Color image forming apparatus

Abstract: A color image forming apparatus, in which a belt-shaped image carrier (7) is stretched and conveyed around rotating rollers (3,4) on which image exposure is conducted by a plurality of exposure units (20,21,22,23), which are disposed on a plurality of step-formed supporting surfaces (42,52,62,72) below the lower surface of the image carrier (7). The stepped formation (42,52,62,72) of the exposure unit supporting surfaces avoids interference between optical paths of laser beams for image exposure, and facilitates compactness of the apparatus. The belt-shaped image carrier (7) and the supporing surfaces are provided at a relative angle of 5-30 degrees.

Description: The invention relates to a color image forming apparatus in which a toner image is formed on an image forming body by means of electrophotography and the toner image is transferred onto a transfer sheet so that the image is obtained, and more particularly relates to a color image forming apparatus such as a printer, copier and facsimile in which a plurality of charging means, exposure means and developing means are provided around the image forming body and a color image can be obtained when the image forming body is rotated by one revolution. A large number of methods and apparatus have been conventionally proposed to form a color image by means of electrophotography. From the viewpoint of the accuracy of the color register and the resolution of the color image, techniques disclosed in JP-A-75850/1985, JP-A-76766/1985, and JP-A- 95456/1985 are excellent. According to the aforementioned techniques, a color image can be obtained in such a manner that a latent image forming process according to the number of separate colors of a document image and a developing process are repeatedly conducted on a drum-shaped image forming body so that toner images of each color can be superimposed on the surface of the aforementioned image forming body, the color toner image is transferred onto a transfer sheet, and the transferred image is fixed. In the aforementioned techniques, only one optical laser device is used as exposing means to form a latent image in accordance with the number of separate colors, and a color image is formed on the drum-shaped image forming body when it is rotated a plurality of times. Instead of the aforementioned drum-shaped image forming a belt-shaped image forming body is disclosed in JP-A- 102261/1987, in which a photoconductive material is coated or vapor-deposited on a flexible belt. This belt-shaped image forming body is provided between rotating rollers including a drive roller, so that space is effectively utilized and the apparatus can be made compact. Accordingly, the belt-shaped image forming body is very effective when a color printer, for example, is designed. However, in order to form a color image on the belt-shaped image forming body using only one optical laser system provided on the periphery of the belt-shaped image forming body, it is necessary to rotate the aforementioned belt-shaped image forming body a number of times, which increases the time and cost of the copying operation. In order to solve the aforementioned conventional problem, a color image forming apparatus can be considered which is characterized in that a plurality of optical laser units are disposed in parallel below the belt-shaped image forming body, and a color image is formed by one revolution of the aforementioned image forming body. With such color image forming apparatus, it is not necessary to rotate the belt-shaped image forming body a plurality of times, so that a color image can be formed like that of a black and white image. Consequently, such apparatus is advantageous in that the copying time is shortened, the life of the apparatus is prolonged, and the apparatus can be operated easily. However, the following problems are caused in such color image forming apparatus: (1) Wide space is necessary to install the plurality of optical laser systems, so that the size of the apparatus becomes large, (2) In order to form a color image, high accuracy of color register is required. However, in case of an aforementioned apparatus, optical laser systems are dependently provided, so that adjusting operations such as positioning for writing-in timing by scanning light, positioning of each optical laser unit, parallelism of scanning light, and adjustment of scanning width, become very complicated, so that it is actually difficult to conduct adjustment of high accuracy. Furthermore, in order to solve the a.m. conventional problem a color image forming apparatus is also known (FP-A-0 090 595) in which a plurality of optical laser units are disposed radially around a cylindrical image forming body so that a color image is formed by one revolution of the cylindrical image forming body. However, such color image forming apparatus has the same problems as the color image forming apparatus having a belt shaped image forming body, i.e. wide space is necessary to install the plurality of optical laser systems and installation of the optical laser systems is difficult so that clear color images of high resolution cannot be formed stabily. The object of the present invention is to provide a color image forming apparatus which is compact and which can stably form a clear color images of high resolution. In order to solve this object a color image forming apparatus is proposed having the features of claim 