MECHANISMS IN IMAGE RECORDING APPARATUS

Abstract: In image recording apparatus, for recording toner images on recording sheets, a gap between a sheet intruding guide (12) and a surface of a photoconductive drum (11) is kept to a predetermined value (A), within allowable tolerances, by positioning the sheet intruding guide (12) together with an image transcription unit (13) placed under the drum (11) and constantly urged (22) towards the drum surface so that spacing between the drum surface and the image transcription unit (13) is maintained constant. The sheet intruding guide (12) can be lifted up and held away from the image transcription unit when the drum (11) is to be removed, and then released and set back to its regular position mechanically when the drum or a replacement drum is put back in its regular position.

Description: The present invention relates to mechanisms in image recording apparatus and is particularly concerned with a sheet transferring mechanism of an image transfer unit provided in electrophotographic or electrostatic recording apparatus. Electrophotographic recording apparatuses, such as electric printers or duplicators, are widely used for electrically recording letters and/or images onto recording sheets. Electrophotographic recording apparatus, which will be called simply "recording apparatus" hereinafter, comprises, for instance, a photoconductive drum; a charger for electro-statically charging a surface of the photoconductive drum; a light irradiating unit for irradiating an optical image onto the surface of the photoconductive drum so that a latent image is formed on the surface of the charged photoconductive drum; a developing unit for developing the latent image by using toner so that a toner image appears on the surface of the photoconductive drum; an image transfer unit for transferring the toner image onto a recording sheet; a sheet transferring mechanism for transferring recording sheets into and out of the image transfer unit one by one, in synchronization with the rotation of the photoconductive drum; an image fixing unit for fixing the toner image transferred onto a recording sheet; and a cleaner for cleaning off toner left on the surface of the photoconductive drum after the transfer of the toner image onto a recording sheet has been completed. The sheet transferring mechanism, around or in the vicinity of the image transfer unit, includes a sheet delivery guide for guiding a recording sheet into the image transfer unit, and a sheet removal guide for guiding a recording sheet out of the sheet transfer unit. The sheet delivery guide is located at a sheet entrance of the image transfer unit for delivering a recording sheet into the image transfer unit and the sheet removal guide is located at a sheet exit of the image transfer unit for removing the recording sheet from the transfer unit after a toner image has been transferred onto the recording sheet. In the recording apparatus, a gap provided between the sheet delivery guide and the photoconductive drum and a gap provided between the sheet removal guide and the photoconductive drum are very significant. The values (dimensions) of these gaps are determined in consideration of various factors such as the speed of rotation of the photoconductive drum, the size and the physical properties of the recording sheets, a delivery point of the recording sheet to the rotating surface of the photoconductive drum, and a delivery angle of the recording sheet to the surface of the rotating photoconductive drum at the delivery point. If the gaps vary with rotation of the photoconductive drum so as to depart from allowable values, the tip and/or the end of the delivered recording sheet may "flutter" in the image transfer unit, causing the toner image transferred onto the recording sheet to lack uniformity, which is detrimental to the quality of the recorded image on the recording sheet. Furthermore, in recording apparatus, units such as the photoconductive drum and the cleaner may be unified into one unit called a "drum unit". This is done to allow users of the recording apparatus to easily exchange these units when some of the units are worn. Actually, the drum unit has to be exchanged periodically, particularly in consideration of the limited life of the photoconductive drum. However, the image transfer unit is difficult to include in the drum unit because dust such as the toner powder worn off from the surface of the photoconductive drum and sheet trash from recording sheets accumulate in the image transfer unit before the image transfer unit has reached the end of its useful life. This means that the image transfer unit must be periodically cleaned up, to maintain its correct operation. Accordingly, whenever the process unit (drum unit) is exchanged or the image transfer unit is cleaned, great attention must be paid to maintaining the gaps