METHOD OF FORMING AN INK IMAGE AND PRINTING THE FORMED IMAGE

Abstract: The present invention provides a method of forming an ink image with a thermo-sensitive ink by using a printing plate that is used in dry lithographic printing, and printing the ink image onto an object. An ink remover is placed via a thermo-sensitive ink layer that is in a heated state on a printing surface of a printing plate having a part receptive to ink and a part capable of repelling ink. The printing plate and the ink remover are separated after they are cooled so that the ink layer on the part capable of repelling ink is transferred to the ink remover. The ink image obtained on the printing plate or the ink remover is transferred to an object. According to this method, printing that does not require maintenance that has been essential to conventional dry lithographic printing becomes possible.

Description: The present invention is concerned with a method of forming an ink image using a printing plate of the kind used in dry lithographic printing, and to a method of printing the formed image. Many problems of lithographic printing caused by the use of damping water were solved by the development of dry lithographic printing (see "Balance between Damping Water and Ink" by MUTSUO KOBAYASHI in JAPAN PRINTER, Vol. 69, No. 5, pages 31 to 36, (1986)). The printing surface of a dry lithographic printing plate consists of image areas receptive to ink and non-image areas capable of repelling ink thereby allowing the ink to adhere selectively to the image areas only. Since the non-image areas require high ink releasability, they are made of a material having a low surface energy, such as silicone elastomers and fluorine compounds (see "Dry Lithography" by TSUGUO YAMAOKA in JAPAN PRINTER, Vol 60, No. 2, pages 9 to 17 (1977)). Dry lithographic printing is carried out by supplying ink to the printing surface of such a printing plate by means of an ink roller, and transferring the ink adhered to the image areas to an object. In lithographic printing it is necessary to use a special ink whose viscoelasticity has been adjusted. The cohesion of the special ink is adjusted to be greater than the adhesion between the ink and the non-image areas so that the ink hardly adheres to the non-image areas. However, in such conventional dry lithographic printing, maintenance is essential during and after the printing operation. The ink used in dry lithographic printing will solidify due to oxidative polymerization when left in the air and it is therefore necessary to clean many parts of the printer after the printing operation. On the other hand, since several hours are required to enable the ink transferred to the object to solidify completely, the printed objects cannot be placed one over another immediately after printing or ink offset will occur. The cohesion of the ink decreases with an increase in temperature so that when the temperature of the printing surface rises during printing, the ink tends to adhere to the non-image areas, resulting in scumming on the object being printed. It is therefore necessary to monitor the temperature of the printing surface of the printing plate continuously during printing. In conventional dry lithographic plates, the ink-repelling areas of the printing surface are generally provided with a silicone coating. Another printing method using a silicone which has both ink-repelling capability and ink-receptive capability is disclosed in U.S. Patent 3,554,836. In the method described in this patent, an image formed by thermosensitive ink on a silicone elastomer is transferred to an object to be printed. The silicone simultaneously has the functions of holding the ink image thereon and of transferring the ink image to the object. An object of the present invention is to provide a method of forming ink images and a method of printing that enable a large number of copies to be printed at low running cost and at high speed, that do not require frequent cleaning and maintenance of the printer, and that can be carried out using conventional dry lithographic printing plates. According to one aspect of the present invention, there is provided a method of forming an ink image, which comprises the steps of: forming a layer of a thermo-sensitive ink that is heated to a readily flowable state over the printing surface of a printing plate, the printing surface having areas receptive to ink and areas capable of repelling ink, the ink receptive areas or the ink repelling areas corresponding to the desired image; applying an ink remover sheet material over the ink layer while the latter is still hot; cooling the assembly of printing plate, ink layer, and ink remover until the ink layer is in a cohesive state; and separating the printing plate and the ink remover by increasing