An image forming apparatus

Abstract: An image forming apparatus including a transfer material supporting device. An elastic member, a movable supporting member, a deformation preventing member or a friction member is provided between a transfer material supporting sheet and a frame constituting the transfer material supporting means or adjacent to the mounting position of the transfer material supporting sheet on the frame. By doing so, the transfer material supporting sheet is prevention from twisting or the like, so that images having good quality can be provided stably.

Description: The present invention relates to an image forming apparatus of an electrophotographic process or electrostatic recording type, provided with an image transfer device for transferring an image formed on an image bearing member onto a transfer material supported on a flexible transfer material supporting sheet. Various devices have been proposed and have been practically used to transfer onto a transfer material a toner image formed on an image bearing member in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus. In one of them, the transfer material is supported on a rotatable transfer material supporting means, and a transfer corona discharge is applied from an inside of the supporting means toward the outside (toward the image bearing member), so that the toner image on the image bearing member in the form of a photosensitive drum or the like is transferred onto the transfer material. Such a type of transfer device is particularly suitable for a color image formation since plural color images can be superposedly transferred onto the transfer material in good order. In this case, however, it is required that the transfer material is uniformly contacted to the image bearing member to prevent occurrence of local non-transfer portion. In order to accomplish this, it is usual that a part of a periphery of a cylindrical member is cut away to form an opening, which is covered with a flexible transfer material supporting sheet made of insulating material and having a rectangular configuration to establish the transfer material supporting means. The transfer material supporting means is rotated in synchronization with the image bearing member. The four sides of the rectangular transfer material supporting sheet have been bonded to the cylindrical member to cover the opening. Although it is possible with this structure to uniformly contact the transfer material to the image bearing member, it involves the following drawbacks. When the four sides of the flexible transfer material supporting sheet are bonded, it is difficult to bond it without slack, bend or inclination of the supporting sheet. The bonding work requires long time, and is not efficient. This applies to the case where an end of the transfer material supporting sheet is confined by a rigid member along the length of the transfer material. Particularly in order to avoid inclination or bend, it has taken a significantly long time. When the part of the periphery of the cylindrical member is cut-away to provide the opening, a frame is formed which comprises a pair of rings and a connecting portion in the form of an elongated plate connecting the rings. Since the transfer material supporting sheet is stretched in the form of a part cylinder to cover the opening, the surface of the transfer material supporting sheet becomes wavy when the transfer material is stretched or shrunk due to the changes in the temperature and/or the humidity or due to the repeated transfer operation. If a wavy surface is produced, the transfer material is not properly attracted or it is attracted with positional deviation, when the transfer material is supported on the supporting sheet. In addition, the image components in different colors are not registered in good order on the transfer material; the image is not uniform due to insufficient image transfer; or there occurs local non-transfer portions. It is desirable that the transfer material supported on the flexible transfer material supporting sheet is in face-to-face contact during the transfer action. Particularly in the transfer system using a corona discharger as in a charging type transfer type, if the contact between the transfer material and the image bearing member during the transfer operation is a line contact, the image transfer of the toner image onto the transfer material and the separation of the transfer material from the image bearing member occur simultaneously, with the result of unstable transfer action, and therefore, extreme non-uniformity and insufficient image transfer efficiency. In order to accomplish the face contact, Japanese Patent Application Publication No. JP-A-53 34468 discloses that a part of the transfer material supporting sheet is pressed away from the inside of the supporting frame at a position upstream of an image transfer position where the image bearing member and the transfer charger are faced to each other, with respect to the direction of the peripheral movement of the transfer drum. This is most often used. However, the freedom of deformation of the supporting sheet is different between the portions fixed to the frame and the other portions, and therefore, the surface contact is different depending