EP0554981A1

EP0554981A1 - A method and apparatus for enhancing color fidelity in a printing process

Info

Publication number: EP0554981A1
Application number: EP93300364A
Authority: EP
Inventors: Stuart W. Volkers
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1992-01-31
Filing date: 1993-01-20
Publication date: 1993-08-11
Also published as: JPH05281863A

Abstract

A method and apparatus for enhancing color fidelity in a printing process employing an intermediate member (10) wherein a developing unit (9) deposits a colorless and transparent material directly onto the intermediate member before transfer of any color toner images thereto. Alternatively, a developing unit first deposits the colorless and transparent material on a latent image member. The colorless and transparent material is then transferred to the intermediate member before transfer of any color toner images thereto. Color toner images are then transferred, in registration, onto the intermediate member over the colorless and transparent material, at each of a plurality of image producing stations (20,18,16,14). The resulting combined image, together with at least some of the colorless and transparent material, is then transferred (52) to a copy sheet. The colorless and transparent material provides a glossy appearance to the final image on the copy sheet.

Description

This invention relates generally to an electrostatographic printing process and apparatus, and more particularly concerns a method and apparatus for enhancing color fidelity in a printing process involving the transfer of a multicolor toner image from an intermediate transfer member to a support sheet. In the present application, the term color means any hue including such hues as black, cyan, yellow, and magenta, and combinations thereof. Also, the term printing as used herein encompasses methods and apparatuses for electrostatographic printing and copying.
In the process of electrostatographic printing, an electrostatic latent charge pattern is reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent charge pattern. Electrography utilizes an insulating medium to form without the aid of electromagnetic radiation, the electrostatic latent charge pattern. In both of the foregoing processes, the electrostatic latent image is developed with color toner particles which are ultimately transferred to a sheet of support material. Hereinafter, the present invention will be illustrated in the context of electrophotographic printing processes and machines, but it is to be understood that the present invention is also suitable in electrographic processes and apparatuses.
A class of single color and multicolor electrophotographic imaging systems employs an intermediate transfer member. In these systems, a photoconductive surface is charged to a substantially uniform potential. The charged photoconductive surface is exposed to a filtered light image to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. The resultant electrostatic latent image is then developed with toner particles corresponding in color to the subtractive primary of the filtered light image. For example, when a red filter is employed, the electrostatic latent image is developed with cyan toner particles. The cyan toner powder image is then transferred to an intermediate transfer member such as a belt or drum. The foregoing process is repeated for a green filtered light image which is developed with magenta toner particles and a blue filtered light image which is developed with yellow toner particles. Each differently colored toner powdered image is sequentially transferred in superimposed registration with one another from the photoconductive surface to the intermediate member. The developed electrostatic images are then transferred from the intermediate member to a sheet of support material and affixed thereto. These systems may also use three or four photoconductive drums or belts in lieu of a single photoconductive member. Examples of electrophotographic printing systems employing an intermediate transfer member can be found in US-A-4,935,788; 4,515,460; and 3,957,367.
The total transfer of a multi-layer color toner image from an intermediate transfer member to a support sheet is essential to achieve color fidelity. Typically, however, the transfer process fails to transfer some of the toner particles that are in contact with the intermediate member (i.e., the transfer process fails to transfer some of the toner particles of the layer closest to the intermediate member). In a single color transfer from the photoconductive member to the intermediate member, it may be theoretically possible to detect and account for the amount of toner not transferred by adjusting other process steps, e.g., charging, exposing, development. However, after all the various toner images of a multi-layer color image are transferred to the intermediate member, each of the color toner layers is in intimate contact depending upon the sequence of application and the color being produced. Therefore, if total transfer is not achieved to the support sheet, hue shifts may result. These shifts might be predicted by computer and then compensated for during the exposure process in a pixel by pixel manner. However, attempts to adjust a parameter that has an overall effect, such as adjusting the developer bias of the cyan toner to attain a certain red hue, will in turn, create hue shifts in other colors.
Various approaches have been devised to enhance the appearance of a color copy. The following disclosures may be relevant to various aspects of the present invention:
