Apparatus for coating articles by means of electrostatically charged articles

Abstract

An apparatus for coating articles or the like by means of electrostatically charged particles comprising at least two electrodes associated with an atomizer nozzle for these particles and a regulatable or controllable high-voltage generator for delivering a respective potential of different magnitude but the same sign to both electrodes. A stabilizing stage is provided between the high-voltage generator and the electrodes, this stage maintaining constant the potential difference between the electrodes at a predetermined value over the entire regulation range of the operating voltage of the high-voltage generator with regard to the articles.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a new and improved construction of apparatus for coating articles or the like by means of electrostatically charged particles and comprising at least two electrodes associated with an atomizer nozzle for these particles, and a regulatable high-voltage generator which delvers to both electrodes a respective potential of different magnitude but the same sign.

With such type equipment it is known that there are formed two electrostatic fields. One of these fields, hereinafter referred to as the transport field, exists between the electrodes on the one hand and on the other hand the articles to be coated which as a general rule are grounded. The field intensity of this transport field is essentially dependent upon the operating voltage of the high-voltage generator with regard to the article, i.e. the absolute value of the potential delivered by the high-voltage generator to the electrodes as well as upon the spacing between the electrodes and the article. Further parameters which influence the local value of the field intensity of the transport field are, for instance, the geometric shapes of the articles or the electrodes and the atomizer nozzle respectively. The field intensity of the transport field is regulated from case to case to an optimum value by adusting the high-voltage generator especially if the transport field is not solely decisive for the transport of the charged particles to the article to be coated. Quite to the contrary, for this transport there is decisive both the propellant gas used for the atomization of the particles as well as also the transport field.

The second field which is formed at such equipment exists between the electrodes and is hereinafter referred to as the "charging field". Its field intensity is dependent upon the potential difference between both electrodes which normally is only a fraction of the absolute value of the electrode potential, and upon the spacing between both electrodes which, in comparison to the spacing between the electrodes on the one hand and the article on the other hand, is very small. Therefore as a general rule the field intensity of the charging field attains higher values than the field intensity of the transport field. This charging field is decisive for the charging of the particles. It should be self evident that the charging of the particles is of paramount importance for the quality of the attainable coating. Therefore attempts are made to maintain this charging and therefore also the field intensity of the charging field always at a maximum value.

However with the heretofore known equipment of the previously mentioned type this is not possible because the setting of the working or operating voltage at the high-voltage generator automatically also influences the field intensity of the charging field. In other words with the heretofore known apparatuses the field intensity of the charging field is a function of the field intensity of the transport field and therefore cannot have an optimum value for all cases.

SUMMARY OF THE INVENTION

Hence it is a primary object of the present invention to provide an improved construction of apparatus for coating articles or the like by means of electrostatically charged particles in a manner not associated with the aforementioned drawbacks and limitations of the prior art proposals.

It is another important and more specific object of the invention to provide an improved apparatus of the previously mentioned type wherein the field intensity of the charging field remains practically constant throughout the entire regulation range of the absolute voltage of the high-voltage generator.

Now in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the proposed apparatus of this development is manifested by the features that there is provided a stabilization stage between the high-voltage generator and the electrodes and which maintains constant at a predetermined value the potential difference between the electrodes throughout the entire regulation range of the operating voltage of the high-voltage generator with regard to the articles.

In this connection it is to be understood that this stabilization stage can contain a diode operating in the cut-off range and the breakdown voltage of which essentially corresponds to the predetermined value.

If for the apparatus there is provided a high-voltage generator possessing a voltage divider arrangement or voltage multiplier circuit arrangement, of which arrangement two different stages are connected with the electrodes, when it is advantageous to bridge both of these different stages by means of the diode.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawing wherein:

FIG. 1 is a schematic illustration of an exemplary embodiment of apparatus designed according to the teachings of the invention; and

FIG. 2 schematically illustrates part of a variant embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Describing now the drawings, with the equivalent or apparatus 10 illustrated in FIG. 1 there will be recognized an atomizer nozzle 11 constructed as a tube or pipe. The atomizer nozzle 11 is charged in the direction of the arrow 12 with a suspension of particles in a propellant gas. At the region of the outlet of the atomizer nozzle 11 there are provided four electrodes 13, 14, 15 and 16, of which two are always arranged in spaced relationship from the remaining two. In the exemplary embodiment the electrodes 13 and 15 and the electrodes 14 and 16 are arranged in offset relationship in the peripheral direction by 90.degree. with respect to one another. Both of these electrode pairs can be located at the same height in the atomizer nozzle 11 or also offset in lengthwise direction. At a spacing D from the atomizer nozzle 11 there is located an article or object 17, hereinafter simply referred to as article, which is grounded by means of the conductor or line 18. This conductor 18 as a general rule is formed by the support or holder by means of which the article 17 is held in front of or moved past in front of the atomizer nozzle 11.

