Abstract:
An ink jet printer including: a sheet support plate having suction holes formed in a sheet support surface thereof; a sheet advance mechanism adapted to advance a sheet step-wise in a sheet advance direction over the sheet support surface; and an imaging system adapted to form an image by depositing liquid ink on the sheet, the ink being allowed to dry-out while the sheet is advanced over the sheet support plate, wherein the suction holes are arranged such that their influence on the ink drying process is essentially uniform over the whole area of the image.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an ink jet printer including: 
        a sheet support plate having suction holes formed in a sheet support surface thereof;     a sheet advance mechanism adapted to advance a sheet step-wise in a sheet advance direction over the sheet support surface; and     an imaging system adapted to form an image by depositing liquid ink on the sheet, the ink being allowed to dry-out while the sheet is advanced over the sheet support plate.        
 
         [0005]     In an ink jet printer, a sheet support plate containing suction holes is frequently used for supporting an image receiving sheet and holding the sheet flat on the sheet support plate. By applying a subatmospheric pressure via the suction holes to the bottom side of the sheet, the sheet may be drawn against the top surface of the sheet support plate. The suction holes should be evenly distributed over the surface area of the sheet support plate, so that an essentially uniform suction is applied to the sheet. On the other hand, in view of manufacturing considerations, the number of suction holes should not be too large.  
         [0006]     EP-A 0 409596 discloses a printer wherein the suction holes are arranged in a regular, non-slanting pattern, with the suction holes of neighboring rows being off-set from one another by one-half pitch.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to improve the image quality of images printed with an ink jet printer of the type indicated above.  
         [0008]     According to the present invention, the suction holes are arranged such that their influence on the ink drying process is essentially uniform over the whole area of the image.  
         [0009]     The present invention is based on the observation that the suction holes may retard or accelerate the ink drying process, and that this may have a visible effect on the printed image.  
         [0010]     For example, in a hot-melt ink jet printer, a sheet, e. g. a sheet of paper, is advanced over the sheet support plate while the image is being printed. At room temperature, the hot-melt ink is solid, and it is therefore necessary that the ink is heated in the printer above its melting point, before it can be jetted onto the paper. In order to obtain a suitable and constant amount of spreading of the ink, the temperature of the sheet support plate and hence the temperature of the paper is controlled such that the ink cools down at an appropriate rate. However, at the positions of the suction holes, the properties of the sheet support plate, especially its heat conduction and its heat capacity, differ from the surrounding parts of the plate. Thereby, heat dissipation is reduced at sheet portions that are positioned above the suction holes. This has negative effects on the scattering of light, the scratch resistance and the glossiness of the printed image. Especially the difference in glossiness is visible in the printed image.  
         [0011]     Similarly, in an ink jet printer in which the ink drying process involves evaporation of a solvent, the flow of air drawn-in through the suction holes may give rise to an accelerated evaporation of the solvent.  
         [0012]     If the suction holes are arranged in a regular pattern of rows and columns, with the columns extending in the sheet advance direction, then certain areas of the printed image will repeatedly pass over the suction holes of a column, whereas the intervening portions of the image will never be affected by the suction holes. As a result, differences in glossiness and the like are visible as a regular pattern on the printed image. For this reason, the invention proposes to arrange the suction holes such that essentially all portions of the image will pass over the suction holes an approximately equal number of times and will thus be affected by the suction holes in essentially a like manner.  
         [0013]     A suitable arrangement would, for example, be a random distribution of the suction holes over the area of the sheet support plate. However, in terms of production efficiency, a regular pattern of suction holes is preferred. Thus, the object of the present invention can be achieved by a slanting pattern in which the columns of suction holes form an angle with respect to the sheet advance direction. As a result, an image portion that has passed over one suction hole will not pass over the next or the next few suction holes of the same column, because these suction holes are laterally offset. After a certain number of advance steps, of course, the image portion may pass over a suction hole of a neighboring column. However, the pattern may be arranged such that, until then, the ink has already dried-out to a sufficient extent and will no longer be affected by the presence of the suction hole.  
         [0014]     The pattern of suction holes may be adapted to the length of the sheet advance steps such that for any two suction holes that are aligned in the sheet advance direction, the mutual distance of the suction holes in the sheet advance direction is a non-integral multiple of the step length. As a result, when an image portion has rested over a suction hole for a certain time in the interval between two sheet advance steps, this image area will not come to rest over the next suction hole after the next advance step, but will be offset from that hole in the sheet advance direction. Thus, as long as the ink has not dried sufficiently, any point of the printed image will either rest over a suction hole only once or will never rest over a suction hole at all, but no point of the image will rest over a suction hole several times, and this may assure a sufficient uniformity of the ink drying process.  
