Printing apparatus

Abstract: A printing apparatus is disclosed. A toner image is formed on an image carrier (2). A solvent supply device (5) supplies a solvent to the toner image (4) in a direction toward such a side that the toner image (4) makes contact with an image bearing medium (9), the solvent lowering the viscosity of the powder image. A transfer device (6) presses the image bearing medium (9) against the toner image (4) of which viscosity is lowered by the solvent, thereby transferring the toner image (4) to the image bearing medium (9).

Description: BACKGROUND OF THE INVENTION This invention relates to a printing apparatus, such as a copying machine, a printer and a facsimile, in which the viscosity of a toner image is reduced to a low value by use of a solvent on said toner before transfer of the toner image to obtain a hard copy. Typical examples of conventional recording systems utilizing toners include electrophotography and electrostatic recording. In such systems, an electrostatic latent image is formed on a charge carrier such as a photoconductive medium and an electrostatic recording medium, and this latent image is developed by charged toner having a polarity opposite to that of the latent image, so as to obtain a toner image. Then, the toner image is fixed into a final recorded form. In electrostatic recording, the electrostatic latent image is formed on electrostatic recording paper by a recording electrode or the like, and this latent image is developed by the toner. This electrostatic recording paper serves as the charge carrier and an image bearing medium. Therefore, the electrostatic recording system obviates the step of transferring the toner image from the charge carrier to the image bearing medium. Conversely, electrophotography inevitably requires the step of transferring the toner image from the photosensitive medium (charge carrier) to the image bearing medium, and the step of fixing the thus transferred image. The present invention is devised so as to improve the step of transferring a toner image, for example, in electrophotography, or the step of fixing a transferred image. Examples of known systems for transferring a toner image from a charge carrier to an image bearing medium include an electrostatic transfer method, an adhesion transfer method and pressure transfer method. First, the electrostatic transfer method will be described. Examples of such electrostatic transfer methods include a corona transfer method and a bias transfer method. In the corona transfer method, an image bearing paper sheet is set on the charge carrier, and a charge of a polarity opposite to that of the toner charge is applied by a corona charger to the paper on the reverse side of the charge carrier, thereby transferring the toner image to the paper sheet under Coulombic force. Conversely, in the bias transfer method, an image bearing paper sheet is set on a toner image on a photoconductive drum, and a conductive electrode drum to which direct current is applied is pressed against the reverse side of the paper sheet so that the toner image is transferred onto the paper sheet by Coulombic force. However, when using the corona transfer method so as to transfer the toner image, a problem has been encountered such that the toner transfer efficiency depends greatly on the electrical resistance of the image bearing sheet. Particularly in a high humidity condition, the image bearing sheet absorbs moisture so that its electrical resistance decreases, and as a result, the toner image can not be transferred to the image bearing sheet at all. This presents a serious problem. Furthermore, during corona transfer, the toner image is susceptible to scattering, resulting in the problem that the printed image is dim Further, in an electrophotographic apparatus employing the corona transfer, high resistance toner is used. It is said that the critical resolution, when using the high resistance toner, is equal to four to five times as large as the particle size of the toner particles. Therefore, a printed image of high resolution can not be expected. In an ordinary two-component development using high resistance toner, the so-called edge effect problem has been encountered. In order to overcome the degrading of the image quality caused by the use of high resistance toner, it has been proposed to use electrically-conductive toner. Such a method is disclosed, for example, in U.S. Patent No. 3,639,245. However, when conductive toner is used, transfer can not be carried out by the corona transfer method. Therefore, when conductive toner is used, adhesion transfer method using an intermediate medium, or other methods should be employed. Next, the adhesion transfer method will now be described. In the adhesion transfer method, the toner is transferred utilizing the adherent or sticky force of a image bearing sheet. Alternatively, when transferring the toner to ordinary paper, the toner image is transferred to an intermediate transfer medium, and then is again transferred therefrom to ordinary paper. Such an adhesion transfer method is described, for example, in Japanese Patent Examined Publication No. 46-41679. However, at the step of retransferring the toner image from the intermediate transfer medium to the image bearing sheet and fixing the thus transferred image, the toner on the intermediate transfer medium is heated by a heating roller so as to be rendered viscous. Therefore, the intermediate transfer member is heated by the heating roller, and the photoconductive member