Multicolor ink jet printing method and printer

A multicolor ink jet printing method which includes the steps of moving a carriage is moved relative to a recording medium in a main scanning direction and a subscanning direction orthogonal to the main scanning direction, printing each color with a different group of nozzle heads mounted on the carriage, wherein at least one color is printed with a larger number of nozzle heads than the other colors, and printing the at least one color with a larger printing resolution in the main scanning direction than the other colors.

BACKGROUND OF THE INVENTION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 02079284.2 filed in Europe on Oct. 9, 2002, which is herein incorporated by reference.

1. Field of the Invention

The present invention relates to a multicolor ink jet printing method, wherein a carriage is moved relative to a recording medium in a main scanning direction and a subscanning direction orthogonal to the main scanning direction, and each color is printed with a different group of nozzle heads mounted on the carriage, and wherein at least one color is printed with a larger number of nozzle heads than the other colors. The present invention further relates to a printer for carrying out the present method.

2. Related Art

A typical multicolor ink jet printer for printing, for example, with the four colors yellow (Y), magenta (M), cyan (C) and black (K), comprises at least four nozzle heads, i.e., at least one for each color. Each nozzle head has a row of nozzles arranged in the subscanning direction. Thus, when the carriage is moved back and forth across the recording medium in the main scanning direction, each nozzle head prints a swath on the recording medium during each pass of the carriage. In a single-pass printing method, the recording medium, e.g. paper, is fed in the subscanning direction by an amount corresponding to the width of the swath after each pass of the carriage. When the nozzle heads are aligned in the main scanning direction, the different color components are superimposed one upon the other during each pass, and the desired hue of the image is obtained by subtractive color composition.

However, a so-called “banding” phenomenon may occur because the order in which the color components are superimposed depends on the direction of movement of the carriage, and a change in this order leads to slight differences in the obtained hue of the printed image, and, as a result, the hue differences are visible in the form of bands or stripes on the printed image. This problem may be overcome by staggering the nozzle heads in the subscanning direction, so that the different color components are printed in different passes of the carriage and the order in which the colors are superimposed will always be the same, irrespective of the direction of movement of the carriage.

A similar banding phenomenon may also occur when two or more nozzle heads for the same color, e.g. black, are staggered in the subscanning direction in order to increase the printing speed. In this case, banding is due to minor differences in the properties of the different nozzle heads, e.g. differences in the optical densities obtained therefrom. This phenomenon can be mitigated by employing a multi-pass printing method. In a two-pass mode, for example, each printhead prints only every second pixel in each line in a first pass, and the gaps are filled-in by the second nozzle head in the subsequent pass. As a result, the pixel patterns produced by the two nozzle heads are interleaved, and the differences in the properties of the nozzle heads are smoothened out, so that a high image quality is achieved, although at the cost of reduced production.

U.S. Pat. No. 6,257,699 discloses a printing method and a printer of the type indicated above. The printer has three staggered nozzle heads for the colors yellow, magenta and cyan, and another group of three nozzle heads for black. The nozzle heads for black are mounted on the carriage in reverse orientation as compared to the other three nozzle heads and are also staggered in the subscanning direction, such that the staggered rows of nozzle heads overlap in the subscanning direction and one of the nozzle heads for black is aligned with the nozzle head for cyan in the main scanning direction. This printer can be operated in different print modes, including (a) a multi-pass color printing mode, wherein only one of the three black nozzle heads is used in combination with the nozzle heads for the other three colors, (b) a single-pass black and white printing mode, wherein all three black nozzle heads are used and the other nozzle heads are disabled, and (c) a mixed mode, in which black is printed in a multi-pass mode while the other colors are printed in a single pass mode.

WO-A-00/58102 discloses a printer having four nozzle heads for the colors yellow, magenta, cyan and black aligned in main scanning direction and one additional nozzle head for black which is offset from the other nozzle heads in the subscanning direction. The two black nozzle heads are used to speed-up black and white printing. When printing with all four colors, only the offset black nozzle head is used, so that black is printed in another pass than the other three colors. The purpose of this measure is to reduce smears by providing a larger time delay between the time at which a pixel is printed in yellow, magenta and/or cyan and the time at which a black dot is superimposed on this pixel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a printing method and printer capable of achieving an improved quality of color images without causing a substantial loss in production.

According to the invention, this object is achieved by a method of the type indicated above, wherein at least one color is printed with a larger printing resolution in a main scanning direction than the other colors.

