EP0488792A2

EP0488792A2 - A method and apparatus for printing color images

Info

Publication number: EP0488792A2
Application number: EP91311125A
Authority: EP
Inventors: John F. O'brien
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1990-11-29
Filing date: 1991-11-29
Publication date: 1992-06-03
Also published as: JPH04291368A; CA2050640A1; EP0488792A3

Abstract

A plurality of color images are formed on a predetermined area of a uniformly charged image receiver (12) using a plurality of ionographic printing heads (22, 24, 26, 28). Formation of the first image is effected by selectively discharging the predetermined area with subsequent development thereof with a toner of a first color. Subsequent to image development the image receiver (12) is recharged to the uniform charge level. The foregoing process is repeated for each different color toner used. In the case of the last developed image the recharging step is omitted. By using ionographic imaging devices (22, 24, 26, 28) for forming the images, the printer disclosed herein is not limited to using toners which are transparent to electromagnetic radiation.

Description

The present invention relates to color printing and more particularly to direct writing imaging using ionographic printhead structures for color printing.
The primary configurations of interest for forming color images are a tandem xerographic engine system and a multiple cycle belt arrangement using four developer housings, one for each cycle.
The tandem engine has maximum throughput for a given process speed but requires a maximum number of parts as well as four separate ESS-ROS devices, four separate image receivers and four separate cleaning systems for removing residual toner from the image receivers. This extra cost attributed to duplicate systems and parts is likely to impact system reliability.
The four cycle system minimizes the number of systems and parts and may result in maximum reliability at minimal cost. However, the four cycle system minimizes the page throughput for a given process speed. Moreover, placing all the developer housings in juxtaposition adjacent one belt maximizes subsystem interactions which can cause image quality defects due to materials interaction problems.
Of the various electrostatic printing techniques, the most familiar is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A less familiar form of electrostatic printing uses ions deposited on an electroreceptor. In ionographic devices such as that described in US Patent No. 4,524,371 granted to Sheridon et al. or US Patent No. 4,463,363 granted to Gundlach et al, an ion producing device generates ions to be directed past a plurality of modulation electrodes to an imaging surface. In one type of ionographic device, ions are produced at a coronode supported within an ion chamber, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit. Ions directed through the chamber exit are deposited on a charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other ionographic devices exist which operate similarly, but do not rely on a moving fluid stream to carry ions to a surface.
US Patent No. 4,879,194 granted to Christopher Snelling discloses a method and apparatus using ion projection to form tri-level latent electrostatic images on a charge retentive surface. The tri-level image described therein comprises two image areas and a background area. One of the images is developed with black toner while the other one is developed with a colored toner to thereby form a highlight color image.
US Patent No. 4,660,059 granted to John F. O'Brien discloses a highlight color printing apparatus in which a document is printed in at least two different colors. Ions are projected onto the surface of a receiving member to record at least two electrostatic latent images thereon. Each of the electrostatic latent images recorded on the receiving member is developed with different color marking particles. The different color marking particles are transferred substantially simultaneously from the receiving member to the document to print the desired information thereon.
US Patent No. 4,731,634 granted to Howard M. Stark discloses a method and apparatus for rendering latent electrostatic images visible using multiple colors of dry toner or developer for developing black and at least two highlight color images in a single pass of the imaging surface through the development stations of the apparatus. Two of the toners are attracted to only one charge level on the charge retentive surface to form the black and one highlight color image and two toners are attracted to a third image level to form the second highlight color.
US Patent No. 4,833,503 granted to Christopher Snelling on May 23, 1989 discloses a multi-color printer wherein the printing process is improved by using a charging unit prior to exposures subsequent to the first exposure to enhance uniformity of photoreceptor potential, i.e., neutralization of the previous image charge. In order to effect imaging through previously developed toner images, the '503 patent proposes the use of toner materials which are transparent to electromagnetic radiation.
As will be appreciated, a printing system which does not require multiple engines and which can create color images in a single pass is highly desirable.
It is an object of the invention to provide a printing system which does not require multiple image receptors and cleaning systems and which color images in a single pass.
Accordingly, the present invention provides a method of printing color images including subjecting a charged image receiver to ions to form thereon a latent electrostatic image and developing said latent electrostatic image using a toner, characterised by charging said image receiver to a uniform charge level, selectively subjecting a predetermined area of said uniformly charged image receiver to ions from imaging means to thereby form a first latent electrostatic image thereon which has a charge level different from said uniform charge level, developing said first latent electrostatic image using a toner of a first color, recharging said image receiver whereby said image receiver contains a uniform charge level, selectively subjecting said predetermined area of said uniformly charged image receiver to ions from a further imaging means to thereby form a second latent electrostatic image thereon which has a charge level different from said uniform charge level, and developing said second latent electrostatic image using a toner different in color from said first color.
