Abstract:
A printing apparatus having an imaging member having an external surface and a series of substantially equal sized cells located over such surface. The printing apparatus includes an ink jet device for selectively ejecting droplets of ink, having marking particles and liquid, into the cells of said imaging member in a desired image-wise ink pattern, a mechanism for filling the cells of said imaging member with a dielectric fluid after said ink jet device ejects droplets of ink into said cells, and transfer mechanism including a device for fractionating the marking particles in the ink from the liquid and transferring the image-wise marking particle pattern to a receiver member.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to printing of ink jet images, and more particularly to printing of ink jet images where a pigmented ink is jetted onto a primary image member to create a gray-scale image and liquid is separated from pigment particles and removed prior to transfer to a receiver member. 
   BACKGROUND OF THE INVENTION 
   High-resolution digital input imaging processes are desirable for superior quality printing applications, especially high quality color printing applications. As is well known, such processes may include electrostatographic processes using small-particle dry toners, e.g., having particle diameters less than about 7 micrometers, electrostatographic processes using solvent based liquid developers in which the particle size is typically on the order of 0.1 micrometer or less, and ink-jet processes using aqueous or solvent based inks. 
   The most widely used high-resolution digital commercial electrostatographic processes involve electrophotography. Although capable of high process speeds and excellent print quality, electrophotographic processes using dry or liquid toners are inherently complicated, and require expensive, large, complex equipment. Moreover, due to their complex nature, electrophotographic processes and machines tend to require significant maintenance. 
   Ink jet technology may be used to deposit fluid materials on substrates and has numerous applications, mainly in printing. Ink jet printers function by depositing small droplets of fluid at desired positions on a substrate. There are various ink jet printing technologies. 
   Typical ink jet inks suffer from several problems that limit their usefulness in commercial printing. Inks are generally dye based and subject to fading. The inks have to have low viscosity to minimize plugging of the ink jet nozzles. As a result, the ink must be jetted onto specially formulated absorbent papers. The absorbency of the drops results in low image density and image spread. Clay coated graphic arts papers commonly used in the printing industry to print high quality images cannot be used in ink jet engines because the ink would run on the nonabsorbent clay coated paper. Rather, for high quality imaging, expensive and limited types of papers are used with ink jet printing. 
   Another limitation to using ink jet technology for commercial printing applications is that all ink has to be dried. This requires a lot of energy to evaporate the water or organic solvents used in the ink and generates a lot of moisture and/or solvent vapors. 
   To overcome problems associated with fading and low image densities associated with dyed aqueous-based inks, pigmented aqueous-based inks have been disclosed in which a pigmented material is colloidally dispersed. Typically, a relatively high concentration of pigmented material is required to produce the desired highest image densities (D max ). Exemplary art pertaining to pigmented aqueous-based inks includes U.S. Pat. No. 6,143,807 (Lin et al.) and U.S. Pat. No. 6,153,000 (Erdtmann et al.). Generally, pigmented inks have a much greater propensity to clog or alter the jet(s) orifice(s) of a drop-on-demand type of ink jet head than do dyed inks, especially for the narrow diameter jets required for high resolution drop-on-demand ink jet imaging, e.g., at 600 dots per inch. Drop-on-demand printers do not have a continuous high pressure in the nozzle, and modification of the nozzle behavior by deposition of pigment particles is strongly dependent on local conditions in the nozzle. In continuous ink jet printers using pigmented inks, the relatively high concentrations of pigment typically affects the droplet break-up, which tends to result in non-uniform printing. 
   In co-pending U.S. patent application Ser. Nos. 11/445,712; 11/446,467; 11/445,713; 11/445,714; 11/445,566, an apparatus, a method, and an ink that can be used to produce high quality at production rates suitable for use in a graphic arts establishment are disclosed. This can be described in brief. Ink having electrically charged marking particles in a dielectric solvent is jetted onto an electrically conducting primary imaging member. The inked image is then passed by a fractionator that subjects the ink to an electric field that drives the marking particles towards the primary imaging member and skives the supernatant solvent off the image, thereby leaving a concentrated ink in the areas corresponding to the image. The charged particles are then pressed into a nip formed by the convergence of the primary imaging member and a receiver. The receiver could be a final image receiver such as paper that is pressed against a roller located on the non-image receiving side of the paper, etc. Alternatively, the receiver could be a transfer intermediate which would receive the image at this point and subsequently transfer it from the intermediate to a final receiver. The image is then transferred from the primary imaging member to the receiver by applying an electrostatic field of such magnitude and polarity as to drive the marking particles towards the receiver. 
