DE102007047158A1 - Method for generating printed images, involves generating loading image of printed image on photo conductor by illumination of photo conductor - Google Patents

Abstract

The method involves generating a loading image of a printed image on a photo conductor (1) by illumination of the photo conductor. The loading image is dyed with charged toner by tribo jumps (6,9) arranged in developer stations (5,8) and lying at a pre-voltage or bias voltage. A photo conductor area is not reloaded below the toner image by a transfer unit (12).

Description

The development of on a charge image carrier, z. As a photoconductor drum, applied charge images of images to be printed in an electrophotographic printing or copying device according to the toner jump principle is known (see, for example. Research Disclosure 331080 ; US 4,868,600 ). In this principle, a toner cloud of toner particles is generated in the space between the developer roller (jump roller) and charge image carrier (hereinafter called development area) by applying an AC voltage and / or DC voltage from the toner particles according to the charge images on the charge image carrier and thus color it. Multi-color printing requires the charge images to be sequentially colored with toner particles of different colors. For this purpose, a corresponding number of developer stations can be arranged along the charge image carrier ( US 5,828,933 ; GB 2 343 144 A ). There is a risk that when the toner cloud is generated in the development area according to the toner jump principle, toner particles also impinge on the charge image carrier and at least partially replace a toner layer of a different color previously applied by a preceding developer station. However, this falsifies the previously printed color image and contaminates the developer mixture of this developer station.

The The problem underlying the invention is a method indicate the toner images already present on the charge image carrier, z. B. at serial pressure, are not affected and subsequent Developer stations are not contaminated.

This Problem is solved according to the features of the claim 1 and of claim 23 solved.

If at one along a circumferential and at an initial potential preloaded photoconductor arranged on the same side printing units are each a character generator and one after the tribo-jump principle working developer station, through each of which a charge image a print image is generated on the photoconductor and this charge image by a bias voltage arranged in the developer station (Biasspannung) Jumping roller with charged toner to the toner image is colored, there is a risk that one through a first printing unit generated toner image in the subsequent printing unit is damaged by the toner of the first toner image is removed by toner of the subsequent printing unit and in the developer station of the subsequent printing unit passes.

in the The following will explain the solution of the Problems as an example of a printing device with two in series arranged printing units assumed. The first one generates Print unit, a first charge image, the first toner image of a first print image is developed and the subsequent second printing unit a second charge image corresponding to the second toner image of a second print image is developed. However, the solution is not up Restricting printing devices with two printing units, it can also be transferred to printing devices with more than two printing units become.

The Danger of carryover of toner from a first toner image in the developer station of a subsequent printing unit can thereby can be reduced that by the first printing unit with toner developed charge image (first toner image) after leaving this Pressure unit and before reaching the next second Druckein unit together with the photoconductor by a Umladeeinheit to a Umladepotential is transferred, which is the transition of toner of the first toner image in the developer station of the second printing unit prevents. Thereby is achieved that the toner of the first toner image is not in the Developer station of the subsequent printing unit passes.

further developments The invention will become apparent from the dependent claims.

In a first embodiment of the invention, the Umladepotential correspond to the initial potential of the photoconductor.

A Further improvement can be achieved if the transhipment potential in terms of amount greater than that Initial potential of the photoconductor is selected.

If the photoconductor with the first toner image before transhipment through a first intermediate exposure unit is discharged again and then by the recharging unit is charged to a Umladepotential, the amount is greater than the initial potential, the Liability of the first toner on the photoconductor significantly increased and moving that toner to the developer station of the subsequent ones Printing unit largely prevented.

This Effect is further enhanced when passing through the first intermediate exposure unit the photoconductor is discharged into saturation, so that after the charge, the potential of the toner image in terms of amount is greater than the Umladepotential the photoconductor.

The development of the second charge image by the subsequent printing unit can be improved by that before reaching the next Printing unit after reloading through a second intermediate exposure unit of Fotolei ter is discharged to a potential corresponding to the initial potential of the photoconductor.

This can be specifically achieved when the potential of the photoconductor after the second intermediate exposure via a charge sensor is measured and the intermediate exposure regulated by the measurement signal becomes.

• – Dieses Ziel kann in einem ersten Verfahren erreicht werden, wenn nach der Umladung des Fotoleiters durch die Umladeeinheit der Zeichengenerator der nachfolgenden zweiten Druckeinheit in einem Bildbereich, in dem das erste und das zweite Druckbild erzeugt werden soll, den Bildbereich außerhalb des ersten Tonerbildes mit dem Inversbild des ersten Ladungsbildes belichtet und zudem in diesem Bildbereich das Ladungsbild des zweiten Druckbildes auf dem Fotoleiter erzeugt. Hier ist es besonders vorteilhaft, wenn durch den Zeichengenerator der zweiten Druckeinheit nur an dem Randbereich des zweiten Ladungsbildes eine Belichtung mit dem Inversbild des ersten Ladungsbildes erfolgt. Die Belichtung des Fotoleiters mit dem Inversbild des ersten Ladungsbildes kann mit Hilfe eines nach dem Zeichengenerator angeordneten Ladungssensors geregelt eingestellt werden. Vorteilhaft ist es, wenn bei der Inversbelichtung ein Bereich um das vorhergehende Tonerbild von der Belichtung ausgenommen wird. Wenn als Zeichengenerator ein LED-Zeichengenerator verwendet wird, kann die Inversbelichtung als punktweise Belichtung durchgeführt werden und einstellbar der Bereich um das vorhergehende Tonerbild von der punktweisen Belichtung ausgenommen werden. Z. B. kann die Ausnahme der punktweisen Belichtung um das vorhergehende Tonerbild ein PEL betragen.
• – Dieses Ziel kann in einem zweiten Verfahren erreicht werden, wenn als Zeichengenerator ein LED-Zeichengenerator verwendet wird, mit dem einstellbar der Randbereich des zweiten Ladungsbildes durch punktweise Belichtung (Rand-PEL-Belichtung) auf ein optimales Ladungspotential für die Entwicklung des zweiten Ladungsbildes eingestellt wird, z. B. auf Anfangspotential umgeladen wird. Bei dieser Lösung kann die Breite des Randbereiches durch die punktweise Belichtung durch den Zeichengenerator eingestellt werden. Zudem kann die Intensität der Umladung im Randbereich durch die Einstellung der Intensität der Belichtung der LED's des Zeichengenerators festgelegt werden. Dann ist die Erzeugung des zweiten Ladungsbildes unabhängig vom ersten Tonerbild, zudem können Druckbilder mit feinen Zeichen und Rastern besser erzeugt werden. Bei benachbart liegenden Druckbildern, die derart zueinander angeordnet sind, dass ein Teil des Tonerbildes des vorhergehenden Druckbildes in den Randbereich des folgenden Druckbildes hineinragt, ist es vorteilhaft, wenn die Belichtung durch den Zeichengenerator derart erfolgt, dass dieser Teil des vorhergehenden Tonerbildes nicht belichtet wird.

