Abstract:
A device and method for inking a charge pattern of a carrier medium. The device includes a toner sprayer that generates and directs a mixed toner stream onto a surface of an application element. The mixed toner stream includes a number of charge toner and air particles and forms a toner layer when the mixed toner stream contacts the application element surface. The application element is movably positioned in close proximity to the carrier medium for transferring the charged toner particles onto the charge pattern of the carrier medium for inking the same.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to an apparatus and a method for inking a charge image via a toner sprayer for electrographic printer or copier or other like devices with high image carrier speed. 
     2. Description of the Prior Art 
     For developing electrostatic charge images or patterns printer devices having high image carrier speed, i.e. with an image carrier speed of 1 m per second and higher, developer systems are known that work with the assistance of two-component magnetic brush systems, with conductive single-component magnetic toner systems, with insulating non-magnetic single-component toner from a toner-air fluid, and with liquid developing systems. 
     The known two-component magnetic brush systems work with a mixture of toner particles and ferromagnetic carrier particles that triboelectrically charge one another. The soft-magnetic carrier particles form a brush in a magnetic field that is brought into direct contact with the charge image, for example on a photoconductor. The charged toner particles are deposited from the brush on the surface of the charge image-carrying element according to the distribution of the electrical field and of the toner charge. In order to achieve a high-quality image development, the toner particles must be very uniformly charged and the partially conductive brush must not discharge the photoconductor in contact. These conditions can only be met with great difficulty, particularly given a high developing speed and high toner throughput. Due to partial discharge of the photoconductor or inadequately uniform charging of all toner particles, undesired toner deposits, what is referred to as the background, therefore often occur. Due to the direct mechanical contact of the magnetic brush with the charge image-carrying element, moreover, a structuring of the toner image (what is referred to as brush marks) that has already been deposited or a fraying of image edges or a cavitation of large areas to be uniformly developed can arise. 
     Conductive one-component magnetic brush systems work without a carrier material. The toner particles themselves contain a ferromagnetic component. The toner particles form a brush structure in the magnetic field, charged toner particles being respectively located at the ends thereof. The charging of the toner particles ensues by charge transport from one toner particle to another proceeding from the carrier drum. A disadvantage of this developing method is that the ferromagnetic material component noticeably limits the color reproducibility of these toners. Moreover, the inking power (optical density) is often limited in that, due to the conductivity of the particle brush, only the respectively last particles are deposited and too weakly charged particles are picked up again. Further, the conductivity of the toner particles leads to limitations in the transfer from the photo conductor onto the ultimate image carrier onto a further intermediate carrier, particularly given electrostatic toner transfer. 
     When developing with insulating, non-magnetic one-component toner from a toner-air fluid, as known from E European Patent Document No P 04 94 454 B 1, toner is fluidized in a uniform air stream and charged in this fluid by a corona discharge, The charged toner is deposited on a conductive drum to which it adheres by electrostatic forces. The toner can leap from the drum surface onto a surface with a charge image corresponding to the topical electrical field distribution and can thus develop an image-by-image toner image. One or more additional drums can be inserted between the aforementioned drum and the surface carrying the charge image in order to separate the toner particles having the correct charge polarity and the adequate charge amount from toner particles that are less charged or are charged with the wrong polarity. Given this development process, the obtainable image quality is physically limited both in view of the uniformity of the inking distribution as well as in view of the detail sharpness. 
     Added outlays also arise for stabilization of the fluid, the stability thereof decreasing with the function duration as a consequence of the electrical interactions. 
     In liquid developing systems, fine toner particles are dispersed in a carrier fluid in which they electrostatically charge compared to the particle environment in the liquid. The developer liquid is usually brought into contact with the surface of the element carrying the charge image (for example, photoconductor). The charged toner particles migrate in the direction that is prescribed by the electrical field and the toner charge. The “damp” toner image is then transferred onto the ultimate image carrier (usually paper). The remaining liquid then escapes from the toner image (ultimate image carrier) into the ambient air. This is a considerable disadvantage when the liquid is an organic solvent like the other isopars employed as carrier liquid. Liquid toner systems are also known wherein a highly concentrated solution of highly charged particles leaps—under the influence of an electrical field—from an application drum surface onto those surface regions of the element carrying the charge image to which the electrostatic field vector points, this being formed of the field vector and the toner charge in the liquid. These systems have likewise the disadvantage that organic solvents are employed as carrier liquids. 
