Abstract:
A powder spray coating discharge assembly ( 230, 260, 216, 228, 232 ) for connection to an electrostatic spray coating gun ( 210 ), the gun ( 210 ) having a gun body ( 212 ), means for connecting to a supply of coating powder and means for supplying a voltage ( 20 ) at first and second potentials respectively to first ( 292 ) and second electrical connections ( 294 ) each for connection to a respective one of a discharge electrode ( 232 ) and a counter electrode ( 260 ), the means for supplying the voltage ( 20 ) comprising: a variable voltage power supply ( 114 ) having an input connected to an electrical power source ( 110 ), an output connected to each of the first and second electrical connections ( 292, 294 ), a control circuit ( 128 ) for controlling the variable voltage power supply ( 20 ) and means ( 120 ) for sensing an output load, wherein the control circuit ( 128 ) is adapted to adjust the variable voltage power supply ( 20 ) to reduce the voltage and current in proportion to a sensed increase in load, or vice-versa.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a National Stage entry of International Application No. PCT/GB2007/050518, filed Aug. 31, 2007, the entire specification, claims and drawings of which are incorporated herewith by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to electrostatic powder spray coating apparatus, and in particular, but not exclusively, to electrode arrangements for electrostatic powder spray coating apparatus. 
         [0004]    2. Introduction 
         [0005]    Electrostatic powder spray coating apparatus can be used for depositing powder coatings on substrates. A known electrostatic powder spray coating apparatus generally comprises a nozzle through which powder can be sprayed and a discharge electrode located adjacent to the nozzle. The discharge electrode is maintained at an elevated electrical potential with respect to a grounded workpiece so that an electromagnetic field is created between the discharge electrode and the workpiece. As the powder passes through the field, it becomes charged, so that it is attracted to, and adheres to, the workpiece. 
         [0006]    Electrostatic spray guns of the corona discharge type operate by the discharge of a very high DC voltage, typically between 30 to 100 kV. This high voltage at the discharge electrode or electrodes results in a discharge current which creates ionized air through which the sprayed powder passes as it is conveyed in an air stream to the earthed product to be coated. As the powder passes through the ionized air some charge is transferred from the ionized air to the powder and this causes the powder to be attracted to anything at a lower potential, for example the earthed product. 
         [0007]    There are 3 commonly recognized drawbacks with this method of charging, namely “Faraday cages”, “back ionisation” and “orange peel”. 
         [0008]    A Faraday cage is where the charged powder follows the electrostatic lines of force between the discharge electrode and the earthed work piece. These lines of force can be beneficial to create a wrap around effect and coat areas of the product which face away from the spray gun e.g. back surfaces, but they can cause a problem when coating into recesses as the lines of force are established to the outer edges of recesses and will not penetrate inside. This can make it very difficult to coat the inside of such recesses. 
         [0009]    Back ionisation is where the excess charge from the ionized air is entrapped into the powder layer deposited onto the surface of the product, and as the charge is all of the same polarity, repulsion effects can cause charge concentrations particularly in thicker coatings, which can erupt from the deposited powder layer to leave holes and craters in the finished and cured powder film. 
         [0010]    Orange peel is where the finished cured powder film has an uneven rippled effect like a fine hammer finish. This is believed to be caused largely by excess charge from the ionized air being attracted to the surface of the deposited powder and when the powder melts during the curing process, this excess charge is drawn towards the surface of the substrate causing indentations in the finished powder film. 
         [0011]    In summary, the known arrangement has the disadvantage that that the nature, i.e. the strength and distribution, of the field depends largely on the geometry of the workpiece and the proximity of the discharge electrode to the workpiece. Where the workpiece has a complex shape such as sharp edges, undercuts etc., the field tends to concentrate at those regions, which can give rise to preferential coating, and hence, an uneven coating thickness in those areas. The field lines are also established strongly between the discharge electrode and the external edges of recesses thus creating “Faraday Cages” within the recesses which are notoriously difficult to coat as the field is weak or non existent within deep recesses. 
         [0012]    In order to charge the coating powder effectively, significantly more charge is generated by the discharge electrode in terms of free ions than is actually required to charge the powder. These free ions are attracted to the earthed workpiece and although most are “neutralised” by the contact with earth, a significant quantity become entrapped in the powder layer or remain on the surface of the powder layer insulated from earth by the powder and can give rise to graters or holes in the finished powder film or an “orange peel” effect on the finished surface. 
         [0013]    There are various ways in which these 3 problems can be minimized, for example the operator can make continual adjustment to the discharge voltage and/or discharge current to suit the distance of the gun to the product and the type of product/powder being sprayed. This is not very practical on a busy production line. 
         [0014]    Many spray guns/control systems have pre-selected settings of charge for different types of product, e.g. flat sheets; complex parts with Faraday Cages or Recoating where an insulating powder film already exists on the product. Although this can be useful, most products do not fall simply into one category and to change presets while spraying one part or between many different parts is also not very practical. 
         [0015]    Many spray guns/control systems now operate with “Constant Current” circuits whereby the discharge current will rise as the spray gun approaches the earthed product but only to a preset value. If the gun is taken closer, the discharge voltage will automatically reduce progressively which reduces the overall charge as the gun approaches the product. Although this helps, it is the current which represents the amount of free ions or ionized air and this does not reduce as the gun distance reduces. 
         [0016]    One system which addresses this is described in U.S. Pat. No. 6,274,202 whereby the discharge current as well as the discharge voltage, reduce as the spray gun approaches the product. 
