Method and apparatus for forming color images

Abstract: In a color image forming method and apparatus utilizing electrophotography, a toner image bearing film (14) is passed around a photosensitive member (11) and moved past a transfer device, the photosensitive member (11) is uniformly charged, and is then exposed to light images corresponding to different color components of the image. Developing devices (17,19,21) are provided facing the toner image bearing film (14) passing around the photosensitive member (11), and toners of different colors are then made to adhere to the toner image bearing film (14) because of the latent image on the photosensitive member (11). Thus, the images of color toners are formed on the toner image bearing film (14), and then transferred to a recording paper. The images of color toners may be transferred one by one, or simultaneously. In the latter case, images of the colors, except or including the last-developed one, are temporarily fixed on the toner image bearing film, by application of heat, and then transferred to the recording paper.

Description: BACKGROUND OF THE INVENTION This invention concerns a method and apparatus for forming color images by means of electrophotography according to US-A-3,937,572. Conventionally, when forming color images by means of electrophotography, a photosensitive drum and a transfer drum carrying recording paper are made to rotate together a plurality of times, and images on the photosensitive drum are transferred the same number of times onto the recording paper on the outer surface of the transfer drum so as to overlay each other. Fig. 1 is a simplified drawing of a color electrophotographic copier employing the conventional color image-forming method (as shown for example in Japanese Patent Application Publication No. 34467/1988; and Proceedings of the Inst. of Electrostatics Japan, Vol. 9, No. 4. pp. 253-261, 1985). In the figure, the surface of a photosensitive drum 1 which rotates at a fixed speed in the counterclockwise direction, is electrostatically charged with a specified polarity by a corona discharge device 2. When an image of the original is formed by the exposure system 3 on the electrified surface of the photosensitive body, the charge on the photosensitive body is lost according to the density of the respective parts of the image of the original, and electrostatic latent images are thereby formed. These latent images are then rendered visible by a developing device 4. In the case of a color copier, this developing device 4 comprises at least three devices, namely a device for developing yellow images 5, a device for developing magenta images 6, and a device for developing cyan images 7. The exposure device 3 is therefore provided with blue, green and red color separation filters. The developing device 4 may also be provided with a device for black development in order to adjust the color tone if necessary. Developing is performed for each color separately. First, when a color image of the original is projected on the photosensitive body using the blue filter, an electrostatic latent image consisting of yellow element which is the complementary color to blue is formed thereon. This is then rendered visible by the yellow developing device 5. The yellow developing device 5 contains a developing agent of a yellow toner and an iron powder carrier. These two components are mixed together by stirring, and the toner is triboelectrified with a polarity opposite to that of the charge on the electrostatic latent image. Both the toner and carrier are attached onto the developing roller 5a, and are transported to the developing area which is near the photosensitive drum 1. Next, the green and red color separation filters are selected, the magenta and cyan toners are developed, and the toner images are copied onto a recording paper 9 via a transfer drum 8 to form color images on the paper. However, in the conventional color image-forming method, the outer circumference of the transfer drum 8 must be arranged to be longer than the length of the recording paper 9. If for example the largest size of the recording paper 9 which can be recorded is B4 (JIS Standard), its dimension in the longer direction is 364 mm. In order that the recording paper of the B4 size can be wound onto the transfer drum 8 without superposing, the outer diameter of the transfer drum 8 must be not less than 116 mm. Because a mechanism required to hold the recording paper 9 at a fixed point must also be provided on the circumference thereof, the transfer drum 8 must have larger dimensions than the above, and the color copier becomes bulky. Further, the mechanism required to load the recording paper 9 on the transfer drum 8, form a recorded image on the paper 9, and release the paper 9, is complex. The cost of the copier therefore increases, and the paper frequently jams. Moreover, in the above conventional method of forming color images, costly and bulky color separation filters had to be used. and a means of selecting them also had to be provided. SUMMARY OF THE INVENTION This invention aims to eliminate need for the transfer drum and the mechanism to load the recording paper on the photosensitive member, form a recorded image on the paper and release the paper, as well as to reduce the frequency of paper jamming. This invention also aims to facilitate the back and forth transport of the recording paper after transfer and fixing, by having toner images transferred to and fixed on the recording paper by means of thermal energy, thereby enabling a roller to be pressed against the recording paper to which the toner images have been transferred. This invention also aims to eliminate need for separate filters and means for selectively using them, through the use of a toner image bearing film having the functions of the color filters, so that the cost of the color image copier and the area which it occupies can be reduced. According to the invention, a