Patent Publication Number: US-2005122364-A1

Title: Inkjet printers

Description:
The invention relates to printing images on substrates using an ink jet printer.  
      In the use of Inkjet printers to print images onto paper or other substrates there is sometimes a problem with the ink nozzles blocking up with dust or debris. When this happens a single line is either not printed on the paper or the position of the line printed is displaced and often blurred. This leads to a defect in the printing. For normal inkjet printing of ink on paper a single line defect is not normally seen by the eye. The blockages can be cleared by ejecting large amounts of ink through the nozzle and washing the blockage away. This is normally called flushing and the ink is dumped into a waste tank. It is typical for inkjet printers to do this flushing periodically as either a rectifying or preventative measure.  
      When, however, the object bring printed is not tolerant to a missing line, for instance the printing of UV resist to define a UV mask for a printing plate, then periodically rectifying a blocked nozzle is not acceptable as a whole plate is written off when a single defect occurs. One solution to the problem is to flush the nozzles at the end of each pass of the plate. It is not normally the case that a rapid blocking of the nozzles occurs where one second a nozzle is clear and the next it is completely blocked or greatly obscured so such a strategy could be adopted. Unfortunately this is wasteful in both time and ink. The time is wasted in the time taken to flush the nozzles and clearly the ink flushed is wasted.  
      In accordance with the present invention, a method of printing an image on a substrate comprises controlling an inkjet printer head having a plurality of nozzles to print the image on a substrate in a series of swaths; periodically checking for full or partial blockages in a nozzle of the inkjet printer head by causing each nozzle to print the same predetermined pattern on a substrate and analysing the patterns to determine any differences between them and to identify a blocked or partially blocked nozzle if its printed pattern differs by more than a predetermined amount from that of the other nozzle(s), wherein the checking step is carried out following each swath of the inkjet printer head across the substrate.  
      The invention can detect when nozzles are completely blocked and also when they are partially blocked or beginning to block. In the case of a partial blockage the direction of the ink droplet is altered giving the droplet a sideways motion (along or transverse to the scan direction) and sometimes the size of drop deposited changes. The predetermined pattern or “feducial marks” are arranged and measured so that small changes in direction of fire of each nozzle can be detected and the size of drop from each nozzle can be detected. Once this discrepancy in nozzle performance has been detected the nozzle can then be flushed at the earliest opportunity, e.g. prior to the next swath, before the change in performance becomes large enough to significantly affect the finished pattern on the substrate being printed on.  
      The predetermined pattern or “feducial marks” can include a sequence of dots and in order to simplify subsequent processing, these dots are preferably equally spaced apart. In that event, the analysis step can comprise analysing the spaces between the dots in the different patterns.  
      Preferably, the predetermined pattern includes a continuous line portion.  
      Although the analysis method can be carried out manually, preferably the method further comprises, prior to the analysing step, scanning the patterns with a detector to generate an electronic representation of the patterns. In this way, the analysis can be carried out electronically and automatically.  
      The electronic analysis can be achieved using a variety of techniques including a Fourier Transform or wavelet analysis.  
      Conveniently, at least some of the nozzles print the predetermined patterns alongside one another for ease of comparison although one or more of the nozzles could print its pattern spaced longitudinally from another pattern. In either event, this enables differences between the patterns printed by different nozzles to be easily compared, thus revealing any discrepancies in one of the patterns due to a faulty nozzle.  
      The analysis step may comprise detecting the absence of part of the predetermined pattern. For example, the absence of a dot in the pattern could be detected.  
      In some cases, the nozzles are controlled to print their respective patterns at predetermined positions relative to a datum, the analysis step comprising detecting an offset between a printed pattern and its expected position with respect to its datum.  
      In some cases, the predetermined pattern and printed image may be printed using a marker ink in order to increase contrast between the predetermined pattern and the underlying substrate and increase signal to noise ratio. 
    
    
      An example of a method according to the invention will now be described with reference to the accompanying drawings, in which:  
       FIGS. 1, 2  and  3  illustrate three different examples of printed predetermined patterns or feducial marks;  
       FIG. 4  is a plan of a substrate showing the feducial marks on an enlarged scale; and,  
       FIG. 5  is a schematic diagram of apparatus for carrying out the method. 
    
