Wet type image forming apparatus

Abstract: A wet type image forming apparatus comprises: a photo-sensitive drum (1) on which a latent image is formed and developed into a toner image with liquid-phase toner; an intermediate transferring medium (5) onto which the toner image is transferred; a pressurizing roll (8) which abuts a recording sheet (9) against the intermediate transferring medium (5) to transfer the toner image onto the recording sheet; means for reducing the potential of the toner image transferred onto the intermediate transferring medium; and means (20) for applying a mold releasing agent to the intermediate transferring medium (5) before the toner image is transferred from the image bearer (1) onto the intermediate transferring medium (5), whereby an image high in picture quality can be printed on a recording sheet (9) independently of the smoothness of the recording sheet, and transferring of the toner image can be achieved with low transferring voltage.

Description: This invention relates to a wet type image forming apparatus. In a wet type image forming apparatus an electrostatic latent image formed on an electrostatic latent image bearer is developed with liquid toner, and transferred onto a recording medium such as a recording sheet, to form an image. The liquid toner means a liquid toner including electrified toner particles containing pigment which are dispersed in an insulation solvent. In a conventional wet type image forming apparatus, its latent image bearer is made up of a metal drum, and a latent image bearing layer formed on the metal drum. A latent image formed on the latent image bearer is developed into a toner image, which is then transferred onto a recording sheet while being brought into contact with the latter. (cf. Electrophotographic Society Publication "Transition and Tendency of the Wet type Copying Machine", 26, 3 (1987), pp 270 - 276). The conventional wet type image forming apparatus is still disadvantageous in the following points: With the apparatus, even if the latent image is developed into a toner image with high fidelity, the toner image is not satisfactorily transferred onto a recording sheet if the latter is low in smoothness. In other words, if the recording sheet has an uneven surface, in a transfer process, some of the toner forming the toner image is not brought into contact with the recording sheet, as a result of which the image printed on the recording sheet is low in picture quality. Furthermore, the toner image developed on the electrostatic latent image bearer is wet containing a solvent component in addition to the toner. Therefore, when the toner image is brought into contact with the recording sheet, the solvent may spread thus collapsing the image. In addition, the apparatus needs a high voltage power source providing a voltage of several kilo-volts to transfer a toner image from a toner bearer to the recording sheet. Various teachings are known with regard to some of the aforementioned problems. For example, documents US-A-4 007 041 and WO-A- 90 08 984 (state of the art according to Article 54 (3) EPC) disclose an electrophotographic printing process, in which an image forming drum transfers a formed image to a secondary drum, which then impresses the image upon a recording medium; document EP-A-0 307 022 discloses an electrostatic transfer roller for transferring a toner image from a photo conductor surface to a recording sheet; and document US-A-4 341 455 discloses a belt mechanism for transferring magnetic and conductive toner from a dielectric surface to plain paper. Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional wet type image forming apparatus. More specifically, an object of the invention is to provide a wet type image forming apparatus which can form an image high in picture quality on a recording sheet independently of the smooth of the latter, and transferring a toner image can be achieved with a relatively low transferring voltage. These objects are solved by the wet type image forming apparatus according to claim 1. Further advantageous features of the apparatus are evident from the dependent claims. The foregoing object and other objects of the invention have been achieved by the provision a wet type image forming apparatus in which an electrostatic latent image formed on an image bearer is developed with liquid toner, and is then transferred onto a recording medium, which, according to the invention, comprises: first toner image transferring means for electrostatically transferring a toner image developed on the image bearer onto an intermediate transferring medium; and second toner image transferring means for abutting a recording sheet against the intermediate transferring medium to transfer the toner image from the intermediate transferring medium onto the recording sheet. In the apparatus, the intermediate transferring medium