GB2351700A

GB2351700A - Solvents removed by suction in lithographic printing

Info

Publication number: GB2351700A
Application number: GB0011530A
Authority: GB
Inventors: Yusuke Nakazawa; Kazuo Ishii; Eiichi Kato; Sadao Osawa
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1999-05-17
Filing date: 2000-05-12
Publication date: 2001-01-10
Also published as: JP2000326479A; GB0011530D0

Abstract

In lithographic printing, an image based on signals of image data is directly formed on a plate material 9 mounted on the plate cylinder 11 of a printing machine by an ink-jet recording head 2 from which an oil-based ink is ejected utilising an electrostatic field. Suction is applied to the under side of the plate material during the image formation to remove the solvent from the ink. The plate material can be permeable to air and a suction unit (not shown) can suck air and solvent from the interior of plate cylinder 11.

Description

2351700 LITHOGRAPHIC PRINTING METHOD AND APPARATUS The present invention

relates to a lithographic printing method and apparatus for effecting digital plate-making on a printing machine. More particularly, the present invention relates to a plate-making and printing method and apparatus in which an oil-based ink is employed and which results in high quality images on both a lithographic printing plate and prints.

In the field of lithographic printing, ink receptive areas and ink repellent areas are formed on a surface of a printing plate in accordance with an original image. Printing ink adheres to the ink receptive areas. ordinarily, hydrophilic areas and oleophilic areas (ink receptive areas) are formed in image patterns on the surface of a printing plate, and the hydrophilic areas are converted to oil-based ink repellent areas by applying dampening water thereto.

Conventional image formation on a printing plate is carried out by exposing a silver halide photographic film with the desired image in an analog or digital manner, exposing a photosensitive material (plate material) containing a diazo resin or a photopolymeri z able polymer to the silver halide photographic f ilm, and removing the photosensitive material by dissolving out the non-image areas. This removal process is carried out mainly by using an alkaline solution.

with recent improvements in digital recording technology and the demand for more efficient printing processes, various 1 methods where digital image information is directly recorded on a plate material have been proposed. These methods include technologies referred to a CTP (computer-to-plate) and a DDPP (digital direct printing plate) method. These methods typically involve an image recording system having a photon mode or heating mode using a laser beam. Some of these methods have been put to practical use.

However, after the image is recorded on a plate using either the photon mode or the heating mode, the non-imaged areas are dissolved out by treating the plate with an alkaline developer. This method results in an alkaline waste liquid discharge, which is environmentally undesirable.

As a means of effecting the printing process at an enhanced ef f iciency there is proposed a system in which image recording is effected on the press. The foregoing method involving the use of laser may be employed. However, this method requires an expensive and large-sized apparatus. Thus, a system utilizing an ink jet method which employs an inexpensive and compact image recording apparatus has been attempted.

JP-A-4-97848 (The term "JP-Y' as used herein means an Olunexamined published Japanese patent application") discloses a method which comprises forming a oleophilic or hydrophilic image on a plate drum which is hydrophilic or oleophilic on the surface thereof instead of the conventional plate cylinder by an ink jet process, and then removing the image after printing to clean the plate drum. However, this method is disadvantageous in that the desired removability of printed 2 image (i.e., cleanability) and press life cannot be accomplished at the same time. In order to form a printed image having a prolonged press life on the plate cylinder, it is necessary that an ink containing a resin in a relatively high concentration be used. Thus, the ink jet recording means for forming a printed image uses a resin solution as an ink. Accordingly, the resin can be easily solidified due to the evaporation of solvent at the nozzle, deteriorating the stability in the ejection of ink. As a result, a good image can hardly be obtained.

Further, JP-A-64-27953 discloses a plate-making method which comprises recording an image of a oleophilic wax ink on a hydrophilic plate material by an ink jet recording process. In this method, the printing plate is disposable. Thus, it is not necessary that the image be removed after printing. Further, this method gives a high stability in ejection. However, this method is disadvantageous in that since the image is formed of a wax, the resulting image area has a reduced mechanical strength. This method is also disadvantageous in that the adhesion of the image area to the hydrophilic surface of the plate material is insuffi cient, reducing the press life.

The present invention has been worked out paying attention to the foregoing problems. it is an object of the present invention to provide a lithographic printing method and apparatus for use with a digital recording system which requires no development processing. It is another object of the present 3 invention to provide a lithographic printing method and apparatus capable of providing a large number of prints having sharp images of high quality in a simple and inexpensive manner.

The above objects of the present invention are accomplished by the following aspects (1) to (19) of the invention:

(1) A lithographic printing method comprises the steps of forming an image based on signals of image data directly on a plate material mounted on the plate cylinder of a printing machine by an ink jet recording method in which an oil-based ink is ejected utilizing an electrostatic field to form a printing plate, and then effecting lithographic printing, wherein suction is conducted at the other side of the plate material during the image formation.

(2) In the lithographic printing method according to Aspect (1), the plate material is permeable to air.

(3) In the lithographic printing method according to Aspect (1) or (2), the oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance of 10' Q-cm or more and a dielectric constant of 3.5 or less.

(4) A lithographic printing apparatus which operates by a method comprises the steps of forming an image based on signals of image data directly on a plate material mounted on the plate cylinder of a printing machine by an ink jet recording method in which an oil- based ink is ejected utilizing an electrostatic field to form a printing plate, and then effecting lithographic

4 T v printing, wherein there is incorporated a means for effecting suction at the other side of the plate material during the image formation.

(5) In the lithographic printing apparatus according to 5 Aspect (4), the plate material is permeable to air.

(6) In the lithographic printing apparatus according to Aspect (4) or (5), the oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance of 109 Q-cm or more and a dielectric constant of 3.5 or less.

(7) The lithographic printing method and apparatus according to any one of Aspects (1) to (6), comprises a device for fixing said ink.

is (8) The lithographic printing method and apparatus according to any one of Aspects (1) to (7), comprises means for removing dust from the surface of the plate material before and/or during formation of the image on the plate material.

(9) In the lithographic printing method and apparatus according to any one of Aspects (1) to (8), main scanning is ef f ected by the rotation of the plate cylinder having the plate material mounted thereon during formation of the image on the plate material.

(10) In the lithographic printing method and apparatus according to any one of Aspects (1) to (9), the ink jet recording device comprises a single channel head or a multiple channel head and subsidiary scanning is effected by sliding the head along the axis of the plate cylinder.

(11) In the lithographic printing method and apparatus according to any one of Aspects (1) to (9), the ink jet recording device comprises a full-line head having a length which is approximately equal to the width of the plate cylinder.

(12) The lithographic printing method and apparatus according to any one of Aspects (1) to (11), comprises means for supplying the ink into the head of the ink jet recording device.

(13) The lithographic printing method and apparatus according to any one of Aspects (1) to (12), comprises means for supplying the ink into the head of the ink jet recording device as well as means for recovering the ink from the head so as to re-circulate the ink.

(14) The lithographic printing method and apparatus according to any one of Aspects (1) to (13), further comprises stirring means in an ink tank housing the oil-based ink.

(15) The lithographic printing method and apparatus according to any one of Aspects (1) to (14), comprises means for controlling the temperature of the ink in the ink tank housing the oil-based ink.

(16) The lithographic printing method and apparatus according to any one of Aspects (1) to (15), comprises means for controlling the concentration of the ink.

(17) In the lithographic printing method and apparatus accordingto anyone of Aspects (1) to (16), the ink jetrecording head is provided releasably from the plate cylinder and there is provide a means for separating the recording head from the 6 plate cylinder except during formation of the image on the plate material.

(18) The lithographic printing method and apparatus according to any one of Aspects (1) to (17), comprises a means 5 for removing paper dust produced during lithographic printing.

(19) The lithographic printing method and apparatus according to any one of Aspects (1) to (18), comprises means for cleaning the ink jet recording head at least after plate making.

Embodiments in accordance with this invention will now be described with reference to the accompanying drawings; in which:- Fig. 1 is a schematic diagram illustrating the entire configuration of a on-press recording type lithographic printing apparatus used in the present invention; Fig. 2 is a schematic diagram illustrating the configuration of the recording portion of the on-press recording type lithographic printing apparatus used in the present invention; Fig. 3 is a schematic diagram illustrating an example of the head provided in the ink jet recording device used in the present invention; Fig. 4 is a schematic sectional view of a portion close to the ink ejector of Fig. 3; Fig. 5 is a schematic sectional view of a portion close to the ink ejector in another example of the head provided in the ink jet recording device used in the present invention; 7 1 1 Fig. 6 is a schematic front view of a portion close to the ink ejector of Fig. 5; Fig. 7 is a schematic diagram illustrating an essential part of a further example of the head provided in the ink jet 5 recording device used in the present invention; Fig. 8 is a schematic diagram of the head of Fig. 7 f rom which regulating panels are removed; Fig. 9 is a schematic diagram illustrating an essential part of a still further example of the head provided in the ink jet recording device used in the present invention; and Fig. 10 is a schematic diagram illustrating the entire conf iguration of a press recording type f our-color lithographic printing apparatus as an example of the copying machine used in the present invention.

In the present invention, the image formation is carried out by an ink jet recording method in which an oil-based ink is ejected utilizing an electrostatic field onto a plate material provided on the plate cylinder of a printing machine.

The ink jet recording method for use in the lithographic printing method and apparatus of the invention involves the use of an ink having a high resistivity having resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in an insulating solvent. In operation, the ink is subject to electrostatic f ield at the ejection position so that 8 1 T condensates of the resin particles are formed at the ejection position. The condensates are then ejected by an electrostatic means. Specifically, a method described in W093/11866 may be used.

When the foregoing method is applied to lithographic printing, the resin particles are ejected in the form of highly concentrated condensates, making it possible to print dots on the plate material to a suf f icient thickness. In this manner, an image made of condensed resin particles having a sufficient press lif e is f ormed on the plate material as a recording medium. Further, since the resin particles are ejected in the form of highly concentrated condensates, and the droplet thus ejected has a small content of solvent, the ink can dry quickly, making it possible to prevent dots f rom blurring and hence form an image to a high precision.

In the ink jet recording method used in the invention, the size of the ejected ink droplets is determined by the size of the ejection position and the electrode and the application condition of the applied electric field. in accordance with the ink jet recording method, minute ink droplets can be formed without reducing the ejection nozzle diameter or the ejection slit width. By controlling the application condition of the applied electric field, the diameter of dots formed on the plate material can be controlled. Accordingly, image formation can be controlled so as to provide a minuteness and a prolonged press life without clogging the ink head.

