Method and apparatus for surface pretreatment of articles

The invention relates to a method and an apparatus for surface treatment of workpieces by means of electrical discharge.

If workpiece surfaces coated, painted, or to be bonded, a pretreatment is frequently required to be removed by the contaminants from the surface and thereby the - changing the molecular structure so that the surface with liquids such as adhesives, coatings and - in particular for workpieces made of plastic like can be wetted.

A known method for the pretreatment of plastic films is that they are allowed to act a corona discharge on the film surface. For this purpose, the film is passed through a narrow gap between the corona electrodes. However, this method is only applicable to relatively thin films. In addition, there may be an undesirable pretreatment of the back of the film, for example, when an air bubble is located between the rear electrode and the film, in which takes place a further discharge.

For pretreating the surface of thicker films or solid workpieces is a corona nozzle described in DE 43 25 939 C1, in which emerges a oscillating or circumferentially guided air stream between the electrodes, so as to obtain a surface discharge region in which the surface to be treated of the workpiece with the corona discharge tufts can be painted over. However, this corona nozzle is not suitable for the forehand Lung workpieces having a relatively deep relief as inside corners, deep grooves and the like are difficult or impossible to achieve with the two-dimensionally extended discharge zone of this nozzle are. Moreover, this known nozzle has a corona relatively complex and bulky construction, since a motor drive is required for the generation of the oscillating or rotating air stream.

The object of the invention is to provide a method for forehand Lung workpiece surfaces by electric discharge, which can also be applied to the workpiece surfaces with a relatively complicated relief, and provide an apparatus for performing this process.

This object is achieved by the method according to claim 1 and by a device according to claim. 4

According to the invention a collimated beam of a reactive medium is generated by plasma discharge while supplying a working gas and the surface to be treated is exposed to the beam so produced.

The process is suitable for treatment of both conductive and non-conductive workpieces, in particular workpieces of plastic. Furthermore, it has been found that it is possible in the manner described above, a beam generating, on the one hand so chemically active that an effective surface pretreatment is achieved, on the other hand, however, can besitztzen such a low temperature that even sensitive surfaces are not damaged.

Further advantages of the method are that a virtually ozone-free pre-treatment can be carried out and can be an unwanted pretreatment the back reliably exclude. In non-conductive workpieces an undesirable surface charging is avoided.

A generator for generating the beam is formed by a tubular nozzle of an electrically insulating material, which is flowed through by the working gas. The mouth of the nozzle is surrounded by a ring electrode and inside the nozzle a pin electrode is attached, the tip is set back axially relative to the mouth of the nozzle. The plasma arc thus extends from the tip of the rod electrode substantially in the axial direction of the nozzle tube, ie parallel to the flow of the working gas to the ring electrode. In this way can be produced before the mouth of the nozzle, an intensive, focused and relatively sharply focused beam, which allows an effective and uniform treatment also inaccessible workpiece surfaces. By suitable choice of the distance between the tip of the pin electrode and the mouth of the nozzle tube, the beam formation can be adjusted as needed.

Advantageous developments and refinements of the invention result from the dependent claims.

The nozzle tube is preferably made of ceramic material and is surrounded on the outer periphery with an electrically conductive jacket which is electrically connected to the ring electrode or formed in one piece with this and extends approximately at the level of the tip of the pin electrode at the opposite end. In this case, a corona discharge can be produced with relatively low tension through the ceramic material produced therethrough by which the arc discharge is ignited. To turn on the beam thus only the operating voltage needs to be highly regulated, and it is not substantially increased ignition voltage required.

The working gas - for example, air or argon - is preferably introduced at the level of the rod electrode in the nozzle tube, to flow swirl shaped by the nozzle tube. In the nozzle tube then forms a uniform vortex whose vortex core channels the arc. Even if not exactly co-axial alignment of the rod electrode in the nozzle tube is thus obtained a very stable arc, which in the form of a single sharply defined branch along the axis of the nozzle pipe from the top of the pin electrode extends to about the mouth of the nozzle tube and only then in several sub-branches fan out, which lead radially to the ring electrode. The point where the arc fans out in the sub-branches, forms an almost point-like source of the reactive jet. The "focus" and divergence of the beam can be in this case, by varying the working gas throughput influence without the geometric configuration of the generator must be changed.