is made up of an elastic layer which comprises a dielectric layer with a smooth surface to which a toner image is borne. In the apparatus, the intermediate transferring medium has an elastic layer which is able to absorb the solvent of a liquid toner, and a pressurizing roll is provided to bring the toner image formed on the intermediate transferring medium into contact with the recording sheet. Furthermore, the image bearer on which a latent image is developed into a toner image with liquid-phase toner; the intermediate transferring medium onto which the toner image is transferred; and the pressurizing roll which abuts a recording sheet against the intermediate transferring medium to transfer the toner image onto the recording sheet, which further comprises: a corotron for reducing the potential of the toner image transferred onto the intermediate transferring medium; and means for applying a mold releasing agent to the intermediate transferring medium before the toner image is transferred from the image bearer onto the intermediate transferring medium. The nature, principle and utility of the invention will become more apparent from the following detailed description and the appended claims when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters. In the accompanying drawings: Fig. 1 is an explanatory diagram showing the arrangement of a printer; Fig. 2 is a sectional view showing an intermediate transferring drum of the invention; Fig. 3 is a perspective view showing an intermediate transferring drum; Fig. 4 is an explanatory diagram showing a transferring region in the printer; Fig. 5 is a perspective view, with parts cut away, showing a pressurizing roll employed in a printer; Fig. 6 is a sectional view showing the arrangement of a printer, according to the invention; Fig. 7 is an explanatory diagram for a description of the reduction of the potential of a toner image in the printer according to the invention; and Fig. 8 is a sectional view showing the arrangement of a printer, according to the invention. Preferred embodiments of this invention will be described with reference to the accompanying drawings. An electrophotographic printer is shown in Fig. 1. In the printer, a recording sheet 9 is supplied from a sheet supplying unit 101 and conveyed to a transferring position while being synchronized with the rotations of a photo-sensitive drum 1 which acts as an image bearer and an intermediate transferring drum 5 by timing rollers 102. A charging unit 2, an exposing unit 3 (or a laser scanner in the embodiment), a developing unit 4, the intermediate transferring drum 5, a cleaning unit 6, and a discharging unit 7 are arranged around the photo-sensitive drum 1. The recording sheet on which a toner image has been transferred is delivered by a conveying belt 112 to a stacker 111 through a fixing unit 11. In the printer thus constructed, a developing operation is carried out with a liquid toner which is prepared by dispersing charged resin in an insulating carrier liquid solvent; that is, a wet type developing operation is carried out. The liquid toner in the developing unit 4 is applied to the photo-sensitive drum with developing rollers 41, and excess liquid toner is removed from the drum with a squeezing roller 42. The photo-sensitive drum 1 is formed by providing a photo-sensitive layer on an aluminum pipe. In the printer, an image transferring operation is carried out as follows: A potential difference is provided between the intermediate transferring drum 5 and the aluminum pipe of the photo- sensitive drum 1, so that a toner image on the photo-sensitive drum is transferred onto the intermediate transferring drum through electrostatic field. Since the intermediate transferring drum has a smooth cylindrical wall, the toner image is transferred from the photo-sensitive drum onto the intermediate transferring drum with high fidelity. In this operation, because the pressure applied between the intermediate transferring drum and the photo-sensitive drum may be small, the photo-sensitive drum is preventable from being degraded. Thereafter, the recording sheet 9 is abutted against the intermediate transferring drum 5, and is then pushed against it by a pressurizing roller 8, so that the toner image is transferred onto the recording sheet. The intermediate transferring drum 5, as shown in Fig. 3, comprises: a hollow metal drum 50; and a conductive elastic layer 51 formed on the outer wall of the metal drum 50. In transferring the toner image from the photo-sensitive drum 1 to the intermediate transferring drum 5 (hereinafter referred to as "a primary toner image transferring operation", when applicable), a potential difference is provided between the metal base of the photo-sensitive drum and the hollow metal drum of the intermediate transferring drum to form a transferring field between the elastic layer 51 and the