Thus, the present invention makes it possible to provide a large number of prints of clear images.

9 1 1 The ink jet recording method of the invention is ef f ected utilizing an electrostatic field - A strong electric field is preferably applied to the ink so that the ink is ejected. when the electric field thus applied is not suf f icient, it is likely that a good ejectability cannot be obtained. Thus, the electric field to be applied is preferably about 1 x 105 V/cm or more. on the contrary, when the electric field thus applied is too high, dot split takes place or satellites are produced, showing a tendency toward image quality drop. Thus, the electric f ield to be applied is preferably about 1 x 108 V/cm or less. More preferably, the electric field to be applied is from 2 x 105 V/cm to 5 x 108 V/cm.

Examples of the configuration of on-press recording type lithographic printing apparatus for use in the implication of the lithographic printing method according to the invention will be described hereinafter.

Fig. 1 is a diagram illustrating the entire configuration of a press recording type monochromatic single-sided lithographic printing apparatus. Fig. 2 is a schematic diagram illustrating the configuration of the recording portion including a controller, an ink supplier and a mechanism for moving a head toward or away in the on-press recording type lithographic printing apparatus. Figs. 3 to 9 each illustrate an ink jet recording device installed in the on- press recording type lithographic printing apparatus shown in Figs. 1 and 10. Fig. 10 is a diagram illustrating the entire configuration of a press recording type four-color single-sided lithographic printing apparatus according to the invention.

1 The printing process according to the invention will be described hereinafter in connection with the entire configuration of press recording type monochromatic singlesided lithographic printing apparatus shown in Fig. 1. As shown in Fig. 1, a on-press recording type lithographic printing apparatus 1 (hereinafter referred to as "printing apparatus") has a plate cylinder 11, a blanket cylinder 12 and an impression cylinder 13. The transferring blanket cylinder 12 is arranged so as to be pressed against the plate cylinder 11 at least during lithographic printing. The impression cylinder 13 for moving a printing ink image which has been transferred to the blanket cylinder 12 to a printing paper P is arranged pressed against the blanket cylinder 12.

The plate cylinder 11 is usually made of a metal. The surface of the plate cylinder 11 is plated with chromium to enhance its abrasion resistance. The plate cylinder 11 may have a heat insulator material on the surf ace thereof as described later. The plate cylinder 11. is permeable to air. In this arrangement, air is sucked by a suction unit (not shown) from the interior of the plate cylinder 11 so that a plate material 9 is attracted by the plate cylinder 11. To this end, the plate cylinder 11 is made of a porous material having a large number of fine pores or pierced with holes. The air which has been sucked by the suction unit is then preferably freed of solvent by a solvent removing unit bef ore discharged out of the printing apparatus.

On the other hand, the plate cylinder 11 is preferably grounded because it acts as a counterelectrode to an electrode 11 of the ejection head during ejection under an electrostatic field. When the substrate of the plate material is a good insulator, it is preferable to provide a conductive layer on the substrate of the printing material. In this case, the 5 conductive layer is preferably grounded to the plate cylinder. In a case where a heat insulator is provided on the plate cylinder 11 as described above, recording is more easily accomplished by providing the plate material with a ground. Examples of the ground employable herein include a known conductive brush, leaf spring, and roller.

The printing apparatus 1 also has an ink jet recording device 2 which ejects an oil-based ink onto the plate material 9 mounted on the plate cylinder 11. The ink is ejected in accordance with image data transmitted from an arithmetic and control unit 2 1, to thereby form an image on the plate material.

The printing apparatus 1 further includes a dampening water supplier 3 installed therein for supplying dampening water onto the water receptive layer (non-image area) of the plate material 9. Fig. 1 illustrates a Morton process water supplier as a typical example of the dampening water supplier 3. other examples of the dampening water supplier 3 employable herein include known apparatus such as synchronous process water supplier and continuous process water supplier. The printing apparatus 1 further includes a printing ink supplier 4 and a fixing device 5 for adhering the oil-based ink image formed on the plate material 9. Additionally, a plate surface oil-desensitizing device 6 may be installed depending on the type of plate material 9 for increasing the hydrophilic 12 1 1 properties of the surface of the plate material 9. The printing apparatus I also has a removing unit 10 for removing dust present on the surf ace of the plate material before and/or during the process of recording the image on the plate material 9. The removing unit 10 can ef f ectively prevent ink f rom adhering to the plate material 9 via dust settling between the ejection head and the plate material 9 during the plate-making process. Examples of the dust remover include a contact method using a brush or a roller, in addition to a conventional non-contact method involving suction, blowing or electrostaticity. in the present invention, the removal method is preferably one that uses suction, blowing or a combination thereof. In this case, an air pump commonly used for paper feeder may be used for this purpose.

An automatic plate material supplying device 7 by which the plate material 9 f or printing is f ed automatically to the plate cylinder 11, and an automatic plate material discharging device 8 by which the plate material 9 is removed from the plate cylinder 11 after the printing process may finish. Examples of the press having these devices, which are known as auxiliary devices for press, include HAMADA VS34A, B452A (produced by HAMADA PRINTING PRESS CO., LTD.), TOKO 800OPFA (produced by Tokyo Koku Keiki K.K.), RYOBI 3200ACD, 320OPFA (produced by Ryobi Imagics Co., Ltd.), AMSIS Multi5150FA (produced by Nippon AM Co., Ltd.), Oliber 266EPZ (produced by Sakurai Graphics Systems Co., Ltd.), and Sinohara 661V/IVP (produced by Shinohara Shoji K.K.). Further, a blanket cleaner 14 may be installed. The use of these devices 7, 8 and 14 can make the 13 printing operation simpler and shorter, so that the effects of the invention can be further enhanced. Further, a paper dust generation inhibiting device 15 may be installed in the vicinity of the plate cylinder 13, making it possible to prevent paper dust from adhering to the plate material. The paper dust generation inhibiting device 15 can operate by humidity control, suction by air or electrostaticity, or the like.

The arithmetic and control unit 21 receives image data from, e.g., an image scanner, a magnetic disk device or an image data communication device, and not only carries out color separation but also processing of the separated data into appropriate numbers of pixels and gradations. In addition to these operations, the control unit 21 calculates dot area percentage in order to enable the recording of oil- based ink images in halftone dots by means of an ejection head 22 (see Fig. 2 explained in detail hereinafter) with which the ink jet recording device 2 is equipped.

Furthermore, as described below, the arithmetic and control unit 21 controls the movement of ejection head 22 and the time at which the oil-based ink is ejected and, if desired, the timing of the rotation of the plate cylinder 11, the blanket cylinder 12, the impression cylinder 13, etc.

A method of preparing a printing plate using the printing apparatus 1 is described in detail below with reference to Fig.

1 and a portion of Fig. 2.

The plate material 9 is f irst mounted on the plate cylinder 11 using the automatic plate material supplying device 7. The plate material 9 is brought into close contact with and 14 fixed firmly to the plate cylinder by means of a well-known mechanical device such as a plate end gripping device or an air suction device, or by a well-known electrostatic device. Due to this firm f ixation, the end of the plate material 9 is prevented from flapping against and damaging the ink jet recording device 2 during the recording process. Also, it is possible to prevent the plate material 9 from scraping against the ink jet recording device by using an arrange which brings the plate material into close contact with the plate cylinder only in the neighborhood of the recording position of the ink jet recording device. Specifically, the arrange may be, for example, holddown rollers disposed on both upstream and downstream sides of the recording position of the plate cylinder. Further, an arrangement may be provided such that the end of the plate material is kept away from the ink supplying roller, making it possible to inhibit stain on the surf ace of the printing plate and hence reduce the number of sheets of waste paper. Specifically, hold-down rollers, guides, electrostatic attraction, etc. are effective.

During image recording, air is sucked from the interior of the plate cylinder so that the air-permeable plate material of the invention is attracted by the plate cylinder.

Using an air pump or the like, suction is effected at a pressure of 600 mmHg or less, preferably 500 mmHg or less, more preferably 400 mmHg or less so that the solvent component of the ink droplets attached to the surface of the plate material is sucked off on the other surface thereof.

Image data from a magnetic disc device or the like is given to the arithmetic and control unit 21. The arithmetic and control unit 21 then calculates the ejection position of oil-based ink and dot area percentage at the ejection position according to the image data thus inputted. The arithmetic data input to the arithmetic and control unit 21 is temporarily stored in a buf f er. The arithmetic and control unit 21 instructs the rotation of the plate cylinder 11 and, at the same time, switches on an ejection head moving device 31 which moves the ejection head 22 towards or away from the plate cylinder 11. The distance between the ejection head 22 and the surface of the plate material 9 mounted on the plate cylinder 11 is maintained during recording at a desired value by mechanical distance control, e. g., using a contact roller or by controlling the ejection head moving device 31 in accordance with signals f rom an optical distance detector. Such distance control makes it possible to prevent irregularities in dot diameter due to looseness of the plate material. Such distance control also ensures no change in dot diameter even when the press is subjected to vibration. Thus, satisfactory plate making can be accomplished.

For the ejection head 22, a single channel head, a multiple channel head or a full-line head can be used. Main scanning is carried out by rotating the plate cylinder 11. In the case of a multiple channel head or full-line head having a plurality of ejectors, the head is arranged so that the ejectors are aligned in the axial direction of the plate cylinder 11. In the case of a single or multiple channel head, 16 according to instructions from the arithmetic and control unit 21, the head is moved in the direction parallel to the axis of rotation every time the plate cylinder 11 rotates. The oil-based ink is ejected from the head towards the plate material 9 mounted on the plate cylinder 11 at a position and with the dot area percentage determined by operations performed by the control unit 21. As a result, a dot image with gradations corresponding to the original is recorded with the oil-based ink on the plate material 9. Thus, air suction is effected on the other surface of the plate material at least during image recording, whereby the solvent in the ink which has been attached to the plate material is sucked from the other surface of the plate material, making it possible to prevent dots from blurring on the plate material and hence form an image having f ine lines. These operations are continued until the oil-based ink image corresponding to one-color information of the original is formed on the plate material to prepare a printing plate. In the case of a full-line head having substantially the same length as the width of the plate cylinder, on the other hand, an oil-based ink image corresponding to one-color information of the original is formed on the printing plate by one rotation of the plate drum. As described above, the plate cylinder is rotated to effect main scanning so that positional precision in the main scanning direction is enhanced and high-speed recording becomes feasible.

