EP0779560A2 - Electrostatic latent image developing method and apparatus - Google Patents

Abstract

The invention relates to a method and a device for developing an electrostatic latent image, which has been produced on a surface (4) of a movable intermediate carrier (2), by means of electrically charged dielectric color particles (6) which are separated by a gap (5) between the Surface of the intermediate carrier and a surface (3) of a developing device (1) are transported. According to the method of the invention, the gap (5) is largely and loosely filled with color particles (6) and the following voltage differences between the developing device and non-image areas on the intermediate carrier are successively generated along the transport path of the color particles through the gap: a first voltage difference, which is substantially zero, a second voltage difference in which the color particles in the non-image areas are completely separated from the intermediate support, and a third voltage difference which is smaller than the second voltage difference and in which the color particles which face the non-image areas still differ are at a distance from this surface. In this way, small voltage differences in the electrostatic latent image are sufficient to achieve an adequate and high-contrast transfer of printing ink to the intermediate carrier. <IMAGE>

Description

The invention relates to a method and an apparatus for developing an electrostatic latent image which has been formed on a surface of a movable intermediate carrier by means of electrically charged dielectric paint particles which are transported through a gap between the surface of the intermediate carrier and a surface of a developing device .

Such a method and the corresponding device are e.g. B. known from xerography and are used for development in laser printers, copiers, etc. In xerography, a photoconductor drum is electrically charged and exposed, thereby creating a latent charge pattern on the photoconductor drum that corresponds to the print density distribution of the image to be printed or copied. The latent charge image is then developed by supplying toner to the photoconductor drum which is attracted to and adheres to charged image areas on the photoconductor drum. The photoconductor drum forms an intermediate carrier for the developed toner image, which is then transferred to and fixed on a substrate such as paper.

The toner is supplied to the intermediate carrier from a developing device which, for. B. is a cylinder or a belt that runs past the intermediate carrier at a more or less small distance. Depending on whether the toner jumps over the gap between the developing device and the intermediate carrier or whether it touches the Sub-carrier is transmitted, a distinction is made between the so-called "jumper development" and the so-called "contact development".

An example of the "jumper development" is shown in US 39 97 688. In the technique described therein, as indicated in the preamble of claims 1 and 7, the toner consists of dielectric pigmented particles with a diameter between 5 and 20 microns. The developing device is a belt which rotates around several cylinders and which passes through the intermediate carrier, a photoconductor drum, at a distance which is many times larger than the diameter of the toner particles. The toner particles adhered to the belt by frictional electricity bounce under the action of an electric field across the gap between the belt and the photoconductor drum, leaving non-electrically charged areas of the electrostatic charge pattern colorless. The electric field on the side of the tape comes from an electrode with a pointed edge around which the tape is guided. This creates a non-uniform field that is strongest in the area of the edge. This provides sufficient field strength to detach the toner particles from the belt without strikethrough between the belt and the photoconductor drum. The change in direction as the ribbon passes the edge also increases the distance between adjacent toner particles of the color layer in the gap and reduces the cohesive forces between the toner particles so that less force is required to remove the individual toner particles from the layer.

In both the "jumper development" and the "contact development", however, the voltage differences between image areas and non-image areas of the electrostatic charge pattern be relatively high in order to obtain a sufficiently high-contrast toner image. This is not a problem if the charge pattern is formed by exposing a photoconductor that has been uniformly charged in advance to a few hundred or a thousand volts, e.g. B. in a photocopier or a laser printer.

Digital techniques have recently emerged in the field of printing technology in which a charge pattern is formed by a multiplicity of charge generators which are arranged in the pixel spacing and which are driven individually in accordance with the image information to be printed. Such a method is known from US 47 92 860. As described therein, an intermediate carrier has a surface on which a multiplicity of mutually insulated and individually chargeable microcells are arranged. A thermoplastic two-component ink is used as the printing ink, which is transferred to the intermediate carrier in the molten state. In this case too, relatively high voltages are required at the microcells if the printing ink is to be transferred with sufficient ink coverage. Therefore, the charge generators are formed by a special emitter array that is able to generate voltages of many hundreds of volts on the microcells. The effort involved could be reduced if less large voltage differences in the charge pattern were necessary.

