Abstract:
A holddown for a hard copy device comprises a member having a surface and plural vacuum zones. Each of the vacuum zones defines a cavity in the surface having at least one port therethrough, and each cavity is defined by a sidewall circumscribing the cavity. At least one of the cavities has sidewall with a first section at a first height relative to the surface and a second section at a second height relative to the surface.

Description:
BACKGROUND  
         [0001]    Hard copy devices process images on media, typically taking the form of printers, plotters (employing inkjet or electron photography imaging technology), scanners, facsimile machines, laminating devices, and various combinations thereof, to name a few. These hard copy devices typically transport media in a sheet form from a supply of cut sheets or a roll, to an interaction zone where printing, scanning or post-print processing, such as laminating, overcoating or folding occurs. Often different types of media are supplied from different supply sources, such as those containing plain paper, letterhead, transparencies, pre-printed media, etc.  
           [0002]    In some kinds of hard copy apparatus a vacuum apparatus is used to apply a suction or vacuum force to a sheet of flexible media to adhere the sheet to a surface, or to stabilize the sheet relative to the surface, for example, for holding a sheet of print media temporarily to a platen. Such vacuum holddown systems are an economical technology to implement commercially and can improve machine throughput specifications and the quality of the print job. There are a variety of vacuum platen systems.  
           [0003]    As wet ink is deposited onto media the surface of the media may be distorted. This distortion of the media that results from interactions between the wet ink and the media, can impact the ability of vacuum holddown systems to reliably stabilize the media, and can likewise have an adverse impact on print quality.  
         SUMMARY OF THE INVENTION  
         [0004]    A holddown for a hard copy device comprises a member having a surface and plural vacuum zones. Each of the vacuum zones defines a cavity in the surface having at least one port therethrough, and each cavity is defined by a sidewall circumscribing the cavity. At least one of the cavities has sidewall with a first section at a first height relative to the surface and a second section at a second height relative to the surface. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a semi-schematic perspective view of selected portions of a hard copy device, here for purposes of illustration an inkjet printer illustrating a vacuum platen according to an illustrated embodiment of the present invention.  
         [0006]    [0006]FIG. 2 is partial cross sectional view of the illustrated embodiment of a vacuum platen showing several vacuum zones contained within the platen and illustrating a sheet of dry media supported on the platen, taken along the line  2 - 2  of FIG. 1.  
         [0007]    [0007]FIG. 3 is a partial cross sectional view of the illustrated embodiment of a vacuum platen showing several vacuum zones contained within the platen and illustrating a sheet of wet media supported on the platen, taken along the line  2 - 2  of FIG. 1.  
         [0008]    [0008]FIG. 4 is a partial cross sectional view of the illustrated embodiment of a vacuum platen taken along the axis that is transverse to the view of FIG. 2, and illustrating a sheet of dry media in the media interaction zone, taken along the line  4 - 4  of FIG. 1.  
         [0009]    [0009]FIG. 5 is a partial cross sectional view as in FIG. 4, and illustrating a sheet of wet media in the media interaction zone after ink has been applied to the media and the media is exhibiting cockle. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0010]    Some kinds of hard copy apparatus that employ inkjet printing techniques, such as printers, plotters, facsimile machines and the like, utilize a vacuum device either to support print media during transport to and from a printing station (also known as the “print zone” or “printing zone”), to hold the media at the printing station while images or alphanumeric text are formed, or both. The vacuum device applies vacuum force or suction to the underside of the media to hold the media down, away from the pens, to improve print quality. As used herein, the terms “vacuum force,” is used generally to refer to a suction force applied to media. Other terms may be used interchangeably with vacuum force, such “vacuum,” “negative pressure,” or simply “suction.” Moreover, for simplicity in description, the term “media” refers generally to all types of print media, including for example individual sheets of paper or paper supplied in a roll form.  
         [0011]    The inkjet printing process involves manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent media. Inkjet pens typically 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 selectively fires ink droplets, each droplet typically being only a tiny 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. 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 across the paper. Thus, a single sweep scan of the pen across the paper can print a swath of dots. The paper is advanced incrementally relative to the inkjet printheads to permit a series of contiguous swaths.  