Pond, Xerox Disclosure Journal, Vol. 2, No. 5, p. 17 (September/October 1977), describes depositing a layer of fine colorless toner particles on the electrostatic image of a photoconductor, followed by depositing a second layer of larger color toner particles.
Drappel et al., EP-A-0424093 describes forming a "peel layer" on an imaging device. Latent images are formed and developed on the "peel layer," and the "peel layer" is subsequently simultaneously removed from the imaging member and transferred and affixed to a substrate. The use of a transparent waxy toner is disclosed.
Sako et al., J02201453 (English language abstract; published August 9, 1990), describes developing the electrostatic latent image formed on an image carrier with chromatic toners, then developing the entire surface of an image forming region with colorless, transparent toner.
Sako et al., J02201452 (English language abstract; published August 9, 1990), describes mixing a color toner for making an electrostatic latent image visible with a colorless, transparent toner.
Tagawa et al., J02140757 (English language abstract; published May 30, 1990), describes developing what seems to be color toner images over the entire surface of a transfer member by using a colorless toner containing wax of a low melting point as a release agent.
Hirano et al., J63080269 (English language abstract; published April 11, 1988), describes a developer composed of a color toner and a colorless, transparent toner.
Kawabata, J63058374 (English language abstract; published March 14, 1988), describes an image forming method which develops the surface of the photosensitive body with a colorless, transparent toner.
Bares, Xerox Disclosure Journal, Vol. 16, No.1, p. 69 (January/February 1991), describes applying a transparent (unpigmented) toner as a final finishing step.
Fukushima et al., US-A-3,901,698 describes a method of reversal development using two developers wherein the first developer consists of colorless or white, positively charged, toner particles.
Mammino, US-A-4,064,285 describes a method of decalcomania wherein the polymeric "subbing" layer is colorless.
Clemens, US-A-4,066,802 describes a method of decalcomania wherein the interposed polymeric sheet is colorless.
The present invention is directed to a method of enhancing color fidelity in an electrostatographic printing process comprising the following steps carried out within an electrostatographic printing apparatus. The first step involves depositing a colorless and transparent material on an intermediate transfer member. Then at least one color toner image is transferred to the intermediate member wherein the color toner image overlays the colorless and transparent material. The last step involves transferring the color toner image from the intermediate member to a support sheet.
The present invention is also directed to a multicolor printing apparatus. The apparatus contains receiving means for receiving a plurality of latent images and image means for recording the plurality of latent images on the receiving means. Developing means is provided for developing the plurality of latent images with color toner to form a plurality of color toner images. An intermediate member is also present to receive the plurality of color toner images. There are deposition means for depositing a colorless and transparent material on the intermediate member before transfer of the plurality of color toner images thereto, and also first transfer means for transferring each of the color toner images from the receiving means to the intermediate member to form a multicolor toner image over the colorless and transparent material. The apparatus also contains second transfer means for transferring at least the multicolor toner image from the intermediate member to a support sheet.
A method and apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:-
Figs. 1 (a)-(d) are a diagrammatic representation of the process steps of the invention.
Fig. 2 is a schematic illustration of an exemplary printing apparatus incorporating the features of the present invention.
In Fig. 1(a), a colorless and transparent material 2 in the form of a layer is deposited on intermediate transfer member 4, which may be a belt, drum, or plate. It is preferred that the particles of the colorless and transparent material are charged, especially to a polarity opposite of that of the intermediate member (e.g., in Fig. 1(a), the particles of colorless and transparent material 2 have a positive charge and intermediate member 4 has a negative charge). The particles of the colorless and transparent material may be charged by any means known to those skilled in the art including triboelectrification.
The phrase colorless and transparent material encompasses any material which permits operation of an electrostatographic printing system without undue jamming, which is preferably transferable, which is colorless and transparent after transfer or fixing to the support sheet, regardless of its initial appearance. Some suitable materials may initially have some color and/or be opaque, but will turn colorless and transparent during the electrostatographic printing process. The colorless and transparent material may include impurities, but these should not tint or reduce the clearness of the material in a significant manner. Utilizing a colorless and transparent material is advantageous when it is transferred to the support sheet since the colorless and transparent nature of the material provides a glossy appearance without affecting the hue of the color toner image or darkening or tinting the background portions of the support sheet.