the curvature of the latter, the ink layer on the areas of the printing surface capable of repelling ink adhering to the ink remover and being removed from the printing surface. According to another aspect of the invention, there is provided a modification of the foregoing method, in which the thermo-sensitive ink layer is formed on the ink remover sheet material to form an ink sheet; the ink sheet is placed on the printing plate with the ink layer in contact with the printing surface thereof and the assembly is heated until the ink is in a readily flowable state; the assembly is cooled until the ink layer is in a cohesive state; and the printing plate is separated from the ink remover by increasing the curvature of the latter, the ink layer in the areas of the printing surface capable of repelling ink remaining adhered to the ink remover and being removed from the printing surface. According to a further aspect of the invention, there is provided a method of printing, which comprises transferring the ink image obtained on the printing plate or on the ink remover by either of the foregoing methods, to an object. For the better understanding of the invention, preferred embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figs. 1 (a) through (e) are schematic sectional views showing the steps of a printing method according to the present invention. Figs. 2 (a) and (b) are schematic sectional views showing the steps of another printing method according to the present invention. Fig. 3 is a schematic sectional view showing a transfer step using a transfer medium in still another printing method according to the present invention. Referring to Figs. 1(a) to (e), Fig. 1 (a) shows a printing plate 1 having an ink receiving body 2, parts of which are covered by an ink repelling layer 3. The step of forming a thermo-sensitive ink layer on the printing plate 1 is shown in Fig. 1 (b). The printing plate 1 is heated by means of a heated metal plate 8. A thermo-sensitive ink is supplied to the printing surface of the heated printing plate 1 and a thin ink layer 6 is then formed by a knife edge 7. If the viscosity of the ink is low at this point, non-image areas 4 corresponding to the ink repelling layer 3 will sometimes repel the ink, whereas the ink adheres to image areas 5 where the ink repelling layer is not present. The thickness of the ink layer formed greatly influences the resolution and the image density of the ink image that will be obtained. Qualitatively, the resolution of the ink image obtained will increase as the thickness of the ink layer is reduced. On the other hand, the image density of the ink image obtained will increase as the thickness of the ink layer is increased. Further, if the ink layer is excessively thick, the ink will be removed from the whole of the printing surface of the printing plate by the ink remover in the subsequent step, so that no ink image is obtained. These phenomena are related to the surface condition of the image areas and the non-image areas, the adhesive force between the ink remover surface and the ink, and the cohesion of the ink. In the next step, an ink remover sheet material 9 is placed on the melted ink layer 6 as shown in Fig. 1 (c). At this point, it is preferred to apply pressure to the upper side of the ink remover by a pressure roll or the like so as to make the thickness of the ink layer uniform. If the ink layer is made uniform in this way, it is not necessary to control the thickness of the ink layer by using a knife edge or the like in step (b) as mentioned above. The heated metal plate 8 is then removed so that the assembly of the printing plate 1, the ink layer 6, and the ink remover 9 cool down. After the ink layer 6 has cooled to a temperature at which the cohesion of the ink has increased sufficiently, the ink remover 9 is separated from the printing plate 1 as shown in Fig. 1 (d). The ink on the non-image areas 4 is transferred to the ink remover 9 to become an ink layer 10 on the ink remover 9. Soiling on the non-image areas can be very substantially reduced as compared with conventional dry lithographic printing. An important characteristic of the present invention lies in the fact that the formation of the ink layer is carried out when the ink is in a readily flowable state and the transfer of the ink is effected when the ink is in a substantially cohesive state. The temperature at which the release is effected depends largely on the materials and properties of the printing plate surface, the ink, and the support. For instance, when a wax ink for thermal ink-transfer printing with a softening point of 70°C is used, the ink layer is formed as a liquid layer at about 100°C and the ink remover is separated after the ink has cooled to about 30°C at which temperature the ink is in a