         [0012]    Stationary, page-wide inkjet printheads or arrays of printheads (known as “page-wide-arrays” or “PWA”) are also used to print images on media, and the illustrated embodiment of a vacuum platen may be utilized in hard copy devices using PWAs.  
         [0013]    A phenomenon of wet-colorant printing is “paper cockle.” Simply described, cockle refers to 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, primarily as a result of physical and chemical interactions between the ink and the paper, and the operating conditions that exist in the printer. Paper printed with images has a greater amount of ink applied to it relative to text pages, and is thus more saturated with colorant than simple text pages and exhibits greater paper cockle. Colors formed by mixing combinations of other color ink drops form greater localized saturation areas and also exhibit greater cockle tendencies. Cockle can adversely affect the quality of a print job, and therefore reducing and managing the effects of paper cockle are important in maintaining high quality printing.  
         [0014]    As inkjet printheads 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 has an impact on print quality. Several aspects of ink control can be addressed to improve the quality of a print job and to reduce printing errors. For instance, by controlling the printhead to paper spacing (known as PPS) so that variations in PPS are reduced, randomness in the manner in which ink is deposited can be reduced. Also, if cockle occurs away from the pens, the likelihood of pen to paper contact that can damage the pens and smear images is reduced.  
         [0015]    The semi-diagrammatic illustration of FIG. 1 shows pertinent portions of a hard copy device, illustrated for purposes herein as a representative inkjet printer  10  in which an illustrated embodiment of a vacuum platen assembly  12  may be used. For purposes of clarity and to illustrate the embodiments of the invention more clearly, many features of the printer structure and chassis are omitted from the figures. Although the vacuum platen assembly is illustrated with respect to its embodiment in one specific type of printer, the vacuum platen assembly may be embodied in numerous different types of printers and recorders.  
         [0016]    Referring to FIG. 1, inkjet printer  10  includes a vacuum platen assembly identified generally with reference number  12 . The vacuum platen assembly is mounted in a chassis (not shown) in an operative position to receive recording media  14 , such as individual sheets of paper or paper from one or more sources of media such as paper trays. The vacuum platen assembly  12  is mounted adjacent one or more media interaction device(s), here inkjet cartridges  16  and  18 , which in a printer are supported by and movable on a shaft (not shown) for reciprocating movement past the media along an axis that extends transverse to the media feed axis. The cartridges  16  and  18  are mounted in a carriage assembly, also not shown, which supports the inkjet cartridges above media  14 . A media interaction head, in the case of an inkjet printer a printhead (also not shown) may be attached on the underside of the cartridge. The printhead may be a planar member having an array of nozzles through which ink droplets are ejected onto the adjacent media. The cartridge is supported on the shaft so that the printhead is precisely maintained at a desired spacing from media  14 .  
         [0017]    The carriage assembly may be driven with a servo motor and drive belt, neither of which are shown, but which are under the control of a printer controller. The position of the carriage assembly relative to print media  14  is typically determined by way of an encoder strip that is mounted to the printer chassis and extends laterally across the media, parallel to the shaft on which the inkjet carriage may be mounted. The encoder strip extends past and in close proximity to an encoder or optical sensor carried on the carriage assembly to thereby signal to the printer controller the position of the carriage assembly relative to the encoder strip.  
         [0018]    In FIG. 1, the “X” axis is defined as the axis along which inkjet cartridges  16  and  18  reciprocate on the supporting shaft, which as noted is not shown. The “Y” axis is transverse to the X axis, and is the axis of media travel as the media is fed through a media interaction zone  20 , which in the case of an inkjet printer is more specifically identified as a printzone where ink is applied to the media. The “Z” axis in FIG. 1 is the axis that extends vertically upward relative to the ground plane.  
         [0019]    As noted, many structural features in the printer are omitted from the drawings to clearly illustrate the embodiment of the invention. For example, printer  10  includes numerous other hardware devices and would of course be mounted in a printer housing with numerous other parts included in the complete printer.  
