Developer mixing apparatus having four ribbon blenders Number:7,426,361 from the United States Patent and Trademark Office (PTO) owispatent

Title: Developer mixing apparatus having four ribbon blenders

Abstract: A developer mixing apparatus, comprising: a housing including a chamber that holds developer; a first ribbon blender disposed within the chamber and elongate along a first longitudinal axis; and, a second ribbon blender disposed within the chamber and elongate along a second longitudinal axis, wherein the first and second ribbon blenders move developer in different directions.

Patent Number: 7,426,361 Issued on 09/16/2008 to Thompson,   et al.

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Inventors:	Thompson; Paul E. (Webster, NY), Stelter; Eric C. (Pittsford, NY), Guth; Joseph E. (Holley, NY)
Assignee:	Eastman Kodak Company (Rochester, NY)
Appl. No.:	11/217,916
Filed:	September 1, 2005

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Current U.S. Class:	399/254 ; 399/252; 399/256; 399/258; 399/263
Current International Class:	G03G 15/08 (20060101)
Field of Search:	399/252,254,256,258,263

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References Cited [Referenced By]

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U.S. Patent Documents

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Foreign Patent Documents

	06 208295		Jul., 1994		JP
	2005 024799		Jan., 2005		JP

Primary Examiner: Gray; David M.
Assistant Examiner: Walsh; Ryan D
Attorney, Agent or Firm: Suchy; Donna P.

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Claims

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The invention claimed is:

1. A developer mixing apparatus, comprising: a housing enclosing a chamber that holds developer; a first ribbon blender disposed within the chamber and elongate along a first longitudinal axis, the first ribbon blender including a first intermediate portion and first marginal portions spaced from each other along the longitudinal axis on either side of the first intermediate portion, the first marginal portions including first elements that, upon rotation about the first longitudinal axis, move developer with a first direction of developer flow along the first longitudinal axis within one of the first marginal portions and an opposite first direction of developer flow along the first longitudinal axis within another of the first marginal portions; a second ribbon blender disposed within the chamber and elongate along a second longitudinal axis and adjacent the first ribbon blender, the second ribbon blender including a second intermediate portion and second marginal portions spaced from each other along the second longitudinal axis on either side of the second intermediate portion, the second marginal portions including second elements that, upon rotation about the second longitudinal axis, move developer with a second direction of developer flow along the second longitudinal axis within one of the second marginal portions and an opposite second direction of developer flow along the second longitudinal axis within another of the second marginal portions; a third ribbon blender disposed within the chamber and elongate along a third longitudinal axis and adjacent at least one of the first ribbon blender and the second ribbon blender; and a fourth ribbon blender disposed within the chamber and elongate along a fourth longitudinal axis and adjacent at least one of the first ribbon blender, the second ribbon blender, and the third ribbon blender.

2. The apparatus of claim 1 wherein: the third ribbon blender is adjacent the first ribbon blender; the fourth ribbon blender is adjacent the second ribbon blender; and the third ribbon blender, is adjacent the fourth ribbon blender.

3. The apparatus of claim 2 wherein: a toner replenisher is disposed intermediate the first ribbon blender, the second ribbon blender, the third ribbon blender and the fourth ribbon blender.

4. The apparatus of claim 1 further including: a toner replenisher disposed intermediate the first ribbon blender, the second ribbon blender, and the third ribbon blender.

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Description

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FIELD OF THE INVENTION

The invention relates to electrographic printers and apparatus thereof. More specifically, the invention is directed to processes and apparatus for a developer mixer and related methods of mixing as well as to a powder coating apparatus and related methods of mixing.

BACKGROUND

Electrographic printers use a developer mixing apparatus and related processes for mixing the developer or toner used during the printing process. The term "electrographic printer," is intended to encompass electrophotographic printers and copiers that employ dry toner developed on an electrophotographic receiver element, as well as ionographic printers and copiers that do not rely upon an electrophotographic receiver. The electrographic apparatus often incorporates an electromagnetic brush station, to develop the toner to a substrate (an imaging/photoconductive member bearing a latent image), after which the applied toner is transferred onto a sheet and fused thereon. Related prior art can be found in U.S. Pat. Nos. 4,473,029 and 4,546,060, and U.S. Patent Application Nos. 2002/0168200 and 2003/0091921.

