EP0932852B1 - Method of multicolor electrophotographic printing with unipolar toner - Google Patents

Description

The invention relates to a method for electrophotographic Print a print image with multiple colors on one Carrier, wherein the printed image at least a first picture element with the color of the carrier, at least a second picture element with a second color and at least a third picture element with a third color. The first picture element becomes a first surface element of a photoconductor layer assigned. The photoconductor layer and a given one Electrode layer with reference potential is in one photosensitive layer system included. The reference potential is usually the zero potential. The second picture element becomes a second surface element and the third picture element a third surface element of the photoconductor layer assigned.

Such a method is e.g. from the US patent 4,078,929 known. The method described there carries the Name "tri-level process". A disadvantage of this method is that only toner particles of two colors in one printing process can be applied. In addition, the developer stations to apply the toner particles together operated opposite polarities. The requires additional constructive measures at the developer stations, so that the developer stations are not similar are built up. So that are the individual developer stations or their components and the toners either interchangeable, and the manufacturing and operating effort is higher than with similar developer stations and like toners.

From Japanese patent application JP-63265255 is a Known printing process in which only one of the toner particles Polarity can be used for the application of three colors. Before developing the second and third colors the potential on the areas of the photoconductor not yet covered each lowered by a total exposure. At the same time becomes the potential of the most recently developed areas raised.

A method is known from US Pat. No. 5,155,541 known in the techniques of color photography and the trilevel process be combined. Between two development steps With the same toner polarity, the potential is lowered by a total exposure with a lamp.

Methods are disclosed in the published patent application DE 44 08 978 A1 known for image generation, in which between developer steps a total exposure with the same toner polarity and possibly also an increase in potential becomes. Various methods are explained in which the total exposure either before that after the potential increase is carried out.

From Japanese patent application JP 08 044140 A is a Printing method for a multi-color printing known, in which A total exposure was carried out between two developer steps is to the potential on not yet developed Lower surfaces of the photoconductor.

The object of the invention is a simple method and a improved, simple printer for printing specify at least three colors with high print quality.

This task is carried out in a method of the type mentioned at the beginning Kind solved in that the specified in claim 1 Process steps are carried out. Compared to the known "Tri-level processes" have the color particles in the invention the first color and the second color at Develop the same polarity. This measure will achieved that the developer stations for applying the Color particles of the first color and the second color essentially can be constructed in the same way. The number various assemblies and components in the printer by the invention compared to known printers with dissimilar built developer stations reduced. It the invention results in a simple structure Printer.

In the invention, the surface elements are negative Initial potential loaded and then different exposed such that the third surface element after exposure has a third potential, the second surface element a higher amount than the third potential second potential and the first surface element one opposite the second potential is higher in terms of amount Has. This different exposure is called pictorial Expose called. This gradation of potential ensures that each color is assigned exactly one potential value is. Another imagewise exposure step in which Area elements irradiated with different light energies can be omitted, since already after a pictorial Exposure step a clear assignment between potential values and colors.

After imagewise exposure in the invention Surface elements in a first development step Color particles of the third color developed. In doing so negatively charged color particles of the third color only on the third surface elements deposited. On the other surface elements no color particles are deposited. The third Surface elements have at the time of this development step the smallest potential in terms of amount. As a result is a development of unloaded surface elements (discharged area development). In the invention, the Color particles negatively charged to selective deposition to facilitate on the third surface elements enable.

After the first development step, the surface elements arranged near a light source in the invention. The Arranging can e.g. by passing the surface elements the light source or by moving the light source past the surface elements can be achieved. But also a static one Arranging the surface elements opposite a light source with homogeneous light distribution is possible. there either the surface elements assigned to the print image of the layer system at the same time opposite the light source arranged or the surface elements are successively arranged opposite the light source, e.g. Surface elements, the picture elements are assigned to a line at the same time can be exposed.