   By transferring concentrated ink, the ink neither runs nor soaks into the paper to any significant extent. This allows one to achieve a high-density image, minimize image spread, and also allows the use of clay-coated papers. Moreover, the amount of solvent that has to then be removed has been greatly reduced, thereby reducing energy consumption and minimizing vapor emissions. 
   There is prior art that addresses the problem of having excessive fluid in the image by forming the image on an intermediate, then transferring the image to a receiver. U.S. Pat. No. 5,099,256 discloses the use of cylinder specifically coated with a silicone polymeric material in combination with a drop on demand print head. U.S. Pat. No. 6,736,500 discloses the use of a coagulating agent that increases the viscosity of the ink jet ink and improve transfer and image durability. U.S. Pat. Nos. 6,755,519 and 6,409,331 teach methods for increasing ink viscosity such as via UV cross-linking or evaporation. None of these patents address the formation of a multi-color image. 
   U.S. Pat. Nos. 6,761,446; 6,767,092; 6,719,423; and 6,761,446 refer to forming images on separate intermediates, then transferring the images in register to form a four-color image on a receiver. While these patents address the problem of excess fluid in a four-color image, the process of registration of the component images from separate intermediates involve complex and expensive mechanisms. The situation is further complicated if receivers of different thickness and/or surface properties need to be used. In addition, the receiver path to accommodate successive transfers to form the multi-color image is relatively long, affecting cost and reliability. 
   A major difficulty encountered with the ink jet developers that use organic solvents is that they tend to spread. This would result in image degradation problems such as a lack of sharpness and resolution. To mitigate this problem, the use of a cellular primary imaging member has been proposed in co-pending U.S. patent application Ser. No. 11/446,467. 
   Cellular structures are commonly used in printing. For example, in gravure printing, a gravure roller or plate is used. The roller or plate includes a cellular pattern that corresponds to the image to be printed, with the size of each cell corresponding to the amount of ink required to produce a certain density. The gravure roller or plate is produced for each specific image to be printed. It should also be noted that only approximately half of the ink transfers to a receiving member in gravure printing, with the rest remaining in the cells. The residual ink in the cells does not create a problem for traditional gravure printing because identical images are produced in register with the gravure roller or plate. This would, however, create problems for digital printing, where the images may not be produced in register and may, in fact, differ from each other. 
   Another type of cellular member commonly used in the printing industry is the anilox roller. This device has a periodic array of uniformly sized cells that are used to apply ink in a non-image-wise fashion to an inking member. 
   A cellular structure can be used as a primary imaging member to minimize the spread of ink droplets jetted from an ink jet nozzle. However, unlike a conventional gravure roller or plate that is job specific, it is highly preferable to use a universal primary imaging roller or plate that can be used for many different images. Such primary imaging roller or plate is capable of having all marking material removed with the printing of each image so that the primary imaging member can be effectively used in digital printing applications without undue stress applied to a cleaning subsystem that would have to remove all residual ink. 
   To use a cellular or textured imaging member, hereafter referred to as a TIM as a primary imaging member in an ink jet printing press and to minimize the spread of the ink droplets, the ink droplets must be injected into each cell to form a positive image, with the gray scale controlled by varying the amount of ink injected into a cell. The varying amounts of ink in the uniformly sized cells creates a problem transferring the ink because the receiver does not contact the ink. 
   SUMMARY OF THE INVENTION 
   In view of the above, this invention is directed to using a TIM in a digital printing press wherein jetting of ink onto the TIM in a manner for both good transfer and the ability to have a gray scale. In such a printing press, liquid is separated from pigment particles prior to transfer to a receiver member. The printing apparatus includes a textured printing or imaging member (TIM) including a series of substantially equal sized cells located over the surface of the printing member. An ink jet device selectively ejects droplets of ink having marking particles and a dielectric liquid into the cells of the printing member in a desired image-wise ink pattern. The cells are then filled with a dielectric liquid, preferably a clear or colorless dielectric fluid. The marking particles are then transferred to a receiving member by forming a nip between the receiving member and the TIM and applying an electrostatic field to urge the particles to the receiving member, leaving behind a substantial portion of the dielectric fluid. The fluid is thereafter removed from the cells in any suitable known manner and can be filtered or otherwise purified and recycled. 
