Inkjet printing method and printing apparatus Number:6,679,597 from the United States Patent and Trademark Office (PTO) owispatent Inkjet printing method having the following steps: ejecting an oily ink comprising particles to a printing medium with use of an electrostatic field according to image data signals to form an image directly on the printing medium; and fixing the image to obtain a printed matter, wherein a prevention of an aggregation and/or a precipitation of the particles is conducted at least during ink circulation, or an aggregate and/or a deposit of the particles formed at least due to a suspension of ink-flow is redispersed.<H apparatus, printing, Inkjet, printing, 6679597.html, Patent, pto, 6679597

Title: Inkjet printing method and printing apparatus

Abstract: Inkjet printing method having the following steps: ejecting an oily ink comprising particles to a printing medium with use of an electrostatic field according to image data signals to form an image directly on the printing medium; and fixing the image to obtain a printed matter, wherein a prevention of an aggregation and/or a precipitation of the particles is conducted at least during ink circulation, or an aggregate and/or a deposit of the particles formed at least due to a suspension of ink-flow is redispersed.

Patent Number: 6,679,597 Issued on 01/20/2004 to Ohsawa, et al.

Inventors: Ohsawa; Sadao (Shizuoka, JP), Nakazawa; Yusuke (Shizuoka, JP), Naniwa; Mutsumi (Shizuoka, JP)
Assignee: Fuji Photo Film Co., Ltd. (Kanagawa, JP)

Appl. No.: 10/100,902

Filed: March 20, 2002

Foreign Application Priority Data


Mar 21, 2001 [JP]			P. 2001-080722
Mar 30, 2001 [JP]			P. 2001-101235

Current U.S. Class: 347/95 ; 347/89

Current International Class: B41J 2/06 (20060101); B41J 2/04 (20060101); B41J 2/175 (20060101); B41J 002/17 (); B41J 002/18 ()

Field of Search: 347/84-86,89,95,100,104 101/463.1,450.1

References Cited [Referenced By]

U.S. Patent Documents


4607261	August 1986	McCann et al.
6454401	September 2002	Naniwa et al.

Foreign Patent Documents


0770486	May., 1997	EP
05249783	Sep., 1993	JP
10-286939	Oct., 1998	JP

Other References

Abstract of JP 2000-280438 A (Fuji Photo Film Co. LTD) (Publication Date, Oct. 10, 2000) from Patent Abstracts of Japan, vol. 2000, No. 13, Feb. 5, 2001. .
Abstract of JP 10-286962 A (NEC NIIGATA LTD) (Publication Date, Oct. 27, 1998) from Patent Abstracts of Japan, vol. 1999, No. 1, Jan. 29, 1999..

Primary Examiner: Meier; Stephen D.
Assistant Examiner: Do; An H.
Attorney, Agent or Firm: Sughrue Mion, PLLC

Claims

What is claimed is:

1. Inkjet printing method comprising: ejecting an oily ink comprising particles to a printing medium with use of an electrostatic field according to image data signals to form an image directly on the printing medium; and fixing the image to obtain a printed matter, wherein a prevention of an aggregation and/or a precipitation of the particles is conducted at least during ink circulation, or an aggregate and/or a deposit of the particles formed at least due to a suspension of ink-flow is redispersed.

2. The inkjet printing method according to claim 1, wherein the oily ink comprises: a nonaqueous solvent having a specific resistance not less than 10.sup.9 .OMEGA.cm and a dielectric constant not higher than 3.5; and colored particles dispersed in the nonaqueous solvent.

3. An inkjet printing apparatus comprising: an image-forming means for forming an image directly on a printing medium according to image data signals; and an image-fixing means for fixing the image formed by the image-forming means to produce a printed matter, the image-forming means being an inkjet recording unit comprising a recording head that ejects an oily ink comprising particles with use of an electrostatic field, wherein at least one aggregation and/or precipitation-preventing means is equipped in an ink-flow channel of the oily ink in an ink circulation, the aggregation and/or precipitation-preventing means being for a prevention of aggregation and/or precipitation of the particles, or a redispersing means is equipped, the redispersing means being for redispersing of the particles which are in a state of aggregation and/or precipitation formed due to a suspension of ink-flow.

4. The inkjet printing apparatus according to claim 3, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means is located just in front of an ink-ejecting part of the recording head.

5. The inkjet printing apparatus according to claim 3, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means comprises from a group of agitation, dispersion, mixing and jetting.

6. The inkjet printing apparatus according to claim 5, wherein the group of agitation, dispersion, mixing and jetting is applied individually or in combination.

7. The inkjet printing apparatus according to claim 6, wherein the group of agitation, dispersion, mixing and jetting is applied with a fixed interval, with a non-fixed interval or continuously.

8. The inkjet printing apparatus according to claim 3, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means is in the form of a cartridge.

9. The inkjet printing apparatus according to claim 3, wherein the oily ink comprises: a nonaqueous solvent having a specific resistance not less than 10.sup.9 .OMEGA.cm and a dielectric constant not higher than 3.5; and colored particles dispersed in the nonaqueous solvent.

10. The inkjet printing apparatus according to claim 3, which further comprises a dust-removing means that removes dusts present on a surface of the printing medium prior to and/or during printing.

11. The inkjet printing apparatus according to claim 3, wherein the image forming is carried out by moving the printing medium through a rotation of a counter drum arranged in a position facing the recording head with the printing medium interposed between the recording head and the drum.

12. The inkjet printing apparatus according to claim 11, wherein the recording head is of a single-channel or multi-channel type and the image forming is carried out by moving the recording head in the direction parallel to the axis of the counter drum.

13. The inkjet printing apparatus according to claim 11, wherein the recording head is of a full-line type having a width substantially equal to that of the printing medium.

14. The inkjet printing apparatus according to claim 3, wherein the image forming is carried out by transporting the printing medium inserted between at least a pair of capstan rollers.

15. The inkjet printing apparatus according to claim 13, wherein the recording head is of a single-channel or multi-channel type, and the image forming is carried out by moving the recording head along the direction perpendicular to the moving direction of the printing medium.

Description

FIELD OF THE INVENTION

The present invention relates to an inkjet printing method and printing apparatus forming an image directly on a printing medium based on electrostatic inkjet recording with use of an oily ink and being capable of achieving a high print quality and a large printing speed. More specifically, the invention relates to a prevention of the aggregation and/or precipitation of the particles in the oily ink and a redispersion of the ink used for such a method.