in contact with the intermediate transfer medium must also be heated. As a result, the photoconductive medium undergoes thermal effects, which causes adverse effects such as a variation in the charge characteristics of the photoconductive medium and a shortened service life of the photoconductive medium. Next the pressure transfer method will now be described. In the pressure transfer method, pressure-fixing toner is used, and an image bearing paper sheet is held between a photoconductive drum and press roller. The toner is transferred and fixed to the image bearing sheet by pressure, thus simultaneously effecting the transfer and the fixing. In this method, since a high pressure is applied to the apparatus, the apparatus is required to have an increased mechanical strength, and the drive torque is also increased. Therefore, a problem has been encountered in that only the type of photosensitive members having a high pressure resistance can be used. Furthermore, the image bearing paper sheet is subjected to calendering because a high pressure is applied thereto. For this reason, a gloss develops on the surface of the image bearing sheet, and the fixing ability is inferior to that of the other fixing methods. (b) Examples of conventional fixing methods include a thermal fixing method, a pressure fixing method and a solvent vapor fixing method. First, the thermal fixing method will now be described. In the thermal fixing method, the fixing is carried out by melting toner by means of a heating roller. This method is widely used at present. In this method, when electric power is supplied to the apparatus to initiate its operation, there is required a certain period of time for warming up the apparatus. Also, a large amount of thermal energy is required for effecting the fixing. For these reasons, a large capacity power source is needed for the heating roller, which results in the problem that the electric power consumption of the apparatus is large. Furthermore, fixing is carried out by heat, and therefore even if it is desired to increase the fixing rate, this can not be achieved because the temperature of the heating roller can not be increased from the viewpoint of safety. Thus, this method suffers from a drawback that recording can not be carried out at a high speed. Next, the pressure fixing method will now be described. Toner used in this pressure fixing method is made of a pressure sensitive material, the viscosity of which is lowered when pressure is applied to the toner. Examples of such toner materials include paraffin, wax and rubber-like soft polymer. In this method, a high pressure is applied to the toner to lower its viscosity, so that the toner can penetrate into the fibers of the image bearing paper sheet, thereby carrying out the transfer and fixing. This method presents problems such that the calendering of the image bearing sheet produces a gloss on the printed image and lowers the fixing ability as described above, that the weight balance of the copying machine is not good due to heavy weight, resulting in less portability, and that impact noise is produced when the paper sheet is discharged from the copying machine. Next, the solvent vapor fixing method will now be described. In this method, solvent vapor dissolves a toner image on an image bearing paper sheet, thereby fixing the toner image to the image bearing sheet. When it is considered that this method is used in various environments, several problems are encountered. Namely, it is difficult to control the pressure of the solvent vapor to a constant level. It is necessary to provide means for preventing the leakage of the solvent vapor. It is difficult to use cut paper sheets. The conventional printing apparatii which have been widely available employ a combination of the aforesaid electrostatic transfer (the corona transfer method in particular) and thermal fixing. (c) Japanese Patent Unexamined Publication No. 62-67576 discloses a prior art method in which a toner image formed on an image carrier is dissolved by a solvent so as to make the toner image viscous, and the viscous toner image is transferred to an image bearing medium through adhesion. This method will now be described. First, a toner image is formed on a photoconductive drum by an ordinary electrophotographic process. Then, a toner powder-dissolving drum (hereinafter referred to as "dissolving drum") coated with a solvent capable of dissolving the toner is located opposite to the photoconductive drum with a space therebetween. Then, the toner image on the photoconductive drum is electrostatically transferred to the dissolving drum by a transfer charging device mounted within the dissolving drum. The toner thus transferred to the dissolving drum is dissolved by the solvent and is made to be viscous. In this condition, when a paper sheet is brought into intimate contact with the toner image, the toner image is adhesion-transferred to the paper sheet by the adherent or sticky force of the viscous toner. However, the printing apparatus of this type has the following problems: (1) A sharp printed image can not be obtained. The reason for this will be described. If there is a gap between the photoconductive drum and the dissolving drum when the toner image on the photosensitive drum is to be electrostatically transferred to the dissolving drum, the toner image is scattered because of a scattering phenomenon as is well known in the art. As a result, the transferred toner image on the dissolving drum is dim. Thus, the above-mentioned problems