XEROGRAPHIC MULTI COLOUR COPYING

Abstract: Apparatus for reducing the contamination of one dry toner or developer by another used for rendering visible latent electrostatic images formed on a charge-retentive surface, such as a photoconductive imaging member (12), causes the otherwise-contaminating dry toner to be attracted preferentially to the charge-retentive surface (12) in its inter-document and peripheral areas. The dry toner or developer so attracted is subsequently removed from the surface (12) at a cleaning station (F).

Description: This invention relates generally to the rendering of latent electrostatic images visible using dry toner or developer of different colors, and more particularly to the reduction of contamination of one color toner or developer by the or another. The invention can be utilized in the art of xerography or in the printing arts. In the practice of xerography, it is the general procedure to form an electrostatic latent image on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor (P/R). The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation. This charge pattern is made visible by developing it with a toner, which is a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or transferred to a receiving sheet to which it is fixed. This method of forming and developing charge patterns is described in US-A-2,297,691. Other methods of forming and developing electrostatic images are disclosed in US-A- 2,647,464, 2,576,047 and 2,825,814. Modern business and computer needs often make it advantageous and desirable to reproduce originals which contain two colors, so that the copy reproduced also contains two colors. An accounting report having certain information highlighted in a second color is one example of a type of document which would desirably be copied in two colors. Computer-generated cathode ray tube (CRT) displays are another example in which it is sometimes desirable to reproduce an image in two colors. For instance, it is sometimes desirable that those portions of the CRT display image representing permanent forms are reproduced in a first color, and those portions of the image representing variable information are reproduced in a second color. Several useful methods are known for making copies having two colors. Some of these methods make high-quality images in two colors. However, there is room for improvement. In particular, it is desirable to reduce contamination of one dry developer or toner by another. One method of two-color reproduction is disclosed in US-A-3,013,890 in which a charge pattern of either positive or negative polarity is developed by a single, two-colored developer. The developer comprises a single carrier which supports both triboelectric positive and negative toner. The positive toner is a first color, and the negative toner is of a second color. The method develops positively-charged image areas with the negative toner, and negatively-charged image areas with the positive toner. A two-color image occurs only when the charge pattern includes both positive and negative polarities. Two-color development of charge patterns is disclosed in US-A-3,045,644 disclosing the development of both positive and negative charge patterns. The development system is a set of magnetic brushes, one of which applies relatively-positive toner of a first color to the negatively-charged areas of the charge pattern, and the other of which applies relatively-negative toner to the positively-charged areas. Methods and apparatus for making colored xerographic images using colored filters and multiple development and transfer steps are disclosed in US-A- 3,832,170 and 3,838,919. US-A-3,816,115 discloses a method for forming a charge pattern having charged areas of a higher and lower strength of the same polarity. The charge pattern is produced by repetitively charging and imagewise exposing a coated xerographic plate to form a composite charge pattern. Development of the charge pattern in one color is disclosed. A method of two-color development of a charge pattern, preferably with a liquid developer, is disclosed in US-A-4,068,938. This method requires that the charge pattern for attracting a developer of one color be above a first threshold voltage, and that the charge pattern for attracting the developer of the second color be below a second threshold voltage, which is below the first threshold voltage. Both the first and second charge patterns have a higher voltage than does the background. Still another method of creating two-color images is disclosed in US-A-4,078,929, which utilizes a charge pattern of only one polarity on an imaging surface. The charge pattern includes charged areas at one voltage level corresponding to background, and charged image areas at two other voltage levels different from the background level. One of the image voltages is greater in magnitude than the background voltage, and the other is smaller. The charge pattern is developed with toner particles of two colors. The toner particles of one of the colors are positively charged, and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively-positive and relatively-negative carrier beads. The carrier beads support, respectively, the relatively-negative and relatively-positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness. As disclosed in US-A-4,403,848, a multi-color printer uses an additive color process to provide either partial- or full-color copies. Multiple scanning beams, each modulated in accordance with distinct color image signals, are scanned across the printer's photoreceptor at relatively widely separated points, there being buffer means provided to control timing of the different color image signals to ensure registration of the color