EP0932850B1 - Multicolor electrophotographic printing device with bipolar toner - Google Patents

Description

The invention relates to a method for electrophotographic Print a print image with multiple colors on one Image carrier.

From US Pat. No. 4,078,929 a method for known electrophotographic printing with two colors. This The process is also called the "tri-level process". at According to this method, a printed image contains at least a first one Image element of a first color and at least a third Image element of a third color. A second picture element of the Print image has a background color of the image carrier, so that no toner particles can be applied.

The first picture element becomes a first surface element Assigned photoconductor layer. The photoconductor layer and one a predetermined reference potential leading electrode layer are contained in a light-sensitive layer system. The The reference potential is usually the zero potential. As well the second picture element becomes a second surface element and the third picture element a third surface element of the Assigned photoconductor layer. A disadvantage of the tri-level process is that only toner particles of two colors in one Print image can be deposited. A color mix with The use of three basic colors is therefore not necessary to repeat the Procedure excluded. There are several for color mixing Printing operations possible, with one printing operation following the steps Charging, imaging, and developing.

From European patent application EP 0 606 141 A2 a printer known for electrophotographic printing a print image with multiple colors can be used. With a printer after the named A European layered coating system is used that consists of four layers. To this layer system too manufacture, an increased effort is necessary because instead of two layers four layers are to be arranged one above the other.

Furthermore, two photosensitive components contained in the coating system must be used Layers can be exposed with two light beams each have different wavelengths. In a first variant of the printer according to the European The photoconductor is published twice on one Exposure station passed for imagewise exposure. This reduces the printing speed on the Half. An exposure station must be used in both runs provide two different light beams. The effort for exposure therefore doubles e.g. regarding generating the exposure signals and with respect to the Optical system requirements regarding aberrations.

In a second variant of the aforementioned European patent application becomes the photosensitive in one pass Layer system led past two exposure points. On but each of these exposure points still need two Exposure beams are provided. The requirements the imaging accuracy of the optical system used increase further with the second variant. So be to the different wavelengths of the two exposure beams for the first exposure station TE light waves (transverse electromagnetic) used and in the second Exposure station TM light waves (transverse magnetic).

A printing method is known from US Pat. No. 5,155,541, in which Exposure and de-entangling techniques of color photography and Tri-level process for printing at least three Colors have been combined.

The object of the invention is a method for electrophotographic Print a print image with at least three Specify colors that are a high quality print allowed, and that on a simply constructed printer can be carried out.

This task is accomplished by a process with the characteristics of Claim 1 solved. Compared to the known method from the above-mentioned laid-open publication EP 0 606 141 A2 are image elements with at least in the invention in the printed image three different colors in the development steps applied paint particles included. Despite the waiver for subtractive color mixing in one printing process in the method according to the invention additive color mixing are generated by color particles arranged side by side. By the method according to the invention is only one imagewise exposure step with a single light frequency and polarization type light required so that a simple imagewise exposure is performed. The Photosensitive layer system can be used in the process according to Invention can be constructed simply. Compared to document EP 0 606 141 A2 can print at least three colors Photoconductor layer and an intermediate layer can be saved. A subtractive color mixing can be done by clicking on the Print image another print image in a subsequent printing process is printed as precisely as possible. It will additional printed image on the same photoconductor after removal the first printed image or on another photoconductor generated.

The invention is based on the knowledge that the print quality with multiple exposures to print a printed image decreases because of inevitable aberrations and positioning of the layer system with tolerances it cannot be guaranteed that both exposure steps for precise positioning of the picture elements of the printed image. Therefore, the invention only performed an imagewise exposure step. Moreover In the invention, aberrations are minimized by that the light-sensitive layer system used only an electrode layer carrying a predetermined reference potential and contains a photoconductor layer, e.g. are mechanically and electrically connected over a large area.

In the invention is loaded after the application of Color particles of the respective color on at least one of the Surface elements the potential on this surface element increased in amount. The light source with the uniform Light distribution only needs a lower light energy radiate because the potentials only by a smaller amount have to be lowered. This allows the through Make good use of the initial potential given the contrast range.

In a first embodiment of the invention, this includes Print image at least one first image element of a first Color, at least a second picture element with the color of the Carrier, at least a third picture element of a third Color and at least a fourth picture element of a fourth Colour. So there is also an additive color mix with three Colors in particular also possible if a white carrier is used. On the second 3ildelement can in the rare case in which all picture elements of the Print image are covered with color particles. In this case all the second picture element or a second surface element is omitted relevant measures, referred to below are.

The first picture element is a first surface element, the second picture element a second surface element, the third Picture element a third surface element and the fourth picture element a fourth surface element of the photoconductor layer assigned. Since the surface elements differ from picture elements Colors exposed only once in the invention and thus e.g. the above repositioning for a second exposure is not required can be guaranteed that the surface elements are exactly aligned with each other are. With three colors, when using colors, those in the color space a sufficiently large distance from each other have an additive color mix of many others Perform colors. For example, as the first color red, as second color blue and third color green. Positional means that there are adjacent surface elements not or only slightly cover and that between neighboring surface elements also no or almost none White spaces are created. By covering surface elements, the picture elements are assigned to different colors it leads to unwanted toner overlays that lead to a lead to poor print image. Through gaps between The color of the base material is unwanted to picture elements visible, which also creates a bad print image. In the invention, the unique pictorial Expose with a high degree of accuracy a covering and an Avoidance of gaps avoided. The consequence is one high print quality.

In the first embodiment of the invention, the Surface elements after a previous charge on Initial potential exposed differently in such a way that after the fourth surface element exposes a fourth potential has, the third surface element compared to the fourth Potential third third potential, the second Area element in relation to the third potential in terms of amount higher second potential and the first surface element a higher amount than the second potential has first potential. This will expose differently also known as imagewise exposure. This gradation of potential it is achieved that each color has exactly one potential value assigned. Another imagewise exposure step, where the surface elements with different Light energy to be irradiated can be dispensed with as it already does after a single image-wise exposure step, a clear one There is an assignment between potential values and colors.