mentioned above within allowable limits, which is not always easy to do. Therefore, it would be of distinct benefit if the gaps can be always kept constant easily even when the exchange or the cleaning is carried out by users. A side view of an image transfer unit and a previously proposed sheet transferring mechanism around the image transfer unit are illustrated in Fig. 1. In Fig. 1, a photoconductive drum 1, which will be called simply a "drum 1" hereinafter, is rotated in direction D1 counterclockwise around an axis 1'. An image transfer unit 3 is placed just under the drum 1, providing a space 4, and a toner image on the drum 1 is transferred onto a recording sheet as the recording sheet passes through the space 4. The recording sheet is sent to the space 4 through a sheet delivery guide 2 composed of a guide frame 5 and a plastic film 6 fixed on the guide frame 5. The upper surface of the plastic film 6 is parallel to the axis 1' and the tip edge, toward the surface of the drum 1, of plastic film 6 is also parallel with the axis 1' and provides a gap A to the surface of the drum 1. A recording sheet sent to the upper surface of the plastic film 6 is guided to the rotating surface of the drum 1 through the gap A and rolled into the space 4 in such a manner that the recording sheet sticks fast to the rotating surface of the drum 1. Therefore, when a recording sheet arrives in the space 4, the toner image on the surface of the rotating drum 1 is transferred to the surface of the recording sheet by electrostatic force provided by the image transfer unit 3. The image transfer unit 3 for example is composed of :- a corona charger 8 which produces an electrostatic force by which the toner image on the surface of the rotating drum 1 is transferred onto the surface of a recording sheet; and a corona discharger 9 for discharging charge produced on the recording sheet so that the recording sheet having been stuck onto the surface of the drum 1 can be easily peeled off of the surface of the drum 1 after the transfer is completed. The corona charger 8 and the corona discharger 9 have wires 7 and 7' respectively for making them operate properly, and they are separated by a separating wall 77. On the upper edge of the separating wall 77, a sheet removal guide 10 consisting of a plurality of guide pieces is fixed, as is illustrated in the partial plan view of the image transfer unit 3 given in Fig. 2. The guide pieces are arranged in a plane parallel to the axis 1' aslant to the sheet transferring direction D₂ so that on opposite sides of a central line they are arranged in different directions slanted at the same angle to the direction D₂, as shown in Fig. 2. This is for removing a recording sheet from the sheet removal guide 10 straight along the direction D₂. In Fig. 1, the axis 1', the sheet delivery guide 2, the image transfer unit 3 and the sheet removal guide 10 are all fixed to a base frame, which is not depicted in Fig. 1, of the recording apparatus. Therefore, it is very hard to maintain the values of the gaps A and B with high accuracy, given that the drum 1 must be periodically removed to be exchanged or for cleaning the inside of the image transfer unit 3. For example, even though the gaps A and B are determined so as to be 0.5 mm ± 0.2 mm and 1.5 mm ± 0.3 mm respectively for obtaining the high quality of the transferred image on the recording sheet, it is very hard to set the gaps A and B with the above tolerances. This can be understood when it is considered that usually the eccentricity of the drum 1 is 0.15 mm, the positioning accuracy of the wire 7 or 7' is 0.1 mm, the positioning accuracy of the sheet delivery guide 2 is 0.1 mm, and furthermore there may be a slight variation of the radius of the drum 1. It cannot be said to be impossible to manufacture recording apparatus with such high accuracy; however, manufacturing cost becomes extremely high, and this high cost is of course unsatisfactory. Patent Abstracts of Japan, Vol. 6, No. 255, (P-162) [1133], 14th December 1982, (JP-A-57 151954) discloses electrophotographic recording apparatus having transfer corotron (10) which, together with a transfer paper conveyance unit (12) and a transfer separation unit (9) on which the corotron is mounted, can be positioned proximate the surface of the photosensitive drum (1) or can be positioned away from that surface, e.g. to allow clearance of a jam. Sheet guides (2, 3) are provided between which sheets are conveyed to the drum. When the corotron is moved away from the drum surface a lower sheet guide plate (3) is moved away from the drum by action of a slide link (7) from the plate (3) to the transfer separation unit (9). When the corotron is proximate the drum surface, the lower sheet guide plate (3) is positioned against that surface, urged by a spring 4. When a sheet is conveyed to the drum, the sheet is between the guide plate (3) and the drum, so there is a gap between guide plate and drum equal to the sheet thickness. EP-A-0 082 254 discloses a corona charger 34 which is positioned against a photosensitive drum 6 by pushing the corona charger 34 vertically towards the photosensitive drum 6 by means of a coil spring 37, so that guide rollers 38, 39 contact the photosensitive drum 6 in order to maintain a gap between the cylindrical surface of the photosensitive drum and corona charger 34 at a predetermined value. US-A-4 609 276 discloses a corona charger 6 which is provided in a fixed state in a fixed position in an image recording apparatus. A lower transfer guide 11, for guiding sheets to the photosensitive drum (3), is movable. A cam 19 can raise the guide 11 to the drum and lower the guide 11, away from the drum, to allow drum dismounting or mounting. According to the present invention there is provided a recording apparatus for transferring a toner image onto a recording sheet, the recording apparatus comprising:- a photoconductive drum rotatable about a drum axis, the drum having a cylindrical surface on which a toner image is produced when the apparatus is in use; image transfer means arranged to transfer the toner image from the cylindrical surface onto the recording sheet; and guide means arranged to guide a recording sheet to the image transfer means for transfer of the toner image to the sheet; characterised in that the apparatus comprises:- first positioning means arranged to urge the image transfer means towards the cylindrical surface of the drum so that an image transfer space between the cylindrical surface and said image transfer means is maintained at a predetermined value, within allowed tolerances, when the photoconductive drum is rotated, the toner image on the cylindrical surface being transferred onto the recording sheet whilst the recording sheet passes through the image transfer space as the drum is rotated; and in that the apparatus further comprises:- second positioning means effective, together with the image transfer means positioned by the first positioning means, to position said guide means, to maintain a gap between the cylindrical surface and said guide means to a predetermined value, within allowed tolerances, which value is greater than the recording sheet thickness. Recording apparatus, for example electrophotographic or electrostatic recording apparatus, embodying the present invention can provide for increased quality of the transferred image on a recording sheet, avoiding or reducing occurrence of faulty image transfer such as shear, luck or light-and-shade of the transferred image, even when removal of units such as a drum unit or the cleaning of an image transfer unit is carried out periodically. Recording apparatus embodying the present invention can provide for easier exchange of units such as a drum unit and easier cleaning of units such as an image transfer unit, without the need to pay particular attention to the quality of the transferred image on a recording sheet. Recording apparatus embodying the present invention can provide for improvement in relation to the task of exchanging units such as a drum unit or the task of cleaning units such as an image transfer unit, so that those tasks can be performed more easily than usual. In recording apparatus embodying the present invention, the gaps between a sheet delivery guide and a drum and between a sheet removal guide and the drum are maintained with high accuracy. To ensure high accuracy for those gaps, the following measures are adopted in an embodiment of the invention, in relation to a mechanism related to the sheet delivery guide, the image transfer unit and the sheet removal guide:- (1) The image transfer unit, in previously proposed apparatus fixed to a base frame of the recording apparatus, is made free from the base frame and arranged so as always to be urged towards the surface of the photoconductive drum of the apparatus, and is provided with rollers so that a space between the surface of the drum and the image transfer unit is always maintained constant during rotation of the drum even if the axis of the drum is eccentrically fixed to the base frame and/or the radius of the drum varies; wherein, pushing force urging the image transfer unit toward the drum is provided by means such as springs. A horizontal pedestal, referred to as a canopy top, is provided on a unit case of the image transfer unit. The sheet delivery guide is provided at an end part of a long arm mounted on an axis carried by the base frame, so that the sheet delivery guide tends to move (swing) downwards. A sliding face directed downwards is provided at an edge portion at the end of the long arm so that the sliding face always contacts a horizontally flat surface of the canopy top, urged by the weight of the long arm itself pivoting around the axis. In this way, the