Ink jet printer and its printing method

Abstract

 Ink is flied under condition that ink flies in mist form according to relation of electric field value between a recording electrode and an opposite electrode and physical property value of the ink. Mist amount of the ink is adjusted thereby a dot is provided with tone without varying the diameter, and high density of the small diameter picture element enables the natural and smooth image formation.

Description

Field of the Invention and Related Art Statement

[0001] The present invention relates to an ink jet printer wherein ink drops are flied by electrostatic means thereby printing is performed, and more particularly to an ink jet printer having structure suited to printing with gradation and its printing method.

[0002] In the prior art, an ink jet printer exists wherein ink drops are selectively flied by electrostatic means, the flied ink drops adhere to a recording medium so as to form dotd, and an image is formed by selective gathering of the dots. In such an ink jet printer, since the dot diameter on the recording paper is uniform, the image cannot have gradation in usual method. Consequently, various manners have been thought in order to give gradation to the image. As one of these, so-called dither method is frequently used in the prior art. That is, one picture element is divided into a plurality of matrices and a dot is formed in each matrix. In order to obtain the picture element of high density, the number of dots formed within the picture element is increased. On the contrary, in order to obtain the picture element of low density, the number of dots is decreased. Thus the density of one picture element is arbitrarily varied, and gradation is given to the whole image being gathering of picture elements.

[0003] The above-mentioned dither method is also used as a color printing manner. More concretely, dot formed in each matrix by dividing one picture element is made one of the three primary colors. Selection of one color in the three primary colors is arbitrary. Thereby, color of one picture element as a whole is expressed in one color among the three primary colors or in mixed color of these colors. Particularly, the mixed color can be expressed with variety as the number of matrices is increased.

[0004] On the other hand, as another manner to give gradation to the image, a device having structure that the dot diameter is varied thereby gradation is given to the whole image has been disclosed by the present applicant in Japanese patent application No. 60-153301 (JP A 62-13356) and Japanese patent application No. 60-153490 (JP A 62-13357). That is, strength of the electrostatic force acting to the ink or the acting time is varied thereby the flying amount of ink is varied so as to obtain the dot of any diameter. As an example of the device having structure of varying the acting time of the electrostatic force acting to the ink, FIG. 10 shows a time lapse variation diagram of the top end portion of the recording electrode illustrating relation between the acting time of the electrostatic force and the ink flying state. A recording electrode 20 and an opposite electrode 21 are opposed through a recording paper 22. The recording electrode 20 is formed by an ink impregnated member, and part except for a top end portion 20a is immersed in an ink liquid (not shown). Consequently, the ink liquid is impregnated in the recording electrode 20, and turned round the top end portion 20a and held. In this state, as shown in FIG. 11 for example, the voltage of the pulse width between the time t₁ and the time t₅ is applied to the recording electrode 20 and the opposite electrode 21. The ink liquid held in the top end portion 20a between the time t₁ and the time t₅ becomes ink drops 23 flying in stringy form towards the opposite electrode 21 and adheres to the recording paper 22 so as to form a dot. Because the potential difference is produced between both electrodes and the imk liquid in the top end portion 20a is attracted to the opposite electrode 21 by means of the electrostatic force. Thus the ink liquid flies in the ink drops 23 by the time of the pulse width of the voltage applied between both electrodes. Consequently, the width of the voltage pulse supplied between both electrodes is varied, thereby the flying amount of the ink liquid is varied resulting in variation of the diameter of the dot formed on the recording paper 22. Thus the dot diameter is adjusted regarding each dot thereby the whole image is provided with gradation.

[0005] Technical problems of the prior art in such constitution will now be described. When the dither method is used, since one picture element is constituted by gathering of a plurality of matrices, size of one picture element as a unit to constitute the image becomes large. Consequently, the density of the picture element becomes low and the quality of the image is deteriorated in natural state. Particularly, when the degradation is raised or when the color printing is performed in the color expression with variety, the number of matrices must be increased. In this case, the density of one picture element becomes lower.