toner image bearing film is passed around a photosensitive member, which is uniformly charged, and exposed successively to a light image corresponding to a first color thereby to form an electrostatic latent image of said first color, to a light image corresponding to a second color thereby to form an electrostatic latent image of said second color, and to a light image corresponding to a third color thereby to form an electrostatic latent image of said third color. Developing devices for said first, second and third colors, are provided to face the photosensitive member through the toner image bearing film, so that toner images of said first, second and third colors are developed on the toner image bearing film when it is passing around said photosensitive member. The toner image bearing film is moved past a transfer section, where the toner images are transferred to a recording paper, which is moved in time with the the toner image bearing film. The transfer device comprises a first roller and a second roller juxtaposed with each other. One of the first and second rollers is a heating roller and the other is a pressure roller, so that they also serve as a fixing means. The pressure roller is pressed against the heating roller when the toner image bearing film bearing the toner image of each color passes between the heating roller and the pressure roller, the feeding means moves the recording paper backward after the transfer of the toner image of each color, and the pressure roller is separated from the heating roller when the recording paper is moved backward. In this way, the toner images of the respective colors are successively transferred to and fixed on the recording paper. The toner image bearing film may have a first filter part, a second filter part, and a third filter part which are successively formed along the length of said toner image bearing film, the exposure device is provided facing the photosensitive member through the toner image bearing film, for successively exposing the photosensitive member through the first filter to form a latent image of a first color, through the second filter part thereby to form a latent image of a second color, and through the third filter part thereby to form a latent image of the third color, with the first, second and third filters being filters of colors complementary to the first, second and third colors, respectively, the developing devices successively develop toner images of the first, second and third colors onto the first filter part, onto the second filter part, and onto the third filter part, and the feeding means feeds the recording paper past the transfer device in time with the passage of the of the first, second, and third filter parts past the transfer device, and moves the recording paper backward after the transfer of the toner image from the first and second filter parts, and the toner images of the first, second and third colors are successively transferred to the recording paper. This invention thus offers the following advantages, (1) As the transfer step and fixing step may be combined, there is no need for a transfer drum, nor any need for a mechanism to load the photosensitive member with the recording paper, form a recorded image and release the paper. The apparatus can thus be made more compact, its cost can be reduced, and as its construction permits easy paper feed, the frequency of paper jamming can be reduced. (2) As the photosensitive member is not subject to contact friction as it was in the conventional developing step or cleaning step, the lifetime of the photosensitive member is extended. (3) The perturbation of the image which occurred in the conventional electrostatic transfer process, can be avoided. (4) When transfer and fixing are carried out simultaneously, a roller can be brought into pressure-contact with the surface of the recording paper to which the toner images have been transferred, and the back and forth transport of the recording paper after transfer and fixing can thus be implemented by a simple mechanism. (5) Images can also be copied onto rough paper such as bond paper, which was regarded as difficult in the conventional electrostatic transfer process. (6) Color separation filters to irradiate the photosensitive member with blue, green and red light can be eliminated, means for selecting filters and means for controlling such a device can therefore be eliminated. The cost of manufacturing the color photocopier, and the space it occupies, can therefore be reduced. (7) As there is no need to apply pressure when the toner images developed on the toner image bearing film are melted, there is no offset onto pressure members such as the pressure roller. There is thus no need to clean away toner, maintenance of pressure members is therefore facilitated, and costs can be reduced. (8) As a cleaning step for removing residual toner powder from the photosensitive member can be omitted, the apparatus can be made even more compact, and its cost can be reduced. (9) As the cleaning step can be eliminated, scattering of toner inside and outside the apparatus can be reduced. (10) As there is no need to dispose of toner after cleaning, soil of the operator or his clothing with toner can be avoided, and maintenance work can be reduced. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic drawing showing a color electrophotographic copier employing the conventional color image-forming method. Fig. 2 is a schematic drawing of the color image-forming apparatus of Embodiment 1. Fig. 3 is a drawing showing the state during the transfer and fixing step according to Embodiments 1 and 2. Fig. 4 is a drawing showing the state when the recording paper has returned according to Embodiments 1 and 2. Figs. 5A to 5D are drawings showing the electrifying step, the exposure step, the developing step, and the transfer and fixing step that occur in the case of yellow toner. Figs. 6A to 6D are drawings showing the electrifying step, the exposure step, the developing step, and the transfer and fixing step that occur in the case of magenta toner. Figs. 7A to 7D are drawings showing the electrifying step, the exposure step, the developing step, and the transfer and fixing step that occur in the case of cyan toner. Fig. 8 is a schematic drawing of the color image-forming apparatus of Embodiment 2. Figs. 9A to 9D are drawings showing the electrifying step, the exposure step, the developing step, and the transfer and fixing step that occur in the case of yellow toner. Figs. 10A to 10D are drawings the electrifying step, the exposure step, the developing step, and the transfer and fixing step that occur in the case of magenta toner. Figs. 11A to 11D are drawings showing the electrifying step, the exposure step, the developing step, and the fourth transfer and fixing step that occur in the case of cyan toner. DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment 1 An embodiment of this invention will now be described with reference to Figs. 2 to 7D. Fig. 2 is a schematic drawing of the color image-forming apparatus according to the invention. The illustrated color image forming apparatus comprises a photosensitive drum 11 rotatably mounted on a frame, not shown. The photosensitive drum 11 may be one having a selenium photosensitive body comprising a photosensitive layer on an electrically conducting support, a negatively charged organic photosensitive body, or a positively charged organic photosensitive body. The color image forming apparatus also comprises a transfer and fixing section 45 formed of a heating roller 12 and a pressure roller 13 which are juxtaposed with each other. The heating roller 12 may be of a hollow metal member enclosing a halogen lamp, or one having a heat-emitting body on a metal surface. The surface of the pressure roller 13 is covered with silicone rubber in order to provide heat stability. An endless toner image bearing belt or film 14 is passed around the photosensitive drum 11 and moves past the transfer and fixing section 45. More particularly, the toner image bearing film 14 is in contact, on a first or inner surface thereof, with the peripheral surface of the photosensitive drum 11 over a portion of the photosensitive drum arc, and as the photosensitive drum 11 rotates, the toner image bearing film 14 moves together with the photosensitive drum 11. Where the toner image bearing film 14 moves past the transfer and fixing section 45 it passes around the heating roller 12 and between the heating roller 12 and the pressure roller 13. The pressure roller 13 can selectively assume one of the two positions: a contact position in which it is pressed against the heating roller 12 clamping the toner image bearing film 14, and a release position in which it separated from the heating roller 12. The pressure roller 13 is moved between the two positions by a means to be described later with reference to Fig. 3 and Fig. 4. The elasticity of the silicone rubber layer of the pressure roller 13, the diameter for the pressure roller 13, and the pressing force applied between the pressure roller 13 and the heating roller 12 are so designed as to provide a just enough nip width. As the photosensitive drum 11 rotates its surface sequentially passes various processing sections or devices, namely, a charging or electrifying device 15, an exposure device 16, developing devices 17, 19 and 21, and a discharge lamp 25. It should be noted that some of the processing devices, i.e., the exposure device 16, and the developing devices 17, 19 and 21, are provided to face the photosensitive drum, not directly, but through the toner image bearing film 14. Between the location where the electrifying device 15 confronts the photosensitive drum 11 and the location where the exposing device confronts the photosensitive drum 11, the toner image bearing film 14 is brought into contact with the photosensitive drum 14. Between the location where the developing devices 17, 19 and 21 confront the photosensitive drum 11 and the location where the discharging lamp 25 confronts the photosensitive drum 11, the toner image bearing film 14 is separated from the photosensitive drum 11. The electrifying device 15 is a corona discharge device for providing an electrostatic charge uniformly over the photosensitive surface of the photosensitive drum 11. The electrifying device 15 may alternatively be formed of a brush discharge device. The exposure device 16 exposes the photosensitive drum 11 through the toner image bearing film 14 to a light image or radiation pattern to form an electrostatic latent image on the photosensitive surface of the photosensitive drum 11. The areas which have been irradiated by light are discharged, while the areas or dots of the photosensitive drum which have not been irradiated are kept charged, so the latent image consists of charged areas and discharged areas. This does not mean that each area assumes either of the two distinct state, the charged and the discharged: there can be intermediate state and the degree to which each area is discharged depends on the density of the respective areas of the image. However, in the following description, it is assumed that the image consists of charged areas and discharged areas for the sake of simplicity of illustration. The exposure is repeated the same number of times as the number of the colors of the toners with which the color image is formed. In the embodiment illustrated, toners of the three primary colors, yellow, magenta and cyan are used, so the exposure is repeated three times, and the light pattern at each exposure is for the image of each color component. The toner image bearing film 14 must therefore be transparent to the wavelengths of the light used for the exposure. The exposure device 16 may be a combination of a light source such as a laser or an LED array and an optical imaging system, and in this case, the light pattern for each color