sheet delivery guide is always positioned, to maintain the gap between the surfaces of the sheet delivery guide and the drum with high accuracy and within allowable tolerances. (2) The sheet removal guide is fixed to the image transfer unit, so that the gap between the surfaces of the drum and the sheet removal guide can be kept constant. In this embodiment, the following measure is also adopted:- (3) When the drum or a drum unit including the drum is removed for cleaning the inside of the image transfer unit, the cleaning work can be carried out without disturbance (obstruction) due to the sheet delivery guide through the provision of an arm holder, by which the arm attached to the sheet delivery guide is held in place, on the base frame. This holder serves to keep the sheet delivery guide away from the image transfer unit. When the drum or the drum unit including the drum is reinstalled in its regular position, the arm is released from the holder so that the sheet delivery guide is set to its regular position mechanically with the reinstalling of the drum or the drum unit. By the introduction of measures (1) and (2) to the sheet transferring mechanism and the mounting mechanism of the image transfer unit, the gap between the surfaces of the drum and the sheet delivery guide and the gap between the surfaces of the drum and the sheet removal guide can be maintained at designated values within allowed tolerances. By the introduction of measure (3) to the recording apparatus, removal of the drum and cleaning of the image transfer unit can be carried out easily without the need to pay particular attention to the accuracy of the gaps. Reference is made, by way of example, to the accompanying drawings in which:- Fig. 1 is a schematic cross-sectional view of a conventional sheet transferring mechanism near to an image transfer unit of an electro-photographic recording apparatus; Fig. 2 is a schematic plan view of the conventional image transfer unit; Fig. 3 is a side view schematically showing internal structure of an electrophotographic recording apparatus having a sheet transferring mechanism according to a first embodiment of the present invention. Fig. 4 is a schematic plan view of the sheet transferring mechanism, near to the image transfer unit, according to a first embodiment of the present invention; Fig. 5 is a perspective view schematically illustrating the photoconductive drum and the image transfer unit which is urged towards the cylindrical surface of the photoconductive drum to contact that surface through the rollers; Fig. 6 is a cross-sectional side view schematically showing, to a larger scale, a portion of the sheet transferring mechanism according to a first embodiment of the present invention; Fig. 7(a) is a cross-sectional side view of the electrophotographic recording apparatus having a sheet transferring mechanism according to a second embodiment of the present invention, with an upper frame of the electrophotographic recording apparatus is opened; Fig. 7(b) is a cross-sectional side view of the electrophotographic recording apparatus having a sheet transferring mechanism according to a second embodiment of the present invention, with the upper frame closed; Fig. 8(a) is a perspective view of a holder for holding an arm attached to a sheet delivery guide provided at an entrance of the image transfer unit, according to a second embodiment of the present invention; Fig. 8(b) is a cross-sectional view of the arm attached to the sheet delivery guide when the arm is released from the holder; Fig. 8(c) is a cross-sectional view of the arm attached to the sheet delivery guide when the arm is held by the holder; and Fig. 9 is a perspective view for illustrating a pushing member provided on a unit case of a drum unit. The present invention primarily concerns a sheet transferring mechanism in recording apparatus. Thus, in relation to a first embodiment of the present invention, a sheet transferring mechanism of recording apparatus will be described in some detail with reference to Figs. 3 and 6. A second embodiment of the present invention, in which a mechanism for holding up the sheet delivery guide is provided, for making cleaning or exchanging of an image transfer unit or drum unit easy, will be described with reference to Figs. 7 and 8. In Figs. 3, 4 and 5, the same reference numerals designate the same or similar units or parts. Fig. 3 is a side view of the internal structure of recording apparatus, illustrating primarily the sheet transferring mechanism. In Fig. 3, a sheet delivery guide 12 provided at an entrance to an image transfer unit 13 is attached to an end of a long arm 20 so as to be rotatable around an axis 19 provided at the other end of the arm 20. The sheet delivery guide 12 has a plurality of guide pieces 18 on its surface for guiding a recording sheet to a photoconductive drum 11 which will be