         [0017]    One method which has been used with some success is to locate the discharge electrode and an earthed counter electrode within the body or the nozzle of the spray gun. This generates minimal ionized air and contains the electrostatic lines of force within the spray gun. It is, however very difficult to prevent the counter electrode from becoming contaminated with powder and therefore ineffective after a short while. 
         [0018]    The internal charging nozzle mentioned above can be turned inside out with an earthed counter electrode fitted outside and behind a conventional corona charging nozzle. This counter electrode can take the form of a single earthed metal rod or pin electrode or can be a series of earthed pins in an annular array either pointing forward towards the nozzle or tangentially out from the nozzle. This is usually an “add on” offered by many spray gun manufacturers and is usually used to achieve a good high quality, smooth finish. In general the charging of the powder is slower and less efficient. The counter electrode is usually mounted approximately 100 mm. behind the discharge electrode in an attempt to keep the discharge voltage at a maximum. This system will cease to be effective if the nozzle (discharge electrode) is taken closer to the product than the distance between the discharge electrode and the counter electrode, say 100 mm. as is often the case when spraying by hand. Another drawback with this method is that the discharge current is usually very high, typically around 100 μA which can cause powder to fuse onto the corona discharge needle due to the hot corona “glow” at the discharge point, this will reduce the charging efficiency and can lead to sparking due to capacitive discharge. Although in many cases the operator will be able to limit the maximum discharge current from the control system, this will invariably not happen as it tends to be the inclination of most operators to turn controls to maximum in an attempt to improve productivity. Another problem with running maximum Discharge voltage and current continuously is that more consideration must be given to the reliability of the highly stressed high voltage components and also the heat generated by the electronic parts of the spray gun. 
       SUMMARY OF THE INVENTION 
       [0019]    It is an object of this invention to address one or more of the above problems and/or to provide an improved electrostatic powder spray coating apparatus. 
         [0020]    A first aspect of the invention provides a powder spray coating discharge assembly for connection to an electrostatic spray coating gun, the gun having a gun body, means for connecting to a supply of coating powder and means for supplying a voltage at first and second potentials respectively to first and second electrical connections each for connection to a respective one of a discharge electrode and a counter electrode, the means for supplying the voltage comprising: a variable voltage power supply having an input connected to an electrical power source, an output connected to each of the first and second electrical connections, a control circuit for controlling the variable voltage power supply and means for sensing an output load, wherein the control circuit is adapted to adjust the variable voltage power supply to reduce the voltage and current as a function of a sensed increase in load, or vice-versa. 
         [0021]    Preferably, the control circuit is adapted to adjust the variable voltage power supply to reduce the voltage and current in proportion to a sensed increase in load, or vice-versa. 
         [0022]    The invention preferably enables the automatic setting of charging parameters when using a counter electrode, such that the charge is automatically set as a function of distance between the counter and discharge electrodes. Since the charging parameters can be made to depend on the load, the charging parameters can be varied automatically to compensate for transient changes, such as fluctuations in the powder throughput, atmospheric conditions etc. 
         [0023]    A second aspect of the invention provides a powder spray coating discharge assembly for connection to an electrostatic spray coating gun, the gun having a gun body, means for connecting to a supply of coating powder and means for supplying a voltage at first and second potentials respectively to first and second electrical connections each for connection to a respective one of a discharge electrode and a counter electrode, the assembly comprising: an electrically insulating spacer having means for connecting to a gun body, and a conduit for the passage of coating powder; a nozzle having means for connecting to the spacer and having an aperture for the discharge of coating powder; a discharge electrode located downstream of the discharge aperture of the nozzle and electrically connectable one of the first or second electrical connections; and a counter electrode located between the discharge electrode and the portion of the spacer which is adapted to engage the gun body, the counter electrode being electrically connectable to the other of the first and second connections. 
         [0024]    A third aspect of the invention provides an electrostatic powder spray coating apparatus comprising a gun having a body, an electrically insulating spacer connectable to the body, a nozzle connectable to the spacer through which a stream of powder is sprayable, a discharge electrode located downstream of the nozzle, and a counter electrode between the nozzle and the body-spacer interface. 
         [0025]    A fourth aspect of the invention provides an electrostatic powder spray coating system comprising at least one electrostatic powder spray coating gun comprising a body, an electrically insulating spacer connected to the body, a nozzle connected to the spacer through which a stream of powder is sprayable, a discharge electrode located downstream of the nozzle, and a counter electrode located upstream of the nozzle, the counter electrode being positioned downstream of the body-spacer interface, and at least one user interface. 
         [0026]    A fifth aspect of the invention provides a powder spray coating discharge assembly for connection to an electrostatic spray coating gun, the gun having a gun body, means for connecting to a supply of coating powder and means for supplying a voltage at first and second potentials respectively to first and second electrical connections each for connection to a respective one of a discharge electrode and a counter electrode, the assembly comprising: an electrically insulating spacer having means for connecting to a gun body, and a conduit for the passage of coating powder; a nozzle having means for connecting to the spacer and having an aperture for the discharge of coating powder; a discharge electrode located downstream of the discharge aperture of the nozzle and electrically connectable to the first electrical connection; and a counter electrode located externally of the spacer, the counter electrode being electrically connectable to the second electrical connection; wherein the means for supplying a voltage at first and second potentials comprises a control circuit arranged to automatically adjust the discharge voltage and current at the first and/or second electrical connection as a function of any one or more of the group comprising: the distance between the discharge electrode and the counter electrode, the powder throughput, the powder type, the electrode condition and atmospheric conditions. 