toner image on the photo-sensitive drum 1 thereby to transfer the toner image onto the intermediate transferring drum 5 in an electrostatic mode. In this case, the elastic layer 51 of the intermediate transferring drum 5 being smooth, the toner image on the photo-sensitive drum 1 is, in its entirety, brought into contact with the intermediate transferring drum. Therefore, the primary toner image transferring operation is free from the conventional difficulty that transferring the toner image is not completely achieved because a part of toner forming the toner image cannot come into contact with the transferring medium due to the unevenness of the transferring medium. In transferring the toner image from the intermediate transferring drum 5 onto the recording sheet 9 (hereinafter referred to as "a secondary toner image transferring operation", when applicable), the pressurizing roll 8 is pushed against the intermediate transferring drum 5 through the recording sheet 9, as a result of which the elastic layer 51 is deformed according to the irregularity of the surface of the recording sheet 9. Then, all the toner forming the toner image borne to the intermediate transferring drum can put in close contact with the uneven surface of the recording sheet. Therefore, the toner image is mechanically transferred onto the recording sheet 9 with high fidelity independently of the smoothness of the recording sheet 9. In order to facilitate the secondary toner image transferring operation, the elastic layer 51 of the intermediate transferring drum 5 is made of a material excellent in mold releasing characteristic. As the pressurizing roll 8 is abutted against the intermediate transferring drum 5, it is unnecessary to drive the pressurizing roll 8; that is, the pressurizing roll 8 is turned with no slip with the intermediate transferring drum 5. Accordingly, the recording sheet 9, while being held between the pressurizing roll 8 and the intermediate transferring drum 5, is conveyed with no slip with the intermediate transferring drum 5. This will prevent the occurrence of a difficulty that, when transferred, the image is lowered in quality because of the irregular conveyance of a recording sheet such as the jittering of a recording sheet. A potential difference may be provided between the pressurizing roll 8 and the intermediate transferring drum 5; that is, also in the secondary toner image transferring operation, electrostatic energy may be additionally utilized. Fig. 4 is an explanatory diagram showing a primary toner image transferring region. In this case, a transferring field E is as follows: E = {(V H - V P )/ε s }/ (d o /ε o + d t /ε t + d m /ε m + g/ε s ) where V H is the surface potential of the toner layer, V P is the transferring voltage applied to the metal base of the intermediate transferring drum, d o /ε o is the effective dielectric thickness of the transferring medium, or the effective dielectric thickness of the elastic layer of the intermediate transferring drum, d m and ε m are the thickness and dielectric constant of the photo-sensitive layer of the photo-sensitive drum, respective, d t and ε t are the thickness and dielectric constant of the toner layer on the photo-sensitive drum, respectively, g is the gap between the toner layer and the elastic layer, and ε s is the dielectric constant of the solvent of liquid toner. In order to perform the primary toner image transferring operation satisfactorily, it is necessary to give a high transferring field. In the invention, liquid-phase toner is employed. Therefore, d t /ε t and g/ε s are about 1µm, and are smaller than d m /ε m which is about 7µm, thus less affecting the transferring filed E of Equation (1). The degree of freedom of setting d m /ε m is not so large because of the charging, photo-sensitive and developing characteristics of the photo-sensitive drum. Therefore, the transferring field E of Equation (1) depends greatly on the effective dielectric thickness d o /ε o . That is, in order to obtain a high transferring field with a low transferring voltage, it is essential to reduce the effective dielectric thickness d o /ε o . In the secondary toner image transferring operation, the elastic function of the elastic layer 51 is utilized to bring the intermediate transferring drum 5 into close contact with the recording sheet 9. In this case, the elastic layer 51 should be relatively large in thickness because, if the thickness of the elastic layer 51 is small, its sufficient elastic effect cannot be expected. That is, the elastic layer should be 0.5 to 2.5 mm in thickness for a sufficient elastic effect. As was described above, it is desired in the primary toner image transferring operation that the effective dielectric thickness of the elastic layer 51 is small, whereas it is desired in the secondary toner image transferring operation that the thickness of the