EP0809720B1 - Pulse-modulated d.c. voltage application process - Google Patents

Description

The present invention relates to a method and an apparatus for Coating objects using direct current.

1. Funkenbildung auch unterhalb der Lackoberfläche an der zu beschichtenden Blechoberfläche.

2. Starke Elektrolyse.

3. Schichtdickenabnahme.

4. Flockenbildung in der Schaumschicht und an den Blechkanten.

5. Nach Erzeugung eines Abrisses ist eine höhere Reduzierung der Spannung erforderlich, um bei dem nächsten zu beschichtenden Teil diese Erscheinung sicher zu vermeiden.

Methods for the deposition of layers on objects by means of a more or less pronounced pulsation voltage are known from the prior art. Using thyristor-controlled rectifiers, for example, unregulated voltage peaks in the microsecond range are generated. These voltage peaks are pure interference pulses and are not used as a reproducible method to influence the deposition result. When working with poorly smoothed thyristor rectifiers, the following disadvantages are also symptomatic:

1. Spark formation also below the paint surface on the sheet metal surface to be coated.

2. Strong electrolysis.

3. Decrease in layer thickness.

4. Flocculation in the foam layer and on the sheet edges.

5. After creating a tear, a higher reduction in the tension is necessary in order to reliably avoid this phenomenon in the next part to be coated.

From Brown, William B. (Journal of Paint Technology Vol. 47. No. 605. June 1975) it is known to have a rectangular pulse shape in the seconds range Generate interruption (shutdown) of the separation current. This procedure has some disadvantages. The specified pulse times are in the range of seconds, preferably up to 3 - 20 seconds. During these longer breaks, Dissipated heat and thereby the sheet resistance increased. On the other hand, sets also a redissolving effect and thus a softening of the deposited film and the removal of the gas bubbles due to the paint flow. This has one Reduced film resistance.

The reduction of heat development and the peak current must be in this Fall by slowly increasing the voltage. You start with pulse-shaped square-wave voltage immediately with full coating voltage, so must the power output of the rectifier can be more than doubled. Increase with it especially the cost of the rectifier.

The rectifier generators currently on the market point to this significant disadvantages. Depending on the design, they have one Ripple, which depends on the type and quality of rectification and smoothing of the AC input voltage depends (see Vincent, Journal of Coatings Technology Vol. 62, No. 785, June 1990). In addition, this residual ripple is load-dependent, i.e. one obtains a feedback about the coating process itself Ripple is then only interpreted as a disturbance.

From T. Ito and K. Shibuya, Metal Finishing, April 1967, pp. 48-57, "Anodic Behavior in Electrophoretic Coating of Aluminum Alloys "is known to be pulsed Generate signals by more or less slippery smoothed alternating current. Procedures with AC separations are also from the German Laid-open specification 1646130 and British patent application 1376761 known. Here, anode sheets are used as rectifiers. The Due to the special coating, anode sheets are only in one direction passes current.

US-A-3 702 813 describes coating objects using of direct current, the alternating current is superimposed, where among other things the ratio the current strengths can be selected so that a voltage always in the same direction prevails.

FR-A-1 534 494 also deals with the coating of objects and discloses various current profiles for this purpose. These either change theirs Direction or go back to zero, or there is a constant overlay of AC and DC instead.

However, all of the methods described have significant shortcomings to date. In particular, tear behavior, wrapping, layer thickness and film disturbances are e.g. in electrocoating etc. depending on the level of tension. In the In practice, it is regularly chosen so that a adequate cavity coating with minimal necessary outer layer thickness is achieved. To save paint material and thus costs in painting, is u.a. endeavors, with reduced outer layer thicknesses, an adequate To get wrap. With the current products and today's ones above described technology, there are limits to this development.

The present invention has therefore set itself the task Device for electrochemically coating objects for To provide systematically with the paint film properties and the application properties can be influenced, e.g. at reduced outer layer thicknesses to get sufficient wrap, or to to achieve pre-networking in the application.