         [0027]    Surprisingly, it has been observed, when using the present invention, that by setting an external counter electrode in a similar relationship to the discharge electrode of a conventional corona discharge nozzle, but behind the nozzle and relatively close thereto (by 50 mm away), and using similarly low discharge voltage and current (say, 40 kV and 20 μA, respectively) the charging of the powder remains adequate. This runs contrary to conventional wisdom, in which it is believed that a minimum threshold discharge voltage and current are required to cause the powder to charge adequately. Advantageously, by reducing the discharge voltage and current, the three aforementioned problems (Faraday cage, back ionization and orange peel) can be greatly reduced due to the electrostatic field being established to the counter electrode and not to the product, which results in little or no current flowing to or through the product. 
         [0028]    Further, by moving the discharge electrode and counter electrode closer together than in conventional counter electrode systems, the system is less likely to be rendered ineffective when the manual spray gun is moved close to the product. By using the much lower discharge voltage and current there is less likelihood of developing fused powder on the discharge electrode due to the corona glow and there will be less stress to the high voltage components and less heating effects to the electronics leading to greater reliability. 
         [0029]    A power source is preferably provided for electrically biasing the discharge electrode with respect to the counter electrode. 
         [0030]    Preferably, the maximum voltage that the power source can apply to the discharge and/or counter electrode is limited to 100 kV, and more preferably to 70 kV. In a most preferred embodiment of the invention, the maximum voltage that the power source can apply to the discharge and/or counter electrode is limited to 40 kV 
         [0031]    The discharge current is preferably limited. Preferably, the maximum discharge current at the discharge and/or counter electrode is less than 100 μA, and preferably less than 70 μA. In a most preferred embodiment, the maximum discharge current at the discharge and/or counter electrode is substantially 20 μA. 
         [0032]    Preferably, the counter electrode is positioned between the nozzle and the spacer. 
         [0033]    Positioning the counter electrode so close to the discharge electrode is contraindicated by the prior art, since it is believed that the high voltage required to create a sufficient field to charge the powder cannot be maintained between closely spaced electrodes without the risk of sparking. 
         [0034]    Surprisingly, it has been noted that substantially the same powder charging can be obtained using a less intense field (i.e. with a lower voltage and/or current) if the discharge and counter electrodes are positioned closer together. 
         [0035]    In a preferred embodiment the apparatus may further comprise a controller for controlling the voltage and/or current applied to the discharge electrode. 
         [0036]    The spacer can be releasably connectable to the body. Preferably, the nozzle and spacer are integrally formed. 
         [0037]    The control circuit may comprise one or more user interfaces, which enable a user to set the desired process parameters. Additionally, a power sensing feedback or alternatively circuit, which monitors the actual process parameters and/or adjusts the power supply to compensate for any deviation between the desired and actual parameters is preferably provided. 
         [0038]    The power sensing/feedback circuit, where provided, may comprise a voltmeter and/or an ammeter connected to the discharge and/or the counter electrode. The sensed outputs from the volt meter and/or the ammeter can be sent to a microprocessor. The microprocessor, where provided, is preferably configured to monitor and/or log the sensed outputs and/or to perform processing operators in response thereto. 
         [0039]    The control circuit may comprise a power controller for varying the output voltage. The power controller may comprise a potential divider, and/or means for varying the current, such as a variable load/resistor. 
         [0040]    The controller preferably has various operating modes which can be selected by the user for different coating applications. Such modes may include: constant voltage and/or constant current control, and/or fixed voltage control and/or fixed current control, and/or proportional energy control. 
         [0041]    The first mode, fixed voltage control, is where the user moves the discharge electrode far away from the workpiece and/or specifies a nominal operating voltage. The power control circuit adjusts the discharge voltage to the specified parameter and locks it. Thus, in use, as the discharge electrode is moved towards or away from the workpiece, the discharge current rises and falls. 
         [0042]    The constant current mode enables the user to set a desired operating current. However, in this modes, the current is allowed to rise to a maximum set value as the gun moves towards an earthed object. When this maximum is reached, it is held at that pre-set value and the voltage is reduced proportionally if the gun is moved yet closer to the earthed object. 
         [0043]    Finally, the proportional energy control mode enables the user to specify a desired energy, which corresponds to a desired charge on the powder. Thus, the control module is free to select any discharge voltage and current provided the energy i.e. the product of the voltage and current, remains proportional to the load resistance. 
         [0044]    Automatic Energy Control will set the voltage and current levels automatically to optimum settings for the distance between the electrode and counter electrode without the need for operator adjustment. 
         [0045]    In any of the above modes, the control circuit can be set to control the voltage/current at the discharge (or counter) electrode with respect to either the counter (or discharge) electrode or the workpiece. 
         [0046]    Thus, the control circuit may allow the user to select from a variety of operating modes. Of particular relevance to the invention is proportional energy control with respect to the counter electrode. Additionally, fixed voltage mode with respect to the workpiece, fixed voltage mode with respect to the counter electrode, fixed current mode with respect to the workpiece, fixed current mode with respect to the counter electrode, constant voltage mode with respect to the workpiece, constant voltage mode with respect to the counter electrode, constant current mode with respect to the workpiece, and constant current mode with respect to the counter electrode, are alternative operating modes. 
         [0047]    In all modes, the control circuit preferably provides a safety shut-off that shuts off the power supply in the event of an earth leak, an arc discharge or short circuit. 
