1. Field of the Invention
The present invention relates to an ink jet printing apparatus that forms an image on a print medium using an ink jet print head having arrays of print elements or ink ejection nozzles. The invention also relates to a printing method for the ink jet printing apparatus.
2. Description of the Related Art
As technologies associated with copying machines, information processing devices such as word processors and computers, and communication devices advance, an ink jet printing apparatus that forms digital images on a print medium from information input from these devices by using an ink jet print head are becoming increasingly widespread. To increase a printing speed the ink jet printing apparatus employs nozzle columns made up of many arrayed print elements, each having an ink ejection opening and an ink path. Further, to enable color image printing, it is general practice to use a print head formed with multiple nozzle columns.
The ink jet printing apparatus can be grouped largely into two types: serial type and line type. The serial type printing apparatus uses a print head having a plurality of print elements arrayed in a direction of a print medium feed. An image is progressively formed by repetitively alternating a main scan operation, in which a print head is moved in a direction crossing the print medium feeding direction as it prints, and a sub-scan operation, in which the print medium is fed a predetermined distance equal to a width of a strip of area printed by the main scan. The serial type ink jet printing apparatus is characterized by its relatively small size and low cost.
The line type printing apparatus uses an elongate print head (line type elongate print head) having print elements or nozzles arrayed in line longer than a width of an image to be formed. A print medium is moved relative to the print head only once in a direction crossing the nozzle array direction to form an image. Therefore, compared with the serial type printing apparatus that performs the printing scan operation many times, the line type printing apparatus can form an image much faster. There are increasing demands on the ink jet printing apparatus for higher image quality and faster printing speed and, to meet these requirements, efforts are being made to develop a technology for integrally fabricating nozzles in the print head at high density. Under these circumstances expectations are growing for a printing apparatus equipped with such a line type elongate print head.
In recent years, demands are growing for further improving the printing speed and resolution by making an ink volume of each dot smaller. To meet these demands, one type of ink jet printing system currently in wide use generates thermal energy in each nozzle to cause film boiling in ink to form and expand a bubble and thereby eject an ink droplet. This system has many advantages, including a relative ease with which to reduce the volume of each ink ejection and integrally form nozzles in arrays at high density and an excellent response to the print signal.
Reference 1: Japanese Patent Application Laid-open No. 8-025693 (1996)
Reference 2: Japanese Patent Application Laid-open No. 2002-096455
The printing apparatus using small-volume ink dots, however, may encounter new problems, such as variations in dot landing positions and unstable ejections. For example, when an image is formed by using a print head that has many nozzles arrayed at high density, each ejecting small droplets of 10 pl or less, a phenomenon is observed in which ink droplets ejected from those nozzles at ends of the print head deviate greatly inwardly from their intended landing positions. This phenomenon is referred to as an “end dot deflection.”
FIG. 1 illustrates the “end dot deflection” phenomenon. Here are shown dots formed on a print medium by print elements or nozzles located at the ends of a print head that has one column of nozzles. For simplicity only those dots formed by the end nozzles are shown in the form of solid black circles. In reality, however, all nozzles perform a 100% printing. In this example, a serial type printing apparatus is applied and a line shown as “paper feed boundary” in the figure represents a boundary between two printing scans. An upper row of dots shown above this line represents a row of dots printed by the lowermost nozzle in a first printing scan and a lower row of dots below this line represents a row of dots printed by the uppermost nozzle in a second printing scan. The print head forms dots on a print medium by ejecting ink onto the print medium at a predetermined drive frequency as it moves from left to right in the figure.
As can be seen from the figure, dots printed by the end nozzles, i.e., those dots printed by the uppermost end nozzle and the lowermost end nozzle in two scans, are shown to have landed at proper positions close together at the beginning of the printing scan. However, as the scan proceeds, the upper dot row and the lower dot row gradually part from each other, forming a blank or white line on the image.
FIG. 2 schematically shows trajectories of ink droplets ejected from the print head during the scan. In FIG. 2, as it moves in a direction perpendicular to a plane of the drawing, the print head ejects ink droplets from its nozzles toward the print medium as indicated by arrows. As shown in the figure, the ink droplets ejected from the end nozzles of the print head deflect toward the central part of the head. This tendency has been observed to become conspicuous when an image is formed using very small ink droplets and when a printing density is high. However, if a print head with a high nozzle density and small ink droplets is used, this phenomenon does not occur as long as the printing density is not high enough.
While the above explanation concerns a case of the serial type printing apparatus, the “end dot deflection” phenomenon of course occurs with the line type print head, too. In the line type print head, it is common that a plurality of nozzle substrates each having a plurality of nozzles arrayed at high density are arranged in a direction of print width, as shown in FIG. 6. Thus, the “and dot deflection” occurs with the nozzles situated at the ends of each nozzle substrate, with a blank line formed at a position between the nozzle substrates. It is noted that the method of arranging a plurality of these nozzle substrates as described above and the phenomenon of blank line produced between the nozzle substrates are not peculiar to the line type printing apparatus. Also in the serial type, this arrangement is adopted when an elongate print head is used and therefore the blank line phenomenon results.