The invention has for its object to provide a development technique that enables sufficient and high-contrast transfer of printing ink to an intermediate carrier, which has an electrostatic latent image with relatively small voltage differences between the different image areas.

This object is achieved according to the invention in a generic method by the characterizing features of claim 1 and in a generic device by the characterizing features of claim 7. Advantageous embodiments of the invention result from the features of the subclaims.

According to the invention, the ink particles are transported through the gap between the surface of the intermediate carrier and a surface of a developing device so that they more or less fill the gap, but no pressure is exerted on the ink particles. The invention can thus be understood as an intermediate model between "jumper development", in which there is essentially empty space in the gap, and "contact development", in which the colored particles are pressed against the intermediate carrier.

The invention provides a development technique which manages with relatively small voltage differences in the electrostatic latent image on the intermediate carrier, e.g. B. about 40 volts. Such voltages can be generated in a simple, reliable and inexpensive manner by means of conventional electronics.

The invention enables a latent electrostatic image with this property to be developed into a color image which enables adequate color coverage in image areas and which has no background coloration in non-image areas. The latter problem, the uncontrolled transfer of color particles to areas that are supposed to remain color-free, is particularly detrimental to conventional "contact development" techniques. In addition, according to the invention, the intensity of the color coverage can be controlled excellently, so that a very fine and faithful reproduction of gray levels is possible.

According to the invention, an electric field is generated in the gap, which changes in a certain way along the transport path of the color particles. For this purpose, either a tension on the surface of the developing device along the transport path is changed, or the stresses on the surface of the intermediate carrier are changed together along the transport path.

For a more detailed explanation it is assumed that the color particles transported in the gap are negatively charged, e.g. B. by friction electricity. Under this condition, in the following, voltages and charges are understood to mean positive voltages or charges unless otherwise stated. For example, the image areas on the intermediate carrier are under a certain (positive) tension, depending on the desired later gray level at this point, in order to attract the negatively charged color particles thereon. If the paint particles were charged positively, the voltages mentioned would be negative. For the sake of simplicity, a voltage of zero volts is chosen as the reference voltage, which generally corresponds to the earth potential. In a practical embodiment, this reference voltage can be shifted positively or negatively to the earth potential, the remaining voltages being changed accordingly while maintaining all voltage differences.

First, the voltages are adjusted so that the colored particles pass a substantially field-free area in which they can be distributed over the entire gap width. The only field from which a noteworthy effect originates in this area is the field which originates from charge islands on the surface of the intermediate carrier, ie from the image areas of the electrostatic charge image. These charge islands have a charge that is opposite to the charge of the color particles, so that some of the paint particles are attracted to it. The remaining color particles can remain in the loosely distributed state or adhere to the surfaces of the intermediate carrier or the developing device by means of local forces, provided they come close to them.

In the case where the voltage on the surface of the developing device is changed along the transport path, this voltage is next increased from zero or a value around zero to a value of a few 100 volts. This voltage can be chosen so high that there is just no breakdown between the developing device and the intermediate carrier. This creates a field in which the color particles in the non-image areas detach from the surface of the intermediate carrier, but partially adhere to them in the loaded image areas. In order for all of the color particles to lift off the surface of the intermediate carrier in the non-image regions, the close-acting forces to which color particles are exposed, which have adhered to the surface of the intermediate carrier in the preceding, essentially field-free region, must be overcome.

These close-acting forces are the Van der Waals forces, intermolecular forces with a maximum range in the range of a few tens of nanometers, and the so-called image forces. The Van der Waals force on a particle that is in the vicinity of a surface is referred to below as the adhesive force. Imaging power is the force on a charged particle in the vicinity of a conductive surface, which corresponds to the attraction of an oppositely charged particle, which can be imagined as a mirror image on the other side of this surface. The image power is inversely proportional to the square of the distance of the particle center from the surface and becomes negligible in the technique described herein small if this distance is greater than the range of the adhesive forces. Therefore, the adhesive forces and the image forces are collectively referred to herein as proximity forces, with a range in the range of a few tens of nanometers.

