Abstract:
A thermal dye transfer printer reduces print time by printing sequential color section from a donor web onto a receiver sheet while the donor web and receiver sheet travel in forward or reverse directions relative to a printer head. A first colored image is formed on the receiver by printing as the donor web and the receiver sheet transit the printer head in a first direction. The donor web is indexed to bring the trailing edge of a second color section in registration with the trailing edge of the first colored printed image on the receiver web portion. Then the image receiver web transits the printer head in the opposite direction and the second color is printed on the receiver sheet in combination with the receiver sheet. The steps are repeated for all color patches.

Description:
FIELD OF THE INVENTION 
   This invention relates to printers and, in particular, to multicolor dye transfer printers. 
   BACKGROUND OF THE INVENTION 
   Digital photography is highly competitive with conventional photography. One disadvantage of digital photography is the quality and durability of prints of images taken with a digital camera. While computer screens display vivid images, photographers still want hard copies of their pictures. Conventional prints from photofinishers are far superior to most prints made from home based printers because many home based printers use ink jet technology. Ink jet printers are low cost devices and they provide a range of prints, some of which are unacceptable, others that fade quickly, and some that have good color and long life. One of the better printers for color digital photography images is the thermal dye transfer printer. It creates an image from sequential patches of different colors and applies a clear, protective coating to the finished print. These printers reproduce excellent images that are quite durable and generally superior to images made with ink jet printers. 
   However, thermal dye transfer printers are inherently slow. Ink jet printers simultaneously deposit different color inks to make an image. In contrast, thermal dye transfer printers deposit only one color at a time. Their speed is further reduced by the conventional process of returning the printed paper to its initial position before a second color is printed on the paper. In order to print three colors and a clear coat on a paper, a printer shuffles the paper back and forth seven times: one time for each color or layer and one time to reload for the three subsequent colors or layers. There is a need to make thermal dye transfer printers quicker and to reduce the time it takes to make a color print using a thermal dye transfer printer. 
   Thermal dye transfer printers are also popular in printing kiosks. The Eastman Kodak Company markets and sells a line of printing kiosks that provide users with thermal dye transfer prints of digital photographs. The kiosks are user friendly and have touch screens with menu driven programs for showing a digital camera user how to make prints of digital images. 
   Nevertheless, printing thermal dye transfer images is inherently slow. A state of the art 4″×6″ thermal dye transfer printer takes between 11 and 12 seconds to make a print. In order to give consumers a net printing time of about five or six seconds, kiosks are equipped with two 4″×6″ printers. The printing operation alternates between the two printers so that the average time per print is about five or six seconds. 
   That solution imposes a high cost of capital equipment on each kiosk. There is still an unsolved problem of economically reducing the net print time. Studies show that about half of the 11 to 12 second print time is spent in handling receiver paper and dye transfer rolls. Hence, even if the actual time of image transfer was zero, the handling time for the receiver and donor webs would be at the current net time experienced by consumers. A zero image print time is impossible, but even a 50% improvement would still leave the consumer with an average print time of about eight seconds. Therefore, even a 50% reduction in image print time, by itself, will not materially reduce the time experienced by consumers or allow the kiosk to print with only one 4″×6″ printer. 
   SUMMARY OF THE INVENTION 
   The invention provides both an apparatus and a process for rapidly printing images with two or more colors. The invention is particularly useful with thermal dye transfer printers that include sequential sections of colored or clear donor material. In a conventional web, the donor material includes sequential sets of sections of yellow, magenta, cyan and clear. The clear section has a transparent protective layer that also transfers via heat. The individual colored or clear sections are printed one at a time onto the receiver sheet. With the invention, a section of a color donor web is registered opposite a receiver sheet prior to transfer of the donor material to the receiver sheet. A printer head moves relative to a platen to engage and disengage the donor web. The printer head urges the donor web against the receiver sheet that is supported on a platen. A controller energizes the printer head and drives the donor web and receiver sheet in order to transfer the donor material to the receiver sheet. After transferring one color, the apparatus stops and disengages the print head from the donor material. The donor material indexes to the next section of a different or transparent color and registers the next section with the printer head and the image receiver. The printer head re-engages the donor web and presses the web against the receiver sheet that is supported on the platen. The donor web and receiver sheet are then driven in a direction opposite to the first printing operation in order to deposit the second color or transparent layer. The above steps are repeated as many times as there are sequential sections of donor material in order to complete the printing operation. 