is produced by electrical signals representing the image of each color. Such electrical signals are supplied from a controller 50 which performs the overall control of the apparatus. The exposure device 16 may alternatively be a combination of an illuminating device illuminating an original document and an optical system for directing the light reflected at the surface of the original to the photosensitive surface of the drum 11 through filters of colors complementary to the colors of the toners. In such a case, the selective placement or insertion of the filters are made by means not shown under control of the controller 50. The developing devices 17, 19 and 21 are the ones for applying toners of yellow (Y), magenta (M) and cyan (C). These developing devices 17, 19 and 21 are installed facing the outer surface of the toner image bearing film 14 which moves in close contact with the photosensitive drum 11, as described above. They are respectively provided with developing agent supports 17a, 19a and 21a for yellow (Y), magenta (M) and cyan (C) toners 18, 20 and 22 being attached on the supports and transported so as to develop the toner images on the outer surface of the toner image bearing film 14. The developing devices may be a binary (two-component) magnetic brush developer, a unitary (one-component) magnetic brush developer, or a unitary non-magnetic brush developer. The developing devices 17, 19 and 21 are successively activated by the controller 50 in accordance with the color of the latent image which has just been formed by the exposure device 16. The discharge lamp 25 is provided so as to face the part of the photosensitive drum 11 which has just separated from the toner image bearing film 14 after developing. the function of the discharge lamp 25 is to irradiate the entire photosensitive drum to dissipate all the charges on it thereby making it ready for next operation. As described earlier, the toner image bearing film 14 passes around the photosensitive drum 11, and the transfer and fixing section 45. The toner image bearing film 14 also passes a fixing cleaner 23 and then a discharge brush 24. The cleaner 23 is provided in apposition to the heating roller 12, but after the transfer and fixing section 45. The function of the cleaner 23 is to remove any residual toner from the toner image bearing film 14 after transfer of the toner image to a recording paper 27 to be described later. The discharge brush 24 is provided in contact with the toner image bearing film 14. The function of the discharge brush 24 is to remove any residual electrostatic charge from the toner image bearing film 14. Pinch rollers 26a and 26b are disposed being pressed against the pressure roller 13, on both sides of transfer and fixing section 45, such that the recording paper 27 is wound on the pressure roller 13 over a sufficient portion of the pressure roller arc. The paper feed system for the recording paper 27 is comprised of a paper feed cassette 40, a paper pick-up roller 41, a paper advance roller 42 and a paper eject roller 43. Their operation, i.e., forward and backward rotation are controlled by the controller 50. Now the mechanism for moving the pressure roller 13 between the two positions is described with reference to Fig. 3 and Fig. 4. As illustrated, a frame 28 supporting the heating roller 12 and a frame 29 supporting the pressure roller 13 are supported such that they can rotate freely on a pivot 30. The pressure roller 13 is pressed against the heating roller 12 by the tensile force of a spring 31 attached to the frames 28 and 29. A solenoid 32 serves to apply a back-and-forth motion to a shaft 33, and causes the frame 29 to rotate, such that the pressure roller 13 is pressed against or separated from the heating roller 12. When the pressure roller 13 is in the contact position, the transfer and fixing process is carried out with a recording paper 27 being passed between the pressure roller 13 and the toner image bearing film 14 on the heating roller 12. When the pressure roller 13 is in the release position, the recording paper 27 may be moved backward, e.g., after transfer of toner image of one color and to be ready for transfer of toner image of another color. The excitation and de-excitation of the solenoid 32 is controlled by the controller 50. In place of the solenoid 32, any other actuator may be used to selectively move the pressure roller 13 between the two positions. The way in which a color image is formed by the above apparatus will now be described. The photosensitive drum 11 and the heating roller 12 are rotated at a constant peripheral speed in the directions shown by the arrow in the figure by a drive mechanism not shown. As a result, due to the friction with the photosensitive drum 11 and the heating roller 12, the toner image bearing film 14 is moved in the direction shown by the arrow. The image-forming processes will be described with reference to Fig. 5A to Fig. 7D. In the electrifying step shown in Fig. 5A, the surface of the photosensitive drum 11 is directly charged uniformly by the corona discharge device 15. The figure shows the case where a selenium photosensitive body is used. In this case, when a high voltage is applied to the corona discharge device 15, the surface is charged positively. In the subsequent exposure shown in Fig. 5B, a light image corresponding to the yellow image signal produced by the exposure device 16 irradiates the photosensitive layer 11a of the photosensitive drum 11 to form an electrostatic latent image for the yellow component of the color image. Next, in the developing step shown in Fig. 5c, reversal development is used, and a means 132 of applying a bias potential is connected across the electrically conducting support 11b of photosensitive drum 11 and the toner support 17a, to apply a bias potential. the discharged areas of the latent image on the photosensitive drum 11 attract the positively charged yellow toner 18, and the