BACKGROUND OF THE INVENTION

As printing methods of forming a print image on a printing medium on the basis of image data signals, the methods based on electrophotography, thermal dye sublimation, thermal melting transfer and inkjet recording are known.

Electrophotography requires processes for forming an electrostatic latent image on a photosensitive drum by charging and exposure, and the system tends to become complicated requiring an expensive apparatus.

In thermal transfer processes, the apparatus is inexpensive, but suffers from a high running cost and the generation of waste as the processes use an ink ribbon.

In contrast, inkjet processes require inexpensive apparatuses and enjoy a low running cost because a direct printing is performed on a printing medium whereby the ink is ejected only onto image areas needed for image formation.

As a method of applying the inkjet technology to printing system, Japanese Patent Laid-Open No. 286939/1998 discloses a printing method comprising adding an inkjet printing apparatus to a rotary press machine, and additionally printing variable numbers or marks on the same printed matters with the inkjet system.

It is further desirable that a printing system can print high-quality image information such as photographic images. Unfortunately, however, with the conventional ink technique that ejects an aqueous or organic solvent-based ink containing dyes or pigments as the colorant by pressure, liquid droplets containing a large amount of solvent are ejected and thus tend to cause blur in the printed image when an expensive dedicated type of paper is not used.

Accordingly, high quality printed images cannot be obtained when ordinary non-dedicated printing stocks or plastic sheets, which are non-absorbent media, are used for printing.

As one of the inkjet techniques, there is known an image-forming method ejecting ink melted and liquefied by applying heat to an ink material that is solid at ambient temperature. By using this type of ink, the blur of the printed image is mitigated, but due to the high ink viscosity during ejection, it is difficult to eject fine droplets, thus the individual printed dot has a large area as well as a large thickness. Accordingly, the formation of high-resolution images is quite difficult.

Furthermore, in image recording by an inkjet process, there take place various problems such as pipe or head choking caused by the precipitation and aggregation of the particulate ingredients in the ink, thus making ink ejection unstable, deteriorating image quality and at the worst terminating ink ejection. In cases where the size of the dispersed particles is large, they tend to sediment when the ink is stationary whereby ink ejection at a constant particle concentration and thus normal image recording become impossible. Furthermore, in some cases, ink ejection completely stops.

Furthermore, after ink-flow is suspended in inkjet recording, aggregates or deposits of the particulate materials in the ink, or foreign matters such as dust sometimes act to choke the ink-flow pipe or the head, thus causing various problems such as unstable ink ejection which leads to image quality deterioration, and at the worst termination of ink ejection. In cases where the size of the dispersed particles is large, they tend to sediment when the ink is stationary whereby ink ejection at a constant particle concentration and thus normal image recording become impossible.

SUMMARY OF THE INVENTION

The invention has been devised by taking notice of the above-cited problems; the object of the invention is to provide an inkjet printing method and printing apparatus which can consistently output sharp and crisp prints by an inexpensive and simple process free of developing treatments, and which cope with digital signals.

As a result of eager investigation of the present inventors for solving the above problems, the present invention has been attained by the following means (1) to (21). (1) Inkjet printing method comprising: ejecting an oily ink comprising particles to a printing medium with use of an electrostatic field according to image data signals to form an image directly on the printing medium; and fixing the image to obtain a printed matter, wherein a prevention of an aggregation and/or a precipitation of the particles is conducted at least during ink circulation, or an aggregate and/or a deposit of the particles formed at least due to a suspension of ink-flow is redispersed. (2) The inkjet printing method as described in (1) above, wherein the oily ink comprises: a nonaqueous solvent having a specific resistance not less than 10.sup.9 .OMEGA.cm and a dielectric constant not higher than 3.5 and; and colored particles dispersed in the nonaqueous solvent. (3) An inkjet printing apparatus comprising: an image-forming means for forming an image directly on a printing medium according to image data signals; and an image-fixing means for fixing the image formed by the image-forming means to produce a printed matter, the image-forming means being an inkjet recording unit comprising a recording head that ejects an oily ink comprising particles with use of an electrostatic field, wherein at least one aggregation and/or precipitation-preventing means is equipped in an ink-flow channel of the oily ink in an ink circulation, the aggregation and/or precipitation-preventing means being for a prevention of aggregation and/or precipitation of the particles, or a redispersing means is equipped, the redispersing means being for redispersing of the particles which are in a state of aggregation and/or precipitation formed due to a suspension of ink-flow. (4) The inkjet printing apparatus as described in (3) above, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means is located just in front of an ink-ejecting part of the recording head. (5) The inkjet printing apparatus as described in (3) or (4) above, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means comprises a step selected from agitation, dispersion, mixing and jetting. (6) The inkjet printing apparatus as described in (5) above, wherein the steps of agitation, dispersion, mixing and jetting are applied individually or in combination. (7) The inkjet printing apparatus as described in (6) above, wherein the steps of agitation, dispersion, mixing and jetting are applied with a fixed interval, with a non-fixed interval or continuously. (8) The inkjet printing apparatus as described in any one of (3) to (7) above, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means is in the form of a cartridge. (9) The inkjet printing apparatus as described in any one of (3) to (8) above, wherein the oily ink comprises: a nonaqueous solvent having a specific resistance not less than 10.sup.9 .OMEGA.cm and a dielectric constant not higher than 3.5 and; and colored particles dispersed in the nonaqueous solvent. (10) The inkjet printing apparatus as described in any one of (3) to (9) above, which further comprises a dust-removing means that removes dusts present on a surface of the printing medium prior to and/or during printing. (11) The inkjet printing apparatus as described in any one of (3) to (10) above, wherein the image forming is carried out by moving the printing medium through s rotation of a counter drum arranged in a position facing the recording head with the printing medium interposed between the recording head and the drum. (12) The inkjet printing apparatus as described in (11) above, wherein the recording head is of a single-channel or multi-channel type and the image forming is carried out by moving the recording head in the direction parallel to the axis of the counter drum. (13) The inkjet printing apparatus as described in any one of (3) to (12) above, wherein the image forming is carried out by transporting the printing medium inserted between at least a pair of capstan rollers. (14) The inkjet printing apparatus as described in (13) above, wherein the recording head is of a single-channel or multi-channel type, and the image forming is carried out by moving the recording head along the direction perpendicular to the moving direction of the printing medium. (15) The inkjet printing apparatus as described in any one of (3) to (14) above, wherein the recording head is of a full-line type having a width substantially equal to that of the printing medium. (16) The inkjet printing apparatus as described in any one of (3) to (15) above, wherein the inkjet recording unit further comprises an ink-feeding member that feeds the oily ink to the recording head. (17) The inkjet printing apparatus as described in (16) above, which further comprises an ink-recovery means that gathers the oily ink from the recording head and circulates the oily ink. (18) The inkjet printing apparatus as described in any one of (3) to (17) above, wherein the inkjet recording unit further comprises an agitating means for agitating the oily ink in an ink tank that stores the oily ink. (19) The inkjet printing apparatus as described in any one of (3) to (18) above, wherein the inkjet recording unit further comprises a contrlooing means for controlling the temperature of the oily ink kept in a ink tank that stores the oily ink. (20) The inkjet printing apparatus as described in any one of (3) to (19) above, wherein the inkjet recording unit further comprises an ink concentration-controlling means that controls the concentration of the oily ink. (21) The inkjet printing apparatus as described in any one of (3) to (20) above, which further comprises a cleaning means that cleans the recording head.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1]

