Abstract:
Apparatus for non-interactive, dry powder development of electrostatic images on an imageable surface with developer material; including a housing containing developer material; a resonating donor member, spaced from the imageable surface, for transporting developer material to a development zone adjacent the imageable surface, the resonating donor member forming a cloud of developer material in the development zone to develop the images.

Description:
Cross-reference is made to concurrently filed patent applications, Ser. No. 09/438,208 entitled;APPARATUS AND METHOD FOR NON-INTERACTIVE ELECTROPHOTOGRAPHIC DEVELOPMENT, by Kristine A. German, et al., Ser. No. 09/438,212 entitled; APPARATUS AND METHOD FOR NON-INTERACTIVE ELECTROPHOTOGRAPHIC DEVELOPMENT, by Dale R. Mashtare, et al., and Ser. No. 09/438,599 entitled, APPARATUS AND METHOD FOR NON-INTERACTIVE ELECTROPHOTOGRAPHIC DEVELOPMENT, by Dale R. Mashtare, et al. 
     The invention relates generally to an electrophotographic printing machine and, more particularly, to the non-interactive development of electrostatic images. 
     BACKGROUND OF THE INVENTION 
     Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing the document being produced. This records an electrostatic image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic image is formed on the photoconductive member, the image is developed by bringing a developer material into effective contact therewith. Typically, the developer material comprises toner particles bearing electrostatic charges chosen to cause them to move toward and adhere to the desired portions of the electrostatic image. The resulting physical image is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration. 
     Development may be interactive or non-interactive depending on whether toner already on the image may or may not be disturbed or removed by subsequent development procedures. Sometimes the terms scavenging and non-scavenging are used interchangeably with the terms interactive and non-interactive. Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color, or cross-contaminating the color toner supplies. This invention relates to such image-on-image, non-interactive development. 
     U.S. Pat. No. 4,868,600 to Hays et al. discloses a non-interactive development system wherein toner is first developed from a two-component developer onto a metal-cored donor roll and thereafter disturbed into a powder cloud in the narrow gap between the donor roll and an electrostatic latent image existing on a photoreceptor surface. Development fields created between the donor roll core and the electrostatic latent image harvest some of the toner from the cloud onto the electrostatic image, thus developing it without physically disturbing any previously deposited toner layers. In this method the powder cloud generation is accomplished by thin, AC biased wires strung across the process direction and within the development gap. The wires ride on the toner layer and are biased relative to the donor roll core. 
     U.S. Pat. No. 4,557,992 to Haneda et al. describes a non-interactive magnetic brush development method wherein a two component developer consisting of magnetically soft carrier materials is carried into close proximity to an electrostatic image and caused to generate a powder cloud by the developer motion due, in part, by the inclusion of an AC voltage applied across the gap between the developer sleeve and the ground plane of the electrostatic image. Cloud generation directly from the surfaces of a two component developer avoids many of the problems created by wires. However, in practice such methods have been speed limited by their low toner cloud generation rate. 
     U.S. Pat. No. 5,409,791 to Kaukeinen et al. describes a non-interactive magnetic brush development method employing permanently magnetized carrier beads operating with a rotating multipole magnet within a conductive and nonmagnetic sleeve. Magnetic field lines form arches in the space above the sleeve surface creating chains of carrier beads which follow these magnetic field lines. The carrier chains are held in contact with the sleeve and spacing between the developer sleeve and a photoreceptor surface is sufficiently large to maintain the carrier bead chains out of direct contact with the photoreceptor surface. As the core rotates in one direction relative to the sleeve, the magnetic field lines beyond the sleeve surface rotate in the opposite sense, moving chains in a tumbling action, which transports developer material along the sleeve surface. The strong mechanical agitation very effectively dislodges toner particles generating a rich powder cloud, which can be developed to the adjacent photoreceptor surface under the influence of development fields between the sleeve and the electrostatic image. U.S. Pat. No. 5,409,791 is hereby incorporated by reference. 
     A problem with non-interactive development methods is achieving good solid region development while maintaining good fine line development and vice versa. Many non-interactive development methods function by generating a powder cloud in the gap between a photoreceptor and another member which serves as a development electrode. It is generally observed that this gap should be as small as possible, on the order of 0.010 inches or less. Generally, the larger the gap, the larger become certain image defects in the development of fine lines and edges. As examples of these defects: lines do not develop to the correct width, lines near solid areas are distorted, and the edges of solids are softened, especially at corners. It is understood that these defects are the result of lateral components of the electric field lines occurring due to the charge patterns existing on the imagewise discharged photoreceptor. Electrostatic field lines emanating from the photoreceptor reach up from the latent electrostatic image patterns of lines and at the edges of solid areas and arch back toward the adjacent photoreceptor regions. These lateral components of the electric field lines result in displacement from the intended pathway of the charged toner particles and in incomplete development of the latent electrostatic images. Defects due to the electrostatic field arches are less serious in interactive two component development subsystems because toner particles can be delivered through these field arches by carrier particles. Nor are they an issue in interactive single component development because a strong, cross-gap AC field is superposed which impart sufficient toner particle velocity toward the photoreceptor to overcome the aforementioned field arch patterns. 