attracted yellow toner 18 is attached to the outer surface of the toner image bearing film 14 directly over the discharged areas. More specifically, electric lines of force are generated between the developer support 17a and the discharged areas of the latent image on the photosensitive drum 11, and toner image bearing film 14, passing through the toner image bearing film 14. The particles of the yellow toner 18 travel along these electric lines of force and adhere to the outer surface of the toner image bearing film 14, and are kept adhering there by a relatively weak electrostatic force. Finally, the toner image on the toner image bearing film 14 is moved past the transfer and fixing section 45 where the transfer and fixing step shown in Fig. 5D takes place. In synchronism with the toner image on the toner image bearing film 14, the recording paper 27 is fed to the transfer and fixing section 45. More specifically, the recording paper is supplied from a paper supply cassette 40, being picked up by a pick-up roller 41, and is advanced by the paper advance roller 42, in synchronization with the toner images on the toner image bearing film 14, such that the leading edge of the recording paper (to be precise, the leading edge of the area in which the color image is to be reproduced) comes into contact with the leading edge of the area of the toner image bearing film 14 in which the toner image is formed. The toner image bearing film 14 and the recording paper 27 are moved at the same speed. The recording paper 27 and the toner image bearing film 14 are held between the pressure roller 13 and the heating roller 12. The yellow toner image on the toner image bearing film 14 is therefore melted by the heat of the heating roller 13, whereupon the pressure causes melted toner 18 to permeate the fibers of the recording paper 27. This transfers and fixes the toner images so as to form a yellow image on the recording paper 27. When the transfer and fixing process occurs, the magnetization of the solenoid 32 is interrupted, and the pressure roller 13 is pressed against the heating roller 12 by the tensile force of the spring 31 so as to hold the toner image bearing film 14 and the recording paper 27 between it and the heating roller 12, as shown in Fig. 3. When the image produced by the yellow toner 18 has been transferred and fixed up to the trailing edge of the image, the solenoid 32 is excited (Fig. 4). As a result, the movable shaft 33 is pulled in, the frame 29 rotates around the pivot 30, and the pressure roller 13 is separated from the heating roller 12. The pressure roller 13 is then rotated in the reverse direction (the direction opposite to the direction in which it is rotated during transfer and fixing), the recording paper 27 returns, so that it is ready for transfer and fixing of the next color toner. After the image has been transferred to the recording paper 27, a small amount of yellow toner 18 may remain on the toner image bearing film 14, however this will be wiped off by the cleaner 23 which is brought into contact with the heating roller 12. Further, a discharge brush 24 removes any residual static electricity from the toner image. In this way, the toner image bearing film 14 is cleaned and electrically discharged, and may thus be used again. After the developing step has been completed, the photosensitive drum 11 separates from the toner image bearing film 14, and is irradiated by a discharge lamp 25 so as to dissipate any residual electrostatic charge on the photosensitive drum. The drum then returns to the electrifying step, and may thus be used again. When the transfer and fixing process for an image of yellow toner 18 is complete, an image of magenta toner 20 is developed on the paper via an electrifying step (Fig. 6A), an exposure step corresponding to the magenta image signal (Fig. 6B), a developing step (Fig. 6C) and a transfer and fixing step (Fig. 6D). When the transfer and fixing processes for the images of yellow toner 18 and magenta toner 20 are complete, an image of cyan toner is transferred and fixed via an electrifying step (Fig. 7A), an exposure step (Fig. 7B), a developing step (Fig. 7C), and a transfer and fixing step (Fig. 7D). In this way, toner images of the respective colors are transferred to and fixed on the recording paper 27 successively. During the transfer and fixing, the pressure roller 13 is pressed against the heating roller 12, and the recording paper 27 is fed in synchronism with the toner image bearing film 14. Between the transfer and fixing of the toner image of one color and the transfer and fixing of the toner image of another color, the pressure roller 13 is separated from the heating roller 12, and the recording paper 27 is moved backward. Apart from the yellow toner 18, the magenta toner 20 and the cyan toner 22, black toner may also be used. In the transfer and fixing step, the toner image bearing film 14 is heated by the heating roller 13 to reach a temperature of about 160° C. It must therefore be heat resistant, must have insulating properties to a certain extent, and must also be transparent to the source light wavelengths used in the exposure step. From these considerations, the toner image bearing film used may be formed of a material such as polyester, polyimide, polyetherimide, polyethersulfone or polyetheretherketone. In the above embodiment, from the consideration of transfer efficiency in the transfer and fixing step, a film of Teflon (Trademark, Polytetrafluoroethylene)-coated polyimide is used. The transfer efficiency is then approximately 100%, there is less load on the cleaner 23, and the life of the cleaner 23 can be extended. Further, from consideration of the electric lines of force generated in the developing step, it is desirable that the thickness of the toner image bearing film 14 be no greater than 200 µm; while from considerations of tensile strength and ease of handling, it is desirable that its thickness be not