Abstract:
A developer system, including: a developer housing having a sump containing developer material including toner particles; a developer member rotatably mounted in the housing for transferring toner particles to a latent image on the photoreceptive member in a development zone; a pickup auger, positioned in an auger channel, for transporting and delivering developer material to the developer member, along a path adjacent to the developer member, the pickup auger having a first end portion and a second end portion, and the pickup auger includes a plurality of blades extending along the length of thereof, the plurality of blades being mounted on a core having a core size, the core size being adapted and arranged in the auger channel to maintain a constant developer material distance from the developer member along the length the auger channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Reference is made to commonly-assigned copending U.S. patent application Ser. No. 11/263,371, filed concurrently herewith, now U.S. Publication No. 2007/0098448, entitled DEVELOPER HOUSING DESIGN WITH IMPROVED SUMP MASS VARIATION LATITUDE, by Steven C. Hart and Ajay Kamar; copending U.S. patent application Ser. No. 11/263,370, filed concurrently herewith, now U.S. Publication No. 2007/0098451, entitled XEROGRAPHIC DEVELOPER UNIT HAVING VARIABLE PITCH AUGER, by Steven C. Hart and Ajay Kamar; copending U.S. patent application Ser. No. 11/262,577, filed concurrently herewith, now U.S. Publication No. 2007/0098458, entitled XEROGRAPHIC DEVELOPER UNIT HAVING MULTIPLE MAGNETIC BRUSH ROLLS WITH A GROOVED SURFACE, by Ajay Kumar, Keith A. Nau, David A. Reed, Jonathan D. Sadik, and Cory J. Winters; copending U.S. patent application Ser. No. 11/262,575, filed concurrently herewith, now U.S. Publication No. 2007/0098456, entitled XEROGRAPHIC DEVELOPER UNIT HAVING MULTIPLE MAGNETIC BRUSH ROLLS ROTATING AGAINST THE PHOTORECEPTOR, by Michael D. Thompson, James M. Chappell, Steven C. Hart, Patrick J. Howe, Ajay Kumar, Steven R. Leroy, Paul W. Morehouse, Jr., Palghat S. Ramesh, and Fei Xiao; and copending U.S. patent application Ser. No. 11/262,576, filed concurrently herewith, now U.S. Publication No. 2007/0098457, entitled XEROGRAPHIC DEVELOPER UNIT HAVING MULTIPLE MAGNETIC BRUSH ROLLS ROTATING WITH THE PHOTORECEPTOR, by James M. Chappell, Patrick J. Howe, Michael D. Thompson, and Fei Xiao, the disclosures of which are incorporated herein. 
   BACKGROUND 
   This invention relates generally to the development of electrostatic images, and more particularly concerns a two component development apparatus having a variable pitch auger to improve pickup latitude in developer housing. 
   Generally, the process of electrophotographic printing includes sensitizing a photoconductive surface by charging it to a substantially uniform potential. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to a desired image. The selective dissipation of the charge leaves a latent charge pattern that is developed by bringing a developer material into contact therewith. This process forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet. Finally, the powder image is heated to permanently affix it to the copy sheet in image configuration. 
   Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles having an electrostatic charge so that they will be attracted to, and adhere to, the latent image on the photoconductive surface. 
   There are various known development systems for bringing toner particles to a latent image on a photoconductive surface. These are: single component, two component, and hybrid systems. Additionally the single component and hybrid systems may be either scavenging or scavengeless; two component development systems are almost always scavenging. The term scavenging or scavengeless denotes whether the development method would disturb any previously developed image already on the photoconductive surface. if any previously developed image is left undisturbed, the system is scavengeless. 
   Single Component Development Systems: A (scavenging) single component development system uses a donor roll for transporting charged toner to the development nip defined by the donor roll and the photoconductive surface. The toner is loaded onto the donor roll by direct contact with a toner reservoir and sometimes with the assistance of a toner loading brush or foam roll. The donor roll rotates to bring the charged toner into the development nip. Using a combination of AC and /or DC electrical biases, the toner is moved from the donor roll to the photoconductive surface. Thus, the toner is developed on the latent image recorded on the photoconductive surface. 
   A scavengeless single component development system is physically similar to a scavenging single component system except that it uses a donor roll with a plurality of electrode wires closely spaced therefrom in the development zone. An AC voltage is applied to the wires detaching the toner from the donor roll and forming a toner powder cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image. 
