Patent Publication Number: US-6210055-B1

Title: Method and apparatus for cockle reduction in print media

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a continuation of application Ser. No. 09/345,689 filed on Jun. 30, 1999 now U.S. Pat. No. 6,079,882. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to hard copy methods and apparatus, more particularly to print media handling and, more specifically to a print media transport belt used in a wet colorant printing apparatus such as an ink-jet printer. 
     2. Description of Related Art 
     For convenience of explanation, while the present invention is useful for all wet colorant hard copy apparatus, it is described with respect to an ink-jet printer. The use of this exemplary embodiment is not intended as a limitation on the scope of the invention nor should any such intention be implied as the invention can be adapted to implementations using other wet colorant printing techniques, e.g., pigment-based and dye-based inks used in ink-jet printing, Wet toner laser printing systems, and the like, also synonymously referred to hereinafter as a “wet dye” printing; the terms “colorant” or “dye” shall be construed to encompass both color and black and grey scale wet printing techniques. 
     The art of the exemplary embodiment in 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 assignee&#39;s  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, incorporated herein by reference for general background. 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). Further details to facilitate an understanding of the present invention are provided below with respect to FIG.  1 . For convenience of description, print media of all shapes, sizes, and varieties are referred to hereinafter simply as “paper.” 
     A well-known phenomenon of wet-colorant printing is “paper cockle,” the irregular surface produced in paper by the saturation and drying of ink deposits on the fibrous medium. As a sheet of paper gets saturated with ink, the paper grows and buckles in a seemingly random manner. Paper printed with images are more saturated with colorant than simple text pages and thus exhibit great paper cockle. Colors formed by mixing combinations of other color ink drops form greater localized saturation areas and also exhibit greater cockle tendencies. 
     In general, the prior art has approached the problem by using ribs that support the sheet of paper at various points along the width. In the state of the art, ink-jet printhead to paper separation is only about 0.5 to 1.5 millimeters. These ribbed paper support constructs allow any cockle growth to grow down away from the printing mechanism instead of toward it where contact could occur and cause further problems. Moreover, the use of ribbed paper support constructs alone is not adequate for larger dimensioned print zones. In order to hold print media flat under larger printheads a different holddown is required, such as a vacuum platen. 
     It has been discovered that media fiber directionality has a significant influence on the directionality of cockle growth. Papers generally have a grain in one direction longer than the other. Rib support constructs may thus be inadequate, depending on the nature of the media being printed. Feeding a sheet into a printing station of the hard copy apparatus with an orientation that minimizes cockle is impractical since it would limit the end-user&#39;s ability to select different media types. 
     There is a need for a paper transport and printing station paper holding device to substantially reduce and for practical purposes substantially eliminate problematical paper cockle. 
     SUMMARY OF THE INVENTION 
     In its basic aspects, the present invention provides a wet colorant hard copy device for reducing print media cockle. The device includes a belt mechanism for transporting print media through a wet colorant printing zone. The print media has a first surface and a second surface. The print media first surface adheres to at least one belt of the belt mechanism during transport. The print media is maintained in a substantially planar orientation on the belt mechanism at least through the printing zone such that the wet colorant is deposited on the print media second surface. The belt mechanism is splayed from a first cross dimension perpendicular to direction of travel of the print media as the print media enters the printing zone to a second cross dimension perpendicular to direction of travel of the print media as the print media exits the printing zone. The second dimension is greater than the first dimension. The belt mechanism includes at least one belt having a belt surface of a material for contacting the first surface wherein the belt surface has a coefficient of friction sufficient for gripping the first surface for applying lateral forces thereon or mechanisms for exerting a vacuum force on the first surface, or both. 
     In another basic aspect, the present invention provides a method for compensating for paper cockle in a wet colorant hard copy apparatus having a printing zone. The method includes the steps of: transporting a sheet of paper in a substantially planar orientation along a paper path through the printing zone, and gradually stretching the sheet of paper perpendicularly to the paper path from at least a point of entry of the printing zone in the paper path to at least a point of exit of the sheet from the printing zone in the paper path. The stretching includes stretching the sheet of paper at least over a distance greater than a paper path length of the printing zone when the ink dry time so requires. 
