Patent Application
20150002569
1. Field of the Invention
The present invention relates to an inkjet printing apparatus and a printing method therefor, and particularly relates to controlling the color of an ink dot to be printed with a plurality of types of inks in an overlapping manner.
2. Description of the Related Art
It is known that in an inkjet printing method, the color (color appearance) of a dot on a print medium which is printed with a plurality of color inks in an overlapping manner varies depending on the order in which these inks are ejected and an ejection time difference. Japanese Patent No. 4343481 discloses that nozzles for ejecting a plurality of color inks are placed at different positions in a conveyance direction of a print medium to have the same order in which the plurality of inks are ejected, and to keep constant an ejection time difference between the plurality of inks in any scan area. This can suppress a color difference.
However, the technique for reducing a color difference as disclosed in Japanese Patent No. 4343481 is basically to have the same order in which inks of different colors are overlapped (ejected). Accordingly, it is necessary to provide print heads having a specific nozzle arrangement and specialize a printing operation including a scan with the print heads and conveyance of a print medium, and it is difficult to carry out the technique disclosed in Japanese Patent No. 4343481 by using print heads and a printing operation which are generally and widely used. On the other hand, in a case where the colors of an image to be printed can be changed by not only maintaining the color appearance, but also controlling the color appearance to change the color, the degree of freedom of design of the image to be printed can be increased.
An object of the present invention is to provide an inkjet printing apparatus and a printing method therefor which can change the colors of an image to be printed by controlling the color appearance in the case of overlapping inks of different colors to perform printing.
In a first aspect of the present invention, there is provided an inkjet printing apparatus comprising: a print head unit for ejecting at least an ink of a first color and an ink of a second color different from the first color on a print medium; and a print control unit configured to control the print head unit to precedently eject the ink of the first color on the print medium and subsequently eject the ink of the second color on the ink of the first color precedently ejected on the print medium in an overlapping manner; wherein the print control unit controls the print head unit so that a permeation speed Ka at which the ink of the first color permeates the print medium is larger than a permeation speed Ka at which the ink of the second color permeates the print medium.
In a second aspect of the present invention, there is provided an inkjet printing method for performing printing by ejecting inks at least an ink of a first color and an ink of a second color different from the first color on a print medium by a print head unit comprising: precedently ejecting the ink of the first color on the print medium by the print head unit and subsequently ejecting the ink of the second color by the print head unit on the ink of the first color precedently ejected on the print medium in an overlapping manner; wherein the print head unit is controlled such that a permeation speed Ka at which the ink of the first color permeates the print medium is larger than a permeation speed Ka at which the ink of the second color permeates the print medium.
According to the above features, in the case of overlapping inks of different colors and performing printing, it becomes possible to control color appearance so as to change the colors of an image to be printed.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
Embodiments of the present invention will be described in detail below with reference to the drawings.
A belt 4 is suspended between a driven roller 2 rotated by the driving force of a conveyance motor 1 and a driven roller 3 placed away from the roller 2, whereby the belt 4 runs. The print medium 5 such as paper is fed and mounted on the running belt 4, whereby the print medium 5 is conveyed. In
In the present embodiment, the print medium 5 is conveyed at a speed of 5 [m/sec] in a direction of an arrow in the conveyance path. Firstly, the first cyan print head 7 prints dots with a cyan ink. Then, the second magenta print head 8 prints magenta. Then, the third yellow print head 9 prints yellow. Finally, the fourth black print head 10 prints black. In a case where dot printing is completed, the print medium 5 is conveyed to the fixing device 11 and dried and fixed with hot air.
The I/O port 24 is connected to a motor driver 26 for driving the conveyance motor 1, the print heads (7, 8, 9, and 10), and the fixing device 11. In these print heads, ejection heaters (27, 28, 29, and 30) are respectively provided for nozzles for ejecting inks. The ejection heaters impart thermal energy necessary for ejecting inks. Further, the print heads are respectively provided with sub-heaters (35, 36, 37, and 38) for controlling the temperatures of inks in the nozzles. Furthermore, the print heads are respectively provided with temperature sensors (31, 32, 33, and 34) for detecting the temperatures of the print heads.