The invention is based on the knowledge that the developer stations can be constructed in the same way if before one or more further development steps each same or at least similar potential relationships as before the first development step. By the Deposition of the negatively charged color particles in the first development step the potential increases to the third Area elements in terms of amount, because of these area elements charged color particles were applied. That with The third surface element covered with color particles is used in the Embodiment of the invention when arranged over the Light source with uniform light distribution considerably less exposed than the uncovered surface element because the light is not or only weakly weakened by the deposited Color particles penetrate. The potential at first Surface element and on the second surface element, however reduced in amount since the incident light energy is not absorbed by color particles. The potential on the second surface element is the same after exposure The amount of light energy is lower than the current one Potential on the third surface element. Therefore lie now similar conditions for the second surface element like before the first development step for the third Area element passed.

In a second development step in the invention Color particles of the second color on the second surface elements deposited. There will be none on the first 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 color particle-free, when transferring the first surface elements assigned. This has the printed image ultimately picture elements with the color of the carrier.

The first picture element is omitted if. all Image elements of a printed image covered with color particles become. In this case, the first picture element is all omitted or measures relating to the first surface elements.

In the invention, the printed image contains at least one another picture element of another color. The further The picture element becomes a further surface element of the photoconductor layer assigned. With image-wise exposure it will exposed further surface element so that it after Exposure has another potential, which is higher in terms of amount than the second potential and lower in amount than that possibly existing first potential. By repeated Arranging the layer system near the light source or The further potential becomes close to other light sources gradually reduced until the amount is less than the current potentials on those already with toner particles covered surface elements. In this state lie for that other area element similar conditions as before first or before the second development step for the third or second surface element. In another Development step can therefore color particles of the other Color on the further surface element similar to that in the previous development steps are deposited. In particular, the developer stations are in the development steps constructed similarly. Eliminated in manufacturing constructive measures regarding different developer stations. Because the potential is relatively low is developed is z.3. the effort for the electrical Insulation in the developer stations is low. In the invention is the number of developer stations with different ones Colors only by the level of the initial potential limited because of the potentials associated with each color are at least about 300 V apart.

In the invention, after applying negative loaded color particles of the respective color to at least one of the surface elements the potential on this Area element increased in amount, so the differences between the developer stations are even lower. The The potential is expediently increased up to a value that is slightly below the respective amount the next largest potential. By the specialist known measures can be achieved that the Potential relationships between the developer station and the developing surface element in all development steps are essentially the same. It is therefore only to be made one variant of the developer station in the required number, which reduces the manufacturing effort.

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 a negative sign have a positive sign. In addition, instead of negatively charged color particles positively charged color particles used. The invention thus relates to two potential profiles on the surface elements that are only in the Distinguish the sign of the potentials. The technical Effects are the same for both potential courses.

The carrier can in one embodiment of the invention directly or in another embodiment of the invention printed indirectly with the help of an intermediate carrier, from which the color particles are transferred to the carrier become. By using an intermediate support, the photosensitive Layer system can be spared because of the material of the intermediate carrier can be selected so that it at Contact between the intermediate beam and the layer system minimal mechanical stress on the surface of the photoconductor layer is coming. As a carrier e.g. sheet material or continuous paper.

The invention also relates to an electrophotographic Printer with the features of claim 5. The above The effects mentioned regarding the procedure also apply to the printer according to the invention. The printer according to the invention has a simple structure. In particular that is Layer system built from only two layers and it is only one image-wise exposure step per printed image is necessary, so that only one imagewise exposure unit with one simple control is needed.

The invention can be used with a dry toner that is only solid Contains color particles, or executed with a liquid toner in which the color particles are 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 den Flächenelementen des Fotoleiters bei einem Belichtungsschritt und einer Tonerpolarität, und

Figur 4

den Zustand von Flächenelementen des Fotoleiters in verschiedenen Verfahrensschritten.

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 printing unit of the printer with essential functional components,

Figure 3

the course of the potential on the surface elements of the photoconductor during an exposure step and a toner polarity, and

Figure 4

the state of surface elements of the photoconductor in various process steps.

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 Transfer printing corona device (see FIG. 2) onto 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 exists and like a conveyor belt around two pulleys 62 and 64 is performed. The deflection roller 64 is through a drive motor, not shown, driven by the pressure controller 34 and via the control and data bus 40 is controlled. The printing unit 20 is one opaque chassis 66 made of a stable material surround. The chassis 66 has an opening 68 at which the Photoconductor 60 passed 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 can no light from the outside strikes the photoconductor 60 since the entire printer 10 has an opaque panel. A corona device 70 is located opposite the opening 68 arranged with which one located on the photoconductor 60 Toner image is transferred to the carrier material 18. The Corona device 70 is also referred to as transfer printing device.