An optimal result is achieved when the character generator of the following printing unit is controlled so that it sets the area on the photoconductor in which the second charge image is to be formed to a potential which is optimal for the generation of the charge image. This can be z. B. be the initial potential.

• This goal can be achieved in a first method if, after the transfer of the photoconductor by the transfer unit, the character generator of the subsequent second printing unit in an image area in which the first and the second print image is to be generated, the image area outside the first toner image Inverse image of the first charge image exposed and also generated in this image area, the charge image of the second print image on the photoconductor. In this case, it is particularly advantageous if exposure takes place to the inverse image of the first charge image by the character generator of the second printing unit only at the edge region of the second charge image. The exposure of the photoconductor to the inverse image of the first charge image can be regulated by means of a charge sensor arranged after the character generator. It is advantageous if, during the inverse exposure, an area around the preceding toner image is excluded from the exposure. When an LED character generator is used as the character generator, the inverse exposure can be performed as pointwise exposure and the area around the preceding toner image can be adjustably excluded from the pointwise exposure. For example, the exception of the pointwise exposure around the previous toner image may be a PEL.
• - This goal can be achieved in a second method, when an LED character generator is used as the character generator, set with the adjustable the edge region of the second charge image by pointwise exposure (edge PEL exposure) to an optimal charge potential for the development of the second charge image is, for. B. is reloaded to initial potential. In this solution, the width of the edge area can be adjusted by the pointwise exposure by the character generator. In addition, the intensity of the transhipment in the peripheral area can be determined by the adjustment of the intensity of the exposure of the LEDs of the character generator. Then, the generation of the second charge image is independent of the first toner image, moreover, printed images with fine characters and rasters can be better generated. In adjacent print images, which are arranged in such a way that a part of the toner image of the previous print image protrudes into the edge region of the following print image, it is advantageous if the exposure is performed by the character generator so that this part of the preceding toner image is not exposed.

Based of embodiments shown in the figures are, the invention will be further explained.

It demonstrate

1 an electrophotographic printing device,

2 a timing diagram which illustrates, in principle, plotted versus time the electrical potential on a photoconductor in the production of two serial printed images by two printing units without the use of the invention;

3 a pulse diagram accordingly 2 representing the potential on the photoconductor when, between the generation of the two images by the two printing units, the photoconductor has been reloaded;

4 a pulse diagram accordingly 2 representing the potential on the photoconductor when the photoconductor is overcharged during transhipment;

5 a pulse diagram accordingly 2 representing the potential on the photoconductor if the photoconductor has been exposed again before being transposed;

6 a pulse diagram accordingly 2 representing the potential on the photoconductor if, after the transfer, the photoconductor has once again been subjected to intermediate exposure;

7 a pulse diagram accordingly 2 representing the potential on the photoconductor when the character generator of the second printing unit is controlled by a charge sensor;

8th a first printed image after inverse exposure with the first charge image;

9 a representation of a first method in inverse exposure after 8th ;

10 an advantageous improvement of the first method;

11 the first and second printed image with inverse exposure of the second printed image only in its edge region;

12 a representation of a second method for implementing the embodiment of the 11 ;

13 an addition to the second procedure;

14 a pulse diagram accordingly 2 representing the potential on the photoconductor when more than two toner images are serially produced on the photoconductor of the invention;

15 a control circuit for controlling the character generator of the second printing unit.

Out 1 results in the basic structure of an electrophotographic printing device. To a photoconductor 1 , in 1 a photoconductor belt, in the exemplary embodiment two Druckseininheiten DE1 and DE2 are arranged, with which toned with toner charge images on the photoconductor 1 can be generated, which are arranged in series with each other. The photoconductor 1 becomes in the direction of the arrow 2 emotional. Before the photoconductor 1 reaches the printing unit DE1, it is cleaned in a cleaning unit RE, which lies behind a transfer printing station US, in which the toner images are transferred to a printing substrate AT.

Each printing unit DE1, DE2 has a character generator 4 . 7 and a developer station 5 . 8th on. As a character generator 4 . 7 can z. B. an LED character generator can be used, the photoconductor 1 discharges according to the images to be printed by exposure and thus generates charge images of the printed images, which are developed in a developer station. As a developer station 5 . 8th may be a tribo-jump process developer station 5 . 8th whose mode of operation is known and which will be explained again below:
From a developer mixture of carrier and toner is the developer of a jump roller after mixing by a magnetic roller 6 . 9 supplied as a developer roller, which takes over from the magnetic roller charged toner. The toner becomes one between the jump roller 6 . 9 and the photoconductor 1 fed developer gap formed there forms a toner cloud, from the toner to the photoconductor 1 arrives to color the charge images there. To transfer the toner from the toner cloud to the photoconductor 1 to assist is to the Jumpwalze 6 . 9 a bias, a bias voltage applied, the electric field forces between rule jump roller 6 . 9 and the charge images on the photoconductor 1 due to which charged toner is moved toward the charge images.

Operational ( 2 ) becomes the photoconductor 1 first through the charger 3 charged to an initial potential U _a . Then the photoconductor is running 1 in the first printing unit DE1. There is the charged photoconductor 1 in accordance with the images to be printed by the first printing unit DE1 (first printed images) from the character generator 4 exposed, so that first charge images LB1 of the first printed images to be printed on the photoconductor 1 be generated. These charge images LB1 are in the first developer station 5 the printing unit DE1 with toner to first toner images TB1 dyed. Between the character generator 4 and the developer station 5 can be a charge sensor 10 be provided by which the charge on the photoconductor 1 is measured and dependent on the exposure by the character generator 4 and the charging by the charger 3 is regulated.

