Abstract:
An inkjet printer and inkjet printing method of the present invention discharges inks having different physical properties from a plurality of discharge nozzles. At the moment of discharge, the physical properties of an ink differ from the physical properties of an ink discharged from another nozzle. Specifically, a plurality of ink discharge nozzles individually communicate with separate ink reservoirs and inks having different physical properties may be supplied to said reservoirs, or supply inks having identical physical properties are supplied to said reservoirs and the physical properties of the inks accommodated in said reservoirs differ from the physical properties of inks accommodated in other reservoirs. That is, the differences in physical properties of the inks may be expressed as different discharge amounts when a uniform discharge force is exerted on the ink. When the discharge force is changed, ink drops of a size corresponding to said physical properties are discharged. Accordingly, when the physical properties of inks discharged from a plurality of nozzles are changed, there is an increase in the control range of the size of the ink drops that can be stably discharged, thereby producing a broader range of halftones.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an inkjet printer and inkjet printing method, and more specifically relates to an inkjet printer and inkjet printing method wherein ink drops are discharged through a nozzle to form an image on a recording medium. 
     When forming high quality images by high-speed printing via inkjet recording methods, halftones can be effectively represented by changing the diameter of the print dots. Therefore, it is necessary to change the amount of ink discharged from the nozzle in accordance with the halftone level, and conventional methods are known wherein the amount of ink discharged from a nozzle is changed by changing the pressure exerted on the ink. 
     The amount of ink which can be stably discharged from the same nozzle and the variable range of the dot size formed by said discharged ink are limited, making it difficult to obtain images having sufficiently abundant halftones. 
     OBJECTS AND SUMMARY 
     In view of the aforesaid disadvantages, an object of the present invention is to provide an improved inkjet printer and inkjet printing method. 
     A further object of the present invention is to provide a novel inkjet printer and inkjet printing method capable of expressing a broad range of halftones. 
     The inkjet printer and inkjet printing method of the present invention discharges inks having different physical properties from a plurality of discharge nozzles. At the moment of discharge, the physical properties of an ink from one nozzle may differ from the physical properties of an ink discharged from another nozzle. Specifically, a plurality of ink discharge nozzles may individually communicate with separate ink reservoirs and inks having different physical properties may be supplied to said reservoirs, or supply inks having identical physical properties may be supplied to said reservoirs and the physical properties of the inks accommodated in said reservoirs may differ from the physical properties of inks accommodated in other reservoirs. That is, the differences in physical properties of the inks may be expressed as different discharge amounts when a uniform discharge force is exerted on the ink. When the discharge force is changed, ink drops of a size corresponding to said physical properties are discharged. Accordingly, when the physical properties of inks discharged from a plurality of nozzles are changed, there is an increase in the control range of the size of the ink drops that can be stably discharged, thereby producing a broader range of halftones. 
     The inkjet printer and inkjet printing method of the present invention modifies the physical properties of the ink discharged from nozzles. Thus, the physical properties of ink discharged from a nozzle can be modified, for example, in accordance with the halftones of an image being printed. Furthermore, modifiable physical properties of the ink include those that change the size of the ink drops discharged from the same nozzle, e.g., viscosity and surface tension. That is, the size of the ink drop may be adjusted by changing the physical properties of the ink. Therefore, it becomes possible to discharge ink drops of suitably different sizes from the same nozzle, thereby allowing the formation of images having abundant halftones. 
     The inkjet printer and inkjet printing method of the present invention use a first nozzle to discharge large ink drops and a second nozzle to discharge small ink drops, and increase the surface tension of the ink drops discharged from said first nozzle so as to be greater than the surface tension of the ink drops discharged from said second nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of the main part of an inkjet printer; 
     FIG. 2 is a perspective view showing the condition of a printhead supported by a carriage when viewing the carriage from the position of the back-up roller; 
     FIG. 3 is a plan view of a printhead; 
     FIG. 4 is a section view on the IV--IV line of FIG. 3; 
     FIG. 5 is a section view on the V--V line of FIG. 4; 
     FIG. 6 illustrates the change in drop size when the surface tension of the ink and the voltage applied to a piezoelectric element are changed under constant viscosity; 
     FIG. 7 illustrates the change in drop size when ink viscosity and the voltage applied to a piezoelectric element are changed under constant surface tension; 
     FIG. 8 shows the compositions of inks used in experiments; 
     FIG. 9 shows a second embodiment of a printhead. 
    
    
     In the following description, like parts are designated by like reference numbers throughout the several drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments are described hereinafter with reference to the accompanying drawings. 
