DE2163591A1 - Method of forming multicolor images using electrostatic latent images - Google Patents

Description

Patent attorneys Dipl.-Ing. R Weickmann,

Dipl.-Ing. H.Weickmann, Dipl.-Phys. Dr. K. Fincke Dipl.-Ing. F. A-Veickmann, Dipl.-Chem. B. Huber

XPR

8 MUNICH 86, DEN

XEROX CORPORATIONi PO Box 860 820

Xerox Square Möhlstrasse 22, phone number 483921/22

Rochester, NlY, 14605 / V, St.A. . ^<983921/22>

Process for multicolor imaging by means of latent electrostatic images

The invention relates to a method for generating multicolor images by means of latent electrostatic images which an oppositely charged electroscopic developer is applied.

In the conventional electrophotographic method, a photosensitive recording medium with a photoconductive Layer on conductive surface evenly charged electrostatically and then with a reproducible Photo irradiated. Under the influence of the light image, the charge on the photoconductive layer becomes selective dissipated in the irradiated areas, creating a latent electrostatic image. This latent electrostatic charge image is then developed or made visible, in that oppositely charged, finely distributed electroscopic drawing particles are brought to the recording medium in such a way that they are attracted in the image areas. After the image development, the image that is now visible is changed to another Image carrier, for example a sheet of paper or similar, and transferred to it for the production of a permanent image, drawing fixed.

203829/0960

The basic electrophotographic process can also can be used to produce multicolored reproductions, for which the known color subtraction methods can be carried out are. Usually, when using electrophotography, this can be done in such a way that initially color separations of the Original image made with the components red, green and blue will. Each component is then used to record a separate electrostatic latent image on the Surface of a photoconductive layer, and these images are developed with toners, which! Coloring agents Contained in the complementary colors of the primary colors of the I color separations. The recorded red, green, and blue color components is developed with a toner that contains cyan, magenta, or yellow dye. Each developed image is customized to a final one Transferring the carrier sheet, which results in a multicolored reproduction of the original image. As most In general, it is necessary to use expensive and complicated mask and / or or to apply compensation processes to produce a true-to-life color reproduction. There is also the high number of irradiations and image transfers are required, there is also a difficulty in aligning the individual images on each other.

Any disadvantage that occurs with the multicolor subtractive image can certainly be attributed to the quality of the image obtained are diminished in importance. However, since this method is relatively expensive and complicated, it is suitable It is not suitable for practical use, especially when reproducing only one or two functional Colours.

209823/0360

A relatively simple one is disclosed in U.S. Patent 3,094 4-29 electrophotographic method for reproducing a two-tone original image is known. In this procedure a special write head is used for selective imaging on a sheet of zinc oxide, with the two color components recorded in the form of two latent electrostatic images, each with a specific charge density. One selective development of each color component is done by toner particles of different sizes, which are applied to the image carrier one after the other. The larger particles are applied first. Because of their size, they are only attracted and bound in areas that are denser contain toned images so that these areas are developed to the exclusion of the less dense areas. To In the first development step, the smaller toner particles are applied, thereby developing the less dense ones Image areas takes place. A color differentiation is obtained with this method simply by adding other dyes to the respective toner material. Although this procedure eliminates the alignment problems described for most of the does not have subtractive color method, it requires a special recording device and consequently has the contact development always has a certain Farbyer impurity.

The object of the invention is to provide multicolor imaging to simplify the type described above. To this end, the process steps required are to be reduced and no special recording devices are required. Furthermore, the change in color should must be kept to a minimum and, overall, ensure rapid and effective reproduction of multicolored images be.

209829/0960

A method of the type mentioned at the beginning is to solve this this task is designed according to the invention in such a way that each color component is recorded as a latent image with a discrete potential associated therewith that these latent images successively according to decreasing potentials with a correspondingly colored developer are developed and that each development is carried out in such a way that the developer material supplied in each case only In which at least one predetermined potential leads Bxld areas is bound.