The applicant of the present invention has found that these proximity forces are the smallest or the easiest to overcome if the color particles or the surface of the intermediate carrier are designed such that the adhesive force and the image force on color particles which represent the surface of the intermediate carrier touch, are of the same order of magnitude.

The fact that the adhesive forces and the image forces are optimally of the same order of magnitude can be exploited not only in the context of the present invention, but also in any printing techniques in which particles have to be detached from a surface.

After the non-image areas have been completely freed from color particles in the technique of the invention, taking into account or taking advantage of the above-mentioned circumstance, the color particles which are opposite to the non-image areas are located at some distance from it. This is because the force which drives the color particles in the electric field of the gap towards the surface of the developing device increases suddenly as soon as the proximity forces are no longer effective. In the image areas, the number of particles that are more likely to be attracted to the image areas is reduced in favor of the number of particles that are more likely to be attracted to the surface of the developing device.

According to the invention, the tension on the surface of the Developing device again reduced, at most so much that the color particles are still a few tens of nanometers from the surface of the intermediate carrier in the non-image areas, so that the near-acting forces on this surface can just not be effective again. This prevents individual color particles from spontaneously switching to the non-image areas, and there is no background coloring in non-image areas.

On the other hand, the level of cleavage of the color layer in the image areas of the intermediate carrier shifts in favor of color particles that are transferred to the image areas. Therefore, even with slight voltage differences between the image areas and the non-image areas on the intermediate carrier, a high-contrast color transfer is possible, as is required for printing in offset quality. The force on the color particles that adjoin the intermediate supports in the non-image areas has a hysteresis property as a combination of the close-acting forces and the force of the electric field in the gap. This makes use of the invention in order to provide for a given color coverage with lower voltage differences in the electrostatic latent image without having to accept any deterioration in the background of the developed image.

Optimal reproduction of grayscale results when using color particles with an average diameter of between a few µm and 20 µm and a gap width between approx. 10 and 200 µm, the width of the gap being a multiple of the average diameter of the color particles. But it is also possible to make the gap only slightly larger than the diameter of the paint particles, z. B. only one layer of color particles is transported into the gap. Grayscale can still be reproduced under partly because the color particles are not strictly organized in practice and have different sizes. Therefore, the cleavage plane is not to be seen as a sharp boundary, but rather as an area in which there are different probabilities according to a Gaussian distribution that a single color particle is drawn in one direction or the other. According to the invention, very low gray levels can be achieved more easily and uniformly than in the conventional "jumper development", since the threshold voltage is significantly lower.

In order to loosen the paint particles and improve their statistical distribution, an alternating voltage of a few kHz can be superimposed on the third voltage. In the event that the third voltage is 100 volts, for example, the amplitude of the superimposed alternating voltage can be up to 200 volts, so that the third voltage is an alternating voltage with peak values between 0 and 200 volts and an effective voltage of 100 volts.

A rotating cylinder or a belt running around a cylinder can be used as the intermediate carrier. In the preferred embodiment, the surface of the intermediate carrier has a multiplicity of mutually insulated microcells which are individually charged outside the region of the gap. However, the surface of the intermediate carrier can also be a homogeneous dielectric layer on which charge islands have been generated in accordance with the desired print image.

The developing device can have a fixed plate, a fixed or rotating cylinder or a belt rotating around a cylinder. There are various options for transporting the color particles into the gap. For example, it is conceivable that the color particles through Gravity slip into the gap. Or one of the many other transport techniques that are known from development technology is used. For example, the color particles can adhere electrostatically to the intermediate carrier before the voltage on the intermediate carrier is brought to zero in the first region of the gap. Magnetic one-component developers can also be considered.

In the event that cylinders are used as the intermediate carrier or as the development device, the surface curvature, which has an effect on the field strength, may have to be taken into account when dimensioning the stresses in the gap. You get a better overview of the conditions in the gap and, above all, a longer distance on which the color particles in the different areas in the gap can be realigned if a belt is used for the intermediate carrier and / or the developing device, with the belt over the length of the gap runs parallel and synchronously with the opposite cylinder or belt.

In order to be able to change the tension on the developing device along the transport path of the color particles, its surface can have a multiplicity of conductive elements which run transversely to the transport path of the color particles, with adjacent conductive elements being more or less insulated from one another.

