conductive material, e.g. aluminium, consisting of a housing which is designed as a pistol muzzle 17. As shown in fig.3, a transistorized voltage converter of a known type is connected to the transformer 11. The connecting elements in this voltage converter, i.e. the transistors 12,13, the fixed resistors 14 and 15 and the regulating resistor 16 are arranged in the pistol grip 17 which also contains a switch 19 for switching the voltage converter on and off, which switch can be operated with the help of a "trigger" 18. The voltage converter is supplied via a low-voltage cable 34 and the switch 19 a direct voltage from e.g. a 12 volt battery.

At the front end of the gun body 2o, a nozzle 35 is arranged. In the embodiment shown, the nozzle 35 consists of a rudder 36 of electrically insulating material and a deflection body 23 arranged in front of its outlet opening 21 and attached to a rod 24. The rod 24 and the deflection body 23 consists of electrically insulating material, e.g. plastic, and can be designed in one piece. The rod 24 is supported on liners 25 and 26. On one liner 26 is attached a string 27 which is connected to a push rod 28. The string 27 and the push rod 28 can be arranged in corresponding channels resp. bores in the gun body 2o. By shifting the push rod 28, the deflecting body 23 can be shifted in the axial direction with respect to the outlet opening 21 of the shock absorber 36, whereby the opening angle for the cone-shaped beam can be increased or decreases. In the area of the outlet opening 21, an annular field electrode 22, e.g.

of thin wire, that it is surrounded on all sides by insulating material. The field electrode 22 is connected via a conductor 37 to the last step in the multiplier cascade. Instead of this ring-shaped field electrode, it can e.g. tip electrodes are also used, or it can be arranged on the front side of the deflection body, e.g. a field electrode in the form of a tip, which is connected to the conductor 37 over a conductor lying in the rod 24 and one of the liners 25 or 26. Parts of the field electrode, preferably point or line-shaped areas thereof, can also lie freely outside. With the help of this field electrode, an additional electrostatic high-voltage field is maintained between the atomizer 35 and the object to be coated, thereby increasing the spray gun's capacity. Instead of this deflection body atomizer, plastic atomizer nozzles with differently shaped outlet openings can also be used. It is appropriate to use a set of such different, exchange-

just atomize so that the spray gun can be optimally adapted to the coating material in question. For easily combustible material, atomizers are used that do not carry any field electrode at all, for less easily combustible material, atomizers whose field electrode is completely embedded in the insulation material are used, and for very difficult combustible material, they are used

jfor stoves with partially detached field electrode.

Instead of a voltage multiplier cascade it can as

schematically shown in fig.4, one of a chain of resistors 29 is used

consisting of voltage dividers which are grounded at one end and at the other end are connected to a pole on a high-voltage generator 38 arranged outside the gun. The resistors 29 in the voltage divider are in turn arranged on the insulation tube 7 jacket and plugged.

The voltage divider resistors can also be designed as rings that are pushed onto the insulating tube.

Instead of the individual resistance rings, a tube-shaped high-resistance resistance can also be used in which the electrode tips 1...6a are inserted.

A particularly movable hand spray gun is schematically shown in fig.5. A supply container 3o for the coating material is also attached to the gun in fig.1, the lower part of which container is funnel-shaped and opens into an adjustable pneumatic feeding device 32 with a diffuser 31 of a known type. The feeding device 32 is connected via a hose 39 to a small compressed air bottle, and the voltage converter present in the gun muzzle is connected via the low-voltage cable 34 to a battery. The compressed air bottle and batteries can be carried by the handgun user himself.

By the one in fig. 6, a tube-shaped sleeve 2o', which preferably consists of plastic, is coaxially arranged with a rudder 7 consisting of electrically insulating material, which rudder at the rear end comprises a rudder attachment 48 directed obliquely backwards and downwards for supplying the coating material by using a propellant gas stream. The rear end of the insulation rudder 7 is closed with a liner 26 with an axial bore, in which bore a push rod 24 is displaceably stored. The liner 26 is secured in a lid 46 screwed onto the rudder sleeve 20. In the sleeve 2o' there is between the end of the insulating tube 7 and the lid 46 space for the insertion of a ring-shaped transformer 11 for a high-voltage generator 38 (fig.9). The secondary winding of the transformer 11 is connected to a voltage multiplier cascade whose capacitors 9, which are normally connected to diodes 8, are connected in succession in two columns. In each column of capacitors 9, a damping resistor 53 or 53a. The damping resistors 53,53a form, with the associated last capacitor 9a, RC combinations that dampen any oscillations in the event of sparking. As can be seen from fig.6 and fig.8, the two columns of capacitors 9 with their damping resistors 53,53a are arranged on the insulating pipe 7. The free space between the insulating pipe 7 and the sleeve 2o' also contains the diodes 8 in the voltage multiplier cascade.

