Highlight colour imaging

Abstract: A magnetic brush developer apparatus comprises a plurality of developer housings (32, 34) each including a plurality of magnetic brush rolls (35 - 38) associated therewith. Conductive magnetic brush (CMB) developer is provided in each of the developer housings. The CMB developer is used to develop electronically-formed images. The developer conductivity is in the range of 10⁻⁹ to 10-¹³ (ohm-cm)⁻¹. The toner concentration of the developer is in the order of 2.0 to 3.0% by weight; the charge level is less than 20 microcoulombs/gram, and the developer rolls are spaced from the charge-retentive surface a distance in the range of 1.0 to 3.0 mm.

Description: This invention relates generally to the rendering of latent electrostatic images visible using multiple colors of dry toner or developer and, more particularly, to an image forming method including structure for suppressing the development of the fringe fields of complementary tri-level images while developing acceptable line images, notwithstanding the presence of relatively high cleaning fields. The invention can be utilized in the art of xerography or in printing. In conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. 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 toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques. The concept of tri-level xerography is described in US-A-4,078,929 which teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second 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 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. In tri-level xerography, the xerographic contrast on the charge-retentive surface or photoreceptor is divided three, rather than two, ways, as is the case in conventional xerography. The photoreceptor is charged, typically to 900V. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V ddp or V cad, see Figures 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V c or V dad (typically 100v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas are exposed such as to reduce the photoreceptor potential to halfway between the V cad and V dad potentials, (typically 500V) and is referred to as V w or V white . The CAD developer is typically biased about 100V closer to V cad than V white (about 600V),and the DAD developer system is biased about 100V closer to V dad than V white (about 400V). Various techniques have heretofore been employed to develop electrostatic images as illustrated by the following disclosures which may be relevant to certain aspects of the present invention. As disclosed in US-A-3,457,900, magnetic brushes have been designed to give fringe - field or solid-area development by adjusting the conductivity of the carrier. It is also stated therein that they can also be made to tone areas of less charge, and clean areas of greater charge giving what is known in the art as a reverse development. As discussed in US-A-4,397,264, which relates to a conventional xerographic image development system, conductive magnetic brush (CMB) development and insulating magnetic brush (IMB) development systems suffer from limitations in their abilities to meet the full range of copy quality requirements. Specifically, insulating magnetic brush development systems have difficulty in using one developer roller to develop both fine-lines and solid areas. In order to optimize solid area development with an insulating developer material, the spacing between the developer roller and photoconductive surface must be made quite small. However, low-density fine-line development occurs at a larger spacing to take advantage of the accuracy of fringe-field development with insulating materials. This permits development with high cleaning fields so as to minimize background development. As further discussed in the '264 patent, conductive magnetic brush development systems inherently fail to reproduce low density lines faithfully. Conductive developer materials are not sensitive to fringe fields. In order to achieve low-density fine-line development with conductive developer materials, the cleaning field must be relatively low. This produces relatively high background. In tri-level xerography, the images comprise charged-area images and discharged-area images. Such images are commonly referred to as charged area development (CAD) images and discharged area development images, respectively. In a typical configuration where the charge-retentive surface is uniformly charged negative, the CAD image is developed using a charged area development (CAD) system including a positive black toner, with subsequent development of the discharged area using a discharged area development (DAD) system including a negative colored toner. When a CAD image or a background (V white ) region moves past the DAD housing a reverse development or cleaning field is established between the image and the developer rolls of that housing. The magnitude of the field is determined by the difference between the voltage level of the CAD image after development which is approximately equal to the CAD bias voltage V bb (Figure 1b) ,or the background, V white and the bias voltage on the discharged area development (DAD) system which is V cb . The field thus established tends to cause the negative toner to migrate away from the photoreceptor towards the developer rolls. Thus, when a fine-line moves through the DAD developer housing, particularly with its smallest dimension travelling in the process direction, the development field generated by the fine-line doesn't have time to attract enough toner that has drifted away from the photoreceptor surface into the developer back to the charge-retentive surface to develop the DAD fine-line image adequately. Because of the toner's inertia, it takes a finite time for the toner to move in response to a rapidly changing development field, and in the case of a fine-line, perpendicular to the process direction, there may not be sufficient time if the toner has migrated too far into the developer. Thus, line images may be improperly developed. This phenomenon is known as a developer history effect, which in this case is manifest as an underdeveloped fine-line. The shortcomings of CMB and IMB development systems discussed above with respect to conventional xerography have, heretofore, been present in highlight