Abstract:
A method for printing and correction of a printed image defects. The method includes inspection of the printed image and detection of segments of printed image that could contain printed defects. Acquisition of printed image and comparison of the acquired image to an error free image being a source of the printed image facilitates detection and correction of the printed image defects.

Description:
TECHNOLOGY FIELD 
       [0001]    The present method and apparatus relate to inkjet printing. 
       BACKGROUND 
       [0002]    Inkjet printing is a non-impact printing technology where a stream of ink droplets is ejected from an inkjet printhead. Usually, the printhead reciprocates over a printing substrate that also can concurrently or intermittently move in a direction perpendicular to the direction in which the printhead moves. The ink droplets ejected towards the substrate form an image on the substrate. 
         [0003]    Since inkjet printing is a non-impact printing technology it is used to print on a variety of substrates such as paper, plastics, stone, glass and others. The substrates may come in a variety of sizes for example, inkjet printers for home and small office use print on A4 papers where some industrial printers print on substrates of up to 6000×5000 mm. 
         [0004]    In order to enable the handling and display of an image printed on the substrate, in some applications the wet image is cured or dried concurrently with the printing. In other applications, for example, printing on ceramics and glass the printed ink may be fired at a high temperature such that pigments contained in the ink become integral with the substrate. 
         [0005]    Like any other technology, inkjet printing is not an error free process. Sometimes excessive ink reaches the substrate and agglomerates into an ink puddle or forms ink drippings, sometimes errors in movement systems and/or inaccurate substrate handling may cause printed image defects. The defects could be such that the printed image becomes unsalable and there it becomes necessary to produce another copy of the same image on the same substrate. 
         [0006]    The cost of printing an additional image depends on the size of the image and the size of the substrate, printing time and printer cost, ink and substrate cost. For example, rigid substrates such as glass and polished stones are far more expensive than paper or plastic substrates. In some cases reprinting an image may nullify all of the printing shop on particular job profit and even cause a loss. 
       BRIEF SUMMARY 
       [0007]    Presented is a method for printing an image on a substrate, acquiring printed image defects and local correction of the printed image defects prior to curing the printed image. The method includes inspection of the printed image and detection and acquisition of segments of the printed image that may contain printed defects. 
         [0008]    Inspection of the printed image may further include acquisition of a printed image or printed image segment and comparison of the acquired image or segment to an error free source image or segment to facilitate detection and recognition of printed image defects. 
         [0009]    Defects in the printed image may be corrected by erasing a detected/recognized segment of an image containing the defect and reprinting the erased segment of the image. 
         [0010]    Also presented is an example of an inkjet printer in which the present method could be implemented. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    Examples of the method and apparatus will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a simplified plan view of an example of a known inkjet printer; 
           [0013]      FIG. 2  is a simplified plan view of an example of an inkjet printer in which the present method could be implemented; 
           [0014]      FIG. 3  is an example of an ultrasonic ink image erasing device; 
           [0015]      FIG. 4  is an example of an ink image ablating device; and 
           [0016]      FIG. 5  is an example of a printed image correction process. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a simplified plan view of an example of an existing inkjet printer. Inkjet printer  100  includes a print substrate support  104  and a bridge  108 . Print substrate  116  could be a glass plate, a polished marble or granite plate, a metal plate, and a plate made of other materials. A carriage  126  with a printhead  112 , which could be an assembly of a plurality of individual printhead modules that could be such modules as Galaxy PH 256/30 printheads or similar, commercially available from FUJIFILM Dimatix, Inc., NH 03766 U.S.A. The particular printheads have 256 piezoelectric ink dispensing elements and the associated mechanical and electrical components for dispensing droplets of ink onto a print substrate  116 . Carriage  126  with printhead  112  reciprocates (back and forth movement), as shown by arrow  120  over print substrate  116  mounted or placed on printing substrate support  104 . Print substrate support  104  advances print substrate  116  past printhead  112  as shown by arrow  122 . Print substrate support  104  could advance substrate  116  past printhead  112  continuously or incrementally, stopping as each swath is printed and then advancing substrate support  104  with substrate  116  for printing the next swath. 
         [0018]    Adjacent to printhead  112  are mounted ink drying or solidifying energy sources  124 . Drying of ink is usually accomplished by heating air and directing a stream of hot air onto the printed wet ink. Hot air evaporates the fluid component of the ink faster than other methods. In some examples, ink is a radiation curable ink and ink drying energy sources  124  could be replaced by sources of ink curing radiation, for example, ultraviolet (UV) radiation sources. Drying or curing solidifies wet ink and facilitates the later handling of substrate  116  having an image  152  printed onto it. 
         [0019]    Printhead  112  could include a large number of individual printhead modules and in one example it could be a stationary printhead that spans the width of print substrate  116  and incorporates thousands of piezoelectric ink dispensing elements. In such printer architecture, print substrate support  104  could be operative to displace substrate  116  in both printing directions  120  and  122 . Other printhead configurations and ink dispensing elements are possible. 