on the positions in the supporting sheet. Therefore, the setting of the pressing force becomes difficult. This tends to cause non-transfer portions due to the insufficient pressure adjacent to the fixed portions. As a measure for this problem, it is considered that the transfer region is made remote from the fixed portions of the transfer material supporting sheet. In that case, the outer diameter of the transfer material supporting means becomes large with the result of the bulky transfer device. In a transfer device wherein a plurality of transfer materials are supported on a transfer drum, the design thereof is preferably such that the outer diameter of the transfer material supporting means is not large. Making the transfer region remote from the fixed portions, as described above, results in production of non-transferable area in the supporting sheet. This requires additional space for the supporting of the transfer material supporting sheet, and therefore, the outer diameter of the transfer material supporting member is increased. Japanese Laid-Open Utility Model Application No. 149157/1984 discloses a method wherein between the outer periphery of the ring and the transfer material supporting sheet, a first member made of elastic material and a second member at the transfer material supporting sheet side, the second member having a low friction coefficient. This method, however, involves the color component misregistration and the problem arising when the transfer material supporting sheet is bonded to the outer periphery of the frame. Reference is made to the following publications:- 1. EP-A-0281 138 which relates to a color image forming apparatus for sequentially transferring toner images to a paper sheet which is held on transfer means which is rotated in contact with photoconductive means in a predetermined transfer region, separating the paper sheet from the transfer means after the transfer by separator means which is located near the transfer means, and transporting the paper sheet separated from the transfer means to fixing means to complete a color copy. 2. Patent Abstracts of Japan, vol. 9, no. 62 (P-342) (1785), 19th March 1985; JP-A-59195675. This discloses an image transfer cylinder having an elastic body provided on a frame. 3. EP-A-0296 334 discloses an image transfer cylinder having a conductive sheet wrapped around a frame, the portion between the cylinder ends being bulged radially outward beyound the outer periphery of the frame. An object of the present invention to provide an image forming apparatus wherein the transfer material can properly receive the image. It is another object of the present invention to provide an image forming apparatus wherein the transfer device is not bulky. It is a further object of the present invention to provide an image forming apparatus wherein the transfer material supporting sheet can be easily mounted to a frame. It is a further object of the present invention to provide an image forming apparatus wherein the transfer material supporting sheet is not slackened or twisted. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. Figure 1 is a sectional view of an image forming process executing portion of an image forming apparatus including an image transfer device, according to an embodiment of the present invention. Figure 2 is a sectional view of a part of the transfer drum at an image transfer position where a toner image is transferred from the photosensitive drum to the transfer material on the transfer drum. Figure 3 is a sectional view of transfer material supporting means of elastic material, according to the present invention. Figure 4 is a perspective view of the transfer material supporting means. Figure 5 shows an embodiment wherein a friction member is provided for the frame of the transfer material supporting means. Figure 6 is a perspective view of a transfer material supporting sheet bent so as to be mounted on the frame. Figure 7 is a perspective view of the transfer material supporting means having the transfer material supporting sheet mounted on the frame. Figure 8 is a sectional view of the transfer material supporting means shown in Figure 7, showing the dimensional relation. Figure 9 is a sectional view of a transfer material supporting means of Figure 6. Figure 10 is a perspective view of a transfer material supporting means having a spring. Figure 10A is an enlarged view of the supporting member shown in Figure 10. Figure 11 is a sectional view of the transfer material supporting means according to a further embodiment of the present invention wherein an elastic member is added to the transfer material supporting means having the spring. Figure 12 is a sectional view of the transfer material supporting means similar to that shown in Figure 11 but which is added by a friction member. Figure 13 is a sectional view of a transfer material supporting means having a friction member which is different from that of Figure 12. Figure 14 is a sectional view of a transfer material supporting means according to a further embodiment of the present invention wherein it is provided with a movable supporting member at an end of the transfer material supporting sheet. Figure 15 is a sectional view of the transfer material supporting means of Figure 14, wherein a pressing member acts thereon. Figure 16 is a sectional view of a transfer material supporting means, according to a further embodiment of the present invention, wherein the transfer material supporting sheet is provided with a deformation preventing member at an end thereof. Figure 17 shows a further embodiment, wherein an elastic member is added to the transfer material supporting means. Figure 18 shows a further embodiment wherein an elastic member is added to the transfer material supporting means of Figure 15. Figure 19 shows a further embodiment, wherein an elastic element is added to the transfer material supporting means of Figure 16. Figure 20 is a perspective view of the transfer material supporting means of Figure 19. Figure 21 is a sectional view of a transfer material supporting means wherein the elastic element in Figure 19 is enlarged in a direction of the rotation of the transfer drum. Figure 22 is a sectional view of a transfer material supporting means according to a further embodiment of the present invention, wherein it is provided with an elastic element close to an elastic element having the same dimension as a deformation preventing means. Figure 1 shows a color image forming apparatus provided with an image transfer device, according to an embodiment of the present invention. In the apparatus of this embodiment, an image bearing member, more particularly, an electrophotographic photosensitive drum 1 is supported for rotation in the direction of an arrow and is uniformly charged by a primary charger 2. Then, it is exposed to light image 3 in accordance with image information by exposure means including laser beam exposure means (not shown), for example, by which an electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image is visualized into a toner image on the photosensitive drum 1 by a movable developing device 4, for example. The movable developing device 4 has four developing devices 4M, 4C, 4Y and 4B for containing four color developers, i.e., a magenta developer, a cyan developer, a yellow developer and a black developer, and a guide (not shown) for supporting the four developing devices and movable in a horizontal plane. The movable developing device 4 presents to a developing position where it is faced to the outer periphery of the photosensitive drum 1 to develop the electrostatic latent image on the photosensitive drum 1. The visualized image, that is, the toner image on the photosensitive drum 1 is transferred onto a transfer material P contacted to the photosensitive drum 1 by a transfer device 30 which will be described in detail hereinafter. The transfer material P is supplied to the transfer device 30 in synchronism with the image by registration rollers 6a. The surface of the photosensitive drum is cleaned by a cleaning device such that the residual toner on the surface is removed therefrom, and is prepared for the next color image formation process. The transfer device 30 of this embodiment comprises transfer material attracting means 50 for attracting and retaining or supporting the transfer material P on a flexible transfer material supporting sheet 34. The transfer material supporting sheet 34 is made of PVdF. The transfer material attracting means 50 includes an attraction corona charger 51 which is disposed inside the substantially cylindrical transfer drum 30D (transfer material supporting means) and which serves to apply to the backside of the transfer material supporting sheet 34 electric charge having the polarity opposite to that of the toner image on the photosensitive drum 1, and a conductive roller 52 disposed outside the transfer drum 30D. The conductive roller 52 is grounded to function as an opposite electrode for the attraction corona charger 51, and also functions to inject the electric charge into the transfer material P so that the transfer material P is electrostatically attracted onto the transfer material supporting sheet 34. The transfer material P attracted by the transfer material attracting means 51 and 52 is conveyed to the image transfer region where the transfer charger 15 is disposed. The transfer corona charger 15 applies electric charge having the polarity opposite to that of the toner onto the backside of the transfer material supporting sheet 34 in order to transfer onto the first transfer material P the first color toner image, for example, a magenta toner image from the photosensitive drum 1. Subsequently the same latent image is formed on the photosensitive drum 1 and is developed with the first color toner, and the first color toner image is transferred onto the second transfer material in the similar manner. By the time the first transfer material reaches again the position of the conductive roller 52, the conductive roller 52 is released, by which it is retracted away from the transfer material supporting sheet to a position not disturbing the toner image on the transfer material P, by, for example 2 mm or more. Then, a second color toner image formed on the photosensitive drum 1 in synchronism with the first transfer material now having the first color toner image, is transferred onto the first transfer material P by the transfer corona charger 15, and similarly, the second color toner image is also