FIG. 1 is a schematic diagram showing the entire constitution of an inkjet printing unit comprising a control unit, an ink-feeding unit, and a head distancing/approximating mechanism for an inkjet printing apparatus of the invention.

[FIG. 2]

FIG. 2 is a diagram showing the constitution of a printing apparatus that is additionally equipped with an ink-circulating function to the ink-feeding unit depicted in FIG. 1.

[FIG. 3]

FIG. 3 is a bird-eye view of a specific example for the ink-ejecting head depicted in FIG. 1.

[FIG. 4]

FIG. 4 is a diagram used to explain the enlarged cross-section of the ink-ejecting imaging unit depicted in FIG. 3.

[FIG. 5]

FIG. 5 is a diagram schematically showing the cross-section of the vicinity of the ink-ejecting part of another example of the ink-ejecting head.

[FIG. 6]

FIG. 6 is a diagram schematically showing the front view of the vicinity of the ink-ejecting part of still another example of the ink-ejecting head.

[FIG. 7]

FIG. 7 is a diagram schematically showing only a part of still another ink-ejecting head.

[FIG. 8]

FIG. 8 is a schematic diagram of the recording head shown in FIG. 7 from which regulating plates 42 and 42' have been removed.

[FIG. 9]

FIG. 9 is a schematic diagram showing part of the ejecting head for another example having a pair of substantially rectangular-shaped supporting members.

[FIG. 10]

FIG. 10 is a diagram showing an apparatus that is a partial modification of the one shown in FIG. 2.

[FIG. 11]

FIG. 11 is a schematic cross-sectional view showing an aggregation and/or precipitation-preventing member and/or a redispersing member.

[FIG. 12]

FIG. 12 is a schematic cross-sectional view showing another aggregation and/or precipitation-preventing member and/or a redispersing member.

[FIG. 13]

FIG. 13 is a schematic cross-sectional view showing still another aggregation and/or precipitation-preventing member and/or a redispersing member.

[FIG. 14]

FIG. 14 is a schematic cross-sectional view showing still another aggregation and/or precipitation-preventing member and/or a redispersing member.

[FIG. 15]

FIG. 15 schematically illustrates the entire constitution of a web-type apparatus performing a single-sided monochrome printing as an example of the inkjet printing apparatus of the invention.

[FIG. 16]

FIG. 16 schematically illustrates the entire constitution of a web-type apparatus performing a single-sided four-color printing as another example of the inkjet printing apparatus of the invention.

[FIG. 17]

FIG. 17 schematically illustrates the entire constitution of a double-sided four-color printing apparatus as another example of the inkjet printing apparatus of the invention.

[FIG. 18]

FIG. 18 schematically illustrates the entire constitution of a double-sided four-color printing apparatus as still another example of the inkjet printing apparatus of the invention.

[FIG. 19]

FIG. 19 schematically illustrates the entire constitution of a single-sided four-color printing apparatus in which a rolled printing medium is cut and wound around a counter drum for performing printing as another example of the inkjet printing apparatus of the invention.

[FIG. 20]

FIG. 20 schematically illustrates the entire constitution of a printing apparatus in which a sheet-formed printing medium is used, as another example of the inkjet printing apparatus of the invention.

[FIG. 21]

FIG. 21 schematically illustrates the entire constitution of a printing apparatus in which a rolled printing medium is conveyed by being inserted between a pair of capstan rollers as another example of the inkjet printing apparatus of the invention.

[FIG. 22]

FIG. 22 schematically illustrates the entire constitution of a printing apparatus in which a sheet-formed printing medium is conveyed by being inserted between a pair of capstan rollers, as another example of the inkjet printing apparatus of the invention.

DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS

1 Printing medium-feeding roll 2 Dust-removing unit 3 Inkjet recording unit 4 Counter (Imaging) drum 5 Fixing unit 6 Printing medium-winding roll 7 Automatic exhausting unit 8 Cutter 9 Automatic feeding unit 10 Capstan rollers 11 Earth member 21 Image data processing-controlling unit 22 Ejecting head 221 Upper block 222 Lower block 22a Ejecting slit 22b Ejecting electrode 23 Oily ink 24 Ink-feeding unit 25 Ink tank 26, 26' Ink-feeding device 27 Agitating member 28 Ink temperature-controlling member 29 Ink concentration-controlling member 30 Encoder 31 Head distancing/approximating unit 32 Head sub-scanning means 33 First insulating base material 34 Second insulating base material 35 Slanted end of the second insulating base material 36 Upper plane of the second insulating base material 37 Ink inflow channel 38 Ink recovery channel 39 Backing 40 Slot 41 Head body 42, 42' Meniscus regulating plate 43 Ink slot 44 Dividing wall 45, 45' Ejecting point 46 Dividing wall 47 Tip of the dividing wall 50, 50' Supporting member 51, 51' Slot 52 Dividing wall 53 Upper end 54 Rectangular part 55 Upper end of the dividing wall 56 Guiding projection 61, 61' Valve 70 Agitating motor 71 Agitating blade 72 Pump 81 Agitating element 82 Stirrer 83 Ultrasonic wave-applying tub 84 Ultrasonic vibrating element 85 Ultrasonic vibrator 86 Vibrating blades 87 Oscillator M Printing medium

DETAILED DESCRIPTION OF THE INVENTION

In the following, the mode for carrying out the invention will be described in detail.