Subsequently, the ejection head 22 is moved away from the position close to the plate cylinder 11 in order to protect the ejection head 22. During this operation, only the ejection head 17 may be moved away from the plate cylinder 11. However, the ejection head 22 may be moved away from the plate cylinder 11 together with a head subsidiary scanning unit 32 or together with the ink supplier 24 and the head subsidiary scanning unit 32. Alternatively, an arrangement may be made such that the fixing device 5 and the dust remover 10 can be moved away from the plate cylinder 11 in the same manner as the ejection head 22, the ink supplier 24 and the head subsidiary scanning unit 32, whereby the printing apparatus can be used also in ordinary printing.

The device for moving the head towards and away operates so as to keep the recording head at least 500 gm away from the plate cylinder except during image recording. This movement may be ef f ected using a sliding system or a mechanism by which the head is gripped with an arm fixed on a shaft and moved in a pendulum- like motion by turning the shaft. By keeping the head away from the plate cylinder when image formation is not being carried out, the head is protected from physical damage and contamination. As a result, the life of the head can be extended.

The oil-based ink image formed by the head is hardened by heating or using a fixing device 5. Well-known fixing techniques, such as heat f ixing, solvent f ixing and f lash f ixing, can be employed for fixing the ink image. In the case of heat f ixing, irradiation with a halogen lamp or f ixing using a heated roller can be usually used. In such a case, for increasing the fixing efficiency measured may be adopted such as previously heating the plate cylinder, previously heating the plate 18 material, performing the recording under exposure to hot air, using aplate cylinder coated with a heat insulator, or heating the plate material alone by separating the plate material from the plate cylinder only at the time of f ixing. In the case of solvent f ixing, a solvent capable of dissolving the resin component of the ink, such as methanol and ethyl acetate, is sprayed onto the plate material, and the excess solvent vapor is recovered. These measures may be employed in combination of two or more thereof. Flash f ixing using, e. g., a xenon lamp, is well-known as a fixing method for electrophotographic toner, and has the advantage of performing the fixing in a short time.

- It is desirable, at least during the portion of the process f rom formation of the oil-based ink image by means of the ejection head 22 to the fixing of the image with the fixing device 5, for the dampening water supplier 3, the printing ink supplier 4 and the blanket cylinder 12 to be prevented from coming into contact with the plate material 9 on the plate cylinder.

The printing plate thus prepared is then subjected to printing process in the same manner as known lithographic printing method. More specifically, the printing plate 9 having the oil- based ink image formed thereon is given a printing ink and a dampening water to form a printing ink image thereon. The printing ink image thus formed is transferred onto a blanket cylinder 12 rotating in concert with the plate cylinder 11, and then the printing ink image on the blanket cylinder 12 is transferred to printing paper passing between the blanket cylinder 12 and the impression cylinder 13 to conduct printing corresponding to one- color information of the 19 original. After the printing operation, the printing plate is removed from the plate cylinder 11 by an automatic plate remover 8, and a blanket on the blanket cylinder 12 is cleaned with a blanket cleaning device 14 so that it is restored to a printable 5 state.

The ink jet recording device 2 will be described in more detail below.

The image recording portion used in the lithographic printing device of the invention comprises an ink jet head 2, and an ink supplier 24, as shown in Fig. 2. The ink supplier 24 has an ink tank 25, an ink supplying device 26 and an ink concentration controller 29. The ink tank 25 is furnished with a stirrer 27 and an ink temperature controlling device 28. The ink may be circulated through the ejection head. In this case, the ink supplier has a recovering function in addition to the circulatory function. The stirrer 27 inhibits the solid component of the ink from precipitating and aggregating so as to reduce the necessity for cleaning the ink tank. Examples of the ink stirrer include a rotating blade, an ultrasonic vibrator and a circulatory pump. These tools can be used singly or in combination. The ink temperature controlling device 28 is arranged so as to prevent the physical properties of the ink from changing due to change in ambient temperature, thereby ensuring no change in dot diameter so as to form a consistently high-quality image. To control the ink temperature, a well-known method can be adopted. More specifically, the ink tank can be provided with a heating element such as a heater or a Peltie element or a cooling element together with the stirrer so as to make the temperature distribution inside the ink tank uniform, and the temperature is controlled with a temperature sensor such as thermostat. It is desirable that the ink temperature inside the ink tank be from 15'C to WC, and preferably from 200C to 5CC. The stirrer may be used for both purposes of keeping the temperature distribution uniform and for preventing precipitation and aggregation of the solid component of the ink.

For achieving high-quality image formation, it is preferred that the plate-making apparatus of the present invention further be provided with an ink concentration controlling device 29. This device makes it possible to effectively prevent blurring on the plate material and missing or f ading in the printed image due to a decrease of solid concentration in the ink, or changes of dot diameter due to increase of the solid concentration in the ink. Ink concentration control is carried out by optical detection, measurement of physical properties such as electric conductivity or viscosity, or monitoring a number of plate materials subjected to image formation. More specifically, the ink concentration is controlled by feeding concentrated ink from an ink tank for replenishment (not shown) in accordance with output signals from an optical detector, a conductivity measuring instrument and a viscosity measuring instrument provided individually or in combination inside the ink tank, or ink flow course in the case of control in accordance with measurement of physical properties, or based on a number of printing plates made or a frequency of plate-making operations 21 in the case of monitoring the number of plate materials subjected to image formation.

The arithmetic and control unit 21, as described above, not only performs arithmetical operations on input image data and controls movement of the ejection head with the ejection head moving device 31 or the head subsidiary scanner 32, but also receives a timing pulse from an encoder 30 attached to the plate cylinder and carries out operation of the ejection head 22 in accordance with the timing pulse. As a result, positional precision in the direction of subsidiary scanning is improved. During the image recording by the ink jet recording device, the use of a driving unit having a high precision different from the driving unit for printing allows the plate cylinder to be driven in an enhanced positional precision in the direction of subsidiary scanning. During this procedure, the plate cylinder is preferably released mechanically from the blanket cylinder, the impression cylinder and others so that only the plate cylinder can be driven. More specifically, the output from a high precision motor can be subjected to reduction through a high precision gear, steel band or the like to drive only the plate cylinder. During the recording of a high quality image, these means may be used singly or in combination.

The ejection head will now be described in more detail with reference to Figs. 3 to 9. However, the present invention should not be construed as being limited thereto.

Figs. 3 and 4 show an example of an ejection head which is installed in the ink jet recording device. The ejection head 22 has a slit interposed between an upper unit 221 and a lower 22 unit 222, each formed by an insulating substrate, while the tip thereof forms an ejection slit 22a. An ejection electrode 22b is arranged in the slit, and the slit is filled with an ink 23 supplied from an ink supplying device. Examples of the insulating substrate usable for the head include plastics, glass and ceramics. The ejection electrode 22b is formed on the lower unit 222 made of an insulating substrate according to a known method. For instance, the top surface of the lower unit 222 may be provided with a conductive material such as aluminum, nickel, chromium, gold or platinum using a technique such as vacuum deposition, sputtering or electroless plating, and then the conductive material coating is covered with a photoresist. The photoresist is exposed to light via a desired e lectrode pattern and developed to form a photoresist pattern in the form of the ejection electrode 2 2b. Then, the conductive material coating undergoes etching, mechanical removal or a combination thereof to form the ejection electrode 22b.

During operation of the ejection head 22, a voltage is applied to the ejection electrode 22b in accordance with digital signals corresponding to image pattern information. As shown in Fig. 3, the ejection electrode 22b is arranged facing the plate cylinder 11 so as to constitute a counterelectrode, and the plate material 9 is mounted on the plate cylinder as the counterelectrode. Upon application of voltage, a circuit is f ormed between the ejection electrode 22b and the plate cylinder 11 acting as the counterelectrode, and the oil-based ink 23 is ejected from the ejection slit 22a of the ejection head 22 to 23 form an image on the plate material 9 mounted on the plate cylinder 11 as the counterelectrode.

In order to form a high-quality image, it is preferred that the tip of the ejection electrode 22b is made as small as possible. The tip of the electrode is ordinarily shaped so as to have a width of from 5 to 100 lAm, although the tip width may be varied depending on conditions.

For instance, a dot having a diameter of 4 0 Vn can be f ormed on the plate material 9 when an ejection electrode 22b having a tip width of 20 pn is used, the space between the ejection electrode 22b and the plate cylinder 11 as a c ountere lect rode is adjusted to 1. 0 mm, and a voltage of 3 kV is applied for 0. 1 millisecond between these electrodes.

Figs. 5 and 6 respectively show a cross-sectional view and a front view of the vicinity of an ink ejector of another example of the ejection head. Reference numeral 22 in these figures indicate the ejection head. The head has a f irst insulating substrate 33 of a tapered shape. A second insulating substrate 34 is set f acing to and apart f rom the f irst insulating substrate 33. An end portion of the second insulating substrate 34 has a tapered portion 35. The f irst and second insulating substrates are each made of, e. g., plastics, glass or ceramics on a top surface 36 of the second insulating substrate 34, which makes a sharp angle with the slope 35, a plurality of ejection electrodes 22b are provided for forming an electrostatic field in the ejector. The tips of the ejection electrodes 22b extend to the vicinity of the tip of the top surface 36, and protrude beyond the tip of the first insulating substrate 33, thereby

24 forming the ejectors. An ink inflow course 37, defining a pathway for supplying ink 23 to the ejector, is formed between the first and second insulating substrates 33 and 34, and the ink recovery course 38 is formed on the underside of the second insulating substrate 34. The ejection electrodes 22b are formed using a conductive material such as aluminum, nickel, chromium, gold or platinum on the top surface of the second insulating substrate 34 in a conventional manner as described above. The respective ejection electrodes 22b are constructed so as to be in an electrically insulated state.

