Plate making device and printer and printing system using the plate making device

Abstract: A plate material the surface of which is formed of film including as its major component a material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment is used for making a printing plate. The substantially entire surface of the plate material is uniformly exposed to active light and an image is written in a heat mode on the surface of the plate material, which has been exposed to the active light.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a plate making device for a general light printer, particularly to an offset printer, which facilitates making a printing plate and makes it feasible to repeatedly recycle and reuse the printing plate. This invention further relates to a printer and a printing system using such a plate making device. Description of the Related Art Offset printing has been in wide use among others due to its simple plate making step. This printing method is based on immiscibility of oil and water, and oil material, i.e., ink, and foundation solution are selectively held in an imaged region and a non-imaged region, respectively. When a printing medium is brought into contact with the surface of the plate directly or by way of an intermediate member called a blanket, the ink on the imaged region is transferred to the printing medium, whereby printing is effected. A prevailing method of the offset printing involves use of a PS plate which comprises an aluminum base plate and a diazo photosensitive layer formed on the base plate. In the PS plate, the surface of the aluminum base plate is subjected to sand dressing, anodizing and other steps in order to enhance ink receptivity of the imaged region and ink repellency of the non-imaged region, to increase durability against repeated printing and to increase fineness of the printing plate. An image to be printed is formed on the surface of the PS plate thus processed. Accordingly, the offset printing is excellent in durability against repeated printing and fineness in the printing plate as well as simplicity. However as the printed matter spreads wider, there arises a demand for simpler offset printing and there have been proposed various simple printers. The typical examples of such simple printers include printers in which a printing plate is made by use of a silver salt diffusion transfer method such as a "Copyrapid" offset printer available from "Agfa-Gevaer", a printer disclosed for instance in Japanese Unexamined Patent Publication No. 7(1995)-56351, and the like. In such printers, a transfer image can be formed in one step on a plate material and since the transfer image is lipophilic, the plate can be used as a printing plate as it is. However since even such printers require a diffusion transfer development step using an alkali developing solution, there is a demand for further simpler printer which requires no developing step by a developing solution. Thus there have been made attempts to realize a simple printing plate which requires no developing step by an alkali developing solution. In the field of the simple printing plate, which is called a non-processed printing plate since it omits necessity of a developing step, there have been proposed various techniques primarily based on one of principles of (1) forming an image by recording an image on the surface of a plate material by image-wise exposure and thermally decomposing the exposed portion of the plate material, (2) forming an image by rendering lipophilic the exposed portion of the plate material in image-wise exposure by heat mode curing, (3) forming an image by rendering lipophilic the exposed portion of the plate material in image-wise exposure by light mode curing, (4) forming an image by modification of the surface of a plate material through decomposition by light, and (5) forming an image by heat mode melt transfer of an imaged portion. The aforesaid simple offset printers are disclosed, for instance, in United States Patent Nos. 3,506,779; 3,549,733; 3,574,657; 3,739,033; 3,832,948; 3,945,318; 3,962,513; 3,964,389; 4,034,183; 4,081,572; 4,693,958; 4,731,317; 5,238,778; 5,353,705; 5,385,092; and 5,395,729 and EP No. 1068. Notwithstanding their advantage that they use no developing solution in making a printing plate, the aforesaid printers have one or more of the following drawbacks and are practically unsatisfactory. An unsatisfactory difference between the lipophilic region and the hydrophilic region, which results in poor quality of an printed image, poor resolution, which results in difficulty in obtaining a sharp printed image, an insufficient mechanical strength of the surface of a printed image to such an extent that the surface of the printed image is apt to be scratched, which requires provision of protective film or the like and deteriorates simplicity of the printer, and an insufficient durability against printing for a long time. Thus a strong demand for a printing plate which can be easily made and has various properties required in printing is not satisfied yet. As a method of making a non-processed printing plate, there has been disclosed in Japanese Unexamined Patent Publication No. 9(1997)-169098 a method which utilizes a phenomenon that zirconia ceramic is rendered hydrophilic by exposure to light. However zirconia is insufficient in photosensitivity and cannot be sufficiently rendered hydrophilic from its hydrophobic (lipophilic) state. Accordingly the approach is disadvantageous in that the imaged region and the non-imaged region are not sufficiently distinguishable from each other. Further it will be advantageous from the viewpoint of reduction in both cost and waste if a used printing plate can be easily recycled and reused. Simplicity of recycle operation is very important in recycle and reuse of printing plates, and it has been a very difficult problem to simplify the recycle operation. Accordingly there has been little disclosure on a method of overcoming this problem except Japanese Unexamined Patent Publication No. 9(1997)-169098 where the recycle operation is mentioned only on a zirconia ceramic plate material. EP-A-903 223, which constitutes prior art according to Art. 54(3) EPC, discloses a lithographic printing method comprising the steps of exposing a printing plate precursor having on the surface thereof a thin layer comprising TiO 2 , ZnO, SnO 2 , Bi 2 O 3 or Fe 2 O 3 to active light to make the exposed area hydrophilic, and making the hydrophilic area hydrophobic by heat treatment. SUMMARY OF THE INVENTION In view of the foregoing observations and description, the primary object of the present invention is to provide a plate making device which can make without use of alkali developing solution a printing plate which can provide a practically sufficient image quality and can be recycled for reuse. Specifically the present invention provides a plate making device which can make without use of alkali developing solution a printing plate which is excellent in resolution and high in distinguishability between the imaged region and the non-imaged region and accordingly can provide an image of high quality. Another object of the present invention is to provide a printing system using the plate making device. Still another object of the present invention is to provide a printer using the plate making device. After various investigations, we have found existence of a material whose surface changes from a lipophilic state to a hydrophilic state upon exposure to light and returns to a lipophilic state when subsequently subjected to a heat treatment. This invention has been made on the basis of this discovery. That is, in accordance with a first aspect of the present invention, there is provided a plate making device comprising a plate material which is removably provided and has a surface layer formed of film including as its major component a material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment, an exposure means which uniformly exposes the substantially entire surface of the plate material to active light, and an image writing means which writes an image in a heat mode on the plate material which has been exposed to the active light and makes a printing plate. The "material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment" will be referred to as "photo-thermal hydrophilicity convertible material", hereinbelow. The "active light" is light which stimulates the photo-thermal hydrophilicity convertible material to change its surface from a lipophilic state to a hydrophilic state. "To uniformly expose the substantially entire surface of the plate material to the active light" means to expose the plate material to the active light at uniform light energy over the substantially entire surface thereof without practical local nonuniformity of light energy. "To write an image in a heat mode" means an ordinary heat mode imaging as used in this field. For example, it means to heat the plate material in an image-wise pattern by touching a fine heater element to the plate material along the pattern or by image-wise exposing the plate material so that the exposed portion is heated by heat energy derived from light energy absorbed by the exposed portion. In one preferred embodiment of the present invention, the plate material is in the form of a flat plate which is removably mounted on the surface of a drum and the exposure means and the image writing means are disposed around the drum. In another preferred embodiment of the present invention, the plate material is in the form of a printing drum and the exposure means and the image writing means are disposed around the plate cylinder. The image writing means may comprise a thermal recording head or a laser such as an infrared laser. Preferably the photo-thermal hydrophilicity convertible material is titanium oxide or zinc oxide. Further it is preferred that the plate making device be provided with an ink removing means for removing ink remaining on the plate material, more strictly on the printing plate made of the plate material, after printing. In accordance with a second aspect of the present invention, there is provided a printing system comprising a plate making device of the first aspect, at least one printer including a plate support means on which a printing plate removed from the plate making device is removably mounted and an ink supply means which supplies ink to the imaged region of the printing plate, and an ink removing means for removing ink remaining on the printing plate after printing. The ink removing means may be provided either on the printer or the plate making device. It is preferred that the printing system comprises at least four said printers. In accordance with a third aspect of the present invention, there is provided an offset printer comprising a plate making section consisting of a plate material which has a surface layer formed of film including as its major component a material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment, an exposure means which uniformly exposes the substantially entire surface of the plate material to active light, an image writing means which writes an image in a heat mode on the plate material which has been exposed to the active light and makes a printing plate, an ink supply means which supplies ink to the imaged region of the printing plate, and