The invention is characterized by that, in the formation of images by an inkjet method in which an oily ink is ejected by an electrostatic field onto a printing medium fed to a printing apparatus and the oily ink particles are prevented from aggregation and precipitation and/or the oily ink is redispersed.

The inkjet method associated with the invention is one described in PCT Publication W093/11866 wherein use is made of an ink of high electric resistance containing at least colored particles dispersed in an insulating solvent. To such an ink, an intense electrostatic field is applied at an ejecting position to form aggregates of said colored particles there and cause said aggregate to eject by electrostatic means from said ejecting position. As the colored particles eject as highly concentrated aggregates, the ink droplets contain only a small amount of solvent. Due to such a fact, high-density, sharp and crisp images free of blur are formed on a printing stock or a plastic film both designed for printing media.

In the invention, the size of the ejected ink droplets is determined by the dimension of the ejecting electrode and the conditions of electrostatic field application. Thus, by adopting a small ejecting electrode and optimized electrostatic field application conditions, one can realize minute ink droplets without reducing the ink-ejecting nozzle diameter or slit width.

Accordingly, a fine control on minute image formation is possible without accompanying the drawback of head choking with ink. Therefore, the invention provides an inkjet printing method capable of producing printed matters containing sharp and crisp images.

Now, an example of a printing apparatus associated with the invention is explained in detail with reference to FIG. 1.

FIG. 1 schematically shows a structural example of an inkjet recording unit comprising a control unit, an ink-feeding unit and a head approximating/distancing mechanism.

As is shown in FIG. 1, inkjet recording unit 3 used for the present inkjet printing method comprises ejecting head 22 and ink-feeding unit 24.

Ink-feeding unit 24 further contains ink tank 25, ink-feeding unit 26 and ink concentration controlling means 29. Ink tank 25 is provided with agitating member 27 and ink temperature controlling means 28. The ink may be circulated in the head as will be shown in FIG. 2. In such a case, the ink-feeding unit has collecting and circulating functions. Agitating member 27 acts to prevent the ink by agitation from aggregation and precipitation and/or to redisperse the ink by agitation to suppress the precipitation or aggregation of the solid ingredients in the ink. Agitating member 27 includes rotary blades, an ultrasonic vibrator and a circulation pump. One can adopt one or more from these means. A more detailed description will be given later. Ink temperature-controlling means 28 is arranged in such a manner as to secure consistent formation of high quality images by suppressing the change in the ink property as well as the change in the dot diameter caused by the change in the ambient temperature. Various conventionally known methods for ink temperature control may be adopted including provision of a heat-generating or cooling element such as a heater or a Peltier element in the ink tank together with an agitating member that is equipped so as to achieve a uniform temperature distribution within said tank and a temperature sensor exemplified by a thermostat that controls temperatures. The ink temperature is preferably 15 to 60.degree. C., more preferably 20 to 50.degree. C. The agitating member that is equipped so as to achieve a uniform temperature distribution in said tank may be commonly used for the prevention of the precipitation or aggregation of the solid ingredients in the ink.

FIG. 2 shows the structure of ink-feeding unit 24 having an ink-collecting function. As is shown in the figure, ink-feeding unit 24 has, in addition to valve 61, pump 26 to feed ink to ejecting head 22, and ink concentration controlling means 29, circulation-collection pump 26' and valve 61' both used for the circulation and collection of ink from the head. Though there are a variety of aggregation/precipitation-preventing members and/or redispersing means as have been described heretofore, the figure illustrates agitating motor 70 and agitating blades 71. With use of these devices, an ink which contains oily ink particles in a finely dispersed condition free of aggregates or precipitates can be supplied to ink-ejecting head 22. By arranging a filtering member such as a filter just in front of ejecting head 22, one can feed to ejecting head 22 ink in a normal dispersion state containing neither paper fiber nor dust.

To output high quality images, the present ink-ejecting printing apparatus 3 is preferably provided with ink concentration control means 29. Ink concentration can be controlled by optical detection, measuring electrical conductance, measuring physical properties such as viscosity, or by the number of output sheets. In the case of the control based on physical property measurement, an optical detector, an electrical conductance-measuring device or a viscosity-measuring device is installed in the ink tank or the ink flow channel whereby such devices are used individually or in combination, and the control is performed by the output signals thereof. When the ink concentration is controlled by the number of printed sheets, feeding from an ink concentrate tank for replenishment or from an ink carrier tank for dilution, both tanks being not shown in the figure, is controlled based on the number of print and printing frequency.

In the figure, 21 designates an image data processing-controlling unit, which calculates input image data and receives the timing pulses from encoder 30 provided in head distancing/approximating unit 31, a counter drum or capstan rollers and drives the head by the pulses. To conduct printing with ink-ejecting recording unit 3, counter drum 4 is driven with a high-precision driving means. Specifically, for example, the recording drum is driven by decelerating the output of a high-precision motor by means of a high-precision gear or a steel belt. By jointly using one or more of these means, extremely high-quality recording can be conducted.

Image data processing-controlling unit 21 receives image data from an image scanner, a magnetic disc unit and an image data transmission unit, and performs color separation, performs division calculation of proper pixel numbers and gradation numbers on the color-separated data, and distributes them to each head. Further, in order to output oily, halftone inkjet images by using ink-ejecting head 22 of inkjet recording unit 3, area coverage values are calculated, too.

Image data processing-controlling unit 21 controls not only the movement of inkjet ejecting head 22 and the ejection timing of the oily ink, but also the timing for moving the printing medium if necessary. Specifically, image data from a magnetic disc unit and the like are given to image data processing-controlling unit 21. Image data processing-controlling unit 21 performs the calculation of the ejecting position of the oily ink and the dot coverage at that position in accordance with the input image data. These processed data are once stored in a buffer. By using head distancing/approximating unit 31, image data processing-controlling unit 21 moves ejecting head 22 to a position close to the printing medium which is in contact with the imaging drum. The spacing between ejecting head 22 and the surface of the imaging drum is kept at a pre-determined value during recording by mechanical distance control such as with a knocking roller or by the control of a head distancing/approximating unit operated by the signals from an optical gap detector. Ejecting head 22 may comprise a single channel head, multi-channel heads or full-line heads.