solid stated with high cohesion. When the ink remover 9 is separated as shown in Fig. 1 (d), the printing plate 1 having an ink layer 11 adhered to the image areas 5 of the printing plate surface is obtained. A negative image 10 to the ink image 11 obtained on the printing plate 1 is obtained on the ink remover 9. The ink remover 9 is separated from the printing plate 1 by increasing the curvature of the ink remover 9 as shown in Fig. 1 (d). By doing this, the transfer, or the adhesion, of the ink to the image areas can be stabilized. Whether the ink will be transferred to the image areas or not depends, as mentioned before, on the nature of the surfaces of the image areas and the non-image areas, the adhesive force between the ink remover surface and the ink, and the cohesion of the ink. The necessary curvature of the ink remover 9 to effect removal of the ink is therefore related to the adhesion of the ink. If the ink remover 9 is curved with a large curvature as shown in Fig. 1 (d), the transfer of the ink from the printing plate 1 is effected easily. The printing plate 1 with the ink image 11 is then again placed on the heated metal plate 8 and an object 12 is placed on the printing plate 1 while pressure is applied thereto by a pressure roller 13 as shown in Fig. 1 (e). As the result, the ink 11 adhered to the image areas 5 is transferred to the object 12 thereby forming an ink image 14 on the object 12. Alternatively, the printing plate 1 in Fig. 1 (e) may be replaced by the ink remover 9 to obtain an ink image corresponding to the ink pattern 10 on the object 12. In the embodiment illustrated above, although the ink layer 6 is formed on the printing surface of the printing plate 1 by using the knife edge 7, the formation of the ink layer is not limited to this procedure. For instance, an ink layer may be formed on the printing surface of the printing plate by an ink roller to which the ink has been previously applied or an ink roller which has been previously impregnated with an ink. That is, any means of forming a thin layer of a thermo-sensitive ink can be used. In particular, by using a commercially available ink sheet for thermal ink-transfer printing, the supply of the ink to the printing plate surface and the placing of the ink remover can be carried out simultaneously. In the above embodiment, although the printing plate 1 is heated by the heated metal plate 8, any means can be used to heat the printing plate. For instance, the printing plate may be heated by thermal irradiation or by hot air. Alternatively, the ink may be transferred by applying a pressure without heating; this is readily carried out where an ink mainly made of wax is used. Figs. 2 (a) and (b) show schematic sectional views of the steps of a second printing method according to the present invention. The printing plate 1 is the same as that used in the first embodiment. As shown in Fig. 2 (a), an ink sheet 203 having a thermo-sensitive ink layer 205 on a support 204 is contacted with the printing plate 1 so that the ink layer 205 is against the printing surface of the printing plate 1. The assembly of ink sheet 203 and printing plate 1 is passed between a heated roller 201 and a pressure roller 202. After the assembly has cooled, the ink sheet 203 is separated from the printing plate 1 to leave an ink pattern 207 adhered only to the image areas of the printing plate surface. The printing plate 1 and an object 12 to be printed are brought together and passed between the heated roller 201 and the pressure roller 202 as shown in Fig. 2 (b), the ink 207 adhered to the image areas of the printing plate 1 is transferred to the object 12 thereby forming an ink image 208 on the object 12. Fig. 3 is a schematic sectional view a transfer step in a third printing method according to the present invention. In the same manner as in the first and second embodiments, a printing plate 1 having an ink pattern 303 adhered only to the image areas of the printing plate surface is obtained. The printing surface of the printing plate 1 is brought in contact with a heated transfer roller 301 and the ink pattern 303 on the image areas is transferred to the transfer roller 301 to form thereon an ink pattern 304. An object 12 to be printed is contacted with the tranfer roller 301 by means of a pressure roller 302 as shown in Fig. 3 and the ink pattern 304 on the transfer roller 301 is transferred to the object 12 to form thereon an ink image 305. Although in the above embodiment, a transfer roller is used as a transfer medium, the transfer roller may be replaced by a rubber sheet in the form of a belt or a resin film to perform the same transfer effect. The elements used in the printing method of the present invention are not limited to those illustrated in the above embodiments. For example, the