The colorless and transparent material may be a conventional toner resin, in a toner devoid of the usual color pigment or dye, in an otherwise conventional single or two component developer. Single and two component developers are illustrated in US-A-4,758,506; 4,634,649; 4,474,866; 4,407,925; and 4,264,697. Suitable colorless and transparent materials include resin particles that are solid under imaging conditions, such as high molecular weight polyalkylene waxes, including polyethylene, preferably of a molecular weight of from about 500 to about 20,000, and more preferably from about 2,000 to about 10,000, Concord 5000 wax from Concord Chemical Company, Camden, NJ, hydrolyzed polyethylene polymers such as Epolene C-16, Candellila Wax (commercially available from Strahl & Pitsch Company), polyethylene, polypropylene, paraffin wax, microcrystalline wax C, polyalkylenes such as Epolene N-10 and N-12 and the like available from Eastman Kodak Company, oxidized polyethylene waxes such as A-C 629 and A-C 629A available from Allied Signal Corporation, copolymers of ethylene and acrylic acid, such as A-C 580 available from Allied Signal Corporation, and the like, as well as mixtures of these waxes either with or without other additives such as Nujol (a mineral oil) or oleic acid. If the material selected does not exhibit desirable triboelectric charging characteristics, a charge control agent can be added to the material. Any suitable charge control agent can be employed, such as alkyl pyridinium halides, including cetyl pyridinium chloride, distearyl dimethyl ammonium methyl sulfate. In addition, a material such as basic barium petronate, polyisobutylene succinimide, lecithin, and the like can be employed as the charge control agent. The colorless and transparent material can be prepared by methods similar to the preparation of conventional toners, such as heating and mixing the ingredients, followed by the jetting, micronization, and classification steps generally employed in the electrophotographic art for preparing toner particles.
The colorless and transparent material may be part of a liquid developer comprising the colorless and transparent material, a liquid vehicle, and a charge control additive. The colorless and transparent material has been previously described and is present in the liquid developer in an effective amount, preferably from about 0.5 to about 4 percent by weight. The liquid medium may be any of several hydrocarbon liquids conventionally employed for liquid development processes, such as hydrocarbons, including high purity alkanes having from about 6 to about 14 carbon atoms, such as Norpar® 12, Norpar® 13, and Norpar® 15, available from Exxon Corporation, and including isoparaffinic hydrocarbons such as Isopar® G, H, L, and M, available from Exxon Corporation, Amsco® 460 Solvent, Amsco® OMS, available from American Mineral Spirits Company, Soltrol®, available from Phillips Petroleum Company, Pagasol®, available from Mobil Oil Corporation, Shellsol®, available from Shell Oil Company, and the like. Isoparaffinic hydrocarbons are preferred liquid media, since they are colorless, environmentally safe, and possess a sufficiently high vapor pressure so that a thin film of the liquid evaporates from the contacting surface within seconds at ambient temperatures. Generally, the liquid medium is present in a large amount in the developer composition, and constitutes that percentage by weight of the developer not accounted for by the other components. The liquid medium is usually present in an amount of from about 80 to about 98 percent by weight, although this amount may vary from this range provided that the objectives of the present invention are achieved.
Examples of suitable charge control agents for liquid developers include lecithin (Fisher Inc.); OLOA 1200, a polyisobutylene succinimide available from Chevron Chemical Company; basic barium petronate (Witco Inc.); zirconium octoate (Nuodex); aluminum stearate; salts of calcium, manganese, magnesium and zinc; heptanoic acid; salts of barium, aluminum, cobalt, manganese, zinc, cerium, and zirconium octoates; salts of barium, aluminum, zinc, copper, lead, and iron with stearic acid; and the like. The charge control additive may be present in an amount of from about 0.01 to about 3 percent by weight, and preferably from about 0.02 to about 0.05 percent by weight of the developer composition.
The liquid developer generally can be prepared by conventional means such as by heating and mixing the ingredients, followed by grinding the mixture in an attritor in the presence of the selected liquid medium. Subsequently, the charge control agent is added to the mixture to yield the liquid developer. An example of a specific liquid developer which may be suitable for the present invention comprises particles having about 60 percent by weight of a hydrolyzed polyethylene polymer, about 20 percent by weight of candellia wax, and about 20 percent by weight of oleic acid suspended in a liquid isoparaffinic hydrocarbon vehicle such as Isopar®G to a concentration of from about 0.5 to about 4 percent by weight solids. The liquid developer also contains a charge control agent such as OLOA 1200 in an amount of about 0.5 percent by weight of the solids content of the developer.