A suitable length for the tip of the ejection electrode 22b that protrude beyond the tip of the first insulating substrate 33 is 2 mm or less. A reason why such a range of protrusion is preferred is that, if the protrusion is too long, it is dif f icult for the ink meniscus to reach the tip of ejector, resulting in difficulty in ejection of the ink and a decrease in maximum recording frequency. Also, it is preferred that the space between the first and second insulating substrates 33 and 34 be f rom 0 - 1 to 3 mm. A reason why this range is pref erred for the space is that too narrow a space makes supply of the ink difficult, resulting in difficulty in ejection of the ink and a decrease in maximum recording frequency while, on the other hand, too wide a space makes the meniscus unstable, resulting in inconsistent ejection of the ink.

The ejection electrode 22b is connected to the arithmetic and control unit 21. In carrying out recording, a voltage is applied to the ejection electrode in accordance with image information signals from the arithmetic and control unit 21, and thereby the ink on the ejection electrode is ejected to perform image formation on a plate material (not shown) arranged to be f acing to the ejector. The ink inflow course 37 is connected to a device for sending ink from an ink supplying 5 device (not shown) on the side opposite to the ink ejector. Further, a backing 39 is arranged apart from and facing toward the underside, which is the reverse of the ejection electrode side, of the second insulating substrate 34 to form an ink recovery course 38 between the backing and the underside of the second insulating substrate 34. It is preferred that the width of the space of the ink recovery course 3 8 be at least 0. 1 mm. This is because too small a space makes the recovery of ink difficult, resulting in ink leakage. The ink recovery course 38 is connected to an ink recoverer, which is attached to the ink supplying device (not shown).

If a uniform ink flow over the ejector is required, grooves 40 may be provided between the ejector and the ink recoverer. Fig. 6 is a front view showing the vicinity of the ejector of an ejection head. As shown in Fig. 6, a plurality of grooves 40 are provided in the tapered portion of the second insulating substrate 34 from the vicinity of the borders with the respective ejection electrodes 22 to the ink recovery course 38. The grooves 40 are aligned in the lengthwise direction of the ink jet electrode 22b, and have a function for conducting by capillary action a predetermined amount of ink, depending on the opening diameter, present in the vicinity of the tip of each ejection electrode f rom the respective openings on the side of ejection electrodes 22b into the ink recovery course 38.

26 Thus, the grooves 40 function to form an ink flow having a certain thickness in the vicinity of the tip of each ink jet electrode. The groove 4 0 may have any shape as far as the grooves can provide the desired capillary action. However, it' is especially desirable that the width of the grooves is from 10 to 200 pm and the depth thereof is from 10 to 300 gm. The grooves 40 are provided in a number suf f icient f or f orming a unif orm. ink f low over the entire ejection head.

In order to effect formation, e.g., printing of a 10 high-quality image, it is preferred that the tip of the ejection electrode 22b be made as small as possible. The tip of the electrode is ordinarily shaped so as to have a width of from 5 to 100 gm, although the tip width may be varied depending on conditions.

Still another example of the ejection head for use in the present invention is shown in Figs. 7 and 8. Fig. 7 is a schematic diagram illustrating only a portion of the head. The recording head 22, as shown in Fig. 7, has a main body 41 made of an insulating material such as plastics, ceramics or glass, and meniscus regulating panels 42 and 42'. Reference numeral 22b in Fig. 7 indicates an ejection electrode to which a voltage is applied to form an electrostatic field in the ejector. The main body 41 of the head is further illustrated in detail with reference to Fig. 9 wherein the regulating panels 42 and 421 are removed from the ejection head.

The main body 41 of the head has a plurality of ink grooves 43 cut perpendicularly to the edge thereof for the purpose of ink circulation. The grooves 43 each may have any shape so far 27 as the grooves can provide a suitable capillary action sufficient to form a uniform ink flow. However, it is especially desirable that the width of the groove be from 10 to 200 lAm and the depth thereof be from 10 to 300 pm. Ejection 5 electrodes 22b are provided in respective ones of the grooves 43. In each of the grooves 43 the ejection electrode 22b may be arranged so as to cover the entire surf ace of the groove or it may be formed on only a portion of the groove using a conductive material such as aluminum, nickel, chromium, gold or platinum, according to a well-known method as described in the above- described example of the head. Additionally, the ejection electrodes are electrically isolated from one another. Two ink grooves adjacent to each other form one cell, and a separator wall 44 positioned in the center of the cell has an ejector 45 or 45' in the tip. The separator wall 44 is made thinner in the ejector 45 or 45' than in other portions thereof, and the ejector is sharpened. The main body of the head having the configuration method such as mechanical processing or etching of a block of insulating material, or molding of an insulating material. It is desirable that the separator wall in the ejector have a thickness of from 5 to 100 Vn and the sharpened tip thereof have a radius of curvature of from 5 to 50 pm. Further, the tip of the ejector may be slightly cut off as shown in the ejector 45,. In the figure, only two cells are depicted for ease of illustration. A separator wall 46 is disposed between cells. The tip 47 of the wall 46 is cut off so as to be set back compared with the ejectors 45 and 45'. The ink is flowed into the ejection head via ink grooves from the 28 direction indicated by an arrow I with from an ink supplying device (not shown), and thereby supplied to the ejectors. Further, the excess ink is recovered in the direction indicated by an arrow 0 with an ink recoverer (not shown). As a result, fresh ink is always supplied to each ejector. A plate cylinder holding a plate material on the surface thereof (not shown) is arranged so as to face the ejector. while maintaining such a condition, a voltage corresponding to the image information is applied to the ejection electrode, and ink is ejected from the ejector to form an image on the plate material.

Still another example of the ejection head is described with reference to Fig. 9. As shown in Fig. 9, the ejection head 22 has a pair of nearly rectangular plate-shaped support members 50 and 50 1. Each of these support members 50 and 501 is made of an insulating plastic, glass or ceramic plate having a thickness of from 1 to 10 mm, and in one surface thereof there are formed a plurality of rectangular grooves 51 or 511 (not shown) extending parallel to one another. Each of the grooves 51 and 511 desirably has a width of from 10 to 200 pn and a depth of from 10 to 300 gm. In each of the grooves, an ejection electrode 22b is f ormed so as to cover the whole or only a portion of the groove surface. The formation of a plurality of grooves 51 or 5 1 1 in one surface of each support member 5 0 or 5 01 results in the formation of rectangular separator walls 52 between respective pairs of grooves. The support members 50 and 50' are placed together so that the surfaces thereof in which no grooves are formed are brought into contact with each other. Specifically, the ejection head 22 has a plurality of grooves 29 f or distribution of ink over the periphery thereof. The grooves 51 formed in the support member 50 are coupled to corresponding ones of the grooves 511 formed in the support member 501 by way of the upper portion 53 of the ejection head 22. Each rectangular portion 54 that couples together two corresponding grooves is set back a predetermined distance (e.g., 50 to 500 pm) from the top end portion 53 of the ejection head. In other words, each of the separator walls 52 adjoining each rectangular portion 54 on both sides is disposed so that the top end 55 thereof protrudes beyond the adjacent rectangular portions 54. Also, a guide protrusion 56 made of an insulating material as described above is attached so as to protrude beyond each rectangular portion 54, thereby forming the ejector.

When ink is circulated through the ejection head 22 having the structure as described above, the ink is supplied to each rectangular portion 54 via a respective groove 51 formed at the periphery of the support member 50, and the ink is discharged via the grooves 51' formed in the support member 501 opposite the support member 50. In this case, the ejection head 22 is inclined so that the ink supply side of the support member 50 is situated upward and the ink discharge side of the support member SO' is situated downward. By circulating the ink through the ejection head 22 in such a manner, the ink passing across each rectangular portion 54 flows forward along the guide protrusions 56 to form an ink meniscus in the vicinity of the rectangular portion 54 and the protrusion 56. A plate cylinder holding a plate material on the surface thereof (not shown) is arranged so as to face the ejector. With independent ink meniscuses formed on the respective rectangular portions 54, a voltage corresponding to the image information is applied to the ejection electrode, and the ink is ejected from the ejector to form an image on the plate material. A cover maybe attached along the periphery of each of the support members 50 and 50, to cover the grooves, thereby forming pipe-shaped ink flow courses along the periphery of each of the support members 50 and 501. In such a case, since the ink can be made to circulate by way of these ink f low courses, it is not necessary to include the ejection head 22.

The ejection heads as shown in Figs. 3 to 9 can also be provided with a maintenance device such as a cleaner if desired. For instance, in a case where recording has been suspended for a certain period or problems in image quality occur, a device for circulating the ink solvent alone, a device for exerting suction on the ejector while supplying or circulating the ink solvent alone, and a device for wiping the tip of the ejection head with a flexible brush or cloth can be adopted singly or in combination, whereby the solidification of ink in the ejection head or other troubles can be effectively prevented, making it possible to maintain satisfactory recording conditions. Further, a method can be effectively used which comprises disposing the ejection head in a cover filled with a solvent vapor. These methods may be used singly or in combination.

A specific example will be described hereinafter with reference to a press recording type multi-color single-sided lithographic printing apparatus.

31 Fig. 10 is a schematic diagram illustrating the entire configuration of a press recording type four-color lithographic printing apparatus. As shown in Fig. 10, the four-color single-sided printing apparatus essentially consists of four 5 plate cylinders 11, four blanket cylinders 12 and four impression cylinders 13 of the monochromatic single-sided printing apparatus shown in Fig. 1 arranged for each of four colors such that printing is effected on the same surface of printing paper P. Though not shown, the delivery of printing 10 paper from an impression cylinder to another as shown by K is carried out by a known delivery cylinder or the like. Although detailed description is omitted, as can be easily seen in the example of Fig. 10, other multi-color single-sided printing apparatus each essentially comprise a plurality of plate cylinders 11, blanket cylinders 12 and impression cylinders 13 arranged f or each color such that printing is ef f ected on the same surf ace of printing paper P. In the case where only one printing plate is prepared for each plate cylinder, there are provided plate cylinders and blanket cylinders in an amount 20 corresponding to the number of colors to be printed. On the other hand, in the case where a plurality of color printing plates are prepared for each plate cylinder, plate cylinders and blanket cylinders are necessary in an amount corresponding to the value obtained by dividing the number of colors to be 25 printed by the number of printing plates per plate cylinder. For example, when two color printing plates are prepared per plate cylinder, a press consisting of two plate cylinders and two blanket cylinders can be used to effect three- or four- 32 color printing on one side of printing paper. The number of impression cylinders to be installed may be the same as that of plate cylinders. Several plate cylinders and blanket cylinders may have one impression cylinder in common. If desired, the impression cylinder may be provided with a holder for holding printing paper until the desired number of colors are printed.

