Patent Publication Number: US-6209999-B1

Title: Printing apparatus with humidity controlled receiver tray

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for providing high quality ink images on a receiver. 
     BACKGROUND OF THE INVENTION 
     Ink jet printing has become a prominent contender in the digital output arena because of its non-impact, low-noise characteristics, and its compatibility with plain paper. Ink jet printing avoids the complications of toner transfers and fixing as in electrophotography, and the pressure contact at the printing interface as in thermal resistive printing technologies. Ink jet printing mechanisms include continuous ink jet or drop-on-demand ink jet. U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Piezoelectric ink jet printers can also utilize piezoelectric crystals in push mode, shear mode, and squeeze mode. EP 827 833 A2 and WO 98/08687 disclose a piezoelectric ink jet print head apparatus with reduced crosstalk between channels, improved ink protection, and capability of ejecting variable ink drop size. 
     U.S. Pat. No. 4,723,129, issued to Endo et al, discloses an electrothermal drop-on-demand ink jet printer which applies a power pulse to an electrothermal heater which is in thermal contact with water based ink in a nozzle. The heat from the electrothermal heater produces vapor bubble in the ink, which causes an ink drop to be ejected from a small aperture along the edge of the heater substrate. This technology is known as Bubblejet™ (trademark of Canon K.K. of Japan). 
     One problem related to ink jet printing is related to insufficient drying of the ink of the ink droplets that are placed on the ink receiver during or after printing. In the field of ink jet printing, the term “drying” typically refers to drying through evaporation as well as drying by absorption of ink fluid into the receiver. For color and photographic printing, it is often desirable to coat an ink absorption layer on the receiver to assist the ink absorption. When ink drops are not dried fast enough, several image artifacts can occur. One type of image artifact is called coalescence, that is, wet ink drops on the receiver paddled together and causes a glossiness change in the area of the ink paddle. The insufficient drying can also cause wet ink of one color on the receiver to diffuse into the ink area of a different color, which produces a “color bleeding” image artifact. 
     Another problem related to the above is that the image quality and other ink jet printing performance are often sensitive to the environment variation. One cause for this problem is that the relative humidity in the environment affects the amount of the moisture stored in the receiver. With high moisture content, the ability of the ink absorption by the receiver is reduced. The drying rate of ink drops on receiver is decreased. A receiver with high moisture content can also affect how accurately the receiver can be transported. The receiver driving rollers can slip at the interface between the roller and the receiver surface. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a printing apparatus having improved image quality. 
     A further object of this invention is to provide a printing apparatus that provides robust performance in different ambient environment. 
     An additional object of the present invention is to improve the drying of ink on the receiver so that printing productivity is increased. 
     These objects are achieved by apparatus for forming an image on a receiver in response to a digital image, comprising: 
     a) print head means responsive to the digital image for transferring colorant to the receiver; 
     b) receiver supply device for storing the receiver(s) in a humidity controlled environment; and, 
     c) receiver transport mechanism for moving the receiver to a position where the print heads can transfer colorant to the receiver to form an image on the receiver. 
     ADVANTAGES 
     An advantage of this invention is that the variability of the printing performance is reduced relative to changes in the ambient environment. 
     Another advantage of this invention is that the drying of ink drops on receiver is increased so that printing productivity is increased. 
     A further advantage of this invention is that the probability for image artifacts is reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial front view of the ink jet printing apparatus in the present invention diagramatically showing the control electronics; 
     FIG. 2 is a partial side view of the ink jet printing apparatus of FIG. 1; 
     FIG. 3 is a detailed perspective of the receiver supply device of FIG. 2 that is in an enclosed state; 
     FIG. 4 is a detailed perspective of the receiver supply device without the cover and the door in the front; 
     FIG. 5 is a detailed perspective of the receiver supply device wherein the front door is in an open state; and, 
     FIG. 6 is a detailed view of the receiver supply device wherein a receiver is in the process of being transported toward the print heads. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is described with relation to a digital printing apparatus including the treatment the receiver for enhanced image structure. 