         [0048]    A user keypad and/or a visual display unit is preferably provided that enables the user to program the control circuit i.e. set the desired operating mode and/or process parameters. A powder delivery means, comprising, for example, hoses, a filter, a fluidising bed and a pump is preferably provided to deliver the powder from a powder supply (e.g. a carton of dry powder) to the nozzle of the gun during use. 
         [0049]    The powder spray discharge coating assembly according to the fourth aspect of the invention automatically adjusts the discharge voltage and/or current at the discharge and/or counter electrode. Since higher powder quantities/throughputs have the effect of suppressing the corona discharge, a higher voltage for the same electrostatic energy would preferably be applied to the discharge and/or counter electrode. Different powder types may require different discharge voltages and/or currents to achieve sufficient charging, depending on the surface area/powder particle, the powder material, surface roughness etc. Thus, the discharge voltage and/or current is preferably adjustable to compensate for these variations. As the discharge or counter electrode degrades with use (e.g. by wear, attrition, oxidation, thermal cycling etc.) the applied discharge voltage and/or current is preferably automatically adjusted to compensate. Atmospheric conditions, such as temperature, humidity etc may affect the powder charging (e.g. higher humidity will lead to a reduced discharge current for a given discharge voltage since the conductivity of the air will be increased). Accordingly, it is a preferred feature of the invention that the discharge voltage and/or current be adjustable, preferably automatically, to compensate for these variations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]      FIG. 1  is a side view of a prior art electrostatic powder spray coating gun; 
           [0051]      FIG. 2  is a side view of a variant of the prior art electrostatic powder spray coating gun of  FIG. 1  comprising a counter electrode; 
           [0052]      FIG. 3  is a perspective view of the prior art electrostatic powder spray coating gun of  FIG. 2 ; 
           [0053]      FIG. 4  is a side view of another variant of the prior art electrostatic powder spray coating gun of  FIG. 1  comprising a plurality of counter electrodes; and 
           [0054]      FIG. 5  is a perspective view of the prior art electrostatic powder spray coating gun of  FIG. 4 ; 
           [0055]      FIG. 6  is a perspective view of an electrostatic powder spray coating gun in accordance with the invention; 
           [0056]      FIG. 7  is a side view of electrostatic powder spray gun of  FIG. 6 ; 
           [0057]      FIG. 8  is a longitudinal cross-section showing a first possible internal configuration of the nozzle and spacer of  FIG. 7  on A-A; 
           [0058]      FIG. 9  is a longitudinal cross-section showing a second possible internal configuration of the nozzle and spacer of  FIG. 7  on A-A; 
           [0059]      FIG. 10  is a schematic control circuit for the electrostatic powder spray coating gun of  FIGS. 6 to 9 ; and 
           [0060]      FIG. 11  is a perspective view of part of an electrostatic powder spray coating production line incorporating a plurality of guns in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0061]    A prior art electrostatic powder spray gun  10  is shown in  FIG. 1 , which generally comprises a body  12 , a grip  14  and a nozzle  16 . At the bottom of the grip  14 , there is a connector  18  for connecting the gun  10  to a hose (not shown) that supplies powder thereto and to a mains power supply. An upper part of the body  12  houses a power supply unit  20 , which outputs a high voltage (positive or negative) from the incoming electrical power. Inside the body  10 , a conduit  22  internally connects the hose connector  18  to the nozzle  16 . 
         [0062]    Interposed between the nozzle  16  and the body  12  is an electrically insulating annular spacing sleeve  30 , known as a “nozzle nut”. The spacing sleeve  30  is detachable from the body  12 . 
         [0063]    The nozzle  16  comprises an annular plastics sleeve portion  24 , whose aperture  26  is aligned with an end of the conduit  22 . Concentrically aligned with, and protruding partially into, the aperture  26  is a deflector  28 . The deflector  28  has a generally flat, circular front face  29  and a generally cylindrical shaft  31  extending rearwards therefrom with a semi-hyperboloidal rear surface portion  33  forming a flared blend between the shaft  31  and the front face  29 . Thus, powder enters the gun  10  via the hose connector  18 , travels through the conduit  22  and exits the gun at the nozzle  16 . The deflector  28  causes the trajectory of the powder particles to be deflected outwardly, which creates, in this case, a generally conical spray in front of the nozzle  16 . 
         [0064]    The nozzle  16  is integrally formed with the spacing sleeve  30  and, hence, can be interchanged with other nozzles having differently profiled deflectors (by replacing the spacer-nozzle assembly) to obtain different spray patterns. (For example, a nozzle having no or a very small deflector could create a substantially cylindrical jet of powder, or a deflector comprising a slotted aperture could create a rectangular spray pattern.) 
         [0065]    A discharge electrode  32 , in the form of a wire, passes through the deflector  28  and protrudes beyond the front face thereof. The discharge electrode passes through an annular plastics bush  34 , to insulate the discharge electrode  32  from the deflector  28 . A wire  36  connects the discharge electrode  32  to the power supply  20 . 
         [0066]    Since the discharge electrode  32  is fully insulated from the remainder of the nozzle  16  (by virtue of the plastics bush  34 , the plastics annular portion  24  of the nozzle and the plastics spacing sleeve  30 ), it can be maintained at a desired electrical potential with respect to the rest of the gun  10 . 
         [0067]    In  FIG. 1 , a workpiece  38  placed in front of the gun  30  and is connected to ground  40 . An electrical field is therefore created between the discharge electrode  32  and the workpiece  38 , which is represented by chain-dot lines  42  in  FIG. 1 . 
         [0068]    Finally, a trigger  44  is provided on the grip  14  of the gun  10  so that an operator can start or stop the spray coating process as desired. 
         [0069]    Another known setup is shown schematically in  FIGS. 2 and 3 , which is generally the same as the arrangement as that shown in  FIG. 1 , except with the addition of a counter electrode  46 . The counter electrode  46  comprises a straight solid metal rod that is supported at its rear end remoter from the nozzle  16 , by passing through an aperture in a plastics support piece  48  which protrudes above the top of the body  12  of the gun  10 . The front end of the counter electrode  46  extends to a position towards the front of the body  12  of the gun  10  adjacent to the rear end of the spacing sleeve  30 . The counter electrode  46  is connected to ground  40  so that the electrical field  42  preferentially extends backwards from the discharge electrode  32  to the counter electrode  46 , rather than forwards, towards the workpiece  38 . When the gun  10  is at relatively large spacings from the workpiece  38 , i.e. when the discharge electrode-workpiece spacing is greater than the discharge electrode-counter electrode tip  50  spacing (if x&gt;y) then the field will generally extend from the discharge electrode  32  to the counter electrode tip  50 . However, since the electric field will normally try to take the “path of least resistance”, when the gun  10  is moved towards the workpiece  38 , (i.e. so that x&lt;y) then the field may preferentially ground to the workpiece  38 , rather than to the counter electrode  46 . 
         [0070]      FIGS. 4 and 5  show a variant of the gun shown in  FIGS. 2 and 3  in which the counter electrode comprises eight metal pins  46  protruding radially from an annular plastics ring  52 . A metal rod  54  and wires (not shown) inside the annular ring  52  serve to electrically connect the counter electrode pins  46  to ground  40 . 
         [0071]    Preferred embodiments of the invention shall be described, by way of example only, with reference to  FIGS. 6 to 10  of the accompanying drawings. 
         [0072]    An electrostatic spray coating gun  10  according to the invention is shown in  FIGS. 6 and 7 , which generally comprises a body  212 , a grip  214  and a nozzle  216 . At the bottom of the grip  214 , there is a connector  218  for connecting the gun  210  to a hose (not shown) that supplies powder thereto and to a mains power supply. An upper part of the body  212  houses a power supply unit  220 , which outputs a high voltage (positive or negative) from the incoming electrical power. Inside the body  210 , a conduit  222  internally connects the hose connector  218  to the nozzle  216 . 
         [0073]    Interposed between the nozzle  216  and the body  212  is an electrically insulating annular spacing sleeve  230 , known as a “nozzle nut”. The spacing sleeve  230  is detachable from the body  212 . 
         [0074]    The nozzle  216  comprises an annular plastics sleeve portion  224 , whose aperture  226  is aligned with an end of the conduit  222 . Concentrically aligned with, and protruding partially into, the aperture  226  is a deflector  228 . The deflector  228  has a generally flat, circular front face  229  and a generally cylindrical shaft  231  extending rearwards therefrom with a semi-hyperboloidal rear surface portion  233  forming a flared blend between the shaft  231  and the front face  229 . Thus, powder enters the gun  210  via the hose connector  218 , travels through the conduit  222  and exits the gun at the nozzle  216 . The deflector  228  causes the trajectory of the powder particles to be deflected outwardly, which creates, in this case, a generally conical spray in front of the nozzle  216 . 
         [0075]    The nozzle  216  is integrally formed with the spacing sleeve  230  and, hence, can be interchanged with other nozzles having differently profiled deflectors (by replacing the spacer-nozzle assembly) to obtain different spray patterns. (For example, a nozzle having no or a very small deflector could create a substantially cylindrical jet of powder, or a deflector comprising a slotted aperture could create a rectangular spray pattern.) 
         [0076]    A discharge electrode  232 , in the form of a wire, passes through the deflector  228  and protrudes beyond the front face  229  thereof. The discharge electrode  232  passes through an annular plastics bush  234 , to insulate the discharge electrode  232  from the deflector  228 . An internal wire  236  connects the discharge electrode  232  to the power supply  220 . 
         [0077]    Since the discharge electrode  232  is fully insulated from the remainder of the nozzle  216  (by virtue of the plastics bush  234 , the plastics annular portion  224  of the nozzle and the plastics spacing sleeve  230 ), it can be maintained at a desired electrical potential with respect to the rest of the gun  210 . 
         [0078]    A trigger  244  is provided on the grip  214  of the gun  210  so that an operator can start or stop the spray coating process as desired. 
         [0079]    The counter electrode  260  is located between the spacing sleeve  230  and the nozzle  216 . The counter electrode  260  comprises an annular metal ring  262  having a castellated periphery comprising a plurality of identical outwardly projecting castellations  264  separated by identical recesses  263 . Each castellation  264  protrudes radially beyond the periphery  266  of a conical portion  268  of the nozzle  216 . The castellated ring  262  is clamped between the spacing sleeve  230  and the nozzle  216 . 
         [0080]    As can be seen more clearly in  FIG. 7 , in use, the electrical field (shown by chain-dot lines  242 ) emanates from the tip of the discharge electrode  232  and extends backwards towards the castellations  264  of the counter electrode  260 . Such a configuration gives rise to a higher field density (i.e. more field lines  242  per unit area) in the vicinity of the nozzle  216  than the prior art arrangements. Because the field density is higher, a lower voltage needs to be applied, compared to the prior art arrangements to achieve comparable powder charging (i.e. transfer of electrons to the powder particles). 