The invention is based on the observation that the optimal resolution for a color image, especially in main scanning direction, may be different for different colors, especially for yellow, magenta and cyan on the one hand and for black on the other hand. One of the reasons is that the human eye itself has a higher resolution for black and white contrast than for color perception. Another important reason is that, in typical ink systems for color ink jet printers, the black ink has a higher surface tension than the inks of the other colors and, as a result, a black ink droplet forms only a relative small dot on the recording medium, e.g. a film of synthetic resin, whereas ink droplets in the other colors tend to spread out on the surface of the recording medium and form a larger dot, even though the volumes of the ink droplets are all the same. As a result, a high resolution which would be optimal for black is not necessarily optimal for the other colors as well, due to the larger dot size.

According to the present invention, the nozzle heads for one color, e.g. black, which are present in a larger number than the nozzle heads for the other colors are used for increasing the resolution for the black color component, whereas a smaller resolution is used for the other components in one and the same color printing operation. As a result, borderlines between black image areas and white or colored image areas, to which the human eye is particularly sensitive, can be printed smoothly, due to the high resolution, whereas the smaller resolution of the other color components is hardly perceptible to the human eye and, in addition, is adapted to the larger dot size of the colored inks.

Accordingly, in one aspect, the present invention provides a multicolor ink jet printer comprising a carriage adapted to travel relative to a recording medium in a main scanning direction and a subscanning direction orthogonal to the main scanning direction, with a plurality of groups of nozzle heads mounted on the carriage, wherein the groups are assigned to different colors and at least one group (e.g. the one for black) comprises a larger number of nozzle heads than the other groups, and wherein a color print mode is implemented in which the printing resolution in the main scanning direction is larger for the color printed with said one group than for the other colors.

In a preferred embodiment, the nozzle heads of all groups are adapted to print with the same resolution (e.g. 600 dpi) in the subscanning direction. Then, in the color print mode using differential resolution, the resolution in the main scanning direction will also be 600 dpi for black but only a smaller resolution, e.g. 300 dpi, is used for the other colors.

Of course, the printer is also capable of operating in another print mode, single-pass or multi-pass, with maximum resolution (e.g. 600 dpi) in both the main scanning direction and subscanning direction. Conversely, it is possible to use the smaller resolution (300 dpi) not only for yellow, magenta and cyan but also for black. In black and white printing, multi-pass printing is possible to reduce the black and white banding phenomenon.

In a preferred embodiment, not only the black printheads but also the other printheads are staggered in the subscanning direction, so that the order of superposition of the color components is always the same and banding due to hue differences is avoided.

In a particularly preferred embodiment, there are provided at least three, preferably four, nozzle heads in one group (for black) and two nozzle heads in each of the other groups. Thus, at least two nozzle heads are provided for each color, so that high production printing is possible. Nevertheless, by using a staggered arrangement of the nozzle heads in overlapping rows, it is possible to integrate the comparatively large total number of nozzle heads in a relatively compact area of the carriage.

Thus, in another aspect, the invention also provides a multicolor ink jet printer comprising a carriage adapted to travel relative to a recording medium in a main scanning direction and a subscanning direction orthogonal to the main scanning direction, and a plurality of groups of nozzle heads are mounted on the carriage, wherein the groups are assigned to different colors, wherein at least one group comprises at least three nozzle heads and each of the other groups comprises at least two nozzle heads, the nozzle heads of all the groups being staggered in at least three lines perpendicular to the main scanning direction, such that the lines mutually overlap in the subscanning direction.

Preferably, said one group comprises four nozzle heads arranged in one of said slanting lines, and at least one of these nozzle heads is offset relative to each of the other nozzle heads in the subscanning direction. Thus, at least one nozzle head can then be used for bi-directional full color printing without any changes in the print order.

DETAILED DESCRIPTION OF THE INVENTION

As is shown inFIG. 1, a multicolor ink jet printer comprises a carriage10which is guided on guide rods12and, as is known per-se, is driven by a drive mechanism (not shown) to travel along the guide rods12in a main scanning direction X relative to a recording medium14. The recording medium14is fed by feeding means (not shown) in a subscanning direction Y which is orthogonal to the main scanning direction X.

A number (10in this example) of nozzle heads16are mounted on the carriage10and are arranged in a specific configuration, as will be explained in conjunction withFIG. 2. Each nozzle head16is formed integrally with an ink cartridge which is the only part of the nozzle head that is visible inFIG. 1. On the bottom side of the ink cartridge and on the bottom side of the carriage10, i.e. facing the recording medium14, each nozzle head16has at least one row of nozzles and an associated drive system which may for example be formed by piezoelectric actuators and is configured to cause the individual nozzles to expel ink droplets at appropriate timings in accordance with image information of the image to be printed.