On the other hand, in the case where the present invention is implicated in the form of press recording type multi-color double-sided lithographic printing apparatus, the foregoing press is provided with a known printing paper inverting device or has a plurality of plate cylinders 11 and blanket cylinders 12 of the monochromatic single-sided printing apparatus shown in Fig. I arranged for each color such that printing is ef f ected on both surfaces of printing paper P. In the case where only one color printing plate is prepared for each plate cylinder, there are provided plate cylinders and blanket cylinders in an amount corresponding to the number of colors to be printed on both surfaces of printing paper. On the other hand, in the case where a plurality of color printing plates are prepared for each plate cylinder as mentioned above, the required number of plate cylinders and blanket cylinders can be reduced. Further, in the case where several plate cylinders and blanket cylinders have one impression cylinder in common, the required number of impression cylinders may be reduced, too. If desired, the impression cylinder is provided with a holder for holding printing paper until the desired number of colors are printed. The detail of the configuration of this system can be easily 33 inferred from the foregoing example of press recording type multi-color single-sided lithographic printing apparatus and thus will not be described hereinafter.

Another example of the on-press recording type lithographic printing apparatus has two plate cylinders per blanket cylinder, whereby printing is effected on one of the two plate cylinders while image recording is being effected on the other. In this case, it is desirable that the plate cylinder on the part of image recording be driven while being mechanically separated off from the blanket cylinder. In this manner, image recording is made possible without suspending the operation of the press. As can be easily inferred, this mechanism can be applied to press recording type multi-color single-sided lithographic printing apparatus and press recording type multi-color double-sided lithographic printing apparatus. The air suction device according to the present invention can be applied to all types of press and thus is not limited to the present example.

The plate material (printing original plate) which can be used in the present invention will be described in greater detail below.

As the plate material there is preferably a plate material having a waterresistant support such as paper subjected to a water-resistant treatment, a plastic film or paper laminated with plastic, having provided thereon an image-receiving layer. The thickness of the image-receiving layer is ordinarily in a range of from 5 to 30 um.

34 The image-receiving layer includes a hydrophilic layer including an inorganic pigment and a binder and a layer capable of being rendered hydrophilic by an oil-desensitizing treatment.

The inorganic pigment used in the hydrophilic image receiving layer include clay, silica, calcium carbonate, zinc oxide, aluminum oxide and barium sulfate. The binder used includes a hydrophilic binder, for example, polyvinyl alcohol, starch, carboxymethyl cellulose, hydroxyethyl cellulose, casein, gelatin, a salt of polyacrylic acid, polyvinyl pyrrolidone and a methyl ether-maleic anhydride copolymer. Further, in order to impart water-res i stance to the imagereceiving layer, a melamine formaldehyde resin, a urea formaldehyde resin or other crosslinking agents may be added thereto if desired.

The image-receiving layer to which an oil-desensitizing treatment is applied includes, for example, a layer containing zinc oxide and a hydrophobic binder.

The zinc oxide used in the image-receiving layer according to the present invention is any of zinc oxide, zinc white, wet-type zinc white, and activated zinc white as commercially available, as described in Nippon Ganryo Gijutsu Kyokai, ed., "Shinban Ganryo Binran (New Edition of Pigment Handbook)", pp. 319, Kabushiki Kaisha Seiundo (1968).

Specifically, depending on the starting materials and production method, zinc oxide is classified into two groups, that produced by a wet method and that produced by a dry method, which groups are further subclassified into zinc oxide produced by the "French" method (indirect method) or "American" method (direct method).

Suitable examples of zinc oxide include those commercially available from Seido Kagaku Kogyo K.K., Sakai 5 Chemical Industry Co., Ltd., Hakusui Chemical Industries, Ltd., Honjo Chemical K.K., Toho Zinc Co., Ltd., and Mitsui mining & Smelting Co., Ltd. A resin suitable for the hydrophilic binder includes a styrene copolymer,

a methacrylate copolymer, an acrylate copolymer, a vinyl acetate copolymer, polyvinyl butyral, an alkyd resin, an epoxy resin, an epoxy resin, a polyester resin and a polyurethane resin. The resins may be employed individually or as a mixture of two or more thereof.

The content of the resin in the image-receiving layer is from 9/91 to 20/80 in terms of aweight ratio of resin/zinc oxide.

Suitable examples of the oil-desensitizing solution which has heretofore been used for the oil-desensitizing of the image-receiving layer containing zinc oxide include those conventionally known, for example, a treating solution containing a cyan compound such as f errocyanate or f erricyanate as the main component, a cyan-f ree treating solution containing an ammine cobalt complex, phytic acid or a derivative thereof, or a guanidine derivative as the main component, a treating solution containing an inorganic or organic acid capable of forming a chelate with an zinc ion as the main component, and a treating solution containing a water-soluble polymer.

For instance, treating solutions containing a cyan compound include those described, e.g., in JP-B-44-9045 (The 36 term WP-B" as used herein means an "examined Japanese patent application, '), JP-B-46-39403, JP-A-52-76101, JP-A-57-107889 and JP-A-54-117201.

The oil-based ink which can be used in the present 5 invention is described in more detail below.

The oil-based ink used in the present invention is not specifically limited. In practice, however, it is preferably a dispersion including resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical resistance of 10' Qcm or more and a dielectric constant of 3.5 or less.

The surface tension of the oil-based ink is not specif ically limited. In practice, however, the surf ace tension of the oil-based ink which can be used in the present invention is 35 dyne/cm or less, preferably from 15 dyne/cm to 35 dyne/cm, more preferably from 16 dyne/cmto 30 dyne/cm. When the surface tension of the oil-based ink is too high, the resulting oil-based ink may exhibit a deteriorated ejectability On the contrary, when the surf ace tension of the oil-based ink is too low, the resulting oil-based ink can easily spill from the ejection head and may exhibit a deteriorated stability.

The viscosity of the oil-based ink is not specifically limited. In practice, however, the viscosity of the oil-based ink which can be used in the present invention is 15 cP or less, preferably from 0.4 cP to 15 cP, more preferably from 0.5 cP to 10 cP. When the viscosity of the oil-based ink is too high, the resulting oil-based ink may exhibit a deteriorated ejectability. When the viscosity of the oil-based ink is too 37 low, the resulting oil-based ink can easily spill from the ejection head and may exhibit a deteriorated stability.

Further, the particle charge distribution of the oilbased ink (proportion of electric charge on particles in electric charge on the entire ink) is not specifically limited. in practice, however, the particle charge distribution of the oil-based ink which can be used in the present invention is 10% or more, preferably 30% or more, more preferably 40% or more. When the particle charge distribution of the oil-based ink is too low, the resulting oil-based ink can hardly form condensates and thus may exhibit an insufficient press life.

The particle charge distribution is defined by f(electrical conductivity of entire ink) - (electrical conductivity of supernatant liquid obtained by centrifugal separation of ink (at 15, 0 0 0 rpm f or 3 0 minutes))) / (electrical conductivity of entire ink) x 100%. For the evaluation of the foregoing particle charge distribution, electrical conductivity is measured at a frequency of 1 kHz and an applied voltage of 5 V.

Preferred examples of the nonaqueous solvent having an electrical resistance of 10" Q-cm or more and a dielectric constant of 3.5 or less include straight-chain or branched aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons and halogenated products of these hydrocarbons.

Specific examples thereof include hexane, heptane, octane, isooctane, decane, isodecane, decaline, nonane, dodecane, isododecane, cyclohexane, cyclooctane, cyclodecane, benzene, toluene, xylene, mesitylene, Isopar C, Isopar E, Isopar G, 38 Isopar H and Isopar L (Isopar: tradename, a product of Exxon Corp.), Shellsol 70 and Shellsol 71 (Shellsol: tradename, product of Shell Oil Corp.), Amsco OMS and Amsco 460 Solvent (Amusco: tradename, product of American Mineral Spirits Corp.), and silicone oils. They can be used singly or as a mixture of two or more thereof. As to the nonaqueous solvent, the upper limit of the electrical specific resistance value is of the order of 10" 0-cm, and the lower limit of the dielectric constant value is about 1.9.

The reason why the range of the nonaqueous solvent is restricted as described above - is explained below. If the electrical resistance of the nonaqueous solvent used is too far below the above- described range, the resulting ink doesnrt exhibit an optimum electrical resistance and thus exhibits a deteriorated ejectability under the application of an electrical field. On the other hand, when the dielectric constant of the nonaqueous solvent used is too far above the above-described range, the electrical field in the ink can be easily relaxed, making it difficult for the ink to be ejected.

As the resin particles to be dispersed in the nonaqueous solvent as described above are hydrophobic resin particles which are solid at temperature of 350C or less and have good affinity with the nonaqueous solvent. As such a hydrophobic resin, a resin (P) having a glass transition temperature of from - 50C to 1100C or a softening temperature of from 33'C to 140'C is preferred. The more preferable range of the glass transition temperature is from 10'C to 100'C, and that of the softening temperature is from 380C to 1200C. In particular, it is 39 1 preferred for the resin (P) to have a glass transition temperature of from 150C to 800C or a softening temperature of from 380C to 1000C.

By using a resin having such a glass transition temperature or a softening temperature as described above, the affinity of each resin particle with the image-receiving surface of the plate material is enhanced and the resin particles are firmly bonded with each other on the plate material. Thus, the adhesion of the ink image to the plate material is increased and the press life is improved. on the contrary, if the glass transition temperature or a softening temperature of the resin used is beyond the upper and lower limits specif ied above, the af f inity of each resin particle with the image-receiving surf ace of the plate material may be lowered and the bond between resin particles may be weakened.

The weight-average molecular weight (Mw) of the resin (P) is preferably from 1 X 103 to 1 X 106, more preferably from 5 10' to 8 x 10', and still more preferably from 1 x 104 to 5 101.