printing plate is not limited to the particular form illustrated above. An ink receiving layer may be placed on an ink repelling body, or the ink-receptive part and the ink-repelling part may be coplanar. That is, any printing plate conventionally used in dry lithographic printing may be used if it can safely be heated. Suitable printing plates are, for example, those in which development of the image area is effected using a developing solution, such as a photosensitive resin plate, those in which the image is formed by electric discharge or a laser beam, and those in which the image areas are mechanically formed. Although in the above illustrated embodiments, the ink remover is a film in the form of a sheet, an endless belt or a roller may, for example, also be used. Further, in order to improve the adhesion of the ink, the film may be provided with a roughened surface. Further, in the step of separating the ink remover from the printing plate, a support member for the ink remover may be provided. For instance, the ink remover may be removed while it is moved along a support member such as a roller. By doing this, the curvature of the ink remover can be made constant thereby stabilizing the release of the ink. Further, in the step of transferring the ink image on the ink remover or the printing plate to an object having a low surface smoothness, a platelike heated body may be used for pressing instead of the roller. By pressing the printing plate or the ink remover and the object with the tip or edge of a platelike heated body to bring them into firm contact with each other, a high pressure and heat can be readily applied to only a small area. In this way high quality transfer with high resolution may be carried out on to an object having a low surface smoothness. For better understanding of the invention, the following examples are given by way of illustration only. All percentages and parts are by weight unless otherwise indicated. Example 1 An aluminum layer having a thickness of 600 Å was formed on a polyethylene terephthalate film (PET film) having a thickness of 25 µ by vacuum deposition. The aluminum layer was coated with a toluene solution of a silicone resin (KS772 manufactured by Shin-Etsu Chemical Co., Ltd.) containing 1% of a hardener (Catalyst PL-4 manufactured by Shin-Etsu Chemical Co., Ltd.) based on the weight of resin. The resin coat was hardened at 150°C for 5 minutes to form an ink repelling silicone resin layer having a thickness of about 0.2 µ. Thus, a printing plate was produced. The printing plate was mounted on a commercially available electric discharge recording apparatus (UA720K manufactured by NIPPON ALEPH CORPORATION), and characters were recorded on the printing plate at an applied voltage of 50 V. The thus obtained printing plate was placed on a commercially available hot plate that had been heated to about 100°C to heat the printing plate. A solid thermo-sensitive ink comprising 3.5 parts of carnauba wax, 3.5 parts of paraffin wax, and 1 parts of carbon was applied to the printing surface of the printing plate and the ink layer was doctored to a thickness of 10 µ by a knife edge. The viscosity of the ink was 120 cp at 100°C. Some parts of the non-image areas of the printing plate repelled the ink, whereas the ink adhered to the image areas. A PET film having a thickness of 12.5 µ was placed as an ink remover on the ink layer and was pressed on to the latter with a metal roller. The printing plate was then removed from the hot plate and after the printing plate, the ink, and the ink remover had cooled to room temperature (25°C), the ink remover was separated. For this purpose, the printing plate was kept flat and with the ink remover was held so as to have a curvature with a radius of about 1 mm. The ink that adhered to the non-image areas was transfered to the ink remover and the printing plate was left having ink only on the image areas and no ink on the non-image areas. On the other hand, when the printing plate was separated from the ink remover with the ink remover kept flat and with the printing plate held to have a curvature with a radius of about 1 mm, all the ink was transferred to the ink remover without leaving any ink on the printing surface of the printing plate. The above results show that the curvature of the ink remover affects the mechanical relationship of the cohesion of the ink, the surface of the ink remover, and the surfaces of the image areas and the non-image areas as described before. The thickness of the ink was preferably 2 to 10 µ. When the thickness of the ink was below 2 µ, the density of the obtained ink image was low. When the thickness of the ink was over 10 µ, all the ink adhered to the ink remover, that is was removed from the printing plate. Example 2 The printing plate