     U.S. Pat. No. 4,481,903 discloses an apparatus for developing a latent electrostatic on a recording medium. The apparatus contains two brushes serving as charging device for the toner, these brushes having deflectable bristles that mechanically hurl the charged toner particles against a developer drum such that a toner layer is formed on the circumferential surface thereof. The toner adhering to the developer drum is then transported into the region of a developing gap where, after overcoming the developing gap, it inks the latent charge image on the recording medium. 
     SUMMARY OF THE INVENTION 
     An object of the invention is therefore to offer an apparatus and a method for low-disturbance development of a charge image given high image carrier speed in a printer or copier or other like device. 
     In the inventive developing apparatus, a uniform, homogeneous toner layer is first generated by powder coating on an application element, such as a drum. Subsequently, this toner layer is transferred in charge-image dependent fashion onto a carrier medium conducted past in close proximity. In particular, the individual toner particles leap over the developing gap and thus ink the charge image located on the carrier medium. 
     A low-disturbance inking of the charge image is possible on the basis of this principle even given a high image carrier speed (speed of the carrier medium of 1 m/sec. or higher). Both one-component toner as well as multi-component toner can be utilized as toner. The employment of one-component toner has the advantage that the toner can be chromatically mixed in the developer device given employment of the inventive developer device in color printer devices. 
     Toner sprayer device as known from powder coating systems can be employed in the invention. Such powder coating guns are disclosed, for example, by U.S. Pat. Nos. 5,482,214 and 4,802,625 that are incorporated herein by reference. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view of a developer with a toner sprayer device. 
     FIG. 2 is a schematic sectional view of the mouthpiece region of the toner sprayer device with charging device. 
     FIG. 3 is a schematic sectional view of the developer with appertaining deflection device. 
     FIG. 4 is a schematic illustration of a toner sprayer device with a broad spray region. 
     FIG. 5 is a schematic illustration of a toner sprayer device that is movable along an application element. 
     FIG. 6 is a schematic illustration of a toner sprayer device with a plurality of spray nozzles arranged next to one another. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A high-performance electrophotographic printing system (not shown in detail here) having a printing output of more than 200 pages/min. includes one or more of the developer devices shown in FIG.  1 . These developer devices serve the purpose of inking a latent charge image applied on a carrier medium  10  with the assistance of a character generator with toner. The carrier medium  10  can thereby be a photoconductor drum, i.e. a metallic member with a photoconductive surface of, for example, As 2 Se 3  or—as shown here—can be an OPC photoconductor band. Given the reversing developing method employed here, the photoconductor or, respectively, the carrier medium  10  is first charged to −500 V and is then discharged to −50 V character-dependently with the assistance of the character generator. This means that the carrier medium  10  is at approximately −500 V in the region of the carrier medium at which no toner image should arise and is at −50 V in the region of the toner image. 
     However, it is also possible to first charge the carrier medium to +500 V and to then discharge it character-dependently to +50 V. 
     The toner image is inked with the assistance of the illustrated developer device upon employment of one-component toner or of two-component toner as well that comprises toner particles having a size of approximately 10 μm on an arbitrary resin bases such as, for example, polystyrol or polyester and also comprises electromagnetic carrier particles in the case of two-component toner. The toner particles thereby deposit charge image-dependently on the photoconductor. After the developing process, they are transferred onto paper in the usual way and are subsequently fixed, for example in a thermal pressure fixing mechanism or other like mechanism. 