         [0081]      FIG. 8  shows a spacer-nozzle arrangement of  FIGS. 6 and 7  in greater detail. As can be seen in  FIG. 8 , the tubular spacing sleeve  230  is connected at one end to the body  212  of the electrostatic powder spray gun  210 , and at the other end, to the nozzle  16 . The annular castellated counter electrode  260  is clamped between the spacing sleeve  230  and the nozzle  216 . 
         [0082]    The spacing sleeve  230  comprises a thick-walled plastics tube  270  having a through aperture  272  through which, in use, the powder passes. The aperture  272  is also aligned with the conduit  222  that passes through the body  212  of the gun  210 . The spacing sleeve  230  has front and rear axial bosses  274  and  276  at either end, that engage corresponding annular recesses  278  and  280  in the body  12  and nozzle  16 , respectively. The front boss  274  and nozzle recess  278  are complementarily screw-threaded so that they can be screw-threadedly connected to one another. The rear boss  276  of the spacing sleeve  230  comprises a bayonet-type connector that is receivable in the recess  280  of the body  212  of the spray gun  210 . The nozzle  216  also has an annulus  226  that aligns with the annulus  72  of the spacing sleeve  30 . 
         [0083]    The counter electrode  260  is also provided with a central through aperture that receives the front boss  276  of the spacing sleeve  230  to allow the counter electrode  260  to be clamped between the nozzle  216  and the spacing sleeve  230  when the two are screwed together. 
         [0084]    An electrode centraliser  282 , in the form of a thin plastics disc, is clamped between the end  284  of the rear boss  276  and a rear wall  286  of the recess  280  when the spacing sleeve  230  is connected to the body  212 . The plastics disc has a plurality of through holes to permit flow through of powder in use. Extending forwards from the centre of the centraliser  282  and beyond the nozzle  216  is an elongate plastics shaft  288  arranged concentrically with the through aperture  272 . A flared deflector  28  is integrally formed with the front end of the shaft  88 . 
         [0085]    The shaft  288  has a central bore, which receives the discharge electrode wire  232 . The discharge electrode wire  232  protrudes slightly beyond the forward end  290  of the shaft  288  remote from the centraliser  282 . The rear end of the discharge electrode wire  232  passes internally through the centraliser  282  and terminates slightly proud of the rear face  292  of the centraliser to form a contact  294 . Thus, when the spacing sleeve  230  is correctly connected to the body  212 , the discharge electrode wire  232  makes an electrical contact with an output  298  of the power supply. 
         [0086]    A counter electrode wire  100  passes through a channel  101  in the spacing sleeve  230  extending parallel to the longitudinal axis of the spacer sleeve  230 , and makes electrical contact at its front end with the counter electrode  260  and at its rear end to a ground terminal  204  of the power supply  220  located in the body  212  of the gun  210 . 
         [0087]    Thus, by having a bayonet-type fitting, and having the discharge and counter electrode contacts ( 294  and  204 ) at different radial positions, it is possible to ensure that the electrodes can only be connected the correct way. Also, if the gun  10  is used in conjunction with a conventional spacer-nozzle (i.e. without a forward-mounted counter electrode), then only the discharge electrode wire will make contact with the power supply. 
         [0088]    In use, the discharge electrode  232  is electrically biased with respect to the counter electrode using a power supply located within the body of the apparatus. 
         [0089]      FIG. 9  shows a slightly different spacer-nozzle arrangement to that of  FIG. 8 . As can be seen in  FIG. 9 , the front end to the body  312  of the electrostatic powder spray gun  310  has a cylindrical extension tube  311  over which the tubular spacing sleeve  330  slides. The nozzle  316 , which comprises a part-conical portion  313  and a cylindrical portion  315 , is integrally formed with the spacing sleeve  330 . A cylindrical aperture  372 , which extends through the body  312  aligns with an aperture in the nozzle  316  to allow coating powder to be blown therethrough, in use. An annular castellated counter electrode  360  is clamped onto the cylindrical portion  315  of the nozzle  316  using a screw-threaded locking ring  317 . 
         [0090]    The spacing sleeve  230  has an internal screw thread  321  at its rear end, which engages a corresponding external screw thread on the outer surface of the cylindrical extension tube  311  so that they can be screw-threadedly connected to one another. 
         [0091]    An electrode centraliser  382 , in the form of a thin plastics disc, is clamped between the end  384  of the extension tube  311  and a rear wall  386  of the nozzle  316  when the spacing sleeve  330  is connected to the body  312 . The plastics disc has a plurality of through holes to permit flow through of powder in use. Extending forwards from the centre of the centraliser  382  and beyond the nozzle  316  is an elongate plastics shaft  388  arranged concentrically with the through aperture  372 . A flared deflector  328  is integrally formed with the front end of the shaft  388 . 
         [0092]    The shaft  388  has a central bore, which receives the discharge electrode wire  332 . The discharge electrode wire  332  protrudes slightly beyond the forward end  390  of the shaft  388  remote from the centraliser  382 . The rear end of the discharge electrode wire  332  passes internally through the centraliser  382  and terminates slightly proud of the rear face  392  of the centraliser  382  to form a contact  394 . Thus, when the spacing sleeve  330  is correctly connected to the body  312 , the discharge electrode wire  332  makes an electrical contact with an output  398  of the power supply. 
         [0093]    A counter electrode wire  300  passes through a channel  301  in the spacing sleeve  330  extending parallel to the longitudinal axis of the spacer sleeve  330 , and makes electrical contact at its front end with the counter electrode  360  and at its rear end to a ground terminal  304  of the power supply located in the body  312  of the gun  310 . 