The front end of the sleeve 2o', which tapers in cross-section towards the insulation pipe 7, is provided with external threads for screwing on a union nut 41. With the aid of the union nut 41, a tube-shaped spray or spray head 35 is attached to the sleeve 2o' and the insulation pipe 7. The atomizer or spray head consists of electrical insulation pipe pieces made of plastic that are offset one above the other, of which the inner pipe piece 7a has the same inner diameter as the insulation pipe 7 and carries a liner 25 for the push rod 24. The front retracted end 36a of the outer pipe piece 36 forms the conical outlet opening 21 for the powder channel 7b running in the insulation rudder 7 and in the inner rudder piece 7a and a shoulder located inside against which the front end of the inner rudder piece 7a is supported.

In the inner rudder piece 7a, four pointed electrodes 6,6',6a,6'a are inserted which project radially inwards into the powder channel 7b. The four pointed electrodes 6,6',6a,6'a are arranged in a plane perpendicular to the row axis and offset by 90° from each other,

whereby this plane for the electrodes has a distance from the plane of the outlet opening 21 which is smaller than its largest diameter. The arrangement of the pointed electrodes 6, 6', 6a, 6'a appears in fig. 7.

As shown in Fig. 9, the two opposite electrodes 6, '\6-cag J5å, 6' a ; electrically connected) with each other bg via bvejr's wire and 42a*connected to one of— the two at the end' of the -condeimalaiiiulons situated outputs 55 resp. 55a in the voltage multiplier cascade. In this way, it is achieved that during operation of the device, each electrode is at a potential different from the potential of the two neighboring electrodes, and with a sufficiently high potential difference between the electrodes, corona discharges occur which completely fill the powder channel cross-section. In order to facilitate the start of the corona discharges, the pointed electrodes 6, 6', 6a, 6'a are preferably made of tungsten.

In the embodiment shown in Fig. 6, there are tungsten wires arranged as connection lines 42, 42a in the space between the inner rudder piece 7a and the outer rudder piece 36, whose front pointed end as electrodes 6, 6', 6a, 6'a are quickly through the wall i

the inner rudder piece 7a, and whose rear end over a setting ring 45 which carries a high-voltage contact is connected to the capacitor columns' clamps as stated above, which clamps are not drawn in fig.6. A rudder-shaped intermediate layer 43 of electrically insulating material and optionally a second setting ring 44 at the front end of this serve to insulate and retain the tungsten wires 42, 42a on the inner rudder piece 7a.

The push rod 24 stored displaceably in the two liners 25 and 26 (fig.6) carries the deflection body 23 which is arranged in front of the outlet opening 21, by means of which deflection body the coating material, e.g. a plastic powder is atomized in the form of a cone jet, as the opening angle of the cone can be changed by shifting the push rod 24. The push rod 24 consists of e.g. of solar steel to ensure a blocking-free sliding in the relatively long rear lining 26, and the front part is provided with an electrically insulating synthetic cover 24a which extends to the deflection body 23.

If the device described above is to be used as a hand-held spray gun, the tube-shaped sleeve 2o', as shown in fig. 6, a pistol muzzle 17 is screwed on. The pistol muzzle 17 contains a supply rudder 49 which is in connection with the insulation rudder 7 rudder shoulder 48, which supply bit 49 end protrudes from the lower part of the pistol muzzle for attaching a hose 33 for supplying the propellant-coating material mixture. Furthermore, the pistol grip 17 contains a switch 19 for switching on and off

of the high voltage and the coating material supply, which switch can be manually operated via a trigger arm 18. Furthermore, the gun muzzle contains a socket 51 above which the low voltage

none for operation of the high-voltage generator 38 is supplied and e.g. control cables running from the feeder to the switch 19

is connected. Furthermore, the pistol grip can also contain connecting elements such as e.g. transistors and resistors for the high voltage generator 38.

The wires 56 that lead from the socket 51 and from possibly existing connecting elements to the switch 19 and to the transformer 11 are laid in an insulating sleeve 50 attached to the handle plate 47 and in an existing bore 52, so that assembly and connection of the device is easy and quick to implement.

In one embodiment, the largest diameter for the outlet opening 21 was approx. 14 mm, and the pointed electrodes 6.6',

6a, 6'a were at a distance of approx. 12 mm from the plane of the outlet opening. The voltage multiplier cascade was dimensioned such that one output 55a delivered a voltage of 50 kV and the other output 55 delivered a voltage of 45 kV. With these operating voltages, due to the powerful corona discharges between the electrodes, such a good electrical charging of the particles in the coating material was achieved at the same time that the charged particles were so

very fast in the transport field between the electrodes and the grounded object to be coated, that with impeccable coating and a high degree of separation, a surprisingly high material transport capacity could be achieved.