of the conventional electrostatic transfer method have not been solved. (2) To deal with the above scattering problem, it has been considered to effect electrostatic transfer by placing the photoconductive drum in intimate contact with the dissolving drum. However, with the conventional method, a new problem arises such that the resultant printed image has increased void portions and therefore is unclear. The reason for this is that even if a sharp toner image is transferred to the dissolving drum, the toner image made to be viscous by the solvent is adhesion-transferred while being distributed possibly to both photoconductive drum and dissolving drum. (3) The solvent begins to dissolve the toner transferred to the dissolving drum from that surface of the toner making contact with the dissolving drum toward the paper sheet to which the toner is to be transferred. In other words, when viewed from the side of the paper sheet, the toner is dissolved from its inner surface toward its outer surface. Accordingly the toner is made to be viscous earlier at its inner side adjacent to the dissolving drum than at its outer side adjacent to the paper sheet. For this reason, if the solvent supplied to the dissolving drum is in small amounts, the adherent force acts only on the dissolving drum side and does not act on the paper sheet side, and as a result the toner is not transferred to the paper sheet at all, thus failing to obtain a printed image. On the other hand, if the solvent is supplied in increased amounts, the dissolved toner is diffused, and therefore the lines of the printed image become thick, thus affecting the reproduction of thin lines. Anyway, in such a method in which the solvent is applied to the toner from the inner surface of the toner, it is extremely difficult to control; the amount of solvent, and therefore it is difficult to obtain a clear printed image. European patent specification number EP-A-0040128 shows an embodiment (fig. 3) of a printing apparatus having the features of the preamble of claim 1. However, in this apparatus, whilst solvent is supplied to an image bearing medium prior to it contacting with a toner image on an image carrier, transfer of the toner image bearing medium is effected by a transfer potential between the image carrier and a support on which the image bearing medium is conveyed. Solvent is supplied to the image bearing medium directly from a solvent tank by a roller contacting the surface of the image bearing medium. SUMMARY OF THE INVENTION In view of the deficiencies of the prior art, an object of this invention is to provide a printing apparatus which is simple in construction, and enables the transfer and fixing of a toner image with low energy, and can produce a printed image of a high resolution free from toner scattering. Accordingly, the present invention provides a printing apparatus comprising: an image forming means for forming a toner image on an image carrier; a solvent supply means including a solvent coating roller pressed against a surface of an image bearing medium to thereby coat solvent supplied to the roller onto a surface of the image bearing medium before the image bearing medium contacts with the toner image, wherein the solvent lowers the viscosity of the toner image to assist its transfer to the image bearing medium, characterised in that the image bearing medium is pressed by a transfer means into contact with a toner image and the solvent is supplied to the coating roller by a solvent supply felt. The viscosity of the toner image of coloured toner is lowered by the solvent, so that transfer and fixing can be carried out simultaneously. Therefore, the printing apparatus can perform printing at high speed with a lower electric power consumption. Further, in the present invention, irrespective of the conductivity of the toner and the electrical resistance of the image bearing sheet, the toner image is not subjected to toner scattering when effecting the transfer, and the printing apparatus can produce a clear printed image of high resolution. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a basic concept of a printing apparatus in which a solvent is supplied to an image bearing medium prior to said medium being pressed into contact with a toner image on an image carrier (in Fig. 1, the toner image is formed by an electrophotographic process, and the toner image is pressed against an image bearing paper sheet coated with the solvent, thereby effecting transfer and fixing); and Fig. 2 is a schematic view of a specific form of a printing apparatus in accordance with the present invention, said apparatus employing an ion projection electrographic printing process. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The invention will now be described in detail with reference to the drawings. Fig. 1 shows the basic concept in which a solvent is supplied to an image bearing medium prior to said medium being pressed into contact with a toner image on an image carrier. With this arrangement the apparatus can be simple in construction. An apparatus in accordance with the invention employs this construction and will be described with reference to Fig. 2. The method shown in Fig. 2 is advantageous in that the solvent is supplied to the toner image by pressing the powered image against the image bearing medium retaining the solvent. With this arrangement, the solvent supply means can be constituted by a simpler device, and besides high-speed recording can be achieved relatively easily. Fig. 2 Reference is