DK142744B - Apparatus for electrostatic coating of articles with powder coating material. - Google Patents

Description

(11) PUBLICATION 142744 DENMARK (51), nt c, -3i H 01 T § 19/00 (21) Application No. 3236/70 (22) Filed on 23 · Jun. 1970 (24) Life day 23 · Jun. 1970 (44) The application presented and the petition published on 12 June SU. 1 9 <j1

Dl THE RECTORATE FOR

PATENT AND TRADE MARKET (30) Priority cup from it

25 jun. 1969a 97W69, CH

5. jun. 1970, 8482/70, CH

(7i) GEMA AG APPARATEBAU, HerisauerEtrasse 30, St. Gallen, (St. Gallen), CH.

TO Inventor: Karl Buechor, Schneebergstr. 58, St. Gallen, (St. Gallen), CH. ~ (74) Plenipotentiary in the proceedings:

Office of Industrial Excellence at Svend Schönnlng.

(B4) Apparatus for electrostatic coating of articles with powder coating.

The present invention relates to an apparatus for electrostatic coating of articles with powdered coating material of the kind set forth in the preamble of claim 1.

Compared to other coating methods, electrostatic coating of articles is distinguished by the fact that the coating material is first charged electrically and then transported towards the surface to be coated.

This method is of particular interest in powder atomizers.

The powdered coating material is blown through a tube of electrically insulating material and is thereby charged electrically by rubbing and then atomized against a bead body, for example.

In such powder coating devices, the performance, due to the powder coating next to falling powder, is suitably determined as the amount of atomized powder per unit of time and the percentage of separation indicated, by the electric charge of the powder. Difficulties arise in not charging the powdery coating material adequately.

Therefore, it is known to improve the charging of high voltage on electrodes located in the interior of the tube.

By an apparatus known from French patent No. 1,223,451 of the kind mentioned above, the electrode having the lowest potential is connected to ground. In this way, the powder, which is already partially charged or which has already been charged by means of the electrodes, forms deposits on this electrode.

An electrostatic charge of powder by corona discharge in a tube of electrically insulating material is known from German Patent No. 960,395. Hereby is arranged a coaxial and negative high voltage potential coaxial provided with the tube and as a model electrode a metal rotation body, the rotary body in the region at the tip of the spray electrode having a relatively large and smooth surface and thus narrowing the free pulse cross-section to close the free tube cross-section. . This electrode arrangement is therefore very prone to powder deposits. In particular, to clean the model electrode during operation, mechanical cleaning is required by means of movable powder-removing parts or by incorporating abrasive materials into the coating material or surrounding the powder jet with guide air. Also, with this known apparatus, a potential cannot be set between the mouth of the sprayer and the article to be coated.

It is therefore the object of the present invention to improve the apparatus of the kind mentioned in the first place in such a way as to avoid the deposition of powder on the electrode with the lowest potential, so as to obtain an overall charge of the powder in relation to what is that is achieved by known devices.

This is achieved according to the invention in that the apparatus is peculiar to that of the characterizing part of claim 1.

By applying a bias also to the electrodes with the lowest potential, it is avoided that the powder is attracted to this electrode, which thereby maintains its efficiency during operation.

3 142744

Preferably, the electrodes of the invention may be located in the insulating tube so that they extend in front of the tube end or the atomizer located at the tube end at a distance no greater than the diameter of the tube lumen.

At this point the propellant-powder stream has its highest density due to its narrow cross-section and, by forming the corona between the electrodes, precisely in this region, a very uniform and complete charge of the powder is obtained. By placing the electrodes at a short distance from the end of the tube or from the nebulizer located at the tube end, a substantial discharge and deposition of the powder is avoided before leaving the tube.

Preferably, as stated in claim 3, at least 4 electrodes are placed approximately in one plane, the electrodes alternating in the circumferential direction being connected to different potentials. Thereby a very intense corona discharge is obtained in a short zone which is substantially perpendicular to the flow direction of the powder, thereby making the charge very uniform.

The invention will now be described in more detail with reference to the drawing, in which: FIG. 1 shows a device designed as a handgun gun according to the invention for sectional electrostatic coating; FIG. 2 shows a cross section through the front part of the apparatus of FIG. 1, FIG. 3 shows a cross section through the middle part of the apparatus according to FIG. FIG. 4 is a schematic diagram of an amplifier cascade with connected electrodes; and FIG. 5 schematically shows another mode of coupling for supplying the electrodes.

In the embodiment of FIG. 1, in a tubular sleeve 20 ', preferably composed of plastic, is coaxially positioned a tube 7 consisting of electrically insulating material, which at its rear end has an inclined downward and rearward tube connection 48 for supply of coating material by means of of a propellant gas stream. The rear end of the insulating tube 7 is closed by means of an axial bore guide 26 in which a push rod 24 is slidably mounted. The guide 26 is held in place by a closure cover 46 screwed onto the sleeve 20 '. there is space between the end of the tube 7 and the closure cover 46 for inserting an annular transformer 11 of a high voltage generator 38 (Fig. 4). The secondary winding of the transformer is connected to a voltage amplifier cascade, if capacitors 9 connected in common with diodes 8 are connected in succession in two columns. In each capacitor column, an attenuating resistor 53 or 53a is connected between the penultimate capacitor 9a and the last capacitor 9b. Damping. 53 and 53a, together with the associated last capacitors 9a, constitute the RC combination which attenuates the possible oscillations in sparking. As shown in FIG. 1 and FIG. 3, both capacitor columns with their attenuating resistors 53 and 53a are arranged on the insulation tube 7 and the bush 20 'also contains the diodes 8 of the voltage-intensified arc.