   Two Component Development Systems: in a two component development system, a magnetic developer roll (with rotating external shell and an interior magnetic assembly which can be either stationary or rotating) attracts developer from a reservoir. The developer includes carrier and toner. As the external shell rotates and transports the developer material, the developer material is subsequently trimmed or metered to a desired uniform thickness. This layer of material is commonly referred to as a magnetic brush. Further rotation of the external shell advances the developer material into the development nip. In the development nip, the magnetic brush is brought into contact with the photoreceptor. Here, the toner is attracted from the carrier beads to the photoreceptor to develop the latent image. Further rotation of the developer roll returns the carrier beads and unused toner to the developer housing reservoir or sump. 
   Hybrid Development Systems: A hybrid development system is a cross between a single component development system and a two component system. A Hybrid system uses two component developer materials in conjunction with a magnetic developer roll to form a magnetic brush. However instead of developing the image directly with the magnetic brush, the magnetic brush is used to apply a uniform layer of toner onto a donor roll. Then as the donor roll rotates, the toner layer is advanced into the development nip and the latent image is developed in a manner similar to single component systems. A Hybrid System may be either scavenging or scavengeless. 
   Two component systems, either strictly two component or hybrid, require a uniform layer of developer material on the developer roll to function optimally. This layer of material must be provided independent of many factors. In some developer housing designs, developer material is picked up from one auger, trimmed to the desired thickness, used to develop an image or to load a donor roll, and then released into different auger. This results in a gradient in the developer material mass (or volume fill) down the length of the pick up auger region; one end of the auger is nearly full and the other end would be almost empty. One solution known in the prior art to deal with this variation, is to vary the “pick up” magnetic pole strength along the developer roll with a weaker pick up pole strength being used to acquire material in the almost full end of the auger and a very strong magnetic pole strength being used to acquire material from the almost empty end of the auger. An undesirable feature of this approach is that it is difficult to manufacture a magnetic structure with the appropriately varying magnetic strength. 
   A second solution known in the prior art is to simply use a uniform and very strong pickup magnet. An undesirable outcome of this solution is that much more material than necessary would be picked up from the nearly full end of the donor roll. This causes a small non-uniformity in the layer thickness, increases mechanical power requirements needed to rotate the donor roll, increases developer material abuse, and leads to a higher unit manufacturing cost (UMC). 
   SUMMARY 
   There is provided an “upper transport auger” or “pick up auger” with a variable pitch. The optimum pitch variation is linear down the length of the auger. A variable pitch auger can maintain a constant volumetric filling when used in a developer housing where developer material is picked up from one auger, used to develop an image, and then released into different auger. The significance of this is that the distance between the developer material available for “pick up” and the developer roll is kept constant down the length of the roll and auger. Maintaining the “pick up” material supply at a constant (and close) distance from the pickup region of the developer roll. This eliminates the need to overachieve the “pick up” function at one end, or alternatively to manufacture a magnet assembly with a uniformly varying magnetic pick up field strength. This enables the use of lower strength “pick up” magnetic fields and at the same time presents a uniform amount of material to the trim region independent of position down the length of the roll. The lower strength pick up magnetics reduces the mechanical power required to drive the housing, enhances developer roll shell life, and reduces developer material abuse. The uniform amount of material presented to the trim region also improves the MOR uniformity. 
   There is also provided a developer system, comprising: a developer housing having a sump containing developer material including toner particles; a developer member rotatably mounted in said housing for transferring toner particles to a latent image on said photoreceptive member in a development zone; a pickup auger, positioned in an auger channel, for transporting and delivering developer material to said developer member, along a path adjacent to said developer member, said pickup auger having a first end portion and a second end portion, and said pickup auger includes a plurality of blades extending along the length of thereof, said plurality of blades being mounted on a core having a core size, said core size being adapted and arranged in said auger channel to maintain a constant developer material distance from said developer member along the length said auger channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic elevational view of an illustrative electrophotographic printing machine incorporating developer unit having the features of the present invention therein. 
       FIG. 2  is a schematic elevational view showing one embodiment of the developer unit used in the  FIG. 1  printing machine. 
       FIG. 3  is an illustration of the portion of the developer unit of the present disclosure 
       FIGS. 4 and 5  illustrate developer material flow patterns in developer unit used in  FIG. 2 . 
       FIG. 6  is a side view illustrating the developer material flowing in an auger of the present disclosure. 
       FIG. 7  is experimental data. 
