Double-sided printing apparatus

Abstract: A double-sided printing apparatus for printing on both surfaces of a recording medium comprises a first image-forming unit 3 for forming the toner image on one surface of the recording medium, a second image-forming unit 4, provided downstream of the first image-forming unit 3, for forming a toner image on the other surface of the recording medium, a first flash fixing unit 50 for fixing the toner image on the first surface of the recording medium and a second flash fixing unit 51, provided downstream of the first fixing unit, for fixing the toner image on the other surface of the recording medium. An intermediate member 71 is provided between the first fixing unit and the second image-forming unit. This intermediate member may be a shielding member for preventing the light from the first fixing unit from reaching a photo-sensitive body of the second image-forming unit 4, and/or a guide member for guiding the recording medium; in the latter case a charger 40 for charging the recording medium P to prevent offset of the as yet unfixed image onto the guide member may also be provided.

Description: The present invention relates generally to a double-sided printing apparatus for printing on double (i.e. both) sides of a recording medium and, more particularly, to a double-sided printing apparatus in which a plurality of electrophotographic recording units are disposed. Printers are widely utilized as an output apparatus for a computer. Increasingly such printers are in the form of electrophotographic apparatus capable of printing on an ordinary sheet of paper. In response to a demand for saving natural resources in recent years, a double-sided printing apparatus for printing on both sides of the sheet has been required. Then, an apparatus provided with both a printing mechanism for printing on the right side of the recording medium and a printing mechanism for printing on the reverse side of the recording medium, is required for increasing the printing speed. FIG. 7 is an explanatory view showing a prior-art apparatus. This double-sided printing apparatus includes an electrophotographic printing unit (a second image-forming unit) 91 for printing on the right or recto side of a sheet of recording paper P, and an electrophotographic printing unit (a first image-forming unit) 92 for printing on the reverse or verso side of the sheet P. The sheet P is formed as continuous paper perforated to delimit each page. The reverse-side printing unit 92 has a photosensitive drum. The photosensitive drum is charged by a pre-charger and thereafter exposed to a light image by an exposing unit. An electrostatic latent image corresponding to the light image is thereby formed on the photosensitive drum. The latent image on the photosensitive drum is then developed by a developing unit. The developed image on the photosensitive drum is transferred onto the sheet P by a transfer unit. Thus, the image is printed on the reverse side of the sheet P. Printing on the right side of the sheet P is performed similarly. To be specific, the right side printing unit 91 has a photosensitive drum. The photosensitive drum is charged by a pre-charger and thereafter exposed to a light image by an exposing unit. An electrostatic latent image corresponding to the light image is thereby formed on the photosensitive drum. The latent image on the photosensitive drum is developed by a developing unit. Subsequently, the developed image on the photosensitive drum is transferred onto the sheet P by a transfer unit. Thus, the image is printed on the right side of the sheet P. Next, the toner image on the reverse side of the recording sheet P is fixed by a first fixing unit 93. Then, the toner image on the recto side of the recording sheet P is fixed by a second fixing unit 94. Thus, in the double-side printing process, after the toner image has been formed on the recto side, the toner image is formed on the reverse side, and then the fixing process is carried out. In this way the double-sided printing apparatus can be reduced in size. This type of double-sided printing apparatus for printing on continuous paper is disclosed in EP-A-866 384, in Japanese Patent Application Laid-Open Publications Nos. 7-77851 and 8-211664, corresponding to EP-A-631 204 and US-A-5 848 323, resp. There arise, however, the following problems inherent in this prior art. First, flash fixing units for fixing by a flash of light are generally used as the fixing units 93, 94, so that the image can be fixed in a non-contact manner onto the sheet. Therefore, even when unfixed images are fixed in a continuous feeding process, the unfixed images are never disturbed. The flash has, however, a high intensity, and hence leaked flash beams may strike the photosensitive drum of the electrophotographic printing unit, resulting in possible deterioration of the photosensitive drum. Especially, the flash