an ink removing means for removing ink remaining on the printing plate after printing, and a transfer section which transfers ink on the imaged region of the printing plate to a printing medium. It is preferred that the plate material be in the form of a printing drum and the exposure means and the image writing means be disposed around the plate cylinder. The image writing means may comprise a thermal recording head or a laser such as an infrared laser. It is preferred that the offset printer comprises at least four said print making sections. Preferably the photo-thermal hydrophilicity convertible material is titanium oxide or zinc oxide. In the plate making device, the printing system and the printer in accordance with the present invention, the surface of the plate material changes from a lipophilic state to a hydrophilic state when exposed to the active light over the substantially entire area thereof. When the image writing means writes an image in a heat mode on the plate material in this state, only the imaged region, that is, the region exposed to heat by the image writing means, becomes lipophilic, whereby a lipophilic imaged region and a hydrophilic non-imaged region are formed on the surface of the plate material. Thus a printing plate bearing thereon a lipophilic image is made. When the printing plate is set to a printer and ink is supplied on the printing plate, the ink is held only on the lipophilic imaged region and is not held on the hydrophilic non-imaged region, whereby an ink image is formed on the printing plate. The ink image is then transferred to a printing medium. When ink remaining on the printing plate is removed after printing and the printing plate is exposed to the active light over the entire surface thereof in the plate making device, the imaged region returns to the hydrophilic state and the printing plate is restored to the state before imaging. Accordingly in accordance with the present invention, a printing plate can be made only by uniform exposure of a plate material and imaging in a heat mode without necessity of development. Further the printing plate thus made is high in distinguishability between the imaged region and the non-imaged region, which ensures high sharpness of the printed image. Further since the printing plate is restored to the state where it bears thereon no image by uniform exposure of the printing plate to active light after cleaned of ink, the plate material can be repeatedly used, whereby printed matter can be provided at low cost. When the plate material is in the form of a flat plate which is removably mounted on the surface of a drum or in the form of a printing drum and the exposure means and the image writing means are disposed around the drum or the plate cylinder, the uniform exposure and the imaging in the heat mode can be effected only by rotating the drum of the plate cylinder, the plate making device can be compact in size and space can be saved. When the ink removing means is provided on the plate making device, the printing system using the plate making device can be simple in structure since the ink removing step is carried out in the plate making device. When the printing system of the present invention is provided with at least four printers, color printing can be carried out by supplying ink of different colors at the respective printers. Further, in the offset printer of the present invention, the printing plate need not be removed from the printer, and accordingly there is no fear that foreign material such as dust adheres to the printing plate when incorporating the printing plate in the printer as in the case of a conventional PS plate. Further when the offset printer of the present invention is provided with at least four plate making sections, color printing can be carried out by supplying ink of different colors at the respective plate making sections. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view for illustrating a printing system in accordance with a first embodiment of the present invention, Figure 2 is a schematic view showing an example of the thermal recording section, Figure 3 is a schematic view showing another example of the thermal recording section, Figure 4 is a schematic view for illustrating a printing system in accordance with a second embodiment of the present invention, Figure 5 is a schematic view for illustrating a plate making device in accordance with a third embodiment of the present invention, Figure 6 is a schematic view for illustrating a printing system in accordance with a fourth embodiment of the present invention, Figure 7 is a schematic view for illustrating a printing system in accordance with a fifth embodiment of the present invention, Figure 8 is an enlarged view of an important part of the printing system, Figure 9 is a schematic view for illustrating an offset printer in accordance with a sixth embodiment of the present invention, Figure 10 is a schematic view for illustrating an offset printer in accordance with a seventh embodiment of the present invention, Figure 11 is a schematic view for illustrating an offset printer in accordance with an eighth embodiment of the present invention, and Figure 12 is an enlarged view of an important part of the printer. DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is based on the discovery of existence of a material such as titanium oxide or zinc oxide whose surface changes from a lipophilic state to a hydrophilic state upon exposure to light and returns to a lipophilic state when subsequently subjected to a heat treatment, and is characterized in that such nature of the photo-thermal hydrophilicity convertible material is utilized in making a printing plate and recycling the same. Figure 1 shows a printing system in accordance with a first embodiment of the present invention. As shown in Figure 1, the printing system of this embodiment comprises a plate making device 1 and a printer 2. The plate making device 1 comprises an exposure drum 4 around which a plate material 3 in the form of a flat plate having a surface layer containing a photo-thermal hydrophilicity convertible material such as titanium oxide or zinc oxide as a major component is wrapped, an active light exposure section 5 which uniformly exposes the substantially entire surface of the plate material to active light, and a thermal recording section 6 for writing an image in heat mode on the plate material 3 which has been exposed to the active light. These elements are disposed inside a housing body 7. The housing body 7 is further provided with a plate material supply section 11 for supplying a plate material 3 to the housing body 7 and a plate discharge section 12 for discharging a printing plate 3' made by the plate making device 1 as will be described later. The printer 2 comprises a printing drum 15 around which the printing plate 3' is wrapped, an ink/water supply section 16 which supplies ink and fountain solution on the surface of the printing plate 3', an ink washing section 17 which removes ink on the printing plate 3' on the plate cylinder 15 after printing, a blanket 18 as an intermediate member for transferring ink on the printing plate 3' to a sheet of printing paper and an impression cylinder 19 which presses the sheet of printing paper against the blanket 18. These elements are disposed inside a printer housing 20. The printer housing 20 is further provided with a printing plate supply section 21 for supplying the printing plate 3' to the plate cylinder 15 as will be described later. There has been well known that titanium oxide and zinc oxide exhibit photosensitivity. Especially zinc oxide is used to obtain an electrostatic image by exposing to image-wise light when it is charged or applied with an electric voltage. This has been put into practice in an electro-fax in the field of electrophotography. However the property that the state of the surface of titanium oxide and zinc oxide change from lipophilic state to hydrophilic state upon exposure to active light has been newly discovered independently from generation of the photoelectric charge and was not found when use of titanium oxide and zinc oxide in electrophotography was investigated. Especially conception of using such a property for a plate making is completely novel. Titanium oxide and zinc oxide are preferable for forming the plate material 3. However titanium oxide is preferable to zinc oxide in view of sensitivity, i.e., the photosensitivity in change of the nature of the surface. Titanium oxide may be prepared by any known method. For example, it may be prepared by sulfuric acid calcination of ilmenite or titanium slug, or by chlorination under an elevated temperature and subsequent oxygen oxidization of ilmenite or titanium slug. Otherwise titanium oxide film may be formed by vacuum film formation such as vacuum deposition, sputtering or the like of titanium or titanium oxide as will be described later. A layer containing therein titanium oxide or zinc oxide may be formed on the surface of the plate material 3 by any known method. For example, the following methods can be employed. (1) Coating the surface of the plate material with dispersion of fine crystals of titanium oxide or zinc oxide, (2) Coating the surface of the plate material with dispersion of fine crystals of titanium oxide or zinc oxide, and subsequently firing the layer thus formed, thereby reducing or removing the binder, (3) depositing titanium oxide or zinc oxide on the surface of the plate material 3 and (4) Coating organic compound of titanium or zinc such as titanium butoxide and forming a layer of titanium oxide or zinc oxide through hydrolyzing or firing oxidization of the coating. In this invention, a titanium oxide layer by vacuum deposition is especially preferable. In the methods of (1) and (2), fine crystals of titanium oxide may be coated, for instance, by coating dispersion of mixture of titanium oxide and silicone oxide and forming a surface layer or by coating a mixture of titanium oxide and organopolysiloxane or its monomer. Further fine crystals of titanium oxide may be coated in the form of dispersion in polymer binder which can coexist with the oxide. As the binder, various polymers dispersive to fine particles of titanium oxide can be used. As such polymer binder, polyalkylene polymer such as polyethylene, hydrophobic binders such as polybutadiene, polyacrylic ester, polymethacrylic ester, polyvinyl acetate, polyform acetate, polyethylene terephtalate, polyethylene naphthalate, polyvinyl alcohol and polystyrene are preferred and a mixture of these resins may also be used. When carrying out vacuum deposition of titanium oxide in the method (4), a normal vacuum metallizer is evacuated to not higher than 1,33·10 -3 Pa (exp(-5) Torr) and titanium oxide is heated by an electron beam under the condition of oxygen gas pressure of 1,33·10 -4 to 13,33 Pa (exp(-1 to -6) Torr), whereby titanium oxide is evaporated and forms film on the surface of the plate material 3. When zinc oxide is used, zinc oxide film may be formed any known method. It is preferred to use a method where the surface of a zinc plate is oxidized by electrolysis to form zinc oxide film or a method where zinc oxide film is formed by vacuum deposition. Deposited film of zinc oxide may be formed by deposition of zinc or zinc oxide under existence of oxygen gas or by forming