This object is achieved in that an adjustable DC voltage Overlay with adjustable AC voltage components, pulse-modulated becomes.

The adjustable AC voltage components are preferably generated from periodic signals, in particular harmonic vibrations (sine vibrations), which are readily available.
According to the invention, it is possible through suitable circuits to subject the periodic signals to preprocessing, preferably blocking the negative voltage components or rectifying them.

It is further provided according to the invention that the superimposition of the DC voltage with the AC voltage components can be switched on and off in an adjustable duty cycle. In this way, the pulse modulation, as a variation of the conventional coating method with pure direct current, can be limited to specific time segments of the coating, for example the beginning or the end.
The preferred duty cycles of On: Off are the ranges between 10: 1 and 1:10. The duration of the "on" period in which pulse modulation takes place is between 10 ms and 100 s.

The DC voltages used according to the invention are in the Range from 0 to 500 V. Likewise, the ones that come to be superimposed AC components between 0 and 500 V. The Superposition in such a way that the resulting tension does not change direction, that is is a pulse modulated DC voltage. The device according to the invention is however, not limited to this, so that a result can also be obtained AC voltage can be worked if this results in advantages.

The period of the periodic used for the overlay According to the invention, AC voltage components are between 1 and 500 ms. This corresponds to a frequency of 1000 to 2 Hz Frequency worked, which results from the mains voltage, e.g. 50 Hz or their multiples.

For generating a pulse-modulated DC voltage according to the invention there are different possibilities.

A variant is a series connection of an alternating current (stell) transformer with a DC generator.

It is also possible to couple the alternating current (stell) transformer to make a rectifier so that a rectified AC voltage is injected. If doing so between AC source and input of the rectifier is connected to a diode, another becomes Voltage modulation is achieved in such a way that only the positive or only that negative half-waves reach the rectifier.

The pulse modulation can optionally be switched on in such a way that the Coupling the AC voltage components via a mechanical or electronic relay takes place. The latter can be used to achieve a defined Duty cycle via a function generator (i.e. with low current) can be controlled.

Another variant of the generation of a pulse-modulated DC voltage according to the invention is obtained by connecting a function generator to the phase control of a three-phase rectifier. In this way, costs and space for an additional AC generator are eliminated.
The function generator can be a commercially available electronic device. It is preferably implemented as a programmable microprocessor system, particularly preferably by means of a computer with appropriate software, with an analog / digital converter for receiving the control voltage and an output unit for the trigger pulses.

A preferred use of the device according to the invention takes place in the Electro dip painting instead. This depends on the processing time amount of paint deposited directly from the amount of charge that has flowed - and thus indirectly from the immersion voltage. It should be noted that the so-called tear-off stress due to heating and boiling a gas layer arises, which can interrupt the flow of electricity. It is also important to have an even one and sufficient layer thickness of the lacquer even in inaccessible places to get, i.e. sufficient wrap with reduced outer layer thickness. The method according to the invention surprisingly becomes an optimized one Result in relation to this partly conflicting requirements achieved.

In Figur 1 sind der Gleichspannungsgenerator 2 und der galvanisch entkoppelte Wechselstromstelltransformator 1 abgebildet. Gemäß Figur 1 ist die Kopplung, die über einen Schalter c wahlweise ein- und ausgeschaltet werden kann, über den Gleichrichter 3 erfolgt. Je nachdem, ob die Diode b über den Schalter a überbrückt wird oder nicht, werden am Gleichrichter alle Halbwellen oder nur die positiven Halbwellen gleichgerichtet. Die jeweils resultierende pulsmodulierte Spannung ist in Diagramm a) (Schalter a offen) bzw. b) (Schalter a geschlossen, Diode überbrückt) in Figur 1 dargestellt.
Die aktuellen Werte von Strom und Spannung können durch ein Meßsystem 6 erfaßt und überwacht werden. Das Elektrotauchbad ist mit der Ziffer 7 bezeichnet.

Figur 2 zeigt eine Variante der Schaltung von Figur 1, bei der sich anstelle der Elemente a, b und c ein Halbleiterrelais 4 zwischen Stelltrafo 1 und Gleichrichter 3 befindet. Dieses Halbleiterrelais 4 wird durch einen Funktionsgenerator 5 gesteuert. Dadurch wird die Pulsmodulation in einem definierten Tastverhältnis ein- und ausgeschaltet. Das Diagramm a) am unteren Rand von Figur 2 zeigt schematisch die resultierende pulsmodulierte Spannung U_ges in Abhängigkeit von dem Signal U_St des Funktionsgenerators.

Figur 3 gibt eine Schaltung wieder, bei der der Funktionsgenerator 8 in die Phasenanschnittsteuerung 9 eines Thyristorbrückengleichrichters 10 für eine Drehstromquelle 11 eingreift. Hierdurch wird periodisch zwischen zwei Phasenwinkeln F₁ und F₂, die zwei Ausgangsspannungen U₁ und U₂ entsprechen, umgeschaltet. Die Pulse haben dann die in dem Diagramm a) von Figur 3 gezeigte Form von geglätteten Drehstrompulsen bei zwei Spannungsniveaus. Die Restwelligkeit der Signale kann durch die Dimensionierung der Glättung 12 eingestellt werden. Selbstverständlich ist es mit dieser Schaltungsanordnung auch möglich, über den Funktionsgenerator zwischen mehr als 2 Spannungsniveaus umzuschalten.

Figur 4 zeigt eine weitere Variante der erfindungsgemäßen Vorrichtung mit einer Reihenschaltung von Gleich- und Wechselstromgenerator, bei der die Diode 13 hinzugefügt worden ist.

The invention is described in more detail below with reference to the figures:

1 shows the DC voltage generator 2 and the galvanically decoupled AC variable transformer 1. According to FIG. 1, the coupling, which can optionally be switched on and off via a switch c, is carried out via the rectifier 3. Depending on whether the diode b is bridged via the switch a or not, all half-waves or only the positive half-waves are rectified on the rectifier. The resulting pulse-modulated voltage is shown in diagram a) (switch a open) or b) (switch a closed, diode bridged) in FIG. 1.
The current values of current and voltage can be recorded and monitored by a measuring system 6. The electrodeposition bath is marked with the number 7.

Figure 2 shows a variant of the circuit of Figure 1, in which instead of the elements a, b and c there is a semiconductor relay 4 between the transformer 1 and rectifier 3. This semiconductor relay 4 is controlled by a function generator 5. This switches the pulse modulation on and off in a defined duty cycle. The diagram a) at the lower edge of FIG. 2 schematically shows the resulting pulse-modulated voltage U _tot as a function of the signal U _{St from} the function generator.

FIG. 3 shows a circuit in which the function generator 8 intervenes in the phase control 9 of a thyristor bridge rectifier 10 for a three-phase source 11. This periodically switches between two phase angles F ₁ and F ₂ , which correspond to two output voltages U ₁ and U ₂ . The pulses then have the form of smoothed three-phase pulses shown in the diagram a) of FIG. 3 at two voltage levels. The residual ripple of the signals can be adjusted by dimensioning the smoothing 12. Of course, it is also possible with this circuit arrangement to switch over more than 2 voltage levels via the function generator.

FIG. 4 shows a further variant of the device according to the invention with a series connection of direct current and alternating current generator, in which the diode 13 has been added.

In the examples described below, the rectifier circuit used according to Figure 1. The maximum achievable in the experimental setup Current was limited to 6 A on average by the variable transformer. Then by reducing the active area of the metal sheets to be coated reached the required current density.