The colorless and transparent material deposited on the intermediate member may generally take one of two forms. Imaged colorless and transparent material corresponds to the image portion of an original document. Or the colorless and transparent material may be in the form of a layer, having a uniform or nonuniform thickness. Preferably, the colorless and transparent material on the intermediate member is a layer having an area roughly equal to or greater than the receiving side of the support sheet. The colorless and transparent material deposited on the intermediate member, whether imaged or in a layer, can be of any thickness, preferably from about 2-20 microns, and especially from about 2-10 microns. In one embodiment, the colorless and transparent material deposited on the intermediate member is a monolayer. The colorless and transparent material comprises particles having an average diameter of about 0.5-20 microns, especially about 2-10 microns. In another embodiment of the present invention, the colorless and transparent material has an average particle diameter smaller than that of the color toner particles. It is believed that finer particles will better cover the surface of the intermediate member and are harder to dislodge therefrom.
Deposition of the colorless and transparent material on the intermediate member may occur in one of two general manners. In the indirect way, the colorless and transparent material is deposited first on a latent image member (such as a photoconductive member) and is then transferred to the intermediate member. In the direct approach, the colorless and transparent material is deposited directly on the intermediate member, rather than first contacting the latent image member followed by transfer of the colorless and transparent material to the intermediate member. Depositing the colorless and transparent material directly on the intermediate member or at the outset on the latent image member may be accomplished by a variety of techniques. When the colorless and transparent material comprises the toner portion of a single or two component developer, any developing method known to those skilled in the art may be employed. Suitable developing methods include cascade development illustrated in US-A-4,015,561; magnetic brush development illustrated in US-A-3,967,892; fur brush development illustrated in US-A-3,841,265; liquid electrophoretic development illustrated in US-A-4,935,788; scavengeless development illustrated in US-A-5,053,824; and jumping development illustrated in US-A-4,660,059. Examples of other suitable methods for depositing or developing the colorless and transparent material include hot melt, air knife, roll coating, gravure, and wire drawn down.
If the colorless and transparent material is deposited on the latent image member at the outset, transfer of the material to the intermediate member may be effected by any suitable means, including, preferably, creation of an electrostatic field. One technique is to spray the intermediate member with ions. Another technique is to electrically bias the intermediate member to a potential and polarity sufficient to attract the particles of the colorless and transparent material.
In Fig. 1(b), color toner image 6 is transferred from the photoconductive member (not shown) to intermediate member 4, wherein color toner image 6 overlays colorless and transparent material 2. The recording of the electrostatic latent images and development with color toner may be effected by conventional electrophotographic means.
Any conventional color toner may be used in the present invention. The color toner particles preferably have an average particle diameter of about 0.5-20 microns and can be in the form of either dry or liquid developers. Examples of suitable single and two component color developers may be found in US-A-4,758,506; 4,634,649; 4,474,866; 4,407,925; and 4,264,697.
Color toner image 6 may be transferred from the photoconductive member to intermediate member 4 by creating an electrostatic field to effect transfer. One technique is to spray the intermediate member with ions. Another technique is to electrically bias the intermediate member to a potential and polarity sufficient to attract the color toner particles. It is preferred that the color toner particles are charged, especially to the same charge polarity as the particles of the colorless and transparent material (e.g., in Fig. 1 (b), the particles of color toner image 6 have a positive charge). The color toner particles may be charged by any means known to those skilled in the art including triboelectrification.
Typically, a plurality of different co/or toner images are transferred to the intermediate member to form a multicolor toner image over the colorless and transparent material. Each color toner image may be transferred successively in superimposed registration or may be transferred in juxtaposition with one another.
In Fig. 1 (c), support sheet 8 is placed over the charged particles of color toner image 6 and colorless and transparent material 2 and an opposite charge is preferably induced on the back of support sheet 8 (e.g., negative charge). Typically, a corona discharger may be employed to spray the backside of support sheet 8 with ions. Support sheet 8 may be paper, plastic, or any other suitable substrate capable of receiving the color toner image.
