Abstract:
An apparatus and method for dispensing toner in an electrostatographic printer includes an apparatus for feeding powder toward the feed apparatus wherein the feed roller includes a tapered feed roller including a shaft and one or more variable height flutes such that there is more developer volume in the direction of flow as well as a conveyance controller for controlling the powder conveying device, including the one or more tapered feed rollers such that the tapered feed roller preferentially uniformly conveys the powder toward the feed apparatus.

Description:
FIELD OF THE INVENTION 
   The invention relates to electrographic printers and apparatus thereof. More specifically, the invention is directed to an apparatus and method for transporting a powder, such as developer to an image device in an electrostatographic printer. 
   BACKGROUND OF THE INVENTION 
   Electrographic printers and copiers utilizing developer comprising toner, carrier, and other components use a developer mixing apparatus and related processes for mixing the developer and 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 or similar development 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. 
   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 developer 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 intermediate transfer member or 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 electrostatographic copiers and printers, pigmented thermoplastic particles, commonly known as “toner,” are applied to latent electrostatic images to render such images visible. Often, the toner particles are mixed with and carried by somewhat larger particles of magnetic material. During the mixing process, the magnetic carrier particles serve to triboelectrically charge the toner particles to a polarity opposite that of the latent charge image. In use, the development mix is advanced, typically by magnetic forces, from a sump to a position in which it contacts the latent charge image. The relatively strong electrostatic forces associated with the charge image operate to strip the toner from the carrier, causing the toner to remain with the charge image. Thus, it will be appreciated that, as multiple charge images are developed in this manner, toner particles are continuously depleted from the mix and a fresh supply of toner must be dispensed from time-to-time in order to maintain a desired image density. Usually, the fresh toner is supplied from a toner supply bottle mounted upside-down, i.e., with its mouth facing downward, at one end of the image-development apparatus. Under the force of gravity, toner accumulates at the bottle mouth, and a metering device, positioned adjacent the bottle mouth, operates to meter sufficient toner to the developer mix to compensate for the toner lost as a result of image development. Usually, the toner-metering device operates under the control of a toner concentration monitor that continuously senses the ratio of toner to carrier particles in the development mix. 
   It is well known that toner is a powdery substance that exhibits a considerable degree of cohesiveness and, hence, relatively poor flowability. Since the force of gravity alone does not usually suffice in causing toner to flow smoothly from the mouth of an inverted toner bottle, other supplemental techniques have been used to “coax” the toner from the bottle. For example, flow additives, such as silica and the like, have been added to the mix to reduce the troublesome cohesive forces between toner particles. See, e.g., the disclosure of U.S. Pat. No. 5,260,159 in which a “fluidization” agent is added to a developer mix in a development sump to assist the movement of developer therein. While beneficial to a more consistent flow of developer, such substances influence other performance attributes of the development process and their effectiveness is therefore constrained. 
   Development stations require replenishment of toner into the developer sump to replace toner that is deposited on the photoconductor or receiver as well as a magnetic carrier that are mixed together uniformly to form an effective developer. The developer must be mixed and transported to a position where it can be in contact with the latent charged image. If the mixing and/or transport are inefficient or ineffective the printing process is compromised. This can lead to many problems from poor prints to a no prints at all. In electrostatic development stations utilizing carrier, this is especially challenging since the magnetic carrier is affected by many conditions including particle size and orientation. Although the developer can stay near the feed roller at the front of the roller, as the developer with the feed roller encounters an increasing magnetic field is imposed on the developer is attracted away from the feed roller. As the feed apparatus picks up developer from the feed roller the amount of developer left near the rear portion of the feed roller is greatly decreased to the point where there is no developer left to transport to the latent charge image and printing stops. This is not an easy problem to solve since a simple change in developer amount or charge can quickly change conditions within the feed channel. This problem is enhanced since when there is less developer left in the feed channel then the pick-up point becomes even further from the feed roller and since the magnetic force is decreased by multiples as the distance decreases this makes the problem quite significant. 