     In another basic aspect, the present invention provides an ink-jet hard copy apparatus for print media, including: an ink-jet printing mechanism for depositing wet colorant on the print media; and, mounted in the printer in relationship to the printing mechanism, a belt mechanism for sequentially transporting sheets of print media through a printing zone of the apparatus such that the print media has a first surface that adheres to at least one belt of the belt mechanism during the transporting and the print media is maintained in a substantially planar orientation on the belt at least through the printing zone such that the wet colorant is deposited on a print media second surface by the printing mechanism and wherein the belt is splayed from a first cross dimension perpendicular to direction of travel of the print media as the print media enters the printing zone to a second cross dimension perpendicular to direction of travel of the print media as the print media to at least a position wherein the print media exits the printing zone wherein the second dimension is greater than the first dimension. 
     It is an advantage of the present invention that it allows paper to grow due to wet colorant saturation, yet keeps the paper substantially flat. 
     It is an advantage of the present invention that it combines the functions of a paper transport and a paper platen. 
     It is another advantage of the present invention that it compensates for paper cockle regardless of print media fiber orientation. 
     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 an ink-jet hard copy apparatus in accordance with the present invention. 
     FIG. 2 is a perspective view of the vacuum transport device in accordance with the present invention as shown in FIG.  1 . 
     FIG. 3 is a plan view (top) of a vacuum transport device for the ink-jet hard copy apparatus as shown in FIG.  2 . 
    
    
     The drawings referred to in this specification should be understood as not being drawn to scale except if specifically noted. 
     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. 
     The present invention approaches the problem of paper cockle by stretching the paper as it progresses along the paper path through the wet colorant printing station of a hard copy apparatus. 
     FIG. 1 depicts an ink-jet hard copy apparatus; in this exemplary embodiment, a computer peripheral printer  101  is shown. A housing  103  encloses the electrical and mechanical operating mechanisms of the printer  101 . Operation is administrated by an electronic controller (usually a microprocessor or application specific integrated circuit (“ASIC”), not shown) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC&#39;s; further explanation is not necessary to an understanding of the present invention. Cut-sheet print media  105 , loaded by the end-user onto an input tray  107 , is fed by a paper-path transport  108  in accordance with the present invention to an internal printing station, or “print zone,” where graphical images and alphanumeric text is created. A carriage  109 , mounted on a slider  111 , scans the fed print medium  105 . An encoder  113  is provided for keeping track of the position of the carriage  109  at any given time. One or more individual ink-jet pens, or print cartridges,  115  are releasable mounted in the carriage  109  for easy access (generally, in a full color system, inks for the subtractive primary colors, cyan, yellow, magenta (CYM) and true black (K) are provided). Once a printed page is completed, the print medium is ejected, such as onto an output tray (not shown) or the desktop. As depicted by the labeled arrows in FIG. 1, it is common in the art to refer to the pen scanning direction as the x-axis, the paper feed direction as the y-axis, and the ink drop firing direction as the z-axis. 
     FIG. 2 shows a belt-drive, paper-path transport  108  in accordance with the present invention. Either a single, flexible, paper-path transport belt or, as shown in this exemplary embodiment, a plurality of individual flexible belts,  201  is adapted to act also as a paper platen in the x-y axes print zone (generally indicated by the labeled arrow  200 ) adjacent the scanning carriage  109 . A chassis  203  provides suitable, known manner, mounting for the components of the transport  108  and the pen carriage  109  relative to the print zone  200  in accordance with the needs of a specific design implementation as would be known to persons skilled in the art. 
