Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1999-10-01
2001-08-14
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S480000, C118S629000, C118S631000, C118S671000
Reexamination Certificate
active
06274202
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns control systems for electrostatic powder spraying apparatus.
BACKGROUND OF THE INVENTION
Electrostatic spraying apparatus, especially for use in painting workpieces, generally comprise a duct for conveying gas-borne powder and means for electrostatically charging the powder, whereby it adheres to the workpiece. Typically a corona discharge needle electrode is used to charge the powder.
Corona charging of paint powder is not without its shortcomings although it is a preferred method of spray coating for a majority of powder coaters. Problems associated with Faraday Cages, back ionisation orange peel (pitting) and overcharging have been well documented but advantages of consistency, good charge transfer and fast powder deposition largely outweigh the above-mentioned shortcomings.
The effects of such shortcomings may be reduced by spray nozzle design or by addition of earth robbing electrodes behind the spray nozzle, as well as by careful setting of the spray apparatus position and operating parameters.
Significant improvements can also be achieved by controlling the discharge current instead of the discharge voltage which has proved its worth over 25 years but surprisingly this method is still the exception rather than the rule. With this system the maximum discharge current (&mgr;A) is limited to a value determined by the operator and the discharge voltage (kV) is allowed to float. The result is that as the spray apparatus approaches the workpiece and the set discharge current is reached and stabilises, so the output voltage automatically reduces to relatively low levels, thus limiting the charge when the apparatus is close to the workpiece. This is of great benefit in maintaining charge consistency, penetrating Faraday Cages and reducing back ionisation and orange peel appearance.
With conventional voltage control, where the discharge voltage is set by the operator, the current rises exponentially as the spray apparatus approaches the product which has the opposite effect to that of using current control and can also cause severe “over charging” of the powder and surrounding air and can give rise to potentially dangerous sparks, although safety cut out circuits are usually employed.
Despite the many advantages of current controlled corona charging, it is still not perfect because the output voltage is allowed to float, when the spray apparatus is pulled away from the workpiece the current can fall below its “constant” or controlled setting and the voltage can rise to the maximum available from the generator, usually 80+kV. In some cases this may be too high and create strong field lines, with the resulting Faraday Cage difficulties. As the gun approaches the product, the air space between the gun and the product obviously reduces and will therefore accept a progressively smaller charge from the corona needle in terms of free ions. It is the free ions which charge the air molecules and then transfer the charge to the powder. The closer the spray apparatus is to the workpiece so the greater the proportion of free ions which are attracted to the surface of the workpiece as opposed to dissipating in the surrounding air, or being neutralised by metal walls of the spray booth. This high charge on the surface of the powder is the main cause of the orange peel effect.
An aim of a first aspect of the present invention is to provide an improved method of controlling operation of powder spray coating apparatus whereby the above-mentioned shortcomings of prior art methods may be overcome or at least reduced in effect.
In conventional electrostatic spraying apparatus, the flow of powder from a container to the means for electrostatically charging the powder is controlled by compressed air. As the spray apparatus approaches a workpiece, the surface area which is sprayed reduces and hence the volume of powder paint required also reduces. In practice, as the spray apparatus approaches the workpiece, an operator will manually reduce the volume of powder flowing to the electrostatic charging means by, for example, reducing the pressure of the compressed air. However, since the volume of powder sprayed is controlled manually, this can lead to irreproducible coating results.
An aim of a second aspect of the present invention is to provide an electrostatic spraying apparatus which overcomes the afore-mentioned problem.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method for controlling operation of powder spray coating apparatus in order to improve coating, comprising controlling the output power charging the powder by progressively reducing both the discharge current and discharge voltage as the spray apparatus approaches a workpiece wherein said reduction in the discharge current and discharge voltage commences at a distance of separation between the spray apparatus and workpiece which is at least 100 mm.
Preferably, the powder spray coating apparatus comprises means for automatically reducing the discharged current and discharge voltage as the spray apparatus approaches the workpiece. In a preferred embodiment of the invention, the reduction in the discharge current and discharge voltage is reversible as the spray apparatus moves away from the workpiece. It is preferred that said means for automatically reducing the discharge current and discharge voltage also, automatically, increases the discharge current and discharge voltage as the spray apparatus moves away from the workpiece.
Preferably the discharge current can be set not to exceed an upper limit typically of 50 &mgr;A but possibly alternatively of, say, 30 &mgr;A. It is preferred that, in use, said means for reducing the discharge current and hence the output power comes into operation when the discharge current reaches said pre-set upper limit, ie the threshold discharge current. Similarly, said means for increasing the discharge current when the spray apparatus moves away from the workpiece may stop its operation when said discharge current reaches its pre-set upper limit.
Preferably, the maximum discharge voltage may be set by the apparatus operator, which in turn also controls the normal operating level of the discharge current.
In accordance with a further embodiment of the invention, the powder spray coating apparatus may further comprise means for reducing the volume of powder dispensed by the spray apparatus as the spray apparatus approaches the workpiece. Said reduction in the volume of paint may be in response to a decrease in the output power.
According to a second aspect of the present invention there is provided an electrostatic powder spray coating apparatus comprising an electrostatic charging means and means for controlling the volume of paint sprayed in response to a change in discharge current or discharge voltage of the electrostatic charging means.
As discussed above, with conventional electrostatic spray apparatus where the discharge current is controlled, as the spray approaches a workpiece the discharge voltage reduces. This effect is also observed where the discharge voltage is controlled. Hence, the present invention provides a system whereby the volume of paint sprayed automatically reduces as the spray apparatus approaches the workpiece and therefore reproducible coating results can be obtained. Similarly, as the spray apparatus is moved away from the workpiece, the discharge voltage increases and therefore the volume of paint sprayed will also increase correspondingly.
It is preferred that said control means reduces the volume of paint sprayed in response, to a reduction in the discharge voltage and, conversely, increases the volume in response to an increase in the discharge voltage. The control means is preferably dependent upon the actual value of the discharge voltage. Therefore, at predetermined minimum and maximum values of discharge voltage, the control means may allow, respectively, a minimum and maximum volume of paint to be sprayed Preferably, volume of paint flowing to the electrostatic charging means
Eurotec Surface Coating Systems, Ltd.
Olson & Hierl Ltd.
Parker Fred J.
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