         [0020]    For other hard copy devices, the printer cartridge may be replaced with another type of media interaction head that performs a desired operation on the media in the media interaction zone.  
         [0021]    Media  14  is advanced through print zone  20  with a driven linefeed roller  22 , which forms a linefeed pinch between the linefeed roller and plural linefeed pinch rollers  24 , each of which is mounted on a chassis assembly such as pinch roller guides  26 , and which typically would be spring loaded so they are biased against the linefeed roller. The illustrated embodiment of the invention is typically included within a hardcopy device such as a printer that utilizes inkjet printheads to apply ink to the media. With an inkjet printer the media is incrementally advanced through the printzone  20  in a controlled manner and such that the media advances between swaths of the printheads. A disk encoder and associated servo systems are one of the usual methods employed for controlling the precise incremental advance of the media, commonly called “linefeed.” Typically, one or more printer controllers synchronize and control linefeed and printhead movement, among other printer operations.  
         [0022]    The vacuum platen assembly will now be described in detail. Referring to FIG. 1, vacuum platen assembly  12  comprises a platen plate member  30  that extends laterally across the printer along the X axis and is positioned below the inkjets. The platen plate member  30  is positioned relative to the inkjets  16  and  18  such that it supports the media  14  as the media is advanced past the inkjets. The platen plate member  30  thus defines a support for the media in printzone  20 . The outer, opposite ends of plate member  30 , labeled  32  and  34 , respectively, are mounted to and supported by the printer chassis. The upper surface  36  of platen plate member  30 —that is, the surface that faces inkjets  16 ,  18  (see FIG. 4)—is a substantially planar surface that defines a portion of printzone  20 . A plurality of generally rectangular depressions or vacuum zones  38  is formed in plate member  30 , arranged in a side-by-side array extending across the plate member. Each vacuum zone  38  is formed as a cavity or depression in the plate member that is recessed relative to the upper surface  36  and, as detailed below, is circumscribed by walls. Each of the individual vacuum zones  38  includes a vacuum passageway or port  40  that extends through a lower surface or floor  31  of each vacuum zone and through platen plate member  30  into a chamber  42  located beneath plate member  30 . Chamber  42  fluidly couples the upper surface  36  and vacuum zone  38  with a vacuum source, shown here generically as a vacuum fan  43 . The number of ports  40 , their size and shape, and their distribution pattern in the vacuum zones  38  may vary depending on the design specifics of a particular implementation. In the illustrated embodiment, the ports  40  comprise an essentially linear array of circular apertures.  
         [0023]    In the embodiment illustrated in FIGS. 1 through 5 each vacuum zone is shown as being generally rectangular in shape. It will be appreciated that the geometric configuration of each vacuum zone depends upon many factors such as the type of hardcopy device, the type of platen, etc., and accordingly that that the vacuum zones may be formed in other geometric configurations, including non-rectangular forms and forms defined by curved wall sections.  
         [0024]    With reference to FIG. 4, platen plate member  30  includes a downwardly depending frame member  44  that extends completely around the plate member to define the boundary of chamber  42 . Frame member  44  is fluidly sealed to a complementary upwardly extending frame member  46  that communicates with vacuum source  43 , which as noted may take the form of a vacuum fan, as shown, or a similar blower, pump or the like. It will be appreciated that vacuum source  43  is illustrated generally and is in fluid communication with chamber  42 . The vacuum source may be remotely located for convenience of design. The preferred vacuum source is an electrically operated fan that draws air through ports  40 , into chamber  42  and through the fan. Frame members  44  and  46  are preferably interconnected such that they form an airtight seal. Rubber gaskets or O-ring seals and the like may be used to facilitate the seal.  
