Abstract:
A method for ink jet printing with light-curing ink, which includes the steps of printing a first ink dot of a color, irradiating the first ink dot with light, printing a second ink dot with the same color and which at least partially overlaps with the first ink dot, and irradiating the second ink dot with light.

Description:
This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 05110073.3 filed in Europe on Oct. 27, 2005, the entire contents of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a method for providing ink jet printing with light-curing ink and a printer for carrying out this method. 
   An ink jet printer typically comprises a carriage that is movable in reciprocation in a main scanning direction across a recording medium, e.g., a sheet of paper. The carriage has mounted thereon a number of printheads, at least one for each color, so that dots of liquid ink may be expelled onto the surface of the paper, and a stripe or swath of the image is printed during each pass of the carriage. Then, the paper is advanced in a sub-scanning direction, so that the next swath may be printed. 
   The ink that has been deposited on the paper should be cured, i.e., caused to solidify, relatively quickly, so that the image will not be damaged in subsequent handling or processing steps. Solvent-based inks may be cured by letting the solvent evaporate from the ink. A hot melt ink jet printer uses ink that is solid at room temperature and is melted in the printhead, and curing is achieved by allowing the ink to cool down. In a print process using light-curing ink, the solidification of the ink is induced by irradiating the printed ink dots with light, preferably UV light. This has the advantage that the curing process can be accelerated and the curing time can be finely controlled, thereby controlling the amount in which the printed ink dots flow out on the paper. 
   However, the light rays used for irradiating the ink dots must be strong enough to penetrate into the volume of the ink. This requirement is particularly challenging when taking into account that the colorants, i.e., the dyes and/or pigment, of some ink may have a low transparency for light rays. Examples of ink with low transparency colorants are black ink, as the black colorant may be highly absorptive for the light rays, and white ink, as the white colorant may be highly reflective for the light rays. Even when the irradiation with light is used only for “pinning” the ink dots, i.e., for solidifying only a certain ink layer at the surface and at the rim of the dot, the light has to penetrate into the ink to some extent. Thus, the power of UV lamps used for curing the ink increases with increasing printing speed and also with decreasing transparency of the ink. As a result, especially the low transparency of black ink and white ink necessitates the use of relatively strong and expensive UV lamps. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to reduce the power required for the lamps that are used for curing the ink in an ink jet print process with light-curable ink. 
   According to the present invention, this object is achieved by a method comprising the following sequence of steps for printing an individual pixel: printing a first ink dot of color, irradiating the first ink dot with light, printing a second ink dot of the same color at least partially overlapping with the first ink dot and irradiating the second ink dot with light. The present method has the advantage that each pixel is composed of a plurality of ink layers which have a reduced thickness, so that light rays with relatively low intensity are sufficient for penetrating through this ink layer and curing or pinning the same, before the next layer of the same color is deposited. Thus, it is possible to use inexpensive irradiating lamps which have only a relatively small amount of power. 
   In a conventional ink jet printer configured for bidirectional printing with light-curing ink, the carriage may carry two UV lamps that are arranged at both ends of the row of printheads, and the lamps are activated alternatingly in the forward and return passes of the carriage, so that the ink dots will always be cured immediately after they have been printed. Alternatively, both lamps are activated both in the forward and return passes of the carriage. In a preferred embodiment of the invention, the carriage has at least two printheads for the low transparency ink, and a third irradiation lamp is disposed between these two printheads. Thus, regardless of the direction of travel of the printhead, the first dot of a low transparency ink that has been printed with one of the printheads will be cured with the intervening lamp, and the second dot of the same low transparency ink that has been printed on top of the first one will be cured with one of the two outer lamps. The fact that, in this case, three lamps are needed instead of only two is over-compensated by the cost savings that is achieved by reducing the power of the lamps. 
   It will be understood that the concept described above may also be used for ink colors other than the low transparency ink colors such as black and white, if desirable. Likewise, the duplicate sequence of printing and curing may also be used in applications where dots of different colors are printed one on top of the other. 
   When the printer is operated in a multi-pass mode, the first dot may be printed in a first pass, and the second dot on top of the first one may be printed in a later pass of the carriage. Then, it is sufficient to use only one printhead per color, and two lamps are sufficient for bidirectional printing. 
   In another embodiment, two printheads for the same color are used for printing the first and second dots, but these printheads are offset relative to one another in the sub-scanning direction, and two irradiating lamps are arranged to extend over both of the two printheads, so that the method according to the present invention can be performed in a high-speed two-pass print mode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described in conjunction with the drawings, wherein: 
       FIG. 1  is a schematic perspective view of the essential elements of an ink jet printer; 
       FIGS. 2 and 3  are diagrams illustrating a print process according to the present invention; and 
       FIGS. 4 and 5  are diagrams illustrating arrangements of printheads and irradiating lamps according to other embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As is shown in  FIG. 1 , an ink jet printer comprises a carriage  10  that is movable back and forth along a guide rail  12  in a main scanning direction X across a recording medium  14  that is advanced step-wise in a sub-scanning direction Y. The recording medium may be a sheet or a web. The carriage  10  comprises two sets of printheads,  16 ,  18 , two outer irradiating lamps  20 ,  22  embracing the two sets of printheads, and a third central irradiating lamp  24  disposed between the two sets of printheads  16 ,  18 . Each set of printheads comprises two printheads  26  for black ink and three further printheads  28  for the colors cyan, magenta and yellow. The printheads  26  for black ink are arranged adjacent to the central irradiating lamp  24 . 