1. Improvements of this apparatus are subject matter of the dependent claims. The invention provides improvements in arrangement and structure of the plurality of optical laser units. The color image forming apparatus of the present invention is composed in such a manner that a belt-shaped image forming body is wrapped around rotating rollers and image exposure is conducted on the belt-shaped image forming body by a plurality of optical laser units, wherein the plurality of optical laser units are disposed on a plurality of step-like supporting surfaces in parallel with each other. In the color image forming apparatus of the present invention, the first set of image forming means having a charger for Y-toner image formation and a developing unit including Y-toner, the second set of image forming means to form an M-toner image, the third set of image forming means to form a C-toner image, and the fourth set of image forming means to form a BK-toner image, are installed in parallel with each other below the lower surface of the belt-shaped image forming body provided between a drive r ller and an idle roller in order to form a toner image of one color at each revolution of the image forming body. While conventional laser units are disposed vertically or radially, each optical laser unit is disposed horizontally in this structure, wherein the optical laser units are installed on the step-like supporting surfaces in parallel with each other so that the laser beams emitted from each optical laser system cannot interfere with each other. The optical laser devices and the support surfaces are preferably integrated into one unit so that a detachable optical unit can be formed, which can be easily attached to and detached from the apparatus body for maintenance or renewal. When the apparatus is structured in the manner mentioned above, before the optical unit is assembled into the apparatus body, each optical laser device can be precisely set onto the support surface so that conditions such as writing-in timing, parallelism of beams and beam width can be satisfied. When the optical laser device is renewed, it is not necessary to reset the positioning of each optical system, the parallelism of scanning beams and the scanning width. All the operator has to do is to set a new standard unit. The optical device in the optical unit is detachably engaged with a groove on the support surface and pushed by a spring so that it can be closely contacted with the support surface. When necessary, the optical device is replaced. The optical devices preferably meet the same requirement so that they can be interchangeable. In order to avoid interference between the optical paths of the laser beams for image exposure, the optical unit including the optical laser device is preferably installed at an angle of 5 to 30°, and more preferably at an angle of 10 to 20°, with regard to the lower surface of the belt-shaped image forming body. In the manner described above, a plurality of optical laser devices can be effectively assembled into a small space in the apparatus body. It is preferable that the belt-shaped image forming body, the plurality of developing units, the charger and the cleaning unit are integrally formed into one unit so that they can be integrally attached to and detached from the apparatus body. The paper conveyance system is composed of a paper feeding cassette, a paper feeding roller and a paper conveyance passage. In order to make the apparatus compact, it is preferable that the paper feeding cassette and the paper feeding roller are provided below the optical unit and the paper conveyance passage is provided on the side of the optical unit. IN THE DRAWINGS Fig. 1 is a sectional view of a color printer of one embodiment of the present invention, Fig. 2 is a plan view showing the arrangement of the optical laser devices of the printer as shown in Fig. 1, Fig. 3 is a perspective view showing the Y-optical laser device of the color printer as shown in Fig. 1, and Fig. 4 is a time chart of the image forming process. The color printer shown in Fig. 1 is composed of a writing system B to conduct image exposure on a photoreceptor according to external signals A, an image forming system C, a paper feeding system D, and a paper discharging system E including a fixing unit. The image forming system C comprises a photoreceptor unit 2 which is detachably provided to the apparatus body 1, wherein the photoreceptor unit 2 can be detached form the apparatus body upward in Fig. 1. The photoreceptor unit 2 comprises a belt shaped photoreceptor 7 which is provided between a drive roller 3 on the transfer side and an idle roller 4 on the cleaning side, and which is guided by a guide plate 5 provided on the tension side and a guide roller 6 provided on the loose side; chargers 10, 11, 12 for Y, M, C which are provided below the photoreceptor 7; developing units 20, 21, 22, 23 for Y, M, C, BK; a pre-cleaning exposure lamp 8; a discharger 9; and a cleaning unit 25, wherein they are integrally provided in the photoreceptor unit 2. The cleaning unit 25 includes a cleaning blade 26 and a cleaning brush 27 for removing toner which remains on the photoreceptor surface while the blade is separated from the surface. A screw conveyor 29 is also provided which conveys the toner scraped off by the cleaning blade 26 and the brush 27 to a used toner recovery chamber 28 installed between the drive roller 3 and the idle roller 4 in the