Abstract:
An ink-jet apparatus employs a platen having an array of vacuum ports that are each filtered. The filter is constructed to provide restricted-airflow rates which remain uniform when the platen is either fully covered or partially uncovered. The filter mechanism provides both airflow restrictions such that ink drop flight trajectories in the printing zone are unaffected, acoustic dampening of the vacuum pump is provided, and vacuum pressure is kept relatively high proximate the print media edges.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to ink-jet hard copy apparatus and methods of operation and, more specifically to a low flow vacuum platen with minimal airflow induced drop directionality errors. 
     2. Description of Related Art 
     The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the  Hewlett - Packard Journal , Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in OUTPUT HARDCOPY [sic] DEVICES, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). As providing background information, the foregoing documents are incorporated herein by reference. Further details of basic ink-jet printing technology are also set forth below in the Detailed Description of the present invention with respect to FIG.  1 . 
     It is known to use a vacuum induced force to adhere a sheet of flexible material to a surface, for example, for holding a sheet of print media temporarily to a transport system or platen. [Hereinafter, “vacuum induced force” is also referred to as “vacuum induced flow,” “vacuum flow,” or more simply as just “airflow,” “vacuum” or “suction,” as best fits the context.] Such vacuum holddown systems are a relatively common, economical technology to implement commercially and can improve hard copy apparatus throughput specifications. For example, it is known to provide a rotating drum with holes through the surface wherein a vacuum type airflow through the chamber formed by the drum cylinder provides a suction force at the holes in the drum surface (see e.g., U.S. Pat. No. 4,237,466 for a PAPER TRANSPORT SYSTEM FOR AN INK JET PRINTER (Scranton)). [The term “drum” as used hereinafter is intended to be synonymous with any curvilinear implementation incorporating the present invention; while the term “platen” can be defined as a flat holding surface, in hard copy technology it is also used for curvilinear surfaces, e.g., as the ubiquitous typewriter rubber roller; thus, for the purposes of the present application, “platen” is used generically for any shape paper holddown surface—stationary or movable—as used in a hard copy apparatus.] Permeable belts traversing a vacuum inducing support have been similarly employed (see e.g., Scranton and U.S. Pat. Appl. Ser. No. 09/163,098. by Rasmussen et al. for a BELT DRIVEN MEDIA HANDLING SYSTEM WITH FEEDBACK CONTROL FOR IMPROVING MEDIA ADVANCE ACCURACY (assigned to the common assignee of the present invention and incorporated herein by reference)). 
     Generally in a hard copy apparatus implementation, the vacuum device is used either to support cut-sheet print media during transport to and from a printing station of a hard copy apparatus, to hold the sheet media at the printing station while images or alphanumeric text are formed (also known as the “print zone” or “printing zone”), or both. [In order to further simplify description of the technology and invention, the term “paper” is used hereinafter to refer to all types of print media and the term “printer” to refer to all types of hard copy apparatus; no limitation on the scope of the invention is intended nor should any be implied.] 
     In essence, the ink-jet printing process involves digitized, dot-matrix manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent paper. One or more ink-jet type writing instruments (also referred to in the art as an “ink-jet pen” or “print cartridge”) include a printhead which generally consists of drop generator mechanisms and a number of columns of ink drop firing nozzles. Each column or selected subset of nozzles (referred to in the art as a “primitive”) selectively fires ink droplets (typically each being only a few picoliters in liquid volume) that are used to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given matrix column print position on the paper (referred to as a picture element, or “pixel.”). Horizontal positions, matrix pixel rows, on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of dots. The paper is stepped to permit a series of contiguous swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Page-wide ink-jet printheads are also contemplated and are adaptable to the present invention. 
