Wireless, scavengeless hybrid development

Abstract: A scavengeless development system in which toner (44) is detached from a donor belt (42) and attracted to latent electrostatic images carried by an image receiver (10) positioned adjacent the belt (42). Generation of a controlled powder cloud of toner particles (44) is effected using AC electric fields created by an AC voltage source (104) and an embedded electrode structure (102) stationarily positioned behind the donor belt (42). Unused toner is removed from the donor belt (42) in a manner - using a conductive cleaning brush (106) connected to a DC bias (108) - which avoids history effects due to accumulation of highly charged, relatively small toner particles (44) on the donor belt.

Description: This invention relates to non-interactive or scavengeless development system for developing latent electrostatic images with toner and to an associated developing method. The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques. The present invention is especially suited for use in highlight color printing systems. One form, tri-level imaging, of highlight color imaging described in US-A-4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. In one embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. The viability of printing system concepts such as tri-level, highlight color xerography requires development systems that do not scavenge or interact with an image already rendered visible with toner. Since commercial development systems such as magnetic brush development and jumping single component development interact with the image receiver, a previously toned image will be scavenged by subsequent development. Great care is required to optimize the development materials and process conditions for minimum interaction. Since the present commercial development systems are highly interactive with the image bearing member, there is a need for scavengeless or non-interactive development systems. US-A-5,053,824 describes a scavengeless development apparatus in which a donor belt is employed to convey toner from a supply source to a development zone. Wires contacting a toner layer on a donor roll is proven method of scavengeless xerographic development. The wires are problematic, in that, they are difficult to mount in a consistent reproducible manner and they are prone to contamination from agglomerates or debris. Such contamination results in banding and streaks on the output copies. Stable toners layers on a donor roll are also difficult to achieve. To avoid the history effects due to the accumulation of highly charged, small toner particles on the donor, unused toner should be continuously removed followed by freshly deposited toner. The toner can be scraped from the donor with a blade but this forces the effective toner throughput to levels beyond the capacity of small sump development systems. Stressed development manifests itself by degradation of the toner charge distribution and the appearance of background density. Brush cleaners also can not handle high throughput and keeping the brush clean is difficult and complicated. The present invention provides a developer apparatus according to claim 1 of the appended claims Preferably, a brush development electrode positioned with the loop of the donor belt serves as a brush development electrode while a brush cleaner electrode, also disposed within the loop delineated by the belt, serves to remove unused toner from the belt in order to avoid history effects. The invention further provides a method of developing electrostatic latent images according to claim 7 of the appended claims. Preferably, the step of electrically biasing said electrode structure comprises means for applying an AC voltage thereto. Preferably, the steps of electrically biasing said development and cleaning brushes comprises using DC voltage sources having opposite polarities. The invention further provides a highlight colour printer incorporating a developer according to any of claims 1 to 6. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary schematic illustration of a developer apparatus according to the present invention; and Figure 2 is schematic illustration of a printing apparatus incorporating the developer apparatus of Fig. 1. As shown in Figure 2, a printing machine incorporating the invention may utilize a charge retentive member in the form of a photoconductive belt 10 consisting of a photoconductive surface and an electrically conductive substrate and mounted for movement past a charging station A, an exposure station B, developer station C, transfer station D and cleaning station F. Belt 10 moves in the direction of arrow 16 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. At development station C, a development system, indicated generally by the reference numeral 30 advances developer materials into contact with the electrostatic latent images. The development system 30 comprises first and second developer apparatuses 32 and 34. The developer apparatus 32 comprises a housing containing a pair of magnetic brush rollers 36 and 38. The rollers advance developer material 40 into contact with the latent images on the charge retentive surface which are at the voltage level V C . The developer material 40 by way of example contains red toner. Appropriate electrical biasing is accomplished via power supply 41 electrically connected to developer apparatus 32. A DC bias of approximately 400 volts is applied to the rollers 36 and 37 via the power supply 41. The developer apparatus 34 (Figure 2) comprises a donor structure in the form of a belt 42. The donor structure 42 conveys charged toner particles 44 deposited thereon to a development zone 46 where the toner particles are formed into a toner cloud for selective deposition on images contained on the the charge retentive surface. The developer in this case comprises black toner. Further details of the developer apparatus 34 will be provided hereinbelow. As illustrated in Figure 1, the donor belt 42 is supported for movement through the development zone 46 (i.e. space between the charge retentive surface and the donor belt) by a drive roll 80, idler roll 82 and a development shoe 84. The drive roller 80 is driven via a drive motor 86 operatively connected thereto in a well know manner. The spacing or gap between the charge retentive surface and the donor belt is in the order of 127 to 508 µm (5-20 mils). The belt 42 is a seamless construction and preferably has a thickness of 25.4 to 76.2 µm (1 to 3 mils) and is fabricated from carbon loaded Tedlar or Kapton (Trademarks of E.l. duPont de Nemours & Co.) having a resistivity of the order of 10 7 ohm-cm. Charged toner particles are conveyed to a loading zone 87 using a magnetic brush device 88 which comprises a stationary magnet assembly 90 and a rotatable sleeve 92. The stationary magnet assembly comprises a plurality of alternately polarized pole pieces 94. A two component developer comprising carrier beads 93 and toner particles 44 is contained in a supply sump 96 from which it is conveyed by the sleeve 92 to the loading zone 87. A conductive brush development electrode 100 positioned behind the donor belt 42 serves to effect deposition of charged toner particles onto the donor belt. To this end, when negatively charged toner is utilized, a positive DC voltage in the order of 200 to 400 volts provided by a DC voltage source 101 is applied to the conductive brush 100. The toner particles are conveyed by the belt to the development zone 46 where the toner particles are formed into a toner cloud for effecting scavengeless development of latent electrostatic images on the charge retentive surface. An AC voltage in the order of 600-800 volts peak depending upon the belt thickness is applied to a pair of electrodes 102 electrodes by an AC voltage source 104. The electrodes are embedded in the development shoe 84 which is fabricated from a dielectric material or otherwise structured to electrically isolate the electrodes 104. A conductive brush cleaning electrode 106 also disposed inside the loop formed by the donor belt 42 serves to repel unused toner particles from the donor belt back onto the sleeve 92. The conductive cleaning brush 106 is electrically biased using a DC voltage source 108 to a negative voltage in the order of -200 to -400 volts. Such removal of toner from the donor belt is followed by a fresh deposit of toner by the development electrode 100. Thus, a stable toner layer on a donor roll is provided which avoids the history effects due to the accumulation of highly charged, small toner particles on the donor. While the developer apparatus 32 has been disclosed as a magnetic brush system, developer apparatus 34 could be used in its place. Also, while the development of discharged area images was discussed as being effected prior to charged area development the sequence of image development can be reversed.