     FIG. 1 is a perspective view of the main part of an inkjet printer. As shown in FIG. 1, an inkjet printer 100 is provided with a base 50. A pair of side panels 51 are disposed in opposition one to another at predetermined spacing on base 50. A back-up roller 54, guide rod 53, and pole screw 52 are suspended in parallel array between the side panels 51. The back-up roller 54 and the pole screw 52 are supported on the side panels 51 so as to be rotatable, and are connected to drive motors 56 and 55, respectively. The guide rod 53 and pole screw 52 support a carriage 2. 
     The carriage 2 is provided with through-holes having a spiral channel provided therein, such that carriage 2 is capable of reciprocating movement in the arrow a direction guided by guide rod 53 in conjunction with the rotation of pole screw 52 via the engagement of pole screw 52 and the spiral channel provided in the through-holes when the pole screw 52 is inserted through the through-holes. The carriage 2 is provided with a printhead (fully described later) on the surface opposite the back-up roller 54. Printing of an image is accomplished by discharging ink drops from the printhead and adhering the discharged ink to a recording sheet transported thereto in conjunction with the rotation of the back-up roller 54. During a printing operation, a recording sheet is gradually transported in a direction perpendicular to the arrow a direction by back-up roller 54, and carriage 2 repeats a reciprocating movement in the arrow a direction to form an image of one picture part. 
     FIG. 2 is a perspective view showing the condition of a printhead 10 supported by the carriage 2 when viewing the carriage 2 from the direction of the back-up roller 54. As shown in FIG. 2, a regulating plate 5 is mounted on carriage 2 via a machine screw 7 so as to be rotatable. The edge of regulating plate 5 on the side opposite the machine screw 7 engages a receptor 8 protruding from carriage 2, such that the regulating plate 5 is pressed against the surface of carriage 2 facing back-up roller 54. A projection 9 bend in the direction of back-up roller 54 is provided on regulating plate 5, such that the tip of an adjustment screw 3 passing through the projection 9 is screwed into a threaded hole formed in a projection 12 protruding from carriage 2. A spring 11 is provided between projections 9 and 12 and exerts an upward force on projection 9. In this construction, it is possible to adjust the position of the printhead by adjusting the adjustment screw 3. 
     FIG. 3 is a plan view of printhead 10, FIG. 4 is a section view on the IV--IV line of FIG. 3, FIG. 5 is a section view on the V--V line of FIG. 4. The construction of printhead 10 is described below with reference to the aforesaid FIGS. 3 through 5. 
     Printhead 10 comprises a first head portion 13 to discharge large ink drops, and a second head portion 14 to discharge small ink drops. The first head portion 13 and the second head portion 14 are integratedly formed and overlaid on partition 18, oscillating layer 20, and substrate 22. (See FIG. 4) 
     A cover panel 16 is formed of metal or synthetic resin or the like, and is subjected to fine processing on the side confronting partition 18 via electroforming, photolithography or the like to form on the first head portion 13 and the second head portion 14 a plurality of ink cavities 26 to accommodate various inks 24, nozzles 28 to discharge ink 24 from the various cavities 26, ink supply reservoirs 30 to accommodate replenishment ink 24, and ink inlets 32 to connect the various ink cavities 26 to the ink reservoirs 30. As shown in FIG. 3, the ink cavities 26 of the first head portion 13 and the second head portion 14 are formed so as to be mutually parallel and extend in a direction toward head portion 13 and head portion 14. The ink reservoirs 30 are formed on bilateral sides and are centered on center line 34 medial to ink cavities 26, and are connected to an ink tank not shown in the illustrations. In the present embodiment, the nozzle 28 of the first head portion 13 and the nozzle 28 of the second head portion 14 have identical nozzle diameters. 
     Partition 18 may be a thin film formed of metal or synthetic resin, and is fixedly attached between cover panel 16 and oscillating layer 20. It is desirable that partition 18 is fixedly attached under the application of a predetermined tension. 
     Oscillating layer 20 comprises a well known piezoelectric material, the top and bottom surfaces of which are provided with a common electrode, and a conductive metal layer (not shown) used as a individual electrode, and is fixedly mounted between partition 18 and substrate 22. The common electrodes and individual electrodes are connected to a print signal control circuit (not illustrated), and a predetermined voltage is applied between said common electrodes and said individual electrodes. The electrode pull out method of the electrodes may utilize various methods and are not particularly restricted, e.g., the electrodes may be pulled out by using a conductive material to use the partition 18 as a common electrode, and patterning a individual electrode on the surface of substrate 22. The oscillating layer 20 is divided into vertical channels 58 and horizontal channels 60 via a dicing process, and provided with piezoelectric member 42 corresponding to each ink cavity 26, partitions 44 positioned between adjacent piezoelectric members 42, partitions 45 positioned between mutually confronting piezoelectric members 42, and elements are separated by a circumscribing wall 46. The piezoelectric members 42 are polarized via the application of a high voltage between the common electrode and individual electrode of the top and bottom surfaces under high temperature. The piezoelectric members 42 may have a monolayer construction, or may have a laminate construction comprising a plurality of overlaid piezoelectric materials and electrodes. 