   One of the features of this invention is that the apparatus and method print in two directions. As such, the invention reduces the number of times a given receiver sheet transits the print path in the apparatus. In conventional printing apparatus, the receiver sheet transits in the forward and reverse direction to print each color. In other words, the printer sheet advances past the print head, stops and returns to its initial position before the next color prints onto the receiver sheet. In contrast, the invention prints on the receiver sheet in both directions. In a conventional thermal dye transfer printer, a receiver sheet transits the printer head at least seven times: four times in one direction for printing and three times in the opposite direction for reloading prior to printing. However, with the invention, the receiver sheet transits the printer head only four or at most five times. Thus, the invention provides more rapid printing and fewer steps. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of an apparatus showing printing in the forward (first) direction; 
       FIG. 2  is a schematic of an apparatus showing printing in the reverse (opposite) direction; 
       FIG. 3  is a plan view of a portion of a web showing two complete sets of color sections; and 
       FIGS. 4A–4L  illustrate the reciprocating operation of the receiver sheet. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Turning to  FIG. 1 , there is shown a schematic of a thermal printer  10  for performing the alternating printing of the invention. The printer  10  has a donor web supply spool  26  that supports a donor web  35  of thermal transfer donor material. The donor web  35  extends along a path that includes the donor web supply spool  26 , a first idler roller  21 , a first stripping plate  22 , thermal print head  23 , a second stripping plate  24 , a second idler roller  25  and donor web take-up spool  20 . Image receiver web  45  travels back and forth along a path  4 . The arrows of path  4  show the forward direction in  FIG. 1 ; the arrows of path  4  in  FIG. 2  show the reverse direction. Image receiver web  45  may be any suitable material, cloth or paper including but not limited to special paper for receiving thermal dye transfer images of digital photographs. The image receiver web  45  travels back and forth along path  4  that includes a pair of forward drive rollers  30 , a freely rotating support platen roller  29 , and a pair of reverse drive rollers  28 . When the print head  23  engages the donor web  35  and receiver web  45 , friction between the two webs is strong enough for the drive rollers  30  and  28  to move the two webs together past the print head  23 . The driver rollers have relatively powerful motors or gear trains that provide high enough torque to move the webs  35  and  45 . In contrast, torque applied to the supply spool  26  and take-up spool  20  is just enough to prevent slack in the donor web  35 . In operation, forward drive rollers  30  pull the donor web  35  and receiver web  45  from right to left and driver rollers  28  pull the webs  35  and  45  in the opposite direction. The donor web  35  passes over and contacts the print head  23 . The image receiver web  45  is disposed between the donor web  35  and a free turning platen roller  29 . Spools  20 ,  26  and  27  have suitable drive motors (not shown) and/or drive trains for turning the spools in clockwise or counterclockwise directions to accommodate driving the webs  35  and  45  in forward and reverse directions. 
   The printer  10  has suitable circuits, sensors, integrated circuits, processors, memory, operating and application software, for operating and controlling the printer  10  and the individual components thereof. In particular, the controller  60  raises and lowers the print head  23 , selectively operates the heater elements in the print head  23  that transfer donor material from the donor web  35  to the receiver web  45 , operates the drive rollers  28 ,  30  to move the receiver web  45  in the forward (right to left) and reverse (left to right) directions, operates the supply spool  26  and take-up spool  20  to move the donor web  35  in forward or reverse directions. Controller  60  has leads  61  and  65  that that connect the controller  60  with sensors and actuators at the supply spool  26  and take-up spool  20 . Other leads  62 ,  64  connect the controller  60  to the drive rollers  30 ,  28 . Lead  63  connects the controller  60  to the print head and carries signals for actuators that raise and lower the print head and also selectively operate the heating elements in the print head. Another lead  66  connects the controller  60  to receiver web spool  27 . 