   A reason for providing two printheads  26  for black ink in each set of printheads is to be able to accommodate a larger amount of black ink in the printheads, since the user frequently wants to print only black and white images, which consumes the black ink more rapidly than colored inks. Another advantage is that the two printheads  26  of each set of printheads may be staggered such that the black images can be printed with a higher resolution. However, the printheads  26  are always arranged such that one printhead  26  of the set  16  is capable of printing an ink dot right on top of a dot that has been printed with one of the printheads  26  of the other set  18 , and vice versa. Likewise, the printheads  28  of both sets of printheads are arranged to print ink dots on identical pixel positions, and the sequence of colours in the two sets  16  and  18  is mirror-symmetric. 
   As is known in the art, each printhead  26 ,  28  comprises an array of nozzles that are facing the recording medium  14 , so that a plurality of pixel lines can be printed in one pass of the carriage  10 . The irradiating lamps  20 ,  22  and  24  are arranged to face the recording medium  14  so as to irradiate the ink dots that have just been printed with ultraviolet light in order to cure or pin the ink. Ink dots that have been printed with one of the printheads of the set  16  will be cured either with the lamp  20  or with the lamp  24 , depending on the direction of travel of the carriage  10 . Similarly, ink dots that have been printed with one of the printheads of the set  18  will be cured either with the lamp  22  or with the lamp  24 . 
   The operation of the printer illustrated in  FIG. 1  will be explained in greater detail in conjunction with  FIGS. 2 and 3  which show a simplified embodiment in which each of the two sets  16 ,  18  has only a single printhead  26  for black ink (K). The colors assigned to the printheads  28  are designated as Y, M and C. 
   In  FIG. 2 , the carriage  10  moves in a positive X-direction, i.e., to the right the drawing. The printheads and lamps that are active in this state have been highlighted by bold lines. These are the four printheads of the set  16 , the black printhead  26  of the set  18  and the lamps  22  and  24 . During the travel of the carriage, the printhead  26  of the set  16  prints first dots  30  in subsequent pixel positions. One of the dots  30  is just being printed, another one is being cured under the lamp  24  (symbolized by hatching), and a third one has already been cured and is now in a position underneath the printhead  26  of the second set  18 , and this printhead is used for printing a second black dot  32  on top of the first one. The second dot  32  is cured by means of the lamp  22 , as is symbolized by another pair of dots  30 ,  32 . Thus, a black pixel is composed of two dots which have only a comparatively small thickness and are cured in subsequent curing steps. As a result, for a given printing speed, the power of the lamps  22 ,  24  that is required for sufficiently curing the ink can be reduced significantly. 
     FIG. 3  illustrates a state in which the carriage  10  moves in a negative X-direction. Here, the first dots  30  are printed with the printhead  26  of the second set  18 , and the second dots are printed with the printhead  26  of the set  16 , and the lamps  24  and  20  are used for curing. 
   The lamps  20 ,  22  and  24  may be Hg-lamps, Xe-flash lamps, and even blue or UV LEDs may be considered in view of the relatively low power. 
   The printing method that has been shown for black ink in  FIGS. 2 and 3  may equivalently be applied to inks in the other colors or even to combinations of inks in different colors, if desired. 
     FIG. 4  illustrates a modified arrangement of printheads  26 ,  28  and lamps  20 ,  22  on the carriage  10 . Here, only the printheads  26  for black ink have been provided in duplicate, so that the method according to the present invention can be applied only for black ink. The printheads  28  for colored ink are configured to expel droplets that have twice the volume of the droplets of black ink expelled from the nozzles  34  of the printheads  26 . 
   The printheads  26  are offset relative to one another in the sub-scanning direction Y. The first dot  30  of black ink is printed with the printhead  26  that is aligned with the printheads  28  and is cured either with the lamp  20  or the lamp  22 . Then, the recording medium  14  is advanced by an amount “d”, so that the second dots  32  are printed on top of the first dots by means of the other printhead  26 . The second dots  32  are cured either with the lamp  20  or the lamp  22 , the length of which is so large that they extend over both printheads  26 . 
   Instead of providing two printheads  26 , it is also possible to provide a single printhead having twice the length of the printheads  28 . 
   In the embodiment shown in  FIG. 4 , the first and second dots are printed in two subsequent passes of the carriage  10 . As an alternative, the second black printhead  26  may be spaced apart from the first printhead by an integral multiple of “d”, so that the second dots are printed in a later pass of the carriage. Then, it will be preferable to provide separate lamps for curing the second dots  32 . 
   In the arrangement shown in  FIG. 4 , two lamps  20  and  22  are sufficient. The same holds true for the embodiment shown in  FIG. 5 , where, similarly as in  FIGS. 2 and 3 , two printheads  28  per color are provided in a mirror-symmetric arrangement. 
   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.