With this method, an original image with differently colored information can be exposed to a uniformly charged photoconductive recording surface are used, each color component of the original with one associated with it specific charge potential is recorded. Then the plate is moved through a development station, on which a plurality of developing units are provided, the number of which corresponds to the number of distinguishable colors of the original image is equivalent to. An electrical control device can be associated with each developing unit, so that only such color components are developed which lead at least a predetermined potential. The recorded Images are developed in a sequence of decreasing potential values, making them different and distinguishable specific colors for each color component on the recording medium be generated.

The invention is illustrated below with reference to one of the figures to illustrate its further features and advantages illustrated embodiment described. Show it:

209829/09 6 0

_ 5 -

1 shows a schematic and perspective illustration an automatically operating electrophotographic reproduction machine which, according to the invention, the Enables multicolor imaging

Fig. 2 is an enlarged side view of one of the Machine according to ITig. 1 development units used,

3 shows a partial section of a dispenser roll used for the development unit according to FIG. 2 and

Figures 4 to 5 are graphical representations of the properties the development processes in automatic multicolor imaging in a machine Fig. 1.

In Fig. 1 is a schematic of an automatically operating electrophotographic Reproduction machine shown, produced with the multicolor images of an original according to the invention can be. The method according to the invention is suitable in the same way for a wide variety of applications, which is why the following description of a Embodiment should not be understood as limiting. The machine shown in Fig. 1 includes a rotatable mounted drum 10 with a photoconductive surface 11. This preferably consists of a photosensitive Material which shows a relatively panchromatic sensitivity behavior towards visible light.

The drum is rotated in the direction of the arrow shown and thereby transports the photoconductive surface past a number of process stations one after the other! The first station in the direction of rotation of the drum is a charging station A at which a corona generator 12 is provided is as exemplified by U.S. Patent

209829/0960

2 836 725 is known. This generator runs across the drum and serves to uniformly electrostatically charge the drum surface to a relatively high potential.

The charged photoconductive drum surface is then moved past an exposure station B, at which a movable one Optics 15 between an object mirror 16 and a Image mirror 17 is arranged. That with the ones to be reproduced. Original image 18 provided with information is stationary on. a support plate 19 is arranged, by means of a movable lamp arrangement 20, its surface elements are successively illuminated. The optics 15 are guided in a similar movement over the illuminated surface elements and focuses the light image reflected on them via the mirror system onto the moving photoconductive drum surface, whereby an image is generated on this. Further details of this type of scanning system can be found in U.S. Patent 3,062,0 °.

After recording the information, the drum surface becomes led to a development station C, on which two developing units 25 and 26 are provided are. As. will be explained, each developing unit brings a toner with a predetermined coloring agent onto the drum surface so that the original image recorded there is developed.

After the development, the now visible image is attached to a Image transfer station D passed, at which it was electrostatic is transferred onto a tape 27 which is the final image carrier. This is done by a usual Transfer corotron 28, which is designed similar to the charging device already described. The image carrier

209829/0960

tape is attached to the image transfer station at a speed passed that at surface speed the drum matches. A transport roller is used for this, which is rotated in such a way that the material to be transferred is smeared Image is prevented ..

After the image transfer, the image carrier tape is guided past an image fixing station E. This is where the bond between two rollers passed through which are exposed to heat and applying pressure to the belt so that the colored toner image is fixed. A heat fixing device, which works with pressure, iä: for example through U.S. Patent 3,256,002 is known.

The last processing station in the direction of drum rotation is a drum cleaning station F, at which a rotatably mounted fiber brush 30 near the photoconductive drum surface is arranged. The brush rotates in the direction shown and brings the brush fibers into moving contact with the drum surface so that any residual toner particles still present are removed from it will.

Per se, any suitable development method for practicing the invention can be used, preferably however, such a development is carried out, the one Relatively little background development guaranteed in order to keep color distortions to a minimum. Under the name "Background" is understood to mean those areas of the original image that do not contain any image information. Unintentionally "or accidentally, unwanted toner particles can sometimes appear randomly in these background areas during the Development, resulting in poor image quality of the reproduced image. At a

209829/0960

Multicolor processes lead to such background development to changes in color.