In another aspect of the invention there is provided an apparatus for printing color images including imaging means for subjecting a charged image receiver to ions to form thereon a latent electrostatic image and developer means for developing said latent electrostatic image using a toner, characterised by charging means for charging said image receiver to a uniform charge level, said imaging means serving for selectively subjecting a predetermined area of said uniformly charged image receiver to ions to thereby form a first latent electrostatic image thereon which has a charge level different from said uniform charge level, said developer means serving for developing said first latent electrostatic image with a toner of a first color, further charging means for recharging said image receiver whereby said image receiver contains a uniform charge level, and further imaging means for selectively subjecting said predetermined area of said uniformly charged image receiver to ions to thereby form a second latent electrostatic image thereon which has a charge level different from said uniform charge level, and further developing means for developing said second latent electrostatic image with a toner different in color from said first toner.
By using ionographic printing structures, one for each color to be printed, for printing images on an intermediate receiver belt the present invention provides a printing system which does not require multiple image receptors and cleaning systems and which can produce color images in a single pass.
A uniformly charged image receiver is employed to form successive images using separate ion sources. Each image is developed using a different color toner. Subsequent to the development of each of the images except for the last one, the image receiver is recharged to the original charge level of the uniformly charged image receiver. The toners forming the image are then transferred to a substrate and passed through a fuser to effect adherence of the toners to the substrate.
By forming a plurality of images using ionographic printing devices, the present invention is not limited to the use of toners which are transparent to electromagnetic radiation. Thus, the ion images can be formed on top of previously developed images.
The invention will be described further by way of example in the accompanying drawings which is a schematic illustration of a color printing apparatus according one embodiment of the invention.
Disclosed in Figure 1 is one embodiment of a color printing device or apparatus generally indicated by reference character 10.
The printing apparatus 10 comprises a charge retentive image receiver in the form of a dielectric belt 12 having a dielectric constant of 3 about 2.5 to 6. The belt 12 is supported for movement in an endless path by a pair of rollers 14 and 16, the latter of which is operatively coupled to a motor 18 for effecting such movement.
A corona discharge device 20 is provided for uniformly charging the belt 12 to a negative 1500 volts.
A plurality of ion generating devices 22, 24, 26 and 28-positioned in the path of movement of the belt 12 serve to form discharged area images on a predetermined area of the belt 12. In the process of forming each of the images the image receiver is discharged to approximately -800 volts in the image area. The first ion generating device 22 is used to form a first image on the image receiver which is developed by a first scavengeless development system 30.
Any other suitable development system, for example, magnetic brush development, could be used to develop the first image but it is preferred, in order to minimize developer interaction with an already formed image, to use a scavengeless system as disclosed in US-A 4,868,600 granted to Hays et al on September 19, 1989. Because a discharged area image is developed by the development system 30 it is referred to as a Discharged area Development or a DAD system. As disclosed, there are provided three other scavengeless development systems 32, 34 and 36.
The developer systems 30, 32, 34 and 36 are each negatively biased to a voltage level intermediate the uniform charge level of -1500 and the image voltage level of -800 volts (1500 -700). A DC bias voltage of approximately -1400 volts is supplied by DC power sources 38, 40, 42 and 44 for biasing each of the development systems. In the embodiment disclosed, negatively charged toner is supplied by the donor roll structures of the development systems 30, 32, 34 and 36. Thus, in each instance of image development, the negative toner particles are deposited on the less negatively charged areas (i.e. image areas) of the image receiver and repelled from the higher charged areas.
Recharging corona devices 46, 48 and 50 each positioned downstream of the development system with which it is associated are utilized to neutralize the toner images prior to the formation of the second, third and fourth images. Neutralization of an image has the effect of raising the image charge level of -700 volts to -1500 volts, the voltage level to which the image receiver had been uniformly charged. Recharging also tacks the toner image to the image receiver thereby helping to minimize image disturbance as the image passes subsequent developer systems.
The belt may be encoded adjacent one edge thereof as indicated by reference character 52 to provide timing marks for synchronizing the timely actuation of the ion generating devices as well as other components of the printing apparatus. To this end, signals generated in a conventional manner by a sensor 54 positioned to sense the timing marks serve as inputs to an electronic subsystem, ESS 56. The ESS is operatively connected by suitable circuity (not shown for sake of clarity) to the operative components of the printer including the ion printhead structures.
A pretransfer corona device 58 serves to condition the toner and image receiver for efficient transfer of the toner image to a substrate 60 such as plain paper. A transfer corona structure 62 effects efficient toner image transfer to the substrate 60.
A cleaning system 64 is provided for removing residual toner from the image receiver.
A heat and pressure fuser comprising a fuser roll 66 and a pressure roll 68 permanently affixes toner powder images to the image receiver. Preferably, the fuser assembly includes a heated fuser roller adapted to be pressure engaged with a back-up roller with the toner powder images contacting the fuser roller. In this manner, the toner powder image is permanently affixed to an image receiver.