     SUMMARY OF THE INVENTION 
     The present invention obviates the problems noted with achieving good solid region development while maintaining good fine line development, by providing an apparatus for non-interactive, development of electrostatic images. For, Charge Area Development (CAD) image voltages are set just above the residual voltage development of electrostatic images on an imageable surface with developer material; including a housing containing developer material; a resonating donor member, spaced from the imageable surface, for transporting developer material to a development zone adjacent the image receiving member, the resonating donor member forming a cloud of developer material in the development zone to develop the images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view, in section, of a four color xerographic reproduction machine incorporating the non-interactive developer of the present invention. 
     FIG. 2 is an enlarged side view of the developer unit of the present invention. 
     FIG. 3 is an enlarged view of the developer roll shown in FIG. 2 . 
     FIG. 4 is a perspective view of a cylindrical rotatable resonating assembly in accordance with the present invention; 
     FIG. 5 is a cross sectional view taken along a diameter of one embodiment of a cylindrical resonating assembly in accordance with the present invention, illustrating a radially excited uniform waveguide transducer segment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 of the drawings, there is shown a xerographic type reproduction machine  8  incorporating an embodiment of the non-interactive development system of the present invention, designated generally by the numeral  80 . Machine  8  has a suitable frame (not shown) on which the machine xerographic components are operatively supported. As will be familiar to those skilled in the art, the machines xerographic components include a recording member, shown here in the form of a translatable photoreceptor  12 . In the exemplary arrangement shown, photoreceptor  12  comprises a belt having a photoconductive surface  14 . The belt is driven by means of a motorized linkage along a path defined by rollers  16 ,  18  and  20 , and those of transfer assembly  30 , the direction of movement being counter-clockwise as viewed in FIG.  1  and indicated by the arrow marked P. Operatively disposed about the periphery of photoreceptor  12  are charge corotrons  22  for placing a uniform charge on the photoconductive surface  14  of photoreceptor  12 ; exposure stations  24  where the uniformly charged photoconductive surface  14  constrained by positioning shoes  50  is exposed in patterns representing the various color separations of the document being generated; development stations  28  where the electrostatic image created on photoconductive surface  14  is developed by toners of the appropriate color; and transfer and detack corotrons (not shown) for assisting transfer of the developed image to a suitable copy substrate material such as a copy sheet  32  brought forward in timed relation with the developed image on photoconductive surface  14  at transfer assembly  30 . In preparation for the next imaging cycle, unwanted residual toner is removed from the belt surface at a cleaning station (not shown). 
     Following transfer, the sheet  32  is carried forward to a fusing station (not shown) where the toner image is fixed by pressure or thermal fusing methods familiar to those practicing the electrophotographic art. After fusing, the copy sheet  32  is discharged to an output tray. 
     At each exposure station  24 , photoreceptor  12  is guided over a positioning shoe  50  so that the photoconductive surface  14  is constrained to coincide with the plane of optimum exposure. A laser diode raster output scanner (ROS)  56  generates a closely spaced raster of scan lines on photoconductive surface  14  as photoreceptor  12  advances at a constant velocity over shoe  50 . A ROS includes a laser source controlled by a data source, a rotating polygon mirror, and optical elements associated therewith. At each exposure station  24 , a ROS  56  exposes the charged photoconductive surface  14  point by point to generate the electrostatic image associated with the color separation to be generated. It will be understood by those familiar with the art that alternative exposure systems for generating the electrostatic images, such as print bars based on liquid crystal light valves and light emitting diodes (LEDs), and other equivalent optical arrangements could be used in place of the ROS systems such that the charged surface may be imagewise discharged to form an electrostatic image of the appropriate color separation at each exposure station. 
     A suitable controller is provided for operating the various components of machine  8  in predetermined relation with one another to produce full color images. 
     Referring now to FIGS. 2 and 3 in greater detail, developing station  26  includes a developer housing  44  defining a chamber  76  for storing a supply of developer material therein. A toner dispensing cartridge (not shown) dispenses toner particles downward into a sump area occupied by the auger. The auger loads toner onto developing member  41 . 
