BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing an “end-deviation”;
FIG. 2 is a graph showing test results that the inventors performed to check the degree of the “end-deviation”;
FIG. 3 is a view showing a print state in the case of actually printing an image with the printing head which generates the end-deviation;
FIG. 4 is a graph showing a relationship between an ejection volume and an end-deviation amount;
FIG. 5 is an explanatory schematic view of a multi-pass printing method;
FIG. 6 is a view showing mask patterns which are improved to avoid the end-deviation;
FIG. 7 is a schematic view showing a deflection state in an ejecting direction different from the end-deviation;
FIG. 8 is a graph showing a relationship between a position of an ejection port and a print position deviation;
FIG. 9 is a view showing a print state in the case where an image is actually printed by one-pass with use of the printing head in the print state shown in FIG. 8;
FIG. 10 is a schematic perspective view showing a main part of an inkjet printer according to an embodiment of the present invention;
FIG. 11 is a cross sectional view of an ejection portion of a printing head;
FIG. 12 is a block diagram illustrating a control constitution of the inkjet printer according to the embodiment of the present invention;
FIG. 13 is a view showing the printing head, which is observed from an ejection port surface side, according to a first embodiment of the present invention;
FIG. 14 is a view showing a print state in performing a two-pass type multi-pass printing;
FIGS. 15A and 15B are views of mask patterns employed for the two-pass type multi-pass printing of the first embodiment respectively;
FIG. 16 is a view of a printing head according to a second embodiment of the present invention, which is observed from an ejection port surface side;
FIG. 17 is a graph showing print permission rate of large and small ejection volumes relative to an input density signal;
FIG. 18 is a view of a mask pattern employed for an ejection port array having a large ejection volume of the second embodiment;
FIG. 19 is a view of a mask pattern employed in the reference example; and
FIG. 20 is a view explaining a problem in the case of performing tow-pass type multi-pass printing with use of the mask pattern of FIG. 19.
DESCRIPTION OF THE EMBODIMENTS
An embodiment of the present invention will be described below citing a serial type inkjet printer having printing heads provided with a plurality of ejection port arrays as an example.
FIG. 10 is a schematic perspective view showing a main part of an inkjet printer according to the embodiment of the present invention. In FIG. 10, the reference numeral 502 denotes a carriage, and printing heads 1 and ink tanks for supplying ink of four colors thereto are changeably mounted on the carriage 502.
The ink of four colors are printable via the printing head 1, and cyan ink, magenta ink, yellow ink and black ink are respectively supplied from the ink tanks. The printing head 1 is positioned and changeably mounted on the carriage 502, a connector holder (electrical connecting part), in which a driving signal, etc., is transmitted to the printing head 1 via a connector, is provided on the carriage 502.
The carriage 502 moves along a guide shaft 503 provided in an apparatus main body while being guided and supported in a main scanning direction. Driving force of a main scanning motor 504 is transmitted to a motor pulley 505, a following pulley 506 and a timing belt 507, and thus the carriage 502 moves, and a position and a movement amount thereof are controlled.
A print medium 508 such as a sheet of paper or plastic thin plate is conveyed so as to pass through a position (print part) opposite a ejection port surface of the printing head 1 by rotation of two sets of conveying rollers (509 and 510, and 511 and 512). Moreover, the back side of the print medium 508 is supported by a platen (not shown) so that the print medium 508 can form into a flat printing surface in the print part. The ejection port surface of the printing head 1 mounted on the carriage 502 is projected downward from the carriage 502 and held between the two sets of conveying rollers (509 and 510, and 511 and 512) so as to be kept parallel with the print medium 508.
FIG. 11 is a cross sectional view of an ejection portion of the printing head 1. In FIG. 11, the reference numeral 24 denotes a substrate composed of a silicon wafer. The substrate 24 is a part of an ink flow path constituting member, and serves as an electrical thermal converter (heater), ink flow-pass and supporting body of a material layer forming electrical thermal converters (heaters), ink flow paths and ejection ports. In the embodiment, the substrate 24 may be composed of glass, ceramics, plastic, metal, etc., other than silicon.
Electric thermal converters (heater) 26, which are thermal energy generating means, are arranged on a substrate 24 at pitches of 600 dpi in a sub-scanning direction, on both sides in a longitudinal direction of the ink supplying port 20. Furthermore, the two heater arrays are arranged so as to deviate from each other by a half pitch in the sub-scanning direction.