In the above configuration, the CPU 21 obtains the head temperatures detected by the temperature sensors (31, 32, 33, and 34). Further, the inks in the print heads (7, 8, 9, and 10) are heated by controlling the sub-heaters (35, 36, 37, and 38) to keep the inks at a predetermined temperature. The inks kept at the predetermined temperature are ejected by energizing the print head heaters (27, 28, 29, and 30). Ink heating by the sub-heaters adjusts the ink temperatures to control colors obtained by ejecting the inks on the same pixel of a print medium in an overlapping manner as will be discussed below.
In the above configuration, the cyan ink is firstly ejected from the print head 7 to the print medium 5 which is conveyed from the upstream side to the downstream side. Then, the magenta ink is ejected from the print head 8. Then, the yellow ink is ejected from the print head 9. Finally, the black ink is ejected from the print head 10. In this manner, the cyan (C), magenta (M), yellow (Y), and black (K) inks are ejected in this order, and are ejected (land) on pixels of the print medium 5. Incidentally, in the present embodiment, the sizes of ink droplets ejected from the nozzles of the print heads are 3 [pl].
Next, explanation will be made on control of a color appearance at the time of ejecting color inks on a certain pixel in an overlapping manner according to an embodiment of the present invention. For brevity, explanation will be made on the case of ejecting, in an overlapping manner, the cyan ink from the print head 7 and the magenta ink from the print head 8 out of the inks of the four colors.
The cyan ink from the print head 7 is always firstly ejected on the print medium to permeate the print medium. Thereafter, the magenta ink from the print head 8 is ejected. Hereinafter, the print head 7 for ejecting the cyan ink will also be referred to as “the preceding cyan head” and the print head 8 for ejecting the magenta ink will also be referred to as “the subsequent magenta head.” Further, the cyan ink to be ejected precedently will also be referred to as “the preceding cyan,” and the magenta ink to be ejected subsequently will also be referred to as “the subsequent magenta.” In the present embodiment, an ejection time difference between the preceding cyan and the subsequent magenta is (a distance between the preceding cyan head 7 and the subsequent magenta head)/(a conveyance speed)=0.02 [m]/5 [m/sec]=4 [msec].
Regarding the printing order of the present embodiment, the preceding cyan and the subsequent magenta are always ejected in this order. Accordingly, in printing by a conventional technique, blue tinged with cyan is obtained, and a color reproducible area is large at a cyan side and small and distorted at a magenta side.
On the other hand, according to the embodiment of the present invention, a color appearance at the time of ejecting a plurality of inks in an overlapping manner is controlled regardless of the ejecting order. This can prevent color reproduction whose color gamut is distorted because of a color bias and also makes it possible to intentionally generate the color bias.
Firstly, the present inventors, et al. consider ideal ink permeation or an ideal permeation area which makes it possible to reproduce a color whose color gamut is not distorted.
Explanation will be made on the case of printing dots in forward and backward scans of a print head according to a serial printing method different from the full-line printing method of the present embodiment. In this bidirectional printing, an ink permeates or a permeation area is formed as shown in
In order to avoid the problem of a color shift which is found on a micro level, it is desirable that cyan and magenta inks permeate in a state in which the cyan ink and the magenta ink are fully mixed (colors are mixed). In a case where an ideal permeation area can be realized in which cyan and magenta inks permeate in a state in which the cyan ink and the magenta ink are fully mixed (colors are mixed), even in a case where a highlighted portion is observed on a micro level, only blue dots are observed and the color shift is not recognized.
The left side of
It is understood that in order to obtain the above ideal permeation state, in a process in which the inks permeate the surface of the print medium, the cyan ink and the magenta ink need to permeate in a state in which the cyan ink and the magenta ink are fully mixed in the print medium. However, in the example shown in
On the other hand, according to the embodiment of the present invention, permeation of the preceding cyan and the subsequent magenta is controlled, whereby the cyan ink and the magenta ink permeate the sheet in a state in which the cyan ink and the magenta ink are fully mixed.
Firstly, ink permeation will be described.