images with one another. Each color image is developed prior to scanning of the photoreceptor by the next beam. Following developing of the last color image, the composite color image is transferred to a copy sheet. In an alternative embodiment, an input section for scanning color originals is provided. The color image signals output by the input section may then be used by the printing section to make full-color copies of the original. In US-A-4,562,129 there is disclosed an image-forming method comprising the steps of forming a latent electrostatic image having at least three different potential levels on a photosensitive member, and developing the latent electrostatic image with a developer to obtain a monochromatic or dichromatic copy image, the developer being composed of at least two components of a non-magnetic insulating toner and a high-resistivity magnetic carrier triboelectrically chargeable with the toner and having a high resistivity of at least 10¹² ohm-cm, the carrier being in the form of particles about 5 to about 40 microns in size, prepared by dispersing a magnetic fine powder in an insulating resin in a proportion of 50 to 75% by weight. US-A-4,562,130 relates to a composite image-forming method having the following features: (A) Forming a composite latent electrostatic image of potentials at three different levels by two image exposures, the potential of the background area (non-image area) resulting from the first image exposure being corrected to a stable intermediate potential which is constant at all times by charging the area with scorotron charging means. Accordingly the image can be developed to a satisfactory copy image free from fog. (B) The composite latent electrostatic image is developed by a single developing device collectively, or by two developing devices. In the latter case, the composite latent image is not developed after it has been formed, but the latent image resulting from the first exposure is developed first before the second exposure, and the latent image resulting from the second exposure is thereafter developed, whereby the fog because of an edging effect is prevented, to produce a satisfactory copy image. In US-A-4,346,982, there is disclosed an electrophotographic recording device having means for uniformly charging the surface of a light-sensitive record medium, means for forming latent images on the medium, and means for developing the latent images the means for forming latent images comprising a plurality of exposing means for exposing a positive optical image and a negative optical image in such a manner that the light-receiving region of the negative optical image overlaps the light-receiving region of the positive optical image, whereby a latent image is formed on the surface of the record medium consisting of a first area which does not receive any light and holds an original potential, a second area which receives the light of only the positive image, and holds a reduced potential from that of the original potential, and a third area which receives the light of both the negative and positive images and has its potential reduced to below that of the second area. It is known that positioning the developer housing containing a darker (e.g. black) toner first, and positioning the developer housing containing the lighter colored toner second, minimizes the effect of contamination of one toner by another. It is known to remove contaminants from a second developer housing which has become contaminated by toner from a first housing. Such a feature is disclosed in US-A-4,351,604 in which first and second developing units apply toners of first and second colors respectively to a photoconductive drum carrying a bipolar electrostatic image to form a bicolor image. A small amount of the toner is scraped off the drum in the second developing step and becomes mixed with the second developing toner in the second developing unit. The admixed first toner is separated and removed from the second toner by a separation member in the form of a roller, belt or mesh-covered electrode which is charged to a polarity opposite to the first toner. Thus, the member electrostatically attracts the first toner while repelling the second toner. It is stated in this patent that "In a copying apparatus using liquid toners the first toner may be removed from the second toner by making use of an electrically charged, non-image area of the dielectric member. However, this expedient is not usable in dry copying apparatus."(See col. 1, lines 27-31). JP-A-59-174 860 (abstract) describes a method of removing mixed toner from the second developer housing by making the bias voltage of the second developer housing opposite in polarity to the electrostatic charge of a photoconductive recording body and making unwanted toner adhere to non-image parts of the content image. In JP-A-58-108 551 another method of removing mixed toner from the second developer housing is described. It is also known to remove contaminants such as paper fibers from the toner prior to use for developing the images. As noted hereinabove it is desirable to minimize the contamination of one toner by the other. Such contamination renders the developing characteristics of that system unstable. The present invention provides a method of selectively removing one of two or more dry toners from one of a plurality of developer housings including the steps of moving a charge-retentive surface with latent images thereon past the plurality of developer housings and electrically biassing said one of the developer housings to cause said one of the toners to be attracted selectively to an area of the charge-retentive surface, characterized by providing a surrogate charged image in an inter-document area