[0006] On the other hand, in the method that the dot diameter is varied thereby the whole image is provided with degradation, when the dot diameter is made small, a blank space between adjacent dots becomes wide thereby the image expression is deteriorated in the smoothness.

Objects and Summary of the Invention

[0007] A first object of the invention is to provide an ink jet printer and its printing method to enable natural image expression when the printing with gradation is performed.

[0008] A second object of the invention is to provide an ink jet printer and its printing method to enable the smooth image expression when the printing with gradation is performed.

[0009] In the present invention, in order to attain the foregoing objects, ink is flied under condition of the ink flying in mist form according to relation of the electric field value between a recording electrode and an opposite electrode and resistivity value of the ink, and a number of fine mist particles by the ink are gathered on a recording medium thereby dots are formed. When pulse width of the voltage applied between the recording electrode and the opposite electrode is varied or when the voltage value is varied, the amount of ink flying from the recording electrode is adjusted and the number of mist particles to form the dot is varied, thereby tone is produced on the dot and degrada­tion is produced on the image. In this case, since diameter of each dot as one picture element is constant, spacing between one dot and other dot can be held to a constant spacing with high density so as to enable the natural and smooth image expression.

Brief Description of the Drawings

[0010] 

FIG. 1 is a longitudinal sectional view of a printer head as a first embodimen of the invention;

FIG. 2 is a perspective view of a whole device;

FIG. 3 is a graph illustrating relation of resistivity value of ink and supplu voltage value to state of ink;

FIG. 4 is a side view of a recording electrode illustrating flying state of ink in time lapse state;

FIG. 5 is a graph illustrating pulse width of supply voltage when ink is flied as in FIG. 4;

FIG. 6 is a graph illustrating relation between pulse width of supply voltage and dot density;

FIG. 7(a)(b) is a front view of dot in comparison of the case that pulse width of supply voltage is made long and made short;

FIG. 8 is a longitudinal sectional view of a printer head as a second embodiment of the invention;

FIG. 9(a)(b) is a side view illustrating state of ink flying from a recording electrode;

FIG. 10 is a graph illustrating relation of ratio of density level of each color and black component in a modification of a third embodiment;

FIG. 11 is a side view of a recording electrode illustrating flying state of ink in time lapse state in an example of the prior art; and

FIG. 12 is a graph illustrating pulse width of supply voltage when ink is flied as in FIG. 9.

Description of Preferred Embodiments

[0011] A first embodiment of the invention will be described based on FIGS. 1 through 7(a)(b). A case 1 of cabinet form is installed, and two guide shafts 2 are arranged horizontally within the case 1. A carrier 3 is slidably installed to these guide shafts 2, and a printer head 4 is held on the carrier 3. An opposite electrode 5 of lateral extending form is mounted horizontally along the guide shafts 2 at center portion within the case 1. On rear side of the opposite electrode 5, tracks 7 feeding a recording paper 6 as a recording medium guided between the opposite electrode 5 and the printer head 4 are installed. Operation knobs 8 projecting outward are coupled with these tracks 7.

[0012] The printer head 4 will be herein described in detail based on FIG. 1. External form of the printer head 4 is a case 9 of cabinet form, and ink 10 is stored in the case 9. As the physical properties of the ink 10, resistivity is slightly less than 10⁶ ∼ 10⁷ Ω · cm, viscosity is 30 cp or less, and surface tension is 20 ∼ 30 dyn/cm. Within the case 9, a plurality of recording electrodes 11 immersed in the ink 10 and having top end portions 11a exposed to outside are aligned longitudinally. Each of these recording electrodes 11 is constituted in that polyester fibers are twisted and copper is coated around the polyester fibers thereby conductivity and ink impregnable property are provided. Each recording electrode 11 has sectional diameter of about 1 mm, and the top end portion 11a is tapered. The tip of the tapered top end portion 11a is provided with curvature, and the curvature radius is 50 ∼ 100 µm. The top end portion 11a of the recording electrodes 11 are disposed in opposition to the opposite electrode 5 with spacing of 500 ∼ 100 µm.