In a case where an ink is ejected on a print medium such as a sheet including capillaries, a colorant (a dye or a pigment) which contributes to a color appearance permeates the print medium with a carrier (a solvent or water) which carries the colorant. In this behavior of the ink, the colorant remains and is fixed in the upper layer, and the carrier disappears since the carrier permeates a lower portion of the print medium and disperses and evaporates, and finally, the colorant which contributes to the color appearance is fixed. As a larger amount of the colorant is fixed at a shallower position of the surface layer of the sheet, the colorant appears more intensely.
A permeation speed Ka in a print medium can be represented by the following Lucas-Washburn equation (the equation for capillary permeation).
Further, the present inventors, et al. find that an ink permeation area of a print medium can be made to approach an ideal state by performing control so that the ink permeation speeds Ka of a preceding ink and a subsequent ink have a specific relative relationship.
In the following, as one configuration, the ink permeation speed Ka is controlled via an ink temperature T. Then a relative relationship between the ink permeation speeds Ka of a preceding dot and a subsequent dot is controlled so that the state of the permeation area is set and thus a color is determined.
Firstly, explanation will be made on controlling the ink permeation speed Ka by using the ink temperature T as a control factor.
According to Mathematical Equation 1 above, the permeation speed Ka depends on the viscosity η and the surface tension γ and thus depends on its ratio (γ/η). The present inventors, et al. firstly investigated the temperature dependence of the viscosity η.
Next, the temperature dependence of the surface tension γ was investigated.
The ink permeation speed Ka [ml/m2(msec)12] can be calculated by using the viscosity η and the surface tension γ which are the physical properties obtained according to the ink temperature as described above.
Table 1 shows the ink viscosity 1 [mPa·s], the surface tension γ [mN/m], and the ink permeation speed Ka [ml/m2(ms)1/2] for the varying ink temperatures as described above.
Next, explanation will be made on ink permeation at the ink permeation speed Ka for the print medium which is obtained based on the ink temperature and its ink permeation area.
Likewise, the permeation speed Ka in a case where T=41 [° C.] is 1.26 times as large as the permeation speed Ka in a case where T=25[° C.]. Accordingly, the volume of the ink permeation area increases 2.0 (=(1.26)3) times, and the capillary occupancy rate p by the ink in the print medium is 50%. Therefore, a permeation area 121 obtained in a condition in which T=41 [° C.] becomes the one shown in
The permeation speed Ka in a case where T=60 [° C.] is 1.4 times as large as the permeation speed Ka in a case where T=25 [° C.], the volume of the ink permeation area increases 2.74 (=(1.4)3) times, and the capillary occupancy rate p by the ink is 36%. More specifically,
The embodiment of the present invention defines a relationship between the permeation speeds Ka of the preceding cyan ink and the subsequent magenta ink and a relationship (magnitude relationship) between the capillary occupancy rates p of the preceding cyan ink and the subsequent magenta ink in the print medium based on the ink temperature T as described above. This controls the permeation area when printing is performed with the preceding cyan and the subsequent magenta, thereby controlling the color appearance realized by the inks which are ejected in an overlapping manner.
In Example 1, evaporation in a micro area can be ignored, and permeation of the preceding cyan and the subsequent magenta realizes a color whose color reproduction area is not distorted.
In the permeation state shown in
The permeation area 122 in which the preceding cyan and the subsequent magenta exist in equal amounts in a mixed state as described above becomes the already-described ideal permeation area. Accordingly, it becomes possible to suppress generation of distortion of a color reproduction area caused by the order in which color ink droplets are ejected.
To sum up the conditions for Example 1, in a case where the permeation speed Kap of the preceding ink and the permeation speed Kan of the subsequent ink have relationships Kap>Kan and Kap=21/3(Kan), it is possible to perform printing with stable colors by mixing the preceding ink and the subsequent ink.
In Example 2, a final permeation area of a sheet includes a permeation area in which only the preceding cyan permeates as well as an area in which the preceding cyan and the subsequent magenta exist in a mixed state to realize permeation states.