After the imagewise exposure are in the first embodiment the invention, the surface elements with color particles the first color in a first development step developed. Color particles of the first color only deposited on the first surface elements. On the other No toner particles are deposited on surface elements. The first surface elements have at the time of this development step the greatest potential in terms of amount. In which development process used to apply the color particles the first color is a developing charged area development. At the The first embodiment of the invention is the color particles the first color positively charged to the selective To facilitate depositing on the first surface elements or to enable.

In a second development step, the first Embodiment of the invention with the surface elements Color particles of the fourth color developed. In doing so negatively charged color particles of the fourth color on the fourth surface elements deposited. The fourth surface elements have that at the time of this development step smallest potential in terms of amount. It is the second Development step accordingly to develop a discharged Area elements (discharged area development).

After the first two development steps, the Surface elements in the first embodiment of the invention near a light source with approximately uniform light distribution arranged. Arranging can e.g. by passing the Surface elements at the light source or by passing them the light source can be reached on the surface elements. But also a static arrangement of the surface elements opposite a light source with homogeneous light distribution is possible. Either the surface elements assigned to the print image of the layer system at the same time compared to the Light source arranged or the surface elements are successively arranged opposite the light source, e.g. Area elements, the image elements assigned to a line are exposed at the same time. The invention and the first embodiment is based on the knowledge that more color particles of other colors are deposited can, if similar potential relationships as before second development step. By the Deposition of the negatively charged color particles in the second development step the potential increases to the fourth Area elements in terms of amount. The one covered with color particles first and the fourth surface element covered with color particles are exposed much less than in the invention the uncovered surface elements, as the light is not or only weakly penetrates through the color particles. The Potential on the uncovered second surface element and on the uncovered third surface element, however reduced in amount since the incident light energy is not absorbed by color particles. The potential on the third surface element is after exposure with equal amount of light energy lower than that current potential on the fourth surface element. As a result, are similar for the second surface element Potential relationships before as before the second development step passed for the third surface element. In a third development step in the invention Color particles of the third color on the third surface elements deposited. There are none on the second surface elements Color particles deposited. When transferring the one to the other Colored particles deposited on the carrier in surface elements the carrier remains in areas in a later process step free of color particles when transferring the second surface elements assigned. This has the printed image ultimately picture elements with the color of the carrier.

In a further exemplary embodiment, the printed image contains at least one further picture element of a further color. During the image-wise exposure on the further surface element creates another potential between the first and the third potential or between the second and the third potential. By arranging the Layer system near the light source with approximately even Light distribution or near other light sources will further potential gradually lowered until it amounts is lower than the current potential on the already surface elements covered with color particles. Is this Condition can be reached in a further development step Color particles of the other color on the other Area element can be applied. In the invention, the possible number of different different colors Image elements in a print image only by the height of the initial potential, since the potentials that the individual colors are assigned, at least about 300 V. should be apart.

The invention also relates to a method in which instead of the negative initial potential is a positive initial potential is used, the respective instantaneous potentials on the surface elements instead of the negative sign have a positive sign. In addition, instead of positively charged color particles negatively charged color particles used and instead of the negatively charged color particles positively charged color particles are used. Thus relates the invention relates to two potential courses on the surface elements, which is only in the sign of the potentials differ. The technical effects are at both potential courses are the same.

The carrier can in one embodiment of the invention directly or indirectly in another embodiment With the help of an intermediate carrier, from which the Color particles are transferred to the carrier. By using The light-sensitive layer system can be an intermediate support be spared because of the material of the intermediate carrier can be selected so that there is contact between Intermediate carrier and layer system to minimal mechanical The surface of the photoconductor layer comes under stress. As Carrier becomes e.g. sheet material or continuous paper used.

The invention also relates to an electrophotographic Printer with the features of claim 13. The above The effects mentioned regarding the procedure also apply to the printer according to the invention. The printer according to the invention stands out from that in the mentioned European Printer described in addition to the printer described Characteristics from a simple structure. In particular the layer system is made up of only two layers and it is only one image-wise exposure step per Print image necessary, so that only an imagewise exposure unit with a simple control is required.

The invention can be used with a dry toner that is only solid Contains color particles, or executed with a liquid toner in which e.g. the color particles in a liquid are included.

Figur 1

eine Prinzipdarstellung eines elektrofotografischen Druckers mit wesentlichen elektronischen und mechanischen Funktionseinheiten,

Figur 2

die Druckeinheit des Druckers mit wesentlichen funktionellen Komponenten,

Figur 3

den Potentialverlauf auf dem Fotoleiter bei einem Belichtungsschritt und zwei Tonerpolaritäten,

Figur 4

den Zustand von Flächenelementen des Fotoleiters in verschiedenen Verfahrensschritten, und

Figur 5

einen zweiten Potentialverlauf auf dem Fotoleiter bei einem Belichtungsschritt und zwei Tonerpolaritäten.

The invention is described below using exemplary embodiments. Show:

Figure 1

a schematic diagram of an electrophotographic printer with essential electronic and mechanical functional units,

Figure 2

the printer's printing unit with essential functional components,

Figure 3

the potential curve on the photoconductor in one exposure step and two toner polarities,

Figure 4

the state of surface elements of the photoconductor in various process steps, and

Figure 5

a second potential curve on the photoconductor in one exposure step and two toner polarities.

Figure 1 shows a schematic diagram of an electrophotographic Printer 10 for performing an embodiment of the method according to the invention. Printer 10 has one transport device driven by a motor 12 and a shaft 14 16 for transporting an endless carrier material 18 past a printing unit 20 essentially according to a predetermined printing speed VD. Alternatively to Endless carrier material 18 can be used for a changed transport single sheets can also be printed. The printing unit 20 creates a multicolored toner image, e.g. with the help of a Corona device (see FIG. 2) on the carrier material 18 is transmitted.

After the carrier material 18 on the printing unit 20 in Direction of an arrow indicating the direction of transport 22 has been transported past, it becomes a fixing station 24 fed in which the still smearable toner image with the Backing material 18 with the help of pressure and temperature smudge-proof is merged. Seen in the transport direction 22 in front of the printing unit 20 is a first deflection unit 26 arranged which the carrier material 18 of the printing unit 20th feeds. Another deflection unit 28 stacks the printed one Carrier material 18 on a stack 30. The carrier material 18 is carried out by a stack 32 at the beginning of the printing process the first deflection unit 26 removed. Instead of the two Stacks 30 and 32 also use rolls on which the Backing material 18 is rolled up.

The printing process is controlled by a print controller 34, the at least one microprocessor 36 and a memory 38 contains. The microprocessor 36 works in the memory 38 stored print program and controls the printing process. In addition, the pressure controller 34 also prepares Memory 38 stored image data and transfers the processed image data via a control and data bus 40 to the printing unit 20. The motor 12 is via a control line 42 controlled by the pressure controller 34 so that the Carrier material 18 has a transport speed which in essentially corresponds to the printing speed VD. The pressure controller 34 is connected to a via data lines 44 Input / output device 46 connected, among other things, operating commands entered by an operator at the start of the printing process become.

Figure 2 shows the printing unit 20 of the printer 10 with essential functional components. The printing unit 20 contains a photoconductor 60 made of a flexible layer system consists of two conveyor rollers in the manner of a conveyor belt 62 and 64 is performed. The deflection roller 64 driven by a drive motor, not shown, by the pressure control 34 and the control and Data bus 40 is controlled. The printing unit 20 is from an opaque chassis 66 made of a stable Surround material. The chassis 66 has an opening 68 at which the photoconductor 60 is guided inside the printing unit 20 becomes. Outside of the printing unit 20 is the carrier material 18 guided past the opening 68. Through the opening 68 no light can strike the photoconductor 60 from the outside, because the entire printer 10 is opaque Has disguise. Opposite opening 68 is one Corona device 70 arranged with the one on the photoconductor 60 located toner image on the carrier material 18th is transmitted. Corona device 70 is also called Transfer printing device called.

The photoconductor 60 contains an electrode layer carrying zero potential 72 and one arranged approximately parallel to it Photoconductor layer 74 with the electrode layer 72 over a large area is in mechanical and electrical contact. The Photoconductor 60 is directed through the deflection rollers 62, 64 an arrow 76 moves. This will be a cross to the direction of transport of the photoconductor 60 area strip of the Photoconductor 60 successively on a charging device 78, a character generator 80, a developer station 82 for Deposition of black toner particles, a developer station 84 for depositing blue toner particles, a charger 86, a total exposure unit 88, a developer station 90 for depositing red toner particles, a transfer station 92, the corona device 70, an extinguishing device 94 and passed a cleaning device 96.

The charging device 78 contains a transverse to the transport direction 76 arranged corona device, each one surface strips of the transverse to the transport direction 76 Photoconductor 60, which is in the immediate vicinity of the charging device 78 is so charged that an initial potential VA of approximately -1200 V on the surface of the photoconductor layer 74 arises in the area of the area strip (cf. Figure 3, step S1).

The character generator 80 contains one transverse to the direction of transport 76 arranged row of light emitting diodes, each one Area of the photoconductor lying transversely to the transport direction 76 Illuminate 60 pictorially. The character generator 80 becomes controlled by the pressure controller 34 so that each Image signals for image elements of a line of the printed image at the same time converted into light signals from the light emitting diodes become. The exposure of the photoconductor 60 increases this Potential on the exposed surface elements of the photoconductor 60 since the photoconductor 60 in the exposed areas conducts better, thereby removing charge carriers from the surface of the Photoconductor layer 74 to the electrode layer 72 in the area of exposed areas can flow off. Surface elements, on where black toner particles are to be deposited not exposed; Area elements on which no toner particles should be deposited with a first Light energy exposed; Surface elements on which red Toner particles are to be deposited with a compared to the first light energy higher second light energy exposed and surface elements on which later blue Toner particles are to be deposited with a compared to the second light energy higher third light energy exposed. With increasing light energy this increases Potential on the respective surface elements, i.e. that Potential changes in a positive direction (see FIG. 3, Step S2).

The developer station 82 stores positively charged color particles of the color black K using an auxiliary electrode 120 with a potential VBIAS3 on surface elements that do not were exposed. The exact mechanism of action is based on of Figure 3 explained below (step S3).

The developer station 84 stores negatively charged toner particles the color blue B with the help of an auxiliary electrode 122 a potential VBIAS4 on surface elements that match the third light energy were exposed. The exact mode of action the developer station 84 is also below explained with reference to Figure 3 (step S4).

By depositing the negatively charged blue toner particles the potential on the surface elements with the third light energy were exposed, lowered again, i.e. changed in the negative direction. To the potential on this Lower surface elements even further, i.e. in negative To change direction, the photoconductor 60 on the charger 86 passed. Charger 86 contains a corona wire stretched transversely to the transport direction 76, who has a potential to charge the surface of the Photoconductor layer 74 in the area with blue toner particles covered surface elements to a potential VB5. The Potential VB5 is smaller in magnitude than a current one Potential VR5 on the surface elements with the second Light energy were exposed (see FIG. 3, step S5).

The strip of photoconductor 60 under consideration then turns on passed the total exposure unit 88. The total exposure unit 88 contains a laser diode that crosses in one arranged to the transport direction 76 of the photoconductor 60 Glass fiber array radiates light energy. The fiber optic array is designed so that essentially over its entire length same light energy is emitted. The light of the Total exposure unit 88 can not by already deposited black or blue toner particles shine through it this toner particle is absorbed. Meets the light of the Total exposure unit 88, however, on surface elements of the Photoconductor layer 74 which is not yet covered with toner particles the potential on these surface elements increased, i.e. changed in the positive direction (see FIG. 3, Step S6).

The developer station 90 stores negatively charged toner particles the color red R to the one with the second light energy exposed surface elements of the photoconductor layer 74. An auxiliary electrode 124 with a potential VBIAS7 used. The exact mode of action of depositing the red Toner particles are also shown in FIG. 3 below explained (step S7).

The positively charged black are in the transfer station 92 Reversed toner particles, so that almost all on the photoconductor 60 deposited toner particles are negatively charged. A reloading takes place on all surface elements of the Photoconductor, as a result of which the potentials on the Reduce surface elements, i.e. in the negative direction change (see FIG. 3, step S8). By this measure it is achieved that the transfer of the toner image from the photoconductor 60 on the carrier material 18 with the aid of the corona device 70 is carried out safely (cf. FIG. 3, step S9).

After transferring the toner image using the corona device 70 is now essentially toner particles free photoconductor 60 past the extinguishing device 94. The eraser 94 includes a corona device 98 and an exposure unit 100 through which on the photoconductor 60 existing residual charges are removed.

Toner particles left after the toner image is transferred remaining on the photoconductor 60 are in the cleaning device 96 using a brush 102 from the photoconductor 60 removed. After being transported past the cleaning facility 96 is the strip of the photoconductor under consideration 60 back in a clean initial condition and on about the same potential in all places.

Figure 3 shows the potential profile on the surface of the considered strip of photoconductor 60 in an exposure step and two toner polarities. On the abscissa axis is the time wasted in nine consecutive Time steps S1 to S9 is divided. On the ordinate axis is the potential on the surface of the photoconductor layer 74 with regard to the potential on the electrode layer 72 shown.

In step S1, the potential on the surface of the Photoconductor layer 74 by the action of the charging device 78 shifted in the negative direction to the initial potential VA, which, as already mentioned, has the value of -1200 V.

In step S2, the image-wise exposure is carried out using the Character generator 80, whereby the potential curve shown on the surface of selected surface elements the photoconductor layer 74 sets. Surface elements that later covered with black toner particles, are not exposed. The potential VA shifts these surface elements in the course of step S2 only slightly in the positive direction by a non-suppressable Self-discharge of the photoconductor 60 to a value CC2. The potential on the surface elements with the first light energy to be exposed changes into positive direction to a value VW2 of approximately -800 V. That Potential on the surface elements with the second Light energy being exposed changes in the course of Step S2 in the positive direction to a potential value VR2 of around -400 V. The potential on the surface elements, that were exposed with the third light energy changed approximately in step S2 in the positive direction Potential value VB2 of approximately -100 V.

In step S3, positive black toner particles are passed through the developer station 82 deposited. The auxiliary electrode 120 in the vicinity of the photoconductor 60 has the auxiliary potential VBIAS3 of about -900 V. On the auxiliary electrode 120 are the positively charged black toner particles. Because the potential VBIAS3 is lower than the potentials VW2, VR2 and VB2 these potentials with respect to the potential VBIAS3 positive. The however, positively charged black toner particles can only be deposited on an area that is in relation to the Potentials VBIAS3 has lower potential. It only applies for surface elements that are not exposed in step S2 were. As a result, the are on these surface elements black toner particles. By depositing the positively charged toner particles increase the potential the surface elements covered with black toner particles to a potential value VK3. Through the mentioned not too avoiding self-discharge of the photoconductor 60 increases the potentials VW2, VR2 or VB2 slightly to the potential values VW3, VR3 or VB3.

In step S4, negative blue toner particles are replaced by the Developer station 84 deposited. The auxiliary electrode 122 in the immediate vicinity of the photoconductor 60 has the auxiliary potential VBIAS4 of about -390V. Located on the auxiliary electrode 122 the negatively charged blue toner particles. Since that Potential VBIAS4 higher than the potentials VK3, VW3 and VR3 these potentials are related to the potential VBIAS4 in the negative direction. The negatively charged blue toner particles can only be deposited on one surface, which is a higher in terms of potential VBIAS4, i.e. in potential shifted in the positive direction. It only applies for surface elements that in step S2 with the third Light energy were exposed. Consequently, on this Surface elements deposited the blue toner particles. By the deposition of the negatively charged blue toner particles the potential on those with blue toner particles decreases covered surface elements to a potential value VB4. The self-discharge of the photoconductor 60 increases the Potentials VK3, VW3 or VR3 slightly on the potential values VK4, VW4 or VR4.

In step S5, the potential VB4 on the surface of the surface elements covered with blue toner particles with the help charger 86 reduced to about -390 V, i.e. in negative potential direction shifted. By self-discharge of the photoconductor 60, the potentials VK4 increase, VW4 or VR4 in step S5 to the potentials VK5, VW5 or VR5.

In step S6 by the total exposure unit 88 emitted light, the potential VW5 or VR5 on the surface elements not covered with toner particles about 400 V to the potentials VW6 or VR6. The potential on surface elements that in step S2 with the second Light energy has been exposed by further exposure in step S6 to the highest potential on one of the Surface elements. The potentials VK5 and VB5 increase slightly due to the self-discharge of the Photoconductor 60 to the potentials VK6 and VB6. Between Potentials VR6 and VB6 have a difference of about 400 V, so that in the following step S7 similar to step S4 Toner particles are deposited on the surface elements can, which is exposed in step S2 with the second light energy were.

In step S7, negative red toner particles are replaced by the Developer station 90 deposited. The auxiliary electrode 124 in the immediate vicinity of the photoconductor 60 has the auxiliary potential VBIAS7 of about -370 V. Located on auxiliary electrode 124 the negatively charged red toner particles. Analog to the electrical conditions described in step S4 the negative toner particles are deposited on the surface elements, which is exposed in step S2 with the second light energy were. The potentials VK6, VW6 and VB6 increase due to the self-discharge of the photoconductor 60 on the Potential values VK7, VW7 or VB7.