     The substrate 22 is formed of metal or synthetic resin or the like, and is fixedly attached to the oscillating layer 20 via adhesive. 
     In printhead 10 of the aforesaid construction, ink 24 is supplied from an ink tank (not illustrated) to ink reservoirs 30. The ink 24 supplied to the ink reservoir 30 of the first head portion 13 has a higher viscosity, or higher surface tension, or both relative to the ink 24 supplied to the ink reservoir 30 of the second head portion 14. The ink 24 accommodated in ink reservoirs 30 is supplied into the various ink cavities 26 via ink inlets 32. When a predetermined voltage (i.e., print signal) is transmitted from the print signal control circuit (not shown) and applied between the common electrode and the individual electrode, the piezoelectric member 42 is deformed toward the ink cavity 26. The deformation of the piezoelectric member 42 is transmitted to the partition 18, thereby applying pressure on the ink 24 accommodated within the ink cavity 26 and causing ink drops to discharge through ink nozzle 28. Since the ink 24 accommodated in the first head portion 13 has a higher viscosity or higher surface tension or both relative to the ink 24 accommodated in the second head portion 14 at this time as previously described, large ink drops are discharged from the first head portion 13 and small ink drops are discharged from the second head portion 14. Furthermore, the size of the ink drops discharged from the first head portion 13 and the second head portion 14 can be modified by changing the voltage value and application time of the signal voltage applied between the common electrode and the individual electrode in accordance with the image signals. Therefore, the halftone range can be broadened by suitably selecting the first head portion 13 and second head portion 14, and changing the voltage applied between the common electrode and the individual electrodes in accordance with the conditions of an image to be printed. Although the nozzle 28 of the first head portion 13 and the nozzle 28 of the second head portion 14 have identical nozzle diameters in the present embodiment, the reproducible halftone range may be enlarged if the nozzle 28 of the first head portion 13 has a larger diameter than the nozzle 28 of the second head portion 14. 
     Various types of conventional and well known inkjet recording inks may be used as the ink used in the aforesaid printhead 10. For example, aqueous inks comprising water-soluble dyes, water soluble organic solvents, surface active agents and the like may be used. 
     Conventional well-known acidic dyes, direct dyes, basic dyes, reactive dyes and the like may be used as the aforesaid water soluble dyes. 
     Various organic solvents may be used as the aforesaid water-soluble organic solvents for the purpose of adjusting dryness, moisture retention, viscosity, surface tension and the like. Examples of usable solvents include monovalent alcohols such as methanol, ethanol and the like, polyalkaline glycols such as polyethylene glycol, polypropylene glycol and the like, polyvalent alcohols such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, glycerine and the like, polyvalent alcohol ethers such as triethylene glycol monobutyl ether, diethylene glycol monobutyl ether and the like, and cyclic amides such as 2-pyrrolidone, 1-methyl-2-pyrrolidone and the like. The use of the aforesaid polyalkaline glycols, polyvalent alcohols, and polyvalent alcohol ethers are particularly desirable to regulate viscosity and surface tension. 
     Various surface active agents such as nonionic agents, anionic agents, cationic agents and the like may be used as the aforesaid surface active agents for the purpose of preventing the ink from running, and improving ink dischargeability. 
     Other additive such as pH control agents, amphipatic agents, chelating agents, anti-corrosion agents, mildew-proofing agents, quenching agents, oxygen absorbing agents, and other viscosity regulating agents, and surface tension regulating agents may be added as needed. 
     Ink viscosity adjustment may be accomplished by adjusting the type and amount of added water-soluble organic solvent and viscosity regulating agent. Surface tension adjustment may be accomplished by adjusting the type and amount of added water-soluble organic solvent, surface tension regulating agent, and surface active agent. In general, adding water increases surface tension, and adding surface active agent reduces surface tension. 
     Specific experimental examples are described below. 
     (1) Experiment 1 
     The relationship between the surface tension of the ink and the ink drop size was investigated by experiment. The inkjet recording head used in example 1 had a nozzle diameter of 35 μm. Four types aqueous inks (samples E, B, F, and G) having different surface tensions (i.e., s=30, 40, 50, 60 dyn/cm) were produced using the compositions shown in FIG. 8. The viscosity of the inks was uniform (i.e., h=2.0 cp). The piezoelectric member was a laminate type member comprising 28 overlaid layers of piezoelectric sheets having a thickness of 30 μm. Using the aforesaid four types of ink, pulses (rise time and fall time: 5 μsec; pulse width 20 μm) of three types of voltages (i.e., V=20, 30, 40 V) were applied between the common electrode and individual electrode, and the size of the ink dot adhered to a recording medium was measured. Results are shown in FIG. 6. 