As is shown inFIG. 2, the nozzle heads, which are designated in general by the reference numeral16, comprise a group of four nozzle heads K1–K4for black ink, a group of two nozzle heads C1, C2for cyan, a group of two nozzle heads M1, M2for magenta and a group of two nozzle heads Y1, Y2for yellow. The position, orientation and length of the nozzle rows18of each nozzle head16has been indicated inFIG. 2by bold vertical lines. It can be seen that the individual nozzle rows18of the nozzle heads16are offset from one another in the subscanning direction Y by a distance corresponding to their length, so that, when the carriage10moves in the main scanning direction Y, the nozzle rows18sweep over adjacent swaths a–h. By way of example, it may be assumed that the nozzle heads16have a resolution of 600 dpi in the subscanning direction, and each nozzle row18has sufficient nozzles to print 192 lines of image dots simultaneously, said 192 lines forming one of the swaths a–h having a width of 8,128 mm.

The nozzle heads K1–K4forming the group of black nozzle heads are arranged on a straight line which is slanting relative to the main scanning direction X, so that their nozzle rows18are appropriately staggered in the subscanning direction Y. Similarly, the nozzle heads C1, C2, M1and M2of two other groups form another slanting line in parallel to the first slanting line and overlapping therewith in the subscanning direction, so that the positions of nozzle heads M1, M2in the direction X coincide with those of the nozzle heads K1and K2, respectively. Moreover, the positions of the nozzle rows18of the nozzle heads C1and C2in the main scanning direction X coincide with those of the nozzle rows18of the nozzle heads K3and K4, respectively. It will further be observed that the orientation of the nozzle heads M1and M2is inverted relative to those of the nozzle heads K1and K2. This is necessary in order for the nozzle rows of the nozzle heads C1, C2, M1and M2to be arranged on a straight slanting line, irrespective of the comparatively large size of the footprints of the ink cartridges of the nozzle heads and the off-center positions of the nozzle rows18relative to the ink cartridges. Similarly, the nozzle heads Y1and Y2forming the fourth group are disposed in inverted orientation, so that their nozzle rows18form another (short) slanting line and are aligned in the subscanning direction with the nozzle rows of the nozzle heads C1and C2. It should be noted however that the nozzle rows18of the nozzle heads C1, C2, M1, M2, Y1and Y2for the colors cyan, magenta and yellow do not overlap in the main scanning direction. This can be seen more clearly inFIG. 3in which the positions of the various nozzle heads have been shown in diagrammatic form.

The operation of the printer will now be explained with reference toFIG. 2. In a first pass of the carriage10, either in the positive or negative X-direction, the nozzle heads Y1and Y2will print a yellow sub-image or color component of an image to be printed on the swaths a and b. At the end of this pass, the recording medium14will be fed in the subscanning direction Y by the width of the two swaths a, b, i.e. by a distance of 16,256 mm. Then, in the next pass, with the carriage10moving in opposite direction, the nozzle heads M1and M2will print a magenta sub-image on the same swaths a and b, so that each magenta ink dot will overlay a yellow dot that may have been printed at that position in the preceding pass, thereby creating a mixed color. The recording medium is again fed by the same distance, and in the next pass, a cyan sub-image is superimposed by means of the nozzle heads C1and C2. In a print mode with a resolution of 600×600 dpi for each of the four colors, the nozzle heads K3and K4are kept inoperative. At the end of this pass, the recording medium is fed once again, and in a fourth pass, a black sub-image is superimposed by means of the nozzle heads K1and K2. Of course, when the second pass is performed, the nozzle heads Y1and Y2will simultaneously print a yellow sub-image on the subsequent two swaths. In the third pass, the nozzle heads Y1, Y2, M1and M2will operate simultaneously with the nozzle heads C1and C2, and so on.

The nozzle heads K3and K4are used for a print mode in which the resolution for the color components yellow, magenta and cyan is 300×600 dpi, i.e. only 300 dpi in the main scanning direction X but 600 dpi in the subscanning direction Y, whereas the resolution for the black sub-image is 600×600 dpi. In this mode, the frequency with which the nozzles of the individual nozzle heads are fired is only one-half the frequency that has been used in the previously described 600×600 dpi mode. The timings at which the nozzles of the nozzle heads K3and K4are fired are shifted relative to the timings of the nozzle heads C1and C2, so that the black ink dots are positioned in the gaps between the cyan dots. In the fourth pass, the nozzles of the nozzle heads K1and K2are fired at such timings that the black ink dots produced thereby are superimposed on the cyan ink dots formed with the nozzle heads C1and C2and hence in the gaps between the dots formed by the nozzle heads K3and K4. Thus, for the black sub-image, the full resolution of 600 dpi in the main scanning direction X is achieved in two passes.