transferred onto the second transfer material having the first color toner image. In the similar manner, four color toner images are transferred onto the two transfer materials P. As shown in Figure 2, in order to assist the transfer of the toner image, a pressing lever 71 is provided adjacent to and at the upstream side of the transfer corona charger 15 with respect to the rotational direction of the transfer material supporting means. The pressing lever 71 urges the transfer material supporting sheet 34 to the photosensitive drum in the transfer region to increase the contact width of the transfer region so as to stabilize the transfer action of the toner image and to effect tensioning of the sheet 34 in the circumferential direction thereof. The pressing lever 71 is pivotable about a shaft 71a, and the pressing force is determined by a tension spring 71b stretched between the pressing lever 71 and a fixed portion 72. Referring to Figures 3 and 4, the transfer device 30 of this embodiment will be further described. In Figures 3 and 4, the transfer material supporting sheet is shown as being developed in cross-section and is shown in a perspective view, respectively. One end 34A of the flexible transfer material supporting sheet 34 (the leading circumferential end of the sheet 34) is bonded on a rigid member 61 (supporting member for the transfer material supporting sheet) by a double-sided adhesive tape or the like. The rigid member 61 is provided with through holes for screws and recesses for sinking the screw heads. The other end 34B of the supporting sheet 34 (the trailing circumferential end of the sheet 34) is bonded on a rigid member 63 through an elastic member 62 which may be urethane foam, rubber or the like. In this embodiment, the elastic member 62 is provided with recesses to sink the screw heads, and the rigid member 63 is provided with holes for the screws. In addition, the end portions 34a and 34b of the transfer material supporting sheet 34 are provided with holes 34c and 34d for allowing the screw heads to sink. The transfer material having been subjected to the image transfer operation, is separated from the transfer material supporting sheet 34 by separating means, and is conveyed to the downstream fixing rollers 18. The separating means 40, as disclosed in Serial No. 332,721, deforms the sheet 34 by a roller 53 disposed inside the sheet 34 to raise it. Below the raised edge of the transfer material, a separating pawl 42 is inserted to separate the transfer material with movement of the sheet 34. A roller 42a of the separating pawl 42 functions as a spacer for preventing damage to the sheet 34 by the pawl 42. As shown in Figure 5, the transfer device 30 includes cylindrical rings 31 and 32 at opposite longitudinal ends and a connecting member 33 for connecting the rings 31 and 32 which cooperate to constitute a drum frame 35 of the transfer drum 30D. The rings and the connecting member may be integrally formed. The connecting portion 33 is provided with screw holes 33a along two longitudinal lines. The longitudinally inside portions of the outer circumferential peripheries of the rings 31 and 32, which are contacted to the transfer material supporting sheet 34, are provided with friction members 70 bonded thereto, the friction member 70 being made of rubber and having a predetermined frictional coefficient. The combination of the transfer material supporting sheet 34, the elastic member 62 and the rigid member 61 is detachably mountable to the frame 35 having the connecting portion 33. As shown in Figures 6 and 7, when the transfer material supporting sheet 34 is mounted on the drum frame 35, the sheet 34 is curved into a cylindrical form over the drum frame 35, and then the rigid members 61 and 63 are fixed to the connecting portion 33 by threading screws into the holes 33a of the connecting portion 33 through the holes of the rigid members 61 and 62 formed adjacent the leading and trailing end portions of the sheet 34. Since the sheet 34 is mounted on the drum in this manner, it is apparent that there are portions where the sheet 34 is not fixed to the rings 31 or 32. As shown in Figure 8, an interval 1 a between the screw holes of the rigid member 61 and the screw holes of the rigid member 63 (which is equal to the interval between the recesses 34c and 34d of the transfer material supporting sheet 34) satisfies, relative to the interval 1 b between the two lines of the screw holes 33a of the connecting portion 33 when the supporting sheet 34 is mounted on the frame, the following: 1 a = 1 b + α (α > 0) where α is a margin which is properly determined by one skilled in the art in consideration of the expansion and/or shrinkage of the transfer material supporting sheet 34 due to the change in the ambient conditions and in consideration of the color component image registration in the case of plural images are superposedly transferred. In this embodiment, as described above, by not directly bonding the transfer material supporting sheet 34 to the connecting portion 33, the stretched state of the sheet 34 when it is mounted on the drum frame is controllable using the margin α. This makes the sheet 34 exchanging operation easy. According to this embodiment, the transfer material supporting