The second printing unit DE2 becomes the photoconductor 1 through the second character generator 7 exposed according to a second printed image to be printed and the photoconductor 1 unloaded accordingly. The charge images LB2 generated by the second printing unit DE2 are adjacent to the first toner images TB1. The second charge images LB2 are passed through the second developer station 8th developed to second toner images TB2.

The two by the printing units DE1 and DE2 on the photoconductor 1 generated toner images TB1, TB2 are in a transfer station US in a known manner to a substrate AT, z. B. paper, and subsequently fixed in a known manner.

Along the photoconductor 1 Further functional units can be arranged to improve the quality of the printed images. Between the printing units DE1 and DE2, a recharging unit 12 be provided to the photoconductor 1 recharge behind the pressure unit DE1. Furthermore, between the first and second printing units DE1 and DE2, a first intermediate exposure unit 13 be used to the photoconductor 1 to be able to unload. This intermediate exposure unit 13 can in front of the recharging unit 12 lie. Finally, between character generator 7 and developer station 8th the second printing unit DE2 another charge sensor 11 are used by the measuring signal, the exposure by the second character generator 7 and the transhipment through the transhipment unit 12 can be regulated. Finally, can between transfer unit 12 and the second printing unit DE2, a second intermediate exposure unit 14 be arranged to the photoconductor 1 depending on the application to be able to expose again.

Based on 2 to 14 the function of the printing device is further explained. The 2 to 7 . 14 show pulse diagrams in which the course of the electric potential on the photoconductor 1 is applied in the production of printed images by the printing units DE1 and DE2 and this with different use of the units along the photoconductor 1 , It is assumed in the embodiments that the toner is negatively charged and the potentials on the photoconductor 1 are also negative. However, the invention is also positive for positively charged toner and positive potentials on the photoconductor 1 used. In the 2 to 13 it is only shown how two print images of different colors are successively and juxtaposed by the printing units DE1 and DE2. 14 shows the case that by another pressure unit (in 1 not shown) a third printed image on the photoconductor 1 is produced. Both 2 to 7 . 14 are above the timing diagrams the func on units of the printing device specified that the photoconductor 1 influence in the manner shown.

2 shows the initial situation when, through the two printing units DE1 and DE2 successively in series toner images TB of two printed images on the photoconductor 1 be produced, the invention is not used. With 2 should be pointed out the problems that occur during operation without invention. Through the charger 3 becomes the photoconductor 1 initially charged to an initial potential U _a , z. B. to about -500 V. Subsequently, the photoconductor 1 through the character generator 4 discharged according to the image to be printed, z. B. to a discharge potential U _e1 of z. B. about -40 V. The generated charge image LB1 is in the next step in the developer station 5 colored with toner T1 of a first color, this process is assisted by the bias voltage at the jump roller 6 z. B. is about -300 V and thereby the movement of the negatively charged toner T1 is supported to the charge image LB1. This results in the first toner image TB1, which is represented symbolically by two toner particles T1. The toner image TB1 has a more negative potential U _t1 compared to U _e1 . The photoconductor 1 is moved on to the printing unit DE2. In the printing unit DE2 becomes the photoconductor 1 adjacent to the toner image TB1 by the character generator 7 exposed according to the image to be printed and thereby discharged again to the discharge potential U _e1 to the charge image LB2. Subsequently, the charge image LB2 in the developer station 8th toned toner T2 of a different color to a toner image TB2.

• – Da das Tonerbild TB1 auch durch die Entwicklerstation 8 läuft, besteht die Gefahr, dass durch die dort im Entwicklerspalt bestehende Tonerwolke aus Toner T2 Toner T1 aus dem Tonerbild TB1 gelöst wird, dieser in die Ent wicklerstation 8 gelangt und dort das Entwicklergemisch verunreinigt.
• – Durch das Herauslösen von Toner T1 aus dem Tonerbild TB1 wird dieses verschlechtert oder ganz zerstört.
• – Weiterhin besteht die Gefahr, dass dadurch bei dem ersten Tonerbild TB1 und damit dem ersten Druckbild eine verstärkte Hintergrundbildung auftritt und dies ebenfalls im nicht umgeladenen Bereich des Fotoleiters 1.

If the generation of the toner images TB corresponding 2 is performed, some problems occur:

• - Since the toner image TB1 also by the developer station 8th runs, there is a risk that is solved by the existing there in the developer gap toner cloud of toner T2 toner T1 from the toner image TB1, this in the Ent winding station 8th and there contaminated the developer mixture.
• - The dissolution of toner T1 from the toner image TB1 this is deteriorated or completely destroyed.
• - Furthermore, there is a risk that in the first toner image TB1 and thus the first printed image increased background formation occurs and this also in the untransferred region of the photoconductor 1 ,

These relationships are in 2 and the following figures indicated that the toner image TB1 toner T1 is missing and the toner image TB2 toner T1 is located.

The disadvantages after 2 are avoided in the invention by various measures, which are described below and which are used according to the desired quality of the printed images.

A first measure may be related to 3 be explained. Here, after formation of the first toner image TB1, the photoconductor becomes 1 along with the toner image TB1 through the transfer unit 12 charged again to a Umladepotential U _h1 , the z. B. the initial potential U _a , and also the toner images TB1 are raised accordingly in the potential to U _t2 . Thus, the potential of the toner image TB1 is more negative compared to the bias voltage of the jump roller 9 , Due to the higher negative toner potential of toner image TB1 the adhesion to the photoconductor becomes on the one hand 1 On the other hand, the repulsive forces of the toner T1 to the toner T2 become larger. The result is that less toner T1 from the toner image TB1 in the developer station 8th is replaced.

Further improvement occurs when appropriate 4 through the transhipment unit 12 the photoconductor 1 is charged to a Umladepotential U _h2 and the first toner image TB1 to the potential U _t3 , which is more negative than the initial potential U _a (overcharge of the toner image TB1).

As a result, a more negative charge of the toner image TB1 is achieved, with the result that its adhesion to the photoconductor 1 is increased. Further, a larger potential difference of the first toner image TB1 becomes the toner T2 in the second developer station 8th (higher repulsive effect) achieved and there prevents transhipment of the toner T1 of the first toner image TB1. With the transhipment unit 12 can the photoconductor 1 z. B. on U _h2 about -870 V charged.

