Methods and apparatus for treating the surface of a workpiece by plasma discharge

A surface of a workpiece is treated by electric discharge. In particular, a concentrated plasma jet is generated through an arc discharge by a non-transferred electric arc while feeding--in a working gas. The surface of the workpiece to be treated is brushed over by the plasma jet.

BACKGROUND OF THE INVENTION 
The invention pertains to a process and apparatus for pre-treating the 
surfaces of workpieces by electric discharge. 
Coating, painting or gluing of workpiece surfaces frequently requires 
pre-treatment of the same to remove impurities and thus change the 
molecule structure of the surface--particularly of workpieces made of 
plastics--so that the surface can be wetted with liquids such as glue, 
paint, etc. 
A known process for pre-treating plastic foil consists of letting a corona 
discharge act on the foil surface. For this purpose the foil is moved 
through a small gap between the corona electrodes. This process, however, 
is only suitable for relatively thin foils. It may furthermore also result 
in an undesired pretreatment of the back of the foil, for example if an 
air bubble is located between the rear electrode and the foil and a 
further discharge takes place inside the air bubble. 
A corona nozzle for the pre-treatment of the surface of thicker foils or 
solid workpieces is described in German Document 43 25 939-C1, with an 
oscillating or circulating air flow that exits between the electrodes that 
produces a vertically expanded discharge zone in which the workpiece 
surface to be treated can be brushed over by the corona discharge brushes. 
This corona nozzle, however, is not suitable for the pretreatment of 
workpieces with relatively deep reliefs, since interior corners, deep 
grooves, etc. cannot be reached at all, or only with difficulty, by the 
flat discharge zone of this nozzle. This known corona nozzle furthermore 
has a relatively complex and bulky design since a motor drive is required 
to produce the oscillating or circulating air flow. 
The objective of the invention is to describe a process for pre-treating 
workpieces by electric discharge that can also be used for workpiece 
surfaces with a relatively complex relief, as well as an apparatus for 
carrying out this process. 
SUMMARY OF THE INVENTION 
This objective is met by the invention which relates to an electric 
discharge process for treating a surface of a workpiece. The process 
comprises the steps of generating, from a working gas, a plasma discharge 
which forms a concentrated jet of a reactive medium, the medium being 
reactive with respect to the surface of the workpiece, and applying the 
jet across the surface of the workpiece. 
The invention further relates to a jet generator for treating a surface of 
a workpiece. The jet generator comprises an electrically insulative nozzle 
pipe having an internal diameter forming a flow channel for working gas. A 
pin electrode is disposed within the nozzle pipe adjacent a rear end of 
the flow channel. A ring electrode extends across a forward end of the 
flow channel and forms a nozzle opening arranged coaxially with the pin 
electrode. The nozzle opening is spaced forwardly from a tip of the pin 
electrode by a distance which is at least two times the internal diameter 
of the nozzle pipe. The flow channel is communicable with a source of 
working gas which flows through the nozzle pipe and the nozzle opening 
from an inlet located adjacent the rear of the flow channel, to generate a 
plasma discharge which forms a concentrated jet of a reactive medium 
extending from the nozzle opening. 
A plurality of independently energizable pin electrodes may be provided to 
form a selected number of jets. 
Thus, according to the invention, a concentrated jet of a reactive medium 
is generated by means of a plasma discharge while feeding in a working 
gas, and the surface to be treated is exposed to this jet. 
This process is suitable for the treatment of both conductive and 
non-conductive workpieces, particularly for the treatment of workpieces 
made of plastic. Furthermore, it has been shown that a jet can be 
generated in the above fashion that is sufficiently chemically active to 
attain an effective surface pre-treatment, while having a temperature that 
is low enough to avoid harming even sensitive surfaces. 
A further advantage of the process consists of the fact that a virtually 
ozone-free pre-treatment can be performed and that the undesired 
pretreatment of the back side of the workpiece can effectively be 
prevented. The undesired electrification of the surface of non-conductive 
workpieces is prevented as well. 
A generator for producing the jet is formed by a pipe-shaped nozzle of 
electrically insulating material through which the working gas flows. The 
opening of the nozzle is surrounded by a ring electrode, and a pin 
electrode is installed inside the nozzle, the tip of which is axially 
recessed from the opening of the nozzle. The plasma arc thus essentially 
extends from the end of the tip of the pin electrode in an axial direction 
of the nozzle pipe, i.e., parallel to the flow of the working gas, to the 
ring electrode. This permits the generation of an intense, pointed and 
relatively sharply concentrated jet in front of the nozzle opening for an 
effective and even pre-treatment, even of hard-to-reach work-piece 
surfaces. The concentration of the jet may be adjusted as required by 
appropriately adjusting the distance between the tip of the pin electrode 
and the opening of the nozzle pipe. 
The nozzle pipe preferably consists of ceramics and has an electrically 
conductive jacket around its outer circumference that is electrically 
connected to the ring electrode or manufactured in one piece with the same 
and extends approximately to the tip of the pin electrode at its opposite 
end. With this design, a relatively low voltage is sufficient to generate 
a corona discharge through the ceramic material to strike the arc 
discharge. To turn on the jet, it is thus only necessary to adjust the 
operating voltage upward, and no significantly increased ignition voltage 
is required. 