first made to a specific form of the invention in which solvent is supplied to an image bearing medium and image transfer is effected. Fig. 2 schematically shows a first embodiment of a printing apparatus of the present invention. In this embodiment, an image forming means for forming a toner image comprises an ion projection electrographic printing process by way of example. Such an ion projection electrographic printing process is disclosed in U.S. Patent No. 4,155,093. In Fig. 2, reference numeral 1 denotes an ion projection print head which applies ions in accordance with an image signal. Reference numeral 2 denotes a dielectric drum serving as an image carrier, the dielectric drum 2 comprising a drum body of an electrically conductive material (e.g. aluminium) and a dielectric layer coated on the outer peripheral surface of the drum body. Preferably, the surface of the dielectric drum 2 should have excellent resistance to solvent and have a smooth surface and low free-surface energy. It is desirable that the surface of the dielectric drum 2 be coated with fluoroplastics, polyolefin or other suitable materials. Reference numeral 3 denotes a developing unit, and reference numeral 4 denotes a toner image formed on the dielectric drum 2. The toner used in this embodiment is a one-component magnetic conducive toner. The ion projection print head 1 and the developing machine 3 constitute in combination an image forming means for forming the toner image 4 on the dielectric drum 2. A press roller (transfer means) 6 is pressed against the dielectric drum 2. Reference numeral 9 denotes an image bearing paper sheet (image bearing medium). The image bearing sheet 9 is, for example, PPC paper of 64 g/m². The press roll 6 cooperates with the dielectric drum 2 to press the toner image (which has been formed on the dielectric drum 2 against the image bearing sheet 9. A cleaning unit 78 removes the residual or untransferred toner, remaining on the dielectric drum 2, from the dielectric drum 2, the cleaning unit 7 comprising a metal blade. Reference numeral 8 denotes a static eliminator for eliminating an electrostatic latent image on the dielectric drum 2. Reference numeral 10 denotes a solvent tank, 11 a solvent supply pump. The constituent or ingredient of the toner whose viscosity is lowered by the solvent is binder resin. A suitable solvent capable of lowering the viscosity of the binder resin is selected. Examples of the combination of the binder resin and the solvent include styrene resin and toluene, acrylic resin and acetone, epoxy resin and dimethyl ethane, and water soluble resin (e.g., PVA (Polyvinyl-Alcohol), CMC (Sodium Carboxymethyl Cellulose) and water. In view of safety and the cost, it is preferred to use water as the solvent. PVA is used as the binder resin, and carbon black and a magnetic material are dispersed in the PVA, and this dispersion is pulverized to produce one-component magnetic conductive toner. The resistivity of the toner used in this embodiment was about 10⁶Ω.cm, and water was used as the solvent. The operation of the printing apparatus of the embodiment of the invention will now be described. First, in accordance with image signals, the electrostatic latent image is formed on the dielectric drum 2 by the ion projection print head 1. This electrostatic latent image is developed by the developing unit 3, so that the toner image 4 is formed on the dielectric drum 2. Concurrently with a toner image being formed on the dielectric drum 2, the solvent is supplied from the solvent coating roller 15 to the image bearing sheet 9, and is coated on the surface of the image bearing sheet 9. In this condition, the image bearing sheet 9 is pressed against the dielectric drum 2 by the press roller 6. At this time, the toner image 4 on the dielectric drum 2 is brought into contact with the solvent on the image bearing sheet 9 so that the lowering of the viscosity of the toner image 4 proceeds on such a side that the toner image 4 makes contact with the image bearing sheet 9, that portion of the toner image 4 which is lowered in viscosity is deformed, and penetrates into the fibers of the image printing apparatus of the present invention. This embodiment is advantageous where paper to be used is limited to have a uniform solvent retention. By virtue of the provision of the solvent supply felt 16, the amount of supply of the solvent to a solvent coating roller 15 is made constant. At this time, since the viscosity of the toner image 4 has been lowered by the solvent, it will be readily appreciated that the pressure applied to the press roller 6 can be much lower than that required in an apparatus utilizing the conventional pressure fixing method. Although the pressure applied to the press roller 6 varies depending on the degree of lowering of viscosity of the toner image 4, this pressure can be reduced to one-several to one-several hundredths of the pressure required for the conventional pressure fixing method. Simultaneously when the toner image 4 is transferred to the image bearing sheet 9 by the pressure, the toner penetrates into the fibers of the image bearing sheet 9 and is therefore fixed thereto. In this condition, the solvent contained in the transferred toner evaporates or is dried through the penetration, thereby providing paper in a final recorded form. At this time, the toner image 4 has begun to become viscous on a side that the toner image 4 makes contact with the image bearing sheet 9, and therefore the amount of the residual (untransferred) toner