   Those skilled in the art understand that the schematic of  FIG. 1  omits details of the controls for operating the printer  10 . However, these controls are generally conventional and may be found in other machines and are otherwise well-known to those skilled in the art. Likewise, this description omits the motors, solenoids and other actuators, sensors and encoders that are used for turning and driving the supply spool  26  and take-up spools  20  and the drive rollers  30  and  28  and receiver web spool  27 . Again, those items are well-known to those skilled in the art. Likewise known to those skilled in the art know of suitable electronics for actuating the heat elements in a linear array of a thermal print head. Those skilled in the art also understand that the thermal print head  23  and the platen roller  29  are kept in close engagement during printing. A linear actuator moves the print head  23  relative to the platen roller  29  in order to permit the donor web  35  to index from one color section to another. 
   With reference to  FIG. 3 , a typical donor web  35  portion shows two sets of a number of sequential sets of color and clear sections. The first set  36 . 1  of sequential sections includes a yellow, magenta, cyan and clear sections identified, respectively, by reference numerals  36 . 1 Y,  36 . 1 M,  36 . 1 CY and  36 . 1 CL. A second set  36 . 2  of sequential sections follows the first set and so on. Each section has a leading edge (L) and a trailing edge (T). In order to provide a full color image with a clear protective coating, the four sections of each set  36 . 1 ,  36 . 2 , etc. are printed, in registration with each other, onto the same portion of the image receiver web  45 . For purposes of explanation, the leading edge is always on the left hand side and the trailing edge is always on the right hand side regardless of the direction of travel of the donor web  35 . 
   The first color is printed in the conventional direction, from right to left as seen by the viewer. See  FIGS. 1 and 3 . Controller  60  raises the print head and actuates the driver rollers  30  to register a portion of the receiver web  45  on the platen roller  29  beneath the print head  23 . Controller  60  actuates supply spool  26  and take-up spool  20  to advance a leading edge of a first (yellow) section  36 . 1 Y of donor web  35  to the print head  23  for registration with the receiver web  45  and for printing a first (yellow) donor color on the receiver web  45 . Thus, in the example shown in  FIG. 3  and  FIGS. 4A–4C , the first (yellow) section  36 . 1 Y is advanced to the print head  23 . There the lower surface of donor web  35  engages the receiver web  45  which is supported by the platen roller  29 . The leading edge LED of the first (yellow) section  36 . 1 Y is registered at printer head  23  with a leading edge LER of an image receiving area on the image receiver web  45 . Controller  60  lowers the print head  23  to engage the donor web  35  with the receiver web  45 . Controller  60  actuates drive rollers  30  and supply spool  26  and take up spool  20  to move the webs  35  and  45  together past the print head  23 . Controller  60  selectively operates heater elements in the print head  23  to transfer donor material from donor web  35  to receiver web  45 . As the webs  35  and  45  leave the print head  23 , stripping plate  22  separates the donor web  35  from the receiver web  45 . The donor web  35  continues over idler roller  21  toward the donor take-up spool  20  and the partially printed portion of receiver web  45  is supported on a guide (not shown). The trailing edge TER of the printed portion of the receiver web  45  remains on the platen roller  29 . 