simply called a "drum 11" hereinafter. The guide pieces 18 lie in a transferring direction of the recording sheet as shown in Fig. 4. The image transfer unit 13 is provided so as to be movable up and down in a vertical radial direction of the drum 11, and has two pairs of rollers 21 which contact side regions 11' of the cylindrical surface of the drum 11 as indicated in Fig. 5; wherein, there is no latent image on the side regions 11' because a photoconductive film is not formed thereon. Fig. 5 is a perspective view of the drum 11 and the image transfer unit 13 which are shown separated. Of course, assembled in the apparatus, the unit 13 is urged towards the cylindrical surface of the drum 11, contacting the drum through the rollers 21. (In Fig. 5, the image transfer unit 13 is illustrated only by its casing. Structures in the casing are not illustrated, they are indicated only by a chain line rectangle, for the sake of simplicity. The same is true of Fig. 4. Further, in Fig. 5, pinch rollers 47 are not shown). Accordingly, the image transfer unit 13 is always positioned so that the space between the surface of the drum 1 and of the image transfer unit 13 is kept constant at a designated value (within allowed tolerances). This facilitates the obtaining of a high-quality transferred image on a recording sheet. In Fig. 6, a cross-sectional side view of the sheet transferring mechanism is partially illustrated. As shown in Fig. 6, the image transfer unit 13 is urged up towards the drum 1 from a base frame 391 of the recording apparatus through springs 22, and the image transfer unit 13 can move up and down guided by guide cylinders 23 in which the springs 22 are installed. As long as the rollers 21 contact the cylindrical surface of the drum 11, a sheet removal guide 24 attached to the image transfer unit 13 is positioned so that a gap B between the surfaces of the sheet removal guide 24 and the drum 11 is kept constant. The rollers 21 always contact the surface of the drum 11 because the force exerted by the springs 22 is sufficiently strong and the rotational speed of the drum 11 is sufficiently slow that contact is maintained. In the image transfer unit 13, the position of the wire 7 in the corona charger 8 with respect to the surface of the drum 1 is important to produce a high-quality transferred image on the recording sheet. The above positioning structure of the image transfer unit 13 is very effective to maintain the correct position of the wire 7 relative to the surface of the drum 11. The end of the sheet delivery guide 12 is supported by a canopy top 26 (a lip or flange providing a horizontal pedestal) attached to a unit case of the image transfer unit 13 and slides on the surface of the canopy top 26 in accordance with the motion of the image transfer unit 13. The rotational speed of the drum 11 is so low that the end of the sheet delivery guide 12 does not leave the canopy top 26. Therefore, as long as the image transfer unit 13 is positioned properly in accordance with the above positioning structure, the gap A between the surface of the sheet delivery guide 12 and the surface of the drum 11 can be set to an approximately constant value. In other words, the gaps A and B can be set simultaneously to a proper value within allowable tolerances. Consequently, in this recording apparatus embodying the present invention, an image is recorded on a recording sheet in accordance with the following procedure, referring to Fig. 3:- (1) The drum 11 is rotated counter-clockwise as shown by a rotational arrow mark D₃ and the surface of the drum 11 is charged by a charger which is not shown in Fig. 3. The latent image corresponding to an image to be recorded is formed on the surface of the drum 11 by a latent image forming unit 14 and the latent image is developed to a toner image by a developing unit 15. (2) On the other hand, the recording sheets are stocked in a sheet cassette unit 17 located under the base frame 391 as shown in Fig. 3. The sheet cassette unit 17 has a structure such that it can be drawn out of the recording apparatus in a leftwards direction in the plane of Fig. 3. The recording sheets are not depicted in Fig. 3, but actually they are set in the sheet cassette unit 17, so as to push downwards a receiving plate 171, as shown by broken lines. The receiving plate 171 is made of spring material so that the recording sheets set in the sheet cassette unit 17 are always urged upwards. Recording sheets are picked up one by one by a pick-up roller 27, so that a picked-up recording sheet jumps over a sheet stopper piece and is sent to a first sending roller 28 through a slit 45 and guide walls 17b and 39z. The recording sheet having arrived at the first sending roller 28 is driven by a pinch roller 28a so as to be sent to a second sending roller 30 through a slit 29 and a sensor 44. The recording sheet having arrived