U.S. Pat. Nos. 6,526,247 and 6,589,703 and U.S. Patent Application Publication Nos. 2002/0168200; 2003/0091921; and 2003/0175053 provide additional description of magnetic brush technology using a rotating magnetic core for use in electrographic development apparatus. An essential feature of magnetic brush technology using a rotating magnetic core is that the magnetic field in the development zone has a rotating magnetic field vector. U.S. Pat. Nos. 6,526,247 and 6,589,703 and United States Patent Application Publication Nos. 2002/0168200; 2003/0091921; and 2003/0175053 are hereby incorporated by reference as if fully set forth herein.

U.S. Pat. Nos. 4,473,029; 4,546,060; and 4,602,863 provide a description of magnetic brush technology using a rotating magnetic core for use in electrographic development apparatus. U.S. Pat. Nos. 4,473,029; 4,546,060; and 4,602,863, and U.S. Patent Application Publication Nos. 2002/0168200 and 2003/0091921 are hereby incorporated by reference as if fully set forth herein.

U.S. Pat. No. 5,400,124 provides a description of magnetic brush technology using a rotating magnetic core and a stationary toning shell for applying toner to an electrostatic image. U.S. Pat. No. 5,966,576 provides a description of an alternate configuration of toning station also having rotating magnetic field vectors, in which a plurality of rotatable magnets are located adjacent to the underside of the development surface of the applicator sleeve to move developer material through the development zone. U.S. Pat. No. 5,376,492 discusses development using a rotating magnetic core and an AC developer bias.

U.S. Pat. Nos. 5,400,124; 5,966,576; and 5,376,492 are hereby full incorporated by reference as if fully set forth herein.

U.S. Pat. No. 5,307,124 discusses pre-charging toner before feeding into the developer sump containing partially depleted two-component developer material. U.S. Pat. No. 5,506,372 discusses a development station having a particle removal device for removing aged magnetic carrier to compensate for the addition of fresh carrier.

Depositing multiple layers of toner on a substrate by direct deposition from a magnetic brush includes U.S. Pat. Nos. 5,001,028 and 5,394,230, which discuss a process for producing two or more toner images in a single frame or area of an image member using two or more magnetic brush development stations with rotating magnetic cores. In this process, a region of an electrostatic receiver is developed with a first toner of a first polarity and then the receiver with a deposit of charged toner particles is passed through a second magnetic brush using a second toner of the first polarity, which deposits the second toner on the receiver. U.S. Pat. Nos. 5,409,791; 5,489,975; and 5,985,499 discuss a process for developing an electrostatic image on an image member already containing a loose dry first toner image with a second toner having the same electrical polarity as the first toner, using rotating magnetic core technology and AC projection toning, where the developer nap is not in contact with the receiver. U.S. Pat. Nos. 5,307,124; 5,506,372; 5,001,028; 5,394,230; 5,409,791; 5,489,975; and 5,985,499 are hereby incorporated by reference as if fully set forth herein.

For depositing multiple layers of toner on a substrate by transfer of the toner from an intermediate transfer member, intermediate transfer medium, or ITM, U.S. Pat. No. 5,084,735 and U.S. Pat. No. 5,370,961 disclose use of a compliant ITM roller coated by a thick compliant layer and a relatively thin hard overcoat to improve the quality of electrostatic toner transfer from an imaging member to a receiver, as compared to a non-compliant intermediate roller. Additional applications of hard overcoats on intermediate transfer members are disclosed in U.S. Pat. Nos. 5,728,496 and 5,807,651, which describe an overcoated photoconductor and overcoated transfer member, U.S. Pat. No. 6,377,772, which describes composite intermediate transfer members, and U.S. Pat. No. 6,393,226, which describes an intermediate transfer member having a stiffening layer. U.S. Pat. Nos. 5,084,735; 5,370,961; 5,728,496; 5,807,651; 6,377,772; and 6,393,226 are hereby incorporated by reference as if fully set forth herein.

U.S. Pat. No. 6,608,641 describes a printer for printing color toner images on a receiver member of any of a variety of textures. The printer has a number of electrophotographic image-forming modules arranged in tandem (see for example, Tombs, U.S. Pat. No. 6,184,911). These include a plurality of imaging subsystems to form a colored toner image that is transferred to a receiver member, the transfer of toner images from each of the modules forming a color print on the receiver member which is fused to form a desired color print. U.S. Pat. Nos. 6,608,641 and 6,184,911 are hereby incorporated by reference as if fully set forth herein.