on the charge-retentive surface whereby the electrical biassing of said one of the developer housings creates an electrostatic field between it and the surrogate charged image for causing said one of the toners to be attracted selectively to the surrogate charged image in the inter-document area whilst the other toner is repelled by the bias. The present invention provides an apparatus for selectively removing one of two or more dry toners from one of a plurality of developer housings including a chargeable surface positioned adjacent to, and outside of, the developer housings and means for electrically biassing said one of the developer housings to cause said one of the toners to be attracted selectively to an area of the charge-retentive surface, characterized by means for providing a surrogate charged image in an inter-document area on the surface of the charge-retentive surface, and transport means for moving the surface past said one of the plurality of housings whereby the electrical biassing of said one of the developer housings creates an electrostatic field between it and the surrogate charged image for causing said one of the toners to be attracted selectively to the surrogate charged image in the inter-document area whilst the other toner is repelled by the bias, and for transporting said one of the dry toners to a downstream cleaning station at which any residual toner is separated from the surface. This invention relates to an imaging process for producing multiple color images. Such a process is also known as highlight color imaging, or tri-level xerography. The present invention is directed to a highlight color process wherein two different-colored images are developed in a single pass of the photoreceptor past a pair of developer housings. In order to carry out the invention, the charge-retentive surface, initially charged to a voltage V₀, is discharged to V₀/2 imagewise in the background (white) image areas, and to substantially-zero or ground potential in the highlight (i.e. color other than black) parts of the image. There are two developer housings, one containing positive toner and the other negative toner. The charge-retentive surface containing the images is moved past these housings in a single pass. Color discrimination in the development of the electrostatic latent image is achieved by electrically biasing the two housings to a voltage which is offset from the background voltage V₀/2, the direction of offset depending on the toner in the housing. One housing contains black developer having triboelectric properties such that it is driven or attracted to the V₀-charged areas of the latent image by the electric field established between the V₀-charged areas of the P/R and the bias voltage level of that biased developer housing. The other housing contains developer that is triboelectrically charged so that it is attracted or driven to the discharged parts of the P/R. Prior to transfer of the two-color image, it is subjected to a pre-transfer corona discharge to condition the toner for effective transfer to a substrate using corona discharge. A practical problem, as noted above, which is encountered in single-pass, highlight color imaging using charged area development (CAD) for one color, and discharged area development (DAD) for the other pass, highlight color, is the accumulation of wrong color/polarity toner in the second development housing. This leads to the development characteristics of the second developer system becoming unsatisfactory. According to this invention, the contamination of the second developer housing with dry toner from the first housing is reduced by conditioning the P/R so that toner that finds its way into the second developer housing, is attracted to the photoreceptor. Such conditioning is effected in the inter-document zone and margins outside the the document area. An electrostatic field for attracting the toner to the P/R in the above-noted areas can be generated in different ways. For example, a corotron is positioned between the two developer housings. The corotron is used to charge the appropriate areas of the P/R to a potential greater than V₀/2. By so doing, the wrong-polarity toner in the second housing is presented with a surrogate charged image area on the P/R, so that the wrong-polarity toner is attracted to the P/R. Alternatively, the same surrogate charged image area can be generated in the second housing by switching the electrical bias thereon to a level below V₀/2. The wrong-polarity toner is then removed at the cleaning station. The foregoing examples of field creation are effected with the developer housing for developing the charged area of the photoreceptor being first, and the housing for developing the discharged area being second. The positions of the two developer systems (i.e DAD and CAD) are reversed in a second embodiment of the invention, in which the field is established by discharging the P/R in the non-image areas with an illumination source after these areas pass the first developer housing. Alternatives to using the illumination source are to shift the bias on the second developer housing, and to use a corona discharge device in lieu of the illumination source. The present invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a schematic view of an electrophotographic printing machine of the present invention; Figure 2 is a plot of photoreceptor potential illustrating single-pass, highlight color latent image characteristics, and Figure 3 is schematic illustration of a modified form of the invention illustrated in Figure 1. In this specification, 'black' is regarded as being a color. Figure 1 schematically depicts the various components of an illustrative electrophotographic printing machine