     Referring to FIG. 1, an ink jet printing apparatus  10  in accordance to the present invention includes a computer  20 , control electronics  25 , print head drive electronics  30 , ink jet print head  31 - 34  respectively for printing black ink (K), cyan ink (C), magenta ink (M), and yellow ink (Y), a plurality of ink reservoirs  40 - 43  for providing the color inks to the print heads  31 - 34 . As described herein, the term “colorant” will be understood to include inks such as pigmented or dye based materials. The colored inks can also include more than one concentration for each color (e.g. cyan and magenta), and green, orange, gold and other colors. The ink jet printing apparatus  10  further includes a receiver transport motor  70  and a roller  65  for transporting an ink receiver  80  across a platen  90  past the ink jet print heads  31 - 34 . The ink receiver  80  can be provided in the form of a web supplied from a receiver roller, or in cut sheets, as shown in FIGS. 1 and 2. The print heads  31 - 34  are fixed to a holder  45 . The print heads  31 - 34  and the holder  45  are translated by a belt  56 , a pulley mechanism  57 , and a print head translating motor  71  along the gliding rail  54  in the fast scan direction (as indicated in FIG.  1 ). The gliding rail  54  is supported by supports  55 . The print heads  31 - 34  can take various forms known in the art, for example, piezo-electric or thermal ink jet print head. The inks stored in the reservoirs  40 - 43  are supplied to the print heads  31 - 34 , respectively. The receiver  80  is supported by the platen  90 . 
     A digital image is applied to the computer  20 . Alternatively, the computer  20  can produce this digital image. The digital image is processed in the computer  20  by image processing algorithms such as tone scale conversion, color mapping and halftoning, all well known in the art. The computer  20  sends electrical signals according to the image data to the print head drive electronics  30  which controls the print heads  31 - 34  in a conventional manner. During each printing pass, the computer  20  controls the control electronics  25  to operate the receiver transport motor  70  and the print head translating motor  71 . Under the control of the computer, the receiver is positioned for a line of pixels of colorant to be formed and then the print head translating motor  71  moves the ink jet print heads  31 - 34  in a fast scan direction (shown in FIG. 1) and the print head drive electronics  30  operates the ink jet print heads  31 - 34  so that colorant provided from the reservoirs  40 - 43  is caused to be delivered as droplets to the receiving surface of the receiver  80 . More particularly, the print heads  31 - 34  transfer ink drops  100  to the receiver  80  during each printing pass, which forms ink spots  110  on the receiver  80 . After each printing pass, the receiver  80  is transported by the receiver transport motor  70  under the control of the control electronics  25  in the slow scan direction (FIG.  2 ). Each printed image is typically formed by a plurality of printing passes. 
     FIG. 2 shows a partial side view of the ink jet printing apparatus  10  in the present invention. The receiver  80  is placed on the platen  90 . The receiver  80  is transported by the receiver transport motor  70  in the slow scan direction. The receiver transport motor  70  includes a roller  65  under the control of control electronics  25 . For clarity, only a black ink jet print head is shown in FIG.  2 . After each movement along the slow scan direction, the ink jet print heads  31 - 34  ejects ink drops  100  which forms ink spots  110  on the ink receiver  80 , and thereby forming a swath of image pixels along the fast scan direction as described in relation to FIG. 1. A receiver supply device  200  is provided upstream to the receiver transport direction to supply the ink receivers  80  over the platen  90  toward the ink jet print heads  31 - 34 . The ink receiver  80  can be provided in the form of a web supplied from a receiver roller, or in cut sheets. For the web, a cutter (not shown) will be required to cut the ink images printed on the receiver to proper sizes before or after printing. 
     FIG. 3 is a detailed perspective of the receiver supply device  200  that is in an enclosed state. The receiver supply device  200  includes a housing  210 , a cover  220  and a door  230 . The door  230  is shown to be closed so that moisture outside of the receiver supply device  200  can be prevented from coming in contact with the receivers  80 . The receiver supply device  200  is shown in more detail without the cover  220  and the door  230  in the front in FIG.  3 . Referring to FIG. 4, a receiver tray  240  is contained in the housing  210 . A stack of ink receivers  80  is placed in the receiver tray  240 . The receivers are pushed up by a spring (not shown) so that the top sheet of the ink receivers  80  is in contact with a picker roller  250 . The picker roller  250  is mounted on a shaft  260  that is driven by a motor  270 . The motor  270  is controlled by control electronics  25 . 