       FIG. 8  is a side view illustrating the developer material flowing in another embodiment of an auger of the present disclosure. 
   

   DETAILED DESCRIPTION 
   While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
   Inasmuch as the art of electrophotographic printing is well known, the various processing stations employed in the  FIG. 1  printing machine will be shown hereinafter schematically and their operation described briefly with reference thereto. 
   Referring initially to  FIG. 1 , there is shown an illustrative electrophotographic printing machine incorporating the development apparatus of the present invention therein. The electrophotographic printing machine employs a belt  10  having a photoconductive surface  12  deposited on a conductive substrate. Belt  10  moves in the direction of arrow  16  to advance successive portions of photoconductive surface  12  sequentially through the various processing stations disposed of throughout the path of movement thereof. Motor  24  rotates belt  10  in the direction of arrow  16 . Roller  22  is coupled to motor  24  by suitable means, such as a drive belt. 
   Initially, a portion of belt  10  passes through charging station A. At charging station A, a corona generating device, indicated generally by the reference numeral  26  charges photoconductive surface  12  to a relatively high, substantially uniform potential. High voltage power supply  28  is coupled to corona generating device  26  to charge photoconductive surface  12  of belt  10 . After photoconductive surface  12  of belt  10  is charged, the charged portion thereof is advanced through exposure station B. 
   At exposure station B, a controller receives the image signals from Print Controller representing the desired output image and processes these signals to convert them to signals transmitted to a laser based output scanning device, which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. Preferably the scanning device is a laser Raster Output Scanner (ROS)  36 . Alternatively, the ROS  36  could be replaced by other xerographic exposure devices such as LED arrays. 
   After the electrostatic latent image has been recorded on photoconductive surface  12 , belt  10  advances the latent image to development station C. At development station C, a developer unit, indicated generally by the reference numeral  38 , develops the latent image recorded on the photoconductive surface. Developer rolls  40  and  41  are mounted, at least partially, in the chamber of the developer housing. The chamber in the developer housing stores a supply of developer material. In one embodiment the developer material is a single component development material of toner particles, whereas in another, the developer material includes at least toner and carrier. 
   With continued reference to  FIG. 1 , after the electrostatic latent image is developed, belt  10  advances the toner powder image to transfer station D. A copy sheet  70  is advanced to transfer station D by sheet feeding apparatus  72 . Preferably, sheet feeding apparatus  72  includes a feed roll  74  contacting the uppermost sheet of stack  76  into chute  78 . Chute  78  directs the advancing sheet of support material into contact with photoconductive surface  12  of belt  10  in a timed sequence so that the toner powder image developed thereon contacts the advancing sheet at transfer station D. Transfer station D includes a corona generating device  80  which sprays ions onto the back side of sheet  70 . This attracts the toner powder image from photoconductive surface  12  to sheet  70 . After transfer, sheet  70  continues to move in the direction of arrow  82  onto a conveyor (not shown) that advances sheet  70  to fusing station E. 
   Fusing station E includes a fuser assembly, indicated generally by the reference numeral  84 , which permanently affixes the transferred powder image to sheet  70 . Fuser assembly  84  includes a heated fuser roller  86  and a back-up roller  88 . Sheet  70  passes between fuser roller  86  and back-up roller  88  with the toner powder image contacting fuser roller  86 . In this manner, the toner powder image is permanently affixed to sheet  70 . After fusing, sheet  70  advances through chute  92  to catch tray  94  for subsequent removal from the printing machine by the operator. 
   After the copy sheet is separated from photoconductive surface  12  of belt  10 , the residual toner particles adhering to photoconductive surface  12  are removed therefrom at cleaning station F. Cleaning station F includes a rotatably mounted fibrous brush  96  in contact with photoconductive surface  12 . The particles are cleaned from photoconductive surface  12  by the rotation of brush  96  in contact therewith. Subsequent to cleaning, a discharge lamp (not shown) floods photoconductive surface  12  with light to dissipate any residual electrostatic charge remaining thereon prior to the charging thereof for the next successive imaging cycle. 
   It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine incorporating the development apparatus of the present disclosure therein. 
   Referring now to  FIG. 2 , there is shown an embodiment of the present disclosure in greater detail. The overall function of developer unit  100  is to apply marking material, such as toner, onto suitably-charged areas forming a latent image on an image receptor such as belt  10  (a portion of which is shown), in a manner generally known in the art. In various types of printers, there may be multiple such developer units, such as one for each primary color or other purpose. 