         [0015]     The suction holes may be arranged in rows, preferably in equidistant rows, that are, for example, perpendicular to the sheet transport direction. This is particularly useful in a hot-melt ink jet printer in which the sheet support plate is temperature-controlled by means of a fluid circulated therethrough. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Preferred embodiments of the present invention will now be described in conjunction with the drawings in which:  
         [0017]      FIG. 1  is a schematic perspective view of a hot-melt ink jet printer;  
         [0018]      FIG. 2  is a partial cross section of a sheet support plate in the printer shown in  FIG. 1 ; and  
         [0019]     FIGS.  3  to  5  are a partial top views of a sheet that is advanced over suction holes of the sheet support plate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     As is shown in  FIG. 1 , a hot-melt ink jet printer comprises a platen  10  which is intermittently driven to rotate in order to advance a sheet  12 , e. g. a sheet of paper, in a direction indicated by an arrow A over the top surface of a sheet support plate  14 . A number of transport rollers  16  are rotatably supported in a cover plate  18  and form a transport nip with the platen  10 , so that the sheet  12 , which is supplied from a reel (not shown) via a guide plate  20 , is paid out through a gap formed between an edge of the cover plate  18  and the surface of the sheet support plate  14 .  
         [0021]     A carriage  22  which includes a number of ink jet printheads (not shown) is mounted above the sheet support plate  14  so as to reciprocate in the direction of arrows B across the sheet  12 . In each pass of the carriage  22 , a number of pixel lines are printed on the sheet  12  by means of the printheads which eject droplets of hot melt ink onto the sheet in accordance with image information supplied to the printheads. For the sake of simplicity, guide and drive means for the carriage  22 , ink supply lines and data supply lines for the printheads, and the like, have not been shown in the drawing.  
         [0022]     The top surface of the sheet support plate  14  has a regular pattern of suction holes  24  which pass through the plate and open into a suction chamber  26  that is formed in the lower part of the plate  14 . The suction chamber is connected to a blower  28  which creates a subatmospheric pressure in the suction chamber, so that air is drawn-in through the suction holes  24 . As a result, the sheet  12  is drawn against the flat surface of the support plate  14  and is thereby held in a flat condition, especially in the area which is scanned by the carriage  22 , so that a uniform distance between the nozzles of the printheads and the surface of the sheet  12  is established over the whole width of the sheet, and a high print quality can be achieved.  
         [0023]     The droplets of molten ink that are jetted out from the nozzles of the printheads have a temperature of 100° C. or more and cool down and solidify after they are deposited on the sheet  12 . Thus, while the image is being printed, the heat of the ink must be dissipated with a sufficient rate that should be essentially uniform for the whole area of the sheet  12 . To dissipate the heat, the temperature of the sheet  12  is controlled via the sheet support plate  14  by means of a temperature control system  30  which circulates a temperature control fluid, preferably a liquid, through the plate  14 . The temperature control system includes a circulating system with tubes  32  that are connected to opposite ends of the plate  14 . One of the tubes passes through an expansion vessel  33  containing a gas buffer for absorbing temperature-dependent changes in the volume of the liquid. As will be readily understood, the temperature control system  30  includes heaters, temperature sensors, heat sinks, and the like for controlling the temperature of the fluid, as well as a pump or other displacement means for circulating the fluid through the interior of the sheet support plate  14 .  
         [0024]     The sheet support plate  14 , which has been shown in cross-section in  FIG. 2 , is made of a material, such as a metal, having a relatively high heat conductivity and also a relatively high heat capacity. A number of elongated cavities  34  are formed in the interior of the plate  14  so as to extend in parallel with one another and in parallel with the direction (B) of travel of the carriage  22  between opposite ends of the plate  14 , where they are connected to the tubes  32  through suitable manifolds. Each cavity  34  is delimited by a top wall  36 , a bottom wall  38  and two separating walls  40 . The top walls  36 , together, define the top surface  42  of the plate  14  which is machined to be perfectly flat. Between each pair of two separating walls  40 , which delimit to adjacent cavities  34 , a hollow space  44  is formed, through which the suction holes  24  pass through into the suction chamber  26 . Since the suction holes  24  are arranged in a slightly slanting pattern, as is shown in  FIGS. 1 and 3 , only one of the suction holes  24  can be seen in section in  FIG. 2 .  
         [0025]      FIG. 3  shows the pattern in which the suction holes  24  are arranged in the surface of the sheet support plate  14 . These suction holes form a regular pattern with rows R and columns C. The rows R extend in parallel with the direction B and hence also in parallel with the cavities  34  formed in the interior of the sheet support plate  14  ( FIG. 2 ). However, the columns C are inclined relative to the sheet advance direction A.  