Test program for examples 1 to 5:

Coating of metal materials with various paints (commercial products from BASF Lacke und Farben AG) Qualities: FT 85-7042 CATHODIP ® FT 82-7627 CATHOGUARD ® FT 82-7640 CATHOGUARD 350 ® FT 25-7225 CATHOGUARD 1008 ®

Deposition conditions:

Beispiel 1:

Zwei 10 ms Pulshalbwellen in 20 ms (quasi 100 Hz)

Beispiel 2:

Eine 10 ms Pulshalbwelle in 20 ms (quasi 50 Hz) Schalterstellungen a) + b) mit 0,30, 60, 150, 250 V

Beispiel 3:

1 Pulshalbwelle; 10 s Pulsspannung, 110 s Gleichspannung (Pulse: 60, 150, 250 V)

Beispiel 4:

1 Pulshalbwelle; 10 s Gleichspannung, 110 s Pulsspannung (Pulse: 60, 150, 250 V)

Beispiel 5:

1 Pulshalbwelle; 60 s Gleichspannung, 60 s Pulsspannung (Pulse: 60, 150, 250 V)

DC voltage: voltage series up to demolition in 20 V steps

Example 1:

Two 10 ms pulse half-waves in 20 ms (quasi 100 Hz)

Example 2:

A 10 ms pulse half-wave in 20 ms (quasi 50 Hz) switch positions a) + b) with 0.30, 60, 150, 250 V

Example 3:

1 pulse half-wave; 10 s pulse voltage, 110 s DC voltage (pulses: 60, 150, 250 V)

Example 4:

1 pulse half-wave; 10 s DC voltage, 110 s pulse voltage (pulses: 60, 150, 250 V)

Example 5:

1 pulse half-wave; 60 s DC voltage, 60 s pulse voltage (pulses: 60, 150, 250 V)

Evaluation: tear-off tension, layer thickness SD Test results: Example 1:

Pulse modulation with two pulse half-waves is set (frequency quasi 100 Hz, cf. Diagram a) in Figure 9). The results are shown in Figure 5 and the Tables 1 and 2 (column 1) reproduced. Up to a strength of 60 V, the Breaking tension determined by the peak tension reached. Partially the pulse proportion increased to 250 V. As a result, peak voltages could be reached that are sometimes 40 - 50 V above that of a pure DC voltage separation.

Example 2:

Pulse modulation with a pulse half-wave was set (frequency quasi 50 Hz, cf. Diagram b) in Figure 9). The results are shown in FIG. 6 and in FIG Tables 1 and 2 (column 2) reproduced. By reducing the Pulse frequencies became significantly higher peak voltages for all products possible. This effect already started with voltage pulses of 30 V and took off increasing pulse strength. The difference increased with voltage pulses of 150 - 250 V. between the breakdown voltage of a DC voltage separation and the possible Voltage peaks to values of 70 - 80 V. The layer thickness at 20 V below The breakdown voltage decreased with increasing pulse proportion.

Example 3:

There were pulse-modulated DC voltage (quasi 50 Hz) followed by 10 s 110 s pure DC voltage coatings carried out (diagram c) in FIG 9). The results are shown in Figure 7 and Tables 1 and 2 (column 3) and are similar to that of Example 2, in which the DC voltage during the entire coating of voltage pulses was superimposed.

Example 4:

There was a coating with 10 s DC and then 110 s DC voltage with superimposed pulse voltage (quasi 50 Hz) carried out (Diagram d) in Figure 9). The corresponding results are shown in Tables 1 and 2 (column 4). In contrast to example 3, the Voltage pulses only switched on after a coating time of 10 s. Through this Variation, a further increase in the peak voltage could be achieved. at FT 82-7627 expressed this effect in improvements of a maximum of 20 V; at FT 82-7640 20-40 V higher voltage peaks occurred. The clearest FT 25-7225 showed a change with voltage increases up to 60 V.

Example 5:

There were 60 s DC voltage and 60 s DC voltage with superimposed Pulse voltage set (diagram d) in Figure 9). The results were with Example 4 identical (see Column 5 of Tables 1 and 2).

Example 6:

A series resistor was integrated in the test setup. The results are there Figure 8 again. By using the series resistor, the otherwise became observing reduction in layer thickness with increasing pulse voltage amplitude up to 150 V no longer detected. Tables 3 and 4 show those relating to FIG. 8 associated data.