Such a printer includes two or more single-color image forming stations or modules arranged in tandem and an insulating transport web for moving receiver members such as paper sheets through the image forming stations, wherein a single-color toner image is transferred from an image carrier, i.e., a photoconductor (PC) or an intermediate transfer member (ITM), to a receiver held electrostatically or mechanically to the transport web, and the single-color toner images from each of the two or more single-color image forming stations are successively laid down one upon the other to produce a plural or multicolor toner image on the receiver.

As is well known, a toner image may be formed on a photoconductor by the sequential steps of uniformly charging the photoconductor surface in a charging station using a corona charger, exposing the charged photoconductor to a pattern of light in an exposure station to form a latent electrostatic image, and toning the latent electrostatic image in a development station to form a toner image on the photoconductor surface. The toner image may then be transferred in a transfer station directly to a receiver, e.g., a paper sheet, or it may first be transferred to an ITM and subsequently transferred to the receiver. The toned receiver is then moved to a fusing station where the toner image is fused to the receiver by heat and/or pressure.

In a digital electrophotographic copier or printer, a uniformly charged photoconductor surface may be exposed pixel by pixel using an electro-optical exposure device comprising light emitting diodes, such as for example described by Y. S. Ng et al., Imaging Science and Technology, 47th Annual Conference Proceedings (1994), pp. 622-625.

A widely practiced method of improving toner transfer is by use of so-called surface treated toners. As is well known, surface treated toner particles have adhered to their surfaces sub-micron particles, e.g., of silica, alumina, titania, and the like (so-called surface additives or surface additive particles). Surface treated toners generally have weaker adhesion to a smooth surface than untreated toners, and therefore surface treated toners can be electrostatically transferred more efficiently from a PC or an ITM to another member.

As disclosed in the Rimai et al. patent (U.S. Pat. No. 5,084,735), in the Zaretsky and Gomes patent (U.S. Pat. No. 5,370,961) and in subsequent U.S. Pat. Nos. 5,821,972; 5,948,585; 5,968,656; 6,074,756; 6,377,772; 6,393,226; and 6,608,641, use of a compliant ITM roller coated by a thick compliant layer and a relatively thin hard overcoat improves the quality of electrostatic toner transfer from an imaging member to a receiver, as compared to a non-compliant intermediate roller. U.S. Pat. Nos. 5,084,735; 5,370,961; 5,728,496; 5,807,651; 5,821,972; 5,948,585; 5,968,656; 6,074,756; 6,377,772; 6,393,226; and 6,608,641 are hereby incorporated by reference as if fully set forth herein.

A receiver carrying an unfused toner image may be fused in a fusing station in which a receiver carrying a toner image is passed through a nip formed by a heated compliant fuser roller in pressure contact with a hard pressure roller. Compliant fuser rollers are well known in the art. For example, the Chen et al. patent (U.S. Pat. No. 5,464,698) discloses a toner fuser member having a silicone rubber cushion layer disposed on a metallic core member, and overlying the cushion layer, a layer of a cured fluorocarbon polymer in which is dispersed a particulate filler. Also, in the Chen et al. U.S. Pat. No. 6,224,978 is disclosed an improved compliant fuser roller including three concentric layers, each of which layers includes a particulate filler. Additional fusing means known in the art, such as non-contact fusing using IR radiation, oven fusing, or fusing by vapors may also be used. U.S. Pat. Nos. 5,464,698 and 6,224,978 are hereby incorporated by reference as if fully set forth herein.