When a single channel head or a multi-channel-type head is used as ejecting head, the ejecting part(s) is (are) arranged substantially in parallel to the conveyance direction of the printing medium. And main scanning is performed by the movement of the ejecting head in the axial direction of the counter drum, while sub-scanning is performed by the rotation of the counter drum to thereby effect image recording. These movements of the counter drum and the ejecting head(s) are controlled by image data processing-controlling unit 21, and the head(s) ejects (eject) an oily ink on the printing medium on the basis of the ejecting position and the dot coverage obtained by the calculation cited above. Thus, a dot image is formed on the printing medium with the oily ink corresponding to the density distribution of the original. This action continues until a predetermined ink image completes on the printing medium.

On the other hand, when ejecting heads 22 are of a full-line-type having a length substantially equal to the width of the drum, the ejecting parts are arranged substantially perpendicular to the conveyance direction of the printing medium. And with the printing medium passing the imaging point by the rotation of the counter drum, an image composed of the oily ink is formed to provide a printed matter.

After completion of printing, the ejecting head 22 is driven to retreat from the position close to the imaging drum for protection whereby only ejecting head 22 may be recessed or together with ink-feeding means 24.

This distancing/approximating member 31 acts to separate the recording head by at least 500 .mu.m apart from the image recording drum 4 except during imaging. Such a separating action may be performed with a sliding mechanism, or with an arm fixed to a certain axis, around which the arm is rotated to cause a pendulum-like movement of the head. With such a head retreat during its suspended period, the head is protected from physical damage or contamination, thus achieving a long life.

Next, ejecting head 22 will be explained with use of FIGS. 3 to 9, which are used to describe ink-ejecting head 22 equipped in the inkjet recording unit shown in FIG. 1. However, the scope of the invention is not restricted to the examples to follow.

FIGS. 3 and 4 illustrate an example of a head equipped in the inkjet imaging unit. Ejecting head 22 has ink-ejecting slit formed between upper block 221 and lower block 222, both made of insulating base materials, and the tip of the head forms ejecting slit 22a. Ejecting electrode 22b is arranged in the slit, and the slit is filled with ink 23 fed from an ink-feeding unit. As the insulating base material, plastics, glasses or ceramics can be used. Ejecting electrode 22b can be fabricated by well-known methods such as a method comprising vacuum deposition, sputtering or electroless plating of an electrically conductive material including aluminum, nickel, chromium, gold or platinum on lower block 222 made of an insulating base material, coating a photo-resist thereon, exposing the photo-resist through a mask of prescribed electrode pattern, developing the exposed photo-resist to develop a photo-resist pattern of ejecting electrode 22b, and etching the conductive material imagewise, or a method based on mechanical removal of the conductive material, or combinations of these methods.

To ejecting electrode 22b of ejecting head 22 is applied a potential modulated by the digital signals representing an image pattern. As is shown in FIG. 3, an image-recording drum is arranged so as to face and act as the counter electrode of ejecting electrode 22b, and a printing medium is loaded on the image-recording drum. With voltage application, an electric circuit is formed between ejecting electrode 22b and the image-recording drum acting as the counter electrode, thus causing oily ink 23 to eject from ejecting slit 22a of ejecting head 22, and an image is formed on the printing medium loaded on the image-recording drum.

The width of electrode 22b should be as small as possible for high quality image formation. Though the specific numerical value differs depending on the conditions such as electrode spacing and applied voltage, the tip of from 5 to 100 .mu.m in width is generally used.

For instance, when the tip of ejecting electrode 22b is 20 .mu.m wide, a 40 .mu.m size dot can be formed on printing medium 9 with the distance of 1.0 mm between electrode 22b and imaging drum 4 acting as the counter electrode under the application of 3 kV between these two electrodes for 0.1 msec.

FIGS. 5 and 6 depict schematically the cross-sectional and front views of the vicinity of the ink-ejecting part in another type of ejecting head, respectively. In the figures, symbol 22 indicates an ejecting head, which has a first insulating base material 33 of tapered shape. A second insulating base material 34 faces this first insulating base material 33 with an intervening space, and at the tip of this second insulating base material 34 is formed beveled part 35. These first and second insulating base materials are made of, for example, plastic, glass or ceramic. On the upper plane 36 forming an acute angle with beveled part 35 of second insulating base material 34 are provided a plurality of ejecting electrodes 22b as electrostatic field-forming means at the ejecting parts. The tips of these plural electrodes 22b extend to the vicinity of the upper plane 36 described above, and protrude beyond the end of first insulating base material 33, thus forming ink-ejecting parts. The space between the first and second insulating base materials 33 and 34 makes ink inflow channel 37 as means of supplying ink 23 to the ejecting point, and ink recovery channel 38 is formed under the lower side of second insulating base material 34. Ejecting electrodes 22b are formed on second insulating base material 34 with an electrically conductive material such as aluminum, nickel, chromium, gold or platinum according to any conventional method well known in the art as described above. Each electrode 22b is formed so as to be electrically insulated from each other. The length by which the tip of ejecting electrode 22b protrudes beyond the end of insulating base material 33 should not exceed 2 mm. The reason of restricting the protrusion length to the above range is that, if this length is too large, the ink meniscus will not reach the end of the ejecting electrode thus making ink-ejection difficult, or lowering the recording frequency. The clearance between first and second insulating base materials 33 and 34 is preferably from 00.1 to 3 mm. The reason of restricting the clearance to the above range is that narrower clearances than this range make ink-feed difficult, and also cause the drop of recording frequency, and that broader spaces make the ink meniscus unstable, causing ink ejection inconsistent. The above ejecting electrode 22b is connected to image data processing-controlling unit 21, which, during printing, applies voltage to the ejecting electrode to cause the ink on the ejecting electrode to eject. In this way, imaging is performed on a printing medium (not shown in the figure) arranged to face the ejecting point. The direction opposite to the ink droplet ejecting direction of inflow channel 37 is connected to the ink-feeding means of the ink-feeding device not shown in the figure. Backing 39 is provided on the counter side to the surface of second insulating base material 34 opposite to the surface on which the ejecting electrodes are formed with a clearance therebetween which forms ink recovery channel 38. The clearance of ink recovering channel 38 is preferably 0.1 mm or larger. The reason why the clearance is restricted in the above range is that if the clearance is too narrow, the ink recovery becomes difficult leading to ink leakage.