The front end of the sleeve 20 'which is tapered to the insulating tube 7 has an external thread for screwing on a screw nut 41. By means of the screw nut 41, a tubular atomizer or spray head 35 is attached to the sleeve 20' and the insulating tube 7. - or the spray head consists of interlaced, electrically insulating tubes of plastic, of which the inner tubing 7a has the same diameter as the insulating tube 7, and carries a star-shaped guide 25 for the push rod. The front retracted end 36a of the outer pipe piece 36 forms the outwardly extending outlet opening 21 of the powder channel 7b extending in the insulating pipe 7 and the inner pipe piece 7a and an inner abutment on which the front end of the inner pipe piece 7a rests.

In the inner tube piece 7a, 4 tapered electrodes 6, 6 ', 6a, 6a' are inserted which project radially inwards into the powder channel 7b. The 4 pointed electrodes 6, 6 ', 6a, 6a' are disposed at 90 ° relative to each other in a plane perpendicular to the tube axis, this electrode plane being a distance from the plane of the outlet opening 21 smaller than its largest diameter. The arrangement of the tip-shaped electrodes 6, 6 ', 6a, 6a' is shown in FIG. 2nd

As FIG. 4 shows at each time the two opposite electrodes 6, 6 ', 6a, 6a' are electrically connected to each other and each connected through a conduit 42 and 42a to one of the two voltage amplifier cascade outputs 55 at the end of the two capacitor columns or 55a. In this way, each electrode during operation of the apparatus is obtained at a potential different from the potential of the two adjacent electrodes, and at a sufficiently large potential difference between the electrodes occurs the corona discharge which fills the powder edge cross-link completely. To facilitate the initiation of the corona discharge, the tip-shaped electrodes 6, 6 ', 6a, 6a' are preferably made of tungsten.

In the embodiment shown in FIG. 1, as connecting conduits 42, 42a in the space between the inner tube piece 7a and the outer tube piece 36 are tungsten wires, whose forward pointed end as electrodes 6, 6 ', 6a, 6a1 are passed through the dividing wall of the inner tube piece 7a and the rear end over a high voltage contact carrying position 45, as previously indicated, is connected to those of FIG. 1 for the sake of clarity, capacitor column connections are not plotted. For insulating and firmly holding the tungsten wires 42, 42a on the inner tube piece 7a, a tubular intermediate bearing 43 of electrically insulating material and optionally a second position 44 is provided on the front end thereof.

The push rod 24, displaceable in the two guides 25 and 26 (Fig. 1), carries the bead body 23 which is located in front of the exit opening 21, at which the coating material, e.g. a plastic powder is atomized, in the form of a syringe cone, the opening angle of the syringe cone can be changed by displacement of the pressure rod 24. The pressure rod consists, e.g. of silver steel to permit a blockage-free slide in the relatively long rear guide 26, and its forward portion has an electrically insulating plastic cover 24a extending to the bell body 23.

If the apparatus described above is to be used as a hand spray gun, the tubular bush 20 'as shown in FIG. 1 screws a pistol grip. The pistol grip 17 includes a supply pipe 49 connected to the insulating tube 7 for the insulating tube 7, the end of which attaches to a hose 33 for supply of propellant coating material mixture extending beyond the bottom of the pistol grip. Further, the gun grip 17 includes a hand-operated switch 19 over a trigger for switching on and off the high voltage and coating material supply, a plug 53 over which the low voltage for operation of the high voltage generator 38 is supplied, and e.g. Connect control lines outgoing from the feed device to the switch 19.

Finally, the gun grip may also contain such coupling elements as transistors and resistors of the high voltage generator 38.

142744 6

Wires 56 are routed from a plug 51 and from any coupling elements present to the switch 19 and to a transformer 11. The wires 56 are housed in an insulating sleeve 50 attached to the grip plate 47 and in a bore 52 provided in the grip plate 47 for mounting and Wiring in the appliance is easy and quick to perform.

In one embodiment, the largest diameter of the exit opening 21 was approx. 14 mm and the pointed electrodes 6, 6 ', 6a, 6a' were removed approx. 12 mm from the exit opening plane. The voltage amplifier cascade was laid so that one output 55a carried a voltage of 50 kV and the other output 55 a voltage of 45 kV.

With these drive voltages, the strong corona discharges between the electrodes charged the coating material particles electrically well, and the charged particles were passed so well in the transport field between the electrodes and the grounded object to be coated that a surprisingly high separation rate could be set at faultless coating.

FIG. 5 schematically shows another embodiment of the high voltage supply of the pointed electrodes 6, 6 ', 6a, 6a'.

To the output 55a of a high voltage generator 38 is connected over a line 42a one of the opposite electrodes 6a, 6a '. Both of the other opposite electrodes 6, 6 'are connected over a line 42 to a high-conductivity resistor 54, the other of which is on the ground. By using this coupling device, a differential voltage is established on the electrodes which causes an increased corona discharge at the electrode tips.

The free space created after fitting between the tubular sleeve 201 and the insulating tube 7 is expediently molded with an electrically insulating molding resin.

For a gun, there may be several nebulizer or syringe bodies of different length, shape and size of the exit aperture as well as in the number and arrangement of the electrodes so that the coating process can be optimally adapted at any time to the requirements determined by the nature of the coating material and the shape. of the objects to be coated.