         [0025]    A rib member separates each vacuum zone  38  from the next adjacent vacuum zone  38  and extends upwardly from floor  31  of the vacuum zones. With reference to FIG. 1, vacuum platen assembly  12  includes two different types of rib members, which differ from one another in their respective heights relative to floor  31 . Turning to FIG. 2, the first type, referred to herein as major ribs, are labeled with reference number  50 . The major ribs  50  have an upper surface  52  that is coextensive and coplanar with upper surface  36  of platen plate member  30 . The second type, referred to herein as minor ribs, are identified with reference number  54 . The minor ribs have an upper surface  56  that is below the level of upper surface  36 . The “height” of major ribs  50 , measured from the floor  31  of a vacuum zone  38  (see FIG. 4), is thus greater than the relative “height” of minor ribs  54 . This orientation of the major ribs  50  relative to the minor ribs  54  is shown in FIG. 2, where the level of upper surface  36  is illustrated schematically and where it may be seen that the upper surfaces  52  of major ribs  50  are separated from the upper surfaces of  56  or minor ribs  54  by a distance D.  
         [0026]    Again referring to FIG. 1, major ribs  50  may alternate with minor ribs  54 . However, as detailed below, printer  10  is designed to accommodate several different sizes of media and it is generally preferred that the lateral media edges rest on a major rib as the media is advanced through the printzone  20 , unless the media is of a type that is wide enough that it extends completely across the vacuum zones, as illustrated in FIG. 1. As such, in some instances two major ribs  50  may be located immediately adjacent one another, as illustrated in FIG. 1 with respect to the two major ribs nearest outer end  32  of platen plate member  30 .  
         [0027]    Each vacuum zone  38  is thus a generally rectangular depression formed in platen plate member  30 . Each vacuum zone is defined by a front and rear wall, and by opposed side walls. The front and rear walls of each vacuum zone—front and rear referring to the walls of each vacuum zone that extend in the direction along the X axis, and “front” being the front end of the printer—are labeled with reference numbers  58  and  60 , respectively. FIG. 4. Front walls  58  and rear walls  60  are all of the same height and terminate at upper surface  36 . The side walls of each vacuum zone—that is, the walls that extend along the Y axis and thus divide one vacuum zone  38  from the next adjacent vacuum zone or zones  38 —are defined by ribs  50  and  54 , except at the two vacuum zones that are at the outermost lateral ends of the platen, in which case one of the side walls is defined by the wall that defines part of the platen rather than a rib.  
         [0028]    The effect of the variable rib heights defined by the major ribs  50  and minor ribs  54  will now be described with reference to a sheet of media  14  as it advances through the printzone  20 . Beginning with FIG. 1, media  14  is shown as being a standard sized cut sheet such as an 8½×11 inch sheet of paper. The outer lateral edges of media  14 , here labeled  61  and  62 , respectively, extend laterally across platen plate member  30  beyond the outermost vacuum zones  38  such that the outer edges of the paper rest on upper surface  36  laterally outwardly of the outermost vacuum zones. It will be appreciated that as noted above, the printer is designed to accommodate several different kinds of media that have several different widths. The media  14  shown in FIG. 1 is one of many kinds of media that may be used with the illustrated embodiment of a vacuum platen, and is shown for illustrative purposes only. The outer edge  62  of the media, regardless of the size of media being used, will usually be aligned on the platen in the position shown in FIG. 1.  
         [0029]    The vacuum source  43  is either activated as the leading edge  64  of media  14  is advanced by linefeed roller  22  through printzone  20 , or is activated prior to the leading edge entering the printzone to induce a flow of air from the upper surface of the platen into the vacuum zones  38  and through ports  40  into chamber  42 . Referring to FIG. 3, linefeed roller  22  feeds media  14  onto upper surface  36  adjacent rear wall  60  so that an effective seal is formed between the media and the vacuum zone as the media advances forwardly enough that the media leading edge travels over the front wall  58  and the media thus covers the entire vacuum zone  38 .  