zinc film in an atmosphere without oxygen and subsequently oxidizing the zinc film by heating it to 700° C in the air. Either of titanium oxide film and zinc oxide film should be 0,1 nm to 1 µm (1 to 10000 Å) in thickness and preferably 1 nm to 1 µm (10 to 10000 Å). In order to prevent strain due to interference of light, it is preferred that the film be not larger than 0,3 µm (3000 Å) in thickness. In order to ensure satisfactory photo-activity, it is preferred that the film be not smaller than 5 nm (50 Å) in thickness. Though titanium oxide may be of any crystal form, anatase titanium oxide is preferred for its high sensitivity. As is well known, anatase can be obtained by firing titanium oxide under a selected condition. Amorphous titanium oxide and/or rutile titanium oxide may mingle with anatase titanium oxide. However preferably anatase titanium oxide exists at least in 40% and more preferably at least in 60% for the aforesaid reason. The layer containing therein titanium oxide or zinc oxide generally should contain 30 to 100% by volume of titanium oxide or zinc oxide, and preferably not smaller than 50%. More preferably the layer comprises a continuous layer of titanium oxide or zinc oxide, that is, contains 100% of titanium oxide or zinc oxide. Doping with a certain kind of metal is sometimes effective for enhancing the phenomenon that hydrophilicity of the surface changes upon exposure to light. For this purpose, doping with metal which is weak in ionization tendency such as Pt, Pd, Au, Ag, Cu, Ni, Fe or Co is preferable. Doping with a plurality of these metals may be employed. When the volume fraction of titanium oxide or zinc oxide is small, sensitivity of change in hydrophilicity of the surface deteriorates. Accordingly it is preferred that the layer contains titanium oxide or zinc oxide in at least 30%. The plate material 3 may be of various materials and may be in various forms. For example, the plate material 3 may comprise a base member of various materials and a layer of various photo-thermal hydrophilicity convertible materials such as titanium oxide, zinc oxide and the like formed on the surface of the base member in various ways such as those described above. The base member may be a metal plate, a flexible plastic sheet such as polyester or cellulose ester, or a paper sheet such as waterproof paper, polyethylene/paper laminate, or impregnated paper. As the metal plate, an aluminum plate, a stainless steel plate, a nickel plate and a copper plate are preferable. The metal plate may be flexible. More specifically, when a layer of titanium oxide or zinc oxide is formed on a base member, the base member may be of various materials so long as it is dimensionally stable. For example, a paper; a paper sheet laminated with plastic such as polyethylene, polypropylene, or polystyrene; a metal plate such as of aluminum, zinc, copper or stainless steel; plastic film such as cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate, or polyvinyl acetal; and a paper sheet or plastic film laminated with or deposited with the above listed metals may be employed. Polyester film, aluminum plate and a SUS plate which is resistant to corrosion on the printer are preferable. Among those, an aluminum plate is most preferable owing to its excellent dimensional stability and inexpensiveness. The aluminum plate may be of pure aluminum or of aluminum alloy containing therein a fine amount of impurity elements such as silicon, iron, manganese, copper, magnesium, chromium, zinc, bismuth, nickel, titanium or the like. The content of such impurity elements in the aluminum alloy is generally 10% by weight at most. Though pure aluminum is most preferred, perfectly pure aluminum is difficult to produce. The aluminum base member need not be of a particular composition and may be any known aluminum plate. The base member employed in the present invention is generally about 0.05mm to 0.6mm in thickness, preferably 0.1 to 0.4mm and more preferably 0.15 to 0.3mm. The surface of the aluminum plate is roughened. If necessary, the surface is degreased to remove rolling oil thereon by use of surfactant, organic solvent or an alkaline solution before roughening the surface. The surface of the aluminum plate may be roughened by various methods. For example, the surface may be mechanically roughened, may be roughened by electrochemical dissolution or may be roughened by selective chemical dissolution. The mechanical roughening may be effected any known method such as ball grinding, brushing, blasting or buffing. The electrochemical roughening may be effected, for instance, by applying an AC current or a DC current in hydrochloric acid electrolyte or nitric acid electrolyte. Further the surface may be roughened by a combination of mechanical roughening and electrochemical roughening as disclosed in Japanese Unexamined Patent Publication No. 54(1979)-63902. After the surface is roughened, the aluminum plate is subjected to alkaline etching treatment and neutralization treatment as required and then subjected to anodizing process, as desired, in order to enhance water retention characteristics and/or resistance to wear of the surface. In anodizing process of the aluminum plate, various electrolytes which forms porous oxide film may be used. As such electrolyte, sulfuric acid, hydrochloric acid, nitric acid, chromic acid or mixture of these acids is generally used. The concentration of the electrolyte may be suitably determined depending on the kind of the electrolyte used. The condition of anodizing depends upon the kind of the electrolyte used and cannot be determined sweepingly. However electrolyte concentration of 1 to 80% by weight, electrolyte temperature of 5 to 70° C, current density of 5 to 60A/dm 2 , electric voltage of 1 to 100V and electrolysis time of 10 seconds to 5 minutes are generally suitable. When the amount of anodized film is less than 1.0g/m 2 , durability against printing becomes insufficient and/or the non-imaged region on the printing plate 3' becomes apt to be scratched, which results in adhesion of ink to the scratched portions. The plate material 3 having a surface layer of titanium oxide or zinc oxide is originally lipophilic and is ink receptive. However when exposed to active light, the surface of the plate material 3 becomes hydrophilic and comes to repel ink. When an image is written in heat mode, for example, by image-wise touching a heater element to the surface of the plate material 3 or by image-wise exposing the same to light which can be converted to heat energy, the imaged region is rendered lipophilic and comes to receive ink. By writing an image on the surface of the plate material 3, the printing plate 3' is made. Then the printing plate 3' is brought into contact with offset printing ink, thereby forming a printing surface where the non-imaged region retains fountain solution and the imaged region retains ink. When a printing medium is brought into contact with the printing surface, the ink on the surface is transferred to the printing medium, whereby printing is effected. The phenomenon that the surface of the photo-thermal hydrophilicity convertible material changes from a lipophilic state to a hydrophilic state upon exposure to light and returns to a lipophilic state when subsequently subjected to a heat treatment, on the basis of which the present invention is made is very remarkable. As the difference between the lipophilicity of the imaged region and the hydrophilicity of the non-imaged region increases, the non-imaged region and imaged region becomes more distinguishable from each other and the printing surface becomes clearer and the durability against repeated printing is enhanced. The degree of difference between the lipophilicity and hydrophilicity can be represented in terms of the contact angle with a droplet of water. As the hydrophilicity increases, the droplet of water spreads wider and the contact angle with the droplet becomes smaller. To the contrast, when the surface is water repellent (i.e., lipophilic), the contact angle becomes larger. That is, the plate material having a layer of the photo-thermal hydrophilicity convertible material such as titanium oxide or zinc oxide originally has a large contact angle with water but the contact angle is sharply reduced when the surface layer is exposed to active light, and the surface of the plate material comes to repel ink which is lipophilic. The surface of the plate material recovers its lipophilicity when heated and accordingly by image-wise heating the surface, an ink receptive, water repellent imaged region and a water receptive, ink repellent non-imaged region are formed on the surface, whereby a printing plate is formed. The active light exposure section 5 of the plate making device 1 will be described hereinbelow. In the printing system of this embodiment, the surface of the plate material 3 is uniformly exposed to active light prior to forming a lipophilic image. The uniform exposure of the surface to the active light may be effected in a surface exposure system in which the entire area of the surface is exposed at one time, in a slit exposure system in which the active light is projected onto the surface through a moving slit, or in a beam scanning system in which the surface is two-dimensionally scanned by a beam of the active light. In the beam scanning system, so long as the scanning pitch is fine enough not to obstruct printing, exposure in the beam scanning system may be considered to be uniform exposure. When the light source used is a laser, the beam scanning system is suitable and when the light source used is an incoherent divergent type light source such as an incandescent lamp or a discharge tube, then the surface exposure system including the slit exposure system is suitable. The active light which excites the film containing therein titanium oxide or zinc oxide as a major component is light in the sensitive wavelength range for the oxide. In the case of anatase titanium oxide, the sensitive wavelength range is not longer than 387nm, in the case of rutile titanium oxide, the sensitive wavelength range is not longer than 413nm, and in the case of zinc oxide, the sensitive wavelength range is 387nm. Accordingly, a mercury vapor lamp, a tungsten halogen lamp, other metal halide lamps, a xenon lamp and the like may be used as the active light source. A helium cadmium laser lasing at 325nm and a water-cooled argon laser lasing at 351.1 to 363.8nm can be also employed as the active light source. In gallium nitride lasers whose emissions at an ultraviolet to near ultraviolet region have been confirmed, an InGaN quantum-well semiconductor laser lasing at 360 to 440nm and an optical waveguide MgO-LiNbO 3 laser having periodic domains reversals lasing at 360 to 430nm can be used. In the case of zinc oxide, spectral sensitivity may be increased by any known method and the light sources listed above may be used. Further other lamps having spectral distribution in the increased range such as a tungsten lamp may also be used. As the surface layer is kept exposed to active light, the photo-thermal hydrophilicity convertible material such as titanium oxide or zinc oxide changes its state from a lipophilic state to a hydrophilic state and when all the photo-thermal hydrophilicity convertible material changes to the hydrophilic state, the degree of hydrophilicity is not increased any more even if exposure to the active light is further continued. A preferred amount of active light to which the surface layer is to be exposed depends upon the property of the surface layer and a target level of distinguishability between the imaged region and the non-imaged region. In the case of a surface layer of titanium oxide or zinc oxide, the preferred amount of active light is generally 0.05 to 100 J/cm 2 , preferably 0.05 to 10 J/cm 2 and more preferably 0.05 to 5 J/cm 2 . The degree of change to the hydrophilic state of the photo-thermal hydrophilicity convertible material depends upon the total amount of active light to which the photo-thermal hydrophilicity convertible material is exposed. For example, exposure for 100 seconds at 10mW/cm 2 results in the same effect as exposure for 1 second at 1W/cm 2 . Said range of the amount of light gives rise to no problem either in the surface exposure system nor in the beam scanning system. The thermal recording section 6 of the plate making device 1 will be described, hereinbelow. An image region is formed on the plate material 3, the entire surface of which has been rendered hydrophilic, by image-wise heating the surface of the plate material 3 in the thermal recording section 6. The structure of the thermal recording section 6 is shown in Figure 2. As shown in Figure 2, the thermal recording section 6 comprises a thermal recording head 41 which is moved along the surface of the plate material 3 in close contact therewith and a head driver 42 which image-wise moves the thermal recording head 41 according to an image signal S from an edit/layout means 40 which generates an image signal S representing an image to be printed. The thermal recording head 41 has a plurality of fine heater elements arranged like an array or a matrix in the direction of the axis of rotation of the exposure drum 4 and writes an image in heat mode on the surface of the plate material 3 line by line or lines by lines while the plate material 3 is rotated together with the exposure drum 4. The region touched by the heater elements (imaged region) is rendered lipophilic with the region not touched by the heater elements (non-imaged region) kept hydrophilic. Figure 3 shows another example of the thermal recording section 6. As shown in Figure 3, the thermal recording section 6 comprises an infrared laser 43 which emits an infrared laser beam toward the surface of the plate material 3 and a laser driver 44 which drives the infrared laser 43 to modulate the infrared laser according to an image signal S from an edit/layout means 40 which generates an image signal S representing an image to be printed. The infrared laser 43 causes the modulated infrared laser beam to scan the surface of the plate material 3 in the direction of the axis of rotation of the exposure drum 4 and writes an image in heat mode on the surface of the plate material 3 while the plate material 3 is rotated together with the exposure drum 4. The region exposed to the infrared laser beam (imaged region) is rendered lipophilic with the region not exposed to the infrared laser beam (non-imaged region) kept hydrophilic. Though, in the example shown in Figure 3, the infrared laser beam is directly modulated by controlling the infrared laser 43, the infrared laser beam may be modulated by a combination of an infrared laser and an external modulator such as acoustooptic element. The thermal recording section 6 need not be limited to those described above. For example, the image may be written by use of a photothermic conversion head for converting light energy to heat energy, or by projecting heat radiations such as flash light to the surface of the plate material 3 through a mask which is non-transparent to heat radiations except the portions corresponding to the image to be recorded with or without use of a slit. The heat radiations may be momentary flash light of high intensity generated by momentary discharge of electricity stored in a mass capacitor. The amount of exposure is preferably 0.05 to 10 J/cm 2 and more preferably 0.05 to 5 J/cm 2 . The photosensitivity for the photo-thermal hydrophilicity convertible material to change from a lipophilic state to a hydrophilic state is different from that of zirconia ceramic disclosed in Japanese Unexamined Patent Publication No. 9(1997)-169098 in both the characteristic and the mechanism. On such photosensitivity of zirconia ceramic, it is disclosed that a laser beam of 7W/µm 2 is required. This value corresponds to 70 J/cm 2 when the duration of the laser beam is assumed to be 100 nanoseconds, which means that the photosensitivity of zirconia ceramic is lower than that of titanium oxide by one figure. Though not fully clarified, the mechanism by which titanium oxide changes the state of its surface is assumed to be a photo-dislocation reaction of lipophilic organic deposit and differs from that of zirconia ceramic. However since zirconia ceramic changes the state of its surface from a lipophilic state to a hydrophilic state upon exposure to light and from a hydrophilic state to lipophilic state when heated as titanium oxide or zinc oxide, also zirconia ceramic can be employed in this invention. After writing an image in heat mode on the surface of the plate material which has been exposed to active light, the plate material 3 can be used as a printing plate 3' without development. Though the non-imaged region of the resulting printing plate 3' is sufficiently rendered hydrophilic, the printing plate 3' may be subjected to, if desired, post treatment by use of a rinse solution containing surfactant, aqueous solution and the like and/or a grease insensitizing solution containing acacia gum and/or starch derivative. For example, a flusher solution is coated on the surface of the printing plate 3' by wiping the surface with sponge or absorbent wadding soaked with the flusher solution, by dipping the printing plate 3' in a vat filled with the flushing agent or by use of an automatic coater. It is preferred that the thickness of the coating of the flushing solution be uniformed by a squeegee roller, a squeegee blade or the like after coating. The amount of the coating is generally 0.03 to 0.8 g/m 2 (by dry weight). Then the treated printing plate 3' is discharged from the plate making device 1 and is wrapped around the plate cylinder 15 of the printer 2. Thereafter ink and fountain solution are supplied from the ink/water supply section 16 and fountain solution and ink are respectively held by the non-imaged region and the imaged region. The ink image on the printing plate 3' is transferred to the blanket 18 from the printing plate 3' and then to a sheet of printing paper from the blanket 18. As can be understood from the description above, the printing system of this embodiment, in particularly, the plate making device 1 in the printing system is advantageous over the conventional offset printer or plate making device in various points. First it is simple to handle. Further chemical processing using an alkaline developing solution is not necessary as well as wiping, brushing and the like which are conventionally required, which prevents environmental pollution by discharge of a developing solution. The ink washing section 17 of the printer 2 will be described, hereinbelow. After end of printing, the printing plate 3' is cleared of ink at the ink washing section 17. This is done by washing out ink adhering to the printing plate 3' by use of hydrophobic petroleum solvent. As such solvent, aromatic hydrocarbons such as kerosine are commercially available as a printing ink solvent. Further benzol, toluol, xylol, acetone, methyl ethyl ketone and mixtures of this solvent may be used. The printing plate 3' which has been cleared off ink can be reused as a plate material 3 by another uniform exposure to active light unless it is exposed to a high temperature. Though how many times the plate material 3 can be recycled has not been clear and is considered to be limited by unremovable stain, practically unamendable blemishes on the surface and/or mechanical deformation of the plate material, it can be recycled at least 15 times. The operation of the printing system of this embodiment will be described, hereinbelow. A plate material 3 is supplied to the housing body 7 from the plate material supply section 11 and then is wrapped around the exposure drum 4. Then active light is emitted from the active light exposure section 5 and the entire surface of the plate material 3 is uniformly exposed to the active light, whereby the entire surface of the plate material 3 is rendered hydrophilic. Thereafter an image is written in heat mode on the surface of the plate material 3 at the thermal recording section 6, whereby a printing plate 3' bearing thereon a lipophilic imaged region and a hydrophilic non-imaged region is made. The printing plate 3' is removed from the exposure drum 4 and is discharged by the plate discharge section 12. The printing plate 3' is conveyed to the printing plate supply section 21 manually or by a conveyor means (not shown). The printing plate 3' is further supplied to the plate cylinder 15 by the printing plate supply section 21 and is wrapped around the plate cylinder 15. Thereafter ink and fountain solution are supplied to the surface of the printing plate 3' from the ink/water supply section 16, whereby fountain solution and ink are respectively held by the non-imaged region and the imaged region. The ink image on the printing plate 3' is transferred to the blanket 18 from the printing plate 3' and then to a sheet of printing paper supplied between the blanket 18 and the pressure drum 19 in the direction of arrow A (Figure 1). After end of printing, ink remaining on the surface of the printing plate 3' is removed by the ink removing section 17 and the printing plate 3' is demounted from the plate cylinder 15. Then the printing plate 3' is discharged through the printing plate supply section 21. The discharged printing plate 3' is conveyed to the plate material supply section 11 manually or by a conveyor means (not shown) for reuse. As can be understood from the description above, in the printing system of this embodiment, the printing plate 3' can be made only by uniformly exposing the surface of the plate material 3 to active light and writing an image in a heat mode without necessity of development. Further the printing plate 3' thus made is high in distinguishability between the imaged region and the non-imaged region, which ensures high sharpness of the printed image. Further since the printing plate 3' can be restored to the state where it bears thereon no image by uniform exposure of the printing plate to active light after cleaned of ink, the plate material 3 can be repeatedly used, whereby printed matter can be provided at low cost. Further in this embodiment, since the plate material 3 is wrapped around the exposure drum 4 and the active light exposure section 5 and the thermal recording section 6 are disposed around the exposure drum 4, the uniform exposure and the imaging in the heat mode can be effected only by rotating the exposure drum 4 and accordingly the plate making device 1 can be compact in size, whereby space can be saved. A concrete example of the present invention will be described hereinbelow. A rolled aluminum plate, 0.30mm thick, of JISA1050 aluminum