less than 10 µm. Further, in the above embodiment, thermal-fixing toners are used for toners 18, 20 and 22. Microcapsule pressure-fixing toners, which can be fixed by applying a minute pressure, may also be used. In the exposure step, a toner image bearing film transparent to the light source wavelengths is used, and the exposure is made through the toner image bearing film which moves in close contact with the photosensitive drum. It is however also possible to expose the photosensitive drum directly, and bring the toner image bearing film in contact with the photosensitive drum after the exposure step. In this case, there is no need for the toner image bearing film to be transparent to the light source wavelengths. In the above embodiment, a photosensitive film can be used in place of photosensitive drum 11. Embodiment 2 Another embodiment of this invention will now be described with reference to Figs. 8 to 11D. In these figures, reference numerals identical to those in Figs. 2 to 7D denote identical or similar devices or elements, and their description is omitted. Fig. 8 is a schematic drawing of the color image-forming apparatus of this embodiment. A toner image bearing film 14 is passed around the photosensitive drum 11, and the heating roller 12, as in Embodiment 1, and in addition, around free rollers 135a and 135b. The surface of the toner image bearing film 14 is successively coated to constitute a blue filter part 114a, a green filter part 114b and a red filter part 114c, functioning as blue, green and red filters. When toner images of yellow, magenta and cyan are to be formed, filter parts of the complementary colors, i.e., the blue filter part 114a, the green filter part 114b and the red filter part 114c are used for the exposure. A glass plate 116a is installed above the area where the toner image bearing film 14 is in close contact with the photosensitive drum 11, and a white light source 116b is disposed to irradiate the original 116b laid on the glass plate 116a. The light reflected from the original 116b is then made to form an image on the photosensitive drum 11 by an optical imaging device 116d. The image formed by the optical imaging device 116d at any moment is a representation of an image of a linear area of the original, and as the glass plate 116a is moved, this linear area is shifted along the direction of the movement. Thus, the original 116d is effectively scanned. The glass plate 116a is moved at a constant speed in the direction as indicated by the arrow. The photosensitive drum 11, the heating roller 12 and the pressure roller 13 are rotated in the directions shown by the arrow at a constant peripheral speed, so that the toner image bearing film 14 is moved by friction with the photosensitive drum 11 and the heating roller 12. The rotation of the photosensitive drum 11 and the movement of the toner image bearing film 14 are synchronized with the movement of the glass plate 116a. More specifically, the movement of the toner image bearing film 14 is so timed that when the leading edge of any of the filter parts 114a, 114b and 114c moves past the exposure device, the scanning of the original is started. The movement of the toner image bearing film 14 and the operation of the developing devices 17, 19 and 21 are also so related that the yellow developing device 17 is activated when the blue filter parts 114a is moving past it, the magenta developing device 19 is activated when the green filter parts 114b is moving past it, and the cyan developing device 21 is activated when the red filter parts 114c is moving past it. The necessary control for these timed operations is made by a controller similar to the controller 50 shown in Fig. 2. The image-forming processes will be described with reference to Fig. 9A to Fig. 11D. In the electrifying step shown in Fig. 9A, the surface of the photosensitive drum 11 is directly charged uniformly by the corona discharge device 15. In the subsequent exposure step shown in Fig. 9B, light reflected from glass plate 116a passes through the optical imaging device 116d and the blue filter part 114a of the toner image bearing film 14. For this purpose, the movement of the toner image bearing film 14 is so timed that the leading edge of the blue filter part 114a passes the exposure device 16 as the scanning of the original 116b is started. The blue light B thus color-separated falls on the photosensitive drum 11, and forms an electrostatic latent image for the yellow toner. Next, in the developing step shown in Fig. 9C, yellow toner 18 charged on the developer support 17a, adheres to the blue filter part 114a of the toner image bearing film 14. Finally, in the transfer and fixing step shown in Fig. 9D, the recording paper 27, which is supplied and advanced from the supply cassette 40 in synchronization with the yellow toner image on the toner image bearing film 14 is passed around the pressure roller 13. When transfer and fixing process occurs as shown in Fig. 3, the pressure roller 13 is pressed against the heating roller 12 to hold the toner image bearing film 14 and the recording paper 27 between it and the heating roller 12. The yellow toner 18 adhering to the blue filter part 114a of the toner image bearing film 14 is then melted by the heat of the heating roller 12, and is caused by pressure to permeate the fibers of the recording paper 27. In this way, the yellow toner image is transferred to and fixed on the recording paper 27. When the image produced by the yellow toner 18 has been transferred and fixed up to the trailing edge of the image, the pressure roller 13 is separated from the heating roller 12. The pressure roller 13 is then rotated in the reverse direction, the recording paper 27 returns so as to be ready for the transfer and fixing process of the next color toner. The glass plate 116a is also moved in the reverse direction so as to return to its original position. After an image of yellow toner 18 has been transferred