at the second sending roller 30 is driven by a pinch roller 47 so as to be sent to the image transfer region 31 through the sheet delivery guide 12 and the gap A. The sensor 44 is for sensing the recording sheet passing therethrough for controlling driving timing for the pick-up roller 27, the first sending roller 28 and the second sending roller 30. The control is performed in synchronization with the rotation of the drum 11 for correctly transferring the toner image onto the recording sheet. Then, the toner image is transferred to the recording sheet at the image transfer unit 13. After the image transfer is completed, the recording sheet is transferred to a sheet guide 32 through the gap B and sent to a fixing unit 16 where the toner image is fixed on the recording sheet. On the surface of the sheet guide 32, a plurality of protrusions (33) are provided as shown in Fig. 4. The protrusions are for sending the recording sheet straight to the fixing unit 16. In the above steps (1) and (2), since the gaps A and B are kept constant, within allowable tolerances, the toner image can be transferred to the recording sheet without any fluttering at the front and rear ends of the recording sheet even if the drum 11 rotates eccentrically and/or the surface of the drum 11 is not formed as a perfect circle. (3) After the toner image on the recording sheet is fixed at the fixing unit 16, the recording sheet is transferred by push rollers 35, 36 and 37 through a sheet guide 34 and ejected and laid on a sheet tray 38 in a facedown state. (4) The cylindrical surface of the drum 11 is cleaned by a cleaner 46 after the image transfer is over. In the embodiment of the present invention described above, the overall frame of the recording apparatus is separated into two frames, an upper frame and a lower frame. In Fig. 3, reference numerals 41 and 39 indicate the upper frame and the lower frame respectively. The lower frame 39 includes the base frame 391. Furthermore, in this embodiment, the drum 11, the cleaner 46, the charger which is not depicted in Fig. 3 and the pinch roller 47, facing the second sending roller 30, are unified into a drum unit 200, and the drum unit 200 is installed in the upper frame 41. This situation can be seen clearly in Fig. 7(a) which relates to a second embodiment of the present invention which in relation to these units in the recording apparatus is the same as the embodiment described above. A recording sheet can be individually fed (manually) through an entrance 42. In this case, the recording sheet is transferred directly to the second sending roller 30 as shown in Fig. 3 and thence transferred to the image transfer unit 13 via the same route as described above. Sheet transfer is controlled by a monitoring system using sensors 43 and 44 which detect the sheet passing through the sheet path. The drum unit 200 can be exchanged by firstly lifting up the upper frame 41 as shown in Fig. 7(a) and secondly drawing the drum unit 200 out from the upper frame 41 in a direction perpendicular to plane of the Figure. By doing this, the drum unit 200 can be exchanged without touching any portion around the gaps A and B, which results in keeping the gaps A and B constant in high accuracy. In this embodiment of the invention, the gap A is set to 0.5 mm ± 0.2 mm and the gap B is set to 1.5 mm ± 0.3 mm. These values are determined experimentally with a 70 mm/s recording sheet transfer speed, which corresponds to a rotational speed of 23 rpm for the drum 11. In this case, the distance between the surface of the drum 11 and the wire 7 of the corona discharger 8 is set to 8.5 mm ± 0.1 mm. In this embodiment of the present invention, exchange of the drum unit 200 and the cleaning of the image transfer unit 13 can be carried out by users easily, so that users can accomplish this without the assistance of a maintenance professional. A second embodiment of the present invention provides features which further facilitate the exchanging of the drum unit 200 and cleaning of the image transfer unit 13 for users. When the image transfer unit 13 needs to be cleaned, the upper frame 41 is opened as mentioned above, so that the inside of the image transfer unit 13 can be easily examined. However, the sheet delivery guide 12 is set on the canopy top 26, so that the action of cleaning is obstructed or disturbed by the sheet delivery guide. Therefore, the sheet delivery guide 12 must be moved by manually lifting up the arm 20 and hand-holding the arm during cleaning of the inside of the image transfer unit 13. The second embodiment of the present invention provides means for avoiding these inconveniences. In the second embodiment, a holder 104 is added to a wall standing on the base frame 391, as illustrated in Figs. 7(a) and 7(b): the wall is not depicted in Figs. 7(a) and 7(b). In Figs. 7(a) and 7(b), a cross-sectional side view of the