beams from the first fixing unit 93 may impinge on the photo-sensitive drum of the second electrophotographic printing unit 91 through the sheet P, and the leak may also lead to deterioration of this photosensitive drum. Second, it is necessary for stabilising transport of the sheet that a guide member is provided between the first fixing unit and the second electrophotographic printing unit 91. The guide member is, however, brought into contact with the unfixed image on the sheet, and consequently the toner image is offset by the guide member, with the possible result of disturbance of the unfixed image on the sheet. It is therefore desirable to provide a double-sided printing apparatus capable of preventing deterioration of a photoconductive body of an image-forming unit even when a flash fixing unit used. It is also desirable to provide a double-sided printing apparatus capable of preventing disturbance of an unfixed image whilst still providing a guide member. According to the invention, a double-sided printing apparatus comprises a first image-forming unit for forming a toner image on the first surface of the recording medium, a second image-forming unit, provided downstream of the first image-forming unit, for forming a toner image on the second surface of the recording medium a first fixing unit for fixing the toner image on one surface of the recording medium (preferably the first surface), a second fixing unit, provided downstream of the first fixing unit, for fixing the toner image on the other surface of the recording medium (preferably the second surface), a guide member, provided between the first fixing unit and the second image-forming unit, for guiding the recording medium, and a charger for charging the recording medium to prevent offset of an unfixed image onto the guide member. In such a double-sided printing apparatus according to this further aspect of the invention, the provision of the guide member for guiding the recording medium between the fixing unit and the image-forming unit allows stabilisation of the transport of the recording medium in the fixing unit. The guide member comes into contact with the unfixed image on the recording medium, and the unfixed image might therefore adhere to the guide member. The charger may therefore attract the unfixed toner image on the recording medium more securely onto the recording medium, thereby preventing the unfixed image on the recording medium from adhering to the guide member. In some embodiments the guide member acts as a shielding member, provided between the first fixing unit and the second image-forming unit, for preventing light from the first fixing unit from reaching the photoconductive body of the second image-forming unit. In this way, the shielding member for cutting off the light from the first fixing unit may be provided between the closer second image-forming unit end the first fixing unit. Therefore, even when the first fixing unit is a flash fixing unit for fixing by use of a flash of light, it is feasible to prevent the flash from impinging upon the photosensitive body of the second image-forming unit. The photosensitive body of the image-forming unit can thereby be prevented from deteriorating. Further, even when a fixing unit of the non-contact type is provided as the first fixing unit, the recording medium can be stably transported. Preferably, the guide roller contacts the other (i.e. second) surface of the recording medium. Advantageously, the guide roller rotates at the same speed as the transport speed of the recording medium. The double-sided printing apparatus may further include a cleaning member for cleaning the guide roller. According to an advantageous embodiment of the invention, the charger applies to the recording medium an electric charge having an opposite polarity to the polarity of the toner image on the recording medium. The charging current of the charger is preferably set to fall within the range 200µA to 1200µA. Additionally or alternatively, the double-sided printing apparatus further comprises a control unit for controlling the set value of the charger in accordance with the environment and/or the thickness of the recording medium and/or a development condition. Other features and advantages of the present invention will become readily apparent from the following description taken in conjunction with the accompanying drawings. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to illustrate of the invention: FIG. 1 is a view showing a construction of a double-sided printing apparatus in one embodiment of the present invention; FIG. 2 is a partial enlarged view of the construction in FIG. 1; FIG. 3 is a characteristic diagram showing an offset quantity of a charger in FIG. 1; FIG. 4 is a characteristic diagram showing an Optical Density (OD) value of the charger in FIG. 1; FIG. 5 is a control block diagram of the charger in FIG. 1; FIG. 6 is a view showing a construction of another embodiment of the present invention; and FIG. 7 is an explanatory diagram of the prior art. FIG.1 illustrates a double-sided printing apparatus for effecting prints on both sides of a continuous sheet having feed perforations. A hopper 1 is stacked with unprinted continuous sheet P. The continuous sheet P is perforated to delimit each page. A sheet carrier 2 engages with the feed perforations of the continuous sheet P and thus carries the continuous sheet P in the direction of the arrow. A reverse side printing mechanism (a first image-forming unit) 3 is constructed as an electrophotographic printing mechanism, and prints on the reverse side of the continuous sheet P. This reverse side or verso printing mechanism 3 includes a photosensitive drum 37, a charging unit 30 for charging the photosensitive drum 37, and an LED head 31 for exposing the photosensitive drum 37 to a one-line light image. The LED head 31 is composed of an LED array in which LEDS (light-emitting diodes), in a number corresponding to one complete line, are arrayed. A developing unit 32 develops a latent image on the photosensitive drum 37. The developing unit 32 is constructed as a two-component developing unit for developing with a two-component developer. A transfer charging unit 33 transfers the developed image on the photosensitive drum 37 onto the continuous sheet P. A transfer guide roller 34 presses the continuous sheet P against the photosensitive drum 37 during the transfer process. A cleaner 35 collects residual toner from the photosensitive drum 37. A de-electrifying lamp 36 removes any residual potential out of the photosensitive drum 37. Downstream of the first there is a second, recto, printing mechanism (a second image-forming unit) 4, likewise composed of an electrophotographic printing mechanism, which implements the printing on the right or obverse side of the continuous sheet P. This recto printing mechanism 4 includes a photosensitive drum 47, a charging unit 40 for charging the photosensitive drum 47 with electricity, and an LED head 41 for exposing the photosensitive drum 47 to a one-line light image. This LED head 41 is composed of an LED array in which LEDS, in a number corresponding to one line, are arrayed. A developing unit 42 develops the latent image on the photosensitive drum 47. The developing unit 42 is constructed as a two-component developing unit for developing with two-component developer. A transfer charging unit 43 transfers the developed image on the photosensitive drum 47 onto the continuous sheet P. A transfer guide roller 44 presses the continuous sheet P against the photosensitive drum 47 during the transfer process. A cleaner 45 collects residual toner from the photosensitive drum 47. A de-electrifying lamp 46 removes any residual potential from the photosensitive drum 47. A neutralisation charging unit 70 is provided between the verso printing mechanism 3 and the recto printing mechanism 4, and neutralises the electrical potential on the recto side of the continuous sheet P, assuming the electric potential through the reverse side printing mechanism 3. The transferring operation can thereby be performed with stability in the recto printing mechanism 4. A guide roller 71 is provided to stabilise the behaviour of the sheet P between the recto printing mechanism 4 and the fixing units 50 and 51, which follow the printing units and are described below. The guide roller 71 is provided to the side and a small distance downstream of the photosensitive drum 47 of the recto printing mechanism 4. The guide roller 71 therefore guides the sheet and prevents the light from the fixing unit 50 from impinging upon the photosensitive drum 47. A charger 72 is provided between the recto printing mechanism 4 and the guide roller 71, and applies to the sheet P an electric charge of a polarity opposite to that of the electric charge of the toner image on the sheet P. The force of constraint or attachment of the unfixed toner image with respect to the sheet P is thereby amplified. This makes it feasible to prevent the unfixed image on the sheet P from adhering to the guide roller 71. The fixing unit is constructed of a pair of flash fixing units 50, 51. The first flash fixing unit 50 is provided on the reverse side of the sheet P, and fixes the toner image on the reverse side of the sheet P by a flash. The second flash fixing unit 51 is provided on the right side of the sheet P, and fixes the toner image on the right side of the sheet P by a flash. A folding roller unit 66 for folding the sheet P is provided between the flash fixing units 50 and 51. The sheet P thus undergoes a change of direction of about a right angle, from vertical to horizontal, between the two fixing units. A stacker 6 is stacked with the printed continuous streets P. Scuff rollers 63, 64, 65 guide the sheet P to the stacker 6 from the fixing unit. A swing