In the invention, so-called transfer development is preferred carried out. Here, a uniform layer of finely distributed toner material is first applied to the surface a donor element and then applied this to the photoconductive recording surface provided with the latent image brought up. The photoconductive recording surface and the donor element are located within the development zone arranged so that there is a very specific air gap between them. This has such a width that the toner particles on the donor element pass through the photoconductive recording surface as both elements move cannot touch the development zone. Contactless development takes place by transferring the toner particles across the air gap. The predominant mechanism of activation or removal of the toner particles the surface of the donor element and its attraction to the photoconductive recording surface is the electrostatic Force field present through the electrostatic latent image. Therefore, little or no toner is inadvertently produced deposited in the background areas.

In i "ig. 2 a development mechanism is shown how it is present within the development units 25 and 26. In this system, individual toner particles are initially created evenly applied to a cylindrical donor element 40 and this then continuously on the photoconductive Drum surface 10 is moved past through a development zone 63. The donor element and the recording surface are arranged so that they form an air gap 42, the width of which is typically in a range between approx. 0.025 mm and 0.25 mm.

20 9 829/0960

The donor element consists of a plurality of segments 41 aligned in the longitudinal direction of the donor element, as shown in I ¹ Xg. 3 are shown. Each segment is isolated from the adjacent one by dielectric strips 44 so that it can be provided with a specific charge potential independently of the other segments. Each segment is arranged on a conductive metal base 46, which is preferably made of aluminum. A thin dielectric lacquer coating 47 is provided thereon. A conductive grid 48 is disposed on this insulating coating so that the outer surface of each segment is divided into a plurality of discrete dielectric islands 49, each surrounded by the conductive material.

In practice, all are segmented conductive pads 46 via electrical conductors 50 with a not shown Connected voltage source, and the documents have a reference potential, which in the illustrated case is ground potential can be. Each grid segment 48 is connected to a control device 51 via a line 52. The control device 51 consists of several commutators which are provided on the donor roll axis 53. They share the scope the dispenser roll into several specific electrical areas. Each electrical area is connected to a voltage source 55 connected, which is connected via lines 56 and contact brushes 57. It gives the grid segments that to bypassing their contact, a predetermined potential.

In the illustrated embodiment, the segmented Dispenser roll arranged to move through five electrical areas. These areas include 1. a toner hold-up zone 60, in which toner agglomerates are also removed, 2. a toner loading zone 61, 3. a toner cartridge

209829/0960

- ίο -

measurement zone 62, 4-, a development zone 63 and finally 5. a cleaning zone 64.

First, the dispenser roll is guided past the loading zone 4-5, on which a housing 67 is provided. A lot of negatively charged toner particles are entrained within this housing brought into contact with the surface of the moving donor element. The grid segments that run through this electrical area have a relatively high potential of approx. +350 Volt., So that the dispenser roll attracts a layer of toner particles and holds it in a charged state. Hahe the donor item a vacuum line 68 is provided immediately following the housing 67. This runs across the Surface of the donor element and has a multitude evenly spaced suction nozzles that are a short distance from the surface of the dispenser roll. Through the line will creates a negative pressure, whereby toner agglomerates and loosely adhering toner particles are removed from the donor surface and a relatively uniform layer of toner is created. It has been found that surface protrusions of the toner layer cause undesirable background developments and thus degrade the quality of the development.

A charging zone 61 is provided between the vacuum line and the development zone 63, in which the potential of the conductive grid is reduced to a reference potential or ground potential. Inside the loading zone is a corona generator 69 arranged, which applies a uniform negative charge to the toner particles, whereby each toner particle receives largely the same charge level. The control of the uniformity of the toner layer and the uniformity the charge on the toner particles brings the development system into a state in which there is background development is kept to a minimum.

209 82 9/09 60

Not all agglomerates or loosely held toner particles that get into the background areas of the image during development could be transmitted through the vacuum line removed. To further prevent unwanted background development, a pseudo development system is inside the electrical zone 62 arranged. Here is an electrically biased roller 70, which is preferably a relatively smooth surface, rotated at the same peripheral speed as the donor element. The role introduces Potential of a voltage source? 1, which is approximately the background potential of the latent image and is between +100 and +200 volts. Further, in order to achieve a developing effect here, the outer periphery of the roller 70 has a Distance corresponding to the air gap 42 and the segmented grids passing through this area are obtained approximately the same potential as within the development zone 63. Through this pseudo development zone Toner particles, which would normally be transferred to the background areas of the latent image, on the surface beforehand the roller 70 deposited.