In black and white printing, a two-pass mode is achieved by firing the nozzle heads K1, K2, K3, K4in the same way as described above, with the only difference that the nozzle heads C1, C2, M1, M2, and Y1, Y2are kept inoperative.

Another possible print mode for black and white is a high speed or draft mode in which the resolution is also 600×600 dpi, but the nozzle heads K1–K4are operated in a single-pass mode. Then the printing speed is approximately twice as high as in the two-pass mode, but the image quality may be lower because of the banding produced by the four nozzle heads K1–K4.

Conversely, a very high image quality without any banding may be achieved by employing a four-pass mode in black and white. In this mode, the nozzle head K4will only print every fourth pixel in the first pass, and the remaining pixels will be successively filled-in by the other nozzle heads K3, K2and K1in the subsequent passes.

In order to suppress banding produced by the two nozzle heads provided for each color component, it is also possible to employ a full-color two pass mode with a resolution of 600×600 dpi. In this mode, the recording medium is advanced only by the width of a single swath, i.e. 8,128 mm after each pass, so that the nozzle head Y1will print on the same swath as the nozzle head Y2, and so on. The nozzle heads K3and K4are not used in this mode.

In yet another mode, the resolution is again 600×600 dpi for black and 300×600 dpi for the other colors, with four-pass printing being employed for black and two-pass printing for the other three colors.

It will thus be appreciated that the printer can be operated in a large variety of different modes in order to comply with different quality requirements. In general, the time required for printing an image of a given size will increase with increasing quality. However, the mixed modes employing a resolution of 600×600 dpi for black (two-pass printing or four-pass printing) and 300×600 dpi for the other colors (single-pass or two-pass printing) permit a printing quality which is significantly higher than the quality achieved in a corresponding 300×300 dpi mode which would require the same printing time (with the nozzle heads K3, K4being inoperative).

In addition, the arrangement of the nozzle heads16as shown inFIGS. 2 and 3permits a compact construction of the carriage10in spite of the space requirements for the ink cartridges of the nozzle heads.

FIG. 4illustrates a modified embodiment employing a group of six nozzle heads K1–K6for black and two nozzle heads for each of the other colors. This embodiment permits one-pass to six-pass printing for black and white. In full-color printing, it permits a resolution of 600×600 dpi or 300×300 dpi in a single-pass mode or a two-pass mode with the nozzle heads K3, K4for black and the nozzle heads C1, C2, M1, M2and Y2, Y2for the other colors. Since the nozzle heads K3, K4do not coincide with any of the other nozzle heads, as seen in main scanning direction X, the print order will be independent of the direction of movement of the carriage.

In addition, the following mixed modes are possible for example:a) a resolution of 600×600 dpi for black and 200×600 dpi for the other colors:a1) a three-pass mode for black and a single-pass mode for the other colors.a2) a six-pass mode for black and a two-pass mode for the other colorsb) a resolution of 600×600 dpi for black and 300×600 dpi for the other colors:b1) a two-pass mode for black (using only four of the six nozzle heads) and a single-pass mode for the other colors.b2) a four-pass mode for black (using only four of the six nozzle heads) and a two-pass mode for the other colors.

In the arrangement shown inFIG. 4, the orientation of the nozzle heads K1and K2would be inverted relative to those of the nozzle heads K3, K4, Y1and Y2, in order to avoid interference with the ink cartridges of the nozzle heads K5and K6. In general, a compact arrangement of the nozzle heads is possible as long as, in any position in X-direction, not more than two of the slanting lines defined by the nozzle rows are overlapping.

When, in the above embodiments, the single-pass mode is used for the colors yellow, magenta and cyan, a minor banding might be visible because adjacent swaths in one color component are printed with different printheads. In order to mitigate this effect, the nozzle row18of the nozzle heads in the proposed embodiments each comprise 208 individual nozzles, i.e. 16 nozzles more than the 192 nozzles actually needed, and the nozzle rows are arranged with an overlap of 16 nozzles in the subscanning direction Y, as is shown inFIG. 5. As an example,FIG. 5shows the bottom end of the nozzle row18-C1of the nozzle head C1inFIG. 2and the top end of the nozzle row18-C2of the nozzle head C2. In the overlapping area of 16 nozzles, the pixels of a given line can be printed with either one of two nozzles. For example, nozzle20can print the same pixel line as nozzle20′.