remaining on the dielectric drum 2 is small. As a result, a printed image with a higher transfer efficiency than that achieved by conventional transfer methods can be obtained. The residual toner remaining on the dielectric drum 2 is removed by the cleaning unit 7, and the static electricity is eliminated from the dielectric drum 2 by the static eliminator 8. If the condensate of water is present on the dielectric drum 2 when the above cleaning is to be carried out by the cleaning unit 7, this condensate is also removed from the dielectric drum 2 together with the residual toner. If the cleaning unit 7 is not sufficiently effective in removing the residual solvent from the dielectric drum 2, then it may be effective to provide a heater or the like in the vicinity of the dielectric drum 2 so as to dry by force the residual solvent on the dielectric drum 2. The electrically-conductive toner which heretofore could not be easily transferred and fixed can be easily transferred without scattering, and good and high-efficiency transfer can be achieved regardless of the resistivity of the toner. Reference numeral 17 denotes a solvent supply pipe for impregnating the solvent supply felt 16 with the solvent. The solvent supply pipe 17 is connected to the solvent tank 10 via the solvent supply pump 11, and is held in intimate contact with the upper end of the solvent supply felt 16, as show in Fig. 2. A plurality of apertures are formed in that portion of the solvent supply pipe 16, and the solvent fed from the solvent supply pump 11 flows through these apertures, thereby impregnating the solvent supply felt 16 with the solvent. The solvent supply felt 16 is held in contact with the solvent coating roller 15 so as to form a liquid film of the solvent on the peripheral surface of the roller for transfer to the image of bearing sheet 9. After that, the portion of the transferred toner image 4 which is not lowered in viscosity is lowered by the solvent contained on the paper 9, and finally the toner image 4 is fixed. In this condition, the solvent contained in the transferred toner evaporates or is dried through penetration, thereby providing the paper in a final recorded form. As described above, the solvent is supplied to the toner image on the image carrier simultaneously with the transfer of the toner image. This has an advantage in that the supply of solvent can be carried out by a simple construction. Furthermore, since the solvent is supplied by the coating roller, high-speed printing can be achieved relatively easily. This will be appreciated, for example, for the case with a printing machine. In the embodiment of the invention, although the photoconductive medium or the dielectric medium (both of which are an electric charge-retaining material) is used as the image carrier, the image carrier may comprise any other suitable electric charge carrier medium, or is not limited to such electric charge carrier mediums. Although the printing apparatus of the above embodiment employs an ion projection electrographic printing process or electrographic process, the invention is not limited to such types of apparatii and the invention is applicable to any type of printing apparatii which perform recording, utilizing toner. For example, a toner image may be formed by a so-called magnetography in which a magnetic latent image is developed by a magnetic toner. In the above embodiments, although PPC paper is used as the transfer element, this recording paper may be material other than PPC paper, such as OHP. Further, although the residual solvent on the image carrier is removed by the cleaning unit, a heater or the like can be advantageously provided if the cleaning unit is not sufficiently effective in removing the residual solvent.

Claims: A printing apparatus comprising: an image forming means (1, 3) for forming a toner image (4) on an image carrier (2); a solvent supply means (10, 11, 15, 16, 17, 18) including a solvent coating roller (15) pressed against a surface of an image bearing medium (9) to thereby coat solvent supplied to the roller (15) onto a surface of the image bearing medium before the image bearing medium contacts with the toner image, wherein the solvent lowers the viscosity of the toner image to assist its transfer to the image bearing medium, characterised in that the image bearing medium is pressed by a transfer means (6) into contact with a toner image (4) and the solvent is supplied to the coating roller (15) by a solvent supply felt (16). A printing apparatus as claimed in claim 1, characterised in that the image carrier (2) comprises an electrical charge carrier member, said image forming means comprising latent image forming means (1) for forming an electrostatic latent image on said image carrier and developing means (3) for developing said electrostatic latent image using toner. A printing apparatus as claimed in claim 2, characterised in that the latent image forming means comprises an ion projection print head. A printing apparatus as claimed in claim 2, characterised in that the image carrier (2) comprises an electro-photosensitive member, said latent image forming means comprising electrostatically charging means, and exposure means. A printing apparatus as claimed in claim 2, characterised in that the toner is electrically conductive.

Inventors: MIMA SOICHIRO, NANKOU TAKAHIKO, ONISHI HIROSHI, TAKASHIMA YUJI, MIMA, SOICHIRO, NANKOU, TAKAHIKO, ONISHI, HIROSHI, TAKASHIMA, YUJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: G03G15
IPC codes: G03G15