U.S. Pat. Nos. 5,339,146; 5,506,671; 5,751,432; and 6,352,806 discuss means of forming overcoats on receivers with charged particles in the context of electrographic imaging. U.S. Pat. No. 5,339,146 uses a fusing surface or belt as an intermediate transfer member. This patent discloses mixing a clear particulate material with a magnetic carrier. The clear particulate material is applied using an applicator of a conventional magnetic brush development device. The applicator, using a rotating magnetic core and/or a rotatable shell, moves the developer mixture through contact with the fusing surface to deposit the particulate material on it. An electrical field is applied between the applicator and belt to assist this application. The fusing belt is preferably a metal belt with a smooth hard surface. U.S. Pat. No. 5,506,671 discloses an electrostatographic printing process for forming one or more colorless toner images in combination with at least one color toner image. At each image-producing station an electrostatic latent image is formed on a rotatable endless surface; toner is deposited on the electrostatic latent image to form a toner image on the rotatable surface, and the toner image is transferred from its corresponding rotatable surface onto the receptor element. U.S. Pat. No. 5,751,432 is directed to glossing selected areas of an imaged substrate and, in particular, to creating images, portions of which include clear polymer for causing them to exhibit high gloss thereby causing them to be highlighted. The clear toner may be applied to color toner image areas as well as black image areas. Additionally, the clear toner may be applied to non-imaged areas of the substrate. In carrying out the invention, a fifth developer housing is provided in a color image creation apparatus normally including only four developer housings. U.S. Pat. No. 6,352,806 concerns a color image reproduction machine that includes means for forming an additional toner image using clear colorless toner particles, thereby resulting in a uniform gloss of the full-gamut color toner image.

Additional prior art for electrostatically applied overcoats on images produced by non-electrographic means include: U.S. Pat. No. 5,804,341, which concerns an electrostatically applied overcoat on a silver halide image; U.S. Pat. No. 5,847,738, in which an electrostatic overcoat is applied to liquid ink; and U.S. Pat. No. 6,031,556, which cites an electrostatic overcoat on an image produced by thermal transfer. U.S. Pat. No. 6,424,364 cites use of an electrostatically-applied clear polymer as an undercoat to capture ink jet images which are subsequently fused.

Transfer of charged toner particles to metal substrates, particularly copper or zinc printing plates, from a paper intermediate using electrostatic transfer is disclosed by Sinclair, M., in Printing Equip. Engr. November 1948, p. 21-25.

The toner was used as an acid resist for etching. Transfer of charged toner particles to metal substrates from an intermediate using adhesive transfer is disclosed in: Ullrich O. A., Walkup, L. E., and Russel, R. E., Proc. Tech. Assn. Graphic Arts p. 130-138 (1954). The toner was used as an ink-bearing surface.

Other prior art citing functional uses of toner include U.S. Pat. No. 2,919,179 which discusses using toner transferred directly from a photoconductor to a metallic surface for use as an etch resist. Although several distinct applications are discussed, the description is limited, by way of example, to the discussion of printed circuit boards. U.S. Pat. No. 3,413,716 discloses transfer of toner particles from a photoconductor to a metallic surface to form a resist layer for etching inductors. U.S. Pat. Nos. 2,919,179 and 3,413,716 are hereby incorporated by reference as if fully set forth herein.

Ribbon blenders are used in two-component toning stations. An example of a two-ribbon blender assembly is presented in U.S. Pat. No. 4,634,286 the contents of which are hereby incorporated by reference as if fully set forth herein. As described in that patent, the outer ribbon moves developer material toward the center of the toning station. The inner ribbon moves developer material from the center toward the ends of the toning station. This produces good mixing between inward-flowing and outward-flowing material.

The present invention corrects the imbalances which can occur in inward and outward flow, thereby leading to non-uniform toner deposition on the substrate. The apparatus and related methods keep the different types of developers mixed and transported efficiently as needed, maintaining the correct proportions necessary to produce the high quality prints or powder coatings required by consumer demand. The following invention solves the current problems with developer mixing so that the mixer will work in a wide variety of situations.

SUMMARY OF THE INVENTION

The invention is in the field of mixing apparatus and processes for an electrographic printer and powder coating systems. More specifically, the invention relates to a mixing apparatus and processes that implement mixing in a plurality of directions. The mixing apparatus has a housing with a chamber that holds developer and a first ribbon blender disposed within the chamber and elongate along a first longitudinal axis and a second blender disposed within the same chamber and elongate along a second longitudinal axis. The first ribbon blender has an intermediate portion and marginal portions spaced from each other along the longitudinal axis on either side of the intermediate portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a schematic view of a printer machine according to one aspect of the invention.

FIG. 2 is a schematic representation of a process according to one aspect of the invention.

FIG. 3 is a cross-sectional side view of an electrographic developer mixing apparatus, according to one aspect of the invention, implemented as part of a development station.

FIGS. 4a-4d are cross-sectional top views of embodiments of the FIG. 3 apparatus with parts broken away.

FIG. 5 is a cross-sectional side view of an electrographic developer mixing apparatus, according to a further aspect of the invention, implemented as part of a development station.

FIG. 6 presents a side view of a blender according to an aspect of the invention.

FIG. 7 presents a perspective view of the FIG. 6 blender.

FIG. 8 presents a cross-sectional view of an electrographic developer mixing apparatus according to an aspect of the invention.

FIG. 9 presents a perspective view of a blender according to an aspect of the invention.

DETAILED DESCRIPTION

Various aspects of the invention are presented with reference to FIGS. 1-9, which are not drawn to any particular scale, and wherein like components in the numerous views are numbered alike. Referring now specifically to FIG. 1, a printer machine 10, such as an electrophotographic printer, that implements the electrographic developer mixing apparatus and processes of the invention is presented. The printer machine 10 includes a moving electrographic imaging or receiver member 18 such as a photoconductive belt which is entrained about a plurality of rollers or other supports 21a through 21g, one or more of which is driven by a motor to advance the belt. By way of example, roller 21a is illustrated as being driven by motor 20. Motor 20 preferably advances the belt at a high speed, such as 20 inches per second or higher, in the direction indicated by arrow P, past a series of workstations of the printer machine 10. Alternatively, belt 18 may be wrapped and secured about only a single drum, or may be a drum.

The term "electrographic printer," is intended to encompass electrophotographic printers and copiers that employ dry toner developed on an electrographic receiver element, as well as ionographic printers and copiers that do not rely upon an electrographic receiver. The processes of the present invention may also include a powder applicator for applying powder materials. To this end, reference is hereby made to copending U.S. application Ser. No. 11/075,784 entitled POWDER COATING APPARATUS AND METHOD OF POWDER COATING USING AN ELECTROMAGNETIC BRUSH, filed on Mar. 9, 2005, the contents of which are incorporated by reference as if fully set forth herein.

Printer machine 10 includes a controller or logic and control unit (LCU) 24, preferably a digital computer or microprocessor operating according to a stored program for sequentially actuating the workstations within printer machine 10, effecting overall control of printer machine 10 and its various subsystems. LCU 24 also is programmed to provide closed-loop control of printer machine 10 in response to signals from various sensors and encoders. Aspects of process control are described in U.S. Pat. No. 6,121,986 incorporated herein by this reference.

A primary charging station 28 in printer machine 10 sensitizes belt 18 by applying a uniform electrostatic corona charge, from high-voltage charging wires at a predetermined primary voltage, to a surface 18a of belt 18. The output of charging station 28 is regulated by programmable voltage controller 30, which is in turn controlled by LCU 24 to adjust this primary voltage, for example by controlling the electrical potential of a grid and thus controlling movement of the corona charge. Other forms of chargers, including brush or roller chargers, may also be used.

An exposure station 34 in printer machine 10 projects light from a writer 34a to belt 18 in accordance with parameters supplied from a writer interface 32. This light selectively dissipates the electrostatic charge on photoconductive belt 18 to form a latent electrostatic image of the document to be copied or printed. Writer 34a is preferably constructed as an array of light emitting diodes (LEDs), or alternatively as another light source such as a laser or spatial light modulator. Writer 34a exposes individual picture elements (pixels) of belt 18 with light at a regulated intensity and exposure, in the manner described below. After exposure, the portion of the belt bearing the latent charge image travels to a development station 35, which can apply toner to the belt 18 by moving a backup roller or bar 35a, which will be discussed in more detail below. The exposing light discharges selected pixel locations of the photoconductor, so that the pattern of localized voltages across the photoconductor corresponds to the image to be printed. An image is a pattern of physical light which may include characters, words, text, and other features such as graphics, photos, etc. An image may be included in a set of one or more images, such as in images of the pages of a document. An image may be divided into segments, objects, or structures each of which is itself an image. A segment, object or structure of an image may be of any size up to and including the whole image.