         [0020]    A control computer or controller  128  in  FIG. 1  represents generally a processor  132  and associated memory  136 , and the electronic circuitry and components needed to control the operation of printer  100 . Control computer  128  includes a keyboard  140  through which different instructions and commands could be entered, and a display  142  that could display the commands as well as images to be printed. Control computer  128  further includes a Raster Image Processor (RIP)  144 . RIP  144  receives the digital image to be printed and processes that image into printer control information and printed image data. Control computer  128  controls the movement of carriage  126  and print substrate support  104 . Control computer  128  is electrically connected to printhead  112  to energize the piezoelectric ink dispensing elements of the printhead modules to dispense ink droplets on to substrate  108 . By coordinating the relative position of printhead  112  and substrate  108  with dispensing ink droplets ejection time, control computer  128  produces the desired image on substrate  108  according to the digital print image data. 
         [0021]    A servo system  148  and a pair of encoders (not shown) associated with printhead  112  and substrate support  104  movement directions are providing a reading of printhead  112  and of the image on substrate  108  coordinates. The encoders could provide accurate coordinates of any point or area within the printed image  116 . These coordinates could be used for synchronization of the relative position of printhead  112  and substrate  116  with dispensing ink droplets ejection time. The encoders could be of any type, such as linear encoders or rotary encoders as well as magnetic strip encoders. 
         [0022]    Like any other technology, inkjet printing is not an error free process. Sometimes excessive ink reaches the substrate and agglomerates into an ink puddle which may result in ink smears or drippings. Inaccurate substrate handling could cause printed image defects. The printed image  152  defects  156 , which usually affect a segment of a printed image, could be also caused by excessive ink droplets spontaneously ejected by printhead  112 , ink mist accumulated on the nozzle plate and dropped onto the substrate, ink agglomerations caused by ink bleeding and other defects. 
         [0023]      FIG. 2  is a simplified plan view of an example of an inkjet printer in which the present method could be implemented Inkjet printer  200  allows for printed image defects  156  in situ correction. In one example, carriage of inkjet printer  200  also includes a device  204  facilitating inspection of the printed image. Such a device could be a CCD camera or a video camera. Control computer  128  governs operation of device  204 . Device  204  instantly (on-line) captures the printed image or a segment of the printed image and provides to control computer  128  the captured image information or data. Control computer  128  could include a dedicated printed circuit board  216  or a program operative to check identicalness of at least a segment of a printed image that includes the defect to a segment of an error free digital image stored in memory  136  of control computer  128 . The identicalness between the images could be determined by comparing at least a segment of the printed image data to data of a segment of an error free digital image. Raster Image Processor (RIP)  144  could provide the data of a segment of an error free digital image and CCD camera  204  provides data or information of the captured segment of the printed image. CCD cameras with a large number of pixels such as 1280×960 to 3296×2472 are currently available from a number of vendors and any suitable camera could be employed to capture a segment of the printed image. The information or data on the captured segment of the printed image could be available on a bit map level and it could be compared with a similar level of information or data available from RIP  144 . 
         [0024]    An image erasing device  208  is also mounted on the carriage. Image erasing device  208  could be an ultrasonic scrapper, a magnetostrictive scrapper, a laser ablation device or combination thereof. Image erasing device  208  is operative to erase a segment of the printed image that includes the defect or the defect only by removing a solidified ink layer from substrate  116 . In one example image defect scrapping could be followed by gentle etching of the surface. Adjacent image erasing device  208  could be mounted a vacuum nozzle  212  connected to a source of vacuum (not shown). Vacuum nozzle  212  is operative to suck and remove from substrate  116  and surface of printed image  156  solidified (dry) ink particles that could be present following operation of the image defect erasing device  208 . 
         [0025]      FIG. 3  is an example of an ultrasonic ink image erasing device  208  such as an ultrasonic scrapper. A piezoceramic element  304  or an assembly of piezoceramic elements induces ultrasonic vibrations in a tip  308 . A cable  312  connects image erasing device  208  to a driver (not shown) that provides proper electric AC voltage between 10 VRMS to 100 VRMS to piezoceramic element  304  that converts the electrical energy provided by the driver into ultrasonic vibrations. The driver (not shown) could be located proximate to ink image erasing device  208  or in common with control computer  128  packaging. In some examples a balancing weight  316  reducing influence of tip  308  vibrations on the carriage could be a part of ultrasonic image erasing device  208 . A sharp edge  320  terminates tip  308 . 
         [0026]    In order to remove a segment of a printed image containing defect  156  ( FIG. 1 ), sharp edge  320  of tip  308  is applied to a segment of solidified ink layer  324  containing the printed image defect  156 . Voltage is supplied to piezoceramic element  304 , which in turn vibrates tip  308 . Tip  308  vibrations help to destruct and erase or remove the solidified ink particles  324  from substrate  116  surface. In course or image defects removal or erasure, sharp edge  320  of tip  308  could wear. Tip  308  could be a disposable tip, replaced when it is felt that tip edge does not perform proper the solidified ink layer  324  erasure process. 
         [0027]    Vacuum nozzle  212  ( FIG. 2 ) becomes operative to suck and remove from substrate  116  and surface of printed image  152  ink particles that could be present following the operation of the image erasing device  208 . Cable  312  could also provide a connection to a source of vacuum. 