Specific examples of such a resin (P) include olefin hompolymers and copolymers (such as polyethylene, polypropylene, polyisobutylene, ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer, ethylene methacrylate copolymer and ethylene-methacrylic acid copolymer), vinyl chloride copolymers (such as polyvinyl chloride and vinyl chloride-vinyl acetate copolymer), vinylidene chloride copolymers, vinyl alkanoate homopolymers and copolymers, allyl alkanoate homopolymers, and copolymers, homopolymers and copolymers of styrene and derivatives thereof (such as butadiene-styrene copolymer, isoprene-styrene copolymer, styrene-methacrylate copolymer and styrene acrylate copolymer), acrylonitrile copolymers, methacrylonitrile copolymers, alkyl vinyl ether copolymers, acrylate homopolymers and copolymers, methacrylate homopolymers and copolymers, itaconic acid diester homopolymers and copolymers, maleic anhydride copolymers, acrylamide copolymers, methacrylamide copolymers, phenol resins, alkyd resins, polycarbonate resins, ketone resins, polyester resins, silicone resins, amide resins, hydroxyl and carboxyl-modified polyester resins, butyral resins, polyvinyl acetal resins, urethane resins, rosin resins, hydrogenated rosin resins, petroleum resins, hydrogenated petroleum resins, maleic acid resins, terpene resins, hydrogenated terpene resins, chroman-indene resins, cyclized rubber-methacrylate copolymers, cyclized rubber-acrylate copolymers, copolymers containing a heterocyclic ring containing no nitrogen atom (as the heterocyclic ring, e.g., f uran ring, tetrahydrof uran ring, thiophene ring, dioxane ring, dioxofuran ring, lactone ring, benzof uran ring, benzothiophene ring and 1, 3-dioxetane ring), and epoxy resins.

It is desirable f or the resin particles to be contained in the oil-based ink in an amount of from 0.5 to 20% by weight based on the total ink content. If the amount of the resin particles is too low, the affinity of the ink with the image-receiving layer of the plate material is insufficient, and, as a result, the ink may not f orm images of good quality 41 and the press life tends to decrease. On the other hand, if the proportion of resin particles is increased beyond the above-described range, it may be difficult to form a homogeneous dispersion, and, as a result, the ink clogs the ejection head 5 and stable ink ejection may not be achieved.

For the oil-based ink used in the present invention, it is preferred to include a coloring material together with the resin particles in order to allow easy visual inspection of the resulting printing plate. Such a coloring material may be any of a number of pigments and dyes which have been ordinarily used in conventional oil-based ink compositions and liquid developers for electrostatic photography.

The pigment to be used has no particular restriction, 'and includes both inorganic and organic pigments which are ordinarily used in the field of printing. Examples of pigments usable in the oil-based ink include carbon black, cadmium red, molybdenum red, chrome yellow, cadmium yellow, titanium yellow, chromium oxide, viridian, titanium cobalt green, ultramarine blue, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindolidone pigments, dioxazine pigments, threne pigments, perylene pigments, perylone pigments, thioindigo pigments, quinophthalone pigments, metal complex pigments, and other conventionally known pigments.

As the dyes, oil-soluble dyes are suitable for use in the oil-based ink, with examples including azo dyes, metal complex dyes, naphthol dyes, anthraquinone dyes, indigo dyes, carbonium dyes, quinoneimine dyes, xanthene dyes, cyanine dyes, quinoline 42 dyes, nitro dyes, nitroso dyes, benzoguinone dyes, naphthoquinone dyes, phthalocyanine dyes and metallophthalocyanine dyes.

The pigments and dyes may be used singly, or they can be used in appropriate combinations. It is desirable that they are contained in a proportion of from 0. 0 1 to 5 % by weight based on the total ink content.

Such a coloring material as described above may be dispersed in the nonagueous solvent as dispersed particles separately from the resin particles, or it may be incorporated into the resin particles dispersed in the nonaqueous solvent. In the latter case, the incorporation of a pigment is ordinarily ef f ected by coating the pigment with the resin material of resin particles to form resin-coated particles, while the incorporation of a dye is ordinarily effected by coloring the surface portion of resin particles with the dye to form colored particles.

The average diameter of the resin particles, including colored particles, dispersed in the nonaqueous solvent is pref erably f rom 0. 05 to 5 pm, more pref erably f rom 0. 1 to 1. 0 pm, still more pref erably f rom 0. 1 to 0. 5 [im. The diameter of the particles is determined with a particle size analyzer, CAPA-500 (tradename, manufactured by Horida Ltd.).

The nonaqueous dispersion of resin particles used in the present invention can be prepared using a well-known mechanical grinding method or a polymerization granulation method. In the mechanical grinding method, the materials for forming resin particles are mixed, molten and kneaded, if required, and 43 directly ground into f ine particles with a conventional grinder, and further dispersed in the presence of a dispersing machine (e.g., a ball mill, a paint shaker, a Keddy mill, a Dyno mill). In another mechanical grinding method, the materials for forming resin particles and a dispersion assisting polymer (a covering polymer) are kneaded in advance to form a kneaded matter, then ground into fine particles, and- further dispersed in the presence of a dispersing polymer. Methods of preparing paints or liquid developers for electrostatic photography can be adopted in practice. Details of these methods are described, e.g., in "Toryo no Ryudo to Ganryo Bunsan (Flow of Paints and Dispersion of Pigments)", translated under the supervision of Kenji Ueki, Kyoritsu Shuppan (1971), Solomon, "Paint Science", "Paint and Surface Coating Theory and Practice", Yuji Harasaki, "Coating no Kiso Kagaku (Elementary Course of Coating Science)", Maki Shoten (1977), etc.

For the polymerization granulation method, well-known methods for dispersion polymerization in nonaqueous media can be employed. Details of such methods are described, e.g., in The Newest Technology of Super-Fine Polymer Particles, Chapter 2, edited under the supervision of Soichi Muroi, CMC Shuppan (1991), The Latest Systems r Electrophotographic Development. and Development and Application of Toner Materials, Chapter 3, edited by Koichi Nakamura, Nippon Kagaku Joho K.K. (1985), and K.B.J. Barret, Dispersion Polymerization in organic Mediu John Wiley (1975).

In order to stabilize the particles dispersed in the nonaqueous solvent, the particles are generally dispersed together with a dispersing polymer (also sometimes referred to as "a dispersion stabilizing resin" hereinafter). The dispersing polymer contains repeating units soluble in the nonaqueous solvent as the main component, and a weight-average 5 molecular weight (Mw) thereof is preferably from 1 x 10' to 1 X 106, more preferably from 5 x 10' to 5 x 105.

Suitable examples of the soluble repeating units of the dispersing polymer usable in the present invention include a polymerizing component represented by the following 10 formula(I):

a, % 1 1 --CH-C Xl-R wherein X, represents -COO-, -OCO- or -0-; R represents an alkyl or alkenyl group having from 10 to 32 carbon atoms, preferably an alkyl or alkenyl group having from 10 to 22 carbon atoms, which may have a straight-chain or branched structure and may be substituted, although the unsubstituted form is preferred (e.g., decyl, dodecyl, tridecyl, tetradecyl, hexadecyl, octadecyl, eicosanyl, docosanyl, decenyl, dodecenyl, tridecenyl, hexadecenyl, octadecenyl or linolenyl).

The suf fixes a, and a2, which may be the same or dif f erent, each represents a hydrogen atom, a halogen atom (e. g., chlorine or bromine), a cyano group, an alkyl group having from 1 to 3 carbon atoms (e.g., methyl, ethyl or propyl), -COO-Z, or - CH2COO-Z, [wherein ZI represents a hydrocarbon group having not more than 22 carbon atoms which may be substituted (such as an alkyl, alkenyl, aralkyl, alicyclic or aryl group).

Specifically, Z1 represents a hydrogen atom or hydrocarbon group. Preferred examples of the hydrocarbon group include an unsubstituted or substituted alkyl group having f rom 1 to 2 2 carbon atoms (e. g., methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, hexadecyl, octadecyl, eicosanyl, docosanyl, 2 chloroethyl, 2-bromoethyl, 2-cyanoethyl, 2- methoxycarbonylethyl, 2-methoxyethyl or 3-bromopropyl), an unsubstituted or substituted alkenyl group having from 4 to 18 carbon atoms (e.g., 2- methyl-1-propenyl, 2-butenyl, 2p. entenyl, 3-methyl-2-pentenyl, 1- pentenyl, 1-hexenyl, 2hexenyl, 4 -methyl- 2 -hexenyl, decenyl, dodecenyl, tridecenyl, hexadecenyl, octadecenyl or linolenyl), an unsubstituted or substituted aralkyl group having f rom 7 to 12 carbon atoms (e. g -, benzyl, phenetyl, 3-phenylpropyl, naphthylmethyl, 2 naphthylethyl, chlorobenzyl, bromobenzyl, methylbenzyl, ethylbenzyl, methoxybenzyl, dimethylbenzyl or dimethoxybenzyl), an unsubstituted or substituted alicyclic group having from 5 to 8 carbon atoms (e.g., cyclohexyl, 2cyclohexylethyl or 2cyclopentylethyl) and an unsubstituted or substituted aromatic group having from 6 to 12 carbon atoms (e.g., phenyl, naphthyl, tolyl, xylyl, propylphenyl, butylphenyl, octylphenyl, dodecylphenyl, methoxyphenyl, ethoxyphenyl, butoxyphenyl, decyloxyphenyl, chlorophenyl, dichlorophenyl, bromophenyl, cyanophenyl, acetylphenyl, methoxycarbonylphenyl, ethoxycarbonylphenyl, 46 butoxycarbonylphenyl, acetamidophenyl, prop ion ami dophenyl or dodecyloxylamidophenyl)].

In addition to the repeating units represented by formula (I), the dispersing polymer may contain other repeating units as copolymerizing components. The copolymerizing components may be derived from any monomers as long as they can be copolymerized with the monomers corresponding to the repeating units of formula (I).

The suitable proportion of the repeating unit represented by formula (I) in the dispersing polymer is preferably at least 50% by weight, more preferably at least 60% by weight.

Specific examples of the dispersing polymer include Dispersion Stabilizing Resin (Q-1) used in Examples described hereinafter and commercially available products, e.g., Sorprene 1205 manufactured by Asahi Chemical Industry Co., Ltd.

In preparing the resin (P) particles in the state of an emulsion (latex), it is preferred that the dispersing polymer be added prior to the polymerization.

The amount of the dispersing polymer to be added is from 0.05 to 4% by weight based on the total weight of the ink.

In the oil-based ink employed in the present invention, it is desirable that the dispersed resin particles and colored particles (the particles of coloring material) be electroscopic particles charged positively or negatively.