     As the ink-jet writing instruments—often scanning at a relatively high rate, across the paper—expel minute droplets of ink onto adjacently positioned print media and sophisticated, computerized, dot matrix manipulation is used to render text and form graphic images, the flight trajectory of each drop is critical to print quality. Printing errors (also referred to in the art as “artifacts”) are induced or exacerbated by any airflow in the printing zone. Thus, use of a vacuum platen and vacuum transport device in the printing zone of an ink-jet printer creates an added difficulty for the system designer. 
     There is a need for a vacuum system for use in an ink-jet printing zone which will provide a minimal airflow impact on ink-jet drop flight trajectory. 
     SUMMARY OF THE INVENTION 
     In its basic aspects, the present invention provides a print media vacuum platen system including: a vacuum box, having at least one surface thereof further comprising filter mechanisms for permitting airflow therethrough; associated with the vacuum box, vacuum mechanisms for creating a negative pressure within the vacuum box and inducing the airflow through the filter mechanisms; and mounted adjacently to the filter mechanisms distally from the vacuum box, platen mechanisms for holding print media in a position for ink-jet printing thereon, the platen mechanisms having a plurality of vacuum passages therethrough such that regions of the filter mechanisms form a porous floor for each of the passages. 
     In another basic aspect, the present invention provides a method for providing a substantially uniform airflow across an ink-jet print media vacuum platen associated with a vacuum inducing mechanism. The method includes the steps of: drawing a vacuum through a plurality of vacuum ports distributed across the platen; and filtering the airflow through the ports via an airflow restrictive porous material filter interposed between the platen and the vacuum inducing mechanism. 
     In another basic aspect, the present invention provides an ink-jet hard copy apparatus including: an ink-jet writing instrument associated with a printing zone within the apparatus; an endless loop vacuum belt system for transporting print media to and from the printing zone; a vacuum platen system located proximate the printing zone, the vacuum platen system having a platen, having a plurality of vacuum ports therethrough, a vacuum chamber, having one wall thereof fabricated of a porous material, the one wall being adjacent the platen such that the material forms a flooring for each of the ports, a vacuum device for maintaining a negative pressure within the chamber such that an airflow is established through the vacuum ports into the chamber via the porous material such that a substantially uniform vacuum force is exerted across the media regardless of the number of vacuum ports covered or partially covered by the print media. 
     Some of the advantages of the present invention are: 
     it provides a low flow vacuum system with minimal airflow induced ink drop directionality errors; 
     it provides a substantially uniform vacuum field regardless of degree of platen coverage; 
     it provides a low flow platen that allows vacuum box pressure to remain relatively constant whether or not paper is fully covering the platen, thus compensating for different sized print media; 
     it allows for various media sizes and thicknesses to be held down with substantially the same pressure without requiring a large vacuum source; 
     it reduces acoustic levels caused by a vacuum induced airflow; 
     it provides a platen that is resistant to clogging by ink and paper dust; and 
     it provides improved vacuum holding at paper edges. 
     The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of an ink-jet hard copy apparatus in accordance with the present invention. 
     FIG. 2 (Prior Art) is a planar, overhead view of detail of the top surface of a vacuum platen. 
     FIG. 3 is a schematic depiction of a vacuum platen system used in the present invention as also shown in FIG.  1 . 
     FIG. 3A is a schematic depiction in a planar, overhead view of detail of the top surface of a vacuum platen in accordance with the present invention as shown in FIG.  3 . 
     The drawings referred to in this specification should be understood as not being drawn to scale except if specifically annotated. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable. 
     FIG. 1 is a schematic depiction of an exemplary embodiment of an ink-jet hard copy apparatus  10  in accordance with the present invention. A writing instrument  12  is provided with a printhead  14 , having drop generators including nozzles for ejecting ink droplets onto an adjacently positioned print medium, e.g., a sheet of paper  16 , in the apparatus&#39; printing zone  34 . 