The solution after 4 has the disadvantage that the photoconductor 1 is overloaded and the development of the second toner image TB2 and thus the second print image DB2 is deteriorated, especially when playing fine characters. To prevent this, can be done accordingly 5 be proceeded. Here is the photoconductor 1 before its reloading a first intermediate exposure by the intermediate exposure unit 13 exposed and thereby in saturation on z. B. U _e2 ≈ -20 V further discharged, the potential of the toner image TB1 can be z. B. be discharged to U _t4 ≈ -100 V. If now the photoconductor 1 through the transhipment unit 12 is recharged, the toner image TB1 is charged negative compared to the photoconductor 1 ; z. B. U _h3 -750 V and U _t5 ≈ -800 V. This will overload the photoconductor 1 reduced. The consequence is also that the retransfer of toner T1 of the first toner image TB1 into the developer station 8th is prevented.

Further improvement is achieved if appropriate 6 the potential of the photoconductor 1 is further reduced while the potential of the first toner image TB1 is not changed. This can be achieved by using the photoconductor 1 a second intermediate exposure by a second intermediate exposure unit 14 is exposed and discharged to a Umladepotential U _h4 , which corresponds to the initial potential U _a . In order to reset the initial potential U _a , can via a charge sensor 11 the potential on the photoconductor 1 and the second intermediate exposure unit 14 be regulated so that the photoconductor 1 the initial potential U _a occupies. Thus, the retransmission of toner T1 of the first toner image TB1 into the developer station becomes 8th prevents and optimal charge potential on the photoconductor 1 provided for the development of the second toner image TB2. A disadvantage of this solution is when the toner T1 of the first charge image LB1 transmits light permeable to the light wavelength of the exposure of the second intermediate exposure unit 14 is because then the toner overload of the first toner image TB1 is also lost.

• – In einem ersten Verfahren wird nach Erzeugung des ersten Tonerbildes TB1 der Zeichengenerator 7 so angesteuert, dass er in einem Bildbereich BB, in dem das erste und zweite Druckbild liegen sollen, den Bereich außerhalb des ersten Tonerbildes TB1 mit dem inversen Bild des ersten Ladungsbildes LB1 belichtet. Anschließend wird das zweite Tonerbild TB2, wie bei 6 beschrieben, erzeugt und dazu der Fotoleiter 1 auf ca. –40 V entladen. Diese inverse Belichtung ergibt sich prinzipiell aus 8, bei der ein Bildbereich BB um das erste Tonerbild TB1 gezeigt ist, bei dem das erste Tonerbild TB1 einfarbig dargestellt ist, während der übrige Bereich um das erste Tonerbild TB1 gestrichelt die Belichtung mit dem inversen Ladungsbild LB1 darstellt. Die Belichtung des Bereiches um das erste Tonerbild TB1 wird mit dem zweiten Zeichengenerator 7 durchgeführt. In dem gestrichelten Bildbereich BB kann das Umladepotential U_h6 eingestellt werden und dann durch den zweiten Zeichengenerator 7 das Ladungsbild LB2 erzeugt werden. An Hand der 9a bis 9c wird dieses erste Verfahren weiter erläutert. Die 9a bis 9c zeigen den Bildbereich BB in prinzipieller Darstellung. Dabei ist der Bildbereich BB in Quadrate unterteilt, die jeweils punktweise z. B. mit einem LED-Zeichengenerator belichtet werden können. Jeder Bildpunkt (pixel), der von einer LED des LED-Zeichengenerators 7 erzeugt wird, entlädt punktweise den Fotoleiter 1 zu einem PEL (printed element) auf dem Fotoleiter 1, der mit Toner eingefärbt den Druckpunkt (dot) bildet. Die LED's des LED-Zeichengenerators 7 können auf bekannte Weise so angesteuert werden, dass sie wahlweise einzelne PEL's (in den Quadraten) durch Belichtung erzeugen. In 9a bis 9c ist das Tonerbild TB1 z. B. durch vier PEL's dargestellt. 9a zeigt die Potentialverhältnisse nach der Entwicklung in der Entwicklerstation 5 (7). Im schraffierten Bereich liegt das Potential U_a vor, im Tonerbereich TB1 das Potential U_t4. Der Bildbereich BB wird nun nach der Aufladung durch die Umladeeinheit 12 auf das Potential U_h5 (7) durch den Zeichengenerator 7 mit dem inversen Ladungsbild LB1 belichtet und gleichzeitig das Ladungs bild LB2 erzeugt (9b). Dadurch wird der Bereich der Inversbelichtung vom Potential U_h5 auf das Potential U_h6, z. B. U_a, entladen, während das Tonerbild TB1 nicht beeinflusst wird. Dieses bleibt somit auf dem Potential U_t7 = U_t6, während das Tonerbild TB2 auf das Potential Ute gelegt wird. 9c zeigt den Bildbereich BB, bei dem die beiden Tonerbilder TB1 und TB2 (jeweils Quadrate von vier PEL's) nebeneinander liegen. Um die Tonerbilder TB1 und TB2 liegt der Bildbereich auf dem Potential U_h6.