In order to impart electroscopicity to those particles, wet developer technology for electrostatic photography can be appropriately utilized. Specifically, electroscopicity can be imparted to the particles by using electroscopic materials 47 and other additives as described, e.g., in "Saikin no Denshi Shashin Genzo System to Toner Zairyou no Kaihatsu Kitsuyouka (The Latest Systems for Electrophotographic Development, and Development and Application of Toner Materials) I', pp. 139 - 148, described above, "Denshi Shashin Gijutsu no Kiso to Oyo (The Fundamentals and Applications of Electrophotographic Techniques)", edited by Electrophotographic Society, pp. 497 - 505, Corona Co. (1988), and Yuji Harasaki, "Denshi Shashin (Electrophotography)", vol. 16 (No. 2), p. 44 (1977).

In addition, details of those materials are described, e.g., in British Patents 893,429, 934,038 and 1, 122,397, U.S. Patents 3, 900,412 and 4,606,989, JP-A-60-179751, JP-A-60185963 and JP-A-2-13965. The charge control agent as described above is preferably used in an amount of fromO.001 to 1.0 parts byweight per 1,000 parts by weight of dispersing medium as a carrier liquid. Although, various kinds of additives can be further added, the total amount of additives has an upper limit because it is restricted by the electrical resistance allowable for the oll-based ink used in the present invention. More specif ically, if the ink has an electrical resistance of lower than 109 Q-cm under the condition that the dispersed particles are removed from the ink, the formation of a continuous-gradation image having good quality may become difficult. Therefore, it is necessary that the amount of each additive added be controlled within the above described limitation.

Examples

48 The present invention will be described in greater detail with reference to the following examples, but the present invention should not be construed as being limited thereto.

An example of a preparation of resin particles (PL) suitable for the oil-based ink used in the present invention will be described below.

PREPARATION EXAMPLE 1 PreparatiQn of Resin Particle (PL---4 A mixed solution of 10 g of Dispersion Stabilizing Resin (Q-1) having the structure illustrated below, 100 g of vinyl acetate, and 384 g of Isopar H was heated to a temperature of 700C under nitrogen gas stream with stirring. To the solution was added 0 - 8 g of 2, 2 1 -azobis (isovaleronitrile) (abbreviated as A.I.V.N.) as a polymerization initiator, followed by reacting for three hours. Twenty minutes after the addition of the polymerization initiator, the reaction mixture became white turbid, and the reaction temperature rose to 880C. Further, 0. 5 g of the above- described polymerization initiator was added to the reaction mixture, and the reaction was carried out for two hours. Then, the temperature of the reaction mixture was raised to 10 0 0 C, and stirred for two hours to remove the unreacted vinyl acetate by distillation. After cooling, the reaction mixture was passed through a nylon cloth of 200-mesh to obtain a white dispersion. In the polymerization process, the polymerization rate was 90%. The white dispersion obtained was a latex of good monodispersity having an average particle diameter of 0.23 pm. The average particle diameter was measured by CAPA-500 (manufactured by Horiba Ltd.).

49 Di spers CH3 CH3 -+CH2-C 96 L12_tJ4 COOC12H25 COO (CH2)20CO(CH2)2COOCH2CH = CH2 Mw: 5 x 104 (composition ratio: by weight) A portion of the above- described white dispersion was centrifuged at a rotation of 1 X 104 r.p.m. for 60 minutes and the thus -precipitated resin particles were collected and dried. The weight-average molecular weight (Mw) of the resin particles was 2 x 105 (a GPC value in terms of polystyrene) and the glass transition temperature (Tg) thereof was 38'C.

EXAMPLE 1

An oil-based ink was prepared in the following manner.

Oil-Based Ink IK-1 In a paint shaker (manufactured by Toyo Seiki K.K.), 10 g of copolymer of dodecyl methacrylate and acrylic acid (copolymerization ratio: 95/5 by weight), 10 g of nigrosine and 30 g of Shellsol 71 were placed together with glass beads, and the mixture was dispersed for four hours to prepare a fine dispersion of nigrosine.

A mixture of 60 g (as a solid basis) of Resin Particles (PL-1) prepared in Preparation Example 1, 2.5 g of the above-described dispersion of nigrosine and 0. 08 g of copolymer of octadecene and maleic acid monooctadecylamide was diluted with one liter of Isopar G, thereby preparing oil-based black ink.

An ink tank of an ink jet recording device of a on-press recording type lithographic printing apparatus (see Figs. I and 2) was filled with 2 liters of Oil-Based Ink (IK-1). A plate cylinder pierced with holes having a diameter of 200 pm at a density of about 10/cm' was used. A 900 dpi 64-channel multiple-channel head as shown in Fig. 3 was used as an ejection head. A drop-in type heater and stirring blades were installed for controlling the ink temperature in the ink tank. The ink temperature was set at 3 0 " C, and temperature control was carried out with a thermostat while rotating the stirring blades at 30 r.p.m. Rotation of the stirring blades was also utilized for preventing precipitation and aggregation. Further, a portion of the ink flow course was made transparent, which portion was arranged between a light emission diode (LED) and a light detector, concentration control of the ink was carried out by feeding diluent for the ink (Isoper G) or concentrated ink (the solid concentration of which was adjusted to twice that of Oil-Based Ink (IK- 1)). As the plate material there was used a paper plate material having a hydrophilic image-receiving layer provided thereon as mentioned above.

Wood-free paper having a basis weight of 100 g/m' was used as a substrate and, on both sides of the substrate, a water-resistant layer composed of as main ingredients kaolin and resin components, including polyvinyl alcohol, SBR latex and melamine resin, was provided to form a paper support. A dispersion A prepared from the following composition in the following manner was applied to the paper support in a dry 51 coating amount of 6 g/M2 to form an image-receiving layer, thereby preparing a paper plate material.

Dispersion A Gelatin (1st grade, produced by 3 g Wako Pure Chemical Industries, Ltd.) Colloidal silica (Snowtex C; produced 20 g by Nissan Chemical Industries, Ltd.; 20% aqueous solution) Silica gel (Silysya #310, produced by 7 g Fuji Silysya Chemical Co., Ltd.) Hardener (paraformaldehyde) 0.4 g Distilled water 100 g The foregoing ingredients were subjected to dispersion together with glass beads in a paint shaker for 10 minutes.

The foregoing plate material was then brought into close contact with a plate cylinder with the head as well as the end portion of the plate material being gripped by a mechanical device provided on the plate cylinder. while the dampening water supplier, the printing ink supplier and the blanket cylinder being separated off from the plate material, dust on the plate material surface was removed by air-pump suction. While suction was being ef f ected on the other side of the plate material under conditions such that the pressure in the plate cylinder was 350 mmHg, the ejection head was moved close to the plate material until it reached the recording position. Image data to be printed was transmitted to an arithmetic and control unit. While rotating the plate cylinder and moving the 64channel ejection head, oil-based ink was ejected from the 52 ejection head onto the plate material, thereby forming an image on the plate material. The ejection electrode of the ejection head had a tip width of 10 pm, and the distance between the head and the plate material was kept at 1 mm by utilizing output from an optically gap-detecting device. A voltage of 2.5 kV was always applied as a bias voltage, and a pulse voltage of 500 V was further superimposed for each ejection of ink. The duration of pulse voltage was changed stepwise from 0.2 millisecond to 0 - 05 millisecond in 256 steps, thereby changing the dot area for recording. As a result, a high precision image was obtained free of blurring. The image thus formed on the plate material had no defects due to dust, and deterioration of image quality due to a change in dot size was not observed at all even when the ambient temperature varied during the plate-making procedure and the number of printing plates prepared with the apparatus was increased. In other words, satisfactory plate-making was accomplished.

The image formed on the plate material was hardened by heating with a xenon flash fixing device (made by USHIO INC.) under a luminous intensity of 200 J/pulse, thereby preparing a printing plate. Then, the ink jet recording device was moved away together with the subsidiary scanner from the position close to the plate cylinder and kept apart at a distance of 50 mm from the plate cylinder for the purpose of protecting the ejection head. Thereafter, printing was effected on printing paper using an ordinary lithographic printing method. In some detail, a printing ink and a dampening water were given to the printing plate to form a printing image thereon. The printing 53 ink image thus formed was then transferred to the blanket cylinder rotating together with the plate cylinder. Subsequently, the printing ink image on the blanket cylinder was transferred to a printing coated paper passing through the 5 gap between the blanket cylinder and the impression cylinder.

The print after printing 10,000 sheets had a very clear image without the occurrence of missing, f ading or sharpening of the printed image. After the completion of plate-making, the ejection head was cleaned by supplying Isopar G to the head and dripping the Isopar G from the opening of the head for 10 minutes. Then, the head was stored in a cover f illed withvapor of Isopar G. By this treatment, prints of good quality were provided f or 3 months without any other work f or maintenance.

EXAMPLE 2

As the stirrer, there was used a circulating pump. A 600 dpi full-line ejection head of the type as shown in Figs. 5, 7 and 9 was installed. The pump was used in the present example. One ink reservoir was arranged between the pump and the ink f low-in course of the ejection head, and a second ink reservoir was arranged between the ink recovery course of the ejection head and the ink tank. The ink was circulated by the dif f erence in hydrostatic pressure between those reservoirs in addition to the action of the circulatory pump. Also, a combination of the circulatory pump with a heater was used for controlling the ink temperature, and the ink temperature was set at 350C and controlled with a thermostat. The circulatory pump was further used as stirrer for preventing precipitation and aggregation. The ink flow course was provided with a conductance measuring 54 device, and according to output signals from the device, concentration control of the ink was carried out by diluting the ink or feeding concentrated ink. The same"plate material as mentioned above was similarly mounted on a plate cylinder of a lithographic printing apparatus. However, as the plate cylinder there was used one obtained by working a sintered metal having a porous structure. Dust on the plate material surface was removed with a rotating brush made of nylon. Then, the image data to be printed was transmitted to an arithmetic and control unit. Image forming was carried out by ejecting the oil-based ink from the full-line head onto the plate material while rotating the plate cylinder. As a result, a high precision image was obtained f ree of blurring. The image thus formed on the plate material had no defects due to dust, and deterioration of image quality due to a change in dot size was not observed at all even when the ambient temperature varied during the plate-making procedure and the number of printing plates prepared with the apparatus was increased. In other words, satisfactory plate-making was accomplished.