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 transport direction 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 Depositing blue toner particles, a charger 84, a total exposure unit 86, a developer station 88 for depositing red toner particles, a loading device 90, a total exposure unit 92, a developer station 94 for depositing green toner particles, a transfer station 96, the corona device 70, an extinguishing device 98 and passed a cleaning device 100.

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 (see FIG. 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 surface elements can flow off. Surface elements, on which no toner particles are to be deposited, are not exposed; Surface elements on which green Toner particles are to be deposited with a first exposed to light energy; 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 stronger (i.e. the potential changes in the positive direction) because the Conducting photoconductor increasingly better (see FIG. 3, step S2).

The developer station 82 stores negatively charged color particles of the color blue B using an auxiliary electrode 120 with a potential VBIAS3 on surface elements that with the third light energy were exposed. The exact mechanism of action is explained below with reference to FIG. 3 (Step S3).

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 potential direction. To the potential to lower it even further on these surface elements Photoconductor 60 guided past the charging device 84. The Loading device 84 contains one transverse to the transport direction 76 tensioned corona wire, which has a potential, the one Charging the surface of the photoconductor layer 74 in the area of the surface elements covered with blue toner particles a potential VB4 causes. The potential VB4 is absolute slightly smaller than the current potential VR4 on the surface elements, which exposes with the second light energy were (see FIG. 3, step S4).

The strip of photoconductor 60 under consideration then turns on the total exposure unit 86 passed. The total exposure unit 86 contains a laser diode that crosses in a 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 86 cannot by already deposited blue toner particles emit as it passes through the toner particles is absorbed. Strikes the light of the total exposure unit 86, however, on surface elements of the photoconductor layer 74, which are not yet covered with toner particles, so the potential on these surface elements is increased, i.e. it is changed in the positive direction (see FIG. 3, Step S5).

The developer station 88 stores negatively charged toner particles the color red R with the help of an auxiliary electrode 122 a potential VBIAS6 on surface elements that match the second light energy were exposed. The exact mode of action the developer station 88 is also below explained with reference to Figure 3 (step S6).

By applying the negatively charged red toner particles the potential on the surface elements with the second light energy were exposed, lowered. To do that Lowering the potential on these surface elements further the photoconductor 60 is guided past the charging device 90. The loading device 90 is essentially like the loading device 84 built. The corona wire in the charger 90 has a potential to charge the surface of the Photoconductor layer 74 in the area with red toner particles covered surface elements to a potential VR7. The The amount of potential VR7 is slightly smaller than the current one Potential VB7 on those covered with blue toner particles Area elements (see FIG. 3, step S7).

Thereafter, the photoconductor 60 on the total exposure unit 92 passed. The total exposure unit 92 is in the essentially constructed as the total exposure unit 86. The potential is increased by the total exposure unit 92 the surface elements lowered, which are not yet with toner particles are covered. This applies to the surface elements that with no light energy or with the first light energy were exposed (see FIG. 3, step S8).

The developer station 94 stores negatively charged toner particles of the color green G on the one with the first light energy exposed surface elements of the photoconductor 60. there an auxiliary electrode 124 with the potential VBIAS9 is used. The exact mode of action of applying the green Toner particles are also shown in FIG. 3 below explained (step S9).

In the transfer station 96, the potentials on the Toner particles covered area elements on about the same Lowered value, the potentials on the surface elements shift in negative potential direction (see FIG. 3, step S10). It also has the potential on the surface elements not covered with toner particles lowered. This measure ensures that the transfer of the toner image from the photoconductor 60 onto the carrier material 18 carried out safely with the aid of the corona device 70 becomes.

After transferring the toner image using the corona device 70 is now essentially toner particles free photoconductor 60 past the extinguishing device 98. The eraser 98 includes a corona device 102 and an exposure unit 104 through which on the photoconductor 60 existing residual charges are removed.

Toner particles left after the toner image is transferred on the photoconductor c0 are left in the cleaning device 100 using a brush 106 from the photoconductor 60 removed. After being transported past the cleaning facility 100 is the strip under consideration Photoconductor 60 again in a clean initial state and has roughly the same potential at all points.