Results of Examples 1 to 6:

In all graphics the layer thicknesses are below 20 V below the Breaking tension can be reached, noted on the respective bars. From this is can be seen that with increasing pulse strength the achievable layer thickness with Exception from the test conditions of example 6 decrease. This effect is up to a pulse strength of 150 V a few µm. The concerned Layer thicknesses are summarized in Table 2.

1. Die Summenspannung kann erheblich über die Abrißspannung herkömmlicher Verfahren erhöht werden, bevor ein Abriß auftritt.

2. Die Spannung, die zum Erreichen einer bestimmten Schichtdicke angelegt werden muß, kann durch das erfindungsgemäße Verfahren über die Einstellung des Verhältnisses von Pulsspannungs- und Gleichspannungsanteil in einem weiten Bereich variiert werden. Einfluß des Wechselspannungsanteils auf die Abrißspannung 100 Hz 50 Hz 50 Hz 10s Puls + 110s Gleich. 50 Hz 10s Gleich. +110s Puls 50 Hz 60s Gleich. +60s Puls FT 85-7082 Gleichspannung 400 Volt 380 V 380 V 380 V 380 V Gleichsp. + 30 V Wechselsp. 360-390 V 380-410 V Gleichsp. + 60 V Wechselsp. 340-400 V 360-420 V 360-420 V 380-440 V 380-440 Gleichsp. + 150 V Wechselsp. 300-450 V 300-450 V 300-450 V 320-470 V Gleichsp. + 250 V Wechselsp. FT 82-7627 Gleichspannung 360 V 360 V 360 V 360 V 360 V Gleichsp. + 30 V Wechselsp. 340-370 V 350-380 V Gleichsp. + 60 V Wechselsp. 320-380 V 320-400 V 340-400 V 360-420 V 360-420 V Gleichsp. + 150 V Wechselsp. 260-410 V 280-430 V 260-410 V 300-450 V 300-450 V Gleichsp. + 250 V Wechselsp. 160-410 V 200-450 V 200-450 V 200-450 V 200-450 V FT 82-7640 Gleichspannung 350 V 350 V 350 V 350 V 350 V Gleichsp. + 30 V Wechselsp. 330-360 V 340-370 V Gleichsp. + 60 V Wechselsp. 300-360 V 320-380 V 310-370 V 360-420 V 350-410 V Gleichsp. + 150 V Wechselsp. 240-390 V 260-410 V 240-390 V 300-450 V 300-450 V Gleichsp. + 250 V Wechselsp. 120-370 V 160-410 V 180-430 V 180-430 V FT 25-7225 Gleichstrom 340 V 320 V 320 V 320 V 320 V Gleichsp. + 30 V Wechselsp. 300-330 V 300-330 V Gleichsp. + 60 V Wechselsp. 280-340 V 280-340 V 280-340 V 300-360 V 300-360 V Gleichsp. + 150 V Wechselsp. 240-390 V 260-410 V 300-450 V 300-450 V Gleichsp. + 250 V Wechselsp. Schichtdicke SD, die 20 V unter der Abrißspannung erreicht wird (Variation des Gleichspannungs- und Wechseispannungsanteils) 100 Hz 50 Hz 50 Hz 10s Puls + 110s Gleich. 50 Hz 10s Gleich. +110s Puls 50 Hz 60s Gleich 60s Puls FT 85-7042 Gleichspannung 1 22 µm 22 µm 22 µm 22 µm 22 µm Gleichsp. + 30 V Wechselsp 20 µm 22 µm Gleichsp. + 60 V Wechselsp. 19 µm 20 µm 19 µm 22 µm 22 µm Gleichsp. + 150 V Wechselsp. 18 µm 16 µm 22 µm 19 µm Gleichsp. + 250 V Wechselsp FT 82-7627 Gleichspannung 26 µm 26 µm 26 µm 26 µm 26 µm Gleichsp. + 30 V Wechselsp. 25 µm 24 µm Gleichsp. + 60 V Wechselsp. 25 µm 23 µm 25 µm 25 µm 25 µm Gleichsp. + 150 V Wechselsp. 24 µm 22 µm 23 µm 27 µm 25 µm Gleichsp. + 250 V Wechselsp. 23 µm 21 µm 16 µm 21 µm 17 µm FT 82-7640 Gleichspannung 33 µm 33 µm 33 µm 33 µm 33 µm Gleichsp. + 30 V Wechselsp. 33 µm 33µm Gleichsp. + 60 V Wechselsp. 30 µm 31 µm 28 µm 34 µm 33 µm Gleichsp. + 150 V Wechselsp. 31 µm 27 µm 22 µm 34 µm 27 µm Gleichsp. + 250 V Wechselsp. 17 µm 22 µm 22 µm 19 µm FT 25-7225 Gleichstrom 17 µm 15 µm 15 µm 15 µm 15 µm Gleichsp. + 30 V Wechselsp. 16 µm 13 µm Gleichsp. + 60 V Wechselsp. 16 µm 13 µm 13 µm 14 µm 13 µm Gleichsp. + 150 V Wechselsp. 12 µm 11 µm 15 µm 13 µm Gleichsp. + 250 V Wechselsp.