In Fig. 1(d), color toner image 6 and a portion of colorless and transparent material 2 are transferred to support sheet 8, with residual colorless and transparent material 2a remaining on intermediate member 4. The transfer can be effected by the creation of an electrostatic field. The electrostatic field may be created in a variety of ways. As discussed above, a corona discharger may spray the backside of support sheet 8 with ions. Also, intermediate member 4 may be electrically biased to a potential and polarity sufficient to repel the color toner image and the colorless and transparent material towards support sheet 8. As an alternative to or as an addition to the electrostatic field, pressure may be employed to effect transfer of the color toner image and colorless and transparent material to the support sheet.
At least a substantial portion of the color toner is transferred to the support sheet. Some color toner may remain on the intermediate member, but, preferably, all of the color toner image is transferred to the support sheet. In another preferred embodiment, the amount of residual color toner on the intermediate layer is insufficient to shift the hue of the color toner image. In a further embodiment, there is an increase in the amount of color toner transferred to the support sheet as compared with a conventional process. One skilled in the art may employ various methods to determine whether there is an increase in the amount of color toner transferred to the support sheet or whether hue shift has occurred including the use of visual examination and/or a color densitometer.
It is believed that the colorless and transparent material facilitates the transfer of the color toner image to the support sheet. The colorless and transparent material increases the distance between the intermediate member and the particles of the color toner image, thereby weakening the van der Waals forces between the intermediate member and the color toner image particles. Thus, more of the color toner image is transferred to the support sheet. Increasing the thickness of the colorless and transparent material may transfer more of the color toner image. The magnitude of the charge on the particles of the color toner image may be different than that of the colorless and transparent material.
During transfer of the color toner image to the support sheet, a portion of the colorless and transparent material may also transfer, preferably in a thickness of about 5-15 microns. It is preferred that sufficient colorless and transparent material transfers to provide a glossy finish to the image areas, more preferably, both the image and background areas of the support sheet. Increasing the thickness of the colorless and transparent material may transfer more of the material to the support sheet. However, it is possible that no colorless and transparent material may transfer particularly when such material is spread into a very thin layer such as a monolayer.
Color toner image 6 and colorless and transparent material 2 on support sheet 8 may then be affixed to the sheet support by any means known to those skilled in the art including such techniques as hot roll fusing, cold roll fusing, radiant fusing, and solvent fusing. For some materials, all that may be required to affix the color toner image to the sheet support is a few moments of drying at ambient temperature. It is also contemplated that the affixing step may occur simultaneously with the transfer of color toner image 6 from intermediate member 4 to support sheet 8. Colorless and transparent material 2 may reduce the offset phenomenon during fusing by selecting as material 2 or adding thereto a waxy resin described previously to function as a release agent. Residual colorless and transparent material 2a may optionally be cleaned off intermediate member 4. Color fidelity is achieved since color toner image 6 is completely transferred to support sheet 8. Also, colorless and transparent material 2 provides a glossy appearance to the image and background portions of support sheet 8.
Referring now to Fig. 2, there is shown an exemplary multicolor printing apparatus. Intermediate drum 10 is mounted rotatably on the machine frame. Drum 10 rotates in the direction of arrow 12. Developer unit 9 and four image reproducing stations, indicated generally by the reference numerals 14, 16, 18, an 20, are positioned about the periphery of intermediate drum 10.
A developer unit, indicated generally by reference numeral 9, deposits a layer of colorless and transparent material on the surface of drum 10. Means for adjusting the thickness of the layer of colorless and transparent material are known to those skilled in the art. Developer unit 9 can be of various types, but is generally what is referred to in the art as a magnetic brush development unit. Typically, a magnetic brush development system employs developer roll 11 and a magnetizable developer material (not shown) including magnetic carrier granules having toner particles adhering triboelectrically thereto. The developer material is continually brought through a directional flux field to form a brush (not shown) of developer material. The developer material is constantly moving so as to continually provide the brush with fresh developer material. Development is achieved by bringing the brush of developer material into contact with drum 10. Preferably, developer roll 11 is electrically biased to about 200-600 volts. Drum 10 may be electrically biased to a potential of sufficient magnitude and polarity to attract the colorless and transparent material from developer unit 9. Preferably, drum 10 is electrically biased to about 300-900 volts. Typically, drum 10 is made from a conductive tube, such as aluminum, with an appropriate dielectric coating. A high voltage power supply applies a direct current bias voltage to drum 10 by suitable means such as a carbon brush and brass ring assembly.
