This application claims priority from Japanese Patent Application No. 2002-084407 filed Mar. 25, 2002, which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet print head and an ink jet printing apparatus for ejecting ink onto a print medium to form an image thereon.
2. Description of the Related Art
An ink jet printing apparatus forms an image on a print medium by ejecting ink droplets from a print head mounted in an apparatus body onto the print medium, with the ink droplets adhering to the print medium and fixing in it to produce their intended colors. Recent years have seen a proliferation of a so-called serial scan type ink jet printing apparatus. In this type, an image is formed by alternately repeating two operationsxe2x80x94a printing scan for scanning the print head over the print medium to eject ink onto the medium and a paper feed for moving the print medium or the print head relative to each other in a direction perpendicular to a printing scan direction. The serial scan type ink jet printing apparatus, however, has the following drawback.
In the serial scan type apparatus, a single printing scan can only produce an image of a predetermined printing width for at least one color of ink (this single printing operation is referred to also as a xe2x80x9cone-pass printingxe2x80x9d). Hence, to form an image over the entire print medium requires performing a plurality of printing scans. When in such a system an image of high duty is to be formed, a problem may occur that a boundary portion between an image area formed on the print medium in a certain printing scan and an adjoining image area formed in another printing scan appears light in density.
This problem is considered to occur in the following mechanism. FIGS. 7A to 7D are schematic views showing how an image of high duty is formed during the one-pass printing, as seen in the print head scanning direction. In the figure, reference number 1 represents a print head, 2 a print medium, and e a column of nozzles (also referred to as a xe2x80x9ccolumn of ejection openingsxe2x80x9d) for ejecting ink droplets.
FIG. 7A shows ink droplets adhering to a print medium which were ejected in one printing scan. In the figure, p1 denotes an ink adhering to the print medium. With the elapse of time the ink on the print medium soaks into the medium and fixes there. FIG. 7B illustrates this state and p2 denotes the ink that has soaked into the print medium and fixed there. After the printing scan, the print medium is fed in a direction perpendicular to the printing scan direction of the print head (this operation is called a line feed) and the next printing scan is performed. FIG. 7C shows a state in which the line feed and the second printing scan have been performed. In the figure, a distance that the print medium was fed is indicated by an arrow. This line feed distance is equal to the length of the nozzle column of the print head. Ink droplets adhering to the print medium that were ejected in the second printing scan are indicated by p3.
FIG. 7D shows a state in which the ink that landed on the print medium during the second printing scan has soaked and fixed with elapse of time. As shown by p1 of FIG. 7A and p3 of FIG. 7C, the ink that has just landed on the print medium and has not yet soaked into and fixed in the print medium forms an ink surface that is low at ends and bulges at a center. This is a common phenomenon produced by a surface tension of the ink. In this state, the ink penetrates and fixes in the print medium. Therefore, as indicated by p2 and p4 of FIGS. 7B and 7D, in an image area formed by each printing scan, an amount of ink that fixes at the end portions is less than at other portions and the color of that portions tends to be lighter. Thus, when the printing scan is repeated a plurality of times to form an image of high duty, the end portions of an image area formed by each printing scan appear light. That is, the boundary portions between adjoining image areas are printed lighter than other portions, giving rise to a problem of light stripes showing up in the printed image. In the case of a black ink in particular, since its penetration capability is generally low, it tends to produce a greater density difference between a dot center and a dot end than do color inks. This may result in boundary portions between adjoining image areas printed by different printing scans appearing lighter and in the worst case showing up as white horizontal stripes.
A possible measure to deal with this problem may involve making the line feed distance shorter than the printing width or nozzle column length of the print head. One such example is to design a nozzle-to-nozzle interval (nozzles may also be referred to as xe2x80x9cejection openingsxe2x80x9d) somewhat longer than normal. As a result, the length of the nozzle column used for the one-pass printing becomes somewhat longer than the line feed distance, producing the following advantages.