Image data to be printed is provided by an image data source 36, which is a device that can provide digital data defining a version of the image. Such types of devices are numerous and include computer or microcontroller, computer workstation, scanner, digital camera, etc. These data represent the location and intensity of each pixel that is exposed by the printer. Signals from image data source 36, in combination with control signals from LCU 24 are provided to a raster image processor (RIP) 37. The digital images (including styled text) are converted by the RIP 37 from their form in a page description language (PDL) to a sequence of serial instructions for the electrographic printer in a process commonly known as "ripping" and which provides a ripped image to an image storage and retrieval system known as a Marking Image Processor (MIP) 38.

In general, the major roles of the RIP 37 are to: receive job information from the server; parse the header from the print job and determine the printing and finishing requirements of the job; analyze the PDL (Page Description Language) to reflect any job or page requirements that were not stated in the header; resolve any conflicts between the requirements of the job and the Marking Engine configuration (i.e., RIP time mismatch resolution); keep accounting record and error logs and provide this information to any subsystem, upon request; communicate image transfer requirements to the Marking Engine; translate the data from PDL (Page Description Language) to Raster for printing; and support diagnostics communication between User Applications. The RIP accepts a print job in the form of a Page Description Language (PDL) such as PostScript, PDF or PCL and converts it into Raster, a form that the marking engine can accept. The PDL file received at the RIP describes the layout of the document as it was created on the host computer used by the customer. This conversion process is called rasterization. The RIP makes the decision on how to process the document based on what PDL the document is described in. It reaches this decision by looking at the first 2 K of the document. A job manager sends the job information to a MSS (Marking Subsystem Services) via Ethernet and the rest of the document further into the RIP to get rasterized. For clarification, the document header contains printer-specific information such as whether to staple or duplex the job. Once the document has been converted to raster by one of the interpreters, the Raster data goes to the MIP 38 via RTS (Raster Transfer Services); this transfers the data over an IDB (Image Data Bus).

The MIP functionally replaces recirculating feeders on optical copiers. This means that images are not mechanically rescanned within jobs that require rescanning, but rather, images are electronically retrieved from the MIP to replace the rescan process. The MIP accepts digital image input and stores it for a limited time so it can be retrieved and printed to complete the job as needed. The MIP consists of memory for storing digital image input received from the RIP. Once the images are in MIP memory, they can be repeatedly read from memory and output to an image render circuit 39. Compressing the images can reduce the amount of memory required to store a given number of images; therefore, the images are compressed prior to MIP memory storage, and then decompressed while being read from MIP memory.

The output of the MIP is provided to the image render circuit 39, which alters the image and provides the altered image to the writer interface 32 (otherwise known as a write head, print head, etc.) which applies exposure parameters to the exposure medium, such as a photoconductor 18.

After exposure, the portion of exposure medium belt 18 bearing the latent charge images travels to a development station 35. Development station 35 includes a magnetic brush in juxtaposition to the belt 18. Magnetic brush development stations are well known in the art, and are preferred in many applications. Alternatively, other known types of development stations or devices may be used. Development stations apply marking material onto the electrographic receiver or belt 18. The marking material may be comprised of a number of materials, such as toner, powder, etc. For exemplary purposes, the term toner will be used henceforth to describe the marking material. Plural development stations 35 may be provided for developing images in plural colors, or from toners of different physical characteristics. Full process color electrographic printing is accomplished by utilizing this process for each of four toner colors (e.g., black, cyan, magenta, yellow).

When the imaged portion of the electrographic receiver, or belt 18, reaches the development station 35, the LCU 24 selectively activates the development station 35 to apply toner to belt 18 by moving the backup roller or bar 35a against the belt 18, into engagement with or close proximity to the magnetic brush. Alternatively, the magnetic brush may be moved toward belt 18 to selectively engage belt 18. In either case, charged toner particles on the magnetic brush are selectively attracted to the latent image patterns present on belt 18, developing those image patterns. As the exposed photoconductor passes the developing station, toner is attracted to pixel locations of the photoconductor and as a result, a pattern of toner corresponding to the image to be printed appears on the photoconductive belt 18, thereby forming a developed image on the electrostatic image. As known in the art, conductor portions of development station 35, such as conductive applicator cylinders, are biased to act as electrodes. The electrodes are connected to a variable supply voltage, which is regulated a by programmable controller 40 in response to the LCU 24, there by controlling the development process.