Any suitable electronic and/or mechanical means may be provided to control the extent of deposition on the surface of drum 10 by developer unit 9. For example, developer unit 9 may be electronically and/or mechanically coupled to drum 10 to coat virtually its entire surface. Alternatively, the electronic and/or mechanical means may time the rotation of drum 10 such that the extent of deposition on its surface corresponds generally in area to the receiving side of the support sheet.
It is also possible to selectively develop the surface of drum 10 with colorless and transparent material corresponding to the image portion of the original document. This is accomplished by adapting intermediate drum 10 to be a photoconductive member. A corona generating device charges the surface of drum 10 to a substantially uniform potential. Imaging devices record a complete electrostatic latent image corresponding to all of the informational areas of an original document. This may be accomplished by exposing the charged surface to unfiltered light (in reflected light systems) or to controlled laser emissions (in raster output scanner systems) to record the complete latent image. The imaging process for the colorless and transparent material differs from that for the color toner images since the imaging process for the color toner images generally records only partial latent images. Developer unit 9 develops the complete latent image to form imaged colorless and transparent material. Employing drum 10 as a photoconductive member does not restrict deposition of the colorless and transparent material on drum 10 to selective development. Any suitable electronic and/or mechanical means can be provided to selectively disengage the imaging devices to prevent recordation of a latent image on drum 10 so that developer unit 9 develops on the uniformly charged surface of drum 10 a layer of the colorless and transparent material. Electronic and/or mechanical means may be provided to control the extent of development of the surface of drum 10 by developer unit 9.
In an alternative arrangement, developer unit 9 may be part of an additional image reproducing station in a configuration similar to that of image reproducing stations 14, 16, 18, and 20. In this configuration, the colorless and transparent material deposited on the intermediate drum 10 may correspond to the image portion of the original document. This is accomplished by first recording the complete latent image on the corresponding photoconductive member. Recording the complete latent image may be done by exposing the charged surface to unfiltered light (in reflected light systems) or to controlled laser emissions (in raster output scanner systems). Application of a developing bias voltage to developer unit 9 develops the latent image with the colorless and transparent material. The developed image may be conveyed to intermediate drum 10 by electrically biasing drum 10 to a potential of sufficient magnitude and polarity to attract the colorless and transparent material. If a layer of colorless and transparent material is desired, there is then no need for the imaging step and developer unit 9 deposits the layer over the uniformly charged photoconductive member, followed by subsequent transfer to drum 10. It is understood that the means used to effect transfer of the colorless and transparent material from the photoconductive member to drum 10 may be the same or different means used to transfer the color toner images to drum 10. For example, in printing systems wherein the intermediate member is an endless belt, the same or different corona charging devices may be used to effect transfer of the colorless and transparent material and color toner images from the photoconductive member to the intermediate belt. Any suitable electronic and/or mechanical means may be provided to control the extent of deposition on the surface of the photoconductive member by developer unit 9. For example, developer unit 9 may be electronically and/or mechanically coupled to the photoconductive member to coat virtually its entire surface. Alternatively, the electronic and/or mechanical means may time the rotation of the photoconductive member such that the extent of deposition on its surface corresponds generally in area to the receiving side of the support sheet.
For printing systems having single or multiple photoconductive members, depositing the colorless and transparent material directly onto the intermediate member is preferred because it is generally simplier, cheaper, and faster than indirect deposition, i.e., first depositing the material on a photoconductive member and then transferring it to the intermediate member.
The portion of drum 10 coated with the colorless and transparent material rotates in the direction of arrow 12 to image reproducing station 20, the first of the four image reproducing stations. The four image reproducing stations are substantially identical to one another. The only distinctions between the image reproducing stations are their geometric position and the color of the developer employed therein. For example, image reproducing station 14 uses a black color developer whereas stations 16, 18, and 20 use yellow, magenta, and cyan color developers respectively. Since stations 14, 16, 18, and 20 are similar, only station 20 will be described in detail.