A coated resin layer 29 for introducing the ink into each heater is adhered to the substrate 24. Flow paths 27 and the ink supplying port 20 are formed in the coated resin layer 29, the flow paths 27 each being formed at the position corresponding to the heater, and the ink supplying port 20 being capable of evenly supplying the ink to each flow path 27. A tip of each flow path 27 forms into an ejection port 28 for ejecting ink droplets cased by a film boiling effect by the heater 26.
In the above constitution, a voltage is applied to each heater at a predetermined timing while the printing head is moved in a main scanning direction, and thus the ink droplets supplied from the same ink supplying port 20 can be printed at a resolution of 1200 dpi in the sub-scanning direction.
One kind of ink is supplied to one ink supplying port 20. A plurality of ink supplying ports 20 are juxtaposed on the substrate 24, and various kinds of ink can be respectively ejected from the ink supplying ports 20.
FIG. 12 is a block diagram illustrating a control constitution of the inkjet printer according to the embodiment. In FIG. 12, a controller 700 is a main controller, and includes: a CPU 701 in the form of, for example, a micro-computer; a ROM 702 in which a program, a desired table and other fixed data are stored; and a RAM 703 in which an are a for development of image data, an are a for working, etc., are provided. A mask pattern to be used in the embodiment is stored in the ROM 702. The CPU 701 performs logical AND operation between the image data supplied from a host device 704 and a mask pattern read from the ROM 702, generating print data for a plurality of print scanning. Then, the CPU 701 supplies this print data for each print scanning to a head driver 709.
The host device 704 connected to the exterior of the printer is a supplying source of the image data. However, the device 704 may be a computer for preparing and processing data such as an image to be printed, a reading part for reading the image, etc. Image data, other commands, status signals, etc., are transmitted/received to/from the controller 700 via an interface (I/F) 712.
An operating part 705 is a switch group for receiving an instruction input from an operator, and includes: a power source switch 706; a print switch 707 for instructing the controller to start printing operation; and a recovery switch 708 for instructing the controller to start maintenance processing for the printing head.
A head driver 709 is a driver for driving the electric thermal converters 26 of the printing head 1 in accordance with print data, etc. The head driver 709 includes: a shift register for making the print data align in accordance with the positions of the electric thermal converters 26; a latch circuit for latching at a proper timing; a logic circuit element for operating the electric thermal converters 26 in synchronization with a driving timing signal; a timing setting part for suitably setting a driving timing (ejecting timing) for dot formation positioning; etc.
A sub-heater 712 is provided in the printing head 1. The sub-heater 712 performs a temperature adjustment for stabilizing ink ejecting features. Although the sub-heater 712 may be formed on the substrate 24 of the printing head together with the electric thermal converter 26, this may be attached to a main body of the printing head 1.
A motor driver 711 is a driver for driving the main scanning motor 504, and a motor driver 713 is a driver for driving a sub-scanning motor 714 for generating force for rotating the conveying rollers.
FIG. 13 is a view of the printing head 1 of the embodiment, which is observed from an ejection port surface side. Four ejection port arrays 1302 to 1305 are arranged on the substrate 24. Cyan ink is ejected from the ejection port array 1302, magenta ink is ejected from the ejection port array 1303, yellow ink is ejected from the ejection port array 1304, and black ink is ejected from the ejection port array 1305. Ink droplets of 0.6 pl are ejected from each ejection port. The ejection port array of each color is a pair of arrays with each having 128 ejection ports, that is, 256 in total, at pitches of 600 dpi, and which are arranged so as to deviate from each other by a half pitch.
FIG. 14 is a view showing a print state in performing a two-pass type multi-pass printing with use of the printing head 1. In FIG. 14, the printing head 1 performs ejecting ink while reciprocating in the main scanning direction so that the dots are printed on the print medium.
In a first print scanning, printing is performed for pixels of approximately 50% in forward direction via the 128 ejection ports of each color positioned at the lower half part of the printing head 1. When the first print scanning ends the print medium is conveyed by a length corresponding to half of a print width of the printing head 1 in the sub-scanning direction in FIG. 14.