Ink recovery channel 38 is connected to the ink recovery member of an ink-feeding device not shown in the figure. In the case where a uniform ink flow on the ejecting point is needed, thin grooves 40 may be provided between the ejecting point and the ink recovery channel. FIG. 6 is the front schematic diagram of the vicinity of the ink-ejecting point, in which a plurality of grooves 40 are provided on the bevel of second insulating base material 34 running from the vicinity of the boundary with electrode 22b toward ink recovery channel 38. These plural grooves 40, which are arranged side by side in plurality in the direction of the array of ejecting electrode 22b, act to attract a constant amount of the ink in the vicinity of the aperture in the side of electrode 22b from the aperture in ejecting electrode 22b by a capillary force determined by the electrode aperture size and discharge the attracted ink to recovery channel 38. To achieve these actions, grooves 40 have a function of forming an ink-flow with a constant layer thickness in the vicinity of the tip of the ejecting. As for the shape and size of grooves 40, which are designed so as to exert a sufficient capillary force, the width is made preferably from 10 to 200 .mu.m, and the depth is preferably made 10 to 300 .mu.m. Grooves 40 are provided in a number necessary to form a uniform ink-flow on the entire surface of the head.

The tip width of ejecting electrode 22b should be as small as possible for the formation of high-resolution images. Usually, the tip width of from 5 to 100 .mu.m is preferred, though the specific numerical value differs depending on electrode spacing, applied voltage, etc.

Another example of the ejecting head used in practicing the invention is illustrated in FIGS. 7 and 8. FIG. 7 depicts schematically a part of such a head for explanation. Head 22 consists of head body 41 made of an insulating material such as plastic, ceramic or glass, and meniscus regulating plates 42 and 42'. In the figure, symbol 22b indicates an ejecting electrode that applies voltage for the formation of electrostatic field at the ejecting point. Further, a more detailed description of the head body will be made with reference to FIG. 8 in which meniscus regulating plates 42 and 42' are removed. Perpendicularly to the edge of head body 41, plural ink slots 43 are provided for ink circulation. The shape and size of ink slot 43, which are designed within the range that the capillary force reaches so as to achieve a uniform ink-flow, should preferably be 10 to 200 .mu.m wide and 10 to 300 .mu.m deep. Ejecting electrode 22b is provided in each ink slot 43. These electrodes can be formed on head body 40 made of an insulating material with the use of an electro-conductive material such as aluminum, nickel, chromium, gold or platinum according to the well-known methods cited in the description of the example of the imaging unit to entirely or partly cover the surface of slot 43. Each of the plural ejecting electrodes is electrically isolated from each other. Adjacent two slots form a single cell, and at the tip of dividing wall 44 located in the center of the cell, ejecting points 45 and 45' are provided. At these ejecting points 45 and 45', the dividing wall is fabricated thinner than the remaining area thereof, thus forming sharp edges. Such a structure of the head body can be made by any method known in the art including mechanical processing, etching or molding a block of the insulating material. The thickness of the dividing wall is preferably from 5 to 100 .mu.m, and the diameter of curvature at the sharpened edge is preferably in the range of 5 to 50 .mu.m. The corner of the point may be slightly chamfered such as 45' shown in the figure. The figure depicts only two cells, and the cells are separated with dividing wall 46, and its tip 47 is beveled in such a manner that tip 47 stands back relative to ejecting points 45 and 45'. An ink-feeding device of an ink-feeding unit not shown in the figure supplies ink to the ejecting point via the ink slots from the direction designated by I. Further, excessive ink is collected by an ink recovery means not shown in the figure to the direction designated by O. Thus, the ejecting point is always supplied with fresh ink. In such a state of the head body, the ink is ejected from the ejecting point to a printing medium mounted on an imaging (counter) drum (not shown in the figure) facing the ejecting point by applying signal voltage modulated by image data to the ejecting electrode, and an image is formed on the printing medium.

Still another example of the ejecting head is described with reference to FIG. 9. As is illustrated in FIG. 9, ejecting head 22 has a pair of supporting members 50 and 50' made of substantially rectangular boards of plastic, glass or ceramic with a 1 to 10 mm thickness. On one side of each board are formed plural rectangular slots 51 and 51' (not shown in the figure) running parallel to each other with spacings corresponding to the recording resolution. Each slot 51 or 51' is preferably 10 to 200 .mu.m wide and 10 to 300 .mu.m deep, and in each slot, ejecting electrode 22b is formed that covers the surface of the slot entirely or partly. By forming plural slots 51 and 51' on one surface of supporting members 50 and 50', plural dividing walls 52 result between each slot 51. Supporting members 50 and 50' are bonded together at the surfaces opposite to the planes on which the slots were formed. As a result, on its outer surface, ejecting head 22 has slots 51 and 51' through which ink flows. Slots 51 and 51' provided on each supporting member 50 or 50' are connected together in one-to-one relationship via upper end 53 of ejecting head 22. And rectangular part 54 where the two slots are connected is recessed from upper end 53 of ejecting head 22 by a predetermined distance (50 to 500 .mu.m). In other words, on both sides of each rectangular part 54, there is provided upper end 55 of each dividing wall 52 of each supporting member 50 or 50' in such a manner that the upper end 55 protrudes rectangular part 54. And, from each rectangular part 54, guiding projection 56 made of an insulating material such as those described previously protrudes to form an ejecting point. When an ink is circulated in ejecting head 22 thus constructed, the ink is fed to rectangular end 54 through each slot 51 provided on the outer surface of supporting member 50, and discharged out via each lower slot 51' formed in supporting member 50' arranged in the opposite side. To facilitate a smooth ink flow, ejecting head 22 is slanted by a pre-determined angle so that the feeding side (supporting member 50) be located upward relative to the discharge side (supporting member 50'). When the ink is circulated in this way, the ink passing each rectangular end 54 wets upward along each projection 56 forming an ink meniscus in the vicinity of rectangular end 54 and projection 56. Under the state wherein an independent ink meniscus is formed at each rectangular end 54 with the application of voltage on ejecting electrode 22b according to the image data relative to the imaging drum (not shown in the figure) holding a printing medium thereon and arranged to face the ejecting point, the ink is ejected from the ejecting points and an image is formed on the printing medium. Alternatively, ink can be compulsorily circulated by forming a cover sealing the slots formed on the outer surfaces of supporting members 50 and 50', thus forming a pipe-formed ink flow channel. In this construction, ejecting head 22 need not be slanted.