After the donor roll is free of toner agglomerates, the evenly distributed and uniformly charged toner layer is brought up to the latent electrostatic image on the photoconductive recording surface within the development zone 63, in which development takes place. As will be described in more detail, while the grid of the donor roll receives a potential which 'holds a threshold value at which the toner particles on the donor element activated and are tightened in the ¹ image surface portions on the drum surface in this electric field.

209829/0980

After the donor element has dispensed the required amount of toner during development, what remains on it becomes brought residual toner into the cleaning zone 64. Here will the grid potential is reduced back to reference potential or ground potential, and the surface of the dispenser roll is treated with an AC corona generator 75. This neutralizes the charge of the remaining toner particles. After this process, the toner particles can be easily removed from the surface of the dispenser roll using a brush 76 must be removed so that they can be sucked off.

To produce a color copy, the original is placed on the platen 19 (Fig. 1) and the rotated photoconductive drum surface with a continuous Photograph of the information of the original irradiated. The original document preferably contains. Colors or Shades that have a selective discharge of the photoconductive Allow recording surface to specific charge density values, determined by the reflective properties of the Original image. In general, the photoconductive Layer is more strongly discharged in those areas in which the light of brighter colors acts, while a weaker discharge is generated where the light comes on dark colors is reflected. Therefore, black images are generally considered to have the highest charge density latent images recorded. The white images are recorded with a relatively low charge density, while the others Colors lie between these two extreme values. It is also possible to create a latent image with predetermined Generate charge density values using an original image with information of one color. this happens by controlling the density or the light absorption properties of the original image.

209829/0960

The processes taking place in the method according to the invention are described below on the basis of an original image which has information in two discrete colors. The photoconductive layer records the information when irradiated with two specific charge values, each charge value being one of the two colors of the original image is equivalent to. The information recorded on the recording surface contains an information component and a color component. The images recorded with the higher potential are developed first, with a Toner is used with a first colorant. the Images provided with the lower potential are then developed using a toner with a second coloring agent. The development units 25 and 26 (Fig, T), in which the transfer development carried out are provided for this purpose.

The developing unit 25 that rotated first on the photoconductive drum surface acts, brings toner a first color on the more strongly charged parts of the image surface. For this purpose, the segmented grids of the Donation element provided with a potential if they through the development zone 63 (Fig. 2). This potential lies between the two recorded Image potentials on the photoconductive drum surface, preferably slightly higher than the lowest recorded Image potential. By applying an electrical bias to the Donor element is applied, an electrical control field is applied within the development zone between the photoconductor and generated the donor element which activates and the attraction of the toner particles from the donor element into the Regulated parts of the image. With the so pre-tensioned grid of the donor roll, its potential higher than the lower one recorded image potential, those with higher

209829/0960

Potential values recorded images with a first Color develops while those of lesser potential. just record images that remain undeveloped.

The operation of the first development unit is shown graphically in Fig. 4-. The dark or denser parts of the original image are recorded on the photoconductive surface 10 with a relatively high potential Y *. The lighter colored or less dense image parts of the original are similarly recorded with a lower potential Vp. By applying the bias voltage to the donor roller grids with a threshold potential Y-, which lies between Y ^ and V ^, only those image area parts are developed which were recorded with the higher potential values. These correspond to the darker or denser parts of the image. This development is done with the first color toner.

After the first development step, the drum top is area of the second development unit: 26 supplied »Here a preload is applied to the grid of the dispenser roll, which is reduced to about the background potential on the photoconductive layer when the roller is through the development zone is managed. Usually that is Background potential between +100 and +200 volts. the second development unit works in such a way that toner with a second coloring part in those image areas parts are brought, those with potential values higher are recorded as the background potential.

The second development step is shown graphically in FIG. It can be seen that by reducing the Bias voltage of the value V-, to the background potential V ^ the second developing unit comes into a state

209829/0960

in which the toner of the second color is applied to the image area parts recorded with the lower potential values will. It should also be noted that a certain residual stress also remains in the previously developed areas, which in some cases may be sufficient to activate and attract the toner during the second development as well, so that he gets into the previously developed areas. When the photoconductive recording surface is moved through the second development zone, toner particles can be in advance developed areas are activated and deposited so that greater development is achieved.