In the following second print scanning, printing is performed for the remaining pixels of 50% in backward direction in the image are a, where the printing has already been performed for the pixels of approximately 50% by the first print scanning, via the 128 ejection ports positioned at the upper half part of the printing head 1. In addition, in the second print scanning, the lower half part of the printing head 1 performs printing for pixels of approximately 50% of a blank are a adjacent to the image area. When the second print scanning ends, the print medium is further conveyed in the sub-scanning direction in FIG. 5 by the length corresponding to half of the print width of the printing head 1. An image is formed in stages by alternately repeating the above reciprocation print main scanning for the pixels of approximately 50% and the sub-scanning of the length corresponding to half of the print width. An approximately 50% printing in each print scanning is performed with the mask pattern prepared in advance.
As printing conditions in the embodiment, the moving speed of the carriage in the print scanning was 25 inch/sec, and 100% printing was performed by the printing head for the pixels arranged at pitches of 1200 dpi, and the distance between the print medium and ejection port surface was fixed at 1.0 mm. If the two-pass type multi-pass printing is performed with generally used random mask patterns under such conditions, the print position deviation arises having a tendency as shown in FIG. 8. That is, the density unevenness as shown in FIG. 9 is found on a 100% image output. The random mask pattern in the present description is a mask pattern, in which print permission pixels and print non-permission pixels are arranged and prepared at random so that an average print permission rate of all ejection ports becomes 50%. Accordingly, there is no bias shown in FIG. 6, and the print permission pixels are uniformly scattered over the entire are a.
On the other hand, when printing is performed with the mask patterns shown in FIG. 6 disclosed in Japanese Patent Laid-Open No. 2002-096455, the density unevenness is further increased.
As described referring FIGS. 7 and 8, the position of the dot printed via the ejection port positioned at the middle between the outermost ends and the center is most deviated.
Accordingly, it is considered that if the frequency of ejecting via the ejection ports positioned at vicinity of the middle is reduced, the adverse effects of the print position deviation would be diminished. Regarding a mask pattern in which the print permission rates of the ejection ports positioned at vicinity of the middle is reduced. FIG. 19 is an example of thus mask pattern, in which the print permission rates of the ejection ports positioned at vicinity of the middle is lowered compared with that of the ejection ports positioned at center and both end. Employing thus mask pattern, the print position deviation generated in one print scanning is reduced.
However, if the two-pass type multi-pass printing is performed with the mask pattern of FIG. 19, the complementary relationship between 1-pass and 2-pass is not be realized, that is defectiveness as a mask pattern. For a mask pattern employed in two-pass type multi-pass printing, it is necessary that 1-pass mask pattern corresponding to the lower half part of the printing head and 2-pass mask pattern corresponding to the upper half part of the printing head have a complementary relationship each other. However, the mask pattern of FIG. 19 does not have this relationship. In order to reduce the adverse effects of the print position deviation in multi-pass printing, a mask pattern having a complementary relationship and being able to reduce the print position deviation is demanded. A mask pattern shown in FIG. 15 meets the requirement, in which the print permission rates of the ejection ports positioned at both ends each having a small deviation amount are set higher than those of the ejection ports positioned at the other parts (for example, center parts).
FIGS. 15A and 15B are views of mask patterns employed for the two-pass type multi-pass printing of the embodiment respectively. It is an object of the embodiment to reduce the density unevenness generated in the case where the image is printed with the printing head having the tendency of the print position deviation shown in FIG. 8. Accordingly, a mask pattern is employed in which the print permission rates of the ejection ports positioned at both ends each having a small deviation amount are set higher than those of the ejection ports positioned at the other parts (for example, center parts).
FIG. 15A shows a mask pattern in which the print permission rate is gradually changed in relation to the position of the ejection port, and FIG. 15B shows a mask pattern in which the print permission rate is changed at three stages in relation to the position of the ejection port. In both mask patterns shown in FIGS. 15A and 15B, the print permission rates of both ends are 75%, and the print permission rate of the center is 25%.
When the multi-pass printing is performed with mask patterns 1400 and 1401 in which the print permission rates of the ejection ports at both ends are thus set higher than those of the ejection ports positioned at the other parts, the print position deviation shown in FIG. 8 is reduced even if printing is performed with the printing head for ejecting small droplets of 0.6 pl. Accordingly, an image having no density unevenness and having high definition can be obtained.
Furthermore, although the ejection port arrays 1302 to 1305 for four colors are arranged in the printing head of the embodiment, the mask patterns shown in FIG. 15 are not required to be used for all ejection port arrays. Even if the ejection volume is small, there is a case where the end-deviation becomes more conspicuous than the density unevenness depending on the ink color, or there is a case where both adverse effects become inconspicuous. In such cases, a distribution of the print permission rates of the mask pattern may be changed in accordance with the priority of an image adverse effect included in each color. For example, the mask patterns shown in FIG. 6 may be used in the case where the end-deviation becomes more conspicuous than the density unevenness. In addition, in the case where both end-deviation and density unevenness become inconspicuous the conventional random mask pattern may be used so that the frequency of ejecting of each ejection port is made as equal as possible.