guide 60 swings to assist the folding of the sheet P. Impellers 61, 62 assist the folding of the sheet P. In this double-sided printing apparatus, the verso printing mechanism 3 starts printing in advance of the recto printing mechanism 4 when in double-sided printing mode. Further, the transport path is vertical, and the verso and recto printing mechanisms 3, 4 are provided with this path between them. The footprint of the double-sided printing apparatus can therefore be reduced. FIG. 2 is a partial enlarged view showing the guide roller 71. The guide roller 71 is provided on the right side of the sheet P and is rotatable. This guide roller 71 guides the sheet P from the photosensitive drum 47 of the right side printing mechanism 4. The guide roller 71 is provided in such a position as to stabilize the behaviour of the sheet P in the flash fixing unit 50 positioned on the reverse side of the sheet. Further, the guide roller 71 is also disposed in such a position as to prevent the flash light from the flash fixing unit 50 from impinging upon the photosensitive drum 47. It therefore functions both as a guide and as a shielding member. The guide roller 71 is rotated by a belt 75 using a motor 72a. The direction of rotation of the guide roller 71 is identical with the direction of transport of the sheet P. Further, the velocity of rotation of the guide roller 71 is approximately the same as the velocity of the sheet P. Hence, the guide roller 71 does not produce any resistance against the carrying movement of the sheet P. Accordingly, the unfixed toner image on the sheet P is never disturbed by the guide roller 71. Further, the unfixed toner image can be prevented from being offset and adhering to the guide roller 71. A cleaning blade 73 scrapes off any toner adhering to the guide roller 71. A collecting screw 74 collects the toner scraped off by the cleaning blade 73. By provision of the cleaning member for the guide roller 71 any toner adhered thereto can be scraped off, even when the toner from the sheet does adhere to the guide roller 71. Consequently, although the guide roller 71 guides the sheet with the unfixed toner image, it is feasible to prevent the toner image from being re-transferred onto the sheet P from the guide roller 71. Disturbance of the unfixed image on 5 the sheet P can be therefore prevented or at least minimised. Furthermore, the charger 72 is provided on the opposite side to the guide roller 71, the sheet P being sandwiched in therebetween, in a position just anterior to (upstream of) the guide roller 71. The charger 72 applies an electric charge having a polarity opposite to that of the electric charge of the toner image on the sheet P. The adherence of the unfixed toner image to the sheet can thereby be increased. It is therefore possible to prevent the unfixed toner image on the surface of the sheet P from being offset onto the guide roller 71. Moreover, the surface of the guide roller 71 may be covered with a low-friction material (e.g., a fluororesin). Its resistance against the sheet can thereby be reduced. This also helps to prevent the unfixed toner image from being offset onto the guide roller 71. The life-span of the guide roller 71 can thus be increased. Next, an optimum value of the charging current of the charger 72 will be explained. FIG. 3 is a characteristic diagram of offset quantity versus current. FIG. 4 is a characteristic diagram of an Optical Density (OD) value versus current. FIG. 3 shows what the offset quantity of the guide roller 71 measures when the charging current changes from 0µA to 1600µA in the construction in FIG. 2. In the construction in FIG. 2, a sheet bearing a predetermined quantity of toner images is carried. The charging current value of the charger 72 is varied and the quantity of the toner adhered to the guide roller 71 with respect to each charging current value is measured. The measured toner quantity is then divided by the predetermined quantity, and the result indicated as a percentage. It can be seen from the results of this measurement that the offset quantity is as much as 10% when the charging current value is 0µA. The offset quantity is decreased to 4% when the charging current value is 200 µA. The offset quantity thereafter decreases again down to 2% as the charging current value increases to 600 µA and then remains constant with further increase in charging current value. This result shows that a rise in the charging current value over 200µA contributes to a reduction in the offset quantity. FIG. 4 shows what contamination on a blank area of the sheet is measured when the charging current changes from 0 µA to 1600µA in the construction in FIG. 2. In the construction in FIG. 2, a sheet bearing the toner images is transported. Then the charging current value of the charger 72 is varied and an OD (Optical Density) value on the blank area of the sheet is measured with respect to each