Abstract:
A powder-spraying apparatus having simultaneous internal and external charging, which is suitable for electrostatic powder coating. In order to achieve improved constancy of the deposition efficiency, largely independent of the distance between the powder-spraying apparatus and the work piece, it is proposed to connect both internal high-voltage electrodes and external high-voltage electrodes to a high-voltage source via high-resistance components.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a powder-spraying apparatus having simultaneous internal and external charging, which is suitable for a electrostatic powder coating. 
     Such a powder-spraying apparatus is disclosed in International Patent Application WO 98/24555. The reference teaches a spray gun having a chamber, into which a powder/air mixture can be introduced. The spray gun contains an earth electrode, needle-like internal high-voltage electrodes distributed on a metal ring, and at least one external high-voltage electrode configured as a needle. A high-voltage source configured as a high-voltage cascade supplies high voltage to the electrodes via an electrical connection and the ring. 
     The at least one external high-voltage electrode is referred to as an additional electrode which can be disposed as desired, with which an electric field and an additional corona can be produced outside, these being intended to increase the deposition efficiency. The electric field generated by the external needle produces a force on the electrically charged particles. In addition, the external corona can effect further charging, and the repulsion between the ions and the charged powder particles can lead to broadening of the spray cloud. Which of the physical influencing factors listed are dominant depends on the quantity of powder expelled, the properties of the powder (good or poor chargeability) and a distance from a work piece. 
     In the case of small quantities of powder (up to about 150 g/min) and normally chargeable powders, a sufficiently good quantity of charge can be applied to the powder particles using the internal charging on its own. The electric field built up by the external needle has the effect of a force F=E*q acting on the powder particles. Depending on the voltage on the needle tip, the result is a more or less pronounced edge effect or wrap-around. The width of the spray cloud can be influenced by the level of the current. 
     In the case of greater quantities of powder (greater than 200 g/min), both the internal and the external charging are required for sufficiently high charging of the powder particles. 
     However, the disadvantage of a configuration with a metallic connection between the electrodes is that the current and voltage on the external needle depend to a great extent on the distance between the gun and the work piece. At a small distance, a much greater current flows, and the voltage is lower than in the case of a large distance. As a result, in particular at high powder expulsion quantities, there may be an increase in the tendency that the deposition efficiency increases with a decreasing distance. 
     However, this tendency is not desired in practice. Within a technically expedient range (100 mm . . . 250 mm), the aim is that the deposition efficiency should be uniformly high, irrespective of the distance. 
     In order to produce the high voltage (negative direct voltage), a cascade with a relatively high internal resistance is used. The no-load voltage U 0 , that is the output voltage of the cascade in the unloaded state, normally lies in the range of about −80 kV to −100 kV. The internal resistance of the cascade R i  lies in the range of about 400 MΩ to 600 MΩ. The actual output voltage of the cascade U c , together with the output current I c , is then calculated in accordance with the following equation: 
     
       
         
           U 
           c 
           =U 
           0 
           −I 
           c 
           R 
           i 
         
       
     