photoreceptor unit 2. The photoreceptor unit 2 is attachable to and detachable from the apparatus body 1 in such a manner that the photoreceptor unit 2 is guided by guide rods 30, 31, one end of which is supported by the apparatus body, and which penetrate the aforementioned guide plate 5 to guide the belt-shaped photoreceptor. An optical unit 40 is horizontally disposed below the aforementioned photoreceptor unit 2, wherein the optical unit 40 is detachably provided on the right side in the drawing. The optical unit 40 is composed in such a manner that optical devices 41, 51, 61, 71 for Y, M, C, BK having the same structure, are laterally disposed on horizontal step-like support surfaces 42, 52, 62, 72, so that image exposure can be accurately conducted on the photoreceptor in sequence being timed to the image formation. In order to conduct image exposure, the photoreceptor unit 2 and the lower surface of the belt-shaped photoreceptor 7 are inclined by an angle of 15° with regard to the optical unit 40 which is horizontally installed. The Y-optical device 41 comprises a polygonal mirror 43a, a support stand 43b for the polygonal mirror 43a, a high speed motor 43c, an f- lens 44a, a support stand 44b for the lens 44a, a mirror 45, a cylindrical lens 46, a dust protective glass 47, and a semi-conductor laser unit 48. In the same manner as described above, the M-optical device 51, C-optical device 61, and BK-optical device 71 comprise polygonal mirrors 53a, 63a, 73a, support stands 53b, 63b, 73b, motors 53c, 63c, 73c, f- lens 54a, 64a, 74a, support stands 54b, 64b, 74b, mirrors 55, 65, 75, cylindrical lens 56, 66, 76, dust protective glass 57, 67, 77, and semi-conductor laser units 58, 68, 78. The optical unit 40 is detached from the apparatus body 1 guided by an upper guide 81 and a lower guide 82, when a handle 80 is pulled by an operator. When the optical unit 40 is attached to the apparatus body 1, it is inserted into the apparatus body 1 until the tip of the optical unit 40 comes into contact with a stopper 83. Fig. 2 and Fig. 3 are provided to help a viewer to understand the structure of the optical unit 40. Fine adjustment of parallelism of the image exposure scanning can be conducted by the mirrors 45, 55, 65, 75. The paper feeding system D is provided below the optical unit 40 and on the left side in Fig. 1. The paper feeding system D includes a paper feeding cassette 90, a paper feeding roller 91, a transfer paper conveyance passage 92, conveyance rollers 93, and a transfer electrode 94. The paper discharging system E is provided in the upper position of the transfer paper conveyance passage 92. The paper discharging system E comprises a fixing unit 96 including fixing rollers 95a, 95b, paper discharging rollers 97, a paper discharging passage 98, and a paper discharging tray 99. Referring to Fig. 1 and Fig. 4, the image forming using the aforementioned color printer will be explained as follows: First, when start switch SW is turned on, the belt-shaped photoreceptor 7 is conveyed in the direction of an arrow at a predetermined peripheral speed V. The chargers 10, 11, 12, 13 are simultaneously turned on in the first timing so that a uniform electrical charge is given on the surface of the photoreceptor 7. Then, the developing units 20, 21, 22, 23 are simultaneously turned on in the second timing. Right after that, Y-image exposure is conducted by a semi-conductor laser beam through the Y-laser optical device 41 according to an Y-signal so that a latent image is formed on the photoreceptor, and then the Y-toner image is formed by the Y-developing unit 20. The photoreceptor 7 on which the Y-toner image is carried, passes the M-charging unit 11, and is charged again. In this case, the distance between the image exposure positions on the surface of the photoreceptor 7 is defined as H. M-image exposure is conducted by the optical device 51 according to an M-signal a predetermined time, H/V seconds, later than the previous Y-image exposure, and an electrostatic latent image is formed on the same region on the photoreceptor as the aforementioned Y-toner image, and then the electrostatic latent image is developed by the M-developing unit 21 so that the M-toner image is formed on the aforementioned Y-toner image. After that, the photoreceptor 7 is charged again by the charger 12, and then C-image exposure is conducted on the surface of the photoreceptor 7 by the optical device 61 according to a C-signal H/V seconds later than the previous M-image exposure so that an electrostatic latent image is formed. This electrostatic latent image is developed by the C-developing unit 22, and a C-toner image is formed on the Y-toner image and the M-toner image. In the same manner as described above, a BK-image exposure is conducted by the optical device 71 according to a BK signal, and the thus obtained latent image is developed by the BK developing unit 23 so that a BK-toner image can be superimposed on the toner images previously formed. A DC bias, the potential of which is slightly lower than the charging potential on the photoreceptor 7, and an AC bias are impressed upon the sleeves of the developing units 20, 21, 22, 23. Then, the toner image is developed by means of a non-contact reversal development system using a two-component type of developer suitable for color development, and a dot-shaped color toner image is formed. The color toner image formed on the surface of the belt-shaped photoreceptor 7 