similar to that of Example 1 was subjected to electric discharge recording and was heated to 100°C in the same way as in Example 1. A thermo-sensitive ink comprising 10 parts of polyamide resin (Fuji Kasei Kogyo Co., Ltd.) and 1 part of carbon was applied to the printing plate surface and the ink layer was doctored to a thickness of about 5 µ by a knife edge. A polyethylene terephthalate film having a thickness of 25 µ was placed as an ink remover on the ink layer and was pressed onto it by a metal roller. The printing plate was removed from the hot plate and after the printing plate, the ink, and the ink remover had cooled to 30°C, the ink remover was separated. For this purpose, the printing plate was kept flat and the ink remover was held with a curvature of a radius of about 1 mm. The resulting printing plate only had ink in the image areas. Example 3 10 Parts of urethane resin (Crisvon 7209 manufactured by Dainippon Ink & Chemicals, Inc.), 1.5 parts of fine silica powder, and 87.5 parts of ethyl acetate were mixed and dispersed thoroughly, and 1 part of cross-linking agent (Crisvon NX manufactured by Dainippon Ink & Chemicals, Inc.) was added thereto to form a coating material. A surface of a polyethylene terephthalate film having a thickness of 25 µ was coated with the coating material in an amount of 4 g/m² based on dry weight to form an ink removing sheet having an almost transparent roughened layer. In a similar way to Example 1, an ink layer was formed on the printing surface of the same printing plate as that used in Example 1. The ink removing sheet was placed as an ink remover on the ink layer and pressed on to the latter with a metal roller. The printing plate was then removed from the hot plate and after the printing plate, the ink, and the ink remover had cooled to room temperature (25°C), the ink remover was separated from the printing plate with the ink remover held to have a curvature with a radius of about 1 mm. The resulting printing plate had ink only on the image areas in the same manner as in Example 1. In contrast to the printing plate obtained in Example 1, the printing plate obtained in this Example had an ink layer of uniform thickness. Example 4 Aluminum was vacuum deposited on the roughened surface of the ink removing sheet used in Example 3 to form an aluminum layer having a thickness of 600 Å. The aluminum layer was coated with a toluene solution of silicone resin (KS772 manufactured by Shin-Etsu Chemical Co., Ltd.) containing 0.5 % of a hardener (Catalyst PL-3 manufactured by Shin-Etsu Chemical Co., Ltd.) with respect to the resin. The resin coat was hardened at 150°C for 5 minutes to form a silicone resin layer having a thickness of 0.2 µ. The thus obtained sheet was used as a printing plate. Characters were recorded on this printing plate by using an electro-discharge recording apparatus in the same way as in Example 1. The quality of the recorded characters were better than those of Example 1. A commercially available black thermal ink-transfer sheet (manufactured by FUJI KAGAKUSHI KOGYO CO., LTD.) having a base film with a thickness of 9 µ was placed as an ink remover on the printing surface of the thus obtained printing plate. The assembly was passed between a pressure roller and a heated roller having a diameter of 30 mm. The heated roller was rotated at a linear velocity of 3 cm/s, was subject to a linear pressure of 200 g/cm from the pressure roller, and was heated to about 100°C. After the printing plate and the ink remover had cooled to room temperature (25°C), a tension was applied to the printing plate to keep it flat, while the ink remover was peeled off around a metal roller having a diameter of 8 mm. The printing plate obtained had ink adhered only to the image areas and the ink remover had no ink on the parts corresponding to the image areas of the printing plate. A sheet of paper was placed on the printing plate and the assembly was passed between the heated roller and the pressure roller to transfer the ink on the image areas to the paper; the characters were clearly transferred to the paper. Further, another sheet of paper was placed on the ink remover and the assembly was passed between the heated roller and the pressure roller to transfer the ink on the ink remover to the paper, whereby an image negative to the image areas of the printing plate was obtained on the paper. Example 5 By using a thermal ink-transfer sheet in the same way as in Example 4, an ink image was formed on the printing plate in the same way as described in Example 4. A transfer roller of nitrile rubber heated to 100°C was brought into contact with the printing plate at a linear pressure of 50 g/cm thereby transferring the ink image to the transfer roller. The transfer roller having the ink image thereon was brought into