The conductive elements can be supplied with the voltages just required via sliding contacts, or they are capacitively or inductively coupled to generators which induce the corresponding voltages therein.

The conductive elements need not be completely isolated from one another. In the event that the surface of the intermediate carrier between the conductive elements is not is completely insulating, but has a low conductivity, the voltage curve can be even, and there are no abrupt field changes when the conductive elements z. B. reach a sliding contact. Not only macroscopic means such as e.g. B. conductor strips, but also microscopic structures, as they exist in directional materials that conduct better in a preferred direction than transversely to it.

Further features and advantages of the invention result from the following description of exemplary embodiments and from the drawing, to which reference is made.

Fig. 1

eine Querschnittsansicht eines Entwicklungszylinders und eines zylindrischen Zwischenträgers im Bereich eines Spaltes dazwischen,
und

Fig. 2a bis 2c

verschiedene Phasen des Entwicklungsprozesses in dem Spalt zwischen den Zylindern.

In it show:

Fig. 1

1 shows a cross-sectional view of a development cylinder and a cylindrical intermediate carrier in the region of a gap between them,
and

2a to 2c

different phases of the development process in the gap between the cylinders.

Fig. 1 shows a small section on the circumference of a development cylinder 1 and an intermediate carrier 2, which is also a cylinder. The development cylinder 1 and the intermediate carrier 2 are mounted above or below the figure on a printing press and are driven so that they rotate synchronously or with a defined differential speed in the arrow directions shown. A surface 3 of the development cylinder 1 and a surface 4 of the intermediate carrier 2 lie opposite one another, with a gap 5 between them.

The development cylinder 1 conveys z. B. four layers of paint particles 6 into the gap 5. The color particles 6 are z. B. negatively charged dielectric particles, the z. B. stick by electrostatic attraction in several layers on the surface 3 of the developing cylinder 1. The color particles 6 are shown in a regular arrangement only for simplification and clarity of the drawing; in practice they are more or less statistically distributed. In addition, the paint particles 6 are exaggerated in size compared to the size of the cylinder.

The gap 5 is so wide at its narrowest point between the development cylinder 1 and the intermediate carrier 2 that the ink particles 6 conveyed therein fill most of the gap 5 without being pressed together.

On the surface 3 of the development cylinder 1 there are a plurality of straight conductor strips 8, each of which extends perpendicular to the plane of the figure across the entire length of the development cylinder 1. The conductor strips 8 are arranged distributed over the entire circumference of the development cylinder 1 and are insulated from one another.

Inside the development cylinder 1 or on its sides, in the area of the gap 5 in the circumferential direction of the development cylinder 1, three fixed sliding contacts are arranged one after the other, which successively touch each of the conductor strips 8 when the development cylinder 1 rotates. Voltages U ₀ , U _max and U _E are successively applied to the conductor strips 8 via the sliding contacts.

The surface 4 of the intermediate carrier 2 has a multiplicity of conductive, mutually insulated microcells (not shown in FIG. 1), as described in US Pat. No. 4,792,860 mentioned above. These microcells, their size is chosen in accordance with the desired print resolution, selectively more or less heavily loaded at a point on the circumference of the intermediate carrier 2 that is not visible in the drawing. The surface 4 of the intermediate carrier 2 thus carries an electrostatic charge pattern that corresponds to the desired print image. In the gap 5, color particles 6 are selectively transferred to this charge pattern, so that behind the gap 5 there are color islands 7 of color particles 6 on the surface 4 of the intermediate carrier 2, which correspond to the colored areas of the image to be printed. This developed image is then transferred to another location on the circumference of the intermediate carrier 2 on paper and fixed thereon.

The manner of the ink transfer in the gap 5 will be explained with further reference to FIGS. 2a to 2c, which show the three areas in the gap 5 along the transport path of the color particles 6, in which the stresses on the surface 3 of the intermediate carrier 1 U ₀ , U _max or U _{E can be} created.