According to the results reproduced above, the novel process has the following advantages:

1. The total voltage can be increased significantly above the breakdown voltage of conventional methods before a breakdown occurs.

2. The voltage which must be applied to achieve a certain layer thickness can be varied within a wide range by the method according to the invention by adjusting the ratio of the pulse voltage and DC voltage components. Influence of the AC voltage component on the breaking voltage 100 Hz 50 Hz 50 Hz 10s pulse + 110s equal. 50 Hz 10s equal. + 110s pulse 50 Hz 60s equal. + 60s pulse FT 85-7082 DC 400 volts 380 V. 380 V. 380 V. 380 V. Gleichsp. + 30 V alternating voltage 360-390 V 380-410 V Gleichsp. + 60 V AC 340-400 V 360-420 V 360-420 V 380-440 V 380-440 Gleichsp. + 150 V AC 300-450 V 300-450 V 300-450 V 320-470 V. Gleichsp. + 250 V AC FT 82-7627 DC 360 V 360 V 360 V 360 V 360 V Gleichsp. + 30 V alternating voltage 340-370 V. 350-380 V Gleichsp. + 60 V AC 320-380 V 320-400 V 340-400 V 360-420 V 360-420 V Gleichsp. + 150 V AC 260-410 V 280-430 V 260-410 V 300-450 V 300-450 V Gleichsp. + 250 V AC 160-410 V 200-450 V 200-450 V 200-450 V 200-450 V FT 82-7640 DC 350 V 350 V 350 V 350 V 350 V Gleichsp. + 30 V alternating voltage 330-360 V 340-370 V. Gleichsp. + 60 V AC 300-360 V 320-380 V 310-370 V. 360-420 V 350-410 V Gleichsp. + 150 V AC 240-390 V. 260-410 V 240-390 V. 300-450 V 300-450 V Gleichsp. + 250 V AC 120-370 V. 160-410 V 180-430 V 180-430 V FT 25-7225 direct current 340 V 320 V 320 V 320 V 320 V Gleichsp. + 30 V alternating voltage 300-330 V 300-330 V Gleichsp. + 60 V AC 280-340 V. 280-340 V. 280-340 V. 300-360 V 300-360 V Gleichsp. + 150 V AC 240-390 V. 260-410 V 300-450 V 300-450 V Gleichsp. + 250 V AC Layer thickness SD, which is reached 20 V below the breakdown voltage (variation of the DC and AC voltage component) 100 Hz 50 Hz 50 Hz 10s pulse + 110s equal. 50 Hz 10s equal. + 110s pulse 50 Hz 60s Equal to 60s pulse FT 85-7042 DC 1 22 µm 22 µm 22 µm 22 µm 22 µm Gleichsp. + 30 V alternating voltage 20 µm 22 µm Gleichsp. + 60 V AC 19 µm 20 µm 19 µm 22 µm 22 µm Gleichsp. + 150 V AC 18 µm 16 µm 22 µm 19 µm Gleichsp. + 250 V AC FT 82-7627 DC 26 µm 26 µm 26 µm 26 µm 26 µm Gleichsp. + 30 V alternating voltage 25 µm 24 µm Gleichsp. + 60 V AC 25 µm 23 µm 25 µm 25 µm 25 µm Gleichsp. + 150 V AC 24 µm 22 µm 23 µm 27 µm 25 µm Gleichsp. + 250 V AC 23 µm 21 µm 16 µm 21 µm 17 µm FT 82-7640 DC 33 µm 33 µm 33 µm 33 µm 33 µm Gleichsp. + 30 V alternating voltage 33 µm 33μm Gleichsp. + 60 V AC 30 µm 31 µm 28 µm 34 µm 33 µm Gleichsp. + 150 V AC 31 µm 27 µm 22 µm 34 µm 27 µm Gleichsp. + 250 V AC 17 µm 22 µm 22 µm 19 µm FT 25-7225 direct current 17 µm 15 µm 15 µm 15 µm 15 µm Gleichsp. + 30 V alternating voltage 16 µm 13 µm Gleichsp. + 60 V AC 16 µm 13 µm 13 µm 14 µm 13 µm Gleichsp. + 150 V AC 12 µm 11 µm 15 µm 13 µm Gleichsp. + 250 V AC