When a working gas is used with suitable additives, the generator also for "plasma coating" of surfaces can be used.

To the electrode a high frequency alternating voltage in the order of 10 to 30 kV is preferably applied, which can be superimposed on a small DC component to stabilize the discharge. A to produce this alternating voltage suitable, easily adjustable RF generator is described in DE 42 35 766 C1. Preferably the Ringeleketrode is grounded, so that accidents or damage to electrically conductive workpieces can be avoided by unwanted discharges.

By likewise grounded sheath shielding the discharge process against external interference and a high electromagnetic compatibility (EMC) is achieved the device.

According to an advantageous development of the invention are several in a row or in several rows in a staggered staggered beam generators in a common working head integrated so that area extended workpieces can be treated equally in a rational way. The ring electrodes are formed in this case by all generators common electrically conductive block, in which the individual nozzle pipes are embedded. Any configuration of the generators can be so compact that the cross sections of the individual plasma jets overlap transversely to the relative movement of the working head and the workpiece at the level of the workpiece to be machined surface in the direction.

In the following preferred embodiments of the invention are explained in detail with reference to the drawing.

Show it:<dl id="dl0001"><dt>Fig. 1</dt><dd>a schematic longitudinal section through a beam generator according to the invention; and</dd><dt>FIG. 2</dt><dd>an end view of the working head with several beam generators.</dd></dl>

The beam generator 10 shown in Figure 1 has a cup-shaped housing 12 made of plastic, which is laterally provided with a terminal 14 for supplying a working gas. In the opening of the housing 12 is coaxially held a nozzle tube 14 made of ceramic. Inside the housing 12, a pin electrode is centrally placed 18 copper, protrudes its tip into the nozzle pipe 16th The outer periphery of the nozzle tube is surrounded outside the casing 12 by a casing 20 of electrically conductive material, which forms a ring electrode 22 at the free end of the nozzle tube sixteenth The ring electrode 22 defining a nozzle opening 24, whose diameter is slightly smaller than the inner diameter of the nozzle pipe 16, as at the outlet of the nozzle pipe a certain constriction is achieved.

The sheath 20 and thus the ring electrode 22 are grounded, and between said ring electrode and the pin electrode 18 is by means of a high-frequency generator 26 is an alternating voltage with a frequency in the order of 20 kHz is applied, the voltage can be regulated, and during operation of the beam generator as in of the order 5 to 30 kV.

The connection 14 for the working gas is arranged eccentrically in relation to the casing 12, so that the supplied working gas flows through the swirl-shaped nozzle pipe 16, as indicated by the arrow 28 in FIG. 1 Supported by the constriction at the outlet of the nozzle pipe, a stable gas vortex whose vortex core extends along the axis of the nozzle tube forms.

The electrically conductive jacket 20 covers the housing-side end as to the level of the tip of the pin electrode 18. High regulating the voltage occurs at the tip of the pin electrode 18 initially to a corona discharge. The bluish luminous discharge tufts extend radially on the wall of the nozzle tube 16, and the transport of the charge carriers to the jacket 20 is effected through the ceramic material of the nozzle tube 16. These corona discharge provides the necessary ions, an arc discharge is ignited by the pin electrode 18 to the ring electrode 22 by the voltage increases. When using air as the working gas produces a white-blue luminous arc 30 extending from the tip of the pin electrode 18 into a sharply defined thin channel along the axis of the nozzle tube 16 to about the center of the outlet 24th Only there the arc is divided into several sub-branches 32 which lead radially to the ring electrode 22nd The point at which the axial arc 30 branches to the individual sub-branches 32, at the same time forms the origin of using air as the working gas weak gold glowing "flame" which is tentatively interpreted as plasma jet 34th

This plasma jet 34 is used for the pretreatment of surfaces. In the example shown, the plasma jet is used for the pretreatment of the surface of a workpiece 36 made of plastic in the area of ​​a groove 38. It is recognized that the plasma beam 34 penetrates into the groove 38 so that the otherwise difficult to reach base of the groove can be effectively pre-treated.