     Continuing with the description of operation at each developing station  26 , developing members  41  and  42  are disposed in predetermined operative relation to the photoconductive surface  14  of photoreceptor  12 , the length of developing members being equal to or slightly greater than the width of photoconductive surface  14 , with the functional axis of the developing members parallel to the photoconductive surface and oriented at a right angle with respect to the path of the photoreceptor  12 . Advancement of each developing member carries the developer blanket into the development zone in proximal relation with the photoconductive surface  14  of photoreceptor  12  to develop the electrostatic image thereon. 
     Donor member  41  comprises an interior rotatable harmonic multipole magnetic assembly  43  and an outer sleeve  45 . The sleeve can be rotated in either the “with” or “against” direction relative to the direction of motion of the photoreceptor belt  12 . Similarly, the magnetic assembly can be rotated in either the “with” or “against” direction relative to the direction of motion of the sleeve  45 . Blade  38  is placed in near contact with the rotating donor member  41  to trim the height of the developer bed. A cleaning blade (not shown) is placed in contact with the rotating donor member  41  to continuously remove developer from the donor member  41  for return to the developer chamber  76 . Donor member  41  has a DC power source  203  and an AC power source  204  electrically attached thereto. 
     In operation, donor member  41  primary function is to developed solid areas of the latent image. Donor member  41  is spaced between 0.020″ and 0.050″ from the photoreceptor. A DC voltage by supply  203  is applied to insure background regions of the latent electrostatic image are not developed. For example, in Discharge Area Development (DAD) images, the DC voltage is set to 100 to 500 volts in accordance to of the photoreceptor about 50 to 200 volts. Interactivity is reduced by using low momentum toner i.e. minimizing the applied AC voltage; and by maintaining a relatively large spacing between donor member  41  and photoreceptor  12 . For example, the development system of the present invention can be setup as follows. For donor member  41 , it is desired to have a toner bed height between 0.015″ to 0.045″, this can be accomplished by configuring the pole spacing of the magnetic assembly to give the desired bed height or trim blade  38  could be employed to give the desired bed height. The AC frequency is selected to provide maximum development below interactively which is 1 Khz to 4 Khz. 
     Donor member  42  primary function is to develop remaining fine lines and edges by reducing fringe field effects by employing a close photoreceptor to donor member spacing and a low toner bed height. Since large solid areas are developed by donor member  41  thereby neutralizing major portions of the charge areas of the latent image. This enables improved developability of the fine lines and edge details to be developed by donor member  42 . 
     Donor member  42  is a cylindrical and rotatable resonating assembly as taught in U.S. Pat. No. 5,697,035 which is hereby incorporated by reference. As shown in FIG. 4, the resonator  100  may include a transducer element  90  having a waveguide member  92  which is press fitted or otherwise bonded to the transducer  90 . The transducer  90 /waveguide  92  combination making up the resonator  100  is further mounted on a conductive shaft  89  which is further coupled to a power supply such as an A.C. voltage source  98  generally operated at a frequency between 20 kHz and 200 kHz and typically at a frequency of approximately 60 kHz for providing an electrical bias to drive transducer element  90 . The shaft  89  provides a fixed support for the cylindrical resonator and an axis of rotation for the cylindrical resonator. The transducer  90  is preferably provided in the form of a piezoelectric material which may be fabricated, for example, from lead zirconate titontate or some form of piezopolymer material. The waveguide member  92 , on the other hand, is preferably fabricated from aluminum. Each resonating element includes a waveguide in the form of a so-called uniform waveguide segment having a uniform cross sectional dimension along the width thereof, as shown in the cross-sectional view of FIG.  5 . This figure illustrates a radially excited transducer segment wherein the orientation of the dominant electrical expansion property of the piezoelectric transducer segment is in the direction of the desired transducer output as indicated by the vertical arrows  102  and  104 . In the case of the radially excited uniform waveguide resonator of FIG. 5, piezoelectric transducer element  90  generates electrical expansion which, in turn, produces piston-like motion at the contact surface  99  of the waveguide member  92 . 
     Donor member  42  is loaded with toner by donor member  41  at reload zone  300 . Donor member  42  has a DC bias applied thereto by supply  201 . The donor member  41  is held at an electrical potential difference relative to the donor member  42  to produce the field necessary for toner development onto donor member  42 . The toner layer on the donor member  42  is vibrated thereby generating a cloud of toner particles in the development zone. This cloud develops the remaining fine lines and edges of the latent image. Donor member can be positioned between 0.005″ and 0.015″ from the photoreceptor. 
     An advantageous feature of using a resonating donor member is reduce toner adhesion forces in the development zone which allows the use of low DC fields. Low DC fields which are less than 1 volts/micros compare to 3-4 volts/micros which are employed in some prior art devices which is near air breakdown which causing development noise and toner explosion in the development zone. Another feature of the resonating donor member is it generates a low localized toner cloud. 
     The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.