Head 22 described using FIGS. 3 to 9 can have a maintenance part such as head-cleaning means if necessary. For example, when a suspension period lasts, or when anything unusual on image quality takes place, a desirable condition can be restored by using the means of wiping the tip of the ejecting head with a soft brush or cloth, circulating a pure ink solvent only, or sucking the head along with the feed or circulation of an ink solvent, individually or in combination. Additionally, to prevent ink solidification, it is effective to keep the head in a cover filled with the vapor of an ink solvent, or cool the head to suppress the vaporization of the ink solvent. In the case where the head is contaminated seriously, it is effective to compulsorily suck the ink from the ejecting point, compulsorily introduce air, ink or the jet of an ink solvent from the ink flow channel, or apply ultrasonic wave to the head immersed in an ink solvent, etc. These methods may be used individually or in combination.

Now, the prevention of ink aggregation and/or precipitation and/or the redispersion of ink will be described. When ink in an ink tank stays stationary due to the suspension of ink-flow and the ink particles therein aggregate and/or precipitate, pipe choking or head choking takes place leading to unstable ink ejection. To prevent such choking problems, a homogeneously dispersed state of the ink particles is again restored by preventing the aggregation and/or precipitation and/or redispersing the aggregate or precipitate by one of the actions of agitation, dispersion, mixing or jetting. Each action may be applied individually or in combination depending on the volume as well as the type of ink. Further, the action may be applied at any timing, with a fixed interval or continuously. Although a aggregation and/or precipitation-preventing member and/or a redispersing member arranged at the upstream side of the ink ejecting part can supply homogeneously dispersed ink particles to the ink ejecting part, it is more effective to provide a tubular agitator such as a pipeline mixer or in-line mixer just in front of the ink ejecting part. In cases where the ink is driven to flow after a suspension of ink-flow, it is effective that the aggregation and/or precipitation-preventing member and/or the redispersing members should be activated prior to the start of ink-flow to prevent the aggregates or precipitates from being fed to the ink ejecting part. Further, by providing a cartridge-type aggregation and/or precipitation-preventing member and/or redispersing member interchangeably in the ink-flow path, it becomes possible to select the most proper aggregation and/or precipitation-preventing member and/or redispersing member differing in aggregation and/or precipitation-preventing and/or redispersing action depending on ink volume or type. At the same time, maintainability improves.

Specific examples of the aggregation and/or precipitation-preventing member and/or redispersing member which exhibits an agitating action include an stirrer equipped with disk- or fan-shaped agitating blades rotating at 1 to 3,000 rpm, a homo-mixer which comprises a turbine of special shape capable of rotating at a high speed and a stator having a radial baffle, and agitates aggregates and the like by making use of ink ejection under the pressure difference between the bottom and the upper part of the turbine caused by the high-speed rotation thereof, a pipeline mixer which agitates aggregates and the like by the rotation of agitating wings arranged in an ink-flow path, a magnetic mixer (exemplified by the magnetic mixers and star-head stirrer both manufactured by Tokai Riki Co., Ltd.), an ultra-vibrating blender which agitates and disperse aggregates by ultrasonic vibration, and a lamond stirrer (made by Tokai Riki Co., Ltd.) which comprises two disks each having honeycomb walls, sucks ink from the axial center of the bottom plane along with disk rotation and agitates ink by expelling ink overflowing the honeycomb walls at the side plane.

As the devices that exert a dispersing action, one can mention a homogenizer in which aggregates are dispersed by the rotation of agitating blades (made by Nippon Seiki Manufacturing Co., Ltd.), an ultrasonic homogenizer which disperses aggregates via ultrasonic vibration (made by Nippon Seiki Manufacturing Co., Ltd.), an ultrasonic filtering machine which disperses aggregates by rapidly vibrating a filter plane (made by Ginsen Co., Ltd.), a high-speed disperser (KD mill), an ultrasonic cleaning machine (made by Nippon Seiki Manufacturing Co., Ltd.), and an ultra-vibration stirrer (Ultra-vibrating .alpha.-stirrer made by Nihon Techno Co., Ltd.).

As the devices that exert a mixing action, one can mention a mixing pump enabling homogenization by the function of mixing two liquids (made by Nippon Ball Valve Co., Ltd.), and an inline mixer which mixes ink with plural mixing wings attached to the rotating axis of a vessel (exemplified by Dynamic Mixer made by Nippon Ball Valve Co., Ltd.).

Further, as the devices that exert a mixing action, one can mention an underwater pump (made by Rei-Sea Co., Ltd.).

Each of those devices cited above is preferably employed for the invention in an arbitrarily miniaturized or modified form. These aggregation and/or precipitation-preventing members and/or redispersing members exhibit a single mode of action such as agitation and mixing, but sometimes exhibit plural actions to effectively conduct aggregation and/or precipitation-preventing and/or redispersion.

FIGS. 15 to 20 are schematic diagrams each showing the constitution of a printing apparatus equipped with inkjet image recording apparatus 3 in which an aggregation and/or precipitation-preventing member and/or redispersing member is installed. However, the scope of the invention is not limited to the following constitutional examples.

FIGS. 15 to 20 are schematic diagrams each showing the constitution of a printing apparatus for performing printing by moving a printing medium along with the rotation of a counter drum according to the invention.

FIGS. 15 to 18 are schematic diagrams each showing the constitution of a web-type printing apparatus in which a roll of a printing medium is stretched by means of a counter drum, a printing medium-feeding roll and a printing medium-winding roll or a guide roll. FIG. 15 is a diagram showing a web-type printing apparatus for performing a single-sided, monochromatic printing, FIG. 16 is one for performing single-sided four-color printing, and FIGS. 17 and 18 are ones for performing double-sided four-color printing.

Further, FIG. 19 is a schematic diagram showing a single-sided four color printing apparatus in which a roll of a printing medium is cut into sheets, the resulting sheets being wound around a counter drum, and FIG. 20 is one showing a printing apparatus using a sheet-formed printing medium.

On the other hand, FIGS. 21 and 22 are schematic diagrams each showing the constitution of a printing apparatus for performing printing by holding and conveying a printing medium with a pair of capstan rollers according to the invention. FIG. 21 is a schematic diagram showing a printing apparatus using a roll of a printing medium while FIG. 22 schematically shows the constitution of a printing apparatus using a sheet-formed recording medium.