is transferred onto the surface of a transfer sheet by the action of the transfer electrode 94. The transfer sheet is fed from the paper feeding cassette 90 by the paper feeding roller 91, and conveyed by the conveyance roller 93 through the conveyance passage 92 so that the conveyance of the transfer sheet can be timed to the image formation. The image transferred onto the transfer paper is thermally fixed by the thermal roller 95a and the press roller 95b provided in the fixing unit 96 so that a thermally fixed color image is formed. The transfer sheet onto which the color image has been transferred is conveyed by the discharging rollers 97 through the discharging passage 98 and discharged onto the discharging tray 99. The timing of the image formation is shown in the timing chart of Fig. 4. After transfer, the surface of the photoreceptor 7 is cleaned the blade 26 and cleaning brush 27 of the cleaning unit 25 in order to prepare it for the following image formation. The cleaning blade 26 and the cleaning brush 27 are turned on right after the printer has been started, and turned off after the color image has been formed, wherein the cleaning brush 27 is turned on later than the cleaning blade 26. The aforementioned image forming was repeated using the aforementioned color printer, and 100,000 prints were made. In the middle of the printing operation, the quality of images and accuracy of image superimposition were deteriorated due to the fatigue of the carrier in the developer and the photoreceptor, so that the photoreceptor unit was renewed when 50,000 prints were made. When 100,000 prints were made, consideration was given to the deterioration of accuracy of superimposition, and the optical laser unit 40 was replaced. When the optical laser unit 40 was replaced, it was pulled out-to the right in Fig. 1 by the handle 80, and a new unit of the completely same structure was set and fixed to the apparatus with a lock spring 85, and then the next image formation was conducted. Images of high quality were obtained while adjusting operations of the optical devices 41, 51, 61, 71, were not necessary, and further adjusting operations of parallelism of the laser beams and scanning width were not necessary, either. In this embodiment, the present invention was applied to a color printer. However, the present invention may be applied also to a color copier in which a color scanner is provided to the upper portion of the color printer. As explained above, the color image forming apparatus of the present invention can provide the following effects: The composition of image forming means such as an image forming body, a charging unit, a developing unit and an optical laser unit for image exposure, is quite new, so that the size of the apparatus can be made small and compact and color images can be formed as quick as black and white images. The optical laser system is precisely positioned, so that the accuracy of color image registration is high and color images of high resolution can be stably formed. Further, when the parts of the optical laser system are replaced, the work can be accurately and easily conducted without adjusting the position of each optical laser unit and the parallelism and amplitude of exposure beams.

Claims: Color image forming apparatus, comprising a plurality of optical laser exposure means (41, 51, 61, 71) for exposing images onto an image carrying member (7) in an apparatus body (1), characterized in that , the image carrying member (7) is a belt-type image carrying member stretched around rotatable rollers (3, 4) on which said image carrying member is conveyed and that the plurality of exposure means (41, 51, 61, 71) are disposed on a plurality of step-formed parallel support surfaces (42, 52, 62, 72). Apparatus as in claim 1, wherein the plurality of step-formed parallel support surfaces (42, 52, 62, 72), on which the plurality of exposure means (41, 51, 61, 71) are disposed, are provided at a relative angle to the part of the belt-type image carrying member (7) surface where images are exposed. Apparatus as in claim 2, wherein the relative angle is larger than 5 degrees and less than 30 degrees. Apparatus as in one of claims 1 to 3, wherein the belt-type image carrying member (7), a plurality of developing means (20, 21, 22, 23), charging means (10, 11, 12, 13) and cleaning means (25) are integrally formed into one unit which can be attached to and detached from the apparatus body (1). Apparatus as in one of claims 1 to 4, wherein the plurality of step-formed parallel support surfaces (42, 52, 62, 72) and the plurality of exposure means (41, 51, 61, 71) disposed thereon are integrally formed into one optical unit (40) which can be attached to and detached from the apparatus body (1). Apparatus as in claim 5, wherein a paper conveyance system comprising a paper feeding cassette (90), a paper feeding roller (91) and a paper conveyance passage (92, 93) is arranged such that the paper feeding cassette (90) and the paper feeding roller (91) are provided below the optical unit (40) and the paper conveyance passage (92, 93) is provided on one side of the optical unit (40).

Inventors: FUKUCHI MASAKAZU, HANEDA SATOSHI, IKEDA TADAYASHI, MORITA SHIZUO, SATOH HISAO, FUKUCHI, MASAKAZU, HANEDA, SATOSHI, IKEDA, TADAYASHI, MORITA, SHIZUO, SATOH, HISAO
Applicants: KONISHIROKU PHOTO IND, KONICA CORPORATION
Application year: 1991
Main classification: G03G15
IPC codes: B41J2, H04N1, G03G15