The illustrated apparatus also possesses a high-voltage generator 19 which is constructed as follows. A regulatable or controllable alternating-current voltage source 20 supplies the primary winding 21 of a transformer 23, the secondary winding 22 of which is grounded at one end and at the other end is coupled with a voltage multiplying cascade. This cascade or circuit arrangement possesses two columns of capacitors 24, 25 with intermediately connected diodes 26. As best seen by referring to FIG. 1 the last stage of the cascade is connected through the agency of a current limiting resistor 27 and the conductors or lines 28, 29 and 30 with the electrodes 14 and 16. Both of these electrodes therefore always possess the same potential. The third last stage of the cascade is connected on the other hand via a resistor 31 as well as the conductors 32, 33 and 34 with the electrodes 13 and 15. Both of these electrodes 13 and 15 therefore possess the same potential which however is different from the potential of both electrodes 14 and 16. This potential difference between the electrodes 13 and 15 and 14 and 16 as well as the distance d between the electrodes 15 and 16 are decisive for the field intensity of the charging field which prevails at the region of the outlet opening of the atomizer nozzle 11. Between the conductors 28 and 32, as best seen by referring to FIG. 1, there is connected a diode 35 which bridges or shunts both of the last stages of the cascade. This diode operates in its cut-off range and maintains the charging voltage of the capacitors 24 and 25 of the last two stages of the cascade at a value which approximately corresponds to the diode breakdown voltage. Accordingly the potential difference prevailing between the electrodes 13, 15 and 14, 16 equals the breakdown voltage of the diode 35 and thus is almost constant. This constant potential difference is however independent of the value of the operating voltage of the high-voltage generator 19 which has been set at the alternating-current voltage source 20.

If it is assumed that the regulating range of the high-voltage generator 19 for the coating work to be undertaken is in the order of between 40 and 100 kV, and that the potential difference which is ideal for the charging is in the order of between 5-10 kV, then it is sufficient to select a diode 35, the breakdown voltage of which in the cut-off range is in the order of 5-10 kV. The fact that there exists a voltage drop at both last stages of the voltage multiplier cascade is a phenomenon which can be readily accepted in order to obtain as effective as possible a charging field.

It has been found that with the illustrated apparatus it is possible to optimumly charge the particles even with a slight field intensity of the transport field, so that it is readily possible to electrostatically coat hollow spaces enclosed by conductive walls, in which as is known the transport field cannot extend. The transport of the particles in such hollow spaces occurs primarily by means of the propellent gas.

With the exemplary embodiment depicted in FIG. 2 there will be recognized the conductors or lines 28 and 32 leading to the electrode pairs 14, 16 and 13, 15 which have been omitted in this case as a matter of convenience in illustration. There will also be recognized the resistors 27 and 31 as well as the diode 35. In this case the diode 35 bridges or shunts the last two resistors 36 of an ohmic voltage divider 37 which is grounded at one end and connected at the other end at location 38 to a not particularly illustrated high-voltage source. The mode of operation of the variant embodiment shown in FIG. 2 is essentially the same as the embodiment of FIG. 1 discussed above.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be variously embodied and practiced within the scope of the following claims. Accordingly,

Claims

What is claimed is:

1. An apparatus for coating articles by means of electrostatically charged particles, comprising at least two electrodes operatively associated with an atomizer nozzle for the particles, a regulatable high-voltage generator for delivering to both electrodes a respective potential of different magnitude but the same sign, the improvement comprising a stabilizing stage provided between the high-voltage generator and the electrodes, said stabilizing stage maintaining substantially constant at a predetermined value the potential difference between the electrodes over the entire regulation range of the operating voltage of the high-voltage generator with regard to the articles.

2. The apparatus as defined in claim 1, wherein said stabilizing stage contains a diode operating in the cut-off range and the breakdown voltage of which essentially corresponds to said predetermined value.

3. The apparatus as defined in claim 2, wherein said high-voltage generator is provided with a circuit arrangement having two different stages connected with the electrodes, said diode bridging both of said different stages.

4. The apparatus as defined in claim 3, wherein said circuit arrangement defines a voltage divider-circuit arrangement.

5. The apparatus as defined in claim 3, wherein said circuit arrangement defines a voltage multiplying-circuit arrangement.