FIGS. 8A to 8D show dots ejected from a nozzle column with a longer-than-normal nozzle-to-nozzle interval. In the figure, reference numeral 1 represents a print head, 2 a print medium and e a nozzle column for ejecting ink droplets. FIG. 8A shows ink droplets adhering to the print medium which were ejected in one printing scan. In the figure, p1 denotes an ink adhering to the print medium. With the elapse of time the ink on the print medium soaks into the print medium and fixes there. FIG. 8B illustrates this state and p2 denotes the ink that has soaked into the print medium and fixed there. After the first printing scan, the print medium is fed (line feed) in a direction perpendicular to the printing scan direction of the print head and the next printing scan is performed. Because the nozzle-to-nozzle interval of the print head is set somewhat longer than normal, the line feed distance is shorter than the nozzle column length e. FIG. 8C shows a state in which the line feed and the second printing scan have been performed. In the figure, a distance that the print medium was fed is indicated by an arrow and, as described above, is somewhat shorter than the nozzle column length e of the print head. Ink droplets adhering to the print medium that were ejected in the second printing scan are indicated by p3. Then, the ink that landed on the print medium during the second printing scan also sinks and fixes in the print medium over time, as shown in FIG. 8D.
Since the line feed distance shown in FIG. 7C is equal to the nozzle column length or a difference between the line feed distance and the nozzle column length is smaller than that of FIG. 8C, a comparison between FIG. 8D and FIG. 7D shows that an overlap between p2 and p4 is somewhat larger in FIG. 8D than in FIG. 7D. Thus, as shown in FIG. 8D, the above-described problem that a boundary portion between an image area formed on the print medium by a printing scan and an adjoining image area formed by another printing scan appears lighter than other portions is less likely to occur.
Printing apparatus capable of printing color inks as well as black ink are available in recent years. Some of these printing apparatus have a black ink nozzle column set longer than other color ink nozzle columns in order to reduce a time taken by the printing operation using only the black ink as in a document printing. In this arrangement, when printing is done using only the black ink, all the nozzles of the black ink nozzle column are used, whereas during color printing, only that part of the black ink nozzle column which is almost equal in length to other color ink nozzle columns is used. In such a printing apparatus, in which the length of that nozzle portion in the entire nozzle column which is used for printing is changed according to an image being formed, a problem may arise that lighter horizontal stripes will show up in a printed image at boundaries between adjoining image areas formed on a print medium by separate printing scans, depending on the length of the nozzle portion used for printing. This problem will be explained as follows.
Referring to FIG. 2 and FIG. 3, reference number 1 denotes a print head, 3 a nozzle column for ejecting a black ink, and 4 nozzle columns for ejecting color inks. To solve the problem described above, the black ink nozzle column is formed longer than the color ink nozzle columns. In the black nozzle column 3, the entire nozzles are represented as a nozzle portion e and a part of the nozzle column is denoted a nozzle portion b. The nozzle portion b has one-half the length of the nozzle portion e. The entire nozzles arrayed in each of the color ink nozzle columns are represented as a nozzle portion a. The number of nozzles in the nozzle portion a counted in the column direction is equal to that of the nozzle portion b.
FIG. 2 is a schematic view showing an operation of the printing apparatus when an image is formed using only a black ink. When an image is formed using only the black ink, the whole black nozzle column (nozzle portion e) is used as described above. In the figure, (f1)-p1 represents a position relative to the print head of an image formed with the black ink in one printing scan. This is followed by a line feed of a predetermined distance in a direction indicated by LF. The line feed distance is shorter than the length of the nozzle portion e. The printed image p1 moves to a position (f2)-p1. After this, another printing scan is performed to form an image (f2)-p2.
FIG. 3 is a schematic view showing an operation of the printing apparatus when an image is formed using a black ink and color inks. As described above, when an image is formed using color inks as well as a black ink, the nozzle portion b of the black nozzle column and the nozzle portion a of the color nozzle columns are used. In the figure, (f1)-p1 represents a position relative to the print head of an image formed with the black ink in one printing scan. After this, a line feed of a predetermined distance is carried out in the direction of LF, moving the printed image p1 to a position (f2)-p1. This is followed by another printing scan to form an image at a position (f2)-p1 using color inks and an image at a position (f2)-p2 using a black ink. As a result, in the (f2)-p1 area the image forming using the black ink and the color inks is completed.