         [0026]     In the example shown, the pattern of suction holes  24  repeats itself every five rows, so that, for example, the suction hole  24 - 1  in the first row is aligned in the sheet advance direction A with the suction hole  25 - 5  in the fifth row.  
         [0027]     Also shown in  FIG. 3  is the leading edge of the sheet  12  which has been advanced in the direction A and is now held stationary on the sheet support plate while the carriage  22  ( FIG. 1 ) travels across the sheet in order to print another swath of image pixels. The hot-melt ink that has been deposited on the sheet  12  is cooled through contact with the sheet support plate  14  with an appropriate cooling rate. However, in the area of the suction holes  24  of the first row, the cooling rate is reduced, because, there, the sheet is not in contact with the metal plate  14  which has a high heat conductivity.  
         [0028]     When the carriage  22  has completed its travel, the sheet  12  is advanced again by one step to the position shown in  FIG. 4  and is then again held stationary for printing the next swath of the image. The length S of the sheet advance step has been indicated in  FIG. 4 . Dark spots  24   a  in  FIG. 4  symbolize those areas of the sheet  12  which had covered the suction holes  24  of the first row in the condition shown in  FIG. 3  and for which, consequently, the ink has not been cooled to the same extent as for the rest of the sheet. Since the cooling rate of the ink will also be somewhat decreased at the edges of the suction holes  24 , the diameter of the spots  24   a  may, in practice, be slightly larger than the diameter of the suction holes  24 .  
         [0029]     Thanks to the inclination of the columns C, the spots  24   a  in  FIG. 4  are laterally offset from the suction holes  24  of the second row and do not overlap with these suction holes in the sheet advance direction A.  
         [0030]     When the sheet  12  rests in the position shown in  FIG. 4 , similar spots, for which the cooling rate is decreased, will also be formed on and around the suction holes  24  of the second row. However, since the cooling rate decays exponentially with time, the spots caused by the suction holes of the second row will be somewhat less pronounced.  
         [0031]     In the further course of the print process, the sheet  12  is advanced step-wise with the step width S, and in each step, the suction holes of the first row will cause another row of spots  24   a , and the suction holes in the second and subsequent rows will cause somewhat fainter spots.  
         [0032]      FIG. 5  shows the condition reached when the sheet  12  has been advanced by  5  steps and indicates the spots  24   a  caused by the suction holes of the first row and also the spots  24   b  caused by the suction holes in the second, third and fourth row. It can be seen in the lower part of  FIG. 5  that these spots  24   a ,  24   b  are non-overlapping and are practically uniformly distributed over the surface of the sheet  12 , so that the influence of the suction holes on the cooling rate of the ink is practically uniform over the area of the sheet  12  and will hardly produce any visible effect.  
         [0033]     As is further shown on the left margin of the sheet  12  in  FIG. 5 , the suction hole  24 - 5  in the fifth row does not overlap with the spot  24   a - 1  that has been created by the suction hole  24 - 1  in the state shown in  FIG. 3 . The spot  24   a - 1  is offset from the suction hole  24 - 5  in sheet advance direction A. This is due to the fact that the distance D between the suction holes  24 - 1  and  24 - 5  in  FIG. 3  is a non-integral multiple of the step width S. Thus, although the suction holes  24 - 1  and  24 - 5  are aligned in sheet advance direction A, the spot  24   a - 1  does not come to rest on the suction hole  24 - 5 , so that the cooling process for the ink in this spot will not be retarded once again by the suction hole  24 - 5 .  
         [0034]     In a practical embodiment of the printer, the step width S may be variable and will change when the printer is switched into another print mode, e. g. from a single-pass mode into a two-pass mode. However, since the number of possible step widths S is limited, and the step widths are known in advance, it is possible to select the distance D such that the condition that D is a non-integral multiple of S is fulfilled for all possible step widths.  
         [0035]     In a modified embodiment, especially in an embodiment in which the columns C are not inclined relative to the sheet advance direction A, the distance D (which would then be the distance between to adjacent rows R) may be smaller than the step width S. In this case, the additional condition that S is a non-integral multiple of D should be fulfilled in order to avoid overlapping spots. For example, if the step width S is 17 †mm, and the diameter of the suction holes  24  is 1.0 mm, then the distance D may be selected as 12 mm. Then, it would only be after  12  sheet advance steps that the spot created by the first suction hole would overlap with another suction hole for the first time. During these  12  steps, the ink has had time enough to cool down, so that it would no longer be affected by the second suction hole.  
         [0036]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.