Figure 3 shows the potential profile on the surface of the observed Strip of photoconductor 60 in an exposure step and toner polarity. On the abscissa axis is the time wasted in ten consecutive Time steps S1 to S10 is divided. On the ordinate axis is the potential on the surface of the photoconductor layer 74 regarding 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 the photoconductor layer 74 sets. Surface elements that are not covered with toner particles later should not be exposed. The Potential VA increases on these surface elements in the course of step S2 only slightly by one that cannot be suppressed Self-discharge of the photoconductor 60 to a value VW2. The potential on the surface elements with the The first light energy to be exposed increases to a value VG2 of approximately -800 V. The potential on the surface elements, which are exposed with the second light energy increases in Course of step S2 to a potential value VR2 of about -400 V. The potential on the surface elements with the third light energy were exposed increases in step S2 approximately to a potential value VB2 of approximately -100 V.

In step S3, negative blue toner particles are replaced by the Developer station 82 deposited. The auxiliary electrode 120 in near the photoconductor 60 has the auxiliary potential VBIAS3 from about -390 V. On the auxiliary electrode 120 are the negatively charged blue toner particles. Because the potential VBIAS3 is higher than the potentials VW2, VG2 and VR2 these potentials with respect to the potential VBIAS3 negative. The however, negatively charged blue toner particles can only show up of an area that is deposited with respect to the potential VBIAS3 has higher or positive potential. That is true only for surface elements that in step S2 with the third Light energy were exposed. As a result, on this Surface elements deposited the blue toner particles. By the deposition of the negatively charged toner particles will Potential on the surface elements covered with blue toner particles lowered to a potential value VB3. Through the mentioned unavoidable self-discharge of the photoconductor 60 the potentials VW2, VG2 and VR2 increase slightly the potential values VW3, VG3 and VR3.

In step S4, the current potential VB4 is on the surface of the surface elements covered with blue toner particles reduced to approximately -380 V with the aid of the charging device 84. The self-discharge of the photoconductor 60 increases the Potentials VW3, VG3 and VR3 in step S4 to the potentials VW4, VG4 or VR4.

In step S5 by the total exposure unit 86 emitted light, the potential VW4, VG4 or VR4 the surface elements not covered with toner particles in each case by about 400 V to the potentials VW5, VG5 or VR5 elevated. The potential on surface elements in step S2 were exposed with the second light energy further exposure in step S5 to the highest current one Potential VR5 on one of the surface elements in step S5. The potential VB4 increases slightly due to the Self-discharge of the photoconductor 60 to the potential VB5. There is a difference between the potentials VR5 and VB5 of about 400 V, so that in step S6 similar to step S3 toner particles are applied to the surface elements that have a potential greater than -380 V. However this now affects the surface elements in step S2 were exposed with the second light energy.

In step S6, negative red toner particles are replaced by the Developer station 88 deposited. The auxiliary electrode 122 in the immediate vicinity of the photoconductor 60 has the auxiliary potential VBIAS6 of about -370V. Located on the auxiliary electrode 122 the negatively charged red toner particles. Because the potential VBIAS6 is higher than the potentials VW6, VG6 and VB6, these potentials are negative with respect to the potential VBIAS6. However, the negatively charged red toner particles can only be deposited on an area that is currently a with respect to the potential VBIAS6 higher or positive potential Has. This only applies to surface elements that are in the Step S2 were exposed to the second light energy. As a result, the red ones become on these surface elements Toner particles deposited. By depositing the negative charged red toner particles reduces the potential on the surface elements covered with red toner particles to a potential value VR6. Through the self-discharge of the Photoconductor 60 increases the potentials VW5, VG5 and VB5 slightly to the potential values VW6, VG6 or VB6.

In step S7, the potential VR7 on the surface of the surface elements covered with red toner particles with the help charger 90 is reduced to about -360 V. Through the Self-discharge of the photoconductor 60 increases the potential VW6, VG6 or VB6 in step S7 to the potentials VW7, VG7 or VB7.