Fig. 10 schematically shows another embodiment example for high-voltage feeding of the pointed electrodes 6, 6', 6a, 6'a.

One pair of opposite electrodes 6a, 6'a is connected to the output 55a of a high-voltage generator 38 via a wire 42a. The other two opposite electrodes 6, 6' are connected via a wire 42 to a high-ohmic diversion resistor 54, the other terminal of which is connected to earth. During operation of this switching device, it is established by electric

the trodes a differential voltage which causes an oct corona out-

In charge at the electrode tips.

The free space which, after assembly, is found between the tubular

sleeve 20' and the insulating pipe 7 are appropriately filled with an electrically insulating stop resin.

For a gun, there may be several fore-stoppers

spray heads 35 which are different in terms of both length,

shape and size of the outlet opening as well as the number and arrangement of the electrodes, so that the coating process can be optimally adapted to the type of coating material and the shape of the object to be coated

is added.

Claims (3)

1. Device for electrostatic coating of re- stands with powdered coating material, consisting of a tube (7,7a) of electrically insulating material, one end of which carries a nozzle (35) for the coating material and the other end of which is connected to a feed device (32) for supplying a propellant coating material strbm , and a high-voltage generator with connected electrodes for electrically charging the coating material and maintaining a high-voltage field between the device and the object to be coated, characterized in that "several electrodes (1 ... 6, la ..6aj6, 6', 6a, 6'a), which for the provision of corona discharges are connected to correspondingly different high potentials and that the vaporizer (35) which is arranged on the insulating tube (7) and in a known manner forms a constructive unit together with this, optionally carrying a further electrode (22) which is connected to a step in a voltage divider or voltage multiplier coupling device (38, 29; 38, 8, 9). 2. Device as stated in claim 1, characterized in that each electrode (1 ... 6, la ... 6a) is connected to a step in the voltage divider or voltage multiplier connection device (38, 29, 38, 8, 9) . 3. Device as stated in claim 1, characterized in that the further electrode (22) is completely embedded in insulating material. 4. Device as stated in claim 1/characterized in that the electrode (22) is further surrounded by insulating material except for a small area of the surface. 5. Device as stated in claim 1, characterized in that the electrodes (6 ,6', 6a,6'a) is arranged on the front end of the insulation tube (7a), which end is either designed as a vaporizer (35) or carries this, and that the electrodes have a distance from the tube outlet opening (21) that is nowhere exceeds the opening diameter. 6. Device as stated in claim 5, characterized in that the electrodes (6,6<*>,6a,6<*>a) are interconnected electrically in two groups, of which the first electrode group (6a,5 'a) is connected to the output (55a) of a high-voltage generator (38) and the second electrode group (6,6<»>) is connected to a lower potential. 7. c Device as stated in claim 6, characterized in that the second electrode group (6,6') is connected to earth via a resistor (54). 8* Device as specified in claim 6, character, characterized in that each of the two electrode groups (6a, 6'a or 6.6<*>) is connected to a stage of a voltage divider or voltage multiplier switching device. 90 Device as stated in claim 8 with a voltage multiplier coupling device consisting of capacitors (9) connected one after the other in two columns and rectifiers (8), characterized in that each of the two electrode groups (6a,6'aj 6,6') is connected to a column of capacitors (9) and that each column of capacitors contains a damping resistor (53a,53). 10. Device as specified in claim 2 or 5, characterized in that the electrodes (1 ... 6, let ... 6a/ 6,6', 6a,6'a) are formed by points that protrude from the inner wall of the insulating tube (7,7a). 11. Device as stated in claim 10, characterized in that the pointed electrodes consist of tungsten. 12. Device as specified in claims 2 and 10, characterized in that the pointed electrodes (1 ... 6) are arranged along at least one helical line on the insulation tube (7). 13. Device as specified in claims 5,6 and 10, characterized in that the pointed electrodes (6,6<*>,6a,6<*>a) are arranged in a plane that extends vertically on the insulation tube (7, 7a) axis and are mutually electrically connected in such a way that the electrodes with equal numbers form one electrode group and the electrodes with different numbers form the other group. 14. Device as stated in claim 2,7,8 or 9, characterized in that the step connection elements (29,8,9) for the voltage divider or voltage multiplier connection and the damping resistors (53,53a) are arranged on the insulation board (7) . 15. Device as specified in claims 1, 2 and 14, characterized in that the resistors (29) in the voltage divider coupling are ring-shaped and attached to the insulating pipe (7). 16o Device as specified in claims 2 and 15, characterized in that the voltage divider is a rib-shaped resistance which is connected to the electrodes at certain points. 17. Device as stated in claim 1 with a high-voltage generator (38) containing a voltage converter (12 ... 16), characterized in that the voltage converter (12 ... 16) is placed in a housing on the insulation pipe (7).