The working gas--for example air or argon--is fed into the nozzle pipe, 
preferably near the vicinity of the pin electrode, in a manner so that it 
spins as it flows through the nozzle pipe. A uniform vortex then forms 
inside the nozzle pipe, whereby the arc is channeled inside the core of 
this vortex. Even if the pin electrode is not aligned precisely in a 
coaxial direction inside the nozzle pipe, a very stable arc results that 
extends as a single, sharply delimited branch along the axis of the nozzle 
pipe from the tip of the pin electrode approximately to the opening of the 
nozzle pipe before it splits into several branches radially extending to 
the ring electrode. The point at which the arc splits into its branches 
forms an almost point-shaped source for the reactive jet. In this design, 
the focus and divergence of the jet may also be influenced by varying the 
flow rate of the working gas without having to change the geometric 
configuration of the generator. 
If a working gas with appropriate additives is used, the generator may also 
be used for the "plasma coating" of surfaces. 
A preferred high-frequency alternating voltage in the range of 10 to 30 kV, 
with an optional small superimposed direct voltage component to stabilize 
the discharge, is applied to the electrode. An easily adjustable HF 
generator suitable to produce this alternating voltage is described in 
German Document 42 35 766-C1. The ring electrode is preferably grounded to 
prevent accidents or damage to electrically conductive workpieces caused 
by undesired discharges. 
Since the jacket is grounded as well, the apparatus is shielded against 
exterior interferences and has a high electromagnetic compatibility (EMC). 
According to an advantageous improvement of the invention, several jet 
generators are integrated into a common discharge head in one row, or 
offset in several rows, so that workpieces with large surface areas can be 
evenly pre-treated in an economical fashion. The ring electrodes in this 
design are formed by an electrically conductive block for all generators 
into which the individual nozzle pipes are embedded. The individual 
generators may be placed so close together that the cross sections of the 
individual plasma jets overlap at the level of the workpiece surface to be 
treated in a direction perpendicular to the relative movement of the 
discharge head and workpiece.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
The jet generator 10 shown in FIG. 1 has a pot-shaped housing 12 made of 
plastic with a lateral connection 14 to the supply line for a working gas. 
A nozzle pipe 16 formed of ceramics is held inside the opening of the 
housing 12 in a coaxial position. A pin electrode 18 formed of copper is 
centered inside the housing 12, the tip of which extends into the nozzle 
pipe 16. Outside the housing 12, the outer circumference of the nozzle 
pipe is covered by a jacket 20 formed of electrically conductive material 
forming a ring electrode 22 at the unattached front or lower end of the 
nozzle pipe 16. The ring electrode 22 delimits the nozzle opening 24, the 
diameter of which is somewhat smaller than the interior diameter of the 
nozzle pipe 16 so that the outlet of the nozzle pipe is contracted to a 
certain extent. 
The jacket 20 and therefore also the ring electrode 22 are grounded, and an 
alternating current with a frequency in the range of 20 kHz, the voltage 
of which can be adjusted and is approximately in the range of 5 to 30 kV 
during the operation of the jet generator, is applied between this ring 
electrode and the pin electrode 18 by a high frequency generator 26. 
The connection 14 for the working gas is installed eccentrically in 
relation to the housing 12 (i.e., a center axis of the connection does not 
intersect a center axis of the nozzle pipe 16) so that the supplied 
working gas spins helically as it flows through the nozzle pipe 16, as 
indicated by the helical arrow 28 in FIG. 1. Supported by the contraction 
at the outlet of the nozzle pipe, a stable gas vortex is generated, the 
core of which extends along the axis of the nozzle pipe. 
The electrically conductive jacket 20 extends along the rear or upper end 
at which the housing is mounted, approximately to the tip of the pin 
electrode 18. When the voltage is adjusted upward, an initial Corona 
discharge takes place at the tip of the pin electrode 18. The discharge 
brushes, which have a bluish glow, extend radially to the wall of the 
nozzle pipe 16, and the charge carriers are transported to the jacket 20 
through the ceramic material of the nozzle pipe 16. This corona discharge 
provides the necessary ions to strike an arc discharge from the pin 
electrode 18 to the ring electrode 22 when the voltage is increased. When 
air is used as the working gas, a white-blue glowing arc 30 results that 
extends from the tip of the pin electrode 18 in a sharply delimited, thin 
channel or beam along the axis of the nozzle pipe 16, approximately to the 
center of the outlet 24. Only there does the arc split into several 
branches 32 extending radially to the ring electrode 22. The point at 
which the axial arc 30 splits into its individual branches 32 also forms 
the source of a "flame" that has a slightly gold-colored glow when air is 
used as the working gas and, for the time being, is denoted as plasma jet 
34. 
This plasma jet 34 is used for the pre-treatment of surfaces. In the shown 
example, the plasma jet is used to pre-treat the surface of a workpiece 
36, made of plastic, in the range of a groove 38. It is apparent that the 
plasma jet 34 enters into the groove so that the usually hard-to-reach 
bottom of the groove can be effectively pre-treated. 