In step S8, the strip of the photoconductor under consideration 60 passed the transfer station 92. In the transfer station 92 contains a corona device that has the potential charges on the surface of the photoconductor layer 74. At the Transporting past the potential on all surface elements significantly reduced, the polarity of the black Toner particles on the photoconductor 60 is reversed so that the black toner particles are also negatively charged.

In step S9, the positively charged by the action Corona device 70 contains the toner particles with toner particles covered area elements essentially under Maintaining their position relative to one another on the carrier material 18 transfer. The potential on the surface elements increases the photoconductor 60 to about -400 V.

In a step, not shown, the still existing one Residual charge on the photoconductor 60 by the eraser 94 removed so that the photoconductor 60 on its surface after passing the quench 94 a potential value of about 0 V.

Figure 4 shows the state of surface elements of the photoconductor 60 at the end of steps S1 to S9. Part a of Figure 4 shows a print image 140, the four picture elements 142 to 148 contains. The picture element 142 has the color blue B, which in Figure 4 is represented by a horizontal hatching.

The picture element 144 has the color red R, which is shown in FIG vertical hatching is shown. The picture element 146 has the color black K, which in Figure 4 by an inclined Hatching is shown, its hatching lines around about 45 ° with respect to the horizontal. The Image element 148 has the color white W (color of the carrier material 18) represented by hatching in FIG whose hatching lines are approximately at an angle of 135 ° are aligned with the horizontal.

Part b shows a strip-shaped section 150 of the photoconductor 60. Section 150 is transverse on photoconductor 60 arranged to the transport direction 76. In Figure 4 the Section 150 shown in plan view, with the photoconductor layer 74 is above. By the pressure controller 34 Area elements 152 to 158 on the surface of the photoconductor 60 assigned the picture elements 142 to 148. The picture element 142 the surface element 152 is assigned. The picture element 144, 146 or 148, the surface element 154, 156 or 158 assigned. The assignment is made so that neighboring Area elements also adjacent image elements of the printed image 140 are assigned. In step S1, the charging device 78 on each of the surface elements 152 to 158 generates the initial potential VA.

Part c of FIG. 4 shows the state of the surface elements 152 to 158 after imagewise exposure in step S2. There on the surface element 152 has the greatest third light energy, takes place in the area of the surface element 152 through the Incidence of light well conductive photoconductor layer 74 a charge equalization instead, the potential VB2 of the surface of the surface element 152. The surface element 154 is exposed to the second light energy, which is lower than the third light energy. As a result, the lower potential compared to the potential VB2 VR2 on the surface of the surface element 154. The Surface element 156 does not become during imagewise exposure illuminated. As a result, arises on the surface of the Surface element 156 at the end of the imagewise exposure step S2 a potential VK2 that is only slightly above that Initial potential VA is. On the surface of the surface element 158 turns up after exposure with the first Light energy in step S2 the potential VW2. Since the first light energy is lower than the second light energy, the potential VW2 is lower than the potential VR2.

A surface element not covered with toner particles, which on End of one of the steps S2 to S9 has the highest potential, is indicated by an asterisk in the top right corner of each Area element marked. A not with toner particles covered area element that at the end of one of the Steps S2 to S9 has the lowest potential value by a cross in the top right corner of the concerned Area element marked. In part c of Figure 4 the surface element 152 has the greatest potential and the surface element 156 the smallest potential.

Part d of Figure 4 shows the surface potentials on the Area elements 152 to 158 at the end of step S3. While of step S3, section 150 becomes the developer station 82 transported past. From the above For this reason, black toner particles are only deposited on the surface of the surface element 156, so that this surface element completely covered with black toner particles (45 ° Hatching).

Part e of FIG. 4 shows the surface elements 152 to 158 End of step S4. In step S4, section 150 transported past developer station 84. In doing so for the above reasons, blue toner particles on the Surface element 152 deposited (horizontal hatching), so that now both the surface element 152 and the surface element 156 are covered with toner particles.

Part f of FIG. 4 shows the surface elements 152 to 158 am End of step S6 in which section 150 is even was exposed. Due to the even exposure it happens a potential increase on the surface of the surface elements 154 and 158 that are not covered with toner particles because the incident light the resistance of the photoconductor layer 74 reduced and a partial load carrier compensation between charge carriers on the surface of these surface elements and charge carriers takes place in the electrode layer 72. At the end of step S6, the surface element has 154 the greatest potential on its surface.

Part g of FIG. 4 shows the surface elements 152 to 158 am End of step S7. In the course of this step the Section 150 transported past developer station 90. Red toner particles are deposited for the reasons mentioned above on the surface element 154 (vertical hatching).

The surface elements 152 to 156 are thus with toner particles covered.

Part h of FIG. 4 shows a section 160 of the carrier material 18 at the end of step S9. The toner particles on the Section 150 are essentially below in step S9 Maintaining their mutual position on section 160 of the carrier material 18 transferred. The carrier material 18 has as already mentioned, the color white W (135 ° hatching), so that the print image 140 as a result of the described method with the picture elements 142 to 148 on the section 160 of the Carrier material 18 was printed.

A picture element has e.g. when printing with the printer 10 at a resolution of 600 pixels per 25.4 mm of approximately 0.042 mm, so that the representations in FIG. 4 a strong magnification with a magnification factor of is about 200. The human eye can see the pixels an ordinary reading distance of about 30 cm dissolve individually. This results in color mixing effects. The blue picture element 142 and the red picture element 144 result e.g. the mixed color perceived by the eye violet.

It comes from the three color process described above one goes to a method with n colors by the initial potential VA approximately equal to n times the potential requirement for one individual development step is selected. You also have to in the imagewise exposure at least n different Light energies can be generated per picture element, so that n + 1 different potentials can be generated. The Steps S5 to S7 become n-3 times after step S7 repeated. The letter n is a natural number, which can take the values 4, 5, etc.