     The belts  201  are mounted and stretched tautly between front, grooved rim, pulleys  211  and rear, grooved rim, pulleys  213 . (It should be recognized that many paper path configurations—e.g., front or back or top or bottom paper feed, a single, multi-grooved, pulley wheel or a set of individual pulleys, and the like can be implemented; therefore, the terms like “front” and “rear” used to describe the invention relative to the exemplary embodiments depicted in the FIGURES and not a limitation on the invention itself.) In an implementation using a plurality of belts  201 , the belts&#39; outer surface should be higher than the rims at least at the paper path&#39;s entrance side. The pulleys  211 ,  213  are mounted in a known manner to the frame  203  for rotation about a respective front pulley axle  212  and a rear pulley axle (not seen in this view). One of the axles  212  is driven by a motor (not shown) in any known manner of the state of the art. The pulley rims  202  keep the belts  201  in a substantially parallel alignment as explained further hereinafter. Belts composed of a flexible material such as rubber or other material with a sufficiently high coefficient of -friction as would be known in the art can be employed in accordance with the present invention. Note that the belts  201  may be alternatively supported by a subjacent, flexible belt-support layer  301  where, turning also to FIG. 3, a gap  209  between adjacent belts is wide enough such that having vacuum ports  207 ′ through belt-support layer  301  aligned with each gap provides sufficient vacuum force to adhere the paper firmly to the belts. The ports have a diameter in the range of approximately 0.4 to 1.5 millimeters; however, this size may vary significantly depending on the actual implementation design and will be function of many factors—vacuum pressure, print zone dimensions, and the like as would be known to a person skilled in the art. This configuration may also provide a firmer platen-side surface where the media size is large and the belt dimensions are relatively long in the paper path direction. 
     The functional operation of the belt system  109  is to receive a sheet of paper  105  (FIG. 1) from a known manner paper pick-and-feed mechanism (not shown) associated with the input tray  107 , to move the sheet through the print zone  200  of the hard copy apparatus  101  where the carriage  109  selectively positions the pens  115  with respect to the paper while a printing algorithm fires the pen printheads appropriately to create text and images, and then to eject the printed page from the print zone. 
     Paper transport using a belt drive has many known advantages. A vacuum belt performs extremely well, particularly at holding a sheet of paper substantially planar at the hard copy apparatus printing station. Thus, in the preferred embodiment, the transport  108  comprises a vacuum belt system. Each belt  201  includes a plurality of vacuum ports  207 . A vacuum force in a vacuum chamber  208  formed by the belts  201  is conventionally generated, such as with an appropriately configured exhaust fan (not shown) mounted within the chassis  203  such that the vacuum force is applied to the inner surface, or “vacuum-side surface,” of the belts  201 . The vacuum force is transmitted to the outer surface, or “platen-side surface,” of the belts  201  via each vacuum port  207  or via the gaps  209  between adjacent belts, or both. A cut sheet print media picked and fed from the input tray  107  (FIG. 1) to the belts  201  thus adheres firmly to the belts  201 . Moreover, if individual belts are relatively long in the y-axis, the addition of ribs can be made to assist in guidance. 
     Looking now to both FIGS. 2 and 3, the belts  201  are intentionally splayed from the paper feed entrance to the paper feed exit of the print zone  200 . This paper feed relative direction is depicted as arrow  215  in FIG. 3, With a sheet of paper adhered to the belts  201 , the splayed belt drive keeps and increases tension across the paper in the x-axis (FIG. 1) as the paper moves in the y-axis through the print zone  200  and exits the transport  108 . For an A-size paper printer using a vacuum force pressure against the bottom of the paper sheet in the range of five to twenty inches-water-column (“W.C.” hereinafter), it has been found that a front-to-rear (or entrance-to-exit) splay of approximately one-half inch is sufficient to keep tension across the page and hold the page flat against the platen. Again, these specifications relate only to the exemplary embodiment; the actual optimum splay will be a function of the specific implementation design. 
     In still another alternative embodiment, a single flexible belt having vacuum ports therethrough and spanning the print zone and may be employed provided it meets the requirement for media entrance to media exit splay. 
     As the wet colorant may require a predetermined dry time, depending on factors such as ink composition, paper composition, ambient temperature, and the like as would be known to a person skilled in the art, it is advantageous to continue stretching the paper as it dries. Thus, as shown, the belts  201  continue to splay as the leading edge of the paper moves out of the print zone and toward paper ejection. 
     The foregoing description of the preferred embodiment of is 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. For example, a lateral edge paper holder that stretches the paper as it moves along the paper path can also be employed in accordance with the methodology described herein. 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 state, but rather means “one or more.” 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 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 . . . .”