• – Bei dem ersten Verfahren nach 8 und 9 besteht die Gefahr, dass das Tonerbild TB1 durch die Belichtung durch den Zeichengenerator 7 in einem Randbereich beeinträchtigt wird und dadurch ein Tonerabtrag in diesem Randbereich in die Entwicklerstation 8 erzeugt werden kann. Diese Gefahr kann dadurch vermieden werden, dass ein Bereich BT um das Tonerbild TB1 von der Belichtung durch den Zeichengenerator 7 ausgenommen wird. In diesem Bereich BT bleibt somit das Potential auf U_h5 liegen und damit bleibt die Potentialdifferenz zum Ladungsbild LB2 bestehen. Die Wanderung von Toner T1 vom Tonerbild TB1 zur Entwicklerstation 8 wird damit verhindert. Bei einer vorteilhaften Lösung genügt es, wenn ein Bereich BT von einem PEL um das Tonerbild TB1 von der inversen Belichtung ausgenommen wird. Diese Lösung ist in 10 dargestellt. 10a entspricht 9a, auf die dortige Erläuterung wird verwiesen. Aus 10b ergibt sich, dass durch den Zeichengenerator 7 der Bildbereich BB ausgenommen das Tonerbild TB1 und der Bereich BT von z. B. ein PEL-Breite um das Tonerbild TB1 herum belichtet wird. Damit bleibt der Bereich BT um das Tonerbild TB1 auf dem Potential U_h5 und nur der Rest des Bildbereiches BB wird auf das Potential U_h6 entladen. Das Ergebnis zeigt 10c. Hier liegen die beiden Tonerbilder TB1 und TB2 nebeneinander, der Bereich BT um das Tonerbild TB1 liegt auf dem Potential U_h5, das Potential des restlichen Bildbereiches BB außer dem Tonerbild TB2 liegt auf U_h6.
• – In einem zweiten Verfahren wird nur in einem Randbereich RB des zweiten Ladungsbildes LB2 die Belichtung mit dem inversen ersten Ladungsbild LB1 durchgeführt (11). In 11 ist ein Bildbereich BB gezeigt, bei dem das erste Tonerbild TB1 und das zweite Tonerbild TB2 nebeneinander liegen und nur der Randbereich RB um das Tonerbild TB2 durch den Zeichengenerator 7 belichtet worden ist. Bei Verwendung eines LED-Zeichengenerators als Zeichengenerator 7 kann der Randbereich RB durch diesen punktweise belichtet werden. Jeder Bildpunkt (pixel), der von einer LED des LED-Zeichengenerators 7 erzeugt wird, entlädt punktweise den Fotoleiter 1 zu einem PEL (printed element), der mit Toner eingefärbt den Druckpunkt (dot) bildet. Die LED's des LED-Zeichengenerators 7 können auf bekannte Weise so angesteuert werden, dass sie wahlweise einzelne PEL's durch Belichtung im Randbereich RB erzeugen und damit den Randbereich RB einstellbar entladen. Durch Auswahl der LED's kann die Breite des Randbereiches RB eingestellt werden, durch Einstellung der Intensität der Belichtung das Ausmaß der Entladung. Dieses Verfahren hat den Vorteil, dass kein Einfluss des ersten Tonerbildes TB1 auf das folgende Tonerbild TB2 besteht. Zudem kann der Fotoleiter 1 gezielt auf ein Ladungspotential U_h6 entladen werden, das eine optimale Entwicklung der Ladungsbilder LB2 ermöglicht. Z. B. kann das Ladungspotential U_h6 auf das Anfangspotential U_a eingestellt werden. Mit diesem Verfahren kann dadurch eine Tonerverschleppung von Toner T1 des ersten Tonerbildes TB1 in die zweite Entwicklerstation 8 vermieden werden. Dieses zweite Verfahren ist in 12a bis 12c dargestellt. 12a entspricht 9a; darauf wird verwiesen. 12b zeigt die Belichtung des Bildbereiches BB durch den Zeichengenerator 7; diese ist derart, dass der Randbereich RB um das Ladungsbild LB2 auf das Potential U_h6 entladen wird, der restliche Bildbereich BB dagegen auf dem Potential U_h5 verbleibt. Andere Breiten des Randbereiches RB sind selbstverständlich möglich. Als Beispiel besteht der Randbereich RB um das zweite Ladungsbild LB2 aus zwei PEL. Das Ergebnis ergibt sich aus 12c. Die beiden Tonerbilder TB1 und TB2 liegen nebeneinander; der Randbereich RB um das Tonerbild TB2 liegt auf dem Potential U_h6, der übrige Bildbereich BB auf dem Potential U_h5. Bei dem Verfahren nach der 12 liegt als Beispiel ein Teil des Tonerbildes TB1 innerhalb des Randbereiches RB. Durch die Belichtung des Randbereiches RB durch den Zeichengenerator 7 kann auch das Tonerbild TB1 an seinem Rand beeinträchtigt werden mit der Folge, dass Toner T1 in die Entwicklerstation 8 wandert. Um für diesen Fall den Tonerabtrag im Rand des Tonerbildes TB1 weiter zu unterbinden, kann eine Belichtung des Randbereiches RB des Ladungsbildes LB2 durch den Zeichengenerator 7 entsprechend der 13 durchgeführt werden. 13a entspricht wieder 9a; darauf wird verwiesen. Die Belichtung durch den Zeichengenerator 7 ergibt sich aus 13b. Der Bereich des Tonerbildes TB1, der im Randbereich RB liegt, wird von der Belichtung ausgenommen. Dabei wird aber darauf geachtet, dass um das Ladungsbild LB2 herum eine Entladung auf das Potential U_h6 bestehen bleibt. Wenn z. B. eine Belichtung durch den Zeichengenerator 7 um das Ladungsbild LB2 herum mit zwei PEL erfolgt, und das Tonerbild TB1 in einen Bereich von einem PEL in den Randbereich RB von zwei PEL hineinragt, wird ein Bereich von einem PEL zwischen dem Ladungsbild LB2 und dem Tonerbild TB1 vom Zeichengenerator 7 belichtet. Ein Tonerabtrag beim Tonerbild TB1 wird dann minimiert.