and fixed as described above, a magenta toner image is transferred and fixed via an electrifying step, an exposure step, a developing step, and a transfer and fixing step, as shown in Figs. 10A to 10D. In the exposure step shown in Fig. 10B reflected light passes through the green filter part 114b of the toner image bearing film 14. For this purpose, the leading edge of the green filter part 114b is made to pass the exposure device 16 when the second scanning of the original 116b is started. The green light G thus color-separated falls on the photosensitive drum 11, and forms an electrostatic latent image. In the developing step shown in Fig. 10C, magenta toner 20 on the developer support 19a, adheres to the green filter part 114b of the toner image bearing film 14. In the transfer and fixing step shown in Fig 10D, an image of magenta toner 20 is transferred and fixed over the image of yellow toner 18 which has been transferred to and fixed on the recording paper 27. After images of yellow toner 18 and magenta toner 20 have been transferred and fixed as described above, an image of cyan toner is transferred and fixed via a electrifying step, a exposure step, a developing step and a transfer and fixing step, as shown in Figs. 11A to 11D. In the exposure step shown in Fig. 11B, light reflected from the original 116b passes through the red filter part 114c, which is made to pass the exposure device 16 in time with the commencement of the third scanning of the original 116b. The red light R thus color-separated falls on the photosensitive drum 11, and forms an electrostatic latent image. In the developing step shown in Fig. 11C, cyan toner 22 on the developer support 21a, adheres to the red filter part 114c of the toner image bearing film 14. In the transfer and fixing step shown in Fig 11D, an image of cyan toner 22 is transferred and fixed over the image of yellow toner 18 and the image of magenta toner 22 which have been transferred to and fixed on the recording paper 27, thereby forming a color image. In this way, the toner images on the filter parts are successively transferred to and fixed on the recording paper 27 while the pressure roller 13 is pressed against the heating roller 12. Between the transfer and fixing of the toner image of one color and the transfer and fixing of the toner image of another color, the pressure roller 13 is separated from the heating roller 12, and the recording paper 27 is moved backward. Embodiment 2 can be modified in the same manner as described with reference to Embodiment 1.

Claims: A method for forming color images, comprising the steps of: (a) electrifying a photosensitive member (11); (b) exposing the photosensitive member; (c) developing toner images by adhering toner onto a toner image bearing film (14) which moves such that a part of it is in close contact with said photosensitive member; and (d) transferring and fixing toner images on the toner image bearing film onto a recording paper (27); wherein (e) said electrifying step, exposure step and developing step are carried out for toners of a plurality of different colors so as to form toner images on the toner image bearing film (14); (f) said toner images are successively transferred to and fixed onto the same recording paper (27); (g) said recording paper is advanced in synchronization with the leading edge of each toner image; characterized in that (h) said transfer step and fixing step are carried out at the same time; and (i) said recording paper (27) is reversibly movable between successive transfer and fixing of the toner images of different colors. A method for forming color images according to claim 1, wherein the toner image bearing film (14) and the recording paper (27) travel between two rollers (12, 13) which can take up either a pressure contact position or a release position in said transfer and fixing step, the toner images on the toner image bearing film (14) are transferred and fixed in the pressure contact position, and said recording paper (27) returns to the leading edge of the toner images in the release position. A method for forming color images according to claim 1, wherein said toner is a heat fixing toner. A method for forming color images according to claim 1, wherein said toner is a pressure fixing toner. A method for forming color images according to claim 1, wherein a toner image bearing film (14) transparent to the light source wavelengths is used and the photosensitive member (11) is irradiated through the toner image bearing film (14) which moves in close contact with the photosensitive member. A method for forming color images according to any of the preceding claims, further comprising the steps of: exposing the photosensitive member through a toner image bearing film (14) which moves such that a part of it is in close contact with the photosensitive member; and providing a toner image bearing film with a first filter part (114a), a second filter part (114b), and a third filter part (114c) which are successively formed along the length of said toner image bearing film (14) for successively exposing the photosensitive member (11) through the first filter part to form a latent image of a first color, through the second filter part thereby to form a latent image of a second color, and through the third filter part thereby to form a latent image of the third color, with the first, second and third filter parts being filters of colors complementary to the first, second and third colors, respectively. A method for forming color images according to claim 6, wherein said first, second and third filter parts (114a, 114b and 114c) are blue, green and red filters, respectively. A color electrophotography apparatus comprising: a photosensitive member (11); a transfer device; a toner image bearing film (14) passing around said photosensitive member and moving past said transfer device; an electrifying device (15) provided facing said photosensitive member for uniformly