     FIG. 5 is another detailed perspective of the receiver supply device  200 . The front door  230  is in an open state so that the receivers  80  can be picked by the piker roller  250  and transported out of the receiver supply device  200 . FIG. 6 shows an ink receiver  80  in the process of being picked and transported by the picker roller  250  toward the print heads. The computer  20  determines the timing and the sequence for picking the ink receiver  80 . The computer  20  then controls the control electronics  25  to activate the motor  270  which rotates the shaft  260  and thereby picker roller  250 . Although not shown in FIG. 5 for clarity, as it is well known in the art, the stack of ink receivers  80  are usually stopped by a wall in the front so that only one sheet of ink receiver  80  is picked up at each time. Sometimes, it is also preferred that corner nails can also be provided at the front corners of the receiver tray  240  for separating the top sheet of ink receiver  80  from the rest of ink receiver sheets in a stack. After the ink receiver is transported out of the receiver supply device  200 , the door  230  is closed so that the ink receivers  80  are sealed off from the ambient environment. 
     With the receiver supply device  200  sealed off from the ambient environment to slow down or prevent the inflow of moisture to the receivers, the moisture concentration can be further reduced by placing drying agent in the receiver supply device  200 . Some examples of drying agents include sodium carbonate, sodium sulfate, calcium chloride, zinc chloride, calcium sulfate, silica gel, calcium oxide, and synthetic aluminosilicates. The drying agent can be stored in a bag that is porous to the environment so that moisture can be effectively absorbed. Since the receiver supply device  200  is sealed off from the ambient environment, a small amount (e.g. 10 gram) of drying agent can typically last a long period of time. A user can easily replace the used drying agent by a fresh batch of drying agent. The frequency of the replacement and the amount of each replacement can be adjusted depending on the frequencies of the printing as well as the ambient environment. 
     It is understood that additional drying devices such as heater(s) and fan(s) can be added to the receiver supply device to further reduce the moisture level in the receiver supply device  200 . Such heaters and fans can be controlled by the control electronics  25 . A humidity sensor can also be provided in the receiver supply tray for detecting the humidity conditions for activating the fans and the heaters. 
     An advantage of this invention is that the drying of ink drops on the receiver is greatly improved because the receiver is dryer. As a result, image artifacts such as coalescence and inter-color bleeding are reduced. With the increased drying rate, the receivers with freshly printed ink images can be stacked in much shorter time frame after printing. That is, the system printing productivity is increased. 
     Another advantage of this invention is that the printing performance is less variable relative to different ambient environments. For example, the environment of a printer can vary from an air conditioned room to outside tropical atmosphere (e.g. in a theme park in Florida). The present invention provides consistent printing performance and long receiver storage time in a printer. 
     A further advantage of the present invention is that the surfaces of the ink receivers are kept dry so that the friction properties of the surfaces under a drive roller (such as rollers  65  and  250 ) are insensitive to the environmental humidity changes. As a result, the receiver transport is more robust. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       10  ink jet printing apparatus 
       20  computer 
       25  control electronics 
       30  print head drive electronics 
       31  ink jet print head 
       32  ink jet print head 
       33  ink jet print head 
       34  ink jet print head 
       40  ink reservoir 
       41  ink reservoir 
       42  ink reservoir 
       43  ink reservoir 
       45  holder 
       54  gliding rail 
       55  support 
       56  belt 
       57  pulley mechanism 
       65  roller 
       70  receiver transport motor 
       71  print head translating motor 
       80  ink receiver 
       90  platen 
       100  ink drop 
       110  ink spot 
       200  receiver supply device 
       210  housing 
       220  cover 
       230  door 
       240  receiver tray 
       250  picker roller 
       260  shaft 
       270  motor