1. Field of the invention
The invention concerns a device for electrostatically spraying liquid coating products such as paint of the type having a rotating atomizer head comprising an approximately bowl-shaped part having an annular atomizer edge at which the coating product is atomized by centrifugal force.
The invention is more particularly concerned with means for electrostatically charging the atomized coating product by ionic bombardment.
2. Description of the Prior Art
It is known in electrostatic sprayer devices to have the liquid coating product in contact with a high voltage electrode at the atomizer to charge the liquid before it is atomized. The electrically charged and atomized liquid carries electrical charges with it when it leaves the atomizer to travel towards the object to be coated. This method of charging is of limited efficiency if the liquid is highly insulative because it can then acquire only a low electrical charge through contact with a high voltage electrode. If the liquid is conductive the feed circuit for the liquid coating product, which is grounded, can short-circuit the high voltage generator. The solution to this is to insulate the coating product feed electrically from ground, for example by using an intermediate storage tank insulated from ground and therefore connectable to the high voltage.
It is also known to charge atomized particles in the air by ionic bombardment. In this method the coating product droplets are charged as they pass between the sprayer device and the object to be coated. The bombardment coating method generates atmospheric ions near a high voltage electrode by virtue of the corona effect. These ions are caused to move in the electric field produced between the electrode and a counter-electrode at a different potential. The counter-electrode is often the object to be coated, which is grounded. For highly insulative liquids this method of charging by bombardment is more appropriate than charging by means of an electrode. It is also advantageous for conductive and even highly conductive liquids because it is no longer necessary to insulate the coating product feed electrically from ground potential. This greatly simplifies the circuits feeding the electrostatic sprayer device.
A sprayer device of this kind is described in U.S. Pat. No. 4,852,810, for example, which discloses a system comprising a rotating atomizer head which is grounded and a large number of external electrodes disposed in an annular member of relatively large diameter surrounding the rotating atomizer head. These electrodes are at the high voltage.
A system of this kind has many drawbacks. It is relatively large overall and is easily soiled. The proportion of the current contributing effectively to the charging of the coating product droplets is relatively small because a large proportion is captured by the atomizer itself, in other words, the high voltage generator must supply a very high current. The charge on the coating product droplets depends greatly on the ionization conditions between the electrodes and the object to be coated. It depends also on the geometry of the object (in particular whether it has edges, especially sharp edges) and on the distance between the electrodes and the object to be coated. It depends further on the relative humidity in the area in which the coating product is applied. The charge on the coating product droplets depends additionally on the area over which the atomized liquid impacts on it. The efficiency of deposition is low. Finally, the electrical capacitance of the charging electrodes is high, which increases the risk of electrical arcing.
The invention proposes a rotating atomizer head sprayer device provided with ionic bombardment charging means enabling the above-mentioned drawbacks to be eliminated or reduced.