     The developer device contains a toner sprayer device  11  that can be constructed analogous to a powder coating mechanism. Such powder coating mechanism are disclosed, for example, by U.S. Pat. No. 5,482,214 and 802,625. The toner sprayer generally composed of a delivery tube  12  with a nozzle  13  shown in FIG. 2 that comprises includes an electrode  14  in the form of a corona discharge mechanism in its mouthpiece region. A mixed stream is generated in the toner sprayer device  11  from a toner-air mixture that includes toner particles having a defined toner charge. To this end, toner is dispersed in air in the pump system of the toner sprayer device as in a known powder coating mechanism, and this mixed stream is supplied via the delivery tube  12  to the nozzle  13 , and a directed spray jet is thus produced. This usually occurs in that toner fluidized via an acceleration mechanism with a Venturi nozzle is sucked up from a fluid bed, is uniformly distributed in a transport air stream and accelerated to high speed. The electrode  14  is charged with a voltage of +5 or −5 KV or more and sprays charges onto the toner particles, which then exhibit a toner charge of, preferably, ±10 μC/g through ±30 μC/g. The electrode acting as corona discharge mechanism can be arranged in the mixed stream or in the immediate proximity of the mixed stream  15 . It charges the toner particles in defined fashion. 
     It has been shown that a negative charging promotes the attainment of a high charge level. The toner application efficiency TAW thereby becomes more efficient. The toner application efficiency is defined as the ratio of the mass stream (toner) deposited on the acceptance surface (application element  16 ) to the mass stream (toner) transported in the air stream. 
     Instead of charging with the assistance of an electrode ( 14 ) (corona charging), it is also possible to triboelectrically charge the toner particles in a known way by contact interaction of the toner particles with an interaction surface. 
     An application element  16  is arranged in the region of the mixed stream  15 . In the illustrated case, this is composed of a metal drum with a partially conductive surface of, for example, amorphous carbon, so that the spacing of the toner charge and the mirror charge thereof is great enough in order to enable adhesion of the toner to the application element  16  and small enough so that the required stripping field does not become all too great, since the charging of the photoconductor is limited. It is also possible to employ an endless band instead of a drum-shaped application element  16 . 
     The application element  16  is moved in arrow direction by a motor. A corona device  17  is arranged at the application element, this serving the purpose of charging a toner layer applied on the application element  16  with the assistance of the toner sprayer device  11  with an ion current and thus homogenizing the charge in the toner layer. The surface of the application element  16  is located in tight proximity to the carrier medium  10 , namely at a spacing that can be less than 100 μm, whereby the gap defines the actual developer region or, transfer region  18  in 
     In order to assure this spacing, the carrier medium  10 —the photoconductor band in this case—is guided in this region by a spacer drum  19 . A stripper element  20  of elastic material is located following the transfer region  18  in moving direction, this serving the purpose of stripping residual toner from the application element and supplying it to a toner reservoir via a conveyor device  21 . A further corona device  23  precedes the stripper element  20 . 
     The function of the developer device is now explained in greater detail on the basis of FIG.  1 . 