sheet 34 may be easily mounted to provide a predetermined stretched state (tension force) without relying on the expertise. The difference in the freedom of deformation of the sheet 34 between the fixed portion and the other portion is decreased, so that the possibility of image transfer failure is decreased. In addition, there is disposed a friction member 70 having a predetermined frictional coefficient between the rings 31 and 32 and the supporting sheet 34, and therefore, the sheet 34 is prevented from deviation in the axial direction of the drum 30D. The description will be made as to the frictional coefficient of the friction member 70. If the friction coefficient of the friction member 70 is small, the color component images are not registered, as described hereinbefore. However, if it is too large, the same results as when the opposite ends of the supporting sheet 34 are fixed by the double sided adhesive tape or the like, are produced, so that the local non-transfer occurs, as described hereinbefore. The experiments by the inventors have revealed that urethane rubber or neoprene rubber or the like have good friction coefficient. In this embodiment, the elastic member 62 is used at the trailing circumferential edge of the supporting sheet 34, by which the flexibility of deformation of the sheet 34 is increased adjacent to the fixed end of the sheet 34. When the elastic member 62 is used at both of the leading and trailing ends of the sheet 34, the deformation flexibility of the sheet 34 is further enhanced. Referring back to Figure 1, the supporting sheet 34 is cleaned by a cleaning brush 72 when there is no transfer material P on the sheet 34. At this time, it is urged to the cleaning brush 74 by a back-up brush 74, upon which a gap 76 is produced between the friction member 70 and the sheet 34 at the position where it is urged. The gap 76 is so small that the other portion of the supporting sheet 34 is not deviated by the friction between the friction member 70 and the sheet 34. However, the gap 76 (Figure 9) advances together with the rotation of the transfer drum 30D in the rotational direction of the transfer material supporting sheet 34 from a starting point 34A to the ending point 34B, by which the deformation of the sheet 34 with time of use, due to temperature and/or humidity factors is accommodated. Therefore, the transfer material supporting sheet 34 is prevented from waving, stretching and shrinkage both in the axial and circumferential directions of the transfer drum 30D. Referring to Figure 10, there is shown a transfer drum of an image transfer device according to another embodiment of the present invention. In this Figure, the same reference numerals as in Figures 2 - 9 are assigned to the elements having the corresponding functions. Figure 10A is an enlarged view of the supporting member 37 of Figure 10. In the transfer device of this embodiment, the leading end 34A of the transfer material supporting sheet 34 on the transfer drum 30D is directly fixed by screws 36 or the like to the connecting portion 33 for connecting the rings 31 and 32 constituting the drum frame 35 therewith. The trailing end 34B of the sheet 34 is fixed on a supporting member 37 for supporting the sheet 34. The supporting member 37 is pulled by urging members such as tension springs 38 or the like so that the transfer material supporting sheet 34 is not slacked, and the springs 38 are fixed on the rings 31 and 32, respectively. The longitudinally inside portions of the outer circumferential periphery of the rings 31 and 32 which are contacted to the transfer material supporting sheet 34 are provided with the friction members 70 bonded thereto, similarly to the foregoing embodiment. The same advantageous effects can be provided with the structure of this embodiment. Figure 11 is a longitudinal sectional view of the transfer material supporting sheet mounting portion in a transfer device according to a further embodiment of the present invention. The leading end 34A of the transfer material supporting sheet 34 is mounted through a rigid member 61 to the connecting portion 33 for connecting the rings 31 and 32 to constitute the drum frame 35 of the transfer drum 30D, and the trailing end 34B thereof is mounted on a rigid member 63 through an elastic member in the form of an elongate strip 62. The rigid member 63 is urged by tension springs 38 or the like. The longitudinally inside portions of the outer circumferential peripheries of the rings 31 and 32 which are contacted to the transfer material supporting sheet 34 are provided with the friction members 70 bonded thereto, similarly to the foregoing embodiments. The same advantageous effects can be provided with the structure of this embodiment. Figure 12 is a cross-sectional view of a transfer drum of a transfer device according to a further embodiment of the present invention. The transfer device of this embodiment includes friction members 70a and 70b having different friction coefficients. They are mounted on each of the longitudinally inside portions of the circumferential outer peripheries of the rings 31 and 32 constituting the drum frame 35 of the transfer drum 30D. The friction coefficient of the friction member 70b at the side including the trailing end 