    
      In this example, an inkjet printer, shown schematically at  20  in  FIG. 5 , has three nozzles  21 - 23  in its head  10  connected to a controller  24 . In practice, many more than three nozzles would be provided. The nozzles  21 - 23  are controlled to print an image  12  (FIG.  4 ) onto a substrate  11  such as a printing plate or PCB along substantially parallel tracks as the head scans across the substrate.  
      In some cases, the printing head  10  is scanned (moved) across the substrate  11  while in other examples the head  10  is stationary and the substrate  11  is moved.  
      As mentioned above, the substrate  11  can include a variety of materials but the invention is particularly suitable for use with printing a UV mask onto a printing plate.  
      The printer  20  prints an image  12  in a series of adjacent swathes or scans across the substrate  11 . (Only two swathes  30 A,  30 B are shown in  FIG. 4 .) At the end of each swath, defined by a single scan of the head  10  across the substrate  11 , the inkjet printer is instructed to cause each nozzle  21 - 23  to print a predetermined pattern or feducial marks which, in this example, comprises a solid line followed by eight equally spaced dots.  FIG. 4  illustrates schematically one set of such patterns  13  corresponding to swath  30 A and another set at  14  corresponding to swath  30 B. These patterns are shown at an enlarged scale relative to the image  12 . Although the patterns  13 ,  14  are printed on the substrate  11  in this example, they could be printed on a separate substrate and possibly in a different colour from the image  12 .  
      An example of the pattern  13  is shown in more detail in  FIG. 1  where the dimension “y” corresponds to the scanning direction and the direction “x” corresponds to a direction perpendicular to the scanning direction. In this example, the first nozzle  21  prints feducial marks  1  comprising a solid line  2  and a set of eight dots  3  relative to an unprinted datum  40 . The second and third nozzles  22 ,  23  are controlled to print the same pattern indicated at  1 ′,  1 ″ respectively relative to an unprinted datum  41  and the datum  40  respectively.  
      A variety of different types of analysis can be carried out. In a typical example, a Fourier Transform technique is used where the phase of the fundamental of the series of single dots is determined. Other techniques could be used such as measuring the centre of gravity of each single dot, subtracting the position of each of the dots from the corresponding dot of the same number in the series of another nozzle. Then averaging these differences so that each dot has an average distance between itself and either every other dot or a chosen dot. A dot would then show an error in the x and/or y direction if this distance was different from other corresponding dot to dot differences, thus indicating a possible (partial) blockage of the corresponding nozzle. The line part  2  of the feducial marks can be used also to determine any variation in the x direction.  
      Further processing techniques which can be used include wavelet analysis and Short Term Fourier Transforms.  
       FIG. 2  illustrates an example in which the nozzle  23  is partially blocked so that it has printed the feducial marks out of alignment, as seen at  4 , with the feducial marks  1 ,  1 ′ printed by nozzles  21  and  22  respectively.  
      In the  FIG. 2  example, a Fourier analysis would operate as follows. Each line of dots is segmented into separate segments so each segment contains a single nozzle output. These segments are then individually Fourier transformed with a 2D discreet Fourier transform, or 2D discreet fast Fourier transform.  
      The peak associated with the recurring dot pattern is reviewed and its phase in the y direction determined (x direction is measured using the line). The phase for odd numbered segments should be the same and the phase for even numbered segments should be the same and at a known relationship to the odd numbered segments. Any segment with an unexpected phase would be then considered to ejecting ink at an angle and in need of flushing.  
      An alternative would be to segment the images as before and to sum up the columns of pixels and perform a 1D Fourier analysis looking at the phase of the peak in the same way as before.  
      Wavelet analysis is the same as Fourier analysis but using wavelets instead of sine waves as the basis. In this case, a wavelet is picked that looks like an ink dot. In this way, the analysis can be made to be more sensitive than the standard Fourier transform technique because it is tuned to detecting dots not sine waves.  
      Short term Fourier techniques are similar to the normal Fourier analysis but instead of sampling all the data at once from one segment, the segment is further divided into shorter segments and each segment analysed. This is a technique used in speech analysis and is sensitive to short term changes in frequency and phase. Thus, the noise in dot position along a single line could be analysed. This would show nozzles with intermittent errors.  
      It should also be noted that it is not essential to include a line segment in the feducial mark pattern. As can be seen in  FIG. 3 , each pattern in this example comprises a set of dots only with the dots  5  from nozzle  22  being out of alignment with the dots  26 ,  27  from nozzles  21  and  23  respectively. It should also be noted in this case that all the nozzles have been controlled to print their feducial marks simultaneously.  
      The feducial marks will usually be printed in the same colour as used for printing the image on the substrate  11 , such as a UV mask. In some cases, the colour of the substrate  11  is such that it is difficult to discern the feducial marks. In these cases, the printing ink may include a marker such as a dye which increases contrast with the substrate colour.  
      Although the feducial marks could be analysed manually, preferably, the printed, feducial marks are scanned by a scanning device such as a line scan CCD detector  25 , or a CMOS detector. This enables the marks to be analysed by computer. Thus, signals from the CCD  25  representing the appearance of the patterns  13 ,  14  are converted to digital form in an A/D converter  26  and then forwarded to a microprocessor  27 . The microprocessor  27  analyses the marks within each pattern  13 ,  14  as described above and then if appropriate, causes the controller  24  to unblock one or more of the nozzles  21 - 23  before commencement of the next printing swath  30 .