Claims

A method of printing color images including subjecting a charged image receiver (12) to ions to form thereon a latent electrostatic image and developing said latent electrostatic image using a toner, characterised by charging said image receiver (12) to a uniform charge level, selectively subjecting a predetermined area of said uniformly charged image receiver (12) to ions from imaging means (22) to thereby form a first latent electrostatic image thereon which has a charge level different from said uniform charge level, developing said first latent electrostatic image using a toner of a first color, recharging said image receiver (12) whereby said image receiver (12) contains a uniform charge level, selectively subjecting said predetermined area of said uniformly charged image receiver (12) to ions from a further imaging means (24) to thereby form a second latent electrostatic image thereon which has a charge level different from said uniform charge level, and developing said second latent electrostatic image using a toner different in color from said first color.
A method as claimed in claim 1, characterised in that said imaging means (22) and said further imaging means (24) are each a separate ion source.
A method as claimed in claim 1 or claim 2, characterised in that the use of said imaging means (22) and said further imaging means (24) to subject said predetermined area to ions discharges the charge on the image receiver (12) to thereby form a discharged area image.
A method as claimed in any one of claims 1 to 3, characterised in that the use of said imaging means (22) and said further imaging means (24) comprise using modulated electrode structures.
An apparatus for printing color images including imaging means (22) for subjecting a charged image receiver (12) to ions to form thereon a latent electrostatic image and developer means (30) for developing said latent electrostatic image using a toner, characterised by charging means (20) for charging said image receiver (12) to a uniform charge level, said imaging means (22) serving for selectively subjecting a predetermined area of said uniformly charged image receiver (12) to ions to thereby form a first latent electrostatic image thereon which has a charge level different from said uniform charge level, said developer means (30) serving for developing said first latent electrostatic image with a toner of a first color, further charging means (46) for recharging said image receiver (12) whereby said image receiver (12) contains a uniform charge level, and further imaging means (24) for selectively subjecting said predetermined area of said uniformly charged image receiver (12) to ions to thereby form a second latent electrostatic image thereon which has a charge level different from said uniform charge level, and further developing means (32) for developing said second latent electrostatic image with a toner different in color from said first toner.
An apparatus as claimed in claim 5, characterised in that said imaging means (22) and said further imaging means (24) for selectively subjecting a predetermined area of said uniformly charged image receiver to ions are each a separate ion source.
An apparatus as claimed in claim 5 or claim 6, characterised in that said imaging means (22) and said further imaging means (24) for selectively subjecting a predetermined area of said uniformly charged image receiver (12) to ions comprise means for forming discharge area images.
An apparatus as claimed in any one of claims 5 to 7, characterised in that said imaging means (22) and said further imaging means (24) for selectively subjecting a predetermined area of said uniformly charged image receiver (12) to ions comprise modulated electrode structures.