34B of the transfer material supporting sheet 34 (trailing side of the image) is smaller than the friction coefficient of the friction member 70a at the side including the leading end 34a of the transfer material supporting sheet 34 (the leading side of the image). Because of this difference, the stretching force by the springs 38 is non-uniformly distributed in the transfer material supporting sheet 34 by the friction members 70a and 70b, more particularly, the leading side of the sheet 34 is stretched with relatively strong force, and the trailing side is stretched with relatively small force. Therefore, at the leading side of the image which is important from the standpoint of the registration of the color component images, the transfer material supporting sheet 34 is not deviated; and at the trailing side of the sheet 34, it is more flexible in the circumferential direction to absorb the entire slack of the sheet 34. Figure 13 is a cross-sectional view of the transfer drum in a transfer device according to a further embodiment of the present invention. In this embodiment, the friction member 70 provided in the longitudinally inside portions of the outer circumferential peripheries of the rings 31 and 32 constituting the drum frame 35 of the transfer drum 30D, at which the rings 31 and 32 are contacted to the transfer material supporting sheet 34, has friction coefficient which decreases away from the leading end 34a toward the trailing end 34b with respect to the rotational direction of the transfer material supporting sheet 34. Using such friction member 70, the friction coefficient thereof can be changed continuously, so that the effect of removing the slack of the transfer material supporting sheet 34 is further improved. Referring to Figures 14 and 15, a further embodiment of the present invention will be described. As shown in Figure 14, both of the leading and trailing ends of the transfer material supporting sheet 34 are provided with supporting members 61, through which the transfer material supporting sheet 34 is fixed to the connecting portion 33. To one of the supporting members 61, a rigid movable supporting member 49 is connected through a pin 49a. To the movable member 49, an end of the transfer material supporting sheet 34 is bonded. As shown in Figure 15, even if the urging means 71 urges the transfer material supporting sheet 34, the movable supporting member 49 rotates about the pin 49a so as to permit free deformation of the transfer material supporting sheet 34 even at the position adjacent to the supporting member 61. Therefore, the similar advantages as when the elastic member 62 can be provided. In this embodiment, the transfer material supporting sheet 34 can be fixed on the rigid movable supporting member 49, and therefore, the transfer material supporting sheet 34 is prevented from waving or slacking at the mounting position, it is preferable that the movable supporting member 49 is mounted to the leading end rather than mounting it at the trailing end, because the twisting of the transfer material supporting sheet 34 adjacent to the leading end is more influential to the image than that adjacent the trailing end. Figure 16 illustrates a further embodiment, wherein the opposite ends of the transfer material supporting sheet 34 are bonded on supporting members 61, and adjacent to one of the supporting members 61, a sheet deformation preventing member 78 such as a block member is bonded to the transfer material supporting sheet 34, extending along the connecting portion 33, by which the deformation such as the waving or slack of the transfer material supporting sheet 34 which tends to occur adjacent to the supporting member 61, can be prevented. The block member 78 extends in the longitudinal direction of the transfer material supporting means. In this embodiment, the movable supporting member 49 or the block member 78 is employed only for one of the supporting members 61, but it may be used for each of them. In that case, the advantageous effects are doubled. Referring to Figures 17 and 18, are further embodiment will be described. This embodiment is a modification of Figures 14 and 15 embodiments, and an elastic member 62 is provided between the transfer material supporting sheet 34 and the movable supporting member 49. With this mounting method of the transfer material supporting sheet 34, the movable supporting member 49 moves to the position of the urging lever 71 when the transfer drum 30D rotates, so that the leading portion of the sheet 34 is raised by the lever 71 toward the photosensitive drum 1. At this time, the raising by the urging lever 71 abuts the movable supporting member 49 to the photosensitive drum 1. Since, however, the elastic member 62 is sandwiched between the movable supporting member 49 and the transfer material supporting sheet 34, the impact due to the abutment of the movable supporting member 49 is eased, and therefore, the toner is not fused on the photosensitive drum 1 by the impact. Also, since the leading portion of the transfer material supporting sheet 34 is linearly supported along the axis of the drum 30d by the movable supporting member 49, the waving or the twisting in the axial direction can be prevented even when cut-away portion or portions