color xerography as well. In fact, the problem of not being able to develop low-density fine-lines with CMB developer in the presence of relatively high cleaning fields has resulted in the wide use of IMB developer. However, in a tri-level highlight color system the use of IMB developer has been found to be unacceptable. Its use results in the development of fringe fields in a color different from the rest of the image. Thus, for example, in a system that uses black and red developers, the black images would have a red border around them, while red images would have a black border around them. The development of such fringe fields is caused by the reverse development or cleaning fields established between the developer biases and a complementary image (either developed or latent) on the charge-retentive surface. The colored border around the black image results from the field established because of the difference (|V bb -V cb |, see figure 1b) between the developer biases as the black image passes through the red developer housing while the black border around the red image results from the field established because of the difference (|V bb- V c |) between the bias on the black developer housing and the voltage level of the red latent image on the charge-retentive surface as that image passes through the black developer housing. Since the use of IMB developer has been found to be unacceptable in a tri-level highlight system for the reason noted, and since CMB developer cannot develop line images in the presence of relatively high cleaning fields, it would appear that tri-level highlight color imaging in a single pass is not viable. It has been discovered that the source of the failure of CMB developers in developing line images is attributable to certain properties of conventional developer materials, as well as other aspects of the basic xerographic process. Thus, the development of optically (as opposed to electronically) formed images in a highlight color system using CMB developer yields images that are not totally acceptable. Optical image formation systems typically possess a large degree of flare (non-image-forming light) which makes it difficult to develop fine-lines using CMB developer, particularly when the smaller dimension of the image moves in the process direction. This is because the conductive developer, in the presence of large cleaning fields, moves toward the developer system, i.e. away from the imaging surface. Therefore, there isn't sufficient time for the toner to travel back to the imaging surface to develop the line images adequately. Even when electronically formed images have been used in highlight color systems, the resulting developed images were still not optimally formed. It has been discovered that the effects of adverse cleaning fields could be obviated by modifying the toner concentrations of conventional xerographic developers, which are too small (i.e. 1.0% by weight or less), and by modifying the charge levels of conventional xerographic developers, which are too high (i.e. 25 to 30 microcoulombs/gram), and further by spacing the developer rolls from the charge-retentive surface at a distance the range of 1.0 to 3.0 mm. Briefly, the present invention as claimed uses a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. Conductive magnetic brush (CMB) developer is provided in each of the developer housings. The CMB developer is used to develop electronically formed images. The developer conductivity, as measured in a Gutman conductivity cell, is in the range of 10⁻⁹ to 10-¹³ (ohm-cm)⁻¹. The toner concentration of the developer is 2.0 to 3.0% by weight and the charge level is less than 20 microcoulombs/gram. Additionally, the developer rolls are spaced from the charge-retentive surface a distance in the order of 1.0 to 3.0 mm. The present invention will now be described with reference to the accompanying drawings, in which: Figure 1a is a plot of photoreceptor potential versus exposure, illustrating a tri-level electrostatic latent image; Figure 1b is a plot of photoreceptor potential, illustrating single-pass, highlight color latent image characteristics; Figure 2 is a schematic illustration of a reprographic apparatus for performing the method of the invention, and Figure 3 is a plot of the magnetic fields around the central axis of a two-roll magnetic brush development system incorporated in the apparatus of Figure 2. For a better understanding of the concept of tri-level imaging, a description thereof will now be made with reference to Figures 1a and 1b. Figure 1a illustrates the tri-level electrostatic latent image in more detail. Here V o is the initial charge level, V ddp the dark discharge potential (unexposed), V w the white discharge level and V c the photoreceptor residual potential (full exposure). Color discrimination in the development of the electrostatic latent image is achieved by passing the photoreceptor through two developer housings in tandem, which housings are electrically biased to voltages which are offset from the background voltage V w , the direction of offset depending on the polarity or sign of toner in the housing. One housing (for the sake of illustration, the first) contains developer with black toner having triboelectric properties such that the toner is driven to the most highly charged (V ddp ) areas of the latent image by the electric field between the photoreceptor and the development rolls biased at V bb (V black bias) as shown in Figure 1b. Conversely, the triboelectric charge on the colored toner in the second housing is chosen so that the toner is urged towards parts of the latent image at residual potential, V c by the electric field existing between the photoreceptor and the development rolls in the second housing at bias voltage V cb (V color bias). As shown in Figure 2, for performing the method according to the invention one may utilize a charge-retentive member in the form of a photoconductive belt 10 consisting of a photoconductive surface and an electrically conductive substrate and mounted for movement past a charging station A, an exposure B, developer stations C, transfer station D and cleaning station F. 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 plurality of rollers 18,20 and 22, 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 photoreceptor belt 10. Motor 23 rotates roller 18 to advance belt 10 in the direction of arrow 16. Roller 18 is coupled to motor 23 by suitable means such as a belt drive. As can be seen by further reference to Figure 2, initially successive portions of belt 10 pass through charging station A, at which a corona discharge device, such as a scorotron, corotron or dicorotron 24, charges the belt 10 to a selectively high uniform positive or negative potential, V o . Preferably charging is negative. Any suitable control may be employed for controlling the corona discharge device 24. Next, the charged portions of the photoreceptor surface are advanced through exposure station B. At exposure station B, the uniformly-charged photoreceptor or charge-retentive surface 10 is exposed to a laser based input and/or output scanning device 25 which causes the charge-retentive surface to be discharged in accordance with the output from the scanning device. Preferably the scanning device is a three-level laser raster output scanner (ROS). The ROS output is set via a programmable power supply which is driven by means of a controller via a digital-to-analog converter. Alternatively, the ROS could be replaced by a conventional xerographic exposure device. The photoreceptor, which is initially charged to a voltage Vo, undergoes dark decay to a level V ddp . When exposed at the exposure station B, it is discharged to V w imagewise in the background (white) image areas, and to V c which is near zero or ground potential in the highlight (i.e. color other than black) color parts of the image. See Figure 1a. 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. Preferably, each magnetic brush development housing includes a pair of magnetic brush developer rollers. Thus, the housing 32 contains a pair of rollers 35, 36, while the housing 34 contains a pair of magnetic brush rollers 37,38. Each pair of rollers advances its respective developer material into contact with the latent image. Appropriate developer biasing is accomplished via power supplies 41 and 43 electrically connected to respective developer housings 32 and 34. Color discrimination in the development of the electrostatic latent image is achieved by passing the photoreceptor past the two developer housings 32 and 34 in a single pass, with the magnetic brush rolls 35,36,37 and 38 electrically biased to voltages which are offset from the background voltage V w , the direction of offset depending on the polarity of toner in the housing. One housing e.g. 32 (for the sake of illustration, the first) contains developer with black toner 40 having triboelectric properties such that the toner is driven to the most highly charged (V ddp ) areas of the latent image by the electrostatic field (development field) between the photoreceptor and the development rolls biased at V bb as shown in Figure 1b. Conversely, the triboelectric charge on colored toner 42 in the second housing is chosen so that the toner is urged towards parts of the latent image at residual potential, V c by the electrostatic field (development field) existing between the photoreceptor and the development rolls in the second housing at bias voltages V cb . In an embodiment of the invention, good quality images including line images were produced using developers 40 and 42 which comprise conductive magnetic brush (CMB) developer material with a conductivity in the range of 10⁻⁹ to 10-¹³ (ohm-cm)⁻¹. These developers comprise an insulative toner and a conductive carrier, the conductivity of the carrier being in the order of 10⁻⁹ to 10⁻¹⁰ (ohm-cm)⁻¹. The toner concentration of the developers 40 and 42 is in the order of 2.0 to 3.0% by weight and the charge level is less than 20 microcoulombs/gram. The developer rolls were spaced from the charge-retentive surface in the order of 1.0 to 3.0 mm. In tri-level xerography, the entire photoreceptor voltage difference (|V ddp -V c |, as shown in Figure 1a) is shared equally between the c harged a rea d evelopment (CAD) and the d ischarged a rea d evelopment (DAD). This corresponds to approximately 800 volts (if a realistic photoreceptor value for V ddp of 900 volts and a residual discharge voltage of 100 volts are assumed). Allowing an additional 100 volts for the cleaning fields in each development housing means an actual development contrast voltage for CAD of approximately 300 volts and an approximately equal amount for DAD. In the foregoing case the 300 volts of contrast voltage is provided by electrically biasing the first developer housing to a voltage level of approximately 600 volts and the second developer housing to a voltage level of 400 volts. 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 the stack into a chute which directs the advancing sheet of support material into contact with photoconductive surface 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. Because the composite image developed on the photoreceptor consists of both positive and negative toner, a pre-transfer corona discharge member 56 is provided to condition the toner for effective transfer to a substrate using corona discharge. 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 the belt 10 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 64, which permanently affixes the transferred powder image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser roller 66 and a backup roller 68. Sheet 58 passes between fuser roller 66 and backup 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 a catch tray, also not shown, for subsequent removal by the operator. After the sheet of support material is separated from photoconductive surface of belt 10, the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station F. Subsequent to cleaning, a discharge lamp (not shown) floods the photoconductive surface with light to dissipate any residual electrostatic charge remaining prior to the charging thereof for the successive imaging cycle. The magnetic brush rolls 35 and 36 may comprise any structures that provide a magnetic field that forms the developer material in the housing 32 into a brush-like configuration in the development zone between the rolls 35 and 36 and the charge-retentive surface. This arrangement effects development of one of the two tri-level images contained on the charge-retentive surface. The magnetic brush rolls 37 and 38 on the other hand are constructed such that development of the other of the two tri-level image is accomplished with minimal disturbance of the first image. To this end, the magnetic rolls 37 and 38 comprise magnetic force fields as depicted in Figures 3a and 3b, respectively. As shown therein, the radial force profiles of the these two rolls are such as to cause developer to be picked up from the developer housing 32 and conveyed to the top of the roll 37 where the developer becomes magnetically unconstrained. The developer is moved through the development zone in a magnetically unconstrained manner until it is attracted to the roll 38 because of the radial magnetic forces of that roll. Magnetic poles are designated N (north) or S (south). As illustrated in the drawings, the magnetic fields are plotted around the central axis of a two-roll magnetic brush development system such as the one comprising rolls 37,38. For a multiple-roll development system comprising more than two rolls, roll 38 is replicated. The rolls are driven synchronously in this example, although it is also possible to have independent drive mechanisms for each roller. Figure 3 depicts the radial components, respectively, of the fields of rolls 37 and 38. The development system additionally consists of a sump, or reservoir, of magnetic developer material, and optionally a mixing system, paddle wheel, or other apparatus to maintain the developing properties of the material in the sump. The developer rolls are rotary non-magnetic cylinders or shells having roughened or longitudinally-corrugated surfaces to urge the developer along by frictional forces around fixed internal magnets. The shells are driven synchronously in this example; it is also possible to have independent drive mechanisms for each roller. During the development process of the system, the direction of rotation of the shell around either fixed magnet is counterclockwise as viewed in the drawing. However, the system can also be configured to develop in the clockwise direction with no compromise in performance, depending on the desired properties of the development system with respect to the direction of the photoreceptor (i.e., against-mode or with-mode development). In the case described, the photoreceptor is located above the development rolls. The developer materials are transported from left to right from the sump to roll 37, to roll 38, back to the sump. A broad radial pole 80 of roll 37 (Figure 3) positioned at 6 o'clock serves to lift magnetic developer material from a donor roll in the sump or housing 32. The combination of tangential and radial fields, starting with pole 84, transport the developer material along the surface of the developer roll until about the 11 o'clock position of roll 37. At that point, the developer becomes magnetically unconstrained because of the lack of poles or strong poles in this area to constrain the developer in a brush-like configuration. The developer is moved magnetically unconstrained through the part of the developement zone delineated by the roll 37 and the charge-retentive surface until the developer comes under the influence of a strong radial south pole 86 of the magnet 38. Movement through the aforementioned zone is effected through the cooperation of the charge-retentive surface and the developer shell. The pole 86 serves to effect transition of the developer from the roll 37 to the roll 38 without magnetically constraining the developer so as to cause scavenging of the first image as it passes the second developer housing. As will be observed, the poles following the pole 86 in the clockwise direction are progressively weaker, so that the developer is magnetically unconstrained as it moves through the part of the development zone delineated by the roll 38 and the charge-retentive surface. In view of the foregoing description it should now be apparent that there has been provided an image-forming method which forms tri-level images that are fringe-free and possess a high fidelity (i.e. a faithful reproduction of the original image) even when optically formed.

Claims: A method fob forming fringe-free, tri-level images, including the steps of: forming a tri-level latent electrostatic image on a charge-retentive surface, the image comprising a first image area at a relatively-high voltage, a second image area at a relatively-low voltage, and a background area at a voltage halfway between the levels of the high and low voltages; electrically basing a first developer applicator (32) to a voltage that is offset from the voltage of the background area in the direction of the high voltage of the first image area; electrically biasing a second developer applicator (34) to a voltage that is offset from the voltage of the background area in the direction of the low voltage of the second image area; using the first developer applicator to apply to the charge-retentive surface a first conductive magnetic brush developer having physical properties for lessening the adverse effects of development of low-density fine lines in the presence of high cleaning fields, to develop the first image area in toner of one color, and using the second developer applicator to apply to the charge-retentive surface a second conductive magnetic brush developer having physical properties for lessening the adverse effects of development of low-density fine lines in the presence of high cleaning fields, to develop the second image area in toner of another color. A method as claimed in claim 1, wherein applying the first and second developers includes the step of spacing the developer applicators from the charge-retentive surface by a distance in the range of 1.0 to 3.0 mm. A method as claimed in claim 1 or 2, wherein the conductivity of the developer is of the order of 10⁻⁹ to 10⁻¹³ (ohm.cm)-1. A method as claimed in any preceding claim, wherein the developer is a two-component developer, of which the toner concentration is of the order of 2.0 to 3.0% by weight. A method as claimed in any preceding claim, wherein the charge level of the developers is less than 20 microcoulombs/gram.

Inventors: ALLEN WILLIAM M JR, MAY JEROME E, PARKER DELMER G, ALLEN, WILLIAM M., JR., MAY, JEROME E., PARKER, DELMER G.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1988
Main classification: G03G13
IPC codes: G03G13, G03G15