elastic layer 51 is large. More specifically, it is desired that the elastic layer 51 is 0.5 to 1.5 mm in thickness, and yet the effective dielectric thickness is reduced. In this case where the elastic layer 51 made of the electrically conductive rubber with good unsticking property is employed, the elastic layer 51 is zero (0) in effective dielectric thickness, and therefore a high transferring field can be obtained in the primary toner image transferring operation, and the elastic layer 51 bearing the toner image can be brought into close contact with the recoding sheet 9 in the secondary toner image transferring operation, Fig. 2 shows a sectional view of another intermediate transferring drum. The intermediate transferring drum comprises a hollow metal drum 50 and an elastic layer 51 provided on the outer periphery of the metal drum 50. The elastic layer 51 comprises a rubber layer 54 and an electrically conductive layer 53 supported by the rubber layer 54. The electrically conductive layer 53 may be formed according to a method in which a resin layer containing electrically conductive compound is formed on a rubber layer, or a thin metal layer is formed on a rubber layer by plating or vacuum deposition. With the above construction, a good toner image can be formed on a recording sheet, similarly to the intermediate transferring drum. Fig. 5 is a perspective view, with parts cut away, showing s pressure roll 8 in a printer. As shown in Fig. 5, the pressurizing roll 8 is in the form of a cylinder, and it accommodates a heat source 80 such as a ceramic heater or halogen lamp, to operate also as heat roll. In transferring a toner image, electrostatic energy, pressure and heat are applied at the same time, so that the toner image is fixed to the recording sheet. When the toner image is heated, the toner resin component is molten, as a result of which the mold release characteristic is improved; that is, the toner image is removed from the elastic layer 51 more readily; while, at the contact region of the recording sheet and the toner image, the gaps between fibers of the recording sheet are filled with the molten toner by pressure. That is, the image transferring operation and the image fixing operation are achieved simultaneously, and therefore the fixing unit may be eliminated. As the elastic layer 51 is heated, the solvent absorbed by the elastic layer in the primary toner image transferring operation is evaporated. In the case where the pressurizing roll 8 is used as a heating roll as was described above, it is preferable that the elastic layer 51 of the intermediate transferring drum 5 is a heat-resisting layer. As was described before, in order to perform the primary and secondary toner image transferring operations satisfactorily, silicone rubber is used for fabrication of the elastic layer or silicone coating is applied. This is suitable for ensuring the heat resistance of the elastic layer 51. In the above, the intermediate transferring drum is in the form of a cylinder. However, it may be in the form of an endless belt. In this case also, in the primary toner image transferring operation, transferring the toner image is achieved mainly by the electrostatic energy, and in the secondary toner image transferring operation, a mechanical force is mainly employed. Furthermore, when the belt-shaped intermediate transferring medium and a pressurizing roll which functions also as a heating roller are used in combination, then transferring and fixing the toner image can be achieved simultaneously. Fig. 6 shows the electrophotographic printer of the invention, in which there is provided a toner electric charge removing device 14. In the invention, the toner image formed on the photosensitive drum is first transferred onto an intermediate transferring drum, and the toner image thus formed on the intermediate transferring drum is then secondly transferred onto a recording sheet while the intermediate transferring drum is abutted against the recording sheet. Fig. 7 is a diagram showing reduction of the potential of the toner image on the intermediate transferring drum. The AC corotron 14 is so designed that a tungsten wire 80 0.05 mm in diameter is shielded with a housing 81 in such a manner that the housing 81 is spaced 5 mm from the tungsten wire 86. A voltage of an AC voltage peak to peak 8-9 KV with a frequency of 500 Hz - 1 KHz superimposed on a DC voltage 0 - +300 V is applied to the tungsten wire. The AC corotron is so positioned that it is spaced 1 to 3 mm from the intermediate transferring drum 5. In Fig. 7, the toner transferred onto the intermediate transferring drum 5 sticks to the elastic layer 51 to form a capacitor, thus providing a potential difference with respect to the intermediate transferring drum. As a result, the toner is stuck to the intermediate transferring drum in an electrostatic mode. In this