2a to 2c each show two microcells 9a, 9b on the surface 4 of the intermediate _carrier 2, the microcell 9a having a voltage U ₁ and the microcell 9b _having a voltage U _1min . U is _1min z. B. equals zero and U _{1 is} greater than U _1min , z. B. equal to 40 volts. The microcell 9a forms an image area in which maximum color saturation is desired, and the microcell 9b forms a non-image area to which no color is to be transferred.

In Fig. 2a, the voltage U ₀ on the surface 3 of the developing cylinder 1 is zero or approximately zero, so that the ink particles 6 in the gap 5 are not subjected to any general force. However, a part of the color particles 6 is attracted to the charged microcell 9a, and some colored particles adhere to the microcell 9b and to the surface 3 of the development cylinder 1 solely by means of local forces.

2b, the voltage on the surface 3 of the development cylinder 1 is a positive voltage U _max , which, at a few hundred volts, is substantially greater than the voltage U _{1 of} the microcell 9a and which generally attracts the colored particles 6 to the surface 3 of the development cylinder 1 . The voltage U _max is selected so that the color particles 6 are completely separated from the microcell 9b, even if they have stuck to them in the meantime. The proximity forces on color particles 6, which adhere to the microcell 9b, must therefore be overcome. A portion of the colored particles 6 which have been attracted by the microcell 9a in FIG. 2a is drawn towards the surface 3 of the development cylinder 1 in FIG.

2c, the voltage U _{max is} lowered to a voltage U _E that is less than U _max and greater than / equal to U ₁ . The voltage U _E is selected so that the color particles 6 immediately above the microcell 9b do not just touch it, more precisely that the near-acting forces from the microcell 9b are not yet able to draw color particles 6 to the microcell 9b. Along with the lowering of the voltage of the surface 3 to U _E , more paint particles 6 are again attracted to the microcell 9a, in FIG. 2c two layers of paint particles.

Under these conditions, the surfaces 3, 4 now diverge when the development cylinder 1 and the intermediate carrier 2 continue to rotate. The color layer splits above the microcell 9a at a height H above the microcell 9a. If the width of the gap 5 is L, the height H is approximately obtained by observation of a particle that is in equilibrium between the surfaces 3, 4. The following applies to such a particle 6: $${\text{<figure-callout id="1" label="U" filenames="imgf0001.png" state="{{state}}">U</figure-callout>}}_{\text{1}}{\text{/ H = U}}_{\text{E}}\text{/ (L - H)}$$

, entsprechend der Darstellung in Fig. 2c. Für einen Fall, daß ${\text{U}}_{\text{1}}{\text{= U}}_{\text{E}}$

, würde sich $\text{H = L/2}$

ergeben. Wie man in Fig. 2c sieht, ist die Dicke der übertragenen Farbschicht wesentlich größer als die Dicke der Farbschicht, die in der in Fig. 2b gezeigten Phase von der Mikrozelle 9a festgehalten wird. Die Spaltungsebene in der Farbschicht verschiebt sich zugunsten von übertragenen Farbteilchen in den Bildbereichen, aber nicht zugunsten einer Übertragung von Farbteilchen in den Nichtbildbereichen. Daher kommt man durch das Absenken der Spannung auf der Oberfläche 3 des Entwicklungszylinders 1 mit sehr geringen, durch eine herkömmliche Elektronik erzeugbaren Spannungsdifferenzen zwischen den Mikrozellen 9a, 9b aus, um in Bildbereichen Farbsättigung und in Nichtbildbereichen einen farbfreien Hintergrund zu erzielen.In the event that, for example, U ₁ = 40 volts and U _E = 100 volts, the result is $\text{H = 2L / 7}$

, as shown in Fig. 2c. In the event that ${\text{<figure-callout id="1" label="U" filenames="imgf0001.png" state="{{state}}">U</figure-callout>}}_{\text{1}}{\text{= U}}_{\text{E}}$

, would $\text{H = L / 2}$

surrender. As can be seen in FIG. 2c, the thickness of the transferred ink layer is substantially greater than the thickness of the ink layer which is held by the microcell 9a in the phase shown in FIG. 2b. The cleavage plane in the color layer shifts in favor of transferred color particles in the image areas, but not in favor of transferring color particles in the non-image areas. Therefore, by lowering the voltage on the surface 3 of the development cylinder 1, very small voltage differences between the microcells 9a, 9b that can be generated by conventional electronics are used in order to achieve color saturation in image areas and a color-free background in non-image areas.

