Method for improving the wettability of the surface of articles

The invention relates to a process for the Increasing the wettability of workpieces with fluids by Surface Preparation electric discharge.

If workpiece surfaces coated, painted or to be glued, is often a pre-treatment required by the impurities be removed from the surface and thereby the - particularly in Workpieces made of plastic - the molecular structure is changed so that the Surface wetted with liquids, such as adhesives, paints and the like can be.

A known method for the pretreatment of plastic films is is allowed to act a corona discharge on the film surface. For this purpose, the film is through a narrow gap between the Corona electrodes guided. However, this method is relatively only thin films applicable. In addition, there may be an undesirable Pretreatment of the back of the film come, for example, when between the rear electrode and the film, an air bubble is, in it will proceed further discharge.

To pre-treating the surface of thicker films or solid workpieces A corona nozzle described in DE 43 25 939 C1, in between the electrodes an oscillating or rotating guided airflow outlet, so as to obtain an areal discharge zone in which the treated swept surface of the workpiece with the corona discharge tufts can be. 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 with the planar extent Discharge zone of this nozzle are difficult or impossible to achieve. also has this known corona nozzle a relatively expensive and bulky construction, since for the generation of the oscillating or rotating Airflow, a motor drive is required.

The object of the invention is a method for forehand Lung workpiece surfaces indicate by means of electrical discharge, which is also in is applicable workpiece surfaces with a relatively complicated relief.

This object is inventively achieved in that by plasma discharge while supplying a working gas a collimated beam of a reactive medium is generated and the surface to be treated with this Beam sweeps.

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

Further advantages of the method are that a practically ozone-free Pretreatment can be performed and an undesirable pretreatment the back can be implemented to prevent. In non-conductive workpieces avoids an undesirable surface charging.

A method for generating plasma as such in various embodiments known. For example, US-A-3515839 a plasma torch, which is used for welding, plasma spraying, cutting and the like. US-A-3,849,584 describes a device having a plurality of plasma torches for melting of metals. DE-C-685 455 describes an arc fan for generating extremely high temperatures. In this fan, the working gas is supplied twist-shaped so that it and the pin between an electrode Arc a grid generated flows around eddying. FR-A-1,181,788 describes a method for arc discharge, wherein the protective gas from a transmitted flow around the arc directly to the metal to be eroded becomes. US Patent 3,579,027 describes an ignition of a plasma torch, in which first a conical discharge between the tip of a rod-shaped produced electrode and a surrounding tubular electrode becomes. US-A-2892067 describes a high-temperature plasma burner, wherein blown out the arc by means of the working gas from the nozzle of the burner becomes.

None of these publications contains the proposal, a plasma with relatively low temperature for pretreating workpiece surfaces use.

Advantageous developments and refinements of the inventive method yield themselves. from the dependent claims

The nozzle pipe preferably consists of ceramic material and is on the outer Periphery surrounded with an electrically conductive sheath electrically connected is connected to the ring electrode or formed in one piece with this and at the opposite end to about the level of the tip of the pin electrode extends. In this case already with relatively low voltage through the ceramic material through a corona discharge generated be, through which the arc discharge is ignited. To turn on the Beam needs therefore to be only highly regulated operating voltage, and it is not substantially increased ignition voltage required.

The working gas - for example, air or argon - is preferably in the amount the pin electrode thus introduced into the nozzle tube that swirl shaped by the nozzle tube flows. In the nozzle tube then forms a uniform Eddy whose vortex core channels the arc. Even if not exactly coaxial 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 tip of the rod 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 where the arc fans out in the sub-branches, forms an almost punctate Source of the reactive jet. The "focus" and divergence 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, can be of Generator also used to "plasma coating" of surfaces.

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

By likewise grounded casing is a shield of the discharge process against external interference and a high electromagnetic Compatibility (EMC) reaches the device.

It is advantageous in a multiple Row or in several rows in a staggered staggered beam generators to integrate into a common operating head, so that area extended Workpieces are treated equally in a rational way can. The ring electrodes are in this case by all Generators formed 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 at the height of the workpiece to be machined surface in the Direction overlap transversely to the relative movement of the working head and the workpiece.

In the following preferred embodiments of the invention will become apparent the drawing explained.

Show it:<dl tsize="7"><dt>Fig. 1</dt><dd>a schematic longitudinal section through a Ray generator; 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 plastic, the side with a terminal 14 for supplying a working gas is provided. In the opening of the housing 12 coaxially a nozzle tube 14 held in ceramic. Inside the housing 12 is a centrally Pin electrode 18 arranged in copper, headed into the nozzle pipe 16 protrudes. The outer periphery of the nozzle tube is outside of the housing 12 surrounded by a shell 20 of electrically conductive material, the at the free end of the nozzle pipe 16 a ring electrode 22 is formed. 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 therefore the ring electrode 22 are grounded, and between this ring electrode and the pin electrode 18 is by means of a high-frequency generator 26 is an alternating voltage with a frequency of the order applied at 20 kHz, whose voltage is controllable, and while the operation of the beam generator such as in the order of magnitude 5-30 kV.