     First, a mixed stream  15  in the form of a spray jet of charged toner particles in a transport air stream is produced with the assistance of the toner sprayer device  11 . In the illustrated reverse developing method that is shown, the application element  16  lies at an application potential of −450 V. Due to these voltage relationships in conjunction with the toner charge, the toner particles agglomerate to the surface of the application element  16  in an acceptance region  22 . The agglomeration of the toner particles is thereby promoted by the kinetics (impulse) thereof. The kinetics are in turn dependent on the velocity of the transport air stream that carries the toner particles. The impulse of the toner particles raises the toner application efficiency TAW. They form a uniform, homogeneous layer that has a layer thickness of approximately 1-3 toner layers or more. This layer is charged with an ion current with the assistance of the corona charging mechanism  17  in order to thereby homogenize the charge in the layer. As a result of continued movement of the application element  16 , the acceptance region  22  with the toner layer proceeds into the transfer region  18  with the developer gap, where the toner particles leap charge image-dependently from the application element  16  across the developer gap having a width of approximately 100 μm or less onto the charge image of the carrier medium  10  and ink the latter. In order to facilitate this leap, an auxiliary transfer voltage of, preferably, 200-500 V can be adjacent in the transfer region between the carrier medium  10  and the application element  16 . It is activated during the entire developing duration. The toner particles collect on the charge image of the carrier medium  10  when a suitable toner charge is present, when the charge of the carrier medium  10  is correctly selected and what is referred to as the “jump potential” and the mechanical distance in the developer gap  18  are correctly selected. Given the illustrated exemplary embodiment with reverse developing, the toner charge amounts to between 10 and 30 μC/g with a −450 V potential of the application element. The auxiliary transfer voltage preferably amounts to 200-500 V. On the carrier medium  10 , the toner-free areas lie at −500 V, the latent charge image at about −50 V. The field strength attacking the toner particles in the developer region amounts to about 8×10 −8  N. The toner particles thus deposit charge-dependently in, preferably,  1 - 2  toner layers in the discharged photoconductor region (−50 V). 
     The voltage and charge conditions described given the illustrated exemplary embodiment apply to reverse development, whereby a charged carrier medium is character-dependently discharged with the assistance of the character generator. In the positive developing principle, wherein a discharged carrier medium  10  is charged character-dependent, analogous, inverse voltage conditions apply that, however, are dependent on material. 
     Since, when developing the charge images on the carrier medium, not all toner particles jump onto the carrier medium, these residual toner particles must be removed from the application element  16  during the further course. To this end, they are first exposed to a further corona charging mechanism  23  that loosens the adhesion of the toner particles on the surface of the application element  16 . Subsequently, they are stripped off with the assistance of the stripper element  20  and are resupplied to the toner reservoir via the toner conveying device  21  or are cleaned via a recycling system and then delivered to the toner reservoir. The application element  16  without residual toner is then again sprayed with toner in the acceptance region  22 . This process cycles continuously. polarity impinge the application element  16 , a deflection element that can be added according to the illustration of FIG.  3 . This is essentially composed of an activatable electrical deflection field  26  between a collecting region  24  and a deflection electrode  25 , whereby the pole direction of the field  26  is symbolically shown. The toner sprayer device is thereby directed such that, given absence of the auxiliary electrical field  26 , the mid-point of the jet leads past the application element  16  at a distance therefrom and impinges in the collecting region  24 . 
     When the deflection field  26  is cut in, the toner particles having the correct charge polarity (negative particles) and an adequately high charge amount (10-30 μC.g) are deflected in a curved path  27  onto the application element  16 . Particles having inadequate charge amount and a different polarity fly past the application element ( 16 ) and impinge the collecting region  24  in the region of the jets  28 . This collecting region  24  can be composed of a corresponding plate on which they collect and are then resupplied to the toner reservoir via the conveyor means  21 . 
     In order to produce a uniform toner layer over the entire acceptance region  22  of the application element  16 , the nozzle  13  can be fashioned as a flat nozzle according to the illustration of FIG. 4 with a nozzle tip as disclosed, for example, by U.S. Pat. No. 5,482,214. According to the illustration of FIG. 5, it is also possible to move the toner sprayer device  11 , i.e. the nozzle  13 , along the application element  16  during application with the assistance of a spindle-shaped drive mechanism  29 . It is also possible according to the illustration of FIG. 6 to arrange a plurality of toner sprayer devices ( 11 ) with a plurality of nozzles  13  whose spray area cover the entire acceptance region  22  of the application element  16   
     A uniform toner application onto the application element  16  or the moving acceptance area thereof can also be achieved by a dense atomization of the toner in the environment of the acceptance region  22  of the application element  16 , whereby high toner charges of approximately 10-30 μC/g are required for a fast process management, i.e. with working speeds of the carrier medium  10  of approximately 1 m/sec. and higher. These ton high a precipitation of the toner on the acceptance region  22  of the application element  16  in a very short time, i.e. in a time of less than 0.1 sec. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.