The last voltage U _E also ensures that the non-transferred ink particles 6 adhere to the surface 3 of the development cylinder 1 and are transported out of the gap. The voltage U _E can be maintained or refreshed during the further rotation of the development cylinder 1, so that the surface 3 can take up new ink particles 6 and transport them again from the left into the gap 5, as shown in FIG. 1.

• 1. Die Entwicklungseinrichtung ist ein Band und der Zwischenträger ist ein Zylinder.
• 2. Die Entwicklungseinrichtung ist ein Zylinder und der Zwischenträger ist ein Band.
• 3. Die Entwicklungseinrichtung ist ein Band und der Zwischenträger ist ein Band.

While the embodiment shown in FIG. 1 is based on a cylindrical developing device and a cylindrical intermediate carrier, the developing device and / or the intermediate carrier can also have the shape of an endless belt that extends over a sufficiently long period Section conforms to the opposite cylinder or band, with a gap between them. This results in the following further embodiments:

• 1. The developing device is a belt and the intermediate carrier is a cylinder.
• 2. The developing device is a cylinder and the intermediate carrier is a belt.
• 3. The developing device is a belt and the intermediate carrier is a belt.

Instead of changing the voltage on the developing device between U0, U _max and U _E , the voltage on the opposite side can alternatively also be changed. In this case, the voltages U ₁ and U _{1min are} changed together while maintaining the voltage _intervals between them.

REFERENCE SIGN LIST

1

Development cylinder

2nd

Intermediate beam

3rd

Surface of the development cylinder

4th

Surface of the intermediate carrier

5

gap

6

Color particles

7

Color islands

8th

Conductor strips

9a, 9b

Microcells

Claims (17)