     It can be understood from FIG. 6 that the size of the ink dot (i.e., the ink drop) increases in conjunction with an increase in the surface tension and applied voltage. Although the dot size only changed from 72 to 115 μm when using only an ink having a surface tension of s=40 dyn/cm even when the voltage was varied, if an ink having a surface tension of s=30 dyn/cm is used in the second head portion and an ink having a surface tension of s=60 dyn/cm is used in the first head portion, it is possible to have an inkjet recording head capable of discharging ink drops from 61 to 138 μm, thereby achieving broad dot size modulation. 
     (2) Experiment 2 
     The relationship between ink viscosity and dot size was investigated by experiment. The inkjet recording head used in example 2 had a nozzle diameter of 35 μm. Four types of aqueous inks (samples A, B, C, D) having different viscosities (i.e., h=1.5, 2.0, 3.0, 5.0 cp) were produced using the compositions shown in FIG. 8. The surface tension of the inks was uniform (i.e., s=40 dyn/cm). The piezoelectric member was a laminate type member comprising 28 overlaid layers of piezoelectric sheets having a thickness of 30 μm. Using the aforesaid four types of ink, pulses (rise time and fall time: 5 μsec; pulse width 20 μm) of three types of voltages (i.e., V=20, 30, 40 V) were applied between the common electrode and individual electrode, and the size of the ink dot adhered to a recording medium was measured. Results are shown in FIG. 7. 
     It can be understood from FIG. 7 that the size of the ink dot (i.e., the ink drop) increases in conjunction with an increase in the surface tension and applied voltage. Although the dot size only changed from 72 to 115 μm when using only an ink having a viscosity of h=2.0 cp even when the voltage was varied, if an ink having a viscosity of h=1.5 cp is used in the second head portion and an ink having a viscosity of h=5.0 cp is used in the first head portion, it is possible to have an inkjet recording head capable of discharging ink drops from 61 to 138 μm, thereby achieving broad dot size modulation. 
     The compositions of samples A through G used in experiments 1 and 2 are shown in the table of FIG. 8. 
     Although an ink having high viscosity and high surface tension was used in the first head portion in the description above, it is to be noted that the loss factor increases as the viscosity and surface tension increase because flow resistance increases as the nozzle diameter becomes smaller such that surface tension resistance increases relative to the force of ink discharge. Accordingly, using an ink having lower viscosity and surface tension in the first head portion than the ink in the second head portion may be considered. Rather than supplying ink having different physical properties to the first and second head portions, an ink physical property modification unit 200 may be provided. In this case, ink having identical physical property may be supplied to both head portions, and physical property of the ink accommodated in the ink supply reservoir or ink cavity is modified by said ink physical property modification unit 200. 
     Specifically, the ink physical property modification unit 200 is provided with a container accommodating a viscosity regulating agent or a surface tension regulating agent and a pump to supply the fluid from said container, and adds said viscosity regulating agent or said surface tension regulating agent to the ink when the ink is supplied from the ink tank to the ink reservoir. Of course, water soluble organic solvent or surface active agent having a viscosity regulating action or surface tension regulating action may be used alternatively. 
     The ink physical property modification unit 200 may provide heaters on the various ink reservoirs, ink cavities, or nozzles so as to adjust the viscosity and surface tension of the ink by changing the temperature of the discharging ink. For example, it was verified by experiment that when an ink having a viscosity of 5 cp (sample D above) at 25° C. is heated to a temperature of 40° C., the viscosity is reduced to 2.1 cp. 
     Furthermore, when a piezoelectric material is used which produces a temperature rise when a high voltage, i.e., a voltage of high frequency, is applied to said piezoelectric member so as to change the physical property of the ink, the physical property of the ink may be modified by heat generated by the ink physical property modification unit 200 by actuating the piezoelectric member before the ink is discharged. The present inventors have found through experimentation that although the surface temperature of a piezoelectric member rises only about +3° C. when driven under normal conditions (i.e., 30 V, 5 kHz), the surface temperature of the piezoelectric member can be raised about +25° C. to about 50° C. by applying a pulse at 50 V and 10 kHz with a rise time of less than 1 μsec. 
     The inkjet recording head is provided with two head portions in the previously described embodiments, but it is to be understood that when an ink physical property modification unit 200 is provided to modify the physical property of the discharging ink as described above, the size of discharged ink drops and the size of the dots formed by said drops may be suitably modified as needed by provided only a single head portion. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.