Abstract:
A method of color printing with an ink jet printing head adapted to print one strip for each color and comprising nozzles disposed in distinct groups (22) for each color, with a different number of nozzles from group to group. Each group is separated from the subsequent group by intervals (DCM, DMY) of different magnitude. The method consists of printing strips of a subsequent color only after having printed at least two strips of a first color and superimposing each subsequent color on the first of the two strips printed previously.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of dot printing and to the corresponding ink jet printing head, and more particularly, to a method for improving the perceptible printing quality of graphic images and alphanumeric texts obtained with both black-and-white and colour printing. 
     The dot printing method according the present invention may be applied indiscriminately to any type of ink jet head, and preferably it is applied to a thermal colour ink jet printing head. 
     BACKGROUND OF THE INVENTION 
     From European Patent No. 300 595 there is a known printing method of the above indicated type, in which the printing head contains several nozzles, fed in groups with ink of various colours, for example, Cyan, Yellow, and Magenta. 
     The head is transported in two opposing directions in front of a printing medium, on which the ink drops are deposited in successive passes. 
     During a first pass a strip of a first colour is deposited, Cyan, for example, on which is deposited a second strip of a second colour, Yellow for example, in a second pass, but staggered in defect i.e. with a width less than that of the previous strip, while a next strip of the first colour is deposited alongside the first, with equal width. 
     In successive passes, strips of the first and the second colour are alternately deposited, of which the strip of the second colour is always staggered in defect of the same quantity with respect to that of the first colour. 
     If printing is done in this way, making use of two successive back and forth runs of the head, the ink of the first pass is not yet sufficiently dry when the ink of the second colour is superimposed on it, so that the latter mixes with the underlying ink in an irregular fashion, creating perceptible spots of colour, to the detriment of the print quality. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention seek to print, with colour inks, images of high quality, having none of the inconveniences mentioned above. 
     One embodiment of the present invention prints graphic images and alphanumeric texts by depositing an ink of a second colour on top of an ink of a first colour after a sufficient time to allow the first-deposited ink to dry and not mix with the ink deposited afterwards. 
     Another embodiment of the present invention covers with a successive pass a white zone that may be situated between two adjacent strips, having been left untouched by ink due to possible errors in the feed of the printing medium, for example, a sheet of paper. 
     The invention is defined in the appended claims to which reference should now be made. 
    
    
     This and other characteristics of the invention will appear more clearly from the following description of a new printing method and a preferred embodiment of the new printing head, making reference to the attached drawings and diagrams. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a-e show the successive phases of a printing method embodying the invention, for printing with two and/or three different colours; 
     FIG. 1f shows an intermediate phase in the printing with three colours; 
     FIG. 1g shows a final phase of the printing with three colours; 
     FIG. 2 shows an ink jet printing head suitable for printing according to the method of FIG. 1; 
     FIG. 3 shows a table indicating the number of nozzles for each colour of the printing head of FIG. 2, as used in the successive passes. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With respect to FIGS. 1a-e, on the lefthand side, there is a diagram of the arrangement of the nozzles of the head 20 used in the present invention and described more closely hereafter with respect to FIG. 2. 
     In brief, the head 20 contains three groups of nozzles, indicated by C, M, Y, respectively referring to the three basic colours Cyan, Magenta, and Yellow. The head 20 is mounted on a carriage, moved by its own motor, not shown in the drawings, in two opposite directions, i.e. a forward run and a return run, also known as passes, to deposit drops of ink on a printing medium PM to form a strip. 
     By strip is meant a band or stripe of a certain colour, extending for the whole or part of the run of the head and deposited by emission of ink through all or part of the nozzles of each group. 
     After each pass, the printing medium is moved by an interlinear distance I in a direction S perpendicular to the direction MT of movement of the head 20 (and, thus, to the printing line). The method of dot printing according to the invention comprises the following phases: 
     Phase a): during a first pass 22 (FIG. 1a), the nozzles C deposit a strip 24 of a first colour, for exemple Cyan, of width Wc. 
     Phase b): during a second pass 26 (FIG. 1b), the nozzles C deposit a second strip 28 of the first colour, adjacent to the first strip 24, of width Yc. 
     The second pass 26 can be made during the return run of the head, in a direction opposite to the first pass, or in a successive run in the same direction as the first pass, after an idle run. According to a preferred embodiment of the present method, but not limited hereto, the head performs the printing in the forward and return runs, without performing any idle runs, in order to increase as much as possible the overall printing speed. 
     Phase c): during a third pass 30 (FIG. 1c), the nozzles C deposit a third strip 32 of the same first colour, of width Wc, adjacent to the second strip 28, and at the same time the nozzles M deposit a first strip 34 of a second colour, Magenta for example, of width Wm, greater than the width Wc, totally covering the first strip 24 and only partially the second strip 28 of the first colour. The width Wm of the strip 24 of the second colour exceeds, by a predetermined quantity H, the width Wc of the first strip 24 beneath it. Therefore, the expression holds: 
     