Claims (15)

1. Process for electrochemical coating of objects by means of direct current, in which case the DC voltage is adjustable and is pulse-modulated by superimposition of adjustable AC voltage components such that

   a) the resultant voltage does not change its direction, and

   b) the pulse modulation is restricted to specific time intervals during the coating process.
2. Process according to Claim 1,
   characterized in that the AC voltage components are obtained from a cyclic AC voltage, in particular a harmonic oscillation.
3. Process according to one of Claims 1 or 2,
   characterized in that the AC voltage components are the complete cycle signal, its positive element or the rectified cycle signal.
4. Process according to one of Claims 1 to 3,
   characterized in that the pulse modulation of the DC voltage can be connected and disconnected with an adjustable duty ratio.
5. Process according to one of Claims 1 to 4,
   characterized in that the DC voltage element is between 0 and 500 V.
6. Process according to one of Claims 1 to 5,
   characterized in that the AC voltage element is between 0 and 500 V.
7. Process according to one of Claims 2 to 6,
   characterized in that the cyclic AC voltage has a cycle duration of 1 ms to 500 ms.
8. Process according to Claim 4,
   characterized in that the connection:disconnection duty ratio is between 10:1 and 1:10, the connection duration being between 10 ms and 100 s.
9. Apparatus for producing a pulse-modulated DC voltage for a process according to one of Claims 1 to 8,
   characterized in that said apparatus is formed by connecting an adjustable DC generator in series with an adjustable AC generator which can be switched on and off, with the DC voltage and AC voltage being set such that the resultant voltage does not change its direction.
10. Apparatus for producing a pulse-modulated' DC voltage for a process according to one of Claims 1 to 8,
    characterized in that said apparatus is produced by introducing an AC generator into the DC circuit via a rectifier.
11. Apparatus according to Claim 10,
    characterized in that a diode is connected upstream of the rectifier on the input side, so that only the positive or only the negative half-cycle reaches said rectifier.
12. Apparatus according to one of Claims 9 to 11,
    characterized in that the introduction of the AC voltage can be connected and disconnected via an electronic or mechanical relay, which is driven by a function generator in order to produce an adjustable duty ratio.
13. Apparatus for producing a pulse-modulated. DC. voltage for a process according to one of Claims 1 to 8,
    characterized in that the pulse-modulated DC voltage is produced by connecting a function generator to the phase-gating controller of a three-phase rectifier.
14. Apparatus according to Claim 13,
    characterized in that the function generator is a programmable microprocessor system, preferably a computer with appropriate software, having an analogue/digital converter for receiving the control voltage, and having an output unit for the trigger pulses.
15. Application of the method and/or the apparatus according to one of Claims 1 - 14 to electro-dipping.

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