         [0028]      FIG. 4  is an example of a solidified ink image ablating device. Device  400  includes a laser diode  404  or a laser diode array, a lens  408  focusing laser radiation emitted by laser diode  404  into a spot  412 . A cable  416  connects laser diode  404  to a diode power supply (not shown) and to control computer  128 . In order to remove a printed image defect  156  ( FIG. 1 ), focused laser radiation spot  412  is applied to the printed image defect  156 . Laser radiation energy ablates the solidified ink layer  324  from the surface of substrate  116 . Vacuum nozzle  212  ( FIG. 2 ) becomes operative to suck and remove from the substrate fumes and debris generated by the ablation process. 
         [0029]    In one example, inkjet printhead  112 , device  204  facilitating inspection of the printed image and image erasing device  208  are located on the carriage and rigidly fixed to it. The spatial relation or the relative location between them could be calibrated and stored in memory  136 . Based on the coordinates of printed image defect  156  ( FIG. 1 ) provided by the feedback loop, control computer  128  could position each of the devices in a proper position to perform a desired operation, which could be erasure of a segment of a printed image that does not correspond to the digital error free image. The digital error free image could be stored in memory  136 . 
         [0030]    In one example, a drive of device  204 , independent from the carriage drive, that facilitates inspection of the printed image and image erasing device  208  could be implemented. The movements of the drive of device  204  facilitating inspection of the printed image and image erasing device  208  could be synchronized and each of them could be located in a proper position to perform a desired operation. Control computer  128  or a servo system could control synchronization of the movement of these devices. 
         [0031]      FIG. 5  is an example of a printed image correction process. For printing, substrate  116  on which image  152  is to be printed is placed on print substrate support  104 . Printer  200  ( FIG. 2 ) prints image  152  (block  500 ). Drying or curing device  124  dries or cures the printed image and solidifies the printed ink. Concurrently with image printing, camera  204  scans and captures a segment of a recently printed and solidified image (block  504 ) although a delay of a desired number of image raster lines could exist. In one example, camera  204  communicates the captured information or data to display  142 . Display  142  displays the image and coordinates of the image provided by the encoders and servo system. Printer operator visually inspects the images displayed on display  142  and if a printed image defect is detected or identified, the operator uses keyboard  140  to record coordinates of the defect. Alternatively, the operator could mark by a cursor the segment of image containing the defect. Coordinates of the defect could include at least coordinates of a corner of a segment of image where the defect resides and the size of the defect. 
         [0032]    In one example, camera  204  communicates the captured information or data to control computer  128  which could operate a program or a dedicated printed circuit board  216  ( FIG. 2 ) to check identicalness of at least a segment of a printed image to a segment of an error free image to be printed and stored in memory  136  of computer  128 . Raster Image Processor (RIP)  144  may provide the data of a segment of an error free image to be printed. Absence of identicalness between the compared images of a segment of a printed image to a segment of an error free image indicates on presence of a printed image defect (block  508 ). 
         [0033]    Practically, the printing could continue until a complete image  152  is printed and additional printed image defects, if such will exist, will be detected. Alternatively, printing of image  152  could be discontinued. The spatial relation between the print head  112 , camera  204 , and image erasing device  208  has been determined apriori and the control computer  128  locates image erasing device  208  at one of coordinates defining the location of an image segment containing printed image defect  156 . In one example the coordinates could include image defect description. Image erasing device  208  becomes operative and erases the segment of the image to be corrected (the segment of the image containing the printed image defect) (block  512 ). Vacuum nozzle  212  becomes operative and removes solidified and erased ink particles and other debris of the image erasing process. 
         [0034]    When printer operator visually detects a printed image defect he or she can manually (by using keyboard  140  to key in appropriate coordinates) locate the image erasing device  208  at one of coordinates determining the location of printed image defect. Image erasing device  208  becomes operative and erases the printed image defect (block  512 ). Vacuum nozzle  212  becomes operative and removes ink particles and debris of the image erasing process. 
         [0035]    Following erasure of the printed image defect  156 , control computer  128  resumes the printing process reprinting the erased segment or segments of the printed image  152  and the remaining segments of the image. Once the image is recognized to be defect-free the printed image may be cured. 
         [0036]    The printing system and the method disclosed facilitate printed image defects correction saving significant costs associated with potential waste of printing time, substrate and ink cost. The throughput of the printing system is increased and system usage improved. 
         [0037]    Additionally, it may be appreciated that the printing system and method disclosed are not limited to printed image defects correction only and may be also employed to implement desired changes in already printed and cured images. 
         [0038]    In this case, an original source digital image may be replaced with a digital source image including desired changes. The desired changes may be communicated to a control computer via a feedback loop, which in turn may initiate the erasing and reprinting process as explained hereinabove. 
         [0039]    It will be readily appreciated that the present system and method are not limited by the current examples and could be applied to other applications as well. Various modifications and variations can be made in practicing the present method without departing from the spirit or scope of the apparatus and method described. Rather the scope of the present system and method are defined by the scope of the claims.