   Among the elements of a the developer unit shown in  FIGS. 2 and 3 , which are typical of developer units of various types, are a housing  112 , which functions generally to hold a supply of developer material, as well as augers such as  130 , 132 ,  134 , which variously mix and convey the developer material, and magnetic development rolls  136 ,  138 , which in this embodiment form magnetic brushes to apply developer material to the belt  10 . 
   For the illustrated embodiment wherein the magnetic development rolls  136 ,  138 , are a relatively rigid cylinder, disposed within each magnetic development rolls  136 ,  138  there is a stationary “magnetic structure”  110 ,  111 . The magnetic structure  110 ,  111  is designed to remain in one position while the magnetic development roll rotates around it. The magnetic structure  110 ,  111  includes any number of magnetic members as necessary, and these magnetic members may be in the form of discrete metal magnets, or areas of specific magnetic polarity within a continuous structure, such as in a “plastic magnet.” Conceivably, the magnetic structure  110 ,  111  may comprise electromagnets as well. The purpose of the magnetic structures  110 ,  111  within magnetic development rolls  136 ,  138  is to attract the magnetic carrier from the developer supply and cause the magnetic carrier to magnetically adhere to the surface of the magnetic development roll as a given portion of the surface of magnetic development roll is advanced, with motion of magnetic development roll, towards the development zone. As is well-known in the art of xerography, two-component developer generally functions as follows: the carrier particles, or beads, attracted by the magnets within magnetic structure  110 ,  111 , form filaments of a “magnetic brush”, particularly around the poles defined in the magnetic structure, much in the manner of iron filings. Adhering triboelectrically to the carrier beads is any number of toner particles. The magnetic brush of carrier beads thus serves to convey the toner particles to the development zone. In a typical two-component contact developing system, the magnetic brush with toner particles thereon is brought into direct contact with the surface  12  of the belt  10 , to develop the latent image thereon. 
   Other types of features for development of latent images, such as developer rolls, paddles, scavengeless-development electrodes, commutators, etc., are known in the art and could be used in conjunction with various embodiments pursuant to the claims. In the illustrated embodiment, there is further provided air manifolds  140 ,  142 , attached to vacuum sources (not shown) for removing dirt and excess particles from near belt  10 . 
     FIGS. 4-6  are diagrams for the developer material flow pattern in the housing. The diagrams are topologically correct. The inboard to outboard placement of the features is relationally correct. The location of the “pick up”, trim, handoff, and development functions are logically correct. For the actual placement of the various components/features, please refer to  FIG. 2 . 
   Auger  134  is an upper transport auger located in auger channel  220 . Mixing/pump auger  130  and transport auger  132  are located below auger  134  and are disposed in auger channel  224  and auger channel  226 . Auger  134  receives developer material from the pump section  200  of the mixing/pump auger  130  and developer material moves along portion  202  of the developer material flow pattern. The auger  134  then transports this material from outboard to inboard along the full length of the housing along portion  204  of the developer material flow pattern. The upper developer roll  40  “picks up” material from auger  134  for use in the development process. Any material that is not “picked up” and used to develop the image is ultimately dropped back down into the mixing/pump auger  130  (as illustrated by the downward arrows) at the inboard end of the developer housing along portion  206  of the developer material flow pattern. 
   Now focusing on the developer material, the developer material flows in the lower portion of the housing, spillway  145  is located at an opening near the top of the wall  146  separating the mixing/pump auger  130  from the lower front auger  132 . It is located just before the junction between the mixing section  203  and pump section  200  of the mixing/pump auger  130 . Spillway  145  is an opening defined in wall  146  and acts as a pressure relief vent; if more material is delivered to the pump section  200  of the mixing/pump auger  130  than the pump can utilize, the excess material spills over the wall  146  and into the lower front auger  132 . 
   The mixing/pump auger  130  has several functions. It a) transports material from inboard to outboard along the developer material flow pattern  208 , as shown in  FIG. 4 , b) mixes in the replenisher (replacement toner and carrier) supply delivered at the inboard end, c) pumps developer material up to the upper transport auger  134 , and d) acts as part of the material mass (volume) buffer to accommodate changes in developer sump charge mass (volume). Auger  130  has been designed with a larger pitch to diameter ratio (P/D) preferably by a factor of 2 in the mixing transport section  203  than in the pump section  200 . This results in a larger transport rate in section  203  than in section  200 . Transport rate is the physical displacement of material per unit time. It is expressed in units of mm/sec or units of mm/rev of the auger. Given equal cross sectional filling factors, section  203  will have a larger volumetric flow rate than section  200 . Volumetric flow is the volume of developer material crossing AN imaginary plane per unit time. In an auger, this is equal to the “Transport rate” times the cross sectional area of the filled portion of the auger (channel). 