According to one operating mode, it can be specified that the image areas recorded with the higher potential values are marked with a black toners are developed while those with the lesser ones Potential recorded image areas are developed with a toner of a primary color. Such colors are, for example, red, blue, green, etc. It can happen that part of the toner has a primary color is deposited on previously black developed image area parts in the second development so that the black Image is given a certain color tint. In most cases, however, this effect is not particularly annoying when viewed and can be admitted. A second operating mode can also be carried out. Here contain the used toner coloring agents that are mixed subtractively in the second development step and the final Give copy two specific color components. For example, a first color can pass through to the photoconductive layer a yellow toner can be applied, which is then used in the subsequent development with a magenta toner is overdeveloped so that a red color arises. By adjusting the development tensions of the first Unit to a value that is considerably higher than the lower one

2098 29/0 9 60

Recording voltage, it is possible to have a sufficient To achieve overdevelopment in order to obtain the desired color mixtures.

The foregoing has been a preferred method for easily and efficiently making a color copy of a multicolor Originals, a single exposure and a single image transfer being carried out. The invention was used to develop images on a photoconductive recording surface with negatively charged toner material described. It should be noted, however, that this in no way restricts the invention. Because it is also possible to use oppositely charged toner material, which has a similar change in the correlation of all other charges and potentials. Hence, references are positive or negative Potential in the above description only as an explanation of the functional relationships between the individual To understand elements, in the same way all relationships can also be reversed.

209829/09 60

Claims (11)

Claims (j1 / Method of forming multicolor images by means of latent electrostatic images to which an oppositely charged electroscopic developer is applied, characterized in that each color component is represented as a latent image with a discrete potential associated therewith is recorded that these latent images are successively corresponding to decreasing potentials with a corresponding colored developer are developed and that gede development is carried out so that the respectively supplied Developer material is only bound in the image areas carrying at least one predetermined potential. 2. The method according to claim 1, characterized in that the developed color images simultaneously on a final Image carriers are transferred after the last development step has been carried out. 3. The method according to claim 1, characterized in that Each color component is developed with a developer, which is a subtractive mixture with the developer other color components allowed to result in a predetermined To produce color. 4. The method according to claim 2, characterized in that the transferred partial images on the final image carrier be fixed. 5. Device for performing the method according to a of claims 1 to 4- ,. characterized by several development units (25, 26), which act one after the other on a recording medium (10) and see an electroscope! 209829/0960 ■ - 18 - Apply winder material to it, which has a predetermined color assigned to it for each developing unit (25, 26) and by a device (44, 4-8, 4-9, 51, 57) " for the electrical regulation of each development unit (25, 26) to a predetermined potential within the action of the respective development unit (25, 26) on the recording medium (10). 6. Apparatus according to claim 5, characterized in that an image transmission device (28) for simultaneous Transfer of the developed color images to a final image carrier (27) is provided. 7. Apparatus according to claim 6, characterized in that a fixing device (29) is provided for the transferred images. 8. Apparatus according to claim 5, characterized in that the development units (25, 26) developer material for contain subtractive mixing of its color component with that of the other developing unit. 9. Device according to one of claims 5 to 8, characterized characterized in that a donor element (4-0) is provided in each development unit (25, 26), which is provided with a uniform layer of electroscopic marking particles is provided and with its surface close to the recording medium (10) is arranged so that the marking particles on an existing on the recording medium (10) electrostatic latent image are transferable. 10. Apparatus according to claim 9, characterized in that the donor element (4-0) of the first developing unit 209829/09 6 0 (25) a potential in the range between the highest and the leads to the next lower recording potential corresponding in each case to a color component of the original image. 11. The device according to claim 10, characterized in that two development units (25, 26) are provided and that the second developing unit (26) has a potential between the background potential of the electrostatic latent Image and the image potential of the less recorded image of the recording medium corresponding to a color component (10) leads. 20 9 82 9/0 960