Using the printing plate thus obtained, printing was performed. As a result, the print obtained had a very clear image without the occurrence of missing, fading or sharpening of image even after printing 10,000 sheets. After the completion of plate-making, the ejection head was cleaned by circulating Isopar G therethrough and then bringing nonwoven fabric impregnated with Isopar G into contact with the tip of the head. By this treatment, prints of good quality were provided for 3 months without any other work for maintenance.

EXAMPLE 3

A full-line head as shown in Fig. 7 was mounted as an ejection head on the ink jet recording device of a press recording type four-color lithographic printing apparatus (see Fig. 10) Using a contact roller made of tef lon, the gap was adjusted to 0. 8 mm. 5, 000 sheets of printing plates were then prepared in the same manner as in Example 1 except that the ink concentration was controlled by replenishing the ink tank with concentrated ink in accordance with the number of sheets of printing plates made. As a result, a high precision image was obtained free of blurring. The image thus formed on the plate material had no defects due to dust and was not af f ected by the change of the ambient temperature. As the number of sheets of printing plates made increased, the diameter of dots printed showed some but an acceptable change. The printing plates thus made were also subjected to flash fixing as mentioned above, heat roll fixing (heat roll produced by Hitachi Metals, Ltd -; power consumption: 1.2 kW), fixing by irradiation with light f rom a halogen lamp (Type QIR, produced by USHIO INC.), or f ixing with spray of ethyl acetate. For the heat roll f ixing and f ixing by irradiation with a halogen lamp, heating was ef f ected so that the temperature of the surf ace of the printing plate reached 0 C f or 2 0 seconds. For the f ixing with spray of ethyl acetate, the amount of ethyl acetate sprayed was adjusted to about I g/m2.

As a result, the print after printing 10,000 sheets had a very clear image without the occurrence of fading or sharpening of the printed image. In particular, the fixing time in heat roll f ixing or f ixing by irradiation with light f rom a halogen lamp 56 was drastically reduced by wrapping a heat insulating material (PET f ilm) around the plate cylinder. In this case, the aluminum substrate was grounded through an electricallyconductive brush (Thunderlon, produced by Tuchiya K.K.; 5 resistance: about 10-1 Q-cm) which comes in contact therewith.

EXAMPLE 4

Printing plates were prepared in the same manner as in Example 1 except that as the printing paper there was used wood-free paper. During the printing of 3,000th sheet, the image was made solid on some area due to paper dust. Then, an air suction pump was installed as a paper dust generation inhibiting device in the vicinity of the paper supplier. Then, printing was resumed. As a result, no defectives in printing occurred. The print after printing 5,000 sheets had a very clear image without the occurrence of missing, fading or sharpening of the printed image.

EXAMPLE 5

The same procedure as in Example 1 was performed, except that the plate material was replaced with a plate material provided with an image-receiving layer capable of being rendered hydrophilic upon an oil-desensitizing treatment described below, the non- image area of the printing plate prepared was rendered hydrophilic using a plate surface oil-desensitizing device, the conductive layer of the plate material was grounded by contact with a conductive leaf spring (made of phosphor bronze) during the recording operation, and fixing was carried out by exposing the plate material to hot air.

57 Wood-free paper having a basis weight of 100 g/m' was used as a substrate and, on both sides of the substrate, a polyethylene film was laminated in a thickness of 20 pm to form a water-resistant paper support. On one side of the thus- prepared paper support, a coating for conductive layer having the following composition was coated in a dry coating amount of 10 g/m' to form a conductive layer and further thereon Dispersion B prepared in the manner indicated below was coated in a dry coating amount of 15 g/m' to form an image-receiving layer, thereby preparing a plate material.

Coating fnr uc:tive layej::

A coating was prepared by mixing 5.4 parts of carbon black (30% aqueous dispersion), 54.6 parts of clay (50% aqueous dispersion), 36 parts of SBR latex (solid content: 50%, Tg:

25"C) and 4 parts of melamine resin (solid content: 80%, Sumirez Resin SR-13), and then adding water thereto so as to have the total solid content of 25%. ni-,per-,ion B:

A mixture of 100 g of dry-type zinc oxide, 3 g of Binder Resin (B-1) having the structure shown below, 17 g of Binder Resin (B-2) having the structure shown below, 0. 15 g of benzoic acid and 155 g of toluene was dispersed using a wet-type dispersing machine (Homogenizer made by Nippon Seiki Co., Ltd.

at 6,000 r.p.m. for 8 minutes.

58 Rinder- Besin (B-4 CH3 CH3 1 1 CH2-0 CH2-CH CH2--; 11.3 1 T6-8 - - -, 1 1 COOCH3 CO0CH3 COOH MW: 9 X 103 Binder Resin (B-4 CH3 1 CH2-0 CH2-CH CH2-CH CH2-CH 86 1 N 0 COOCH3 CC)012H25 Cr COOH Mw: 4 x 104 (composition ratio: by weight) The print after printing 5,000 sheets had a very clear image without the occurrence of missing fading or sharpening of the printed image.

According to the present invention, a large number of prints having clear images can be provided. Further, a printing plate of high image quality is directly formed on the press corresponding to digital image data in a stable manner, making it possible to conduct lithographic printing at a low cost and a high speed.

59 1 0

Claims

1. A lithographic printing method comprising the steps of forming an image based on signals of image data directly on a plate material mounted on the plate cylinder of a printing machine by an ink jet recording method in which an oil-based ink is ejected from the head utilizing an electrostatic field to form a printing plate, and then effecting lithographic printing, characterized in that suction is conducted at the other side of said plate material during the image formation.

2. The lithographic printing method according to claim 1, wherein said plate material is permeable to air.

3. The lithographic printing method according to claim 1 Or 2, wherein said oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance of 10' Q-cm or more and a dielectric constant of 3.5 or less.

4. A lithographic printing apparatus which operates by an ink jet recording method using an ink jet recording device which ejects an oil-based ink utilizing an electrostatic field to form an image based on signals of image data directly on a plate material mounted on the plate cylinder of a printing' machine, thereby preparing a printing plate by which lithographic printing is effected, characterized in that there is incorporated a means for ef f ecting suction at the other side of said plate material during the image formation.

5. The lithographic printing apparatus according to claim 4, wherein said plate material is permeable to air.

6. The lithographic printing apparatus according to claim 4 or 5, wherein said oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance to 109 0-cm or more and a dielectric constant of 3.5 or less.

7. The lithographic printing apparatus according to claim 4, 5 or 6, further comprising means for fixing said ink.

8. The lithographic printing apparatus according to any one of claims 4 to 7, further comprising means for removing dust from the surface of the plate material before and/or during formation of the image on the plate material.

9. The lithographic printing apparatus according to any one of claims 4 to 8, wherein main scanning is effected by the rotation of the plate cylinder having the plate material mounted thereon during formation of the image on the plate material.

10. The lithographic printing apparatus according to any one of claims 4 to 9, said ink jet recording device comprises a single channel head or a multiple channel head and subsidiary scanning is effected by sliding the head along the axis of the plate cylinder.

11. The lithographic printing apparatus according to any one of claims 4 to 10, said ink jet recording device comprises a full-line head having a length which is approximately equal to the width of the plate cylinder.

12. The lithographic printing apparatus according to 61 any one of claims 4 to 11, further comprising means for supplying the ink into the head of the ink jet recording device.

13. The lithographic printing apparatus according to any one of claims 4 to 12, further comprising means for supplying the ink into the head of the ink jet recording device and means for recovering the ink from the head so as to re-circulate the ink.

14. The lithographic printing apparatus according to any one of claims 4 to 13, further comprising stirring means in an ink tank housing the oil-based ink.

15. The lithographic printing apparatus according to any one of claims 4 to 14, further comprising means for controlling the temperature of the ink in the ink tank is housing the oil-based ink.

16. The lithographic printing apparatus according to any one of claims 4 to 15, comprising means for controlling the concentration of the ink.

17. The lithographic printing apparatus according to any one of claims 4 to 16, wherein the ink jet recording head is provided releasably from the plate cylinder, and said apparatus further comprising means for separating the recording head from the plate cylinder except during formation of the image on the plate material.

18. The lithographic printing apparatus according to any one of claims 4 to 17, further comprising means for removing paper dust produced during lithographic printing.

19. The lithographic printing apparatus according to 62 A l.

making.

20. A method of lithographic printing substantially as described with reference to the accompanying drawings.

21. A lithographic printing apparatus substantially as described with reference to the accompanying drawings.

-&a G '7

Amendments to the claims have been filed as follows CLAIMS 1. A lithographic printing method comprising the steps of forming an image based on signals of image data directly on a plate material mounted on the plate cylinder of a printing machine by an ink jet recording method in which an oil-based ink is ejected from the head utilizing an electrostatic field to form a printing plate, and then effecting lithographic printing, characterized in that suction is conducted at the other side of said plate material during the image formation.

3. The lithographic printing method according to claim 1 or 2, wherein said oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance of 109 Q-cm or more and a dielectric constant of 3.5 or less.

4. A lithographic printing apparatus which operates by an ink jet recording method using an ink jet recording device which ejects an oil-based ink utilizing an electrostatic field to form an image based on signals of image data directly on a plate material mounted on the plate cylinder of a printing, machine, thereby preparing a printing plate by which lithographic printing is effected, characterized in that there is incorporated a means for ef f ecting suction at the other side of said plate material during the image formation.

f%T c 4- 6. The lithographic printing apparatus according to claim 4 or 5, wherein said oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance to 10' 0-cm or more and a dielectric constant of 3.5 or less.

B. The lithographic printing apparatus according to any one of claims 4 to 7, further comprising means for removing dust from the surface of the plate material before and/or during formation of the image on the plate material.

10. The lithographic printing apparatus according to any one of claims 4 to.9, said--- ink j et recording _ device comprises a single channel head or a multiple channel head and subsidiary scanning is ef f ected by sliding the head along the axis of the plate cylinder.

12. The lithographic printing apparatus according to E'T C, 5, any one of claims 4 to 11, further comprising means for supplying the ink into the head of the ink jet recording device.

15. The lithographic printing apparatus according to any one of claims 4 to 14, further comprising means for controlling the temperature of the ink in the ink tank housing the oil-based ink.

19. The lithographic printing apparatus according to Irallr c JC any one of claims 4 to 18, further comprising means for cleaning the ink jet recording head at least after plate making.