         [0094]    Thus, by having the discharge and counter electrode contacts ( 394  and  304 ) at different radial positions, it is possible to ensure that the electrodes can only be connected the correct way. Also, if the gun  310  is used in conjunction with a conventional spacer-nozzle (i.e. without a forward-mounted counter electrode), then only the discharge electrode wire will make contact with the power supply. 
         [0095]    In use, the discharge electrode  332  is electrically biased with respect to the counter electrode  360  using a power supply located within the body of the apparatus. 
         [0096]      FIG. 10  is a schematic of an alternative control and feedback system for the electrostatic powder spray coating gun of  FIGS. 6 to 9 . A variable voltage power supply  110  is connected to a step-up transformer  112  for converting an AC mains electricity supply (e.g. 230V, 50 Hz or 120V, 60 Hz) into a high tension or ultra-high tension (100-200 kV) supply at the discharge electrode contact  294 . 
         [0097]    The power supplied to the discharge electrode  232  is controlled by adjusting the variable voltage power supply  110 . 
         [0098]    A power sensing circuit  120  is also provided to monitor the discharge voltage and current indirectly. A voltmeter  122  and an ammeter  124 , respectively, monitor the load drawn by the transformer, which is assumed to vary as a function of the load at the discharge  232  or counter electrode  260 . 
         [0099]    The voltmeter  122  and ammeter  124  readings are fed  126  to a microprocessor  128  (via appropriate analogue to digital converters, if necessary) which monitors and logs the respective readings. If the measured voltage or current moves outside specified ranges, then the microprocessor outputs a signal  130  to adjust the variable voltage power supply  110  to bring the voltage and/or current at the discharge electrode  260  back within the specified range. 
         [0100]    In use, the power sensing and power control circuits operate as follows: 
         [0101]    The discharge voltage and current are both limited to predetermined maximum values. The discharge voltage will operate at the maximum predetermined value until the maximum value of discharge current is reached. If the discharge current tries to exceed this maximum value the discharge voltage and current are both reduced proportionately as an inverse ratio of the output load. It can be assumed that the output load is the resistance of the air between the discharge electrode and an earthed object or counter electrode. 
         [0102]    Since the distance between the discharge and counter electrodes is substantially fixed, variations in discharge load can be attributed to variations in powder throughput, atmospheric conditions or the gun closely approaching a grounded workpiece. 
         [0103]    Where a fixed counter electrode is not used, i.e. where the workpiece, rather than the counter electrode is earthed, the greater the distance between the discharge electrode and the workpiece, the higher will be the resistance of the air and the lower will be the load and therefore the discharge current. The closer the distance between the discharge electrode and the workpiece, the lower will be the resistance of the air and the higher will be the load and therefore the discharge current. The controlled reduction of output voltage and current when the maximum predetermined current is exceeded is achieved by reducing the voltage of the low voltage supply to the high voltage generator. 
         [0104]    The control circuit provides a variable voltage power supply that is controlled by a micro-controller. This power supply is fed to a high voltage generator assembly (HVGA) which in turn steps the voltage up by a fixed ratio to generate the high voltage at the gun. The HVGA comprises a circuit, a step-up transformer and a multi-stage voltage multiplier to convert a 12V DC input to a (+ or −) 80 kV DC output voltage. (A 10-stage voltage multiplier is preferred over a 12-stage one as it gives more clearance and therefore reduces the likelihood of electrical breakdown). 
         [0105]    If the HVGA in the gun is assumed perfect with no losses and the step up ratio is fixed, the actual gun voltage is calculated by the micro-controller by multiplying the assumed step up ratio of the HVGA by the controlled power supply voltage. 
         [0106]    The current load is measured on the positive input side of the HVGA by a dedicated analogue circuit. This circuit measures the voltage dropped across a small ohmic value resistor and translates this measurement into a 0 to 5 volt ground referenced signal. The frequency response of this analogue circuit is sufficiently fast to perform real time current control of the power supply. When the micro-controller measures the input current to the HVGA it can determine the effective load on the high voltage side. This is done by dividing the calibrated measured current on the input side of the HVGA by the assumed fixed step up ratio of the HVGA. 
         [0107]    The actual amount of input voltage reduction or fold back when the predetermined output current is exceeded is configurable either by calculation by the microprocessor or by look up tables programmed into the system. This means that the fold back slope or gradient may be altered as necessary. 
         [0108]    With the addition of the counter electrode to the spray gun in close proximity to the discharge electrode of the high voltage gun, and it being physically near to the output of the gun the near field created, causes the HVGA to operate as a high voltage proportional energy source, where the energy is proportional to the load resistance. Furthermore, there is now no immediate electrical interaction with the target being sprayed. In the event of the gun becoming too near to the target the gun current tries to increase, this is sensed by the micro-controller, and the output voltage of the controlled power supply is reduced to maintain the proportional energy of the gun. When the gun voltage is reduced to a level where spraying is not adequately possible, the gun current and voltage is further reduced to prevent any arcing between the gun electrode and the target being sprayed. This mode of reduced current and voltage is a purely safety operating mode and normal powder spraying would not be possible when the target work piece and gun are in close proximity. 
         [0109]    The same control effects can be achieved using conventional analogue circuits. This is also achieved by measuring the voltage dropped across a small ohmic value resistor and using operational amplifiers to provide negative feedback relative to the output current and control an output transistor which provides the low voltage supply to the high voltage generator. 