     One type printing zone input-output paper transport, and a preferred embodiment for the present invention, is an endless-loop belt  32  subsystem. A motor  33  having a drive shaft  30  is used to drive a gear train  35  coupled to a belt pulley  38  mounted on an fixed axle  39 . A biased idler wheel  40  provides appropriate tensioning of the belt  32 . The belt rides over a generic platen  36  in the print zone  34 ; a specific platen subsystem in accordance with the present invention is described in detail hereinafter with respect to FIGS. 3 and 3A, but in general the vacuum platen subsystem is associated with a known manner vacuum induction system  37  (for simplicity of description referred to hereinafter as merely a “pump;” actual induced vacuum force is a function of specific implementation design factors, such as sizes, shapes, thicknesses of the media, and the like as would be known to a person skilled in the art). The paper sheet  16  is picked from an input supply (not shown) and its leading edge  54  is delivered to a guide  50 ,  52  aligned for delivering a leading edge to the belt; an optional pinch wheel  42  in contact with the belt  32  may be used to assist transport of the paper sheet  16  through the printing zone  34  (the paper path is represented by arrow  31 ). While vacuum release downstream of the printing zone  34  may be sufficient to transport the sheet  16  toward the output, an output roller  44  in contact with the belt  32  may optionally be used to receive the leading edge  54  of the paper sheet  16  and continue the paper transport until the trailing edge  55  of the now printed page is released. Belt porosity and vacuum force requirements will be a function of a specific printer  10  design. 
     Referring to both FIGS.  1  and FIG. 2 (Prior Art), a specific type of platen  201  is illustrated. This platen  201  has a top surface  203  over which the belt  32  slides. Slots  205  in the surface  203  are coupled to the subjacent vacuum induction system  37  by through-holes  207  to distribute the vacuum force across the platen  201  to hold the sheet of paper  16 . A region  209  of the sheet of paper  16  is shown covering part of the surface  203  area. When a slot  205  is fully or partially open, as shown, airflow is high through the holes  207  of that slot  205  since the region  209  of paper is not closing the entire slot off from the local atmosphere. This can cause several problems. For example, the airflow into the vacuum box is high for smaller media that leaves a large percentage of the platen surface  203  open. This requires a relatively large vacuum pump  37 . If the surface  203  is mostly open (e.g., when a 3×5-inch card is on a 12×16-inch platen such that there is only about eight percent platen coverage), the pump  37  must provide a very large flow (e.g., 200 CFM or greater) before the appropriate vacuum level (e.g., at least 6-inches H 2 O) is produced in the slots  205  beneath the card. A large vacuum pump is undesirable since it leads to noise problems and increased cost of manufacture. The use of smaller holes  207  weakens vacuum levels in partially open slots  205  and leads to still other problems as smaller holes tend to clog with ink and paper dust. As another example, high airflow is induced around the edge  211  of the paper  209  which disturbs ink droplet flight trajectory from the pen  12  (FIG. 1 only) to the paper  209 . Moreover, the vacuum force exerted on the underside of the paper  209  is diminished in partially open slots which might permit undesirable paper flexing or motion during a printing cycle. 
     Referring now to both FIGS. 1 and 3, illustrations of the details of the vacuum platen system  301  for the hard copy apparatus  10  are shown. The system  301  fundamentally substitutes in the printing zone  34  of FIG. 1 for elements  36  and  37 . Electrical power is supplied in any known manner; further details are not required for an understanding of the present invention. 
     A vacuum creating device, such as a pump or exhaust mechanism,  303  is mounted  305  in any known manner in a vacuum box  307 . Appropriate exhaust (represented by the labeled arrow) manifolding  309  is provided. A sheet of paper  16  is transported along paper path  31  to the printing zone  34  by the perforated transport belt  32 . A platen  311  member is mounted atop the vacuum box  307 . While in the shown embodiment it has been found that incorporating the pump  303  into the vacuum box provides a commercially viable arrangement, it will be apparent to those skilled in the art that the vacuum pump can be remotely located in the printer  10  and coupled to the vacuum box  307 . 
