Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-01-20
2002-08-27
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S015000, C347S041000
Reexamination Certificate
active
06439677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing apparatus and a method of printing. More specifically, the present invention pertains to a printing apparatus that drives a print head to form a raster line, which includes an array of dots arranged in one direction of a printing medium, and carries out a sub-scan, which feeds the printing medium relative to the print head in another direction that crosses the raster line whenever the raster line is formed, thereby printing an image, as well as to a method of such printing.
In this specification, the raster line that includes an array of dots arranged in one direction of the printing medium implies an array of dots created by at least one of the dot-forming elements without a feed in the sub-scanning direction. The direction of the dot array is hereinafter referred to as the main scanning direction, and the direction crossing the dot array as the sub-scanning direction.
2. Discussion of the Background
The printing apparatus, especially an ink jet printer, causes dot-forming elements, such as nozzles, mounted on a print head to spray ink against a printing medium and create dots of the ink on the surface of the printing medium, so as to implement printing. One typical arrangement of the ink jet printer has a print head that scans the surface of the printing medium (sheet of printing paper) to form raster lines.
A known ink jet printer of this arrangement has a nozzle array on the print head, which includes a plurality of nozzles arranged at a predetermined pitch in the sub-scanning direction. The print head of this structure simultaneously prints a plurality of lines by the plurality of nozzles in one main scan (pass).
In the case of the ink jet printer with such a print head, some scatter in properties of the respective nozzles or scatter in pitch between the plurality of nozzles causes the occurrence of banding and thereby deteriorates the picture quality of an image printed on the printing medium.
One known printing technique to prevent such deterioration of the picture quality is constant pitch sub-scan printing. This printing technique is also called interlace printing. The constant pitch sub-scan printing uses a nozzle array on a print head, which includes a plurality of nozzles arranged in the sub-scanning direction at intervals of an integral multiple of a dot pitch that corresponds to the dot of the printing resolution.
The printing medium is fed in the sub-scanning direction by a constant distance corresponding to n dot pitch after each pass, where N nozzles (where N is a positive integer) are arranged in the sub-scanning direction in the nozzle array, n nozzles (where n is a positive integer of not greater than N) are actually driven among the N nozzles arranged in the nozzle array, and the pitch between the nozzles is equal to k dot pitch, which is the minimum pitch of dots created on the printing medium (where k is a positive integer not greater than n and is prime to n).
In the constant pitch sub-scan printing technique, adjoining raster lines in the sub-scanning direction are printed by different nozzles. Even when there is some scatter in properties and pitch of the respective nozzles, this arrangement effectively reduces the deterioration of the picture quality of the printing image due to the scatter and thereby ensures printing of the high picture quality.
In addition to printing of letters and characters, the recent trend requires printing of multi-tone images, such as photographic images, at high quality. A variety of improvements have been given to the ink jet printer to enable printing of fine dots and meet the requirement. A known technique to print a multi-tone image doubles the driving frequency of the ink jet nozzles in the main scanning direction and thereby enhances the dot density in the main scanning direction. A proposed technique to enhance the dot density in the sub-scanning direction decreases the feeding amount of the printing medium in the sub-scanning direction to ensure finer sheet feeding.
Even when the constant pitch sub-scan printing technique is applied to attain high-quality printing, accumulation of errors of sheet feeding (feeding errors) may result in banding. By way of example, in constant pitch sub-scan printing, it is assumed that the upper dot out of two consecutive dots in the sub-scanning direction is printed by a last nozzle included in a selected group of nozzles in a specific pass, whereas the lower dot is printed by a first nozzle included in the selected group of nozzles. In this case, the passes for creating these dots are discontinuous in time. This causes a large accumulated error of sheet feeding between the two dots and facilitates the occurrence of banding.
In the recent ink jet printers, the constant pitch sub-scan printing is implemented by adopting an overlap printing technique that causes dots printed in a subsequent pass to partly overlap the dots printed in a preceding pass, with a view to realizing high-quality printing. If there is a significant time interval between printing of a dot in the preceding pass and the printing of a dot in the subsequent pass, the ink of the dot in the preceding pass is dried up before the dot is printed in the subsequent pass. This results in a poor combination of these two dots and causes a significant difference in printing density, compared with other overlapped portions. This undesirably causes the occurrence of banding.
The above description of the problem of the deteriorating picture quality is in reference to an ink jet printer that creates dots, while the print head moves in the main scanning direction. This problem is, however, not restricted to an ink jet printer, but may be found in any printers that create an image as a set of dots, such as a thermal wax-transfer printer. The problem also arises in the printing apparatuses that carry out the feed in the sub-scanning direction, whereas the print head is not required to move in the main scanning direction.
SUMMARY OF THE INVENTION
The object of the present invention is thus to provide a printing apparatus with a plurality of dot-forming elements arranged in the sub-scanning direction, which solves the problem of the prior art printing apparatuses that create an image as a set of dots and carries out the high-quality printing without deterioration of the picture quality due to banding.
At least part of the above and the other related objects is realized by a printing apparatus that drives a print head to form a raster line, which includes an array of dots arranged in one direction in a printing medium, and carries out a sub-scan, which feeds the printing medium relative to the print head in another direction that crosses the raster line whenever the raster line is formed, thereby printing an image. The printing apparatus includes:
a dot-forming element array that is mounted on the print head and includes N dot-forming elements for creating dots on the printing medium, the N dot-forming elements being arranged at a k dot pitch, which corresponds to a minimum pitch between dots created on the printing medium, in the sub-scanning direction;
a print head driving unit that drives the print head and causes required dot-forming elements in the dot-forming element array to create dots on the printing medium;
a sub-scan control unit that carries out the feed of the printing medium in the sub-scanning direction after each pass of raster creation, which drives the print head driving unit to form at least part of the raster line; and
a dot creation control unit that selects n dot-forming elements (where n is a positive integer of less than N) among the dot-forming element array and causes a raster line to be formed on the printing medium in each pass, the dot creation control unit causing a remaining dot-forming element other than the n dot-forming elements selected among the dot-forming element array create a dot adjoining to two dots, which are created by two passes having a significant time interval between them.
The present inven
Kanaya Munehide
Shimada Kazumichi
Dudding Alfred
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Seiko Epson Corporation
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