material containing 99.5% by weight of aluminum, 0.01% by weight of copper, 0.03% by weight of titanium, 0.3% by weight of iron and 0.1% by weight of silicon was prepared. The aluminum plate was subjected to sand dressing by use of 20% by weight aqueous suspension of 400 mesh "pamistone" (from Kyouritsu Yougyou) and a rotary nylon brush (6,10-nylon) and then washed well. The aluminum plate was further dipped in a 15% by weight aqueous solution of sodium hydroxide (containing 4.5% by weight of aluminum and etched so that aluminum was dissolved in an amount of 5g/m 2 . Then the aluminum plate was washed with running water. After neutralization by 1% by weight nitric acid, the surface of the aluminum plate was roughened by electric charge in a 0.7% by weight aqueous solution of nitric acid (containing 0.5% by weight of aluminum) by use of a rectangular wave alternating voltage (current ratio r=0.90, a current waveform disclosed in an embodiment in Japanese Patent Publication No. 58(1983)-5796). The voltage was 10.5V when the aluminum plate was the anode and 9.3V when the aluminum plate was the cathode, and the current when the aluminum plate was the anode was 160 C/dm 2 . After washing, the aluminum plate was further dipped in a 10% by weight aqueous solution of sodium hydroxide at 35° C and etched so that aluminum was dissolved in an amount of 1g/m 2 , and then further washed. Thereafter the aluminum plate was further dipped in a 30% by weight aqueous solution of sulfuric acid at 50° C to de-smut and washed with water. Then the aluminum plate was subjected to porous anodized film forming process in a 20% by weight aqueous solution of sulfuric acid (containing 0.8% by weight of aluminum) at 35° C by use of a direct current at a current density of 13A/dm 2 . The electrolysis time was controlled so that 2.7g/m 2 of anodized film was formed. The aluminum plate was washed with water and then dipped in a 3% by weight aqueous solution of sodium silicate at 70° C for 30 seconds. The aluminum plate was then washed with water and dried. The aluminum plate thus obtained was used as a base member. The aluminum base member was 0.30 in the reflection density as measured by a Macbeth reflection densitometer and 0.58µm in centerline mean roughness. The aluminum base member was placed in a vacuum metallizer and heated to 200° C. Then the vacuum metallizer was evacuated to 1,33·10 -6 Pa (1.0x10 -8 Torr) and titanium oxide was heated by an electron beam under the condition of oxygen gas pressure of 0,02 Pa (1.5x10 -4 Torr), whereby film of titanium oxide was formed on the aluminum base member. In this titanium oxide film, the ratio of amorphous component, anatase crystal component and rutile crystal component was 2.5/4.5/3 as analyzed by X-ray analysis. The titanium oxide was 75 nm (750 Å) in thickness. The aluminum base member having the titanium oxide film on the surface thereof thus obtained was used as a sample of the plate material 3. The plate material 3 was wrapped around the. exposure drum 4 and was exposed through a slit 10cm wide to light at an intensity of 35mW/cm 2 emitted from "USIO Printing Light Source Unit Unirec URM600 model GH-60201" (Usio Electric) while the plate material 3 was slowly rotated together with the exposure drum 4, whereby the entire surface of the plate material 3 was uniformly exposed to light for 15 seconds. Then the contact angle with water droplet (in air) of the surface was measured by use of a CONTACT-ANGLE METER CA-D (Kyouwa Kaimen Kagaku K.K.). The contact angle was in the range of 5 to 7° over the entire surface. An image was recorded on the surface layer of titanium oxide by touching a heater element array comprising a plurality of thermal heads of 150µm x 150µm arranged at intervals of 250µm, each thermal head comprising a Ta-SiO 2 heating resistor provided with wear-resistant protective layer of sialon. It was confirmed by separate temperature measurement that the temperature of each thermal head reached to 450° C by energizing for 20ms. The recording speed was 400m/s. The printing plate 3' thus prepared was set to a single-sided printer (Oliver 52 from Sakurai) and 1000 copies were offset by use of pure water as fountain solution and Newchampion F gross 85 India ink (from "Dainihon Ink Chemical"). Sharp printed matter was obtained from beginning to end and no damage was observed on the printing plate 3'. Then the surface of the printing plate 3' was washed with printing ink cleaner "Dye-Clean R" (from "Dainihon Ink Chemical") to remove ink remaining thereon. Thereafter the printing plate 3' was wrapped around the exposure drum 4 and was exposed through a slit 10cm wide to light at an intensity of 35mW/cm 2 emitted from "USIO Printing Light Source Unit Unirec URM600 model GH-60201" (Usio Electric) while the printing plate 3' was slowly rotated together with the exposure drum 4, whereby the entire surface of the printing plate 3' was uniformly exposed to light for 15 seconds. Then another image was written under the same conditions as described above on the plate material 3 recycled from the printing plate 3'. The printing plate 3' was set to a single-sided printer (Oliver 52 from Sakurai) and 1000 copies were offset by use of pure water as fountain solution and Newchampion F gross 85 India ink (from "Dainihon Ink Chemical"). Sharp printed matter was obtained from beginning to end and no damage was observed on the printing plate 3'. This process was repeated 5 times. No change in repeatability in the photosensitivity, the sensitivity to thermal recording and the contact angle was observed. This example proves that a lipophilic image can be written on the titanium oxide surface layer of the plate material 3 by touching a thermal head and the plate material 3 can be repeatedly used by removing ink after printing and uniformly exposing the surface layer of the plate material 3 to active light. A printing system in accordance with a second embodiment of the present invention will be described with reference to Figure 4, hereinbelow. In Figure 4, the elements analogous to those in the first embodiment are given the same reference numerals and will not be described here. The printing system of the second embodiment differs from that of the first embodiment in that the plate making device 1 and the printer 2 are housed in one unit 23 and a conveyor means 24 which conveys the printing plate 3' to the printer 2 from the plate making device 1 and to the plate making device 1 from the printer 2 is provided between the plate making device 1 and the printer 2. The operation of the printing system of the second embodiment will be described, hereinbelow. A plate material 3 is supplied to the housing body 7 from the plate material supply section 11 and then is wrapped around the exposure drum 4. Then active light is emitted from the active light exposure section 5 and the entire surface of the plate material 3 is uniformly exposed to the active light, whereby the entire surface of the plate material 3 is rendered hydrophilic. Thereafter an image is written in heat mode on the surface of the plate material 3 at the thermal recording section 6, whereby a printing plate 3' bearing thereon a lipophilic imaged region and a hydrophilic non-imaged region is made. The printing plate 3' is removed from the exposure drum 4 and is conveyed to the printer 2 by the conveyor means 24. The printing plate 3' conveyed to the printer 2 is further supplied to the plate cylinder 15 and is wrapped around the plate cylinder 15. Thereafter ink and fountain solution are supplied to the surface of the printing plate 3' from the ink/water supply section 16, whereby fountain solution and ink are respectively held by the non-imaged region and the imaged region. The ink image on the printing plate 3' is transferred to the blanket 18 from the printing plate 3' and then to a sheet of printing paper supplied between the blanket 18 and the pressure drum 19 in the direction of arrow B. After end of printing, ink remaining on the surface of the printing plate 3' is removed by the ink washing section 17 and the printing plate 3' is demounted from the plate cylinder 15. Then the printing plate 3' is conveyed to the plate making device 1 by the conveyor means 24 for reuse. Though, in the embodiments described above, the ink removing section 17 is provided on the printer 2, it may be provided on the plate making device 1 or may be provided separately from both the plate making device and the printer 2. Further though, in the first and second embodiments, the plate material 3 is removably mounted on the exposure drum 4 and is transferred between the plate making device 1 and the printer 2, the plate material 3 may be the plate cylinder itself and the plate cylinder having a surface layer of the photo-thermal hydrophilicity convertible material may be removably mounted in both the plate making device 1 and the printer 2 so that an image is written on the plate material in the form of the plate cylinder removably mounted in the plate making device 1 in place of the exposure drum 4 and then the plate material in the form of the plate cylinder is transferred to the printer 2 and mounted in place of the plate cylinder 15. Further though, in the first and second embodiments, the plate material 3 is wrapped around the exposure drum 4, the plate material 3 may be kept flat in a plate making device. A plate making device in accordance with a third embodiment of the present invention in which the plate material 3 is kept flat will be described with reference to Figure 5, hereinbelow. In Figure 5, the elements analogous to those in the first embodiments are given the same reference numerals and will not be described in detail here. In the plate making device 1 of this embodiment, the ink washing section 17, which is provided in the printer 2 in the first and second embodiments, is provided in the plate making device 1, and the ink washing section 17, the active light exposure section 5 and the thermal recording section 6 are arranged in series. In Figure 5, the ink washing section 17 comprises a pair of rollers 17A for wiping ink off and a cleaning solution supply section 17B which supplies a cleaning solution. The active light exposure section 6 is provided with a light source 5A for uniformly exposing the surface of the plate material 3. The thermal recording section 6 is provided with a thermal head 6A for writing in heat mode an image on the surface of the plate material 3. The operation of the third embodiment will be described hereinbelow. After printing, the printing plate 3' is conveyed into the plate making device 1 as shown by arrow C in Figure 5, and ink remaining on the surface of the printing plate 3' is removed by the ink washing section 17. Then active light is emitted from the active light exposure section 5 and the entire surface of the printing plate 3' is uniformly exposed to the active light, whereby the entire surface of the printing plate 3' including the imaged region, which has been lipophilic, is rendered hydrophilic and thus the printing plate 3' is recycled to a plate material 3. Thereafter an image is written in heat mode on the surface of the plate material 3 at the thermal recording section 6, whereby a printing plate 3' bearing thereon a lipophilic imaged region and a hydrophilic non-imaged region is made. The printing plate 3' is supplied to the printer 2. Thus even if the plate material 3 is used kept flat, the printing plate 3' can be made only by uniformly exposing the surface of the plate material 3 to active light and writing an image in a heat mode without necessity of development. Further the printing plate 3' thus made is high in distinguishability between the imaged region and the non-imaged region, which ensures high sharpness of the printed image. Further since the printing plate 3' can be restored to the state where it bears thereon no image by uniform exposure of the printing plate to active light after cleaned of ink, the plate material 3 can be repeatedly used, whereby printed matter can be provided at low cost. Though, in the third embodiment, the thermal recording head 6A is used for recording an image at the thermal recording section 6, an infrared laser such as shown in Figure 3 may be employed in place of the thermal head 6A. A printing system in accordance with a fourth embodiment of the present invention will be described with reference to Figure 6, hereinbelow. As shown in Figure 6, the printing system of this embodiment comprises four plate making units 1Y, 1M, 1C and 1B, each equivalent to the plate making device 1 shown in Figure 1, disposed in a housing body 32 in series and four printing units 2Y, 2M, 2C and 2B, each equivalent to the printer 2 shown in Figure 1, disposed in the housing body 32 respectively opposed to the plate making units 1Y, 1M, 1C and 1B. The combination of the plate making unit 1Y and the printing unit 2Y is for printing by yellow ink, the combination of the plate making unit 1M and the printing unit 2M is for printing by magenta ink, the combination of the plate making unit 1C and the printing unit 2C is for printing by cyan ink, and the combination of the plate making unit 1B and the printing unit 2B is for printing by black ink. Since each of the plate making units 1Y, 1M, 1C and 1B is the same as the plate making device 1 shown in Figure 1 and each of the printing units 2Y, 2M, 2C and 2B are the same as the printer 2 shown in Figure 1, they will not be described here. In the printing system of this embodiment, images to be printed in yellow, magenta, cyan and black are written on the plate materials in the respective plate making units 1Y, 1M, 1C and 1B and yellow ink, magenta ink, cyan ink and black ink are respectively supplied to the printing plates in the respective printing units 2Y, 2M, 2C and 2B. The operation of the printing system of the fourth embodiment will be described, hereinbelow. The entire surface of the plate material 3 is first uniformly exposed to the active light and an image to be printed in the corresponding color is written in heat mode on the surface of the plate material 3 in each of the plate making units 1Y, 1M, 1C and 1B. Then the printing plates 3' are supplied to the respective printing units 2Y, 2M, 2C and 2B. Thereafter ink of the respective colors and fountain solution are supplied to the surface of the printing plates 3' from the respective ink/water supply sections, whereby fountain solution and ink are respectively held by the non-imaged regions and the imaged regions of the respective printing plates 3'. The ink images on the printing plates 3' are transferred to a sheet of printing paper in sequence supplied in the direction of arrow D. That is, a yellow ink image is transferred to the sheet of printing paper in the printing unit 2Y, a magenta ink image is transferred to the sheet of printing paper in the printing unit 2M, a cyan ink image is transferred to the sheet of printing paper in the printing unit 2C, and a black ink image is transferred to the sheet of printing paper in the printing unit 2B, whereby a color image is printed on the sheet of printing paper. After end of printing, ink remaining on the surface of the printing plate 3' is removed by the ink washing section in each printing unit and the printing plates 3' are conveyed to the respective plate making units for reuse. Though, in the fourth embodiment, one plate making unit is provided for each printing unit, only a single plate making unit may be provided for all the printing units. In such a case, printing plates 3' for the printing units are made by the single plate making unit in sequence and supplied to the respective printing units from the single plate making unit. After printing, all the printing plates 3' are returned to the single plate making unit. Further though in the fourth embodiment, each of the plate making units 1Y, 1M, 1C and 1B is equivalent to that shown in Figure 1, the plate making units equivalent to that shown in Figure 5 may be employed. In this case, the printing units need not be provided with the ink washing section. A printing system in accordance with a fifth embodiment of the present invention will be described with reference to Figures 7 and 8, hereinbelow. Figure 7 is a schematic view showing the arrangement of the printing system of the fifth embodiment and Figure 8 is an enlarged view of an important part thereof. The printing system of this embodiment comprises a plate making device 33 equivalent to the plate making device 1 shown in Figure 1 provided in a housing body 33 and four printing stations 34Y, 34M, 34C and 34B, each equivalent to the printer 2 shown in Figure 1, disposed in the housing body 33 around an impression cylinder 19. The printing stations 34Y, 34M, 34C and 34B are for printing in yellow, magenta, cyan and black, respectively. Figure 8 shows the printing station 34Y. The other printing stations 34M, 34C and 34B are of the same structure as the printing station 34Y. As shown in Figure 8, the printing station 34Y comprises an ink/water supply section 16 which supplies ink and fountain solution on the surface of the printing plate 3' mounted on a printing drum 15, an ink washing section 17 which removes ink on the printing plate 3' on the plate cylinder 15 after printing, a blanket 18 which is in contact with the impression cylinder as an intermediate member for transferring ink on the printing plate 3' to a sheet of printing paper. The operation of the printing stations 34Y, 34M, 34C and 34B are the same as that of the printer 2 shown in Figure 1 and will not be described in detail here. In the fifth embodiment, images to be printed in the respective colors are written in heat mode on the surfaces of four plate materials 3 in sequence in the plate making device 1. Thereafter ink of the respective colors and fountain solution are supplied to the surface of the printing plates 3' from the respective ink/water supply sections in the respective printing stations 34Y, 34M, 34C and 34B. The operation of the printing system of the fifth embodiment will be described, hereinbelow. The surfaces of the plate materials 3 for yellow, magenta, cyan and black images are uniformly exposed to active light in the plate making device 1 and then the images to be printed in the respective colors are written in heat mode on the surfaces of four plate materials 3 in sequence by the thermal recording section. Then the printing plates 3' thus obtained are supplied to the respective printing stations 34Y, 34M, 34C and 34B in sequence. Thereafter ink of the respective colors and fountain solution are supplied to the surface of the printing plates 3' from the respective ink/water supply sections 16 of the respective printing stations 34Y, 34M, 34C and 34B, whereby fountain solution and ink are respectively held by the non-imaged regions and the imaged regions of the respective printing plates 3'. The ink images on the printing plates 3' are transferred to a sheet of printing paper in sequence supplied in the direction of arrow E in Figure 7. That is, a yellow ink image is transferred to the sheet of printing paper in the printing station 34Y, a magenta ink image is transferred to the sheet of printing paper in the printing station 34M, a cyan ink image is transferred to the sheet of printing paper in the printing station 34C, and a black ink image is transferred to the sheet of printing paper in the printing station 34B, whereby a color image is printed on the sheet of printing paper. After end of printing, ink remaining on the surface of the printing plate 3' is removed by the ink washing section in each printing station and the printing plates 3' are conveyed to the plate making device 1 for reuse. Though in the printing system of the fifth embodiment, the printing plates 3' for all the printing stations are made by the single plate making device 1, one plate making device may be provided for each printing section so that each plate making device makes the printing plate 3' for one printing station. Further though in the fifth embodiment, the plate making device 1 is equivalent to that shown in Figure 1, the plate making device equivalent to that shown in Figure 5 may be employed. In this case, the printing stations need not be provided with the ink washing section. Further though in the printing systems of the fourth and fifth embodiments, color printing is performed by use of four printing units 2Y, 2M, 2C and 2B or four printing stations 34Y, 34M, 34C and 34B, it is possible to perform color printing by use of five or more printing units or stations. Further though, in the fourth and fifth embodiments, the plate materials 3 which are removably mounted on the exposure drums 4 are transferred between the plate making devices 1 and the printing units or stations, the plate material 3 may be the plate cylinder themselves and the plate cylinders having a surface layer of the photo-thermal hydrophilicity convertible material may be removably mounted in both the plate making devices 1 and the printing units or the stations so that an image is written on the plate material in the form of the plate cylinder removably mounted in each of the plate making device 1 in place of the exposure drum 4 and then the plate material in the form of the plate cylinder is transferred to the printing units or stations and mounted in place of the plate cylinder 15. Further though, in the fourth and fifth embodiments described above, the ink removing section is provided on each printing unit of station, it may be provided on the plate making device or unit or may be provided separately from both the plate making device or unit and the printing unit or station. An offset printer in accordance with a sixth embodiment of the present invention will be described with reference to Figure 9, hereinbelow. In Figure 9, an offset printer of this embodiment comprises a printing drum 101 having a surface layer containing a photo-thermal hydrophilicity convertible material such as titanium oxide or zinc oxide as a major component, an active light exposure section 102 which uniformly exposes the substantially entire surface of the plate cylinder 101 to active light, and a thermal recording section 105 for writing an image in heat mode on the plate cylinder 101 which has been exposed to the active light, an ink/water supply section 103 which supplies ink and fountain solution on the surface of the plate cylinder 101 on which the image has been written, an ink washing section 104 which removes ink on the plate cylinder 101 after printing, a blanket 106 as an intermediate member for transferring ink on the plate cylinder 101 to a sheet of printing paper and an impression cylinder 107 which presses the sheet of printing paper against the