FIG. 3 and FIG. 4 schematically show printed dots in an area on a print medium between two nozzle substrates, formed by an elongate print head of the line type or serial type printing apparatus. FIG. 3 represents a case in which the print density of the print head is low (25%) and FIG. 4 a case in which the print density is high (100%).
In FIG. 3, H1100A and H1100B represent nozzle substrates arranged close together. In each of the nozzle substrates, nozzle openings H1105 for ejecting ink are arranged at a pitch of Pn. Of these nozzle openings, the shaded ones are nozzle openings that are activated and represent the print density of 25%. 301 represents a state of dots printed by the shaded nozzle openings while moving the print medium in a vertical direction of the drawing. Right-inclined shade lines and left-inclined shade lines indicate from which nozzle substrates the dots of interest have been ejected. In 301, the printed dots are arranged uniformly at the same pitch as the nozzle pitch Pn.
As in FIG. 3, dots printed at a print density of 100% are shown in FIG. 4. In the printed dot array indicated by 401, a gap is produced between a dot group of right-inclined shade lines and a dot group of left-inclined shade lines. That is, ink droplets ejected from the nozzles at the right end of the nozzle substrate H1100A deflect toward left as they land on the print medium and ink droplets ejected from the nozzles at the left end of the nozzle substrate H1100B deflect toward right.
As described above, in ink jet printing apparatus of recent years that eject small ink droplets at a high resolution, the “end dot deflection” poses a serious problem.
Some countermeasures, though not limited to the “end dot deflection,” have already been proposed to improve image impairments that occur at a boundary between nozzle substrates. For example, Japanese Patent Application Laid-open No. 8-025693 (1996) discloses a method which overlaps an image printed by the print head in one printing scan and an image printed in the next scan by a predetermined amount. According to this method, of the image data printed in the preceding scan, image data in an area that is to be overlapped in the next scan is masked with a random mask pattern. Further, of the image data that is to be printed in the next scan, image data in an area that overlaps the previous scan is masked with an inverted pattern of the previously applied random mask pattern. With this arrangement, the image impairment characteristic of the boundary between the succeeding printing scans is dispersed in an area of a predetermined width, making a boundary line on the image less conspicuous.
This method can be applied also to the line type print head. That is, the ends of the two nozzle substrates are overlapped, with the nozzles in the overlapped portion printing image data masked with the random mask pattern.
However, since the method of Japanese Patent Application Laid-open No 8-025693 (1996) is not intended specifically for the “end dot deflection,” this method may give rise to another problem where the “end dot deflection” phenomenon does not exist. For example, as described in connection with FIG. 3 and FIG. 4, the “end dot deflection” varies in its intensity depending on the print density of the print head. The method of Japanese Patent Application Laid-open No. 8-025693 (1996), however, performs the above-described processing on the boundary portion irrespective of the print density. That is, the boundary portion is always subjected to the processing different than that applied to other areas. Where the “end dot deflection” does not occur, this method may make the boundary portion more distinguished in the form of different texture or dark line. Further, since in the serial type printing apparatus, the greater the overlapping area the lower the printing speed, a problem arises that even a simple image that can be formed with a low print density takes unduly long to print.
There are some printing methods proposed specifically for solving the “end dot deflection” problem. For example, Japanese Patent Application Laid-open No. 2002-096455 discloses a method which, when performing a multipass printing in a serial printing apparatus, involves dividing a nozzle column into a plurality of sub-columns at a predetermined pitch and setting different thinning factors for the different divided sub-columns. With this method, the print density of the nozzles situated at the ends of an area printed in one scan can be set small beforehand. Since the number of dots whose landing positions are deviated from intended positions can be minimized, a blank line such as described in connection with FIG. 1 is rendered indistinguishable.
It is noted, however, that since the method of Japanese Patent Application Laid-open No. 2002-096455 uses a multipass printing as a precondition, it can only be applied to the serial type printing apparatus. Further, since this printing method is intended to print a high-quality image such as photograph using a multipass printing and taking a prolonged time, it cannot be applied to an ink jet printing apparatus that performs a fast printing for industrial applications that this invention is intended to achieve. Further, the method of Japanese Patent Application Laid-open No. 2002-096455 produces differences in the number of ejections or ejection frequency among a plurality of nozzles arrayed in the print head. Those nozzles whose ejection frequencies are high deteriorate in ejection characteristic faster than other nozzles. A print head is determined as not usable when even a single nozzle fails. Thus, the method described in the cited reference, which causes a local portion of the nozzles to print at high frequency, results in a shorter life of the print head.
As described above, in ink jet printing apparatus that perform a high-resolution printing using small droplets, particularly those ink jet printing apparatus that form an image at high speed without performing a multipass printing, the image impairments caused by the “end dot deflection” is not yet resolved.