case, application of positive and negative ions 84 to the toner image, can reduce the electrostatic sticking force acting between the toner 83 and the intermediate transferring drum 5. In transferring the toner image from the intermediate transferring drum 5 onto the recording sheet 9, the pressurizing roll 8 is pushed against the intermediate transferring drum 5 through the recording sheet 9, so that the elastic layer 51 is deformed in correspondence to the fine unevenness of the recording sheet surface. As a result, the toner image is transferred onto the recording sheet 9 with high fidelity, independently of the smoothness of the recording sheet 9. In this operation, the toner image on the intermediate transferring drum 5 is not so affected by the electrostatic sticking force, and therefore the mechanical elastic force and mold release characteristic of the elastic layer 51 allows the toner image to be transferred onto the recording sheet 9 with high fidelity. The toner electric charge removing device is effective in any case where the surface of the elastic layer of the intermediate transferring drum is electrically conductive, or insulative. Particularly, in the case where the elastic layer is insulative, not only the electric charge of the toner, but also the electric charge on the elastic layer can be removed by the toner electric charge removing device. Fig. 8 is a sectional view showing the arrangement of a printer, another embodiment of the invention. This embodiment is different in arrangement from other embodiment only in that mold releasing agent applying unit 20 is so positioned to apply a mold releasing agent to the intermediate transferring drum 5 before the toner image is transferred from the photo- sensitive drum 1 onto the intermediate transferring drum 5 from which the toner has been removed by the cleaning unit 6. In the embodiment, the mold releasing agent is silicone oil. The mold releasing agent applying unit 20 is a silicone oil impregnated pad. The pad is pushed against the intermediate transferring drum 5 to apply silicone oil to a thickness of the order of sub- microns. The toner image is transferred from the photo-sensitive drum 1 onto the intermediate transferring drum 5 which has been coated with silicone oil. Thereafter, the potential of the toner image on the intermediate transferring drum 5 is reduced by the DC corotron 14 and then the toner image is transferred onto the recording sheet 9. In this case, owing to the mold releasing agent, silicone oil, applied to the intermediate transferring drum, the secondary toner image transferring operation is achieved with ease. In addition, the provision of the mold releasing agent applying unit 20 makes it possible to use an elastic material such as urethane rubber low in mold releasing characteristic to form the intermediate transferring drum. In the above-described embodiment, the intermediate transferring medium is in the form of a drum; however, it goes without saying that it may be in the form of an endless belt. As was described above, in the wet type image forming apparatus of the invention, the toner image transferred onto the intermediate transferring roller is further transferred onto the recording sheet which is pushed against the intermediate transferring roller. Therefore, the toner image is transferred satisfactorily onto the recording sheet independently of the surface roughness of the latter. Furthermore, in the invention, the intermediate transferring medium is made up of the elastic layer, which is formed of the electrically conductive rubber having a smooth surface, or which comprises the electrically conductive rubber layer and the relatively thin dielectric layer having a smooth surface. Therefore, with a relatively low transferring voltage, the toner image formed on the electrostatic latent image bearer can be electrostatically transferred onto the intermediate transferring medium with high fidelity, and then the toner image on the intermediate transferring medium can be brought into close contact with the recording sheet which is uneven. That is, the toner image can be satisfactorily transferring onto the recording sheet independently of the surface roughness of the latter. In the secondary toner image transferring operation, the recording sheet is conveyed while being kept in close contact with the intermediate transferring medium; that is, the recording sheet is conveyed with no slip with respect to the intermediate transferring drum. Therefore, the image forming apparatus of the invention is free from the difficulty that the printed image is low in picture quality because of the irregular conveyance of the recording sheet. Furthermore, the toner image transferring operation is stably carried out independently of atmospheric conditions such as temperature and humidity. By using the pressurizing roll incorporating a heat