incorporating the present invention. In as much as the art of electrophotographic printing is well known, the various processing stations employed in the printing machine illustrated in Figure 1 will be described only briefly. As shown in Figure 1, the printing machine utilizes a photoconductive (P/R) belt 10 which consists of a photoconductive surface 12 and an electrically-conductive substrate 14. Belt 10 moves in the direction of arrow 16 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt 10 is entrained about a pair of rollers 18 and 20, the former of which can be used as a drive roller and the latter of which can be used to provide suitable tensioning of the P/R belt 10. Motor 24 rotates roller 18 to advance belt 10 in the direction of arrow 16. Roller 18 is coupled to motor 24 by suitable means such as a belt drive. As can be seen by further reference to Figure 1, initially a portion of belt 10 passes through charging station A. At charging station A, a corona discharge device, such as a scorotron or corotron 25, charges the belt 10 to a high uniform potential, V₀. Next, the charged portion of photoconductive surface is advanced through exposure station B, at which the surface 12 is exposed to a laser-based input and/or output scanning device which causes the charge-retentive surface to be discharged in accordance with the output from the scanning device. The P/R, which is initially charged to a voltage V₀, is discharged to V₀/2 imagewise in the background (white) image areas, and to zero or ground potential in the highlight (i.e. color other than black) parts of the image. At development station C, a magnetic brush development system 30 advances developer materials into contact with the electrostatic latent images. The development system 30 comprises first and second developer housings 32 and 34. The magnetic brush development housings include two magnetic brush developer rollers 36 and 38. Each roller advances its respective developer materials into contact with the latent image. Each developer roller forms a brush comprising toner particles which are attracted by the latent images on the P/R. One of the two developer housings contains positive toner 40 and the other negative toner 42. The charge-retentive surface containing the images is moved past these housings in a single pass. Color discrimination in the development of the electrostatic latent image is achieved by electrically biasing the two housings 32 and 34 to a voltage equal to a voltage which is offset from the background voltage V₀/2. To this end there is provided a dc power supply 35 including a pair of voltage sources V black bias and V c bias . In the case of the housing 32 containing black toner the shift is toward V₀, and in the case of the housing 34 the shift is toward the zero voltage level. In the embodiment of the invention disclosed in Figure 1, the housing 32 contains black developer having triboelectric properties so that it is driven or attracted to the V₀ charged areas of the latent images by the electrostatic field established between the V₀ charged areas of the P/R and the voltage level of the biased developer housing. The housing 34 contains developer that is triboelectrically charged so that it is attracted or driven to to the discharged parts of the P/R. The latent image characteristics are depicted in Figure 2. A practical problem, as noted above, which is encountered in single-pass, highlight color imaging using charged area development (CAD) for one color and discharged area development (DAD) for the other color, is the accumulation of wrong-polarity toner in the second development housing. This leads to the development characteristics of the second developer system becoming unsatisfactory. According to this invention, the contamination of the second developer housing with dry toner from the first housing is reduced or minimized by conditioning the P/R so that toner that finds its way into the second developer housing, is attracted to the photoreceptor. Such conditioning is effected in the inter-document zone and margins outside the the document area. The field for attracting the toner to the P/R in the above noted areas can be generated in different ways. For example, as disclosed in Figure 1, a corotron 44 or other suitable corona discharge device is positioned between the two developer housings 32 and 34. The corotron so positioned is used to charge the the appropriate areas of the P/R to a potential greater than the voltage V₀/2 to which the developer housing 34 is biased. The bias voltage for the corotron is provided by a dc source 46. Thus, any wrong-polarity toner in the second housing is presented with a surrogate charged image area, so that it is attracted to the P/R while the toner 42 is repelled from the P/R in the non-image areas. Alternatively, the same surrogate charged image area can be generated in the second housing by switching the electrical bias thereon to a level below V₀/2. For this purpose a switch 48 is provided which is capable of connecting the developer housing 34 to the power source 35 or a ground connection 50. After being transferred to the P/R in the foregoing manner, the wrong-polarity toner which is attracted to the non-image areas of the P/R is then removed at the cleaning station. The foregoing examples of wrong-polarity toner purging are effected with the developer housing for developing the charged area of the photoreceptor being first in the path of movement of the images, and the housing for developing the discharged area being second in that path. The positions of the two developer systems (i.e. DAD and CAD) are reversed in the embodiment illustrated in Figure 3. In this embodiment, the field for attracting wrong-polarity toner to the