Figure 5 shows a second potential profile on the surface of surface elements of the photoconductor 60 in an exposure step and two toner polarities. The one shown in Figure 5 Potential course applies to a printer that a not shown printing unit 20 ', which is different from the Printing unit 20 differs in that the corona device 70, the charging device 78, the charging device 86, the transfer station 92 and the corona device 98 with opposite ones Operating voltage can be operated. Besides, will instead of the developer station 82, a developer station uses the negative toner particles of black color With the help of an auxiliary electrode with the potential VBIAS3 'of about Deposits +900 V. Instead of the developer station 84 is a Developer station for positively charged blue toner particles used. The auxiliary electrode when depositing the blue toner particles has an auxiliary potential VBIAS4 'of around +390 V. Instead of the developer station 90 becomes a developer station used for positively charged red toner particles. At the An auxiliary electrode is used to deposit the red toner particles an auxiliary potential VBIAS7 'of approximately +370 V.

The potential curve shown in FIG. 5 differs of the potential curve of Figure 3 in that the sign the potentials are reversed compared to FIG. 3. Under The signs made with reference to FIG. 3 apply Statements also for the potential curve in FIG. 5. Instead of steps S1 to S9 there are now steps S1 ' to S9 '. Instead of the potential VA, a is shown in FIG Potential VA 'with opposite signs is used. In addition, changed potentials VK2 'to occur VK7 ', VW2' to VW7 ', VR2' to VR7 'or VB2' to VB7 'to the Place the potentials VK2 to VK7, VW2 to VW7, VR2 to VR7 or VB2 to VB7.

Claims (13)

1. Method for the electrophotographic printing of a print image (140) with a plurality of colours on a carrier (18),
   wherein a layer system (60) is charged to a starting potential (VA),
   at least three different potentials (VK2, VR2, VB2), referred to as first, third and fourth potential, are generated on the layer system (60) by image-wise exposure,
   the third potential (VR2) having a value that is greater in terms of amount than the fourth potential and the first potential (VK2) having a value that is greater in terms of amount than the third potential (VR2),
   in a developing step with colour particles of a first colour (K) and a first polarity, these colour particles are applied onto locations having the first potential (VK3),
   in a developing step with colour particles of a fourth colour (B) of the other polarity, these colour particles are applied onto locations having the fourth potential (VB4),
   the third potential (VR6) is lowered in terms of amount by uniform exposure to a value below the momentary value of the fourth potential (VB6) that is present after said developing step with colour particles of the fourth colour (B),
   and wherein in a developing step with colour particles of a third colour (R) of the other polarity, these colour particles are applied onto locations having the lowered, third potential (VR7),
   characterized in that the potential on surface elements already covered with colour particles is increased in terms of amount at least once before the uniform exposure (step S5).
2. Method according to claim 1, characterized in that the print image (140) contains at least one first image element (146) of a first colour (K), at least one second image element (148) having the colour (W) of the carrier (18), at least one third image element (144) having a third colour (R) and at least one fourth image element (142) having a fourth colour (B),
   a first surface element (156) of a photoconductor layer (74) is allocated to the first image element (146), a second surface element (158) is allocated to the second image element (148), a third surface element (154) is allocated to the third image element (144), and a fourth surface element (152) of the photoconductor layer (74) is allocated to the fourth image element (142),
   the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),
   and in that the following steps are implemented:

   S1) the surface elements (152 to 158) are charged to a negative starting potential (VA) (step S1),

   S2) the surface elements (152 to 158) are differently exposed such that, following the exposure, the fourth surface element (152) has a fourth potential (VB2), the third surface element (154) has a third potential (VR2) that is higher in terms of amount compared to the fourth potential (VB2), the second surface element (158) has a second potential (VW2) that is higher in terms of amount compared to the third potential (VR2) and the first surface element (156) has a first potential (VK2) that is higher in terms of amount compared to the second potential (VW2) (step S2),

   S3) the surface elements (152 to 158) are developed with colour particles of the first colour (K) (step S3),
   wherein positively charged colour particles of the first colour (K) are deposited on the first surface element (156) using a first auxiliary electrode (120) that has a first auxiliary potential (VBIAS3) that is higher in terms of amount than the momentary potential (VW3) on the second surface element (158) and lower in terms of amount than the momentary potential (VK3) on the first surface element (156),

   S4) the surface elements (152 to 158) are developed with colour particles of the fourth colour (B) (step S4),
   wherein negatively charged colour particles of the fourth colour (B) are deposited on the fourth surface element (152) using a second auxiliary electrode (122) that has a second auxiliary potential (VBIAS4) that is higher in terms of amount than the momentary potential (VB4) on the fourth surface element (152) and is lower in terms of amount than the momentary potential (VR4) on the third surface element (154),

   S6) the surface elements (152 to 158) are arranged close to a light source (88) having an approximately uniform light distribution (step S6),
   wherein the first surface element (156) covered with colour particles and the fourth surface element (152) covered with colour particles are exposed substantially less than the non-covered, second surface element (158) and the non-covered, third surface element (154),
   and wherein the momentary potential (VR6) on the third surface element (154) is reduced in terms of amount to a potential (VR6) that is lower in terms of amount than the momentary potential (VB6) on the fourth surface element (152),

   S7) the surface elements (152 to 158) are developed with colour particles of the third colour (R) (step S7),
   wherein negatively charged colour particles of the third colour (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) that has a third auxiliary potential (VBIAS7) that is higher in terms of amount than the momentary potential (VR7) on the third surface element (154) and lower in terms of amount than the momentary potential (VB7) on the fourth surface element (152) and than the momentary potential (VW7) on the second surface element (158).
3. Method according to claim 1, characterized in that the print image (140) contains at least one first image element (146) of a first colour (K), at least one third image element (144) of a third colour (R) and at least one fourth image element (142) of a fourth colour (B),
   a first surface element (156) of a photoconductor layer (74) is allocated to the first image element (146), a third surface element (154) is allocated to the third image element (144) and a fourth surface element (152) of the photoconductor layer (74) is allocated to the fourth image element (142),
   the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),
   and in that the following steps are implemented:

   S1) the surface elements (152 to 156) are charged to a negative starting potential (VA) (step 81),

   S2) the surface elements (152 to 156) are differently exposed such that, following the exposure, the fourth surface element (152) has a fourth potential (VB2), the third surface element (154) has a third potential (VR2) that is higher in terms of amount compared to the fourth potential (VB2), and the first surface element (156) has a first potential (VK2) that is higher in terms of amount compared to the third potential (VR2) (step S2),

   S3) the surface elements (152 to 156) are developed with colour particles of the first colour (K) (step S3),
   wherein positively charged colour particles of the first colour (K) are deposited on the first surface element (156) using a first auxiliary electrode (120) that has a first auxiliary potential (VBIAS3) that is higher in terms of amount than the momentary potential (VR3) on the third surface element (154) and lower in terms of amount than the momentary potential (VK3) on the first surface element (156),

   S4) the surface elements (152 to 156) are developed with colour particles of the fourth colour (B) (step S4),
   wherein negatively charged colour particles of the fourth colour (B) are deposited on the fourth surface element (152) using a second auxiliary electrode (122) that has a second auxiliary potential (VBIAS4) that is higher in terms of amount than the momentary potential (VB4) on the fourth surface element (152) and lower in terms of amount than the momentary potential (VR4) on the third surface element (154),

   S6) the surface elements (152 to 156) are arranged close to a light source (88) having an approximately uniform light distribution (step S6),
   wherein the first surface element (156) covered with colour particles and the fourth surface element (152) covered with colour particles are exposed substantially less than the non-covered third surface element (154),
   and wherein the momentary potential (VR6) on the third surface element (154) is reduced in terms of amount to a potential (VR6) that is lower in terms of amount than the momentary potential (VB6) on the fourth surface element (152),

   S7) the surface elements (152 to 156) are developed with colour particles of the third colour (R) (step S7),
   wherein negatively charged colour particles of the third colour (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) that has a third auxiliary potential (VBIAS7) that is higher in terms of amount than the momentary potential (VR7) on the third surface element (154) and lower in terms of amount than the momentary potential (VB7) on the fourth surface element (152).
4. Method according to claim 2 or 3, characterized in that the print image (140) contains at least one further image element of a further colour,
   the further image element being allocated to a further surface element of the photoconductor layer (74),
   the further surface element being charged to the starting potential (VA) (step S1),
   the further surface element being exposed such given different exposure that, following the exposure, it has a further potential that is higher in terms of amount than the third potential (VR2) and is lower in terms of amount than the second potential (VW2) or, respectively, the first potential (VK2) (step S2),
   in that, given repeated arranging of the surface elements (152 to 158) close to the light source (88) or, respectively, close to a respective light source, the non-covered further surface element is respectively considerably more exposed than surface elements covered with colour particles,
   wherein the potential on the further surface element is reduced in terms of amount, given the respective arranging,
   and in that the surface elements are developed with colour particles of the further colour,
   wherein negatively charged colour particles of the further colour are deposited on the further surface element using a further auxiliary electrode that has a further auxiliary potential that is higher in terms of amount than the momentary potential on the further surface element and lower in terms of amount than the momentary potentials on the other surface elements.
5. Method according to one of the claims 1 to 4, characterized in that the deposited colour particles having a positive polarity are recharged to a negative polarity (step S8).
6. Method according to claim 1, characterized in that the print image (140) contains at least one first image element (146) of a first colour (K), at least one second image element (148) with the colour (W) of the carrier (18), at least one third image element (144) of a third colour (R) and at least one fourth image element (142) with a fourth colour (B),
   a first surface element (156) of a photoconductor layer (74) is allocated to the first image element (146), a second surface element (158) is allocated to the second image element (148), a third surface element (154) is allocated to the third image element (144) and a fourth surface element (152) of the photoconductor layer (74) is allocated to the fourth image element (142),
   the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),
   and in that the following steps are implemented:

   S1) the surface elements (152 to 158) are charged to a positive starting potential (VA') (step S1')

   S2) the surface elements (152 to 158) are differently exposed such that, following the exposure, the fourth surface element (152) has a fourth potential (VB2'), the third surface element (154) has a third potential (VR2') that is higher in terms of amount compared to the fourth potential (VB2'), the second surface element (158) has a second potential (VW2') that is higher in terms of amount compared to the third potential (VR2') and the first surface element (156) has a first potential (VK2') that is higher in terms of amount compared to the second potential (VW2') (step S2'),

   S3) the surface elements (152 to 158) are developed with colour particles of the first colour (K) (step S3'),
   wherein negatively charged colour particles of the first colour (K) are deposited on the first surface element (156) using a first auxiliary electrode (120) that has a first auxiliary potential (VBIAS3') that is higher in terms of amount than the momentary potential (VW3') on the second surface element (158) and lower in terms of amount than the momentary potential (VK3') on the first surface element (156),

   S4) the surface elements (152 to 158) are developed with colour particles of the fourth colour (B) (step S4'),
   wherein positively charged colour particles of the fourth colour (B) are deposited on the fourth surface element (152) using a second auxiliary electrode (122) that has a second auxiliary potential (VBIAS4') that is higher in terms of amount than the momentary potential (VB4') on the fourth surface element (152) and lower in terms of amount than the momentary potential (VR4') on the third surface element (154),

   S6) the surface elements (152 to 158) are arranged close to a light source (88) having an approximately uniform light distribution (step S6'),
   wherein the first surface element (156) covered with colour particles and the fourth surface element (152) covered with colour particles are exposed substantially less than the non-covered second surface element (158) and the non-covered third surface element (154),
   and wherein the momentary potential (VR6') on the third surface element (154) is reduced in terms of amount to a potential (VR6') that is lower in terms of amount than the momentary potential (VB6') on the fourth surface element (152),

