Printing method and printing apparatus

Details Printing method and printing apparatus Printing method and printing apparatus

1999-09-29

2001-03-27

Nguyen, Thinh (Department: 2861)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

C347S009000

Reexamination Certificate

active

06206502

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing method and a printing apparatus for producing an image on a printing medium and more specifically to a printing method and a printing apparatus used in a case of performing so-called multipass printing.
2. Description of the Related Art
An ink jet printing method, because of its variety of advantages such as low noise, low running cost and ease with which to reduce an apparatus size and to produce color images, has found a wide range of applications as in printers, copying machines and the like. Most of such printing apparatuses employs a print head, which integrates a plurality of printing elements (i.e., nozzles for ejecting ink in a case of an ink jet printing method), to improve the printing speed.
The printing apparatus that uses a print head with the plurality of arrayed printing elements is known to produce a stripe like density variation that appears periodically in a subscan direction (the direction in which the printing medium is fed) and may become one cause of deterioration of printing quality. These periodic stripe like density variations are very conspicuous. Possible causes of these stripe like density variations include, in the case of the ink jet printing system, variations in an amount of ink ejected and an ink ejection direction among nozzles, deviations between paper feed and nozzle pitch, and density variations caused by time variations among each scanning operation.
To reduce the stripe like density variations and improve the print quality, a variety of methods have been disclosed.
For example, Japanese Patent Application Publication No. 59-31949 (1984) discloses a method which eliminates the stripe like density variation (hereinafter, simply referred to “a boundary stripe”) that occurs at a boundary (hereinafter, simply referred to “joint”) between respective areas, each of which is printed with respective plurality of scans in a main scan direction (hereinafter, simply referred to as “scans”). In the method, pixels at the lower end of the area printed at a previous scan and pixels at an upper end of an area printed at a current scan are overlapped, and then the overlapped pixels are selectively printed with these two times of scans.
A well-known conventional method for realizing further improvement of print quality is a divisional printing method (multipass printing method). The divisional printing method will be explained as follows.
In a case of a print head founded on an ink jet system, the print head with a plurality of nozzles may have a slight manufacturing error among the nozzles during a process of manufacturing the print head. Such error results in variations in the amount of ink ejected and in the ejection direction among the nozzles on printing, which in turn forms the stripe like density variations on a printed image, thus degrading the print quality. One example of such causing of the stripe like density variation is shown in
FIGS. 1A-1C
.
FIG. 1A
shows that, in the print head with 8 ink ejection nozzles, the volumes of ink ejected from the nozzles and the ink ejection directions differ from one nozzle to another. If the printing is performed using a head having such ink ejection characteristic variations among nozzles, ink dots formed will vary in a size and a landing position among rasters printed respectively corresponding to respective nozzles, as shown in FIG.
1
B. As a result, a blank portion may be formed at a central part of
FIG. 1B
as shown or conversely a portion may be formed where dots are overlapped more than necessary.
FIG. 1C
shows a density distribution on an image printed with such dots. These density variations are recognized as the stripe like density variations that may become the cause of deterioration of the print quality.
On the other hand, the divisional printing method (multipass printing method), rather than printing all pixels in a single pass (or in one scan) of the print head in the main scan direction, prints them in a plurality of scans by using different nozzles in different scans.
FIGS. 2A-2C
explain the multipass printing that uses the same head as used in the method shown in
FIGS. 1A-1C
. As shown in
FIG. 2A
, with respect to the printing area shown in
FIG. 1B
, three scanning operations of the print head are performed, and then, printing each half of this area, which is covered by four vertically arranged pixels, is completed with two scanning operations. In this case, the eight nozzles of the print head are divided into two groups, i.e., upper four nozzles and lower four nozzles. The dots formed with one nozzle in one scan is what conforms to data obtained by thinning the image data of each half area to approximately one-half according to a predetermined method. Then, by feeding a paper at a distance corresponding to four pixels, nozzles different from those used for the first printing face the same printing position and complementarily form dots according to the remaining half thinned data, thus completing the printing. According to this printing method, since one raster (one line of dots in a main scan direction) is printed with inks ejected from different nozzles (in the example shown, two different nozzles), the influence of variations among nozzles can be alleviated and the density variations can be reduced as shown in
FIGS. 2B and 2C
.
This divisional printing method divides image data so that respective image data for the first and second scans are extracted according to respective predetermined rules and the extracted data complement each other. Generally, the extraction is performed using mask processing and a most popular mask is a checker pattern mask where the data is extracted vertically and horizontally at every other pixels like a checker pattern as shown in
FIGS. 3A-3C
. In a unit printing area (in this case, a 4-pixel unit area), the printing is completed with two scans (2 passes), where at the first scan, a checker pattern as shown in
FIG. 3A
is printed, and at the second scan, an inverse checker pattern as shown in
FIG. 3B
is printed.
In the multipass printing method described above, increasing a number of divisions (passes) is effective in further improving the print quality. For example, when a 2-pass printing and a 10-pass printing are compared, the 2-pass printing completes one raster by using two different nozzles whereas the 10-pass printing completes the same raster by using 10 different nozzles. Hence, the degree to which the printed result is affected by the ejection characteristic variations among nozzles is relatively smaller in the 10-pass printing than in the 2-pass printing, and then the overall influence of the characteristic variations on the print quality becomes smaller in the 10-pass printing to that extent. As a result, the stripe like density variations is made inconspicuous.
Regarding the multipass printing method described above, Japanese Patent Application Laid-Open No. 6-143618 (1994) discloses a method which considers a fact that a total numbers of scans applied for completing printing of both side areas of the joint or the boundary portion are greater than the number of scans required to complete printing in other areas, and reduces the print duty in both side areas of the boundary portion, particularly when a printing medium that quickly absorbs ink is used to improve the print quality of the multipass printing method.
In any of the multipass printing methods described above, when there is a discrepancy between the paper feed amount and the nozzle pitch of the print head, the so-called boundary stripe is unavoidably produced. This phenomenon will be explained by referring to
FIGS. 4A and 4B
.
FIGS. 4A and 4B
represent examples of the 2-division printing (2-pass printing) for a case where an 8-nozzle head is used as in
FIGS. 3A-3C
. For simplicity of explanation, it is assumed that the head has no ejection characteristic variations among these 8 nozzles. Further, the mask used for the division printing is checker pattern mask. That is, durin

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