   The next color is printed in the reverse direction, i.e., from left to right. See  FIGS. 2 and 3 . To do so, a second (magenta) section  36 . 1 M of donor web  35  is advanced from spool  26  to the print head  23 . Controller  60  operates the supply spool  26  and take-up spool  20  to drive the second (magenta) section  36 . 1 M so that its trailing edge TED is registered at the trailing edge TER of the printed portion of the receiver web  45  on the platen roller  29 . Controller  60  lowers the print head  23  to press the donor web  35  against the receiver web  45  that is supported on the platen roller  29 . Controller  60  operates the drive rollers  28 , the receiver web spool  27  and the donor supply spool  26  and take-up spool  20  to move the donor web  35  and receiver web  45  together beneath the print head  23 . See FIGS.  2  and  4 D– 4 F. Controller  60  selectively operates heater elements in the print head  23  to transfer the second color (magenta) from the donor web  35  onto the receiver web  45 . The stripping plate  24  separates the webs  35  and  45  from each other and the donor web travels over idler roller  25  for temporary storage on supply spool  26 . 
   The above operations are repeated to transfer the third (cyan) and fourth (clear) sections  36 . 1 CY,  36 . 1 CL to the receiver web. Those operations are shown in  FIGS. 4I–4L . However, as a preliminary step the expended portion  36 . 1  M of the second section is advanced past the print head  23  and onto the take-up spool  20  so that the third (cyan) section  36 . 1 CY may be advanced to and registered with the receiver web at the print head  23 . Once so positioned, the operations described above are repeated to print the third (cyan) and fourth (clear) section onto the receiver web  45 . In a final operation, the printed portion of the image receiver web  45  is cut from the rest of the web  45  and discharged as a finished print of the digital image. Those skilled in the art understand that the above process could begin by predisposing the trailing edge of the yellow portion opposite the trailing edge of the image receiver sheet and performing the first print in the reverse direction. 
   The apparatus and method described above provide an average printing time for a single print of between five or six seconds. As such, the invention may save capital equipment expenses in photo kiosks by allowing the manufacturer to use only one 4″×6″ printer for each machine rather than the two printers that are currently used. As an alternative, kiosks could be equipped with two of the printers using the invention and the net printing time for a set of prints could be further reduced to between two to three seconds by using both machines to alternately make prints. It will be appreciated that one of the printing times and printing rates described in this paragraph are exemplary only and that the invention can be practiced to increase the rate at which any printer of this type can generate images without inherently requiring an increase in printing speed. 
   The invention may be incorporated into existing printer designs by certain modification. The invention requires stripping plates blades on both sides of the printer head; prior art printers need only one stripping plate. Where the prior art printers use one set of drive rollers and drives the image receiver web  45  and donor web  35  in only one (forward) direction, the invention has a pair of such drive rollers on each side of the printer head  23  to drive the pinched image receiver web  45  and donor web  35  through the printer head in opposite directions. Suitable controls and shaft encoders are used on the donor web  35 , supply spool  26 , take up spool  20 , and the drive rollers  28  and  30  accurately register the donor web  35  and the image receiver web  45 . After printing is complete, the printed portion of the image receiver web  45  is cut from the receiver web  45  with a cutter (not shown) to provide a print of the digital image. 
   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  printer 
         20  donor web take-up spool 
         21  idler roller 
         22  first stripping plate 
         23  thermal print head 
         24  second stripping plate 
         25  second idler roller 
         26  donor web supply spool 
         27  receiver web spool 
         28  reverse drive rollers 
         29  support platen roller 
         30  forward drive rollers 
         35  donor web 
         36 . 1  first set of sequential sections 
         36 . 1 Y Yellow sequential section 
         36 . 1 M Magenta sequential section 
         36 . 1 CY Cyan sequential section 
         36 . 1 CL Clear sequential section 
         36 . 2  second set of sequential sections 
         45  image receiver web 
         60  controller 
         61 ,  65  leads to  20 ,  26   
         62 ,  64  leads to  30 ,  28   
         63  lead to print head  23   
         66  lead to receiver web spool  27   
       LED leading edge of donor 
       LER leading edge of receiver 
       TED trailing edge of donor 
       TER trailing edge of receiver