In step S8 by the total exposure unit 92 emitted light, the potential VW7 or VG7 on the surface elements not covered with toner particles about 400 V to the potentials VW8 or VG8. The potential on surface elements that in step S2 with the first Light energy has been exposed by further exposure in step S8 to the highest potential VG8 on one of the Area elements in step S8. The potentials VB7 and VR7 increase slightly due to the self-discharge of the Photoconductor 60 to the potentials VB8 and VR8. Between Potentials VR8 and VG8 there is a difference of about 400 V, so that in step S9 similar to that in steps S3 and S6 toner particles applied to the surface elements that have a potential greater than -360 V. However this now affects the surface elements in step S2 were exposed with the first light energy.

In step S9, negative green toner particles are replaced by the Developer station 94 deposited. The auxiliary electrode 124 in near the photoconductor 60 has the auxiliary potential VBIAS9 from about -350 V. The auxiliary electrode 124 contains the negatively charged green toner particles. Analog to the im Electrical conditions described in step S3 or S6 the negative toner particles on the sheet elements applied in step S2 with the first light energy were exposed. By depositing the negatively charged green toner particles reduce the potential to the surface elements covered with green toner particles onto one Potential value VG9. Increase the potentials VW8, VB8 and VR8 due to the self-discharge of the photoconductor 60 on the Potential values VW9, VB9 or VR9.

In step S10, the strip of the photoconductor under consideration 60 passed the transfer station 96. In the transfer station 96 contains a corona device which is a transhipment of the layer system to about -1200V. When transporting past, the Potentials on all surface elements significantly reduced.

In a step SH, not shown in FIG by the action of the positively charged corona device 70 the toner particles of sheet elements covered with toner particles essentially keeping their location transferred to each other on the carrier material 18. Doing so increases the potential on the surface elements of the photoconductor 60 to about -400 V. The remaining charge on the Photoconductor 60 is removed by the eraser 98 that the photoconductor 60 on its surface after passing the quenching device 98 has a potential value of approximately 0 V Has.

Figure 4 shows the state of surface elements of the photoconductor 60 at the end of steps S1 to S11. 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 green G, which in Figure 4 by an inclined Hatching is shown, the hatching lines of about 45 ° are arranged with respect to the horizontal. The picture element 148 has the color white W (color of the carrier material 18), the in Figure 4 is represented by hatching, the Hatch lines about an angle of 135 ° with respect to Are aligned horizontally.

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 points upwards. By the pressure control 34 surface elements 152 to 158 on the surface of the Photoconductor 60 assigned the picture elements 142 to 148. The The surface element 152 is assigned to the image element 142. The Picture element 144, 146 or 148 becomes surface element 154, 156 and 158 assigned. The assignment is made so that neighboring Area elements also neighboring picture elements of the Print image 140 are assigned. In step S1 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 reduction instead, as a result of which the potential VB2 on the Surface of the surface element 152 adjusts. The surface element 154 is exposed to the second light energy, the is lower than the third light energy. As a result the potential VR2, which is lower than the potential VB2 on the surface of the surface element 154. On the Surface of the surface element 156 turns after the Expose with the first light energy in step S2 Potential VG2. Because the first light energy is lower than is the second light energy, the potential VG2 is lower than the potential VR2. The surface element 158 is at pictorial exposures not illuminated. As a result itself on the surface of the surface element 158 at the end of the imagewise exposure step S2 the potential VW2, that is only slightly above the initial potential VA.

In FIG. 4, a surface element which is not covered with toner particles and which has the highest potential at the end of one of the steps S1 to S11 is identified by an asterisk in the top right corner of the respective surface element. In part c of FIG. 4, the surface element 152 has the greatest potential.
Part d of FIG. 4 shows the surface potentials on the surface elements 152 to 158 at the end of step S3. During step S3, section 150 is transported past developer station 82. For the reasons mentioned above, blue toner particles are only deposited on the surface of the surface element 152, so that this surface element is completely covered with blue toner particles (horizontal hatching).

Part e of FIG. 4 shows the surface elements 152 to 158 End of step S5 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, 156 and 158 that are not covered with toner particles because, as already mentioned, the incident light Resistance of the photoconductor layer 74 is reduced and a partial one Load carrier compensation between load carriers on the Surface of these surface elements and charge carriers in the Electrode layer 72 takes place. At the end of step S5 the surface element 154 has the largest on its surface Potential.