Claims: Apparatus for developing latent electrostatic images on a charge retentive surface (10) with toner, said apparatus comprising: a supply (96) of toner (44); a donor belt structure (42) spaced from said charge retentive surface (10) for conveying toner (44) from said supply (96) of toner to a development zone (46) intermediate said charge retentive surface (10) and said donor belt structure (42); means (80,82,84) for supporting said belt (42) for movement in an endless path whereby said belt delineates an endless loop; said supporting means (80,82,84) including a development shoe (84), said development shoe (84) being supported within said loop and in contact with said donor belt (42) adjacent said development zone (46); an electrode structure (102); means (100) for loading toner particles (44) onto said donor belt structure (42) in a loading zone (87); and electrical bias means (104) for applying a voltage to said electrode structure (102) for forming a cloud of toner particles (44) in said development zone (46), characterized in that said electrode structure (102) is embedded in said development shoe (84). Apparatus according to claim 1 wherein said means for loading said toner particles (44) onto said donor belt structure (42) in a loading zone (87) comprises a conductive development brush (100) supported in contact with said belt (42) within said loop and means (101) for electrically biasing said conductive development brush (100) for attracting charged toner particles (44) to said belt (42). Apparatus according to claim 1 or claim 2 including means (106,108) for continuously removing unused toner particles (44) from said donor belt (42) prior to loading fresh toner onto said donor belt (42). Apparatus according to claim 3 wherein said means for continuously removing unused toner particles from said donor belt (42) prior to loading fresh toner onto said belt (42) comprises a conductive cleaning brush (106) contacting said belt (42) within said loop and means (108) for electrically biasing said cleaning brush (106) for repelling toner particles (44) from said donor belt (42). Apparatus according to any of claims 1 to 4 wherein said electrical bias means (104) for applying a voltage to said electrode structure (102) comprises an AC voltage source. Apparatus according to claim 4 or claim 5 when dependent on claim 4, as far as claim 4 is referring to claim 2 wherein said electrical biases (101,108) for electrically biasing said development (100) and cleaning (106) brushes comprise DC voltage sources having opposite polarities. A method for developing latent electrostatic images on a charge retentive surface (10) with toner, said method comprising the steps of: providing a supply (96) of toner (44); using a donor belt structure (42) positioned adjacent said charge retentive surface (10), conveying toner (44) from said supply (96) of toner to a development zone (46) intermediate said charge retentive surface (10) and said donor belt structure (42); supporting said donor belt (42) for movement in an endless path whereby said belt delineates an endless loop; providing an electrode structure; contacting said donor belt (42) within said loop with a development shoe (84); loading toner particles (44) onto said donor belt structure (42) in a loading zone (87); and electrically biasing said electrode structure (102) for forming a cloud of toner particles (44) in said development zone (46) characterized in that said electrode structure (102) is embedded in said development shoe (84). The method according to claim 7 wherein said step of loading comprises electrically biasing a conductive development brush supported in contact with said belt within said loop for thereby attracting charged toner particles to said belt. The method according to claim 7 or 8 including continuously removing unused toner particles from said donor belt prior to loading fresh toner onto said belt. The method according to claim 9 wherein said step of continuously removing unused toner particles from said donor belt prior to loading fresh toner onto said belt comprises electrically biasing a conductive cleaning brush contacting said belt within said loop.

Inventors: SCHMIDLIN FRED W, SCHMIDLIN, FRED W.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: G03G15
IPC codes: G03G15, G03G21