A solution for this case can 7 be removed. Here is dispensed with the second intermediate exposure. After the intermediate exposure through the intermediate exposure unit 13 The toner image TB1 is passed through the transfer unit 12 reloaded to the potential U _t6 , the potential of the photoconductor 1 is then U _h5 . The second character generator 7 is controlled so that he for the generation of the second toner image TB2 the photoconductor 1 on the Umladepotential U _h6 places z. B. at initial potential U _a . This can be done by the character generator 7 be controlled in different ways and thereby different results can be achieved. The procedures are carried out on the basis of 8th to 13 explained. As an exemplary embodiment, an image area BB is used, in which two print images are to be generated side by side. This will be done accordingly 7 first the charge image LB1 for the first print image by the character generator 4 generates this to the toner image TB1 in the developer station 5 developed. Subsequently, the charge image LB2 for the second print image is formed by the character generator 7 generated and this in the developer station 8th developed to the toner image TB2.

• In a first method, after generation of the first toner image TB1, the character generator 7 is driven in such a way that it illuminates the area outside the first toner image TB1 with the inverse image of the first charge image LB1 in an image region BB in which the first and second print images are to lie. Subsequently, the second toner image TB2 becomes as in 6 described, generated and to the photoconductor 1 discharged to about -40V. This inverse exposure results in principle 8th in which an image area BB around the first toner image TB1 is shown, in which the first toner image TB1 is shown in monochrome, while the remaining area around the first toner image TB1 shows the exposure with the inverse charge image LB1 in broken lines. The exposure of the area around the first toner image TB1 is made with the second character generator 7 carried out. In the dashed image area BB, the Umladepotential U _{h6 can be} adjusted and then by the second character generator 7 the charge image LB2 can be generated. Based on 9a to 9c this first method will be further explained. The 9a to 9c show the image area BB in a schematic representation. In this case, the image area BB is divided into squares, each pointwise z. B. can be exposed with an LED character generator. Each pixel (pixel) generated by an LED of the LED character generator 7 is generated, discharges point by point the photoconductor 1 to a PEL (printed element) on the photoconductor 1 , which forms the pressure point (dot) colored with toner. The LEDs of the LED character generator 7 can be driven in a known manner to selectively generate individual PELs (in the squares) by exposure. In 9a to 9c is the toner image TB1 z. B. represented by four PEL's. 9a shows the potential conditions after development in the developer station 5 ( 7 ). In the hatched area the potential U _{a is} present, in the toner area TB1 the potential U _t4 . The image area BB is now after charging by the recharging unit 12 to the potential U _h5 ( 7 ) by the character generator 7 exposed with the inverse charge image LB1 and at the same time the charge image LB2 generates ( 9b ). As a result, the area of the inverse exposure from the potential U _h5 to the potential U _h6 , z. B. U _a discharged while the toner image TB1 is not affected. This thus remains at the potential U _t7 = U _t6 , while the toner image TB2 is set to the potential Ute. 9c shows the image area BB, in which the two toner images TB1 and TB2 (each squares of four PELs) are adjacent. Around the toner images TB1 and TB2, the image area is at the potential U _h6 .
• - In the first method after 8th and 9 there is a risk that the toner image TB1 will be exposed by the character generator 7 is impaired in an edge region and thereby a toner removal in this edge region in the developer station 8th can be generated. This danger can be avoided by having a region BT around the toner image TB1 from exposure by the character generator 7 is excluded. In this area BT, therefore, the potential remains at U _h5 and thus the potential difference to the charge image LB 2 remains. The migration of toner T1 from the toner image TB1 to the developer station 8th is thus prevented. In an advantageous solution, it suffices if a region BT is excluded from the PEL by the toner image TB1 from the inverse exposure. This solution is in 10 shown. 10a corresponds to 9a , to the explanation there is referenced. Out 10b it follows that by the character generator 7 the image area BB except the toner image TB1 and the area BT of z. B. a PEL width is exposed around the toner image TB1 around. Thus, the area BT remains around the toner image TB1 at the potential U _h5 and only the rest of the image area BB is discharged to the potential U _h6 . The result shows 10c , Here, the two toner images TB1 and TB2 are juxtaposed, the area BT around the toner image TB1 is at the potential U _h5 , the potential of the remaining image area BB except the toner image TB2 is at U _h6 .
• In a second method, the exposure with the inverse first charge image LB1 is performed only in an edge region RB of the second charge image LB2 ( 11 ). In 11 an image area BB is shown in which the first toner image TB1 and the second toner image TB2 are juxtaposed and only the peripheral area RB is around the toner image TB2 by the character generator 7 has been exposed. When using an LED character generator as a character generator 7 The edge region RB can be exposed by this pointwise. Each pixel (pixel) generated by an LED of the LED character generator 7 is generated, discharges point by point the photoconductor 1 to a PEL (printed element), which forms the printing dot (dot) colored with toner. The LEDs of the LED character generator 7 can be controlled in a known manner so that they selectively generate individual PELs by exposure in the edge region RB and thus discharges the edge region RB adjustable. By selecting the LEDs, the width of the peripheral area RB can be adjusted, by adjusting the intensity of the exposure, the amount of discharge. This method has the advantage that there is no influence of the first toner image TB1 on the following toner image TB2. In addition, the photoconductor 1 are selectively discharged to a charge potential U _h6 , which allows optimal development of the charge images LB2. For example, the charge potential U _{h6 can be set} to the initial potential U _a . With this method, a toner entrainment of toner T1 of the first toner image TB1 into the second developer station can thereby be achieved 8th be avoided. This second method is in 12a to 12c shown. 12a corresponds to 9a ; it is referred to. 12b shows the exposure of the image area BB by the character generator 7 ; this is such that the edge region RB is discharged around the charge image LB2 to the potential U _h6 , while the remaining image region BB remains at the potential U _h5 . Other widths of the edge area RB are of course possible. By way of example, the edge region RB consists of the second charge image LB2 of two PELs. The result follows 12c , The two toner images TB1 and TB2 are next to each other; the edge area RB around the toner image TB2 is at the potential U _h6 , the remaining image area BB at the potential U _h5 . In the method of the 12 As an example, part of the toner image TB1 lies within the margin area RB. By the exposure of the edge area RB by the character generator 7 Also, the toner image TB1 may be affected at its periphery with the result that toner T1 in the developer station 8th emigrated. In order to prevent the removal of toner in the edge of the toner image TB1 for this case, an exposure of the edge region RB of the charge image LB2 by the character generator 7 according to the 13 be performed. 13a corresponds again 9a ; it is referred to. The exposure by the character generator 7 turns out 13b , The area of the toner image TB1 located in the peripheral area RB is excluded from the exposure. However, care is taken to ensure that discharge around the charge image LB2 remains at the potential U _h6 . If z. B. an exposure by the character generator 7 around the charge image LB2 with two PELs, and the toner image TB1 protrudes into a region of one PEL in the peripheral region RB of two PELs, a region of one PEL between the charge image LB2 and the toner image TB1 from the character generator becomes 7 exposed. Toner removal on toner image TB1 is then minimized.

The methods according to the invention thus make it possible to deliberately avoid the disadvantages caused by the fact that the first toner image TB1 passes through the second developer station 8th running. An optimum solution is achieved when the toner image TB1 is overloaded and thus higher adhesive forces to the photoconductor 1 and at the same time have a higher repelling effect on the toner T2 in the second developer station 8th used is being developed. Via a charge sensor 11 can be the character generator 7 regulated to be operated so that the photoconductor 1 to an optimal development potential, z. B. the initial potential is reloaded.