source, the transferring operation and the fixing operation can be achieved simultaneously, so that the fixing unit can be eliminated. In the invention, the intermediate transferring medium is made up of the elastic layer which has a smooth surface and is able to absorb the solvent of the liquid-phase toner. Therefore, after the toner image has been electrostatically transferred onto the intermediate transferring medium with high fidelity, the toner image on the intermediate transferring medium can be brought into close contact with the recording sheet which is uneven. Thus, the toner image can be completely transferred onto the recording sheet; that is, the resultant image formed on the recording sheet is excellent in picture quality. Furthermore, as was described above, in the wet type image forming apparatus of the invention, the means for reducing the potential of the toner image transferred onto the intermediate transferring medium is provided; that is, the electrostatic sticking force of the toner image to the intermediate transferring medium can be reduced, whereby the transferring of the toner image onto the recording sheet is readily achieved by the mechanical elastic force and mold release characteristic of the intermediate transferring medium. In addition, employment of the intermediate transferring medium which is made up of the elastic layer whose surface is smooth, the toner image is completely transferred onto the intermediate transferring medium in the primary toner image transferring operation, and in the secondary toner image transferring operation the image is printed excellent in picture quality on the recording sheet. Where the intermediate transferring medium is provided with the mold releasing agent applying means, the toner image transferred onto the intermediate transferring medium is more readily transferred onto the recording sheet, and in addition the intermediate transferring medium can be fabricated by using an elastic material lower in mold releasing characteristic. While there has been described in connection with the preferred embodiments of the invention, it will be obvious to those skilled in the art that various modifications may be made therein without departing from the invention as claimed.

Claims: A wet type image forming apparatus in which an image is formed onto a recording medium (9), said apparatus comprising: a) an image bearer (1) on which an electrostatic latent image is developed into a toner image with liquid toner; b) an intermediate transferring medium (5) having at least one elastic outer layer (51) onto which said toner image is transferred from said image bearer (1); c) first toner image transferring means for electrostatically transferring said toner image developed on said image bearer (1) onto said intermediate transferring medium (5); d) second toner image transferring means (8) which abuts a recording sheet (9) against said intermediate transferring medium (5) for transferring said toner image from said intermediate transferring medium (5) onto said recording sheet (9), comprising a pressurizing roll (8) for bringing said recording sheet (9) into close contact with said intermediate transferring medium (5) to transfer said toner image from said intermediate transferring medium (5) onto said recording sheet; wherein a corotron (14) is provided for reducing the potential of said toner image transferred onto said intermediate transferring medium (5). A wet type image forming apparatus in accordance with claim 1 wherein said elastic layer (51) comprises an electrically conductive layer (53) with a smooth surface. The apparatus as claimed in claim 1 or 2 in which said elastic layer (51) comprises a thin dielectric layer (52) whose surface is smooth and an electric conductive layer (53) supporting said thin dielectric layer (52). The apparatus as claimed in any of claims 1 to 3 wherein said intermediate transferring medium (5) is made up of an elastic layer (51) which has a smooth surface and absorbs the solvent of said liquid toner. A wet type image forming apparatus as claimed in any of claims 1 to 4 wherein said second toner transferring means (8) for transferring the toner image to a record medium (9) incorporates a heat source (80). A wet type image forming apparatus as in any of claims 1 to 5 wherein mold releasing agent applying means (20) are provided for applying a mold releasing agent to said intermediate transferring medium (5) before said toner image is transferred from said image bearer (1) onto said intermediate transferring medium (5).

Inventors: FUJINO MAKOTO C O SEIKO EPSON, MIYAZAWA YOSHINORI C O SEIKO E, YAMAZAKI HIDEO C O SEIKO EPSON, FUJINO, MAKOTO, C/O SEIKO EPSON CORPORATION, MIYAZAWA, YOSHINORI, C/O SEIKO EPSON CORPORATION, YAMAZAKI, HIDEO, C/O SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1990
Main classification: G03G15
IPC codes: G03G15