are formed at the leading side. Figures 19 and 20 show a further embodiment. This embodiment is a modification of the Figure 16 embodiment. In this embodiment, an elastic element 82 is provided between the transfer material supporting sheet 34 and the block member 78. By the mounting method of the transfer material supporting sheet 37, a desired contact area between the photosensitive drum 1 and the transfer material can be assured by the urging lever 71 urging the movable block 78 the leading portion of the sheet 34 through the movable block 78 toward the photosensitive drum 1. Here, the block 78 is stated as being movable, since, as will be understood from the Figure, the block slightly pivots about a portion of the sheet between the supporting member 61 and the block member 78 (longitudinal direction of the transfer material supporting means). The impact by the abutment of the movable block 78 to the photosensitive drum 1 is eased by the provision of the elastic element 82 to prevent the possible toner fusing on the photosensitive drum 1 which can be caused by the impact. By the supporting with the use of the movable block 78, the waving or the twisting in the axial direction can be prevented even when the cut-away portion or portions are provided at the leading edge of the sheet 34. As shown in Figure 21, the width La of the movable block 78 measured in the circumferential direction of the transfer drum 30 is preferably smaller than a width Lb of the elastic element 82 (Lb > La), and that the elastic element 82 is projected from the movable block 78 away from the connecting portion 33. By doing so, the following advantages are provided. The leading end of the transfer material supporting sheet 34 is urged to the photosensitive drum 1 through the movable block 78 by an urging lever 71. When the lever 71 is away from the movable block 78 with the rotation of the transfer drum 30D, the lever 71 immediately abuts the sheet 34. Therefore, there is a likelihood depending on the urging force by the lever 71, though, that the impact by the abutment deviates the transferred toner image onto the transfer material supported by the sheet 34. By making the width Lb of the elastic element 82 larger than the width La of the movable block 78, and by projecting the elastic member 62 from the movable block 78, the lever 71 abuts the elastic member 62 when the lever 71 is away from the movable block 78. Therefore, the impact is absorbed, and only then it abuts the sheet 34, thus easing the impact by the abutment of the lever 71 to the sheet 34. Therefore, the deviation of the transferred toner image on the transfer material supported on the sheet 34 can be prevented. As shown in Figure 22, a small elastic element 82a may be disposed adjacent to the elastic element 82 having the same size as the block 78. By doing so, the sheet deformation occurring between the block 78 and the downstream side of the sheet can be further prevented. The above advantageous effects can be provided similarly by making the width of the.elastic member 62 larger than the width of the movable supporting member 49 to project the elastic member 62 from the movable supporting member 49 in the foregoing embodiments. As described in the foregoing embodiment, in the transfer device having the structure absorbing the movement of the transfer material supporting sheet, the means for separating the transfer material from the transfer material supporting sheet may preferably use the deformation of the sheet. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims: An image forming apparatus, comprising: a movable image bearing member (1); means (2,3,4) for forming an image on said image bearing member (1); a rotatable transfer material supporting means (30) for supporting the transfer material (P), said transfer material supporting means (30) including a transfer material supporting sheet (34), and a frame (35) having ring portions (31, 32) at longitudinal opposite ends of said transfer material supporting means (30) and a connecting portion (33) for connecting the ring portions (31,32) and to provide an opening defined by the ring portions (31,32) and the connecting portion (33), said transfer material supporting sheet (34) being disposed around said frame (35) to cover the opening and having its leading and trailing ends (34A, 34B), with respect to a rotational direction of said transfer material supporting means (30); and means (15) for transferring the image formed on said image bearing member (1) onto a transfer material (P) supported by said transfer material supporting means (30) at a transfer position; characterised in that there is provided a tensioning member (38, 71b) for tensioning said transfer material supporting sheet (34) in a circumferential direction by application of a circumferential force to said transfer material supporting sheet (34). An apparatus as claimed in claim 1, characterised in that one of said leading and trailing ends (34A, 34B) of the transfer material supporting sheet (34) is fixedly mounted to the connection portion (33) of the frame (35) and the other end is mounted by an elastic member (62) connected to said tensioning member (38), the elastic member (62) being in the form of an elongate strip extending along a