• Aufladung (3): U_a ≈ –500 V Bereich: (–400 V → –600 V)
• Belichtung (4/7): U_e1 ≈ –40 V Bereich (–10 V → –100 V)
• Entwicklung (5/8): Jumpspalt ≈ 180 μm Bereich (100 μm → 300 μm)
• 1. Zwischenbel. (13): U_e2 ≈ –20 V Bereich (–10 V → –100 V)
• Betonerter Bereich: Je nach Lichtdurchlässigkeit des Toners bzgl. Wellenlänge von (13)
• Umladung (12): U_h5 ≈ –700 V Bereich (–650 V → –950 V) U_t6 ≈ –750 V Bereich (–700 V → –1000 V)
• 2. Zwischenbel. (7): U_h6 ≈ –500 V Bereich (–400 V → –600) U_t7 (Lichtdurchlässig) ≈ –500 V Bereich (–1000 V → –600 V) U_t7 (Lichtundurchl.) ≈ –750 V Bereich (–700 V → –1000 V)
• Inversbel.: U_h4 ≈ –500 V Bereich (–400 V → –600)

Typical potentials as an example of the method according to 7 :

• Charge (3): U _a ≈ -500 V Range: (-400 V → -600 V)
• Exposure (4/7): U _e1 ≈ -40 V range (-10 V → -100 V)
• Development (5/8): Jump gap ≈ 180 μm range (100 μm → 300 μm)
• 1. Intermediate (13): U _e2 ≈ -20 V range (-10 V → -100 V)
• Concreted area: Depending on the light transmittance of the toner with respect to the wavelength of (13)
• Transshipment (12): U _h5 ≈ -700 V range (-650 V → -950 V) U _t6 ≈ -750 V range (-700 V → -1000 V)
• 2. Intermediate (7): U _h6 ≈ -500 V range (-400 V → -600) U _t7 (translucent) ≈ -500 V range (-1000 V → -600 V) U _t7 (opaque) ≈ -750 V range ( -700 V → -1000 V)
• Inverse: U _h4 ≈ -500 V range (-400 V → -600)

areas with toner T1 from the first print image DB1 are formed by inverse exposure not impaired.

14 shows the timing diagram of an example in which three printing units along the photoconductor 1 are arranged. The first area corresponds to the course of the 7 (up to the dashed vertical line), then the potential profile is displayed when the third pressure unit (in 1 not shown) a toner image TB3 for a third printed image on the photoconductor 1 generated. First, by an intermediate exposure unit 15 the photoconductor 1 unloaded, followed by a recharging unit 16 a transhipment comparable to that of 7 (transfer unit 12 ). In the following steps becomes the photoconductor 1 by a character generator 17 discharged according to the third printed image to a charge image LB3 and this developed in a developer station of the third printing unit to the toner image TB3. The process and the potential conditions correspond to those of 7 ; it is referred to. Subsequently, printed images lying in series on the same principle can be produced by further printing units.

Out 15 results in a basic circuit of a control circuit z. For example, for the character generator 7 , The charge of the photoconductor 1 is through a charge sensor 11 measured. The measurement signal is compared in a comparison circuit VG with a discharge target value SW. The resulting control difference RD is a discharge controller RG, z. B. a PI controller, supplied to the character generator 7 so that it regulates the desired potential (eg according to 7 ) on the photoconductor 1 established.

The invention has been explained with an embodiment in which the toner is negatively charged and accordingly the potentials on the photoconductor are negative. Of course, the invention can also be used in one embodiment with positively charged toner. In the claims, therefore, the potentials on the photoconductor 1 and indicated on the toner images TB in terms of amount.

DE1

printing unit

DE2

printing unit

RE

cleaning unit

US

transfer station

AT

substrate

LB

charge image

TB

toner image

BB

image area

RB

border area a printed image

BT

border area of the toner image

T

toner

1

photoconductor

2

Transport direction of the photoconductor 1

3

Charger

4

character generator

5

developer station

6

jump roller

7

character generator

8th

developer station

9

jump roller

10

charge sensor

11

charge sensor

12

transfer unit

13

Between the exposure unit

14

Between the exposure unit

15

Between the exposure unit

16

transfer unit

17

character generator

18

developer station

VG

comparison circuit

RG

regulator

RD

Control difference

SW

Discharge setpoint

U _a

initial potential of the photoconductor

U _e

discharge potential of the photoconductor

U _t

toner potential

U _h

Umladepotential of the toner image

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 4868600 [0001]
• US 5828933 [0001]
• GB 2343144 A [0001]

Cited non-patent literature

• - Research Disclosure 331080 [0001]

Claims (28)