[0013] A switch circuit 12 is connected to each recording electrode 11. One change-over contact of the switch circuit 12 is earthed to the ground G. Other change-over contact of the switch circuit 12 is connected to the opposite electrode 5 through two power source 13, 14 with the connection neutral point earthed to the ground G. These power sources 13, 14 generate voltage of 1 kV ∼ 3.2 kV between the opposite electrode 5 and the recording electrode 11 when the switch circuit 12 is turned on. Furthermore, an image signal circuit 15 is connected to the switch circuit 12, and a pulse width control circuit 16 as a printing control circuit is interposed between the switch circuit 12 and the image signal circuit 15. The pulse width control circuit 16 generates printing control signal which varies the voltage pulse width of voltage applied to the recording electrode 11 and the opposite electrode 5 in response to the image signal.

[0014] In such constitution, if the image signal is generated from the image signal circuit 15, the switch circuit 12 is turned on and voltage is applied between the recording electrode 11 and the opposite electrode 5. In this case, the ink 10 within the case 9 is soaked into the recording electrode 11 and supplied up to the top end portion 11a. Consequently, the ink 10 supplied to the top end portion 11a is subjected to the electrostatic force and flies towards the opposite electrode 5. The flying ink 10 adheres to the recording paper 6 and forms a dot 17. Diameter of such a dot 17 is about 300 µm. Selective gathering of these dots 17 forms an image on the recording paper 6.

[0015] On the other hand, the ink 10 turning round te top end portion 11a of the recording electrode 11 flies in mist form from the top end portion 11a. Because the voltage from the electrodes 13, 14 is applied between the recording electrode 11 and the opposite electrode 5 in condition of the ink 10 flying in mist form corresponding to the physical properties of the ink 10. More concretely, as shown in FIG. 3, in the region A the ink 10 remaining in the liquid state flies in stringy form, whereas in the region B the ink 10 flies in mist form. For example, if the ink 10 has the resistivity of 10⁷ Ω · cm, voltage of about 3 kV or more is applied between both electrodes thereby flying of the ink 10 in mist form is produced. Although the graph of FIG. 3 shows condition of the ink 10 flying in mist form according to relation of the supply voltage between the recording electrode 11 and the opposite electrode and the resistivity value of the ink 10, other factors actually exist also in order that the ink 10 flies in mist form. That is, the ink 10 flies in mist form according to relation of the physical properties and the supply voltage between both electrodes, and the resistivity value of the ink 10 is one factor only.

[0016] FIG. 4 shows the state of the ink 10 in time lapse where the ink 10 flies in mist form from the top end portion 11a of the recording electrode 11. In this case, the pulse width of the voltage set by the pulse width control circuit 16 is that between the time t₁ and the time t₅ as shown in FIG. 5. Consequently, the ink 10 flies in mist particles 10a between the time t₁ and the time t₅, and then adheres completely to the recording paper 6 and becomes the form as the dots 17 at the time t₈. Thus the flying time of the ink 10 coincides with the pulse width of the voltage. Consequently, in the embodiment, the pulse width of the voltage, i.e., the voltage supply time between both electrodes is varied by the pulse width control circuit 16 thereby the flying time of the ink 10 is varied. As a result of varying the flying time of the ink 10, more concretely, the number of the flying mist particles 10a is varied thereby the density is expressed in the dots 17. If the density of the dots 17 is adjusted throughout the whole image, the image is provided with the gradation. Relation of the voltage supply time and the density of the dots 17 is shown in FIG. 6 in a graph. FIG. 7(a) shows an enlarged view of the dots 17 formed when the voltage supply time is made long, and FIG. 7(b) shows an enlarged view of the dots 17 formed when the voltage supply time is made short. However, FIG. 7 is a model view illustrating concept for comparison, and in the dot 17 of the actual state, each mist particle 10a has smaller diameter and the distribution state has higher density.

[0017] Noticing each dot 17, the density of the dot 17 is produced only by difference of the number of the mist particles 10a as elements to constitute the dot 17, and not dependent on difference of the diameter of the dot 17. Consequently, the outer diameter of the dot 17 is equal regarding all dots 17 in the image. Thus the distance between dots 17 is always held constant, and the density of each dot 17 can be varied. When the distance density between the dots 17 is elevated, the connection between the adjacent dots 17 becomes smooth thereby quite natural gradation expression can be realized.