blanket 106. These elements are disposed inside a printer housing 108. Titanium oxide and zinc oxide are preferable for forming the surface layer of the plate cylinder 101. However titanium oxide is preferable to zinc oxide in view of sensitivity, i.e., the photosensitivity in change of the nature of the surface. The surface layer containing therein titanium oxide or zinc oxide may be formed on the surface of the plate cylinder 101 by any known method. For example, the following methods can be employed. (1) Coating the surface of the plate cylinder 101 with dispersion of fine crystals of titanium oxide or zinc oxide, (2) Coating the surface of the plate cylinder 101 with dispersion of fine crystals of titanium oxide or zinc oxide, and subsequently firing the layer thus formed, thereby reducing or removing the binder, (3) depositing titanium oxide or zinc oxide on the surface of the plate cylinder 101 and (4) Coating organic compound of titanium or zinc such as titanium butoxide and forming a layer of titanium oxide or zinc oxide through hydrolyzing or firing oxidization of the coating. In this invention, a titanium oxide layer by vacuum deposition is especially preferable. In the methods of (1) and (2), fine crystals of titanium oxide may be coated, for instance, by coating dispersion of mixture of titanium oxide and silicone oxide and forming a surface layer or by coating a mixture of titanium oxide and organopolysiloxane or its monomer. Further fine crystals of titanium oxide may be coated in the form of dispersion in polymer binder which can coexist with the oxide. As the binder, various polymers dispersive to fine particles of titanium oxide can be used. When carrying out vacuum deposition of titanium oxide in the method (4), a normal vacuum metallizer is evacuated to not higher than 1,33·10 -3 Pa (exp(-5) Torr) and titanium oxide is heated by an electron beam under the condition of oxygen gas pressure of 1,33·10 -4 to 13,33 Pa (exp(-1 to -6) Torr), whereby titanium oxide is evaporated and forms film on the surface of the plate cylinder 101. When zinc oxide is used, zinc oxide film may be formed any known method. It is preferred to use a method where the surface of a zinc plate is oxidized by electrolysis to form zinc oxide film or a method where zinc oxide film is formed by vacuum deposition. Deposited film of zinc oxide may be formed by deposition of zinc or zinc oxide under existence of oxygen gas or by forming zinc film in an atmosphere without oxygen and subsequently oxidizing the zinc film by heating it to 700° C in the air. Either of titanium oxide film and zinc oxide film should be 0,1mm to 1 µm (1 to 10000 Å) in thickness and preferably 1 nm to 1 µm (10 to 10000 Å). In order to prevent strain due to interference of light, it is preferred that the film be not larger than 0,3 µm (3000 Å) in thickness. In order to ensure satisfactory photo-activity, it is preferred that the film be not smaller than 5 nm (50 Å) in thickness. The surface layer containing therein titanium oxide or zinc oxide generally should contain 30 to 100% by volume of titanium oxide or zinc oxide, and preferably not smaller than 50%. More preferably the surface layer comprises a continuous layer titanium oxide or zinc oxide, that is, contains 100% of titanium oxide or zinc oxide. Doping with a certain kind of metal is sometimes effective for enhancing the phenomenon that hydrophilicity of the surface changes upon exposure to light. When the volume fraction of titanium oxide or zinc oxide is small, sensitivity of change in hydrophilicity of the surface layer deteriorates. Accordingly it is preferred that the surface layer contains titanium oxide or zinc oxide in at least 30%. The plate cylinder 101 may be of various materials and may be in various forms. For example, the plate cylinder 101 may comprise a base drum of various materials and a surface layer of various photo-thermal hydrophilicity convertible materials such as titanium oxide, zinc oxide and the like formed on the surface of the base drum in various ways such as those described above. Otherwise a surface plate comprising a base member and a surface layer of a photo-thermal hydrophilicity convertible material formed on the base member may be fixedly wrapped around the base drum. The surface can be made in the same manner as the aforesaid plate material described in conjunction with the first embodiment. The plate cylinder 101 having a surface layer of a photo-thermal hydrophilicity convertible material such as titanium oxide or zinc oxide is originally lipophilic and is ink receptive. However when exposed to active light, the surface of the plate cylinder 101 becomes hydrophilic and comes to repel ink. When an image is written in heat mode, for example, by image-wise touching a heater element to the surface of the plate cylinder 101 or by image-wise exposing the same to light which can be converted to heat energy, the imaged region is rendered lipophilic and comes to receive ink. Then the plate cylinder 101 bearing thereon the image written in heat mode is brought into contact with offset printing ink, thereby forming a printing surface where the non-imaged region retains fountain solution and the imaged region retains ink. When a printing medium is brought into contact with the printing surface, the ink on the surface is transferred to the printing medium, whereby printing is effected. The active light exposure section 102 will be described hereinbelow. In the offset printer of this embodiment, the surface of the plate cylinder 101 is uniformly exposed to active light prior to forming a lipophilic image. The uniform exposure of the surface to the active light may be effected in a surface exposure system in which the entire area of the surface is exposed at one time, in a slit exposure system in which the active light is projected onto the surface through a moving slit, or in a beam scanning system in which the surface is two-dimensionally scanned by a beam of the active light. The thermal recording section 105 will be described, hereinbelow. An image region is formed on the plate cylinder 101, the entire surface of which has been rendered hydrophilic, by image-wise heating the surface of the plate cylinder 101 in the thermal recording section 105. The structure of the thermal recording section 105 may be the same as the thermal recording section described above in conjunction with the first embodiment. After writing an image in heat mode on the surface of the plate cylinder 101 which has been exposed to active light, the plate cylinder 101 can be used for printing as it is without development. Though the non-imaged region of the resulting printing drum 101 is sufficiently rendered hydrophilic, the plate cylinder 101 may be subjected to, if desired, post treatment by use of a rinse solution containing surfactant, aqueous solution and the like and/or a grease insensitizing solution containing acacia gum and/or starch derivative. For example, a flusher solution is coated on the surface of the plate cylinder 101 by wiping the surface with sponge or absorbent wadding soaked with the flusher solution, by dipping the plate cylinder 101 in a vat filled with the flushing agent or by use of an automatic coater. It is preferred that the thickness of the coating of the flushing solution be uniformed by a squeegee roller, a squeegee blade or the like after coating. The amount of the coating is generally 0.03 to 0.8 g/m 2 (by dry weight). Thereafter ink and fountain solution are supplied to the treated printing drum 101 from the ink/water supply section 103 and fountain solution and ink are respectively held by the non-imaged region and the imaged region. The ink image on the plate cylinder 101 is transferred to the blanket 106 from the plate cylinder 101 and then to a sheet of printing paper from the blanket 106. As can be understood from the description above, the offset printer of this embodiment is advantageous over the conventional offset printer in various points. First it is simple to handle. Further chemical processing using an alkaline developing solution is not necessary as well as wiping, brushing and the like which are conventionally required, which prevents environmental pollution by discharge of a developing solution. The ink washing section 104 will be described, hereinbelow. After end of printing, the plate cylinder 101 is cleared of ink at the ink washing section 104. This is done by washing out ink adhering to the plate cylinder 101 by use of hydrophobic petroleum solvent. As such solvent, aromatic hydrocarbons such as kerosine are commercially available as a printing ink solvent. Further benzol, toluol, xylol, acetone, methyl ethyl ketone and mixtures of this solvent may be used. The plate cylinder 101 which has been cleared off ink can be reused by another uniform exposure to active light unless it is exposed to a high temperature. Though how many times the plate cylinder 101 can be recycled has not been clear and is considered to be limited by unremovable stain, practically unamendable blemishes on the surface and/or mechanical deformation of the plate cylinder 101, it can be recycled at least 15 times. The operation of the offset printer of this embodiment will be described, hereinbelow. Active light is emitted from the active light exposure section 102 and the entire surface of the plate cylinder 101 is uniformly exposed to the active light, whereby the entire surface of the plate cylinder 101 is rendered hydrophilic. Thereafter an image is written in heat mode on the surface of the plate cylinder 101 at the thermal recording section 105, whereby the plate cylinder 101 comes to bear thereon a lipophilic imaged region and a hydrophilic non-imaged region is made. Then ink and fountain solution are supplied to the surface of the plate cylinder 101 from the ink/water supply section 103, whereby fountain solution and ink are respectively held by the non-imaged region and the imaged region. The ink image on the plate cylinder 101 is transferred to the blanket 106 from the plate cylinder 101 and then to a sheet of printing paper supplied between the blanket 106 and the impression cylinder 107 in the direction of arrow A Figure 9. After end of printing, ink remaining on the surface of the plate cylinder 101 is removed by the ink removing section 104. By subsequent uniform exposure to active light, the plate cylinder 101 is returned to the state prior to imaging in heat mode. As can be understood from the description above, in the offset printer of this embodiment, a printing surface can be made only by uniformly exposing the surface of the plate cylinder 101 to active light and writing an image in a heat mode thereon without necessity of development. Further the printing surface thus made is high in distinguishability between the imaged region and the non-imaged region, which ensures high sharpness of the printed image. Further since the plate cylinder 101 can be restored to the state where it bears thereon no image by uniform exposure of the plate cylinder 101 to active light after cleaned of ink, the plate cylinder 101 can be repeatedly used, whereby printed matter can be provided at low cost. Further since the plate cylinder 101 need not be removed from the printer, there is no fear that foreign material such as dust adheres to the plate cylinder 101 when incorporating the printing plate in the