A feedback temperature control is performed by using the sub-heater 35 based on a temperature read from the temperature sensor 31 so that the temperature Tc of the preceding cyan ink ejected from the print head 7 becomes 60 [° C.]. With this control of increasing the ink temperature is increased, as shown in
To summarize Example 2 above, in a case where the permeation speed Kap of the preceding ink is larger than the permeation speed Kan of the subsequent ink (Kap>Kan), it is possible to realize the area of the ideal state in which the preceding ink and the subsequent ink are printed in a mixed state as described above. In addition, in a case where Kap>21/3(Kan), there is formed a permeation area in which the color of the subsequent ink is dominant as well as the permeation area of the above ideal state.
In Example 3, as final permeation areas of a sheet, there are formed an area in which the preceding cyan and the subsequent magenta exist in a mixed state and a permeation area in which only the subsequent magenta permeates. In this example, temperature control is performed so that the temperature Tc of the preceding cyan ink ejected from the print head 7 becomes 30 [° C.]. In a case where the ink temperature is increased, the volume Vc of the permeation area 118 is 1.26 times as large as that of the permeation area 117 in a case where Tc=25 [° C.] (
In a case where the subsequent magenta is ejected in this permeation state, the amount of the subsequent magenta ink which can be further absorbed by the permeation area 118 in which the preceding cyan permeates is 0.26 (=1.26×(1−0.8)) time as large as the amount of the preceding cyan ink which is absorbed by the permeation area in a condition in which the ink temperature is 25 [° C.]. Therefore, if the magenta ink whose temperature is 25 [° C.] is used, the amount of the subsequent magenta ink which can be further absorbed by the permeation area in which the preceding cyan permeates in a condition in which the ink temperature Tc is 30[° C.] is 0.26 time as large as the amount of the preceding cyan ink which is absorbed by the permeation area in a condition in which the ink temperature Tc is 25 [° C.].
To summarize Example 3 above, in a case where Kap>Kan, the area in which the preceding ink and the subsequent ink are printed in a mixed state can be formed as described above. In addition, in a case where Kap<21/3(Kan), there is formed the permeation area in which the color of the preceding ink is dominant as well as the permeation area of the above mixture state.
Incidentally, in the above descriptions, it is assumed that evaporation in the micro area can be ignored. If the evaporation cannot be ignored, the permeation speed Ka can be set in consideration of the evaporation.
According to a second embodiment of the present invention, the ink permeation speed Ka is controlled by controlling the ink temperature with Peltier elements. Incidentally, in the descriptions of the present embodiment, elements which are the same as those of the first embodiment are assigned the same reference numerals and their detailed explanations are omitted.
In the present embodiment, the temperatures T of the preceding ink and the subsequent ink can be adjusted by using the Peltier elements. Control is performed so that the permeation speed Kap of the preceding ink is larger than the permeation speed Kan of the subsequent ink, and Kap=21/3(Kan). In a case where a room temperature is 41 [° C.] and the temperature Tc of the preceding cyan is 41 [° C.], for example, the Peltier elements perform cooling control so that the temperature Tm of the subsequent magenta ink is 25 [° C.]. This can create a state in which the preceding ink and the subsequent ink are printed in a mixed state. The ink permeation speed Ka is controlled by performing cooling in the above manner, whereby it becomes possible to use, as a print head member, a member whose thermal resistance is low and to reduce manufacturing cost.
Water has high surface tension and has characteristics that general paper is not likely to be wet with water. In this respect, it is necessary to add a substance for enabling water to permeate paper easily. In order to increase the permeation speed, it is possible to add a non-ionic surfactant, a polyhydric alcohol derivative, or a monovalent alcohol which has the effect of lowering a contact angle between a paper surface and an ink droplet. These additives can be used to control the viscosity η and the surface tension γ of the ink. An adjustment only has to be made so that the permeation speed Kap of the preceding ink is larger than the permeation speed Kan of the subsequent ink and Kap=21/3(Kan). In this manner, control is performed based on the physical properties of the ink, whereby control of a temperature on the side of a print element becomes unnecessary, and it becomes possible to carry out the invention with a simpler configuration. More specifically, the permeation speed of the ink can be controlled by adjusting the physical properties of the ink.