Part f of FIG. 4 shows the surface elements 152 to 158 am End of step S6. In the course of this step the Section 150 transported past developer station 88. Red toner particles are deposited for the reasons mentioned above on the surface element 154 (vertical hatching). The surface elements 152 and 154 are thus with toner particles covered.

Part g of FIG. 4 shows the surface elements 152 to 158 am End of step S8, in which the section 150 for the second Times evenly exposed. By even exposure as in step S5, there is an increase in potential on the surface of surface elements that are not with Toner particles are covered. This applies to step S8 Area elements 156 and 158. The potential on the area elements 156 and 158 each increase in step S8 400 V increased. At the end of step S8 the surface element has 156 the greatest potential on its surface.

Part h of FIG. 4 shows the surface elements 152 to 158 am End of step S9. In step S9, section 150 transported past developer station 94. In doing so green toner particles on the for the above reasons Surface element 152 deposited (45 ° hatching), so that now the surface elements 152, 154 and 156 covered with toner particles are.

Part i of FIG. 4 shows a section 160 of the carrier material 18 at the end of step S11. The toner particles on the Section 150 are essentially below in step S11 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 S7 to S9 become n-3 times after step S9 repeated. The letter n is a natural number, which can take the values 4, 5, etc.

With the help of an auxiliary electrode with the potential VBIAS3 'of about Deposits +390 V. Instead of the developer station 88, a Developer station for positively charged black toner particles used. The auxiliary electrode when applying the black Toner particle has an auxiliary potential VBIAS6 'of approximately +370 V. The charger 90, the total exposure unit 92 and the developer station 94 is omitted in the printing unit 20 ', because only two colors are printed.

The potential curve shown in FIG. 5 differs of the potential curve of Figure 3 in principle in that the Signs of the potentials reversed compared to FIG. 3 and that only three potential curves are shown. Taking into account the signs, those based on FIG. 3 apply statements made also for the potential courses of the figure 5. Instead of steps S1 to S6 there are now steps S1 'to S6'. Instead of the potential VA, a is shown in FIG potential VA 'of smaller amount with opposite, positive sign used. In addition, the sign changed potentials VR2 ', VR3', VR4 ', VR5' or VR6 'to the Place the potentials VB2, VB3, VB4, VB5 and VB6. Instead of of the potentials VR2, VR3, VR4, VR5 and VR6 occur in the Signs of changed potentials VK2, VK3, VK4, VK5 or VK6. The potentials VW2 ', VW3', VW4 ', VW5 'or VW6' replace the potentials VG2, VG3, VG4, VG5 or VG6 and describe the potential curve on surface elements the photoconductor layer 74, on which, however no toner particles are deposited.

Claims (6)

1. Method for the electrophotographic printing of a printing image (140) having a number of colours onto a substrate (18),

   in which the printing image (140) contains at least a first image element (148) having the colour (W) of the substrate (18), at least a second image element (144) having a second colour (R) and at least a third image element (142) having a third colour (B) and at least a further image element (146) of a further colour (G),

   the first image element (148) is assigned a first area element (158) of a photoconductor layer (74), the second image element (144) is assigned a second area element (154), the third image element (142) is assigned a third area element (152) and the further image element (146) is assigned a further area element (156),

   the photo conductor layer (74) and an electrode layer (72) carrying a predefined reference potential are contained in a layer system (60)

   and in which the following steps are carried out one after another in the specified order:

   S1) the area elements (152 to 158) are charged to an initial potential (VA) of a first polarity (step S1),

   S2) the area elements (152 to 158) are exposed differently in such a way that, following the exposure, the third area element (152) has a third potential (VB2), the second area element (154) has a second potential (VR2) whose magnitude is higher than the third potential (VB2), the further area element (156) has a further potential (VG2) whose magnitude is higher than the second potential (VR2), and the first area element (158) has a first potential (VW2) whose magnitude is higher than the further potential (VG2) (step S2),

   S3) the area elements (152 to 158) are developed with coloured particles of the third colour (B) (step S3),
   coloured particles at the first polarity and of the third colour (B) being deposited on the third area element (152) using a first auxiliary electrode (120), which is at a first auxiliary potential (VBIAS3) whose magnitude is higher than the instantaneous potential (VB3) on the third area element (152) and whose magnitude is lower than the instantaneous potential (VR3) on the second area element (154),