contact with paper at a linear pressure of 200 g/cm thereby transferring the ink on the transfer roller to the paper. As the result, a printed image whose profile was clearer than that obtained in Example 4 was obtained. Example 6 A commercially available black thermal ink-transfer sheet (manufactured by FUJI KAGAKUSHI KOGYO Co., Ltd.) was placed as an ink remover on the printing surface of a commercially available dry lithographic printing plate (of a waterless lithographic positive type manufactured by Toray Industries, Inc.) that had been subjected to platemaking. After the assembly had been heated on a hot plate at a temperature of about 100°C, it was removed and allowed to cool down to room temperature (25°C). The ink remover was separated from the printing plate surface and the resulting printing plate had ink adhered only to the image areas. A sheet of paper was placed on the printing plate and the assembly was placed on a hot plate. A linear pressure of about 100 g/cm was applied to the paper by means of a nitrile rubber roller. When the paper was removed from the printing plate, an image of the image areas of the printing plate was obtained on the paper. Example 7 A silicone rubber (KE45TS manufactured by Shin-Etsu Chemical Co., Ltd.) was applied on a polyethylene terephthalate film having a thickness of 25 µ, and hardened at room temperature to form an ink repelling silicone rubber layer having a thickness of about 2 µ. This film was used as a printing plate. The silicone rubber layer on the printing plate was then partially removed using a sharply tipped edged tool to form image areas. Ink was applied to the resulting printing plate and removed from the non-image areas in the same way as in Example 6. The ink image was then printed onto paper.

Claims: A method of forming an ink image, which comprises the steps of: forming a layer of a thermo-sensitive ink that is heated to a readily flowable state over the printing surface of a printing plate, the printing surface having areas receptive to ink and areas capable of repelling ink, the ink receptive areas or the ink repelling areas corresponding to the desired image; applying an ink remover sheet material over the ink layer while the latter is still hot; cooling the assembly of printing plate, ink layer, and ink remover until the ink layer is in a cohesive state; and separating the printing plate and the ink remover by increasing the curvature of the latter, the ink layer on the areas of the printing surface capable of repelling ink adhering to the ink remover and being removed from the printing surface. A modification of the method according to claim 1, in which the thermo-sensitive ink layer is formed on the ink remover sheet material to form an ink sheet; the ink sheet is placed on the printing plate with the ink layer in contact with the printing surface thereof and the assembly is heated until the ink is in a readily flowable state; the assembly is cooled until the ink layer is in a cohesive state; and the printing plate is separated from the ink remover by increasing the curvature of the latter, the ink layer in the areas of the printing surface capable of repelling ink remaining adhered to the ink remover and being removed from the printing surface. A method according to claim 1 or 2, in which the ink remover sheet material is formed of a resin film. A method according to any of claims 1 to 3, in which the ink-repelling areas of the printing surface are coated with a silicone material. A method according to any of claims 1 to 3, in which the printing plate comprises an ink-receptive support, an electroconductive layer on the ink-receptive support, and an ink-repelling layer on the electro-conductive layer, and the desired ink-receptive image areas are formed by removing the ink-repelling layer therefrom by electric discharge. A method according to any of claims 1 to 3, in which the printing plate comprises an ink-receptive support and an ink-repelling layer on the ink-receptive support, and the desired ink-receptive image areas are formed by removing the ink-repelling layer therefrom by mechanical means or by irradiation with a high energy density laser beam. A method of printing, which comprises transferring the ink image obtained on the printing plate or on the ink remover by the method claimed in any of claims 1 to 6, to an object. A method according to claim 7, in which the ink image is heated and transferred to the object by the tip of a plate-like heating body. A method according to claim 7 or 8, in which the ink image is transferred to a transfer medium and is then transferred from the transfer medium to the object.

Inventors: FUKUI, YASUO, NISHIMURA, YUTAKA, TSUKAMOTO, MASAHIDE
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: B41M1
IPC codes: B41M1, B41F16, B41M5