Development station 35 may contain a two-component developer mix including a dry mixture of toner or powder and carrier particles. Typically the carrier preferably has high coercivity (hard magnetic) ferrite particles. As an example, the carrier particles have a volume-weighted diameter of approximately 26.mu.. The dry toner particles are substantially smaller, on the order of 6.mu. to 15.mu. in volume-weighted diameter. Development station 35 may include an applicator having a magnetic core within a shell, either of which may be rotatably driven by a motor or other suitable driving means. Relative rotation of the core and shell moves the developer through a development zone in the presence of an electrical field. In the course of development, the toner selectively electrostatically adheres to photoconductive belt 18 to develop the electrostatic images thereon and the carrier material remains at development station 35. As toner is depleted from the development station due to the development of the electrostatic image. Additional toner is periodically introduced by a toner replenisher 42 driven by a replenisher motor 41 into development station 35 in response to a replenisher motor control 43. The toner is mixed with the carrier particles to maintain a uniform amount of development mixture. This development mixture is controlled in accordance with various development control processes that use information gathered from various devices, such as a toner concentration monitors 57. Single component developer stations, as well as conventional liquid toner development stations, may also be used.

A transfer station 46 in printing machine 10, including a programmable voltage controller 46a and roller 46b, moves a receiver (such as a sheet S) into engagement with photoconductive belt 18, in registration with a developed image to transfer the developed image to receiver S. Receiver S may be plain or coated paper, plastic, or another medium capable of being handled by printer machine 10, such as a sheet, web, roll, or intermediate for intermediate transfer. Typically, transfer station 46 includes a charging device for electrostatically biasing movement of the toner particles from belt 18 to receiver sheet S. In this example, the biasing device is roller 46b, which engages the back of the receiver S and which is connected to programmable voltage controller 46a that operates in a constant current mode during transfer. Alternatively, an intermediate member may have the image transferred to it and the image may then be transferred to a receiver. After transfer of the toner image to a receivers, it is detacked from belt 18 and transported to fuser station 49 where the image is fixed onto the receiver, typically by the application of heat. Alternatively, the image may be fixed to the receiver at the time of transfer. A fuser entry guide may be implemented between the transfer station 46 and the fuser station, for example, as described in U.S. patent application Ser. No. 10/668,416 filed Sep. 23, 2003, in the names of John Giannetti, Giovanni B. Caiazza, and Jerome F. Sleve, the contents of which are incorporated by reference as if fully set forth herein.

A cleaning station 48, such as a brush, blade, or web is also located adjacent belt 18 behind transfer station 46, and removes residual toner from belt 18. A pre-clean charger (not shown) may be located before or at cleaning station 48 to assist in this cleaning. After cleaning, this portion of belt 18 is then ready for recharging and re-exposure. Of course, other portions of belt 18 are simultaneously located at the various workstations of printing machine 10, so that the printing process is carried out in a substantially continuous manner.

LCU 24 provides overall control of the apparatus and its various subsystems as is well known. LCU 24 will typically include temporary data storage memory, a central processing unit, timing and cycle control unit, and stored program control. Data input and output is performed sequentially through or under program control. Input data can be applied through input signal buffers to an input data processor, or through an interrupt signal processor, and include input signals from various switches, sensors, and analog-to-digital converters internal to printing machine 10, or received from sources external to printing machine 10, such from as a human user or a network control. The output data and control signals from LCU 24 are applied directly or through storage latches to suitable output drivers and in turn to the appropriate subsystems within printing machine 10.

Process control strategies generally utilize various sensors to provide real-time closed-loop control of the electrostatographic process so that printing machine 10 generates "constant" image quality output, from the user's perspective. Real-time process control is necessary in electrographic printing, to account for changes in the environmental ambient of the electrographic printer, and for changes in the operating conditions of the printer that occur over time during operation (rest/run effects). An important environmental condition parameter requiring process control is relative humidity, because changes in relative humidity affect the charge-to-mass ratio Q/m of toner particles. The ratio Q/m directly determines the density of toner that adheres to the photoconductor during development, and thus directly affects the density of the resulting image. An example of charges in operating conditions include system changes that can occur over time include changes due to aging of the printhead (exposure station), changes in the concentration of magnetic carrier particles to the toner as the toner is depleted through use, changes in the mechanical position of primary charger elements, aging of the electrographic receiver, variability in the manufacture of electrical components and of the electrographic receiver, change in conditions as the printer warms up after power-on, triboelectric charging of the toner, and other changes in electrographic process conditions. Because of these effects and the high resolution of modern electrographic printing, the process control techniques have become quite complex.