In the first place, the printing process according to the invention is described with reference to the diagram of the printing apparatus for performing single-sided monochromatic printing on a rolled printing medium shown in FIG. 15.

The inkjet printing apparatus shown in FIG. 15 (hereinafter sometimes referred to as "printing apparatus", too) comprises rolled printing medium-feeding roll 1, dust and paper powder-eliminating member 2, inkjet image recording unit 3, counter (imaging) drum 4 arranged at the position facing image recording unit 3 with a printing medium therebetween, fixing unit 5 and printing medium-winding roll 6.

After the removal of dusts and the like on the printing medium delivered from the printing medium-feeding roll by means of dust and paper powder-removing member 2, an ink is imagewise ejected from the ink-ejecting head (described later) of imaging unit 3 onto the printing medium on imaging drum 4, thus a printing image is recorded. After the image is fixed on the printing medium by fixing member 5, the printing medium which finished printing is wound round printing medium-winding roll 6.

Counter (imaging) drum 4 is comprised of a metallic roll, a roll having an electrically conductive rubber layer on the surface, or an insulating drum made of, e.g., plastic, glass or ceramic, having a metallic layer on the surface thereof provided by vapor deposition or metal plating so as to act as the counter electrode to the inkjet electrode of the ejecting head. Thus, an effective electric field can be formed between counter (imaging) drum 4 and the ink-ejecting part of imaging unit 3. It is also effective to provide a heating member on imaging drum 4 and elevate the temperature of the drum for the improvement of image quality. As the fixing of the ejected ink droplets on the printing medium is accelerated by this measure, blur is further restrained.

Further, the physical properties of the ejected ink droplets on the printing medium are controlled by making the drum temperature constant, leading to consistent and uniform dot formation. For making drum temperature constant, it is more preferred to provide a cooling means, too.

As the method of eliminating dusts and paper powders, a non-contacting one such as suction removal, blow-off removal or electrostatic removal, and a contacting one using a brush or roller can be used.

In the present invention, air suction, blow-off by air or a combination of them is used.

The printing medium M fed out of printing medium-feeding roll 1 is given tension by driving printing medium-winding roll 6, and brought into contact with imaging (counter) drum 4, by which inkjet imaging unit 3 is prevented from damaging by accidental contact with the vibrating printing medium web during imaging.

Alternatively, it is possible to prevent printing medium M from touching inkjet imaging unit 3 by arranging members that bring the printing medium into close contact with the imaging (counter) drum 4 only at a close vicinity of the imaging position of the inkjet recording unit and actuating these members at least when imaging is conducted. Specifically, for example, pressing rollers may be arranged at the upstream and downstream sides of the imaging position on the drum. Specifically, pressing rollers, guides, electrostatic adsorption, etc. are effectively used.

The oily ink image thus formed is enhanced with fixing unit 5. Image fixing can be performed by various methods known in the art such as heat fixing or solvent fixing. As heat fixing, irradiation with an infrared lamp, a halogen lamp or a xenon flash lamp, hot air fixing with a heater or heat roll fixing is usually employed. Flush fixing with use of a xenon lamp is well known as a fixing method for electrophotographic toner images and has an advantage of completing fixing in a short period. When a laminated paper is used, a rapid temperature rise promotes an abrupt moisture vaporization to form unevenness in the paper surface, which phenomenon is often called blistering. Thus, it is preferred for blister prevention to elevate the temperature of the paper gradually by using multiple fixing members whereby the distance from each member to the printing medium or the power supplied to each member is properly changed.

In solvent fixing, a solvent such as methanol and ethyl acetate that can dissolve the resinous ingredient in the ink is sprayed or the medium is exposed to the vapor of such a solvent, and the excessive solvent vapor is collected.

It is desirable to keep the image formed on the printing medium not brought into contact with anything after the oily ink image formation with ejecting head 22 until the step of image fixing with fixing unit 5.

FIGS. 16 to 18 are diagrams each showing the constitutional example of a single- or two-sided four-color printing apparatus.

Since the operating principle thereof is readily understood by the description on the single-sided monochromatic printing apparatus cited hereinabove, further explanation will be omitted. Though in the specification a four-color printing apparatus is shown, the number of colors need not be limited to 4, but optionally chosen depending on need.

FIGS. 19 and 20 illustrate other constitutions according to the invention, and explains a printing apparatus in which an automatic paper-exhausting member 7 is equipped with use of a printing medium M wound around a counter drum 4. FIG. 20 illustrates a constitutional example of an apparatus equipped with automatic paper-feeding member 9 with use of a sheet-formed printing medium. In the following, the example illustrated in FIG. 19 that uses a roll of a printing medium M is described.

In the first place, printing medium M is drawn from printing medium-feeding roll 1, and then loaded onto counter drum 4 after cut to an arbitrary length by means of cutter 8 whereby the printing medium is contacted and fixed to the drum with mechanical means such as leading edge/trailing edge grippers or an air suction device, or electrostatic means to prevent the trailing edge of the medium from flapping to touch inkjet imaging unit 3 during imaging.

Alternatively, it is possible to prevent printing medium M from touching inkjet recording unit 3 by arranging a member that brings the printing medium into contact with drum 4 only near the imaging position of the inkjet imaging unit and by actuating the member at least during imaging. Specifically, for example, pressing rollers may be arranged at the upstream and downstream sides of the imaging position.

Further it is desirable to keep the head apart from printing medium M when image recording is not performed, by which the inkjet imaging unit is effectively prevented from damaging by the contact with the medium.

Inkjet head 22 (shown in FIG. 1) may comprise a single channel head, multi-channel heads or full line heads, and main scanning is performed by the rotation of counter drum 4. When the inkjet head comprises multi-channel heads having a plurality of ink-ejecting parts, the ink-ejecting parts are arranged in parallel to the axis of counter drum 4.

Further, when a single channel head or multi-channel type head is used, image data processing-control unit 21 moves head 22 parallel to the axial direction of the counter drum continuously or stepwise, and an oily ink is ejected onto printing medium M loaded on drum 4 on the basis of the ejection position and the dot coverage obtained by the calculation of image data processing-control unit 21. In this way, a dot image is formed on printing medium M with the oily ink corresponding to the density distribution of the original. This action continues until a predetermined ink ima