Whether it is in here as a plasma jet 34 "flame" designated actually a plasma in the strict sense, ie to be a at least partially ionized medium is not fully secured. It tried to demonstrate the electrical conductivity of the medium, by the ends of two conductors, one of which was connected directly and the other via a light bulb with a battery, the flame was held. However, the light bulb lit up only when one of the branches 32 of the arc jumped on the conductor ends and this union. The conductivity of the plasma beam 34 is thus substantially lower than that of the plasma within the arc. It is possible that in the "flame" only a weakly ionized plasma or a medium containing only free radicals or excited atoms or molecules. However, it was unequivocally established that the plasma jet 34 has the desired effect on pretreating products placed on the beam workpiece surfaces. There were various plastic surfaces, which are not normally wettable with water, brought into the plasma jet 34 and then covered with water. The patients treated with the plasma jet 34 surface areas could then be wetted with water. This effect can be observed even in highly fluorinated polymers such as PTFE. Even metal surfaces could effectively vorhandelt using the plasma jet 34 and, for example, silicone oil residues and the like are removed. In experiments with base metals showed, moreover, that the plasma jet 34 has virtually no oxidizing effect. Even in the treatment of aluminum, there was no formation of an oxide layer.

The temperature of the plasma beam 34 is relatively low. Similar to a candle flame can be the finger with moderate speed by the plasma jet move, without causing burns.

The nozzle tube 16 has in the previously examined embodiments, an inner diameter of about 8 mm, and the axial distance between the tip of the rod electrode 18 and the nozzle orifice 24 is about 55 mm. The inner diameter of the nozzle opening 24 is about 5 mm. Under these conditions, a plasma jet 34, which has a length of about 30 mm and a maximum diameter of about 5 mm. By increasing the distance between the pin electrode and the nozzle opening 24, the bundling and thus the range of the plasma jet can be improved. Also, the constriction at the nozzle opening 24 seems to favor the concentration of the plasma jet.

When the flow rate of the working gas is increased by the nozzle tube 16, the origin of the plasma jet shifted further outwardly, ie, in the direction of the workpiece, and the flame is longer and slimmer. At the same time increases the recognizable in Figure 1 Away-curvature of the radial branches 32 of the arc. If one the other hand, restricts the flow of the working gas, or completely shut down, the axial arc 30 expands until it finally fills almost the entire interior of the nozzle tube. The plasma jet 34 is thereby shorter, and its origin is not point-like, but distributed two-dimensionally over the cross section of the nozzle opening 24th At moderate gas flow rate can be seen that the arc 30 follows within the nozzle tube of the swirl-shaped gas flow. When gradually increasing the gas flow rate the arc 30 is increasingly compressed in the radial direction and fixed on the axis of the nozzle tube. The arc 30 would seem to be channeled through the core of the gas vortex. Due to this effect is obtained at a sufficiently high gas flow rate a very stable plasma beam 34 which emanates from a point-like origin tightly before the center of the outlet 24th By appropriate selection of the position of the workpiece 35 with respect to the beam generator 10 so the location and extent of the treated surface area and the intensity of the plasma treatment can be precisely controlled.

Due to the relatively great length of the plasma beam 34, the distance between the workpiece and the outlet opening 24 of the plasma torch can be selected so large, the damage of the surface by direct action of the branches 32 of the arc is avoided. Similarly, in electrically conductive workpieces, a transfer of the arc to the workpiece is avoided.

As can be seen in Figure 1, the beam generator 10 has a total of relatively small radial dimensions, so that can be packed tightly integrated into a working head several identical beam generators, which is suitable for treating larger workpiece surfaces. An example of such work head 40 shown in FIG. 2 Instead of the jacket 20 in Figure 1, a one-piece metal block 42 is provided here in which the nozzle openings 24 of the individual beam generators are recessed and the same time forms the associated ring electrodes. The nozzle tubes 16 of Figure 1 are - not visible in Figure 2 - sunk into the metal block 42nd

The nozzle openings 24 are offset in the shown example in two parallel rows in staggered and overlapping each other. When the working head 40 is moved in the direction of arrow A in Figure 2 on the surface to be treated of a planar workpiece, thus the workpiece surface can be pretreated with a largely uniform "plasma curtain" whose width control by connecting or disconnecting individual beam generators as needed leaves.