One process control sensor used is a densitometer 76, which monitors test patches that are exposed and developed in non-image areas of the photoconductive belt 18 under the control of LCU 24. Densitometer 76 may include an infrared or visible light LED, which either shines through the belt or is reflected by the belt onto a photodiode in densitometer 76. These developed test patches are exposed to varying toner density levels, including full density and various intermediate densities, so that the actual density of toner in the patch can be compared with the desired density of toner as indicated by the various control voltages and signals. These densitometer measurements are used to control primary charging voltage V.sub.O, maximum exposure light intensity E.sub.O, and development station electrode bias V.sub.B. In addition, the process control utilizes a toner replenishment control signal value or a toner concentration set point value to maintain the charge-to-mass ratio Q/m at a level that avoids dusting or hollow character formation due to low toner charge, and also avoids breakdown and transfer mottle due to high toner charge for improved accuracy in the process control of printing machine 10. The developed test patches are formed in the interframe area of belt 18 so that the process control can be carried out in real time without reducing the printed output throughput. Another sensor useful for monitoring process parameters in printer machine 10 is electrometer probe 50, mounted downstream of the charging station 28 relative to direction P of the movement of belt 18. An example of an electrometer is described in U.S. Pat. No. 5,956,544 incorporated herein by this reference.

Other approaches to electrographic printing process control may be utilized, such as those described in International Publication Number WO 02/10860 A1, and International Publication Number WO 02/14957 A1, both commonly assigned herewith and incorporated herein by this reference.

Raster image processing begins with a page description generated by the computer application used to produce the desired image. The Raster Image Processor interprets this page description into a display list of objects. This display list contains a descriptor for each text and non-text object to be printed; in the case of text, the descriptor specifies each text character, its font, and its location on the page. For example, the contents of a word processing document with styled text is translated by the RIP into serial printer instructions that include, for the example of a binary black printer, a bit for each pixel location indicating whether that pixel is to be black or white. Binary print means an image is converted to a digital array of pixels, each pixel having a value assigned to it, and wherein the digital value of every pixel is represented by only two possible numbers, either a one or a zero. The digital image in such a case is known as a binary image. Multi-bit images, alternatively, are represented by a digital array of pixels, wherein the pixels have assigned values of more than two number possibilities. The RIP renders the display list into a "contone" (continuous tone) byte map for the page to be printed. This contone byte map represents each pixel location on the page to be printed by a density level (typically eight bits, or one byte, for a byte map rendering) for each color to be printed. Black text is generally represented by a full density value (255, for an eight bit rendering) for each pixel within the character. The byte map typically contains more information than can be used by the printer. Finally, the RIP rasterizes the byte map into a bit map for use by the printer. Half-tone densities are formed by the application of a halftone "screen" to the byte map, especially in the case of image objects to be printed. Pre-press adjustments can include the selection of the particular halftone screens to be applied, for example to adjust the contrast of the resulting image.

Electrographic printers with gray scale printheads are also known, as described in International Publication Number WO 01/89194 A2, incorporated herein by this reference. As described in this publication, the rendering algorithm groups adjacent pixels into sets of adjacent cells, each cell corresponding to a halftone dot of the image to be printed. The gray tones are printed by increasing the level of exposure of each pixel in the cell, by increasing the duration by way of which a corresponding LED in the printhead is kept on, and by "growing" the exposure into adjacent pixels within the cell.

Ripping is printer-specific, in that the writing characteristics of the printer to be used are taken into account in producing the printer bit map. For example, the resolution of the printer both in pixel size (dpi) and contrast resolution (bit depth at the contone byte map) will determine the contone byte map. As noted above, the contrast performance of the printer can be used in pre-press to select the appropriate halftone screen. RIP rendering therefore incorporates the attributes of the printer itself with the image data to be printed.

The printer specificity in the RIP output may cause problems if the RIP output is forwarded to a different electrographic printer. One such problem is that the printed image will turn out to be either darker or lighter than that which would be printed on the printer for which the original RIP was performed. In some cases the original image data is not available for re-processing by another RIP in which tonal adjustments for the new printer may be made.

Similarly, according to the invention, the powder particles are developed, although preferably directly deposited as described above in connection