     Turning also to FIG. 3A (to best illustrate features of the present invention, the belt  32  is not shown), the platen  311  surface  313  has an array of vacuum passageways, or ports,  315  distributed across the surface. The distribution pattern can vary depending on the design specifics of a particular implementation. In the exemplary embodiment shown, the pattern is a staggered row and column linear array of substantially circular apertures. The platen  311  can be nearly all open as shown, or mostly solid material depending on the needed vacuum holddown area and force design requirements. Optimally, the paper sheet  16  fully closes subjacent ports  315 , but at edges  16 ′,  16 ″, may only partially close subjacent ports, such as trailing edge  16 ″ in FIG. 3 partially covering port  315 ″. 
     It is preferable that the vacuum ports  315  be large enough so that they do not clog with ink or paper dust. Ports  315  having a diameter in the approximate range of two to seven (2-7) millimeters have been found to be suitable to ink-jet printing conditions. 
     Returning to FIG. 3, the vacuum box  307  has a lid  317  that is essentially an airflow filter. The lid-filter  317  is mounted to segregate and fluidically couple the vacuum box  307  and platen  311 . Airflow induced by the vacuum pump  303  through the platen ports  315  and lid-filter  317  is represented by arrows  319 . There are platen side areas of the filter material  321  that act as a floor of each vacuum port  315  and a relatively large volume of filter material on the vacuum side of the platen  311  to trap such debris without clogging the system. The lid-filter  317  provides flow restriction to enable uniform suction, that is, even vacuum pressure distribution, to the underside the media on the platen  201 . Moreover, no substantial ink mist and paper dust reaches the vacuum pump where it would eventually affect pump  303  operations. Note that with the pattern described, there is an advantage of having fully covered ports closer to the edges of the media. 
     In other words, the ports  315  are large enough not to clog but filtered such that due to the flow restriction effect of the filter material, if one is partially open there will be relatively little airflow through the open portion which could alter ink drop flight trajectories near the paper edges or lead to excessive loss of vacuum pressure at the edges. This is particularly important for high quality graphics and photographic type printing where the user may wish to print the entire page with extremely small margins (also known in the art as “full bleed” printing). 
     This system allows use of a relatively small vacuum pump to maintain a substantially constant vacuum box  307  negative pressure. 
     It has been found that for a preferred embodiment, the lid-filter  317  element itself is layered, or graduated, from being relatively porous (coarse material  318 ) proximate the underside of the platen  311  to relatively dense (fine material  323 ) superjacent to the vacuum box  307 . Air flow through the coarse material region  321  at the floor of each port  315  is freer, removing ink mist, paper dust, and other known ink-jet process contaminants through the ports  315 , particularly via open ports or partially open ports  315 ″. The fine material  323  acts to restrict airflow to desired levels. 
     While use of any filter material that provides an adequate filtering and pressure drop may be employed in implementing the present invention, preferred material for the lid-filter  317  is selected from the group comprising polypropylene, cotton, polyester, PTFE, cellulose or the like paper, or sintered materials such as of plastic or metals. Commercially available G300 and SBMF materials from 3M Corp., St. Paul, Minn., have been successfully employed in accordance with the present invention. Generally, it is believed that materials from 5 gm/m 2  to 500 gm/m 2  may be employed, depending on the requirements of a specific implementation. It has been found that the use of these materials versus use of a more solid screen type material for a lid-filter  317  provides superior acoustic dampening. An approximate range of 0.1 CFM/in. 2  to 1.5 CFM/in. 2 , about three to fifty inches H 2 O can be employed; specific implementations can vary. 
     It will be recognized by those skilled in the art that while the present invention has been illustrated in a substantially planar embodiment, the concept is applicable to curvilinear platen implementation, including vacuum drum designs where the platen, filter (or filters), and vacuum box are concentric constructs. 
     The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather means “one or more.” Moreover, no element, component, nor method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the following claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for. . . . ”