   S7) the surface elements (152 to 158) are developed with colour particles of the third colour (R) (step S7'),
   wherein positively charged colour particles of the third colour (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) that has a third auxiliary potential (VBIAS7') that is higher in terms of amount than the momentary potential (VR7') on the third surface element (154) and lower in terms of amount than the momentary potential (VB7') on the fourth surface element (152) and than the momentary potential (VW7') on the second surface element (158).
7. Method according to claim 1, characterized in that the print image (140) contains at least one first image element (146) of a first colour (K), at least one third image element (144) of a third colour (R) and at least one fourth image element (142) with a fourth colour (B),
   a first surface element (156) of a photoconductor layer (74) is allocated to the first image element (146), a third surface element (154) is allocated to the third image element (144) and a fourth surface element (152) of the photoconductor layer (74) is allocated to the fourth image element (142),
   the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),
   and in that the following steps are implemented:

   S1) the surface elements (152 to 156) are charged to a positive starting potential (VA') (step S1')

   S2) the surface elements (152 to 156) are differently exposed such that, following the exposure, the fourth surface element (152) has a fourth potential (VB2'), the third surface element (154) has a third potential (VR2') that is higher in terms of amount compared to the fourth potential (VB2'), and the first surface element (156) has a first potential (VK2') that is higher in terms of amount compared to the third potential (VR2') (step S2'),

   S3) the surface elements (152 to 156) are developed with colour particles of the first colour (K) (step S3'),
   wherein negatively charged colour particles of the first colour (K) are deposited on the first surface element (156) using a first auxiliary electrode (120) that has a first auxiliary potential (VBIAS3') that is higher in terms of amount than the momentary potential (VR3') on the third surface element (154) and lower in terms of amount than the momentary potential (VK3') on the first surface element (156),

   S4) the surface elements (152 to 156) are developed with colour particles of the fourth colour (B) (step S4'),
   wherein positively charged colour particles of the fourth colour (B) are deposited on the fourth surface element (152) using a second auxiliary electrode (122) that has a second auxiliary potential (VBIAS4') that is higher in terms of amount than the momentary potential (VB4') on the fourth surface element (152) and lower in terms of amount than the momentary potential (VR4') on the third surface element (154),

   S6) the surface elements (152 to 156) are arranged close to a light source (88) having an approximately uniform light distribution (step S6'),
   wherein the first surface element (156) covered with colour particles and the fourth surface element (152) covered with colour particles are exposed substantially less than the non-covered third surface element (154),
   and wherein the momentary potential (VR6') on the third surface element (154) is reduced in terms of amount to a potential (VR6') that is lower in terms of amount than the momentary potential (VB6') on the fourth surface element (152),

   S7) the surface elements (152 to 156) are developed with colour particles of the third colour (R) (step S7'),
   wherein positively charged colour particles of the third colour (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) that has a third auxiliary potential (VBIAS7') that is higher in terms of amount than the momentary potential (VR7') on the third surface element (154) and lower in terms of amount than the momentary potential (VB7') on the fourth surface element (152).
8. Method according to claim 6 or 7, characterized in that the print image (140) contains at least one further image element of a further colour,
   the further image element is allocated to a further surface element of the photoconductor layer (74),
   the further surface element is charged to the starting potential (VA') (step S1'),
   the further surface element is exposed such given different exposure that, following the exposure, it has a further potential that is higher in terms of amount than the third potential (VR2') and is lower in terms of amount than the second potential (VW2') or, respectively, the first potential (VK2') (step S2'),
   in that, given repeated arranging of the surface elements (152 to 158) close to the light source (88) or, respectively, close to a respective light source, the non-covered further surface element is respectively considerably more exposed than surface elements covered with colour particles,
   wherein the potential on the further surface element is reduced in terms of amount, given the respective arranging,
   and in that the surface elements are developed with colour particles of the further colour,
   wherein positively charged colour particles of the further colour are deposited on the further surface element using a further auxiliary electrode that has a further auxiliary potential that is higher in terms of amount than the momentary potential on the further surface element and lower in terms of amount than the momentary potentials on the other surface elements.
9. Method according to one of the claims 6 to 8, characterized in that the deposited colour particles of negative polarity are recharged to a positive polarity (step S8').
10. Method according to one of the preceding claims, characterized in that the deposited colour particles are transferred onto the carrier (18) from the photoconductor layer (74), while essentially retaining their mutual positions.
11. Method according to one of the claims 1 to 9, characterized in that the deposited colour particles are transferred onto an intermediate carrier, while essentially retaining their mutual positions,
    and in that the colour particles are transferred from the intermediate carrier onto the carrier (18), while essentially retaining their mutual positions.
12. Electrophotographic printer (10), particularly for implementing the method according to one of the claims 1 to 11,
    having a light-sensitive layer system (60) that contains an electrode layer (72) carrying a predetermined reference potential and a photoconductor layer (74),
    having a charging means (78) for generating a starting potential (VA, VA') on the photoconductor layer (72),
    an exposure means (80) for the image-wise exposure of the photoconductor layer (72),
    a first developer station (82) for depositing colour particles of a first polarity and a first colour (K) onto the layer system (60),
    a second developer station (84) for depositing colour particles of the other polarity and a second colour (B) onto the layer system (60),
    at least one total exposure unit (88) for uniform exposure (60),
    and having at least one further exposure unit (90) for depositing colour particles of the other polarity and a further colour (R) onto the layer system (60),
    characterized by at least one potential increasing means (86) for the amount-wise increase of only the respectively lowest potential in terms of amount on the layer system (60), after deposition of colour particles.
13. Printer according to claim 12, characterized by a recharging station (92) for charging the deposited colour particles of the first polarity to the other polarity,
    and/or a transfer means (70) for transferring the deposited colour particles from the layer system onto a carrier (18),
    and/or an erase means (94) for erasing a residual charge image on the layer system (60),
    and/or by a cleaning means (96) for cleaning the layer system (60).

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