   S4) the area elements are arranged close to a potential-increasing device (84), the magnitude of the potential on the third area element (152) being increased after the coloured particles of the third colour (B) have been applied,

   S5) the area elements (152 to 158) are arranged close to a light source (86) having an approximately uniform light distribution (step S5),

   the third area element (152), covered by coloured particles, being exposed considerably less than the uncovered first area element (158), the uncovered further area element and the uncovered second area element (154),

   and the magnitude of the instantaneous potential (VR5) on the second area element (154) being reduced to a potential (VR5) whose magnitude is lower than the instantaneous potential (VB5) on the third area element (152),

   S6) the area elements (152 to 158) are developed using coloured particles of the second colour (R) (step S6),
   charged coloured particles at the first polarity and of the second colour (R) being deposited on the second area element (154) using a second auxiliary electrode (122), which is at a second auxiliary potential (VBIAS6) whose magnitude is higher than the instantaneous potential (VR6) on the second area element (154) and whose magnitude is lower than the instantaneous potential (VB6) on the third area element (152) and than the instantaneous potential (VW6) on the first area element (158),

   S7) the area elements are arranged close to a further potential-increasing device (90), the magnitude of the potential on the second area element (154) being increased after the coloured particles of the second colour (R) have been applied,

   S8) the area elements (152 to 158) are arranged close to a further light source (92), the uncovered further area element (156) in each case being exposed considerably more than area elements covered by coloured particles,
   and the magnitude of the potential on the further area element (156) being reduced to a potential (VG8) whose magnitude is lower than the instantaneous potential (VR8) on the second area element (154),

   S9) the area elements (152 to 158) are developed using coloured particles of the further colour (G),
   coloured particles at the first polarity and of the further colour (G) being deposited on the further area element (156) using a further auxiliary electrode (124), which is at a further auxiliary potential (VBIAS9) whose magnitude is higher than the instantaneous potential on the further area element (156) and whose magnitude is lower than the instantaneous potentials on the other area elements.
2. Method according to Claim 1, characterized in that following the development of the area elements with coloured particles of the further colour (step S9) the deposited coloured particles are brought approximately to an equal potential value (step S10).
3. Method according to one of the preceding claims, characterized in that the deposited coloured particles are transferred simultaneously from the photo conductor layer (74) to the substrate (18) whilst essentially maintaining their mutual position.
4. Method according to Claim 1 or 2, characterized in that the deposited coloured particles are transferred to an intermediate substrate whilst essentially maintaining their mutual position,
   and in that the coloured particles are transferred from the intermediate substrate to the substrate (18) whilst essentially maintaining their mutual position.
5. Electrophotographic printer (10), in particular for implementing the method according to one of Claims 1 to 3,

   having a light-sensitive layer system (60), which contains an electrode layer (72) carrying a predefined reference potential, and a photo conductor layer (74),

   having a charging device (78) for producing an initial potential (VA, VA') on the photo conductor layer (74),

   having an exposure device (80) for exposing the photo conductor layer (74) in accordance with an image,

   having a first developer station (82) for applying coloured particles at one polarity and of one colour (B) to the layer system (60),

   having at least two total exposure units (86, 92) for the uniform exposure of the layer system (60),

   having at least two potential-increasing devices (84, 90) for increasing the magnitude only of the potential with the respectively lowest magnitude on the layer system (60),

   and having at least two further developer stations (88, 94) for applying coloured particles at the polarity and of further colours (R, G) to the layer system (60),

   characterized in that the total exposure units (86, 92) and the potential-increasing devices, as well as the further developer stations (88, 94), are arranged in such a way that a strip of the area of the layer system (60) which is located transversely with respect to the transport direction of the layer system (60) is in each case led firstly past a potential-increasing device (84, 90), then past a total exposure unit (86, 92) and after that past a further developer station (88, 94).
6. Printer according to Claim 5, characterized by

   a recharging station (96) for charging the applied coloured particles to approximately an equal potential value,

   and/or a transfer device (70) for transferring the applied coloured particles from the layer system (60) to a substrate (18),

   and/or an erasure device (98) for erasing a residual charge image on the layer system (60),

   and/or by a cleaning device (100) for cleaning the layer system (60).

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