   The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
       FIGS. 1   a  and  1   b  are views of a portion of a textured imaging member (TIM) and details of the cells thereof, for use in the printing apparatus according to this invention, on a significantly enlarged scale; 
       FIG. 2  is a schematic illustration of a preferred embodiment of the printing apparatus according to this invention; 
       FIG. 3  is a side view, in cross-section of a portion of the anilox roller and intermediate member of the printing apparatus according to this invention; 
       FIG. 4  is a side view, in cross-section of a portion of an alternate embodiment of the anilox roller of the printing apparatus according to this invention; and 
       FIGS. 5   a - 5   d  are respective views, in cross-section, showing the sequential operation of the printing apparatus according to this invention as seen in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to this invention, an ink jet mechanism is utilized to write an image, using ink having marking particles, on to the surface of a textured imaging member (TIM), such as a drum, a plate, or alternatively an endless belt. The texture of the TIM is specified so that the ink jet drops are contained in very small wells (cells) that are deep enough to fully contain any ink that is directed toward it. Referring to  FIGS. 1   a  and  1   b , a preferred exemplary structure for a TIM is shown (designated by numeral  12 ) where cells  14  are hexagonally shaped and closely packed. Of course, other shapes for the cells  14 , such as diamond, rectangular, or oval for example, are suitable for use with this invention. The structural relation of the TIM cells  14  prevents ink deposited in the cells from coalescing, which blurs the image, by preventing the ink in the cells from migrating beyond the cell walls. The cells  14  can also correct satellites and jet errors by collecting ink drops within the cell walls. 
   The ink is not a conventional ink jet ink. Rather, it has electrically charged particles in a dielectric solvent, as described in co-pending U.S. patent application Ser. No. 11/445,713. The particles generally include a colorant such as a dye or pigment. Although electrically charged pigment particles can be used, it is preferable to use colorant particles having a colorant and a polymer binder such as polyester, polystyrene, etc. For certain applications, such as protecting the image or imparting a uniform gloss, for example, it is not necessary that the so-called marking particles include a colorant. Further, the particles should not swell, dissolve, or soften in the dielectric solvent. Suitable solvents for use in this invention include various hydrocarbons such as Isopar L and Isopar G, sold by Exxon, various high carbon alcohols, and various mineral oils, soy oil, for example. Low carbon alcohols such as methanol, ethanol, and isopropanol, for example, are too electrically conducting and would not be suitable for this invention. Other dielectric solvents such as dichoromethane, diiodomethane, hexane, heptane, or acetone, for example, would tend to dissolve the polymer and, therefore, would not be suitable for this invention. In addition, the flammability of certain organic solvents would make their use less desirable. Because of its electrical conductivity, water also would not be a suitable solvent. 
   The TIM  12  is an electrically conducting member. The TIM  12  includes cells  14  of uniform size and spacing. The spacing of the cells  14  is determined by the line frequency and image characteristics desired in the final print. The cells  14  must be of sufficient size to hold the entire volume of ink needed to achieve maximum density. The structure of the TIM  12  could be similar to an anilox roller or a gravure roller or plate in which the cells of the gravure roller or plate cover the entire printing area of the plate and are uniform in size. The electrically conducting layer of the TIM  12  can be over-coated with a thin non-conducting layer of material such a ceramic or a low surface energy material such as Teflon provided that the layer is sufficiently thin that a sufficient electric field can be established that will allow the marking particles to be ultimately transferred from the TIM  12  to a receiver, using the process that will be described forthwith. 
   In one embodiment of the invention, the ink  60  described above is jetted into the cells  14  of a TIM  12  with the quantity of the ink jetted into each cell  14  determined by the desired image density on the final print in a manner corresponding to a positive image (see  FIG. 5   a ). The cells  14  are then filled with a dielectric fluid  62 , preferably the same fluid as that of the ink solvent. The fluid can be jetted into specific cells that are already partially filled with ink, or it can be uniformly applied to the surface of the TIM  12  and then skived from the surface, leaving each cell totally filled with a combination of ink and fluid, as shown in  FIG. 5   b . It should be noted that the ink droplets could be jetted in the vicinity of the cells  14  as long as the TIM  12  has a low surface energy coating such as Teflon so that the droplets will spontaneously flow into the cells  14 . The meaning of the term “jetting” as used herein, includes both the instance wherein the droplets are directly jetted into the cells  14  and the instance wherein the droplets are jetted adjacent to the cells  14  and spontaneously flow into the cells  14 . 
   The marking particles in the ink  60  are transferred to a receiver  50  by applying an electrostatic field that urges the particles towards the receiver (see  FIG. 5   c ). For example, the marking particles can be transferred directly to a final receiver, such as paper, by pressing the receiver against the TIM  12  and applying an electrostatic field using known methods such as a biased roller behind the receiver, charging the receiver with a corona or brush, or using other known processes similar to those used in electrophotographic engines. However, to avoid contamination, such as the introduction of paper fibers, for example, upon completion of transfer the cells  14  are cleaned by any suitable cleaning device (not shown) such that the cells  14  are completely empty (see  FIG. 5   d ) the TIM  12  by a final receiver, it is preferable to transfer the image to an intermediate member, as illustrated in  FIG. 3 . 