         Wm=Wc+H 
    
     The quantity H is equal at least to the distance p (elementary pitch) between two consecutive nozzles (FIG. 2) of each group, measured in the direction of alignment of the nozzles themselves. 
     Phase d): during a fourth pass 36 (FIG. 1d), a fourth strip 38 of the first colour, of width Wc, is deposited adjacent to the third strip 32, and at the same time a second strip 40 of the second colour, this time of width W&#39;m equal to the width Wc of the strips of the first colour, is deposited adjacent to the preceding strip 34 of the same colour and totally covering the portion 42 of the second strip 28 of the first colour that has remained uncovered, and a part 43 of the third strip 32 of the first colour. 
     In this way, it is clear that, since W&#39;m=Wc, the second strip 40 of the second colour overflows the third strip 32 of the first colour by the quantity H. 
     Phase e): during a fifth pass 46 (FIG. 1e), there are simultaneously deposited: a fifth strip 48 of the first colour of width Wc, adjacent to the fourth strip 32; a third strip 50 of the second colour, of width W&#39;m, adjacent to the preceding strip 40 of the same second colour; and a first strip 52 of a third colour, Yellow for example, of width Wy, greater than the width W&#39;m (and thus, also greater than the width Wc) and totally covering the first strips 24, 34 of the first and the second colour, and only partially the second strips 28, 40 of the first and the second colour. In this fifth pass 46, the third strip 50 of the second colour is superimposed, as in the previous phase, on the part 54 remaining uncovered of the underlying strip of the first colour and a part 56 of the fourth strip 32 of the first colour. 
     The width Wy of the first strip 52 of the third colour exceeds, in this fifth pass, the width Wm by at least a quantity K, for which the relation now holds: 
     