non-image areas of the P/R is established by discharging the P/R in the non-image areas with an illumination source 52 after these areas pass the first developer housing. Prior to transfer of the two color images, they are subjected to a pre-transfer corotron discharge to bring all the toner to a common sign so it can be transferred to a substrate using corona discharge of the opposite polarity. A corotron 54 is provided for such pre-transfer. As successive electrostatic latent images are developed, toner particles are depleted from the developer material. Toner particle dispensers, not shown, are arranged to furnish additional toner particles to housings 32 and 34 for subsequent use by developer rollers disposed therein. A sheet of support material 58 is moved into contact with the toner image at transfer station D. The sheet of support material is advanced to transfer station D by conventional sheet-feeding apparatus, not shown. Preferably, sheet-feeding apparatus includes a feed roll contacting the uppermost sheet of a stack of copy sheets. Feed rolls rotate so as to advance the uppermost sheet from a stack into a chute which directs the advancing sheet of support material into contact with photoconductive surface 12 of belt 10 in a timed sequence so that the toner powder image developed thereon contacts the advancing sheet of support material at transfer station D. Transfer station D includes a corona-generating device 60 which sprays ions of a suitable polarity onto the back of sheet 58. This attracts the charged toner powder images from photoconductive surface 12 to sheet 58. After transfer, the sheet continues to move, in the direction of arrow 62, onto a conveyor (not shown) which advances the sheet to fusing station E. Fusing station E includes a fuser assembly, indicated generally by the reference numeral 64, which permanently affixes the transferred powder image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser roller 66 and a back-up roller 68. Sheet 58 passes between fuser roller 66 and back-up roller 68 with the toner powder image contacting fuser roller 66. In this manner, the toner powder image is permanently affixed to sheet 58. After fusing, a chute, not shown, guides the advancing sheet 58 to catch tray, also not shown, for subsequent removal from the printing machine by the operator. After the sheet of support material is separated from photoconductive surface of belt 10, the residual toner toner particles and the wrong sign/color toner particles carried by the non-image areas on the P/R are removed therefrom at cleaning station F. Subsequent to cleaning, discharge lamp (not shown) floods the photoconductive surface with light to dissipate any residual electrostatic charge remaining prior to the charging thereof for the next imaging cycle.

Claims: A method of selectively removing one of two or more dry toners from one of a plurality of developer housings (32, 34) including the steps of moving a charge-retentive surface (12) with latent images thereon past the plurality of developer housings (32, 34) and electrically biassing said one of the developer housings (32, 34) to cause said one of the toners to be attracted selectively to an area of the charge-retentive surface (12), characterized by providing a surrogate charged image in an inter-document area on the charge-retentive surface (12) whereby the electrical biassing of said one of the developer housings (32, 34) creates an electrostatic field between it and the surrogate charged image for causing said one of the toners to be attracted selectively to the surrogate charged image in the inter-document area whilst the other toner is repelled by the bias. A method as claimed by claim 1, characterized by moving the surface (12) to a cleaning station (F) for removal of any toner particles from it. An apparatus for selectively removing one of two or more dry toners from one of a plurality of developer housings (32, 34) including a chargeable surface (12) positioned adjacent to, and outside of, the developer housings (32, 34) and means (30) for electrically biassing said one of the developer housings (32, 34) to cause said one of the toners to be attracted selectively to an area of the charge-retentive surface (12), characterized by means (44, 52) for providing a surrogate charged image in an inter-document area on the surface of the charge-retentive surface (12), and transport means (24) for moving the surface past said one of the plurality of housings (32, 34) whereby the electrical biassing of said one of the developer housings (32, 34) creates an electrostatic field between it and the surrogate charged image for causing said one of the toners to be attracted selectively to the surrogate charged image in the inter-document area whilst the other toner is repelled by the bias, and for transporting said one of the dry toners to a downstream cleaning station (F) at which any residual toner is separated from the surface (12). An apparatus as claimed in claim 3, characterized in that said means (44, 52) for providing a surrogate charged image is a charge-varying means (44, 52) positioned intermediate two spaced-apart developer housings (32, 34). An apparatus as claimed in claim 4, characterized in that the charge-varying means (44, 52) comprises an electrostatic discharge device (44). An apparatus as claimed in claim 4, characterized in that the charge-varying means (44, 52) comprises a light source (52). An apparatus as claimed in any one of claims 3 to 6, characterized by means for adjusting the electrical bias on said one of the developer housings (32, 34).

Inventors: PARKER, DELMER GENE, STARK, HOWARD MARK
Applicants: XEROX CORPORATION
Application year: 1987
Main classification: G03G15
IPC codes: G03G15