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 1b 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-5d 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. 1a and 1b, 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 Serial No. ______. 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. 5a). 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. 5b. 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 by applying an electrostatic field that urges the particles towards the receiver (see FIG. 5c). 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. 5d) 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 16a-16d 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 16a-16d 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 1×108 and 1×1011 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 1×108 and 1×1011 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 (<1×106 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 16a-16d 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 16a-16d. 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-16d 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
Patent Application
20070279471