         [0030]    [0030]FIG. 4 illustrates the vacuum platen assembly  12  when media  14  is present and covers the entire vacuum zone  38  but where no ink has been applied to the media and therefore no ink-induced cockle is occurring in the media. In FIG. 4, the leading edge  64  of media  14  has advanced past the forward edge  66  of platen plate member  30 . The vacuum force applied on media  14  causes the media to be deflected downwardly toward the platen, away from the inkjet  16  and effectively forms a sealed chamber in each vacuum zone  38 . Application of vacuum force in this manner tends to hold dry media  14  in a relatively flat orientation on platen plate member  30 , and therefore controls the printhead to paper spacing so that the distance B in FIG. 4 is relatively constant. When the PPS is controlled, randomness in the manner in which ink droplets are deposited on the media is reduced.  
         [0031]    [0031]FIG. 5 is similar to FIG. 4 except it illustrates a sheet of media  14  onto which ink has been applied, and the media is exhibiting cockle as a result of the interactions between the ink and the media. As cockle is formed in media  14 , the vacuum force applied to the media causes the paper to be deflected downwardly into vacuum zones  38  toward floor  31  to a greater extent than shown in FIG. 4. That is, cockle growth occurs in the direction away from the inkjet printheads. Although the cockle results necessarily in slight variations in PPS (distance B) at some points in printzone  20 , the application of vacuum insures that cockle growth is away from the inkjet  16 . It will be noted that each vacuum zone  38  is wider (in the direction along the Y axis) than the width (along the same axis) of the inkjets  16  and  18 . As such, each vacuum zone  38  extends forwardly beyond the forward edge  68  of inkjet  16 . Stated in another way, the front wall  58  of each vacuum zone is positioned forward along the Y axis of the forward edge  68  of the inkjet. This spacing provides an additional distance along the vacuum zone that the media  14  may ride over as cockle forms, yet still be exposed to vacuum force.  
         [0032]    [0032]FIG. 2 is similar to FIG. 4 in that it illustrates media  14  that has no ink applied thereto and is therefore dry, except FIG. 2 is a sectional view taken through several vacuum zones and along the X axis. The vacuum force applied to media  14  causes the media to rest on the upper surfaces  52  and  56  of the alternating major and minor ribs,  50  and  54 . It will be appreciated that the amount of downward deflection in media  14  in FIG. 2 (where the media defines a waveform across the platen) is exaggerated to demonstrate that the alternating rib heights between major ribs  50  and minor ribs  54  define a media receiving and supporting surface that holds the media away from the inkjets to maintain and control PPS. Because vacuum force is applied to the underside of media  14 , the dry media in FIG. 2 is held downwardly in the direction away from the inkjets. As illustrated in FIG. 1, the alternating rib heights between the upper surfaces  52  of major ribs  50  and adjacent upper surfaces  56  of minor ribs  54  defines a media-supporting surface in the printzone that is non-planar, whereas the upper surface  36  of the platen outside of the vacuum zones is planar.  
         [0033]    [0033]FIG. 3 is a view comparable to FIG. 2, except that as in FIG. 5, FIG. 3 illustrates media  14  onto which ink has been applied and which as a result is exhibiting cockle. Again, it will be appreciated that the amount of cockle shown in media  14  in FIG. 3 is exaggerated to demonstrate that the alternating rib heights between major ribs  50  and minor ribs  54  define a media receiving and supporting surface that holds the media away from the inkjets to maintain PPS. Because vacuum force is applied to the underside of media  14 , cockle growth desirably occurs downwardly, in the direction away from the inkjets.  
         [0034]    The non-planar media supporting surface defined by alternating rib heights of the illustrated embodiment allows for increased rib-to-rib spacing between adjacent ribs than if all of the ribs were of the same height. Stated otherwise, a vacuum platen that has ribs that are all of the same height and has the same rib spacing as the illustrated embodiment would require either a greater vacuum force to accomplish the same initial downward bias of dry paper toward the platen, or a higher PPS variation if the same vacuum force were used. By using alternating rib heights and a resulting non-planar media supporting surface, the amount of vacuum force applied may be reduced, thereby lowering the noise levels from the vacuum fans. Moreover, with alternating rib heights, cockle is controlled accurately and the PPS may be decreased, thereby increasing the quality of the print job.  
         [0035]    Although preferred and alternative embodiments of the present invention have been described, it will be appreciated by one of ordinary skill in this art that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.