Abstract:
A vacuum holddown device for a hardcopy apparatus, comprising a printing platen with a pattern of through holes and an underlying vacuum chamber, further comprises a member located therebetween and having a substantially similar pattern of through holes, the member being moveable to align the pattern of holes or not so as to respectively interconnect or disconnect the holes in the printing platen and the vacuum chamber. The member may itself define a secondary chamber with which the holes in the printing platen are in communication when not in communication with the vacuum chamber.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a vacuum holddown device for hardcopy apparatus and in particular to a vacuum mechanism for a printer which permits a change between a higher vacuum during a print phase and a lower vacuum or no vacuum during a print media advance phase.  
         BACKGROUND OF THE INVENTION  
         [0002]    The vacuum in the printzone of a printer needs to be applied during printing to hold the print media in the correct position and at the correct spacing under the printhead. This also serves to reduce expansion of the print media as it absorbs ink (i.e. cockle) and ensures a good print quality. On the other hand, the vacuum needs to be removed or substantially reduced while the print media is being advanced in order to reduce friction and to keep the required media positioning accuracy.  
           [0003]    During the time the vacuum is rising and falling between its high and low values, the printer is idle, since the vacuum is too high to advance the media, but too low to permit satisfactory printing. Thus the problem arises of how to switch the vacuum in the printzone on and off in the minimum possible time so that it does not adversely affect printing time and thus the throughput of the printer.  
           [0004]    In one previous proposal, a fan is used to produce a vacuum in a chamber below a printzone with a plurality of holes in a printing platen between the chamber and the print media. The vacuum pressure can be changed by switching the fan off and on to pressurise and depressurise the chamber. However, this has the disadvantage of taking a long time to change between maximum and minimum pressure levels, typically of the order of tenths of seconds.  
           [0005]    In another previous proposal, two vacuum accumulation chambers are provided, one at a high level and the other at a low vacuum level, and a valve is provided to switch the connection of the main vacuum chamber under the printzone to one or the other of the accumulation chambers. The time taken to pressurise and depressurise the main vacuum chamber depends on the relative size of the chambers and also the power used to generate the vacuum. An efficient arrangement has a high power requirement and the volume of each of the vacuum accumulation chambers needs to be at least one order of magnitude larger than that of the main vacuum chamber, leading to high space requirements.  
         SUMMARY OF THE INVENTION  
         [0006]    Certain aspects of the present invention seek to overcome or reduce one or more of the above problems.  
           [0007]    The present invention seeks to provide a vacuum holddown arrangement which is simple, compact, efficient and inexpensive.  
           [0008]    According to a first aspect of the present invention there is provided a vacuum holddown device for a hardcopy apparatus comprising a printing platen, said printing platen having holes therethrough, said holes being arranged in a predetermined pattern, a vacuum chamber, and a moveable member, said moveable member including a planar portion, said planar portion having holes therethrough arranged in substantially the same predetermined pattern, and said moveable member being moveable between a first position, in which said holes in said moveable member are substantially in alignment with said holes in said printing platen and said holes in said printing platen are in communication with said vacuum chamber, and a second position, in which said holes in said moveable member are out of alignment with said holes in said printing platen and said holes in said printing platen are not in communication with said vacuum chamber.  
           [0009]    According to a second aspect of the present invention there is provided a vacuum holddown device for hardcopy apparatus comprising a printing platen having a set of through holes in a predetermined pattern, a moveable member having a set of through holes in substantially the same pattern, and a vacuum chamber, the moveable member being located between said printing platen and said vacuum chamber.  
           [0010]    According to a third aspect of the present invention there is provided a method of applying two different pressure levels to a printing platen of a vacuum holddown device of a hardcopy apparatus, said platen having a first set of through holes located to be in communication with a vacuum chamber, the method comprising providing a moveable member between said platen and said vacuum chamber, said moveable member having a second set of through holes, the holes in said first and second sets being arranged in substantially the same pattern, and the method further comprising moving said moveable member between a first position and a second position, said holes in said two sets being in substantial alignment in said first position wherein said first set of holes is in communication with said vacuum chamber, and said holes in said two sets being out of alignment in said second position wherein said first set of holes is not in communication with said vacuum chamber. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:  
         [0012]    [0012]FIG. 1 shows an exploded perspective view of the components of a holddown device in accordance with the present invention;  
         [0013]    [0013]FIG. 2 shows an enlarged schematic prospective view of the device of FIG. 1, with the platen plate omitted for reasons of clarity;  
         [0014]    [0014]FIG. 3 is a top view of part of the device of FIG. 1, with the valve plate in a first position;  
         [0015]    [0015]FIG. 4 is a top view similar to FIG. 3 but with the valve plate in a second position; and  
         [0016]    [0016]FIG. 5 is a top view of a modified valve plate.  
