Embodiments of the subject invention relate to improved electrophotographic apparatus and method for controlling electrical memory effects in photoreceptors. More specifically, embodiments relate to apparatus and techniques for substantially reducing a form of electrical fatigue, occurring in such photoreceptors, that cause a xe2x80x9cresidual imagexe2x80x9d of a previous document in subsequent prints of a different document.
Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto an image receiver, such as a substantially uniformly charged photoreceptor. In response to that image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor""s surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The charge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be implemented using a number of different architectures. For example, in a single pass printer a composite final image is produced in one pass of the photoreceptor through the machine. A second architecture is a four pass printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine and wherein the composite color image is transferred and fused during the fourth pass. REaD IOI can also be implemented in a five cycle printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine, but wherein the composite color image is transferred and fused during a fifth pass through the machine.
The single pass architecture is very fast, but expensive since four charging stations and four exposure stations are required. The four pass architecture is slower, since four passes of the photoreceptive surface are required, but also much cheaper since it only requires a single charging station and a single exposure station. Five cycle printing is even slower since five passes of the photoreceptive surface are required, but has the advantage that multiple uses can be made of various stations (such as using a charging station for transfer). Furthermore, five cycle printing also has the advantage of a smaller footprint. Finally, five cycle printing has a decided advantage in that no color image is produced in the same cycle as transfer, fusing, and cleaning when mechanical loads are placed on the drive system.
The residual image phenomenon is observed as a faint image of a previous document in initial copies of a new document after the previous document has been repeatedly imaged on the photoreceptor, i.e., after the photoreceptor has been cyclically charged overall and discharged, repeatedly in registry, by the light pattern from the previous document. This residual image effect is believed to be caused by the accumulation of charges trapped within the charge generating layer of the photoreceptor in an imagewise pattern corresponding to the previous document image. The speed (rate of discharge per unit exposure) of the photoreceptor is modified by this accumulation of trapped charges so that, upon exposure to a new document, the areas of the photoreceptor associated with the previous document pattern are discharged proportionally to their previous history and the new image is developed with toner simultaneously with a ghost of the previous image. It will be readily appreciated that such a ghost image is detractive from the esthetic viewpoint; however, the provision of previous document information in the subsequent document prints presents an even more serious problem when proprietary information is embodied in the previous document.
It is well known that fatigue of the type causing the residual image effect in photoconductive insulator members can be relieved to some extent by application of infrared radiation to, or otherwise heating, such members or by an overall flooding of such members with light (see for example, U.S. Pat. No. 2,863,767). Also, it has been noted that some regeneration of such a fatigued member can be effected by application of an electrostatic charge, of polarity opposite that of the primary (sensitizing) charge, at some time after the development step and before any subsequent sensitizing step of a copy/print cycle (see for example, U.S. Pat. No. 2,741,959). However, in certain electrophotographic apparatus, e.g., one employing a REaD IOI process, in which a photoreceptor is rapidly exposed a large number of times to the same image, and in which the latent image is not completely erased between each subsequent exposure and development step, the residual image problem is more pronounced. Specifically, in the ReaD IOI process, the differential history of each portion of the image area, with parts being charged and recharged at each subsequent station without exposure while others are charged and exposed several times, causes a pronounced residual image problem. In this case, the above-noted prior art techniques have been found impractical and/or to inadequately eliminate residual image, at least in certain such members.
To erase residual electrostatic charge from the photoreceptor, conventional printing machines employ an erase source that either faces the image area on the front surface of the photoreceptor (xe2x80x9cfront erasexe2x80x9d) or faces and penetrates semi-transparent or translucent layers from the rear of the photoreceptor (xe2x80x9crear erasexe2x80x9d). This conventional arrangement generally has been adequate for black and white reproductions and in color machines employing three or more pass architectures. Conventional erase arrangements may be inadequate in certain situations for high quality color reproductions and especially for printing machines employing a single pass image on image architecture (with no erase after every development station). Such conventional erase arrangements may create ghost images (i.e., residual image effect) and slight voltage non-uniformities that result in objectionable color shifts. Thus, there is a need, which the present invention addresses for new apparatus and new methods that can alleviate the above described residual image problem.
Electrostatic charge erase apparatus and methods, as well as other parts of printing machines, are disclosed in U.S. Pat. No. 4,035,750, issued to Staudenmayer et al.; U.S. Pat. No. 5,748,221, issued to Castelli et al.; U.S. Pat. No. 5,848,335, issued to Folkins et al.; U.S. Pat. No. 5,394,230, Kaukeinen et al.; and, U.S. Pat. No. 4,728,985, issued to Nakashima et al.; U.S. Pat. No. 5,778,288, issued to Tabb et al.; U.S. Pat. No. 5,079,121, issued to Facci et al.; and U.S. Pat. No. 5,933,177, issued to Pollutro et al. Reconditioning systems are also disclosed in U.S. Pat. No. 6,208,819, issued to Pai et al.; and U.S. Pat. No. 6,223,011, issued to Abramsohn et al.
To further reduce and/or substantially eliminate residual images, embodiments contemplate use of a multiple emission dischage device. Embodiments comprise a discharge device including a plurality of emitters distributed along the discharge device. A first quantity of the plurality of emitters emit first emissions that can change the charge state of an image receiver, such as the photoreceptor of an electrostatographic printing machine. At least one more quantity of the plurality of emitters emit at least one respective additional emission that can change the charge state of an image receiver. The emissions can be light, ions, or any other suitable type of emissions that can change the charge state of an image receiver.
In embodiments, the emitters are arranged along a single axis. The first quantity of emitters can be interspersed with the at least one additional quantity of emitters, as in an alternating relationship. Thus, the device can be a bar of LEDs arranged so that the first, third, fifth, etc., LEDs belong to the first quantity of emitters and emit a first frequency of light, and the second, fourth, sixth, etc., LEDs belong to a second quantity of emitters emit a second frequency of light. In embodiments employing three emissions, every third emitter can belong to the same group; where four emissions are used, every fourth emitter; where five are used, every fifth emitter; and so forth.
Alternate embodiments have the emitters arranged in rows with each quantity of emitters having its own row or rows. Thus, the device can, for example, take the form of a bar of LEDs arranged in rows along the bar so that the first quantity of emitters is one row of LEDs, a second quantity of emitters is a second row of LEDs, and so forth. Other embodiments could, of course, have the emitters arranged differently, depending on the particular emissions used and the particular environment in which the discharge device is employed.
As mentioned above, the emitters can be LEDs, and it should be apparent to those of skill in the art that any suitable emitter could be used. Examples of such emitters include, but are not limited to, LEDs, gas discharge lamps, excimer/gas discharge lasers, filament lamps, ion beam generators, and broadband emitters. In embodiments, some or all of the emitters can be tunable so that a single quantity of emitters can emit more than one type of emissions. For example, the device could include a bar of tunable LEDs that can selectively emit different wavelengths of light as conditions warrant.
Embodiments of the device can be used to discharge image receivers in various ways. For example, embodiments can be used to image, erase, and/or recondition photoreceptor belts and other image receivers, especially in electrostatographic printing devices, like laser printers and digital photocopiers. In particular, embodiments can be employed to discharge photoreceptors with a single layer responsive to the emissions, whereas prior art multiple wavelength devices encompasses only multiple layer photoreceptors. In such embodiments, the discharge device can be used by providing the device in an electrostatographic printing machine including a photoreceptor, selectively directing emissions from the first quantity of the plurality of emitters at the photoreceptor to induce a first level of discharge of the photoreceptor, and selectively directing emissions from the at least one more quantity of the plurality of emitters at the photoreceptor to induce at least one additional level of discharge of the photoreceptor.
The discharge device can, for example, be arranged as part of a reconditioning station, with the first quantity of emitters achieving a first degree of photoreceptor reconditioning, and second and subsequent quantities of emitters achieving additional degrees of reconditioning. Similarly, the device can be arranged as part of an erase station, with the first quantity of emitters achieving a first degree of photoreceptor erasure, and second and subsequent quantities of emitters achieving additional degrees of erasure. Additionally, the device can be arranged as part of an imaging station, with the first quantity of emitters achieving a first degree of photoreceptor imaging, and second and subsequent quantities of emitters achieving additional degrees of imaging. Further, the device can be configured to achieve more than one of these functions. For example, the device can be arranged as part of an erase station, with the first quantity of emitters achieving a first degree of photoreceptor erasure, and second and subsequent quantities of emitters achieving degrees of reconditioning and/or erasure. The combinations could even include a single station that can image, erase, and recondition.
Embodiments can be deployed, for example, in one or more of an imaging, erase, and reconditioning stations in an electrostatographic printing machine, such as a machine comprising:
(a) a photoreceptor having an image area;
(b) at least one charging apparatus and at least one imaging apparatus that create a plurality of complementary electrostatic latent images on the image area to correspond to an image wherein the creation of the plurality of the complementary electrostatic latent images involves a substantially uniform charging and an imagewise discharge of the image area for each of the complementary electrostatic latent images and results in a variation in the quantity of trapped charges among different portions of the image area, thereby leading to differential residual voltage among the different portions of the image area;
(c) a plurality of complementary electrostatic latent image developing apparatus;
(d) a charge erase device that directs charge dissipation emissions at the photoreceptor to reduce the quantity of the surface charges; and
(e) a reconditioning light source that directs light at the photoreceptor to reduce the variation in the quantity of the trapped charges among the different portions of the image area, thereby creating a more uniform residual voltage among the different portions of the image area.
The at least one charging apparatus refers to for example devices 22 and 36a-c. 
The at least one imaging apparatus refers to for example devices 24 and 38a-c. 
The plurality of complementary electrostatic latent image developing apparatus refers to for example development stations C, D, E, and F.
In embodiments, the inventive printing machine further includes a residual developer cleaning device that removes residual developer particles from the photoreceptor, wherein the charge erase device directs the charge dissipation emissions at the photoreceptor subsequent to the removal of the residual developer particles by the residual developer cleaning device.
A residual developer cleaning device that removes residual developer particles from the photoreceptor, wherein the reconditioning light source directs light at the photoreceptor subsequent to the removal of the residual developer particles by the residual developer cleaning device can also be used.
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