[0018] Moreover, since the dot 17 is a minimum picture element to constitute the image, the high density formation is easy. Regarding this point, from the property of the dot 17 with the density varying for itself, the density can be sufficiently expressed even when the diameter of the dot 17 is made as small as about 300 µm in the embodiment, thereby the image formation at higher density is possible. Consequently, not only the smoothness of the gradation expression but also the smoothness of the whole image by the high density formation of one picture element can be realized, thereby more natural image formation becomes possible.

[0019] In addition, in the embodiment, the ink 10 having the resistivity value of slightly less than 10⁶ ∼ 10⁷ Ω · cm is used. Consequently, as clearly seen from the graph of FIG. 3, the ink 10 flies in mist form by the supply voltage with value of slightly less than 2 kV to about 3 kV. If the value of the supply voltage is increased, spark may be produced between the opposite electrode 5 and the recording electrode 5 resulting in the printing failure. However, the spark is not liable to occur by the supply voltage with value of about 3 kV, thereby the device of high safety can be realized. Moreover, the consumption power can be saved in comparison to the case using the ink 10 of higher resistivity.

[0020] Next, a second embodiment of the invention will be described based on FIG. 8(a)(b). Same parts as those in the first embodiment are designated by the same reference numerals, and the description shall be omitted. In the second embodiment, the image signal circuit 15 is directly connected to the switch circuit 12 not through the pulse width control circuit 16, and a voltage variable circuit 18 is installed between the switch circuit 12 and the power source 13. The voltage variable circuit 18 varies the value of the voltage applied between the recording electrode 11 and the opposite electrode 5 in range of 2 kV ∼ 4 kV. The ink 10 having the resistivity value of slightly less than 10⁶ Ω · cm is used. Consequently, in this embodiment, in place of variation of the pulse width of the voltage, the number of the flying mist particles 10a is varied by varying the voltage value using the voltage variable circuit 18. In this case, in the region where the value of the supply voltage is as high as nearly 4 kV, not only the number of the flying mist particles 10a but also the flying state may be spread. FIG. 8(a)(b) shows such state. Consequently, in the gradation expression in the actual state, not only the variation of the density of the dot 17 in itself but also the variation of the diameter of the dot 17 realized by the spread in the flying state of the mist particles 10a may be utilized.

[0021] A third embodiment of the invention will now be described. The same parts as those of the first embodiment are designated by the same reference numerals, and the description shall be omitted. In the third embodiment, a process color ink of yellow, mazenta and cyan is used as the ink 10. The mist particles 10a of these colors are overlapped on the same dot 17, and an arbitrary color can be expressed by the distribution of the amount of these mist particles 10a. Within one dot 17, the mist particles 10a of each color are distributed uniformly in the area of the dot 17 thereby quite natural color expression becomes possible. Consequently, when the image expression having the smooth gradation by the high density of the dot 17 and the equal diameter of the dot 17 is maintained, the natural color printing becomes possible.

[0022] FIG. 9 shows a modification. When the density level of each color in one dot 17 is assumed that yellow component is at c level, magenta component is at b level, and cyan component is at a level, black component exists at equal amount of each color. The black component at d level lower than the c level being the minimum level is contained in the dot 17, and components at level lower than the d level are excluded regarding other colors. In this constitution, the dot 17 is formed in the same color as the case that components of three colors, yellowm magenta and cyan, only exist at c level, b level and c level respectively.