printer as in the case of a conventional PS plate. Further in this embodiment, since the active light exposure section 102, the ink/water supply section 103, the ink washing section 104 and the thermal recording section 105 are disposed around the plate cylinder 101, the uniform exposure, supply of ink and fountain solution, ink washing and the imaging in the heat mode can be effected only by rotating the plate cylinder 101 and accordingly the offset printer can be compact in size, whereby space can be saved. An offset printer in accordance with a seventh embodiment of the present invention will be described with reference to Figure 10, hereinbelow. As shown in Figure 10, the offset printer of this embodiment comprises four printing units 111Y, 111M, 111C and 111B, each equivalent to the offset printer shown in Figure 9, disposed in a housing body 112 in series. The printing unit 111Y is for printing by yellow ink, the printing unit 111M is for printing by magenta ink, the printing unit 111C is for printing by cyan ink, and the printing unit 111B is for printing by black ink. Since each of the printing units 111Y, 111M, 111C and 111B are the same as the offset printer shown in Figure 9, they will not be described here. In the offset printer of this embodiment, yellow ink, magenta ink, cyan ink and black ink are respectively supplied to the plate cylinder in the respective printing units 111Y, 111M, 111C and 111B. The operation of the printing system of the seventh embodiment will be described, hereinbelow. The entire surface of the plate cylinder 101 is first uniformly exposed to the active light and an image to be printed in the corresponding color is written in heat mode on the surface of the plate cylinder 101 in each of the printing units 111Y, 111M, 111C and 111B. Thereafter ink of the respective colors and fountain solution are supplied to the surfaces of the plate cylinders 101 from the respective ink/water supply sections in the respective printing units, whereby fountain solution and ink are respectively held by the non-imaged regions and the imaged regions of the respective plate cylinder 101. The ink images on the plate cylinders 101 are transferred to a sheet of printing paper in sequence supplied in the direction of arrow B. That is, a yellow ink image is transferred to the sheet of printing paper in the printing unit 111Y, a magenta ink image is transferred to the sheet of printing paper in the printing unit 111M, a cyan ink image is transferred to the sheet of printing paper in the printing unit 111C, and a black ink image is transferred to the sheet of printing paper in the printing unit 111B, whereby a color image is printed on the sheet of printing paper. After end of printing, ink remaining on the surface of the plate cylinder 101 is removed by the ink washing section in each printing unit. By subsequent uniform exposure to active light, the plate cylinders 101 are returned to the state prior to imaging in heat mode. An offset printer in accordance with an eighth embodiment of the present invention will be described with reference to Figures 11 and 12, hereinbelow. Figure 11 is a schematic view showing the arrangement of the offset of the eighth embodiment and Figure 12 is an enlarged view of an important part thereof. The offset printer of this embodiment comprises four printing stations 114Y, 114M, 114C and 114B, each equivalent to the offset printer shown in Figure 9, disposed in a housing body 115 around an impression cylinder 107. The printing stations 114Y, 114M, 114C and 114B are for printing in yellow, magenta, cyan and black, respectively. Figure 12 shows the printing station 114Y. The other printing stations 114M, 114C and 114B are of the same structure as the printing station 114Y. As shown in Figure 12, the printing station 114Y comprises a plate cylinder 101 having a surface layer containing a photo-thermal hydrophilicity convertible material such as titanium oxide or zinc oxide as a major component, an active light exposure section 102 which uniformly exposes the substantially entire surface of the plate cylinder 101 to active light, and a thermal recording section 105 for writing an image in heat mode on the plate cylinder 101 which has been exposed to the active light, an ink/water supply section 103 which supplies ink and fountain solution on the surface of the plate cylinder 101 on which the image has been written, an ink washing section 104 which removes ink on the plate cylinder 101 after printing, a blanket 106 as an intermediate member for transferring ink on the plate cylinder 101 to a sheet of printing paper and an impression cylinder 107 which presses the sheet of printing paper against the blanket 106. The operation of the printing stations 114Y, 114M, 114C and 114B are the same as that of the printer shown in Figure 9 and will not be described in detail here. In the offset printer of this embodiment, yellow ink, magenta ink, cyan ink and black ink are respectively supplied to the plate cylinders in the respective printing stations 114Y, 114M, 114C and 114B. The operation of the printing system of the eighth embodiment will be described, hereinbelow. The entire surface of the plate cylinder 101 is first uniformly exposed to the active light and an image to be printed in the corresponding color is written in heat mode on the surface of the plate cylinder 101 in each of the printing stations 114Y, 114M, 114C and 114B. Thereafter ink of the respective colors and fountain solution are supplied to the surfaces of the plate cylinders 101 from the respective ink/water supply sections in the respective printing stations, whereby fountain solution and ink are respectively held by the non-imaged regions and the imaged regions of the respective plate cylinder 101. The ink images on the plate cylinders 101 are transferred to a sheet of printing paper in sequence supplied in the direction of arrow C in Figure 12. That is, a yellow ink image is transferred to the sheet of printing paper in the printing station 114Y, a magenta ink image is transferred to the sheet of printing paper in the printing station 114M, a cyan ink image is transferred to the sheet of printing paper in the printing station 114C, and a black ink image is transferred to the sheet of printing paper in the printing station 114B, whereby a color image is printed on the sheet of printing paper. After end of printing, ink remaining on the surface of the plate cylinder 101 is removed by the ink washing section in each printing unit. By subsequent uniform exposure to active light, the plate cylinders 101 are returned to the state prior to imaging in heat mode. Though in the offset printers of the seventh and eighth embodiments, color printing is performed by use of four printing units 111Y, 111M, 111C and 111B or four printing stations 114Y, 114M, 114C and 114B, it is possible to perform color printing by use of five or more printing units or stations. Further though, in the sixth to eighth embodiments, the ink washing section 104, the ink/water supply section 103 and the thermal recording section 105 are arranged in this order in the clockwise direction from the active light exposure section, this sections may be arranged in any order. Further though, in the first to eighth embodiments, titanium oxide or zinc oxide is used as the photo-thermal hydrophilicity convertible material, any other photo-thermal hydrophilicity convertible material may be employed.

Claims: A plate making device comprising a plate material which is removably provided and has a surface layer formed of film including as its major component a material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment, an exposure means which uniformly exposes the substantially entire surface of the plate material to active light, and an image writing means which writes an image in a heat mode on the surface of the plate material, which has been exposed to the active light, and makes a printing plate. A plate making device as defined in Claim 1 in which the plate material is in the form of a flat plate which is removably mounted on the surface of a drum and the exposure means and the image writing means are disposed around the drum. A plate making device as defined in Claim 1 in which the plate material is in the form of a plate cylinder and the exposure means and the image writing means are disposed around the plate cylinder. A plate making device as defined in Claim 1 in which the image writing means comprises a thermal recording head. A plate making device as defined in Claim 1 in which the image writing means comprises a laser. A plate making device as defined in Claim 1 in which the material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment is titanium oxide or zinc oxide. A plate making device as defined in Claim 1 further comprising an ink removing means for removing ink remaining on the printing plate after printing. A printing system comprising a plate making device defined in Claim 1, at least one printing unit including a plate support means on which a printing plate removed from the plate making device is removably mounted and an ink supply means which supplies ink to the imaged region of the printing plate, and an ink removing means for removing ink remaining on the printing plate after printing. A printing system as defined in Claim 8 in which the ink removing means is provided on the printing unit. A printing system as defined in Claim 8 in which the ink removing means is provided on the plate making device. A printing system as defined in Claim 8 in which at least four said printing units are provided. An offset printer comprising a plate making section consisting of a plate material which has a surface layer formed of film including as its major component a material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment, an exposure means which uniformly exposes the substantially entire surface of the plate material to active light, an image writing means which writes an image in a heat mode on the surface of the plate material which has been exposed to the active light, thereby making a printing plate, an ink supply means which supplies ink to the imaged region of the printing plate, and an ink removing means for removing ink remaining on the printing plate after printing, and a transfer section which transfers ink on the imaged region of the printing plate to a printing medium. An offset printer as defined in Claim 12 in which the plate material is in the form of a plate cylinder and the exposure means and the image writing means are disposed around the plate cylinder. An offset printer as defined in Claim 12 in which the image writing means comprises a thermal recording head. An offset printer as defined in Claim 12 in which the image writing means comprises a laser. An offset printer as defined in Claim 12 in which at least four said plate making devices are provided. An offset printer as defined in Claim 12 in which the material whose surface changes from a lipophilic state to a hydrophilic state by a photocatalytic reaction and returns to a lipophilic state when subsequently subjected to a heat treatment is titanium oxide or zinc oxide.

Inventors: KAMIYAMA KOJI, KOGUCHI HIDEYUKI, NAKAYAMA TOKAO, KAMIYAMA, KOJI, KOGUCHI, HIDEYUKI, NAKAYAMA, TOKAO
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1998
Main classification: B41C1
IPC codes: B41N1, B41C1