A method for generating juxtaposed on a printing print images using an electrophotographic printing device, in which pre-loaded along a circumferential and an initial potential (U _a) the photoconductor ( 1 ) are arranged on the same side printing units (DE1, DE2) each having a character generator ( 4 . 7 ) and a tribo-jump principle developer station ( 5 . 8th ), wherein at each printing unit (DE1, DE2) by the character generator ( 4 . 7 ) by exposure of the photoconductor ( 1 ) a charge image (LB) of a printed image on the photoconductor ( 1 ) and this charge image (LB) by a in the developer station ( 5 . 8th ) arranged at a bias voltage (bias voltage) Jump roller ( 6 . 9 ) is dyed with charged toner (T), in which the charge image developed by a printing unit (DE) with toner (T) to the toner image (TB) after leaving this printing unit and before reaching the next printing unit (DE) together with the photoconductor ( 1 ) with a first transhipment unit ( 12 . 16 ) is reloaded to a Umladepotential (U _h ), which is greater in magnitude than the bias voltage of the Jumpwalzen ( 6 . 9 ). Method according to Claim 1, in which the charge-reversal potential (U _h1 ) corresponds to the initial potential (U _a ) of the photoconductor ( 1 ) corresponds. Method according to Claim 1, in which the charge-reversal potential (U _h2 ) is greater in magnitude than the initial potential (U _a ) of the photoconductor ( 1 ) is selected. Method according to Claim 3, in which, with an initial potential (U _a ) of 500 V in absolute value, the charge-reversal potential (U _h2 ) is chosen to be 700 V to 900 V in absolute terms. Method according to Claim 4, in which the photoconductor ( 1 ) with the toner image (TB1) before the second charging through an intermediate exposure unit ( 13 ) is discharged further and then by the Umladeeinheit ( 12 ) is charged to a Umladepotential (U _h3 ), which is greater in magnitude than the initial potential (U _a ). Method according to Claim 5, in which the first intermediate exposure unit ( 13 ) the photoconductor ( 1 ) is discharged into the saturation (U _e2 ), so that after the charge, the potential (U _t5 ) of the toner image (TB1) in terms of amount is greater than the Umladepotential (U _h3 ) of the photoconductor ( 1 ). Method according to one of claims 5 or 6, wherein before reaching the next printing unit (DE2) after the transfer by a second intermediate exposure unit ( 14 ) the photoconductor ( 1 ) is discharged to a potential (U _h4 ), the initial potential (U _a ) of the photoconductor ( 1 ) corresponds. Method according to Claim 7, in which the potential of the photoconductor ( 1 ) after the second intermediate exposure via a charge sensor ( 10 ) is measured and the intermediate exposure is controlled by the measurement signal. Method according to Claim 5 or 6, in which the character generator ( 7 ) of the subsequent printing unit (DE2) is regulated in such a way that, for the production of the toner image (TB2), it causes the photoconductor ( 1 ) in a region in which the toner image (TB2) is formed on a charge potential (U _h6 ) for the development of the charge image (LB2) sets. Method according to Claim 9, in which the charge potential (U _h6 ) corresponds to the initial potential (U _a ). Method according to claim 9 or 10, in which after being transhipped by the transhipment unit ( 12 ) in the subsequent printing unit (DE2) by the associated character generator ( 7 ) in an image area (BB) in which the two adjacent printed images are produced, the image area (BB) outside the preceding toner image (TB1) is exposed in an inverse exposure to the inverse image of the preceding charge image (LB1) and by the associated character generator ( 7 ) the charge image (LB2) of the subsequent printed image on the photoconductor ( 1 ) is produced. A method according to claim 11, wherein in the inverse exposure an area (BT) around the previous toner image (TB1) from the exposure is excluded. Method according to Claim 12, in which as a character generator ( 7 ) an LED character generator is used, with which the inverse exposure is performed as a pointwise exposure and adjustably the area (BT) around the preceding toner image (TB1) is excluded from the pointwise exposure. The method of claim 13, wherein the exception the pointwise exposure around the previous toner image (TB1) a PEL is. Method according to claim 9 or 10, in which after being transhipped by a transhipment unit ( 12 ) in the subsequent printing unit (DE2) by the associated character generator ( 7 ) in an image area (BB) in which the two adjacent printed images are generated, the edge region (RB) of the subsequent charge image (LB2) is exposed to the inverse image of the preceding charge image (LB1) and by the associated character generator ( 7 ) the charge image (LB2) of the following the printed image on the photoconductor ( 1 ) is produced. Method according to one of Claims 11 to 15, in which the exposure of the photoconductor ( 1 ) with the inverse image of the preceding print image (DB1) by one after the character generator ( 7 ) arranged charge sensor ( 11 ) is regulated. Method according to Claim 15 or 16, in which, after being transhipped by the transhipment unit ( 12 ) in the subsequent printing unit (DE2) by the associated character generator ( 7 ) in an image area (BB), in which the two adjacent printed images are generated, the edge region (RB) of the subsequent charge image (LB2) is exposed such that there sets a charge potential (U _h6 ), the initial potential (U _a ) corresponds. Method according to Claim 17, in which as a character generator ( 7 ) an LED character generator is used with the adjustable edge region (RB) by pointwise exposure as edge PEL exposure to the charge potential (U _h6 ) is reloaded. A method according to claim 18, wherein in adjacent print images arranged in such relation to each other that a part of the toner image (TB1) of the preceding print image (DB1). into the edge area (RB), the exposure by the character generator ( 7 ) such that this part of the preceding toner image (TB1) is not exposed. Method according to Claim 18 or 19, in which the width of the edge region (RB) is determined by the pointwise exposure by the character generator ( 7 ) is set. Method according to Claim 18, 19 or 20, in which the intensity of the transposition in the edge region (RB) is determined by the adjustment of the intensity of the exposure of the LED's of the character generator ( 7 ). Method according to one of Claims 5 to 21, in which, in the printing units following a second printing unit (DE2), the photoconductor ( 1 ) by an intermediate exposure unit ( 15 ) is transferred to saturation, - the photoconductor ( 1 ) by a transhipment unit ( 16 ) is reloaded to a Umladepotential, the amount is higher than the initial potential (U _a ), - the photoconductor ( 1 ) by a further intermediate exposure unit or a character generator ( 17 ) of the associated printing unit to the charge potential, which corresponds to the initial potential (U _a ), and by the character generator ( 17 ) the charge image (LB) of the respective print image is generated, - the charge image (LB) by a developer station ( 18 ) of the associated printing unit to the toner image of the respective print image is developed. Arrangement for carrying out a method according to one of the preceding claims, - in which, along a photoconductor ( 1 ) at least two printing units (DE1, DE2) are provided for the production of printed images arranged in series, each of which is a character generator ( 4 . 7 ) for generating the charge images (LB) of the images to be printed and a developer station ( 5 . 8th ) for coloring the charge images (LB) with toner on the photoconductor ( 1 ), in which the toner images (TB) are transferred to a printing material (AT) and subsequently fixed. - at which the developer stations ( 5 . 8th ) each have a jump roller ( 6 . 9 ) having a toner cloud in the developer gap between the photoconductor ( 1 ) and Jumpwalze ( 6 . 9 ) and at which a bias voltage is applied with a polarity, due to which the charged toner to the charge images (LB) on the photoconductor ( 1 ) to their coloring. Arrangement according to claim 23, in which units for intermediate exposure (between the printing units (DE1, DE2) 13 . 14 ) and transhipment ( 12 ) are arranged. Arrangement according to Claim 23 or 24, in which, within the printing units (DE1, DE2), between character generator ( 4 . 7 ) and developer station ( 5 . 8th ) each a charge sensor ( 10 . 11 ) is provided. Arrangement according to one of Claims 23 to 25, in which, along the photoconductor ( 1 ) a number of printing units (DE) corresponding to the number of colors to be printed are arranged, between which in each case units for intermediate exposure ( 13 . 14 ) and transhipment ( 12 ) are provided. Arrangement according to one of Claims 23 to 26, in which the character generators ( 4 . 7 ) LED character generators are. Arrangement according to one of claims 23 to 27, - in which a control circuit is provided, by which the charging of the photoconductor ( 1 ) by the character generators ( 7 ), - in which the charge sensors ( 10 . 11 ) the charge of the photoconductor ( 1 ) and deliver a measurement signal dependent thereon, - in which the measurement signal is compared by a comparison circuit (VG) with a discharge setpoint (SW) and the comparison generates a control signal (RG) which corresponds to a PI controller (RG) Trains which generates a control signal for the respective character generators ( 7 ), so that the photoconductor ( 1 ) is discharged to the setpoint.