recording apparatus including the holder 104 according to the second embodiment is illustrated. In Fig. 7(a), the upper frame 41 is opened and the image transfer unit 13 is in its highest position, because it is urged upwardly by the spring 22, and the sheet delivery guide 12 is manually removed from the canopy top 26 and held by making the arm 20 hold to the holder 104. In Fig. 7(b), the upper frame 41 is closed. When the upper frame 41 is closed, the sheet delivery guide 12 is pushed down by a pushing member 101 provided on the upper frame 41 so that the arm 20 is released from the holder 104. The pushing member 101 is attached to the drum unit 200 as shown in Fig. 9. The pushing member 101 is provided on the drum unit by processing or forming a board or plate attached to a unit case of the drum unit for mounting the pinch rollers 47. Two member pieces of the pushing member 101 are provided at two sides of the board as shown in Fig. 9. When the upper frame 41 is closed completely, the sheet delivery guide 12 is placed back to the canopy top 26, and at the same time, the image transfer unit 13 is pushed down to its normal position by the descending drum 11. Figs. 8(a), 8(b) and 8(c) illustrate the holding means 104 of the sheet delivery guide 12. In Figs. 8(a), 8(b) and 8(c), the same reference numerals as in Fig. 7(a) or 7(b) designate the same or similar units or parts as in Fig. 7(a) or 7(b). In Fig. 8(a), a perspective view of the holder 104 is given, with the arm 20 of the sheet delivery guide 12. The arm 20 is supported by the axis 19 on a side wall 392 stood on the base frame 391 of the lower frame 39. The holder 104 is provided on the side wall 392 by using a flat spring 1041 protruded from the side wall 392 through a hole 106 and folded so as to form a roof shape. Fig. 8(b) is a cross-sectional view of the arm 20 released from the holder 104. In this case, the arm 20 is not reached to the protruded flat spring 1041 yet, so that the flat spring 1041 has the roof shape. When the arm 20 is lifted up, the arm 20 reaches the flat spring 1041 and the roof shape of the flat spring 1041 is almost flattened on the side wall 392 as shown in Fig. 8(c), so that the arm 20 is held by the elastic force of the flat spring 1041. When the upper frame 41 is closed, the sheet delivery guide 12 is pushed down by the pushing member 101, releasing the arm 20 from the holder 104. Then, the flat spring is restored as shown in Fig. 8(a) or 8(b). The pushing member 101 is used only for releasing the arm 20 from the holder 104. Therefore, when such release has been effected, the pushing member 101 departs from the sheet delivery guide 12 as shown in Fig. 8(b). As a result, the end of the sheet delivery guide 12 is placed back on the canopy top 26, resting freely thereon. In the above-described embodiments, reference has been made to electrophotographic recording apparatus, however it will be understood that embodiments of the present invention can also be applied to electrostatic recording apparatus. Means similar to those of the above-described embodiments of the present invention, such as the pushing mechanism for the image transfer unit, towards the surface of the photoconductive drum, through rollers, have been disclosed in Japanese patent, JITSUKOSHO No. 57-3000 entitled "Charging type image transcription apparatus", by Tadayuki Kitajima and Shohji Suda on January 19, 1982. However, this patent No. 57-3000 contains no teaching relating to the sheet delivery guide and the maintenance of a proper relationship between the sheet delivery guide and the image transfer unit as in embodiments of the present invention. As indicated above, the sheet delivery guide, particularly the gap A between the surfaces of the sheet delivery guide and of the photoconductive drum is very important for obtaining a high-quality transferred image on a recording sheet. Unless the gap A is kept to a designated value, within predetermined tolerances, a high-quality image cannot be transferred to a recording sheet. In the above embodiments of the present invention, the sheet delivery guide is moved in concert with the motion of the image transfer unit, for keeping the gap A to a designated value within allowable tolerances. In an embodiment of the present invention, a gap between a sheet delivery guide and a surface of a drum is kept to a predetermined value, within allowable tolerances, by positioning the sheet delivery guide together with an image transfer unit placed under the drum and constantly pushed toward the drum surface so that spacing between the drum surface and the image transfer unit is maintained constant. The sheet delivery guide can be lifted up and held away from the image transfer unit when the drum is to be removed and released and set back to its regular position mechanically when the drum or a replacement drum is put back in its regular position.