   The present invention corrects the problem of non-uniform transport of developer from the feed roller to the feed apparatus. The apparatus and related methods described correct the problem of non-uniform developer feed in order to allow the printer to produce the high quality prints or powder coatings required by consumer demand. The following invention solves the current problems with developer feed rollers and will work in a wide variety of situations and with different types of toners, powders, or particles. 
   SUMMARY OF THE INVENTION 
   The invention is in the field of electrographic printers and powder coating systems. More specifically, the invention relates to an apparatus and method for feeding powder toward a feed apparatus wherein the feed roller includes a tapered feed roller comprising a shaft and one or more variable height flutes such that there is more developer volume in the direction of flow as well as a conveyance controller for controlling the powder conveying device. The tapered feed roller preferentially uniformly conveys the powder toward the feed apparatus. The apparatus for transporting powder into a developer station containing at least powder and magnetic carrier including a conveyance housing and the one or more tapered feed rollers with flutes of some specific volume per unit length, along with a stationary magnet in the core of the roller that urges developer into the flute volume. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view, in cross-section, of a reproduction apparatus magnetic brush developer station according to this invention. 
       FIGS. 2   a  and  2   b  show a tapered roller of the magnetic brush development station of  FIG. 1 . 
       FIGS. 3   a - 3   b  are schematics of a portion of the tapered roller of  FIG. 2 , particularly showing other embodiments according to this invention. 
       FIGS. 4   a - 4   b  are schematics of a portion of the tapered roller of  FIG. 2 , particularly showing other embodiments according to this invention. 
       FIG. 5  is a schematic of a portion of the reproduction apparatus magnetic brush developer station according to this invention. 
       FIG. 6  shows the number of flutes and how it influences the amount of developer feed in the direction of the developer flow. 
       FIG. 7  shows the channel depletion effect. 
       FIG. 8  is a schematic showing one embodiment of the present invention that can better balance the developer flow. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an electrostatic printer magnetic brush developer station, according to this invention, sometimes simply referred to as a developer station, designated generally by the numeral  10 . The development station housing  12  encloses a feed apparatus  14  and a powder conveyance device  16  and forms, in part, a reservoir  15  for developer material  17  comprising a powder and a carrier material. A development roller  18  is mounted within the development station housing  12 . The development roller  18  includes a rotating (shown as counterclockwise in  FIG. 1 ) fourteen-pole core magnet  20  inside a rotating (shown as clockwise in  FIG. 1 ) shell  22 . The core magnet  20  and the shell can have many other suitable relative rotations as is known in the art. 
   The quantity of developer material delivered from the reservoir  15  to the development zone  24  is controlled by a metering skive  26 , positioned parallel to the longitudinal axis of the development roller  18 , at a location upstream in the direction of shell rotation prior to the development zone. The metering skive  26  extends the length of the development roller  18  (see  FIG. 3 ). The core magnet  20  does not extend the entire length of the development roller; as such, the developer nap on the shell  22  does not extend to the end of the development roller. The development station  12  houses one or development rollers to move the developer material within the reservoir of the housing  12  from the mixing area to the feed apparatus. 
     FIGS. 2   a  and  2   b  show one or more tapered feed rollers  28  (only one is shown for clarity) each having a shaft  50  and one or more variable height flutes  52  such that there is more developer volume between the flutes as the developer moves in the direction of flow (F). Generally, the feed roller has a rotating outer shell and flutes that can move some specific volume of developer  17  per unit length, along with a stationary magnet  30  in the core of the roller that urges developer  17  into the flute volume  32 , as shown in  FIGS. 2   b .  FIG. 2   a  shows a feed roller flute height ‘d’ increasing in the direction of the developer flow (F). This is sometimes referred to as volume bias. Developer feed uniformity is improved by creating a variable flute height ‘d’ on the feed roller. This can be accomplished by machining a taper on a constant height flute roller as shown in  FIG. 2   b.    
   The magnetic brush development station  10 , according to this invention, uses two augers (see  FIG. 1 ), although a different number could be used in conjunction with the tapered roller. Controller  60  controls the development station including the tapered feed roller  28  as shown in  FIG. 2   g . The controller also controls the powder-conveying device, such that the auger preferentially mixes in the mixing space and transports in the second transport space as the powder is conveyed toward the tapered rollers  28  as shown in  FIG. 1 . The tapered rollers  28  described above allow more developer volume between the flutes  52  as the developer moves in the direction of flow (F). 