         [0110]    In other words, the control circuit provides a means to control the discharge energy (i.e. the voltage and current) in proportion to the proximity of the discharge electrode and counter electrode or earth (e.g. a grounded workpiece). 
         [0111]    A user interface  132  is provided so that an operator can specify the mode of operation and/or the process parameters, e.g. the maximum predetermined discharge current and voltage. The user interface comprises a built-in database of customisable pre-sets so that the user can quickly select the operating parameters for a particular task. The user interface comprises an input device, such as a touch screen and/or a keyboard and/or a pointing device (e.g. a mouse) and a visual display unit. A portable, remote user interface  134  is also provided that is wirelessly connected to the microprocessor. 
         [0112]    In addition, the microprocessor  128  also controls other aspects of the gun&#39;s operation, such as the powder throughput. The microprocessor is, accordingly, operatively connected to control a pump  136  so that the delivery of powder from a powder supply  138  to the nozzle  216  of the gun  210  can be controlled. 
         [0113]    The microprocessor is also configured to “recognise” which particular type of spacing sleeve  230  and nozzle  216  attached to the gun  210 . For example, if the spacer-nozzle arrangement described with reference to  FIGS. 6 to 8  is connected, when the gun is activated, the microprocessor will immediately act to reduce the applied voltage and/or current. However, if a counter electrode arrangement such as that shown in  FIGS. 2 and 4  is used, then when the gun is activated, the microprocessor would not immediately act to reduce the applied voltage and/or current (unless the discharge electrode is close to a grounded object). 
         [0114]    The microprocessor  128  is thus configured to recognise different setups automatically and to limit the available modes of operation and/or the process parameters accordingly. For example, if a counter electrode arrangement such as that shown in  FIGS. 6 to 8  is used, then the maximum current and/or voltage will be limited to prevent sparking, whereas with more spaced-apart electrodes, much higher discharge voltages can be used. Additionally, if no counter electrode is used, then the feedback will need to come from readings taken between the discharge electrode and the workpiece etc. 
         [0115]    The microprocessor also has built in safety functions, such as spark prevention. By constantly monitoring the discharge current and voltage, a rapid increase in discharge current accompanied by a rapid decrease in discharge voltage can be “interpreted” as a short circuit or a spark and a shut-down command can be sent to the power control circuit  114  to temporarily reduce and/or switch off the power supply  110 . 
         [0116]    Finally,  FIG. 11  shows a plurality of electrostatic powder spray guns  210  (as described in relation to  FIGS. 6 to 10 ) ready for use on a production line  140 . 
         [0117]    The production line  140  comprises an overhead conveyor system  142  for conveying workpieces  144  past a series of work stations  146 , each having an electrostatic powder spray coating gun  210  associated therewith. The conveyer system  142  comprises an overhead cable  148  that is passed around spaced apart pulleys  150  and whose ends are connected together to form a continuous loop. Rotation of the pulleys  150  causes the cable  148  to move past each work station  146  in turn. Connected to and hanging from the cable  148  are a number of suspension means  152 . The suspension means  152  each comprise a mechanically driven swivel  154  from which an elongate wire  156  hangs having a hook  158  at the free end thereof. Thus, workpieces  144  can be hooked onto the suspension means  152  and indexed from one work station  146  to the next and/or rotated so that they can be sequentially coated from each side. 
         [0118]    The hook  158 , swivel  154  and cable  148  are all manufactured of metal to ensure earth continuity. 
         [0119]    A primary user interface  132  allows an operator to set the operating parameters for each gun  10 . The primary user interface  132  connects via concealed wires to a series of connector boxes  160 . Thus, in use, each operator can simply plug his gun  10  into the connector box  160  to receive power and control inputs from the microprocessor in the primary user interface  132 . 
         [0120]    In the embodiment shown in  FIG. 11 , the two guns  10  are set up to perform different coating operations, namely priming and top coating. Thus, the guns  10  are connected to different powder supplies  138  and are programmed with different operating parameters from the user interface  132 . 
         [0121]    A hand-held portable computer  134 , with a wireless link to the primary controller  132 , is provided so that operators can inspect the workpiece  144  and make manual adjustments to the gun&#39;s process parameters without having to leave their workstations  146 . A cradle  160  is provided to conveniently store the portable computer  134  when it is not being used. 
         [0122]    The invention is not limited to the details of the foregoing embodiments. In particular, the guns need not be hand-held devices—they could equally be robot mounted or even mounted on fixed stands past which the workpieces move. The production line shown in  FIG. 10  need not be so sophisticated—it could simply comprise a partially enclosed booth at which an operator stands and manually manipulates the item to be coated. 
         [0123]    The counter electrode of the invention could be supplied as an “add on” to a conventional nozzle with an external earth connection in the form of a flying lead to an earth point near the back of the spray gun (as in current practice), or it could be built into a nozzle with an earth connection being made automatically when the nozzle is fitted to the spray gun. The earth electrode contacts could take the form of an earthed metal rod or single point electrode or an annular array of multiple earthed pins as previously described or could take the form of a castellated metal disk where the edges of the castellations form points which act as individual electrodes. 
         [0124]    If a conventional nozzle were fitted without the counter electrode, the control circuit of the invention would preferably detect whether the counter electrode is fitted when the spray gun is energized by monitoring whether a low voltage and current is in use automatically on “switch on” or whether the normal high voltage is being discharged. If no counter electrode is fitted, the electrostatic output would be either disabled or automatically switched to conventional charge