     The maximum output current of the cascade is about 80 μA . . . 120 μA. The use of cascades with greater output voltages and smaller internal resistances is not possible, for reasons of safety. The relevant standards in Europe and America require that the air/powder mixture must not be ignited by electric discharges under any circumstances. Therefore, inter alia, the output of the cascade is limited. Further measures for avoiding ignition are bias resistors and the avoidance of electrode configurations that lead to a high electric capacitance. 
     During powder application, attempts are made to operate at a constant distance. However, for various reasons it is not possible to avoid the distance from the work piece changing (for example at edges). 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a powder-spraying apparatus with internal and external charging that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which improved constancy of the deposition efficiency can be achieved. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a powder-spraying apparatus equipped for simultaneous internal and external powder charging, including: 
     a high-voltage source; 
     an earth electrode; 
     at least one external high-voltage electrode; 
     a plurality of internal high-voltage electrodes; 
     a ring formed of high-resistance material and having a circumference, the plurality of internal high-voltage electrodes uniformly distributed about the circumference of the ring; and 
     connection elements formed of the high-resistance material connecting the high-voltage source to both the at least one external high-voltage electrode and the ring. 
     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 apparatus with internal and external charging, 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational and a corresponding side-elevational view of a configuration of internal and external electrodes including high-resistance connecting parts, according to the invention; 
     FIG. 2 is a complete electrical equivalent circuit diagram of the configuration shown in FIG. 1; 
     FIG. 3 is a circuit diagram of a simplified equivalent circuit; 
     FIG. 4 is a graph showing associated current-voltage characteristics; and 
     FIG. 5 is a sectional view a powder-spraying apparatus known from the prior art, into which the configuration shown in FIG. 1 can be inserted instead of the electrode configuration shown in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The objective of improved constancy of the deposition efficiency is achieved with the powder-spraying apparatus according to the invention by a ratio between internal and external currents being set specifically, and changing only within narrow limits when there are changes in a distance between an electrode and a work piece. At a typical nominal distance of 200 mm from the work piece, about 70% of the current flows via the electrodes for an internal charging, and 30% via an external electrode. 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a front view and a side view of a ring  1  made of a high-resistance material. The high-resistance material used for the ring  1  and further high-resistance components—described below—may be, for example, a plastic which is filled with graphite, carbon black or other conductive materials. A number of needle-like internal high-voltage electrodes  2  are inserted into the ring  1 , distributed uniformly over its internal circumference. The ring  1  can be connected to a high-voltage source (cascade)  6  via a high-resistance rod  3  (see FIG.  2  and FIG.  5 ). A connection point  11  for the rod  3  is located at a center of a ring portion located between two of the electrodes  2 . A needle-like external high-voltage electrode  5  is connected to the rod  3  at a connection point  12  via a high-resistance pin  4 . 
     The current and voltage distribution which can be achieved with such a configuration is determined by the resistance values of the components  1 ,  3  and  4  and the current/voltage characteristic of the gas discharge paths of the: 
     a. needle ring to internal earth electrode path; and 
     b. external needle to work piece path. 
     For improved understanding, FIG. 2 shows the complete electric equivalent circuit of the configuration shown in FIG. 1, which is connected to the high-voltage cascade  6 . The cascade  6  is simulated by an ideal voltage source having a voltage U 0  and a resistance R i . The discharge paths for an internal charging  7  and an external charging  8  are given by their current/voltage characteristics. Portions of the ring  1  located between the internal electrodes  2  are in each case simulated by resistances R 1 . The resistance of the rod  3  is divided by the connection point  12  for the pin  4  into resistances R 2  and R 3 . The rod  4  is simulated by the resistance R 4 . 
     For the purpose of configuring the resistances, a simplification of the equivalent circuit is expedient, and this is shown in FIG. 3. A resistance R a  combines the resistances R i  and R 3 . The components that are relevant to the internal discharge have been combined, to a first approximation, by a resistance R b  and a gas discharge path  9 . R c  is equal to R 4 . 
     Associated typical current/voltage characteristics are illustrated by way of example in FIG.  4 . The characteristic curve  10  applies to the internal discharge, and characteristic curves  11  and  12  apply to the external discharge at two different distances from the work piece. The distance in the case of characteristic curve  11  is smaller than in the case of characteristic curve  12 . 
     The characteristic curves can be approximated using the below listed equations. 
     
       
         The internal discharge− I   i   =C   i ( U   i   −U   i0 ) 2 , 
       
     
     where C i  and U i0  are characteristic variables that depend on a geometric construction of the gun, and depend in particular on the distance of the needle-electrode ring from the internal earth electrode. 
     
       
         The external discharge− I   a   =C   a ( U   a   −U   a0 ) 2 , 
       
     
     where 
     C a  and U a0  are characteristic variables that depend both on the geometric construction of the gun but, to a significantly greater extent, on the distance between the gun and the work piece and also on a shape of the work piece. The result is therefore different characteristics for different positions of the work piece. 
     The characteristic variables C i , U i0 , C a  and U a0  can be determined from the geometric dimensions and the material characteristics, using a numerical field calculation. However, an experimental check is to be recommended. The influence of the quantity of powder delivered can be neglected here. 
     The equivalent circuit is described by the following system of equations: 
     
       
           U   0   =R   a   C   a ( U   a   −U   a0 ) 2 +( R   a   +R   b ) C   i ( U   i   −U   i0 ) 2   +U   i , 
       
     
     and 
     
       
           U   0   =R   a   C   i ( U   i   −U   i0 ) 2 +( R   a   +R   c ) C   i ( U   a   −U   a0 ) 2   +U   a . 
       
     
     Using these equations, the values for the resistances can be optimized. The following guide values having emerged from an exemplary embodiment: 
     
       
           R   a =400 MΩ . . . 600 MΩ, 
       
     
     
       
           R   b =20 MΩ . . . 200 MΩ, 
       
     
     and 
     
       
           R   c =(2 . . . 5)* R   b . 
       
     
     For R c , it is true to a first approximation that the greater R c  is, the greater is the edge effect. 
     The resistance R 1  should lie in the range of about 10 MΩ to about 30 MΩ. This resistance prevents field strength peaks, and hence high currents, occurring in the event of direct contact between the powder and the internal needles. 
     FIG. 5 shows the powder-spraying apparatus disclosed in International Patent Application WO 98/24555. The spray gun disclosed has a chamber  100 , into which a powder/air mixture PL can be introduced. The spray gun contains an earth electrode  20 , the needle-like internal high-voltage electrodes  2  disposed distributed on a metal ring  150 , and the at least one external high-voltage electrode  5  configured as a needle. The high-voltage source  6  configured as a high-voltage cascade supplies high voltage to the electrodes  5 ,  2  via an electrical connection  30  and the ring  150 . The inventive features of FIG. 1 are to replace equivalent features shown in FIG. 5 for providing a better consistency in the delivery of the powder.