A method of developing an electrostatic latent image formed on a surface of a movable submount using electrically charged dielectric paint particles transported through a gap between the surface of the submount and a surface of a developing device.
characterized,
that the gap (5) is largely and loosely filled with colored particles (6), and in that the following tension differences between the surface (3) of the developing device (1) and non-image areas (9b.) successively along the transport path of the colored particles through the gap ) are generated on the surface (4) of the intermediate carrier (2): a first voltage difference (U ₀ - U _1min ), which is essentially zero, so that the color particles are not attracted or repelled essentially electrostatically by the surface of the developing device or the non-image areas, a second voltage difference (U _max - U _1min ) which provides an electric field between the surface of the developing _device and the non-image areas, by means of which the color particles in the non-image areas are completely separated from the surface of the intermediate carrier, and - A third voltage difference (U _E - U _1min ), which is smaller than the second voltage difference and the one provides an electric field between the surface of the developing device and the non-image areas, in which the color particles, which lie opposite the surface of the intermediate carrier in the non-image areas, are still at a distance from this surface. Method according to claim 1,
characterized,
that the different voltage differences are generated by different voltages (U ₀ , U _max , U _E ) on the surface (3) of the developing device (1), with voltages (U _1min ) in the non- _{image areas} (9b) and voltages (U ₁ ) in Image areas (9a) of the surface (4) of the intermediate carrier (2) are kept constant. Method according to claim 1,
characterized,
that the different voltage differences are generated by jointly changing voltages (U _1min ) in the non-image areas (9b) and voltages (U ₁ ) in image areas (9a) of the surface (4) of the intermediate _carrier (2), the surface (3) of the Development device (1) is kept at a constant voltage. Method according to one of the preceding claims,
characterized,
that the third voltage difference (U _E - U _1min ) is selected so that the color particles (6), which adjoin the non-image areas (9b), approach the non-image areas to a maximum of a few tens of nanometers. Method according to one of the preceding claims,
characterized,
that the nature of the color particles (6) and the surface (4) of the intermediate carrier (2) are chosen so that the adhesive force and the image force on color particles that touch the surface are of the same order of magnitude. Method according to one of the preceding claims,
characterized,
that an electrostatic latent image is generated on the surface (4) of the intermediate _carrier (2), in which the voltage difference (U ₁ - U _1min ) between image areas (9a) and non-image areas (9b) is a maximum of approximately 40 volts. Apparatus for developing an electrostatic latent image on a surface of a movable submount, having a developing device having a surface opposite to the surface of the movable submount with a gap therebetween, and means for transporting electrically charged dielectric paint particles through the gap ,
characterized,
that the device for transporting the colored particles (6) is designed to fill the gap (5) largely and loosely with colored particles, and in that the following three successive regions are formed along the transport path of the colored particles through the gap, in which there are different voltage differences between the surface (3) of the developing device (1) and non-image areas (9b) on the surface (4) of the intermediate carrier (2): - A first area with a first voltage difference (U ₀ - U _1min ), which is essentially zero, so that the color particles are essentially not electrostatically attracted or repelled by the surface of the developing device or the non-image areas, a second area with a second voltage difference (U _max - U _1min ) which provides an electric field between the surface of the developing _device and the non-image areas, in which the color particles in the non-image areas are completely separated from the surface of the intermediate carrier, and - A third area with a third voltage difference (U _E - U _1min ), which is smaller than the second voltage difference and which provides an electric field between the surface of the developing device and the non-image areas, in which the color particles, which the surface of the intermediate carrier in are opposite the non-image areas, are still at a distance from this surface. Device according to claim 7,
characterized,
that the colored particles (6) have an average diameter of between a few microns and 20 microns and that the gap (5) between the surface (4) of the movable intermediate carrier (2) and the surface (3) of the developing device (1) between 10 and Is 200 µm wide. Apparatus according to claim 7 or claim 8,
characterized,
that the width of the gap (5) is a multiple of the average diameter of the color particles (6). Device according to one of claims 7 to 9,
characterized,
that the paint particles (6) and the surface (4) of the intermediate carrier (2) are such that the The adhesive force and the image force on color particles that touch the surface are of the same order of magnitude. Device according to one of claims 7 to 10,
characterized,
that the difference between a voltage (U _1min ) on non-image areas (9b) and voltages (U ₁ ) on image areas (9a) of the surface (4) of the intermediate _carrier (2) is a maximum of approximately 40 volts. Device according to one of claims 7 to 11,
characterized,
that the third voltage difference (U _E - U _1min ) is superimposed by an AC voltage of a few kHz. Device according to one of claims 7 to 12,
characterized,
that either the charge of the paint particles (6) is negative and at least one voltage (U _max ) on the surface (3) of the developing device (1) in the second region and one voltage (U _E ) on the surface (3) of the developing device (1 ) are positive in the third area or that the charge of the colored particles (6) is positive and at least one voltage (U _max ) on the surface (3) of the developing device (1) in the second area and one voltage (U _E ) on the surface ( 3) the developing device (1) in the third area are negative. Device according to claim 13,
characterized,
that, in addition, voltages (U ₁ ) on image areas (9a) of the surface (4) of the intermediate carrier (2) are positive for negatively charged color particles and negative for positively charged color particles, and that a voltage (U ₀ ) is present the surface (3) of the developing _device (1) in the first region and a voltage (U _1min ) on non- _{image regions} (9b) of the surface (4) of the intermediate _carrier (2) are equal to or approximately equal to zero. Device according to one of claims 7 to 14,
characterized,
that the intermediate carrier is a rotating cylinder (2) or a belt rotating around a cylinder, the surface (4) of which has a plurality of mutually insulated and individually chargeable microcells (9a, 9b). Device according to one of claims 7 to 15,
characterized,
that the developing device has a fixed plate or a fixed or rotating cylinder (1) or a belt running around a cylinder, with a surface (3) which contains a multiplicity of conductive elements (8) which run transversely to the transport path of the color particles, wherein there is a high or infinitely high electrical resistance between adjacent conductive elements, and in that in each of the different areas along the transport path of the colored particles, a device for generating voltages (U ₀ , U _max , U _E ) in the conductive elements (8 ) is arranged. Device according to claim 16,
characterized,
that the devices for generating the voltages (U ₀ , U _max , U _E ) are sliding contacts which touch the conductive elements (8), or are capacitive or inductive devices for the contact-free induction of voltages in the conductive elements.

Images