   Now focusing on the present disclosure, referring to  FIG. 6 , an “Upper Transport Auger” or “Pick Up Auger” with a variable pitch, it has been found that the optimum pitch variation is linear down the length of the auger  134 . A variable pitch auger maintains a constant volumetric filling in auger channel  220 . The significance of this is that the distance between the developer material in the auger channel  220  available for “pick up” and the developer roll is kept constant down the length of the roll and auger channel. This maintains the “pick up” material supply at a constant (and close) distance from the pickup region of the developer roll thereby eliminating the need to over achieve the “pick up” function at one end. This enables the use of lower strength “pick up” magnetic fields and at the same time presents a uniform amount of material to the trim region independent of position down the length of the roll. The lower strength pick up magnetics reduces the mechanical power required to drive the housing, enhances developer roll shell life, and reduces developer material abuse. The uniform amount of material presented to the trim region improves the MOR uniformity. 
   In operation, material (for use in development) is removed uniformly down the length of the upper transport auger by the upper developer roll  40  at the pickup region. This material is trimmed/metered to a desired layer thickness and utilized to develop an image. After development, the material is delivered to the lower auger, not back into the upper transport auger. Since, the developer material is not returned to the pickup upper transport auger, the auger&#39;s material transport requirement (to supply the developer material to the upper developer roll) decreases linearly down the length of the auger. Material transport for an auger is proportional to the pitch, filled cross sectional area, and rotational speed. Hence, the material transport rate may be decreased linearly and the filled cross sectional area may be held constant if the pitch of the auger is linearly decreased (at the appropriate rate). 
   Applicants have found that a Pitch to Diameter ratio of 0.7 on the outboard (up feed) end and about 0.4 on the inboard (down feed) end of the auger provides an approximately constant cross sectional filling area for nominal conditions. It should be noted that the pitch can be varied stepwise or varied continuously. 
   As illustrated in  FIG. 7 , nominal conditions are: developer mass on roll (MOR) of about 37 mg/cm 2 , roll surface velocity of about 700 mm/sec, auger rotational speed of 800 RPM. 
   There are several benefits. Since, the upper transport auger&#39;s filled cross sectional area in the channel is approximately constant, there is less observed variation in MOR between the inboard and outboard (trimming is a slight function of the amount of material presented to the trim blade). Because the gap between the developer material surface and the developer roll surface is small and uniform, applicants have been able to reduce the strength of the pick up pole magnet. As a result, less material is in general picked up and delivered to the trim region. This reduces the amount of power required to drive the developer roll, reduces wear on both the developer roll surface and developer material itself, and significantly increases the nominal trim blade gap required to meter the desired 37 mg/cm 2  MOR. 
   Now referring to  FIG. 8  which illustrates an alternative embodiment of the present disclosure for maintaining a uniform constant cross sectional filling factor within the pick up auger channel. As illustrated in  FIG. 8 , core  300  has a plurality of blades  302  positioned about core  300 . The core size of the auger is varied to maintain a uniform constant cross sectional filling factor within the pick up auger channel. Preferably the core is round and the root diameter is varied in a fashion so as to compensate for the volume of developer material which has been picked up and used for development. In the case where the volume of developer material used for development is constant down the length of the developer roll, the root diameter. D R , would need to increase and can be determined by the following equation:
 
 D   R ( L )=(( D   0 ) 2   +K×L ) 1/2 .
 
where, L is the distance down the length of the magnetic brush, D 0  is the root diameter of the auger at the edge of the magnetic brush, and K is a function of auger pitch (P), auger rotational period (τ), developer roll surface velocity (V), developer material density (ρ), and developer roll mass per unit area on the roll (MOR).
 
   K is given by: K=4×P×V×MOR×τ/(π×ρ). 
   It should be noted that the two concepts of varying core size and varying Pitch to Diameter ratio can be combined to also produce an useful auger for maintaining a uniform constant cross sectional filling factor within the pick up auger channel 
   It is, therefore, apparent that there has been provided in accordance with the present invention, an Auger that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.