Claims: A recording apparatus for transferring a toner image onto a recording sheet, the recording apparatus comprising:- a photoconductive drum (11) rotatable about a drum axis, the drum having a cylindrical surface on which a toner image is produced when the apparatus is in use; image transfer means (13) arranged to transfer the toner image from the cylindrical surface onto the recording sheet; and guide means (12) arranged to guide a recording sheet to the image transfer means (13) for transfer of the toner image to the sheet; characterised in that the apparatus comprises:- first positioning means (22, 23, 21) arranged to urge the image transfer means towards the cylindrical surface of the drum (11) so that an image transfer space between the cylindrical surface and said image transfer means (13) is maintained at a predetermined value, within allowed tolerances, when the photoconductive drum is rotated, the toner image on the cylindrical surface being transferred onto the recording sheet whilst the recording sheet passes through the image transfer space as the drum is rotated; and in that the apparatus further comprises:- second positioning means (20) effective, together with the image transfer means (13) positioned by the first positioning means (22, 23, 21), to position said guide means (12), to maintain a gap between the cylindrical surface and said guide means to a predetermined value, within allowed tolerances, which value is greater than the recording sheet thickness. Recording apparatus as claimed in claim 1, wherein:- the image transfer means (13) comprises pedestal means (26), said pedestal means having a face; the guide means (12) comprises a sliding face capable of contacting the face of the pedestal means; and said second positioning means comprises first pushing means (20) tending to urge the sliding face into constant contact with the face of the pedestal means (26). Recording apparatus as claimed in claim 2, wherein:- the drum axis of the photoconductive drum (11) is disposed horizontally; the first positioning means (22, 23, 21) positions the said image transfer means (13) in relation to the rotating cylindrical surface of the drum, by urging the image transfer means vertically towards the drum, using second pushing means (22) urging the image transfer unit away from a base frame (391) of the apparatus; and the face of the pedestal means (26) is positioned horizontally. Recording apparatus as claimed in claim 3, wherein:- the first pushing means comprises an arm (20) sustained at an arm axis (19), fixed to said base frame (391), so that the arm is rotatable in a vertical plane around the arm axis, said guide means (12) being provided at the end of the arm so that the sliding face is urged into contact with the face of the pedestal means (26), by the weight of the guide means (12) and the arm (20) turning around the arm axis, and the second pushing means comprises springs (22) stood vertically on said base frame, placed between the image transfer means (13) and the base frame (391). Recording apparatus as claimed in claim 4 further comprising:- a lower frame (39), in which are installed at least the base frame (391) and means comprising the image transfer means (13), the first positioning means (22, 23, 21), the guide means (12) and the second positioning means (20); and an upper frame (41) in which at least the photoconductive drum (11) is installed, the upper frame being capable of being opened, leaving said lower frame horizontal. Recording apparatus as claimed in claim 5, wherein:- the lower frame (39) comprises holding means (104) attached to said base frame (391) and operable to hold the arm (20) so that the guide means (12) are held in a lifted-up disposition, the arm being held by the holding means, after the upper frame (41) is opened; and the upper frame (41) comprises third pushing means (101), operable to push the guide means (12) for setting the sliding face of said guide means back to the face of the pedestal means (26), releasing the arm (20) from said holding means (104), when said upper frame is closed to said lower frame.

Inventors: HOUKI, YOUJI C/O FUJITSU LIMITED, NONOMURA, TOMOHIRO C/O FUJITSU LIMITED, OHYABU, KEIJI C/O FUJITSU LIMITED, WATARAI, SHINICHI C/O FUJITSU LIMITED
Applicants: FUJITSU LIMITED
Application year: 1988
Main classification: G03G15
IPC codes: G03G21, G03G15