longitudinal direction of said transfer material supporting means (30). An image forming apparatus as claimed in claim 2 characterised in that said transfer material supporting means (30) includes a member (61) fixed to said transfer material supporting sheet (34), said fixed member (61) being in the form of an elongate strip extending in the longitudinal direction of said supporting means (30) and lying adjacent said frame (35). An apparatus according to claim 1, 2 or 3 characterised in that said transfer material supporting means (30) rotates in contact with said image bearing member (1) at the transfer position. An apparatus according to any one of claims 1-4, further comprising image transfer means (15) inside the frame (35). An apparatus according to claim 1, characterised in that a pressing lever (71) is disposed in contact with the transfer material supporting sheet (34) at a side thereof remote from said image bearing member, for engaging the transfer material supporting sheet (34) to ensure good contact between the leading edge (34A) of the transfer material supporting sheet (34) and the image bearing member (1). An apparatus as claimed in any one of claims 1-5, characterised in that said transfer material supporting means (30) includes a supporting member (63) for supporting the elastic member (62), the supporting member (63) being provided between the elastic member (62) and the frame (35). An apparatus as claimed in any one of claims 1-7, characterised in that the transfer material supporting sheet (34) is detachably mountable to said frame (35). An apparatus as claimed in claim 8, characterised in that the combination of the transfer material supporting sheet (34), the elastic member (62) and a supporting member (63) for the elastic member is detachably mountable to the frame (35). An apparatus as claimed in any one of claims 1-9, characterised in that the transfer material supporting sheet (34) is unattached to the ring portions (31, 32) at opposite longitudinal ends thereof. An apparatus as claimed in any one of claims 1-5, characterised in that an elastic member (62) is provided at least at one of the leading and trailing ends (34A,34B) of the transfer material supporting sheet (34) with respect to a rotational direction of said transfer material supporting means (30). An apparatus as claimed in claims 2-11, characterised in that the elastic member (62) is of urethane foam or rubber. An apparatus as claimed in any one of claims 5 or 7-12, characterised in that said frame (35) movably supports a trailing end (34B) of the transfer material supporting sheet (34) with respect to a rotational direction of said transfer material supporting means (30). An apparatus as claimed in claim 13, characterised in that the elastic member (62) is supported on the connecting portion (33) together with the transfer material supporting sheet (34). An apparatus as claimed in any one of claims 1-5, characterised in that said transfer material supporting means (30) includes a first elongate elastic element (82) between the transfer material supporting sheet and a fixed block (78). An apparatus as claimed in claim 15, characterised in that the transfer material supporting sheet (34) is provided with a second elongate elastic element (82a) close to the first elastic member (82) across the frame (35). An apparatus as claimed in claim 15 or 16, characterised in that the first elastic element (82) is longer than the fixed block (78) in a circumferential direction of said transfer material supporting means (30). An apparatus as claimed in any one of claims 1-5, characterised in that said transfer material supporting means includes a supporting member (63) for supporting the transfer material supporting sheet (34), and wherein said elastic member (38) is connected between the supporting member (63) and the frame (35). An apparatus as claimed in any one of claims 1-18, characterised in that a friction coefficient between the transfer material supporting sheet (34) and the ring portions (31,32) is larger at a leading side than at a trailing side with respect to the rotational direction of said transfer material supporting means (30). An apparatus as claimed in claim 19, characterised in that a member (70) is provided for varying the friction coefficient in dependence upon the circumferential position. An apparatus as claimed in any one of claims 1-20, characterised in that a separating member (40) is provided for urging and deforming said transfer material supporting sheet (34) when the transfer material is separated from said transfer material supporting means (30). An apparatus according to any one of claims 1-21, characterised in that plural images are transferred from said image bearing member (1) to the transfer material to provide a color image. An apparatus according to any one of claims 1-22, characterised in that attracting means (51,52) are provided for electrostatically attracting the transfer material onto the transfer material supporting sheet (34).

Inventors: KIMURA KOUJI C O CANON KABUSHI, SUZUKI HAJIME C O CANON KABUSH, TAKEHARA YOSHIFUMI CANON KABUS, KIMURA, KOUJI, C/O CANON KABUSHIKI KAISHA, SUZUKI, HAJIME, C/O CANON KABUSHIKI KAISHA, TAKEHARA, YOSHIFUMI, CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15