   Developer feed uniformity is improved by tapering the feed roller. In one embodiment this is achieved using the variable flute height ‘d’ on the feed roller as shown in  FIG. 2   b  and discussed above. This can also be accomplished by varying other features of the tapered feed roller as shown in the two embodiments shown in  FIGS. 3   a  and  3   b  that result in developer feed uniformity and specifically encourage more developer in areas or greater pickup, such as at the second end.  FIGS. 3   a  and  3   b  show a flute  52  with an internal angle α and a flute angle tilt β on individual flutes as well as the flute height ‘d’. Theses features could be combined or used separately to control the volume bias as required. The flutes  52  can also have one or more surface features, such as texture or pockets that might effectively create a bucket type effect, to further move the volume of developer moved toward the feed apparatus. 
   Other embodiments as shown in  FIGS. 4   a  and  4   b  can be used to increase the relative volumes of developer traveling in direction F. These include tapering the shaft diameter or support diameter and/or sloping the whole taper feed roller shaft the required amount to effect the desired total volume increase. This can be done by machining a taper on the flute shaft or cylinder or some other similar method as shown in  FIG. 3 . This variable height is oriented such that the flute height ‘d’ increases in the direction of the developer flow (F) in the channel. Since during operation there is normally more developer at the second or rear end  62  of the feed roller than in the front end  61 , as shown in  FIG. 3 . The tapered rollers compensate for this effect. This is important since when there is less developer left in the feed channel the pick-up point at the surface of the developer in the channel becomes even further from the feed roller. The tapered feed roller allows the lead edge of the feed roller to hold less developer, thereby allowing more developer to move to the rear  62  of the channel resulting in more uniform pick-up by the feed apparatus and thus more efficient and higher quality prints. 
     FIG. 5  shows the developer moving from the first end  61  of the tapered roller  28  to the second end  62  of the tapered roller  28 . The volume of developer at the first end does not normally equal the volume at the second end since there is more space up at the second end but the present invention does try to minimize that difference so that the percent fill (or ratio of a powder volume to total volume) at the first end  61  or first location approaches that at the second end  62  or at a second location. in the feed roller as the powder is conveyed toward the development zone  24  as shown in  FIG. 1 . 
   The addition of these flutes  52  on the feed roller shaft  50  helps urge and keep the developer on the feed roller until such time where the imposed magnetic field of the toning roller would attract the developer to it. This effect is shown in  FIG. 6 .  FIG. 6  shows the number of flutes  52  and how it influences the amount of developer feed in the direction of the developer flow. Note that the feed is increased approximately 2 times when the number of flutes increase from 0 flutes (a bare roller) to 12 flutes. As discussed above, when a non-tapered feed roller picks up developer from the feed channel, the amount of developer in that channel normally decreases, creating a gradient of developer load along the channel as discussed above. This can be so severe as to completely empty the feed channel of developer, effectively stopping the developer circulation in the sump. This effect is shown in  FIG. 7  and is referred to as the channel depletion effect. 
     FIG. 8  shows the effect of different tapered feed roller taper angles A on developer flow and the resulting feed uniformity. The optimal feed taper angles A were generated iteratively and then tested to find the optimum setting to maximize developer flow by position and taper angle. The various taper angles shown in  FIG. 8  simultaneously optimize a maximum mean developer flow and a minimum total range of developer flow from front to rear of the development station housing  12  and reservoir  15 .  FIG. 8  shows various setting that thus optimize the tapered feed roller  28  to significantly improve the uniformity of the developer flow. The taper can be developed by an increasing shaft  50  diameter or alternately increasing flute diameter or a combination the two including both increasing/and or decreasing both together to result in increasing volume in the direction of flow.  FIG. 8  shows that there is a point where the taper no longer increases the volume bias and at that point flow essentially stops. In the embodiment shown in  FIG. 8 , that was when the taper angle A was 0.425 degrees so the desired range was between 0 and less then 0.425 degrees with an optimum between 0 and 0.3 or 0.4 degrees but less then 0.425 degrees for this embodiment. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.