At station 20, a drum 22 having a photoconductive surface deposited on a conductive substrate rotates in the direction of arrow 24. Preferably, the photoconductive surface is made from a selenium alloy with the conductive substrate being made from an electrically grounded aluminum alloy. Other suitable photoconductive surfaces and conductive substrates may also be employed. Drum 22 rotates in the direction of arrow 24 to advance successive portions of the photoconductive surface through the various processing stations disposed about the path of movement thereof.
Initially, a portion of the photoconductive surface of drum 22 passes beneath corona generating device 26. Corona generating device 26 charges the photoconductive surface of drum 22 to a relatively high, substantially uniform potential.
Next, the charged portion of the photoconductive surface is advanced through an imaging station. At the imaging station, an imaging unit, indicated generally by the reference numeral 28, records a partial electrostatic latent image on the photoconductive surface of drum 22. Imaging unit 28 includes a raster output scanner. The raster output scanner lays out the electrostatic latent image in a series of horizontal scan lines with each line having a specified number of pixels per unit length. Preferably, the raster output scanner employs a laser which generates a beam of light rays that are modulated by rotating polygon mirror blocks or solid state image modulator bars. Alternatively, the raster output scanner may use light emitting diode array write bars. In this way, an electrostatic latent image is recorded on the photoconductive surface of drum 22.
Next, a developer unit, indicated generally by the reference numeral 30, develops the electrostatic latent image with a cyan color developer. Like developer unit 9 containing the colorless and transparent material, developer unit 30 may also be a magnetic brush development unit. After development of the latent image with cyan color developer is completed, drum 22 rotates in the direction of arrow 24 to advance the cyan toner image to transfer zone 32 where the cyan toner image is transferred from drum 22 to intermediate drum 10.
At transfer zone 32, the cyan toner image is transferred from photoconductive drum 22 to the portion of intermediate drum 10 coated with the colorless and transparent material, whereby the cyan toner image overlays the colorless and transparent material, preferably in superimposed registration therewith. Drum 10 and drum 22 have substantially the same tangential velocity in transfer zone 32. Drum 10 is electrically biased to a potential of sufficient magnitude and polarity to attract the cyan toner image from drum 22.
After the cyan toner image is transferred to drum 10 at reproducing station 20, drum 10 rotates the cyan toner image to the transfer zone of reproducing station 18 where the magenta toner image is transferred to drum 10, in superimposed registration with the cyan toner image previously transferred to drum 10. After the magenta toner image is transferred to drum 10, drum 10 rotates the transferred toner images to reproducing station 16 where the yellow toner image is transferred to drum 10 in superimposed registration with the previously transferred toner images. Finally, drum 10 rotates the transferred toner images to reproducing station 14 where the black toner image is transferred thereto in superimposed registration with the previously transferred toner images. After all of the color toner images have been transferred to drum 10 in superimposed registration with one another to form a multicolor toner image over the colorless and transparent material, the multicolor toner image and a portion of the colorless and transparent material are transferred to a support sheet, e.g., a copy paper, at the transfer station.
At the transfer station, a copy sheet is moved into contact with the multicolor toner image on drum 10. The copy sheet is advanced to transfer station from a stack of sheets 34 mounted on tray 36, by a sheet feeder 38, or from either a stack of sheets 40 on tray 42, or a stack of sheets 44 on tray 46 by either sheet feed 48 or sheet feeder 50. The copy sheet is advanced into contact with the multicolor toner image on drum 10 beneath corona generating unit 52 at the transfer station. Corona generating unit 52 sprays ions onto the backside of the sheet to attract the multicolor toner image and at least a portion of the colorless and transparent material to the front side thereof from drum 10. After transfer, the colorless and transparent material overlays preferably both the image and background portions of the copy sheet. Based on the present disclosure, optimization of the transfer of the multicolor toner image and the colorless and transparent material to the support sheet may be made by those skilled in the art. After transfer, the copy sheet continues to move in the direction of arrow 54 on a conveyor to a fusing station.
At the fusing station, a roll fusing system, indicated generally by the reference numeral 56, permanently fuses the colorless and transparent material and multicolor toner image to the copy sheet. This forms a multicolor copy having a glossy appearance in both the image and background portions thereof. The roll fusing system includes a heated fuser roller 58 and a back-up roller 60. The rollers are resiliently urged into engagement with one another to define a nip therebetween. After fusing, the copy sheet is advanced by a conveyor to catch tray 62 for subsequent removal from the printing machine by the operator.
Some residual colorless and transparent material and/or color toner may adhere to drum 10 after transfer. The residual matter is removed from the drum surface at a cleaning station, indicated generally by the reference numeral 64.