In addition, the print permission rate of ejection ports positioned at the both end and at the center is not limited to the combination of 75% and 25%. The print permission rate of ejection ports positioned at the both end and at the center may be a combination of 90% and 10% or a combination of 60% and 40%, for example. If a mask pattern in which the print permission rates of the ejection ports positioned at both ends are set higher than those of the ejection ports positioned at the other parts (for example, center parts) is provided, the mask pattern may have applicability to this invention.
Second Embodiment
A second embodiment of the present invention will be described hereinafter. The inkjet printer and inkjet printing head as described with reference to FIG. 10 to FIG. 12 are used in the embodiment similarly to the first embodiment. However, the arrangement of each ejection port is different from that of the first embodiment.
FIG. 16 is a view of a printing head 1, which is observed from a ejection port surface side, used in the second embodiment. Twelve large and small ejection port arrays in total are arranged on a substrate of the embodiment, and 128 ejection ports are arranged in each ejection port array at pitches of 600 dpi. Ink droplets of 2.8 pl are ejected from ejection port arrays C1, C2, M1, M2, Y1, Y2, Bk1 and Bk2, and ink droplets of 0.6 pl are ejected from ejection port arrays C3, C4, M3 and M4. In addition, the cyan ink is ejected from the ejection port arrays C1, C2, C3 and C4, the magenta ink is ejected from the ejection port arrays M1, M2, M3 and M4, the yellow ink is ejected from the ejection port arrays Y1, Y2, and the black ink is ejected from the ejection port arrays Bk1, Bk2.
When an image is thus formed in a plurality of stages of ejection volume regarding one color, print data is adjusted for every ejection port array in accordance with an input density signal.
FIG. 17 is a graph showing print rates of the ejection port arrays of which the ejection volumes are different from each other relative to an input density signal. Here, the print rate of 100% shows a state where the ink droplets are printed for all pixels one by one. Printings with a large dot (2.6 pl) and small dot (0.6 pl) are possible for all pixels. When an image density is low, only the printing with the small dot is performed. When the image density is raised to a certain degree (30% in this case) the printing with the large dot is started, the rate thereof is gradually increased, and simultaneously the rate of the printing with the small dot is gradually reduced. When the image density becomes maximum (100%), all pixels is printed with the large dot.
In the embodiment, the mask pattern 1400 or 1401, in which the print permission rate of the end is higher than that of the center, shown in FIG. 15 is applicable to a ejection port array for which the print position deviation as shown in FIG. 8 is considered to easily arise, that is, the ejection port arrays C3, C4, M3 and M4 each having a small ejection volume. On the other hand, the mask pattern, in which the print permission rate of the end is set lower than that of the center, shown in FIG. 18 is applicable to the ejection port arrays C1, C2, M1, M2, Y1, Y2, Bk1 and Bk2 each of which the end-deviation is considered to easily arise and each of which has a relatively large ejection volume.
According to the embodiment as described above, when an image is printed with a printing head having a plurality of ejection port arrays corresponding to ejection volumes of a plurality of stages, a mask pattern, in which the print permission rates of both ends are set higher than those of the other parts, is made to correspond to the ejection port array having a smaller ejection volume. Thus, the image can be obtained which has no end-deviation and density unevenness and has high definition.
Furthermore, although the two-pass type multi-pass printing is cited in the above embodiments, the present invention is not limited thereto. In this case, when the number of passes is changed, a substantial ejecting frequency of the printing head is also changed. Accordingly, even in the case where the same printing head is used, it can be assumed that the tendency of the print position deviation is changed by changing the number of passes. That is, although the tendency of the print position deviation as shown in FIG. 8 appears and the density unevenness becomes more conspicuous in the two-pass type multi-pass printing, a case, where the end-deviation becomes more conspicuous than the density unevenness, can be assumed in a four-pass type multi-pass printing. In this case, a mask pattern to be employed may be properly changed in accordance with the number of passes.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Laid-Open No. 2006-130790, filed May 9, 2006, which is hereby incorporated by reference herein in its entirety.
Patent Application
20070285450