charging current value. It has been shown from this measurement that the OD value of the blank area is substantially the same as for a state of the complete blank when the charging current value is 0 µA to 1200 µA. When the charging current value is 1400 µA or more, the OD value of the blank area rises. It can be presumed that; if the charging current value is large, the toner image on the sheet might be disturbed enough to over-transfer the toner onto the blank area. As a result of this, it has been shown that when the charging current value is under 1200 µA, no change in the OD value of a blank area is seen. It has been demonstrated from the results in FIGS. 3 and 4 that when the charging current value falls within a range of 200 µA to 1200 µA, the offset to the guide roller 71 can be minimised without disturbing the toner image. Given next is an explanation of how the charging current value of the charger is controlled. FIG. 5 is a block diagram of one embodiment of the present invention. Referring to FIG. 5, an operator initiates operation through a panel 10. The thickness (a consecutive quantity) of the sheet is input from the panel 10. A temperature detector 12 detects the temperature of the apparatus. A humidity detector 13 detects the humidity of the apparatus. A mechanism control unit 11 controls the respective units of the apparatus in accordance with indications given from a system control unit (not shown) as well as from the panel 10. The mechanism control unit 11 includes an MPU 14, a ROM 15, a RAM 16, an I/O port 17 and a D/A converter 18. A high-voltage control unit 21 controls the charging voltage applied to the charger 72 in accordance with a signal given from the mechanism control unit 11. The high-voltage control unit 21 includes a high voltage controller 19 for receiving an ON/OFF indication from the I/O port 17 and a control quantity from a D/A converter 18, and controlling the voltage value of a high voltage power supply 20. In this embodiment, the MPU 14 of the mechanism control unit 11 changes the charging voltage of the charger 72 in accordance with a sheet thickness indication from the panel 10. For example, when the sheet thickness is small, the charging voltage is decreased. When the sheet thickness is large, the charging voltage is increased. Further, the MPU 14 changes the charging voltage of the charger 72 in accordance with the detected temperature from the temperature detector 12. For instance, when the temperature is low, the charging voltage is increased. When the temperature is high, the charging voltage is decreased. Similarly, the MPU 14 changes the charging voltage of the charger 72 in accordance with the detected humidity from the humidity detector 13. For example, when the humidity is low, the charging voltage is increased. When the humidity is high, the charging voltage is decreased. Thus, the charging voltage of the charger 72 is controlled in dependence on the ambient environment and the thickness of the sheet. Therefore, the charging voltage value can be set to an optimum value corresponding to these factors. Furthermore, the electric potential of the toner image on the sheet might change depending on developing conditions such as the surface potential of the photosensitive drum, exposure power, toner density, developing bias voltage and transfer current. Accordingly, the MPU 14 may receive set values of the developing conditions and control the set value of the charging voltage of the charger 72 according to these also. FIG. 6 is a diagram of a construction according to another embodiment of the present invention, showing in outline a variant construction of the guide roller assembly, in which a cleaning roller is used. Referring to FIG. 6, the guide roller 71 is formed as a roller rotationally driven to follow the sheet with which it is brought into contact. A cleaning roller 76 removes any toner adhered to the guide roller 71. Since, as before, the guide roller 71 rotates substantially at the same rotating speed as the sheet P it does not produce any resistance against the movement of the sheet. Accordingly, offset of the toner image on the sheet can be prevented. Further, the cleaning roller 76 cleans toner off the guide roller 71, and hence toner adhered to the guide roller 71 can be removed. Abrasion maintenance of the guide roller 71 may involve referring to the contents of, e.g., a drum counter for measuring the time of rotation of the photosensitive drum and, of a print charge counter for managing the number of prints. Then, with reference to these contents, when the total rotation of the guide roller 71 reaches a desired value, an exchange message is displayed to prompt the user to replace it. In addition to the embodiments discussed above, the apparatus may also be modified as follows: (1) The electrophotographic mechanism using the photosensitive drum has been exemplified as a printing mechanism, but the