Whether this "flame" referred to here as plasma jet 34, really is a plasma 
in the true sense, i.e., an at least partially ionized medium, is not 
completely clear. Attempts were made to prove the electrical conductivity 
of this medium by holding the ends of two conductors into the flame, with 
one conductor directly connected to a battery and the other conductor 
connected to the battery through a light bulb. The light bulb only lit up, 
however, when one of the branches 32 of the arc sparked over to the 
conductor ends and connected the same. The conductivity of the plasma jet 
34 is thus considerably lower than that of the plasma within the arc. It 
is possible that the "flame" consists of only slightly ionized plasma or a 
medium that merely contains free radicals or excited atoms or molecules. 
It has been proven without a doubt, however, that the plasma jet 34 has 
the desired pre-treatment effect on the surface of the workpiece placed 
under the jet. Several plastic surfaces that are not normally wettable 
with water were placed under the plasma jet 34 and subsequently coated 
with water. The surface areas treated with the plasma jet 34 could then be 
wetted with water. This effect is also apparent with strongly fluorinated 
polymers, such as PTFE. 
It was also possible to effectively treat metal surfaces with the help of 
the plasma jet 34 and remove, for example, silicone oil residues, etc. 
Experiments with non-precious metals furthermore revealed that the plasma 
jet 34 has virtually no oxidizing effect. Even the treatment of aluminum 
did not produce any oxide layer. 
The temperature of the plasma jet 34 is relatively low. Similar to a candle 
flame, it is possible to move one's finger through the plasma jet at a 
moderate speed without sustaining any burns. 
In the design examples examined so far, the nozzle pipe 16 has an interior 
diameter of approximately 8 mm, and the axial distance between the tip of 
the pin electrode 18 and the nozzle opening 24 is approximately 55 mm. The 
interior diameter of the nozzle opening 24 is approximately 5 mm. Under 
these conditions, a plasma jet 34 is obtained having a length of 
approximately 30 mm and a maximum diameter of approximately 5 mm. The 
concentration, and thus also the reach, of the plasma jet may be improved 
by increasing the distance between the pin electrode and the nozzle 
opening 24. The contraction at the nozzle opening 24 also appears to have 
a positive effect on the concentration of the plasma jet. 
When the flow rate of the working gas through the nozzle pipe 16 is 
increased, the source of the plasma jet moves further out, i.e., in the 
direction of the workpiece, and the flame becomes longer and thinner. At 
the same time, the outward curvature shown in FIG. 1 of the radial 
branches 32 of the arc increases. When the flow of the working gas is 
decreased or shut off completely, on the other hand, the axial arc 30 
extends until it eventually fills newly the entire interior of the nozzle 
pipe. The plasma jet 34 becomes shorter and its source is no longer 
point-shaped but flatly distributed across the cross section of the nozzle 
opening 24. At a moderate gas flow it is noticeable that the arc 30 
follows the spinning gas flow inside the nozzle pipe. When the flow rate 
of the gas is gradually increased, the arc 30 is increasingly compressed 
in a radial direction and fixed to the axis of the nozzle pipe. It thus 
appears that the arc 30 is channeled through the core of the gas vortex. 
Because of this effect, a very stable plasma jet 34 is obtained that 
originates from a point-shaped source immediately in front of the center 
of the outlet 24, if the gas flow is maintained at a sufficiently high 
level. The location and size of the surface area to be treated, as well as 
the intensity of the plasma treatment can be controlled precisely by 
adjusting the position of the workpiece 35 in relation to the jet 
generator 10 as required. 
Because of the relatively large length of the plasma jet 34, the distance 
between the workpiece and the outlet 24 of the plasma burner can be 
adjusted large enough to prevent any damage to the surface due to a direct 
action of the branches 32 of the arc on the surface. Similarly, a transfer 
of the arc onto electrically conductive workpieces is also prevented. 
As can be seen from FIG. 1, the total radial dimensions of the jet 
generator 10 are relatively small, so that several similar jet generators 
can be tightly packed into one discharge head suitable for the treatment 
of larger workpiece surfaces. An example for such a discharge, head 40 is 
shown in FIG. 2. In place of the jacket 20 in FIG. 1, a one-piece metal 
block 42 was used in this design, inside which the nozzle openings 24 of 
the individual jet generators have been left open and which also forms the 
respective ring electrodes. The nozzle pipes 16 shown in FIG. 1--not 
visible in FIG. 2--are embedded in the metal block. 
In the example shown, the nozzle openings 24 are arranged in two parallel 
rows in offset positions and overlapping each other. When the discharge 
head 40 is moved in the direction of the arrow A in FIG. 2 along the 
surface of a workpiece to be treated, the workpiece surface can thus be 
pre-treated with a virtually even "plasma curtain", the width of which can 
be adjusted as required by turning additional jet generators on or off. 
Although the present invention has been described in connection with 
preferred embodiments thereof, it will be appreciated by those skilled in 
the art that additions, deletions, modifications, and substitutions not 
specifically described may be made without departing from the spirit and 
scope of the invention as defined in the appended claims.