   A preferred embodiment of a printing apparatus  10 , according to this invention, is shown in  FIG. 2 . Referring to  FIG. 2 , the TIM  12  is shown as an anilox roller (with hexagonally shaped, closely packed cells  14 ). For the reasons set forth below, the anilox roller must have an electrode. As shown in  FIG. 3 , the anilox roller  12  may be a steel roller  12 ′ (alternatively may be chrome coated), thus making electrical contact straightforward. That is, the anilox roller  12  is grounded and an intermediate member  20 , further described below, in contact therewith, has an applied electrical bias connected thereto, such as voltage V. Alternatively, the anilox roller, designated by the numeral  80  in  FIG. 4 , may have a structure where a ceramic layer  82  is formed on top of steel (conducting) substrate  84 . The ceramic layer  82  is etched (for example, with a high powered laser) to form the cells  86 . In this case, the ceramic layer  82  is selected to be relatively thin, i.e., about twice the depth of the etched cell. The steel substrate  84  would then serve as the electrical contact. 
   Four basic, substantially identical imaging units designated as  16   a - 16   d  are shown in  FIG. 2 . More or less imaging units may be used if it is desired to create monochrome prints, two or three color spot color prints, or process color prints with four or more color separation images, with or without additional spot color separations. Each of the imaging units  16   a - 16   d  includes an ink jet device  18  that selectively jets ink in an image-wise fashion on to the TIM (anilox roller)  12 , thereby creating a positive image in the cells  14  on the surface of the respective TIM  12 . The image is thereafter fractionated and transferred to an intermediate member  20 , which is preferably compliant, by an electrical field. Although any suitable intermediate member known in the electrophotographic art is suitable for use in this invention, a preferred intermediate member  20  has a volume resistivity between 1×10 8  and 1×10 11  ohm-cm and is preferably compliant. The intermediate member  20  could be a roller or a web. If it is a roller, then the support layer is preferably a conducting cylinder (aluminum, steel core) and the thickness of the compliant layer is preferably greater than 1 mm and less than 15 mm. If the intermediate member is a web, then the support material is preferably a plastic material such as polyimide, polyester, or polycarbonate, and the thickness of the compliant layer is preferably between 100 and 1,000 microns. The substrate preferably has a volume resistivity between 1×10 8  and 1×10 11  ohm-cm. As shown in  FIGS. 2 and 3 , the electrical bias (voltage V) is applied by conducting rollers  21  urging the intermediate web  20  into contact with the TIM rollers  12 . Alternatively, to having bulk resistive properties, the plastic substrate of the intermediate member  20  could be insulated with a thin conductive (&lt;1×10 6  ohm-cm) coated between it and the compliant resistive blanket material. The thin conducting layer acts as the electrical contact. In this case, the TIM rollers  21  apply only pressure and the electrical bias connection to the thin conducting layer is not shown. 
   The transfer process effectively fractionates the developer, transferring the marking particles to the receiver while leaving the bulk of the liquid behind on the TIM  12 . A conditioning unit  22  cleans the cells  14  of the TIM  12  after transfer in order to ready it for receiving the next image. Each imaging unit  16   a - 16   d  creates one color separation image, which is combined in register on the intermediate member  20  to form a desired multi-color image. An optional liquid removal unit  24  is shown that acts to remove excess liquid from the imaged intermediate member  20 . The liquid depleted image carried by the intermediate member  20  is then transferred to a receiver member R (paper or other media) in a transfer zone  26 , and the intermediate member  20  is cleaned by a cleaning unit  28  prior to re-entrance into operative relation with the imaging units  16   a - 16   d . The solvent recovered by the conditioning unit can be filtered or otherwise purified and recycled back into the reservoir for the developer. 
   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. 
   PARTS LIST 
   
       
         12 ,  12 ′ Textured imaging member (TIM) 
         14  Cells 
         16   a - 16   d  Imaging units 
         18  Ink jet device 
         20  Intermediate member 
         21  Conducting roller 
         22  Conditioning unit 
         24  Liquid removal unit 
         28  Cleaning unit 
         60  Ink 
         62  Dielectric fluid 
         80  Roller 
         82  Ceramic layer 
         84  Steel substrate 
       R Receiver member 
       V Voltage