Claims

A method to enhance color fidelity in an electrostatographic printing process comprising the following steps carried out within an electrostatographic printing apparatus:
(a) depositing (9) a colorless and transparent material on an intermediate transfer member;

(b) transferring (32) at least one color toner image to the intermediate member wherein the color toner image overlays the colorless and transparent material; and

(c) transferring (52) the color toner image from the intermediate member to a support sheet.
The method according to claim 1, wherein the step (a) comprises forming the colorless and transparent material on a latent image member-and transferring the colorless and transparent material from the latent image member to the intermediate member.
The method according to claim 1, wherein the step (a) comprises developing the colorless and transparent material directly on the intermediate member.
The method according to any one of claims 1 to 3, wherein the step (a) comprises depositing on the intermediate member a colorless and transparent material having an average particle diameter smaller than that of the color toner.
The method according to any one of claims 1 to 4, wherein the step (b) comprises creating and transferring (20,18,16,14) a plurality of different color toner images to the intermediate member to form a multicolor toner image over the colorless and transparent material.
A multicolor printing apparatus comprising:
(a) receiving means (22) for receiving latent images;

(b) image means (28) for recording the latent images on the receiving means;

(c) developing means (30) for developing the latent images with color toner to form color toner images;

(d) an intermediate member (10) to receive the color toner images;

(e) deposition means (9) for depositing a colorless and transparent material on the intermediate member before transfer of a color toner image thereto;

(f) first transfer means for transferring a color toner image from the receiving means to the intermediate member to form a color toner image over the colorless and transparent material; and

(g) second transfer means (52) for transferring at least the color toner image from the intermediate member (10) to a support sheet.
The apparatus according to claim 6, wherein the receiving means includes one latent image member.
The apparatus according to claim 6, wherein the receiving means includes a plurality of latent image members (20,18,16,14), whereby a plurality of different color images may be transferred to the intermediate member to form a multi-color toner image.
The apparatus according to any one of claims 6 to 8, wherein the deposition means (9) includes means for depositing the colorless and transparent material on a receiving means and means for transferring the colorless and transparent material from the receiving means to the intermediate member.
The apparatus according to any one of claims 6 to 8, wherein the deposition means (9) develops the colorless and transparent material directly on the intermediate member (10).