electrifying the photosensitive member; an exposure device (16; 116c, 116d) for exposing the photosensitive member, successively to light image corresponding to a first color thereby to form an electrostatic latent image of said first color, to light image corresponding to a second color thereby to form an electrostatic latent image of said second color, and to light image corresponding to a third color thereby to form an electrostatic latent image of said third color; developing devices (17, 19, 21) for said first, second and third colors, for successively developing toner images of said first, second and third colors by adhering toner of said first color, toner of said second color, and toner of said third color, onto said toner image bearing film (14) when it is passing around said photosensitive member (11); means for moving said toner image bearing film (14) past said developing devices, and said transfer device; and means (12, 13, 41, 42, 43) for feeding recording paper (27) past said transfer device in synchronization with the movement of said toner images on said toner image bearing film past said transfer fixing device; said transfer device transferring said toner images of said first, second and third colors on the toner image bearing film onto the recording paper; said transfer device comprises a first roller (12), and a second roller (13) juxtaposed with said first roller, said toner image bearing film (14) passes around said first roller and between said first and second rollers, and said feeding means feeds said recording paper (27) between the toner image bearing film (14) on said first roller (12) and said second roller (13); characterized in that one of said first and second rollers is a heating roller (12) and said first and second rollers also serves to fix the toner image on the recording paper; said feeding means (12, 13, 41, 42, 43) being arranged to reversibly move said recording paper (27) between successive transfer and fixing of the toner images of different colors; said first and second rollers (12, 13) being arranged to separate from each other during successive transfer and fixing of the toner images of different colors. Apparatus according to claim 8, wherein said toner image bearing film (14) is an endless film repeatedly passing around said photosensitive member (11) and moving past said transfer device. Apparatus according to claim 8, wherein said toner image bearing film (14) is transparent to the wavelengths of light of said exposure device; and said exposure device is provided facing said photosensitive member (11) through said toner image bearing film. Apparatus according to claim 8, wherein said second roller (13) can selectively assume a contact position in which it is pressed against said first roller (12) and a release position in which it is separated from said first roller. Apparatus according to claim 8, wherein said first roller (12) is a heating roller and said second roller (13) is a pressure roller. Apparatus according to claim 12, wherein said pressure roller (13) is pressed against said heating roller (12) when said toner image bearing film (14) bearing the toner image of each color passes between said heating roller and said pressure roller; said feeding means moves the recording paper (27) backward between the transfer of the toner image of one color and the transfer of the toner image of another color; and said pressure roller is separated from said heating roller when said recording paper is moved backward; whereby the toner images of the respective colors are successively transferred to and fixed on the recording paper. Apparatus according to claim 13, further comprising a cleaner (23) for removing any residual toner on the toner image bearing film (14) after the transfer of the toner image to the recording paper (27). Apparatus according to claim 8, wherein said first roller is a pressure roller and said second roller is a heating roller. Apparatus according to claim 8, wherein said toner image bearing film (14) has a first filter part (114a), a second filter part (114b), and a third filter part (114c) which are successively formed along the length of said toner image bearing film; said exposure device (116c, 116d) is provided facing said photosensitive member (11) through said toner image bearing film, for successively exposing said photosensitive member through said first filter part to form a latent image of a first color, through said second filter part thereby to form a latent image of a second color, and through said third filter part thereby to form a latent image of said third color; said first, second and third filter parts are filters of colors complementary to said first, second and third colors, respectively; said developing devices (17, 19, 21) successively develop toner images of said first, second and third colors by adhering toner of said first color onto said first filter part, toner of said second color onto said second filter part, and toner of said third color onto said third filter part; and said feeding means feeds (41, 42, 43) the recording paper (27) past said transfer device in synchronism with the movement of said first filter part, said second filter part and said third filter part past said first roller (12), and moves the recording paper backward after the transfer of the toner image from said first filter part and after the transfer of the toner image from said second filter part; whereby the toner images of said first, second and third colors are successively transferred to the recording paper. Apparatus according to claim 8, further comprising means (50) for selectively activating the developing devices (17, 19, 21) in accordance with the color of the electrostatic latent image which is then formed on said photosensitive member (11).

Inventors: ITOH SHINICHI C O OKI ELECTRIC, ITOH, SHINICHI,C/O OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1990
Main classification: G03G15
IPC codes: G03G15