Whether an image is to be made using only a black ink or both a black ink and color inks is determined based on image data sent from a host computer. A printer driver running on the host computer displays an operation window for the user to select either a color printing or a black-only printing. When the user makes a selection on the operation window, the printer driver sends a color printing instruction or a black-only printing instruction along with image data to the printing apparatus. The printing apparatus determines the operations of various driving units according to the instruction received. Another arrangement is also available in which, rather than the user selecting either a color printing or a black-only printing, the printing apparatus checks the image data transferred from the host to make a decision. Still another arrangement is available in which a detailed control is performed to switch the black nozzle operation between a long nozzle portion and a short nozzle portion of the black nozzle column according to the image data in each page. That is, in an area of each page to be printed with only a black ink a long black nozzle portion, i.e., entire black nozzle column, is used and, in an area to be printed with color inks as well, a short black nozzle portion equal in length to the color nozzle columns is used.
In a printing apparatus with a means to change the length of a black nozzle portion to be used for printing, it has been proposed to set a nozzle interval a predetermined amount longer than normal to deal with the aforementioned problem of light density portions showing up in a printed image at boundaries between image areas printed by separate printing scans. As explained earlier in conjunction with FIG. 2 and FIG. 3, the length of an activated portion of the black nozzle column differs between the black-only printing and the color printing. Therefore, the difference between the line feed distance and the width (in the line feed direction) of a black printed area also varies. More specifically, the black nozzle column is set somewhat longer than normal by expanding the nozzle intervals uniformly. If it is assumed that the black nozzle column is set longer by t than the normal nozzle column length s, an entire length of the nozzle column is s+t. In a black-only printing, the entire black nozzle column is used and, if the line feed distance is assumed to be s, image areas printed by separate printing scans overlap each other over a distance of t. In a color printing, only the nozzle portion b of the black nozzle column is used, that is, only one-half of the black nozzle column is used. Then, the length of the nozzle portion b is 1/2xc2x7(s+t). Suppose that the line feed distance is s/2. The difference between the line feed distance and the length of the nozzle portion b is only t/2. Thus the overlap between the image areas is only t/2. This means that, if the nozzle interval is expanded to ensure an enough overlap during the black-only printing, the color printing cannot secure a sufficient overlap. Conversely, if the nozzle interval is set so as to cause a sufficient overlapping during the color printing, the amount of overlap at the boundary portions between separate printing scans becomes too large, giving rise to a problem that the overlapped portions may look darker than other portions.
In light of the conventional problems described above, it is an object of the present invention to provide an ink jet print head and an ink jet printing apparatus which can produce a good printed result at all times at boundary portions between image areas printed by separate printing scans even in ink jet printing apparatus in which a range of use of the nozzle column varies according to the printing condition.
In one aspect, the present invention provides an ink jet print head having a plurality of nozzles arrayed in a predetermined direction to form a nozzle column, wherein the nozzle column ejects ink droplets, the ink jet print head comprising: a long nozzle column portion formed in a predetermined portion of the nozzle column, the long nozzle column portion having a wider nozzle interval than those in other portions of the nozzle column.
In another aspect, the present invention provides an ink jet printing apparatus comprising: a ink jet print head having a plurality of nozzles arrayed in a predetermined direction to form a nozzle column, the nozzle column being adapted to eject ink droplets; wherein the ink jet print head is scanned over a print medium a plurality of times in a direction different from the direction of array and a printing scan and a line feed are performed to print on a predetermined image area on the print medium, the printing scan ejecting ink droplets onto the print medium during each scan and the line feed feeding, between each of the plurality of scans, the print medium and the ink jet print head relative to each other in a direction different from the scan direction of the ink jet print head; wherein a portion of the nozzle column in the ink jet print head is a long nozzle column portion whose nozzle-to-nozzle interval is wider than that in another portion of the nozzle column; wherein a width in the line feed direction of each image area printed by a single printing scan of the ink jet print head is longer than a distance that the print medium is fed by one print feed.
With this construction, by arranging the nozzles in the nozzle column such that, in only that portion of the nozzle column always used in any printing condition, such as color printing and black-only printing, its nozzles have a wider nozzle-to-nozzle interval than those of other nozzle portions, the width of each image area printed by a single printing scan can be made a predetermined amount longer than the line feed distance at all times. This arrangement can produce a printed result in which adjoining image areas printed by separate printing scans overlap each other at their boundary portions by a predetermined amount in whatever printing condition.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.