Coating apparatus – With means to apply electrical and/or radiant energy to work... – Electrostatic and/or electromagnetic attraction or...
Reexamination Certificate
1999-06-07
2001-07-03
Crispino, Richard (Department: 1734)
Coating apparatus
With means to apply electrical and/or radiant energy to work...
Electrostatic and/or electromagnetic attraction or...
C239S697000, C239S698000, C427S475000
Reexamination Certificate
active
06254684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a powder-spraying appliance for electrostatic powder coating having a chamber supplied with an air/powder mixture via a supply conduit. At least one high-voltage electrode and an earth electrode are disposed within the chamber for charging the powder.
German Patent DE 27 22 100 C2 discloses a powder-spraying appliance containing at least one chamber. In the chamber, there are disposed a point-discharge electrode opposite to a flow of material and an obtuse-angled counter-electrode having an opposite polarity to the point-discharge electrode and is located upstream at a distance from the latter. In order to improve the charging of the powder particles, a configuration with a plurality of chambers connected in series is provided. It is proposed, moreover, to configure the chamber so as to result continuously in widenings and narrowings which are intended to ensure that material particles are retarded, at the same time being swirled, and are therefore exposed more intensively to the force field. Finally, it is proposed to arrange, so as to be distributed over the circumference of the at least one chamber, orifices, via which additional air is introduced and which reduce the average density of the mixture of air and powder and improve the charging.
Tests have shown that the measures are not sufficient to bring about a coating of powder on a work piece with adequate efficiency. Since the electrodes come directly into contact with the powder, powder is deposited on the electrodes, thus resulting in markedly poorer charging.
It is pointed out, in U.S. Pat. No. 4,811,898, that, in a spraying appliance with a baffle body, the relative position of the baffle body and the electrodes is an essential parameter. Use of external electrodes may be gathered from U.S. Pat. No. 4,228,961.
A powder-spraying appliance is known from Published, European Patent Application EP 0 237 249. The appliance has a chamber, the cross-section of which is larger than the cross-section of a supply conduit for the powder/air mixture. An earth electrode and a plurality of high-voltage electrodes, onto which cleaning air can be blown are disposed in the chamber.
The configuration of the chamber and, in particular, of the electrodes is selected in such a way that a plasma with positive polarity and a plasma with negative polarity are generated. A voltage is applied intermittently to the plasma electrodes. Tests have shown that these measures result in sparking and in the fouling of the electrodes.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a powder-spraying appliance which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which leads to improved charging of the powder particles and thereby to an improved separation rate.
With the foregoing and other objects in view there is provided, in accordance with the invention, a powder-spraying appliance for electrostatic powder coating, including:
a chamber having a cross-section, a length between 7 and 15 cm, a diameter between 2 to 4 cm, an interior, and a main axis;
an outlet disposed downstream of the chamber;
a plurality of high-voltage electrodes disposed in the chamber for internal charging of a powder of a powder/air mixture, the plurality of high-voltage electrodes annularly distributed in a region upstream of the outlet;
a tubular earth electrode disposed centered in the main axis of the chamber and a spatial distance between the tubular earth electrode and the plurality of high-voltage electrodes is between 7 and 14 cm, the tubular earth electrode has an end directed towards the interior of the chamber and the end has a head part formed with rounded edges closing off the end;
a covering made of an insulating material covers the head part;
a supply conduit for conducting the powder/air mixture and has a cross-section being smaller than the cross-section of the chamber, the supply conduit opening into the chamber transversely to a spraying direction resulting in an intensive swirling of the powder/air mixture; and
the tubular earth electrode and the plurality of high-voltage electrodes receiving cleaning air for avoiding powder
deposits on the tubular earth electrode and the plurality of high-voltage electrodes.
It was found that, in particular, a powder deposit on the electrodes may be the cause of an unsatisfactory charging of the powder particles. It was also found that the entire chamber should be filled with corona and intensive swirling of the powder particles should take place in the region of the corona, in order to achieve high overall efficiency. These effects are achieved by the features of the invention.
In accordance with a further feature of the invention, one or more outer electrodes provide for an improvement in charging or for field control and, consequently, for increasing the is efficiency.
Another feature relates to a variable arrangement of a distance between the earth electrode and high-voltage electrodes, with the result that an optimum can be established between good charging and disturbing effects, such as reionization.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a powder-spraying appliance, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
REFERENCES:
patent: 4228961 (1980-10-01), Itoh
patent: 0237249 (1993-06-01), None
patent: 1313511 (1987-06-01), None
Bohme Helmut
Borner Gunter
Nienburg Hans-Christoph
Wittmann Josef
ABB Research Ltd.
Crispino Richard
Greenberg Laurence A.
Lerner Herbert L.
Stemer Werner H.
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