[0023] Further, a fourth embodiment of the invention will be described. The same parts as those in the second embodiment are designated by the same reference numerals, and the description shall be omitted. In the fourth embodiment, while the printer head 4 is moved in the main scanning direction, voltage is applied between the recording electrode 11 and the opposite electrode 5 continuously by an arbitrary time. In this constitution, not only the dot 17 but also the line can be drawn on the recording paper 6. Consequently, the image is formed by combination of plural lines. On the other hand, in the scanning time of the printer head 4, the supply voltage value between the recording electrode 11 and the opposite electrode 5 per unit time is varied by the voltage variable circuit 18. Operation of the voltage variable circuit 18 is by analog signals. Or, the scanning speed is varied stepwise, thereby the pulse width of the supply voltage per unit time is varied. In this constitution, the lines drawn on the recording paper 6 have gradation without varying the thickness. In this case, however, voltage is applied between the opposite electrode 5 and the recording electrode 11 in the range where the value of the supply voltage is as low as nearly 2 kV. Because the thickness of the lines drawn on the recording paper 6 does not become constant if the voltage is applied at the region nearly equal to 4 kV.

[0024] As a modification, the lines drawn on the recording paper 6 may be colored. That is, in similar manner to the third embodiment, the mist particles 10a in respective colors, yellow, magenta and cyan, are formed in arbitrary mixing ratio on the same line. In this constitution, the color variation and the gradation variation can be simultaneously expressed on the lines drawn in the scanning direction.

[0025] Although the pulse width control circuit 16 or the voltage variable circuit 18 has been used as the printing control circuit in the above-mentioned embodiments, the actual use is not limited to these circuits, but other means may be used. For example, the electric amount between both electrodes may be varied by means for varying the current value between the recording electrode and the opposite electrode.

Claims

1. In an ink jet printer wherein an opposite electrode and a recording electrode having a top end portion supplied with ink are opposed through a recording medium, and ink is flied from the top end portion of the recording electrode by the electric field between the opposite electrode and the recoding electrode so as to form an image on the recording medium, printing method of said ink jet printer characterized in that the ink is flied under condition of the ink flying in mist form according to relation of the electric field value between the recording electrode and the opposite electrode and physical property of the ink.

2. Printing method of an ink jet printer as set forth in claim 1, wherein physical property value of the ink is resistivity of the ink.

3. Printing method of an ink jet printer as set forth in claim 2, wherein the ink with low resistivity value is used.

4. An ink jet printer comprising:
      a recording electrode having a top end portion supplied with ink;
      an opposite electrode opposed to the recording electrode through a recording medium:
      a power source applying voltage between the recording electrode and the opposite electrode in voltage range so that the ink flies in mist form; and
      a printing control circuit which varies power amount supplied between the recording electrode and the opposite electrode.

5. An ink jet printer as set forth in claim 4, wherein said printing control circuit is a pulse width control circuit which varies the power amount between the recording electrode and the opposite electrode by varying pulse width of the voltage set between both electrodes.

6. An ink jet printer as set forth in claim 5, wherein the recording medium and the recording electrode are relatively moved and ink is flied continuously in the scanning direction, and the scanning speed is varied stepwise so that the pulse width of the voltage is varied.

7. An ink jet printer as set forth in claim 4, wherein said printing circuit is a voltage variable circuit which varies the power amount by varying the voltage value of the power source.

8. An ink jet printer as set forth in claim 7, wherein the recording medium and the recording electrode are relatively moved and ink is flied continuously in the scanning direction.

9. An ink jet printer as set forth in claim 4, wherein dots are formed on the recording medium by the flying ink, and the dots are selectively gathered so as to perform printing.

10. In an ink jet printer wherein an opposite electrode and a recording electrode having a top end portion supplied with ink are opposed through a recording medium, and ink is flied from the top end portion of the recording electrode by the electric field between the opposite electrode and the recording electrode so as to form an image on the recording medium, printing method of said ink jet printer characterized in that color inks of plural colors are used and flied on the same portion under condition of the color inks flying in mist form according to relation of the electric field value between the recording electrode and the resistivity value of the ink, and the flying amount of each color ink is varied so that the image is provided with the degradation and the color property.

11. Printing method of an ink jet printer as set forth in claim 10, wherein dots are formed om the recording medium by the flying inks, and the dots are selectively gathered so as to perform printing.

12. Printing method of an ink jet printer as set forth in claim 10, wherein the recording medium and the recording electrode are relatively moved and color inks are flied continuously in the scanning direction.

Drawing