The connection 14 for the working gas is eccentric with respect to the housing 12 arranged so that the supplied working gas swirl shaped by the nozzle tube 16 flows, as indicated by the arrow 28 in FIG. 1 Supports through the constriction at the outlet of the nozzle pipe, a stable forms Gas vortices whose Wirbelkem extends along the axis of the nozzle tube.

The electrically conductive jacket 20 covers the housing-side end approximately to the height of the tip of the pin electrode 18. High rules Voltage occurs at the tip of the pin electrode 18 initially to a Corona discharge. extend the bluish glowing discharge tufts radially on the wall of the nozzle tube 16, and the transport of the charge carriers done to the shell 20 by the ceramic material of the nozzle tube 16 therethrough. These corona discharge provides the necessary ions, through the with increasing voltage arc discharge from the pin electrode 18 to Ring electrode 22 is ignited. arises when using air as the working gas a white-blue luminous arc 30, which extends from the tip Pin electrode 18 into a sharply defined thin channel along the axis of the nozzle pipe 16 extends to approximately the middle of the outlet opening 24th Only there the arc is divided into several sub-branches 32, radially lead to the ring electrode 22nd The point at which the axial arc 30 branches to the individual sub-branches 32, also forms the origin a gold weak when using air as the working gas bright "flame" which is tentatively interpreted as plasma jet 34th

This plasma jet 34 is used for the pretreatment of surfaces. in the example shown is the plasma jet for pretreating the surface a workpiece 36 made of plastic in the area of ​​a groove 38. It recognized, that the plasma jet 34 enters the groove 38 so that the otherwise inaccessible bottom of the groove can be effectively treated.

Whether it is in here as a plasma jet 34 designated "Flame" actually a plasma in the strict sense, ie, to an at least partially ionized medium is, is not fully secured. It has been tried, detecting the electrical conductivity of the medium by the Ends of two conductors, one of which is directly and the other via a light bulb was connected to a battery, the flame was kept. The However lightbulb lit only when one of the branches 32 of the arc jumped on the conductor ends and this union. conductivity of the plasma beam 34 is thus substantially lower than that of the plasma within of the arc. It is possible that in the "flame" only a weakly ionized plasma or a medium containing only free includes radicals or excited atoms or molecules. However, it has be proved beyond doubt that the plasma beam 34 the desired pretreating effect on products placed on the beam workpiece surfaces Has. There were various plastic surfaces, which normally non-wettable with water, brought into the plasma jet 34 and then covered with water. The 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 by means of the plasma beam 34 vorhandelt and example of silicone oil residues and the like be freed. 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 through the Plasma jet move, without causing burns.

The nozzle tube 16 has at the previously examined embodiments Inner diameter of about 8 mm, and the axial distance between the Tip of the pin electrode 18 and the nozzle opening 24 is about 55 mm. The inner diameter of the nozzle opening 24 is about 5 mm. Under these Conditions obtained a plasma beam 34, which has a length of about 30 mm and a maximum diameter of about 5 mm has. By Increase the distance between the pin electrode and the nozzle opening 24, the concentration and thus the range of the plasma jet improves will. Also, the constriction at the nozzle opening 24 seems to be the to promote pooling of the plasma jet.

If the throughput of the working gas through the nozzle tube 16 increases is the origin of the plasma jet shifted further outwardly, ie, toward the workpiece, and the flame is longer and slimmer. Same time, the visible in Figure 1 Away-curvature of the radial Branches 32 of the arc to. If one the other hand, the flow of the working gas throttles or shuts off completely, the axial arc expands 30 on, until it finally fills almost the entire interior of the nozzle pipe. The plasma jet 34 is thereby shorter, and its origin is no longer point but surface distributed over the cross section of the nozzle opening 24th At moderate gas flow rate can be seen that the arc 30 within the nozzle tube of the swirl-shaped gas flow follows. With gradual Increase the gas flow rate, the arc 30 in the radial direction increasing compressed and fixed on the axis of the nozzle tube. The arc 30 thus seems channeled through the core of the gas vortex to will. Due to this effect is obtained with sufficiently high gas throughput a very stable plasma jet 34 of a point-like Originating tightly starts before the center of the outlet 24th By appropriate Selecting the position of the workpiece 35 in respect to the beam generator 10 can be treated as the place and the extent of the treated surface area and the intensity of the plasma treatment precisely control.

Due to the relatively large length of the plasma jet 34, the distance can between the workpiece and the outlet opening 24 of the plasma torch so choose great that damage to the surface by direct action of the branches 32 of the arc is avoided. Also, in electrically conductive workpieces, a transfer of the arc on the Workpiece avoided.

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

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