The above-described embodiments use a full-line head and provide feedback based on temperatures detected by the temperature sensors (31, 32, 33, and 34). More specifically, the embodiments use a thermal inkjet system in which the sub-heaters (35, 36, 37, and 38) maintain the ink temperatures and further the head heaters (27, 28, 29, and 30) are driven to apply thermal energy to eject ink droplets. However, another system can be used to carry out the present invention.
The above-described embodiments use a line head. However, the present invention can be applied to a case where a serial printing apparatus performs one-way printing in which printing is performed only in a forward path or a backward path. Further, in bidirectional printing, the ink temperature can be controlled in each printing operation, whereby printing is performed without a difference in color between forward and backward scans, for example.
Further, the above-described embodiments use the four print heads as the plurality of print heads, but the present invention is not necessarily limited to the embodiments. The present invention can use two, three, or five or more print heads. Further, regarding the types of inks, the present invention can be applied to the case of using three colors except black, the case of using five or more color inks, and the case of using a plurality of types of black inks as well as the case of using the four colors, cyan, magenta, yellow, and black.
As described above, the print head can be a thermal inkjet print head which ejects ink droplets by applying thermal energy and can also be a piezo jet print head which uses a piezo element. In a case where the print head is the piezo jet print head, the temperature can be controlled by providing a heater for heating an ink as well as the piezo element for ejecting an ink.
The above-described embodiments use a system for controlling the temperature based on feedback, but a control method used for temperature control is not limited to this and a publicly-known method can be used. For example, the head member and the heat capacity of an ink can be considered and adjusted so that the temperature of the print head is controlled in a self-balancing manner. In this manner, it becomes unnecessary to use parts required to form a feedback loop such as the sensors and the sub-heaters, and the configuration of the present invention becomes simpler.
Incidentally, it is possible to use, as the self-balancing print head used for the present invention, a thermal insulation print head (thermal insulation head) having a structure which will be described below.
Firstly, with reference to
The thermal insulation portion 74 is formed by using a material having a thermal expansion coefficient almost equal to that of the support plate 72 and thermal conductivity lower than that of the support plate 72, such as polyphenylene sulfide (PPS, thermal conductivity: 17 W/mK) which is thermoplastic plastic. A common liquid chamber 78 for storing an ink in a negative pressure state is formed on the upper surface of the thermal insulation portion 74 and is tightly fixed to prevent ink leakage.
Next, explanation will be made on a path for supplying an ink to each print element substrate. An ink introduced from an ink introduction port 76 to the print head flows in the common liquid chamber 78 in a longitudinal direction of the print head (an arrangement direction of the print element substrate) and subsequently flows into the communication port formed in the thermal insulation portion 74. Further, the ink is supplied to ejection port arrays 73 of the print element substrate 71 in an odd-number row through the individual ink supply path 77. The same can be said for a path for supplying an ink to the print element substrate 71 in an even-number row.
Next, explanation will be made on a transmission path for heat generated in the print element substrates 71 in a case where an ink is ejected. A remaining amount obtained by subtracting heat brought to the outside of the print head by ejected ink droplets and motion energy for ejecting the ink droplets from power supplied to the print element substrates 71 for ejecting an ink is used to generate heat in the print element substrate 71. Further, this heat in the print element substrate 71 whose heat amount decreases by the amount of heat released into the air is propagated to the support plate 72. Even in a case where the flow of an ink in the common liquid chamber 78 increases a thermal transmission coefficient, a rate is small at which heat is transmitted to the ink in the common liquid chamber 78 via the thermal insulation portion 74 having high thermal resistance. Accordingly, the heat accumulated in the support plate 72 is absorbed mainly by the ink flowing through the individual ink supply paths 77 or released to the air from the side surface of the support plate 72. Therefore, even in a case where printing is performed at a higher speed and a larger amount of heat is generated, the present embodiment can reduce, to a lower level, the amount of heat transmitted from an upstream side of the print element substrate to a downstream side of the print element substrate via the ink flowing through the common liquid chamber 78 as compared with a conventional inkjet print head.
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 Application No. 2013-135009, filed Jun. 27, 2013, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-135009 | Jun 2013 | JP | national |