     
    
       [0017]    It will be appreciated that the drawings are intended for the purpose of explanation and are not to scale.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Referring to the drawings, FIG. 1 shows a general perspective view of a vacuum holddown device  10  in accordance with the present invention. A generally U-shaped channel member  11  defines a main vacuum chamber  12  of the device. In practice, one end of chamber  12  is closed and a high vacuum source is connected to the other end. A valve plate  20 , of sheet metal material, is slidably located in a guide groove  14 , FIG. 2, extending adjacent to the top of the walls of the U-shaped channel member  11 . A printzone platen  30  is secured to the top of the walls allowing the valve plate  20  to slide between the vacuum chamber  12  and the platen  30 . The upper surface of the platen  30  is provided with surface grooves serving to ensure equal pressure distribution. The platen  30  and valve plate  20  have matching patterns of through holes  32 ,  22  therein, and valve plate is slideable between a first position shown in FIG. 3, in which the holes  32 ,  22  are aligned and a second position shown in FIG. 4, in which the holes  32 ,  22  are not aligned. To assist explanation, both plate  20  and platen  30  are shown in full lines in FIGS. 3 and 4.  
         [0019]    Although component  20  is referred to as a vacuum plate, FIG. 2 shows that it has a peripheral wall  24  so that it resembles a shallow tray around a planar portion  26  of the component  20  defining a major surface thereof. The top surface of wall  24  slides directly against the underside of platen  30  so that a secondary vacuum chamber  34  is formed between the platen  30  and the planar portion  26  of the vacuum plate  20 . It will be noted that chamber  34  is shorter than printing platen  30 , i.e. one end part  24 ′ of wall  24  is located inwardly of the end of vacuum plate  20 . This permits sliding movement of the valve plate underneath the printing platen while keeping the vacuum chamber  34  enclosed.  
         [0020]    A gap is provided in wall  24  which is aligned with a recess or cut out  18  in the wall  16  of channel member  11 . A low vacuum source is connected to the vacuum chamber  34  by means of the thus formed passageway.  
         [0021]    Each of the holes  22  through the vacuum plate is surrounded by a peripheral wall  27 , of the same height as the wall  24 , typically 2 to 3 mm. For reasons of clarity, only one of the holes  22  is shown in FIG. 2. Accordingly when the holes  32 ,  22  are aligned, the top edges of the peripheral walls  27  sealingly engage around holes  32 , so that the main vacuum chamber  12  is directly connected to the upper surface of the platen  30 . Thus, in operation, in the first position of the valve plate  20 , FIG. 3, a high vacuum is applied through the holes  32  in the platen  30  to a print media thereon and a printing operation can occur.  
         [0022]    When holes  32 ,  22  are not aligned in the second position of the valve plate, FIG. 4, the holes  32  through platen  30  are instead in communication with secondary vacuum chamber  34 . Thus a low vacuum is applied to the holes  32  and a media advance operation can occur. The secondary chamber  34  is sealed from the main vacuum chamber, since the top edges of walls  27  engage parts of the platen  30  where there are no holes  32 .  
         [0023]    A drive means in the form of a solenoid (not shown) is provided for linearly sliding the valve plate  20  between the positions shown in FIGS. 3 and 4. Since valve plate  20  is relatively light, it has low inertia and the switching movement can occur within a few milliseconds. Thus, in operation, the valve plate pulsates or reciprocates at a high frequency between its end positions. Stop means (not shown) are provided for determining the end positions of the range of movement of the valve plate.  
         [0024]    An advantage of the above-described arrangement is that the vacuum chambers  12  and  34  are much smaller than those required in prior art devices. Thus less space is required and less power is required to maintain the vacuums.  
         [0025]    Various modifications may be made to the above-described arrangements. For example, it will be noted that wall  24  does not need to extend on all sides of plate  20 . It only needs to be provided to prevent leakage into secondary chamber  34 , and thus in particular at or adjacent the ends of the plate  20 .  
         [0026]    Other means may be provided to guide the linear movements of the valve plate  20 ; for example the walls of channel member  11  may have projections which engage in grooves in the edges of the valve plate.  
         [0027]    The top of the valve plate  20  may be closed by a second planar portion facing planar portion  26 , but leaving holes for the passageways formed by walls  27  in communication with holes  22 . This increases the tightness of the chamber  34  against leaks.  
         [0028]    Other drive means may be provided for the valve plate  20  instead of a solenoid, for example a quickly-reversible motor.  
         [0029]    The vacuum holddown device may be used in connection with any type of printer. It may also be employed in other types of hardcopy apparatus, in particular plotters, scanners, photocopies and facsimile machines.  
         [0030]    In a further modification, the low vacuum source can be omitted and the cut out  18  can be connected directly to the atmosphere (i.e. atmospheric pressure is applied to the underside of the print media as it advances).  
         [0031]    In a further modification, in which atmospheric pressure is also applied during print media advance movements, the vacuum plate  20  is replaced by a flat shim  20 ′, FIG. 5. Thus walls  24 ,  27  are omitted. Accordingly, when holes  32  and  22  are not aligned, the holes  32  are effectively sealed off. In this modification, the design of the grooves in the top surface of the platen  30  needs to be capable of applying sufficient vacuum to the media during printing, but allowing the underside of the print media to quickly attain atmospheric pressure for media advance. This inevitably involves a design compromise, so that the valve plate  20  of the embodiment of FIGS.  1  to  4  is preferred.  
         [0032]    Although the holes  22 ,  32  are shown as being of generally oval shape, they can have other shapes such as circular, square or rectangular.  
         [0033]    The vacuum plate  20  can be arranged to slide transversely of the platen instead of horizontally.  
         [0034]    What has been described and illustrated herein is a preferred embodiment of the invention along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognise that may variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated.