Electric heating – Metal heating – By arc
Reexamination Certificate
2000-07-07
2001-07-17
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121390, C219S121440, C219S121510, C219S121500
Reexamination Certificate
active
06262386
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a plasma nozzle, in particular for pretreating surfaces, the nozzle comprising a tubular casing forming a nozzle channel through which a working gas is passed, an electrode disposed coaxially in the nozzle channel, and a counter electrode surrounding the nozzle channel.
A plasma nozzle of this type is disclosed in DE 195 32 412 A corresponding to U.S. Pat. No. 5,837,958 and serves, for example, for pretreating the surfaces of plastic (synthetic resin) materials such that coating of the surface with adhesive, printing inks and the like is made possible or facilitated. Such a pretreatment is necessary because plastic surfaces can normally not be wetted with liquids and do therefore not accept the printing ink or the adhesive. The pretreatment modifies the surface structure of the plastic material in such a manner that the surface can be wetted with liquids having a relatively large surface tension. The surface tension of the liquids with which surface can just be wetted is an indicator for the quality of the pretreatment.
The known plasma nozzle provides a relatively cool but nevertheless highly reactive plasma jet which has approximately the shape and dimensions of a candle flame and therefore permits also the pretreatment of profiled workpieces having a relatively deep relief. Thanks to the high reactivity of the plasma jet, a very short pretreatment time is sufficient, so that the workpiece can be moved past the plasma jet with a relatively high velocity. The relatively low temperature of the plasma jet therefore permits also the pretreatment of heat sensitive plastic materials. Since no counter electrode on the rear side of the workpiece is necessary, the surfaces of arbitrarily thick block-like workpieces, hollow bodies and the like can be pretreated without difficulties. For an even treatment of larger surface areas, the cited publication purposes an array of a plurality of staggered plasma nozzles. This, however, requires complex installations.
For pretreatment of larger surface areas, DE 298 05 999 U discloses an apparatus in which two plasma nozzles are mounted eccentrically and with parallel axes on a common rotating head, so that, when the surface is scanned with the rotary head, pretreatment is achieved in a stripe which has a width corresponding to the diameter of the rotating head. This apparatus is however not suitable for treating bulged surfaces the radius of curvature of which is in the order of the diameter of the rotating head. Moreover, the eccentric arrangement of at least two nozzles and the relatively high rotary speed lead to the occurrence of forces of inertia and gyroscopic forces when the rotating head is moved along more than one axis, for example with the aid of a robot arm.
In general, the known plasma the nozzles eject the plasma in axial direction of the nozzle channel. In case of workpieces having a complicated shape, this has the drawback that the locations to be treated are sometimes difficult to reach, in particular, when the nozzle is moved along the workpiece by means of a robot.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a plasma nozzle with which the surface portions to be pretreated can be reached more easily.
This object is achieved by a plasma nozzle of the type indicated above, in which the mouth of the nozzle channel is angled relative to the axis of the nozzle channel.
Thus, this nozzle generates a plasma jet which is inclined relative to the axis of the nozzle channel, so that, for example, undercut parts of a workpiece can be reached more easily.
Although the plasma jet is deflected from the original axial direction at the mouth of the nozzle, experiments have shown that this does not impair the stability of the plasma jet and its efficiency in the pretreatment of surfaces.
In one preferred embodiment the casing or at least the part of the casing forming the nozzle channel is rotatable about is longitudinal axis. When the casing is caused to rotate rapidly, and the plasma nozzle is moved past a workpiece, it is therefore possible to treat, within a single pass, a surface stripe the width of which is significantly larger than the diameter of the plasma jet. Since only a single nozzle is used, the complexity of the installation is significantly smaller than in case of the previously described rotating head. In addition, the forces of inertia are greatly reduced because the casing rotates around its longitudinal axis. Thus, a plasma nozzle is provided which has a compact construction and nevertheless permits and efficient plasma treatment of large surface areas.
The angle of deflection of the plasma jet relative to the rotary axis can be selected in accordance with the demand and may for example amount to 90° or more. In this embodiment, the plasma nozzle is particularly suited for pretreatment of the internal surfaces of pipes or tubes. It is possible for example to mount the plasma nozzle inside of the annular gap of an extrusion die, so that an extruded tube may be pretreated right after it has exited from the extruder.
Preferably, the casing is rotatable relative to the electrode and the supply system for the working gas which are mounted inside of the nozzle channel, so that the electrode and the gas supply system can be held non-rotatably and only the surrounding casing is rotated. As a result, no sliding contacts, rotary joints or the like are needed for the supply of the working gas and for the power supply to the electrode. The counter electrode may be formed directly by the rotating casing and is preferably grounded, so that it is not necessary to protect the casing and the associated rotary drive system against contact or touch.
A drive disk or an toothed gear for rotatingly driving the casing may be provided on the outer periphery of the casing.
Like in the plasma nozzle of the type indicated in the preamble, the working gas is preferably swirled, so that it flows through the nozzle channel in vortex fashion, and the electric are formed between the electrode and the counter electrode is channeled in the vortex core until it reaches the region of the mouth of the nozzle channel. Thus, the plasma jet is stabilized, and, inside of the vortex core, the working gas is brought into intimate contact with the electric arc, so that the reactivity of the plasma is enhanced.
In another preferred embodiment the mouth of the nozzle channel is formed in a mouth piece which is inserted in the casing and in which a passage is defined which is inclined relative to the axis of the casing. The passage of the mouth piece may be tapered towards its downstream end.
Preferably, the mouth piece is rotatably supported in the casing by means of a contactless bearing such as a magnet bearing or an aerodynamic bearing.
The counter electrode is preferably formed by the mouth piece, and the contactless bearing defines a gap between the casing and the mouth piece which is so dimensioned that an arc discharge occurs across this gap, thereby to ground the mouth piece.
The contactless bearing may be an axial/radial bearing and the mouth piece may be dynamically biased against this bearing by the working gas flowing through the mouth piece.
Further, the mouth piece may be aerodynamically driven for rotation.
REFERENCES:
patent: 3707615 (1972-12-01), Rotolico
patent: 4877937 (1989-10-01), Muller
patent: 5278387 (1994-01-01), Borne
patent: 5837958 (1998-11-01), Förnsel
patent: 298 05 999 U (1998-06-01), None
Agrodyn Hochspannungstechnik GmbH
Goldberg Richard M.
Paschall Mark
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