When, for example, a plain area filled with the color cyan has to be printed, the nozzle20will omit every 16th pixel in the line, and the missing pixel will be printed with the nozzle20′. Similarly, nozzle24will omit every 12th pixel in the line, and the missing pixels will be printed with nozzle24′. Finally, the last nozzle of the row18-C2, nozzle36, will print only one pixel in the line, and all the other pixels will be printed by nozzle36′. In this way, the image printed with the nozzle line18-C2is gradually merged with the image printed by the nozzle line18-C1, so that any differences between the two nozzle heads C1and C2are smoothened out. The same holds true for the borders between the nozzle rows of the other pairs of nozzle heads such as Y1, Y2and M1, M2, and as well for the borders between the four black nozzle heads K1–K4.

In order to avoid artefacts at the transition between adjacent nozzle rows18, it is important that the nozzle heads16are precisely adjusted in the subscanning direction Y. To this end, electronic adjusting means may be used, as is generally known in the art.

The effect of the mixed mode with a resolution of 600×660 dpi for black and 300×600 dpi for the other colors will now be explained by reference toFIGS. 6 and 7.

FIG. 6shows a pattern of pixels or ink dots corresponding to an image area solidly filled with cyan and superimposed by a vertical black line40. The cyan ink dots42are represented by non-filled circles which have a comparatively large diameter, due to the comparatively low surface tension of cyan ink. The center-to-center distance between adjacent ink dots42in the main scanning direction X is 1/300″, corresponding to the resolution of 300 dpi, and the corresponding distance in the subscanning direction Y is 1/600″. The dots in adjacent lines are shifted relative to one another by 1/600″, so that the ink dots overlap and the image area is completely covered with ink, without leaving any voids.

InFIG. 6, it is assumed that the carriage10has moved in a positive X-direction while printing the shown part of the image. Thus, the cyan image dots42have been printed first by the nozzle head C1, and the black dots44,46forming the black line40are printed on the cyan background. The dots44,46have a smaller diameter, due to the higher surface tension of black ink, and are printed with a resolution of 600×600 dpi. The dots44have been printed with the nozzle head K3inFIG. 2, and the dots46have been printed with the nozzle head K2. A horizontal line48indicates the border between the upper portion of the black line40, for which the two-pass black printing has been completed, and the lower portion for which only the first pass has been performed. The dots46are shown to overlay the dots44.

It can be seen that the high resolution obtained by two-pass printing in black gives a smooth edge of the black line40, as has been highlighted at50.

FIG. 7illustrates a situation analogous to the one shown inFIG. 6for the case where the carriage10has moved in the opposite direction, i.e. in the -X-direction. In this case, the dots44are printed by the nozzle head K3before the cyan dots42are printed by the nozzle head C1. As a result,FIG. 7shows the dots44on top of the dots42. The dots46are printed last and therefore form a top layer.

The fact that the order in which the dots42and44are printed inFIG. 7is reversed in comparison to the order in which they are printed inFIG. 6, may give rise to minor changes in the color hue of the final image. However, these differences will hardly be visible for the following reasons:1. The reversed print order applies only for one half of the black dot, i.e. only to the dots44and not to the dots46.2. The color cyan printed with the nozzle heads C1and C2which are aligned with the black nozzle heads K3and K4is relatively dark, so that the dots42and the dots44have a similar optical density and the reversal of the print order will not cause a significant effect.3. Although the ink dots have been shown as circles in the drawings, it will be understood that the optical density decreases from the center towards the periphery. As can be seen inFIG. 7, the dots44are printed in the gaps between the dots42and overlap only with the faint peripheral portions of the dots42. This also helps to reduce the effects of the reversal of the print order. In contrast, the dots46are disposed centrally on the dots42, but the dots46are, in any case, the last ones to be printed, so that there is no reversal in the print order.

Thus, although the cyan nozzle heads C1and C2are aligned with the black nozzle heads K3and K4inFIG. 1, so that a compact construction of the carriage10is achieved, the image quality will not significantly be impaired by the reversal of the print order.

In the 600×600 dpi mode for all colors, there will be no reversal in the print order, anyway, because the nozzle heads K3and K4are not used.

Similarly, in the embodiment shown inFIG. 4, there will be no reversal in the print order when only the nozzle heads K3and K4are used.