         Wy=Wm+K=Wc+H+K 
    
     where K can be equal to or greater than H. 
     In each of the successive passes, three strips of the three colours Cyan, Magenta, and Yellow are simultaneously deposited by the methods indicated for the fifth pass,; except that the strips of the third colour, Yellow, have a width W&#39;y=Wc (FIG. 1f). 
     At the end of the printing of a colour image, according to the method of the present invention as described above, the last strip 58 (FIG. 1g) of the second colour will have a width W&#39;m decreased by the quantity H with respect to the width Wc, while the width of the last strip 59 of the third colour will have a width W&#39;y, decreased by (H+K). 
     As will be easily observed, the method specifies that each subsequent colour is deposited only after the second pass with respect to the colour previously deposited, so as to allow the latter to dry sufficiently. In fact, the second colour is deposited on top of the first only during the third pass of the printing head (FIG. 1c), while the third colour is deposited on top of the second in the fifth pass of the head (FIG. 1e). 
     In this way, any disuniformity in the mixing of the colours is eliminated, so that the formation of intermediate colours or hues occurs in a very regular way over the entire surface covered thus eliminating spots of different colouration. 
     FIG. 2 shows, in schematic fashion, the new arrangement of the nozzles of an ink jet printing head 20, particularly adapted to colour printing of graphic images and alphanumeric text of high print quality, so that each subsequent colour is superimposed on a different, preceding, colour, to form all the desired intermediate shades, without creating noticeable spots or haloes. 
     The head 20 contains a number of nozzles 22. The nozzles 22 communicate with ink expulsion chambers (not shown), in which a pressure impulses is generated by any of the methods familiar to the art for expelling ink drops from the corresponding nozzles. 
     In the following description we shall refer to an ink jet head of thermal type, although other types of ink jet heads can also be used. 
     The nozzles 22 are arranged in groups, each group being fed with ink of a different colour. 
     In a preferred embodiment, but not limited hereto, the nozzles 22 are subdivided into three groups, respectively indicated C, M, Y, with reference to three inks of different colour, Cyan, Magenta and Yellow. 
     The nozzles of each group are aligned in two columns 124 and 126, parallel to the direction S of feed of the printing medium, and therefore the two columns 124 and 126 are perpendicular to the direction of movement of the head, indicated by MT. 
     The nozzles 22 can also be arranged in a single column, or distributed in more than two columns, however. 
     The nozzles of adjacent columns are staggered in the direction S by a quantity equal to the pitch p, while the groups of nozzles are spaced by a distance that varies from one group to another, as shall be explained hereafter. 
     Moreover, each group contains a number of nozzles that is different from that of the other groups. 
     Indicating by I the width of an interlinear spacing, expressed in number of pitches p, the distribution of the nozzles 22 satisfies the following expressions; NC=I+1; NM=I+1+H; NY=I+1+H+K 
     
         DCM=I+1-H (pitches) 
    
     
         DMY=I+1-(H+K)(pitches) 
    
     where: 
     NC is the number of nozzles of group C (Cyan); 
     NM is the number of nozzles of group M (Magenta); 
     NY is the number nozzles of group Y (Yellow); 
     DCM is the distance between the groups C and M; 
     DMY is the distance between the groups M and Y, expressed in number of pitches p (normally, p is equal to 1/300&#34;), and having (H, K=1,2, . . . ). 
     In the case when I=15 (pitches p) and H=K=1, the number of nozzles of the groups C, M, Y are respectively; 
     
         NC=16; NM=17; NY=18; 
    
     and the spacings between the groups C and M and between the groups M and Y are respectively: 
     
         DCM=15 (pitches p); DMY=14 (pitches p). 
    
     With the numerical values calculated above, it is easy to determine how the printing method should operate. 
     The first strip of Cyan, printed with the 16 nozzles of the group C, has a width of Wc=15/300&#34;. After an interlinear spacing I=15/300&#34;, the second strip of Cyan is printed with the 16 nozzles C, being adjacent to the previous strip. 
     In the third pass, after another interlinear spacing of 15/300&#34;, the nozzles C print another strip of Cyan adjacent ot the preceding one, while the 17 nozzles M print a strip of colour Magenta, of width Wm=16/300&#34;, for which the 17th nozzle prints on top of a portion of the second strip of Cyan, in a width exceeding 1/300&#34;. 
     Continuing with the printing and using interlinear spacings of 15/300&#34;, we reach the fifth pass, in which the 18 nozzles of the group Y (Yellow) print a strip of width Wy=17/300&#34;, which is superimposed on the first strip of Cyan+Magenta,, overflowing onto the second strip by 2/300&#34;, with respect to the width Wc. 
     In the intermediate passes, the strips of the second and third colour are staggered forward with respect to the strips of the first colour. 
     The table of FIG. 3 shows, as an example, the sequence number of the nozzles of each group used for the colour printing of a hypothetical image with 13 passes. 
     It is understood that additions or modifications can be made in the method and the head embodying to the present invention, without leaving the framework of the latter.