use of other photosensitive bodies as a printing mechanism for forming the toner images may also be envisaged; (2) Flash fixing units were used as the fixing unit; however, other fixing units such as a heat roller fixing unit may also be usable; (3) The image on the second surface was fixed after the image on the first surface in the examples; however, it may alternatively be fixed before (upstream of) the image on the surface; (4) The guide roller was shown adjacent to the second surface; however, it may be situated on either side of the recording medium. Furthermore, a simple light-blocking member, having no guiding function may be provided instead of the guide rollers. As discussed above, significant features and effects of embodiments of the present invention are as follows: (1) When fixing the toner images on the obverse and reverse sides, the shielding member is provided between the second image-forming unit and the first fixing unit. Therefore, even when the first fixing unit is provided with a flash fixing unit, it is feasible to prevent the flash light from impinging upon the photosensitive body of the second image-forming unit. The photosensitive body of the image-forming unit can thereby be prevented from being deteriorated. (2) Further, the guide member for guiding the recording medium is provided between the fixing unit and the image-forming unit. With this arrangement, it is possible to stabilize the transport or carrying movement of the recording medium in the fixing unit. The charger is provided for attracting the unfixed toner image on the recording medium, thereby preventing the unfixed image on the recording medium from being adhered to the guide member.

Claims: A double-sided printing apparatus for printing on both surfaces of a recording medium (P), comprising: a first image forming unit (3) for forming a toner image on the first surface of said recording medium; a second image forming unit (4), provided downstream of the first image-forming unit, for forming a toner image on the second surface of the recording medium (P); a first fixing unit (50) for fixing the toner image on one surface of the recording medium (P); a second fixing unit (51), provided downstream of the first fixing unit, for fixing the toner image on the other surface of said recording medium (P); a guide member (71), provided between the second image-forming unit and the first fixing unit, for guiding the recording medium (P); and a charger (72) for charging the recording medium (P) to prevent offset of an unfixed image onto the guide member. A double-sided printing apparatus according to claim 1, wherein the charger (72) applies to the recording medium (P) an electric charge having a polarity opposite to that of the toner image on the recording medium (P). A double-sided printing apparatus according to claim 1 or 2, wherein the value of the charging current of the charger (72) is set to fall within the range 200µA to 1200µA. A double-sided printing apparatus according to any preceding claim, further comprising a control unit (14) for controlling the set value of the charger (72) in accordance with an ambient environment, and/or in accordance with a thickness of the recording medium and/or in accordance with a developing condition of the first and second image-forming units. A double-sided printing apparatus according to any preceding claim, wherein the guide member is a guide roller. A double-sided printing apparatus according to claim 5, wherein the guide roller is provided on the second surface of the recording medium (P), and the charger (72) is provided on the first side of the recording medium. A double-sided printing apparatus according to claim 5 or 6, wherein the guide roller (71) rotates at a speed corresponding to the transport speed of the recording medium (P). A double-sided printing apparatus according to any of claims 5 to 7, further comprising a motor (72a) for rotating the guide roller. A double-sided printing apparatus according to any of claims 5 to 8, further comprising a cleaning member (73, 76) for cleaning the guide roller. A double-sided printing apparatus according to any of claims 5 to 9, wherein the guide roller (71) has a low-friction surface. A double-sided printing apparatus according to any preceding claim, wherein the first fixing unit (50) is a flash fixing unit and the second image-forming unit (4) includes a photo-sensitive body (47), the guide member (71) preventing light from the flash fixing unit reaching the photo-sensitive body. A double-sided printing apparatus according to any preceding claim, in which the recording medium is turned by about a right angle between the two fixing units (50, 51).

Inventors: ADACHI KATSUMI, WADA YOSHINORI, ADACHI, KATSUMI, WADA, YOSHINORI, Wada, Yoshinori, c/o Fujitsu Limited
Applicants: FUJI XEROX CO LTD, FUJI XEROX CO., LTD.
Application year: 2000
Main classification: G03G15
IPC codes: G03G15