An image forming apparatus

Abstract: An image forming apparatus includes separating device (40). A recording material having an image is carried on a recording material carrying member (34). The separating member separates the recording material from the recording material carrying member. The separating member deforms the recording material carrying member in the form of a thin sheet, by pressing a roller (42a) to the recording material carrying sheet, thus facilitate insertion of a separating pawl (42) between the recording material and the carrying sheet. By providing a single separating pawl, the probability of the erroneous separation is minimized. The recording material on the recording material carrying member is efficiently separated therefrom without damage to the recording material carrying member, thus providing a high quality of images.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus including separating means for separating a recording material on which an image is formed, from recording material carrying means for carrying the recording material, more particularly to a color image forming apparatus wherein a plurality of color images formed with toners on an image bearing member are superposedly transferred onto one and the same transfer material, and thereafter, the recording material is separated. In a conventional image forming apparatus such as an electrophotographic copying apparatus wherein a toner image is formed on an image bearing member such as an electrophotographic photosensitive member and is transferred onto a transfer material, the transfer material is conveyed to the image bearing member carrying the toner image and is contacted to the image bearing member by which the toner image is transferred from the image bearing member to the transfer material; and in order to achieve this, there is provided an image transfer device. Referring first to Figure 11, the transfer device 10 includes a transfer material supporting or carrying member 11 usually in the form of a cylinder, a transfer material gripper 12 mounted on the transfer material carrying member 11 to grip a leading edge of the transfer material P supplied to the transfer material carrying member 11 and a transfer material carrying sheet (dielectric material sheet) 14 in the form of high resistance film for carrying the transfer material P, the transfer material carrying sheet being wrapped so as to cover an opening 13 (shown by broken lines) formed in the circumference of the transfer material carrying member 11. With this structure, the leading edge of the transfer material P supplied to the transfer material carrying member 11 is gripped by a gripping pawl which is movable substantially vertically and which constitutes the transfer material gripper 12, and the remaining portion of the transfer material P is retained on the transfer material carrying sheet 14. The transfer material P gripped by the gripper 12 and retained on the transfer material carrying sheet 14 is brought into contact with the image bearing member by the rotation of the transfer material carrying member 11. At the transfer position, a corona discharger disposed inside the transfer material carrying member 11 applies to the backside of the transfer material carrying sheet the electric charge having the polarity opposite to that of the toner, or a conductive roller or the like applies a voltage having the polarity opposite to that of the toner, whereby the toner image is transferred from the image bearing member to the transfer material P. In this image transfer process, the transfer material P now having the transferred image is subjected to AC discharge by an unshown discharger so as to weaken the attraction force between the transfer material P and the transfer material carrying sheet 14, and thereafter, separation pawls A are inserted between the transfer material and the transfer material carrying sheet, so that the transfer material is separated from the transfer material carrying sheet 14. As shown in Japanese Patent Application Publication 27029/1985, for example, a transfer material raising member (not shown) can be used at the gripper 12 in order to facilitate the insertion of the separation pawls A between the transfer material and the transfer material carrying sheet. The conventional transfer device in the image forming apparatus, described above, wherein the transfer material P is gripped by the gripper 12 is advantageous, particularly when used with a color image forming apparatus, in that the transfer material P is positively supported on the transfer material carrying member 11 while the images are superposedly transferred onto the transfer material, and therefore, the registration among the different color images are good. However, when, for example, a plural number of image formations are to be executed continuously, and plural transfer materials P are supported on the transfer material carrying member 11 for the purpose of increasing the image forming speed, both of the gripper and the transfer material raising member are required for the respective transfer materials so as to effect good separating operations, with the result of complicated structure. In addition, if the plurality of the grippers and the raising members are used, no image can be formed at the portion where the gripper and the raising member are disposed when a large size transfer material which is large enough to cover the plural grippers and the raising members is supported on the transfer material carrying member. For this reason, it has not been possible to provide a structure by which the transfer device is usable with plural sizes of the transfer materials. U.S. Patent No. 4,712,906 discloses that a surface of a transfer material is formed of a thin film of an elastomer, and a plurality of sucking chambers are disposed and arranged circumferentially in the transfer drum adjacent to the surface of the drum. The thin film of the elastomer is provided with sucking apertures communicating with the sucking chambers. The transfer material is attracted to the surface of the transfer drum through the sucking apertures so as to retain and carry the transfer material thereon. With this means, it is not necessary for the transfer material to be gripped by the gripper. In addition, if plural sucking apertures are formed spaced in the circumferential direction of the transfer drum, plural transfer materials can be simultaneously retained and carried on the surface of the transfer drum. Japanese Laid-Open Patent Application No. 32079/1980 discloses a transfer drum provided with a surface dielectric layer. In the transfer drum, a corona charger for attracting the transfer sheet (transfer material) is used, and a conductive roller or another charger having a polarity opposite to that of the corona charger for attracting the transfer sheet is disposed facing the corona charger. The transfer material is retained on the dielectric material sheet on the transfer drum surface by electrostatic attraction force and is carried thereon. This transfer drum is advantageous in that it does not require a gripper or sucking device and can retain and carry the transfer sheet on the transfer drum with high efficiency. In these two publications, when the transfer material is to be separated from the transfer drum surface, a separation pawl is inserted between the transfer material and the transfer drum surface. However, when the separation pawl is strongly pressed to the sheet in an attempt to assure the separation, the sheet can be damaged with the result of deteriorated quality of the transferred image, and therefore, that the sheet has to be exchanged frequently. Conventionally, five to nine separation pawls A are used, which are disposed at regular intervals with a predetermined angle so that all of the separation pawls A have comparable separating functions. All of the separating pawls A are brought to the neighborhood of or contact with the transfer material carrying sheet 14 in response to an operation signal produced by a control device (not shown) when the separating timing comes. It separates the transfer material from the transfer material carrying sheet, and is returned to its home position in response to a releasing signal produced by the control device when the separating operation is completed. The conventional separating means of the transfer device involves problems. Even if the edges of the plural separating pawls A are designed to be aligned relative to the transfer material carrying sheet, unavoidable variation in the positional accuracy of the edges during the manufacturing and assembling thereof, can lead to insufficient separation of the transfer material. In addition, when the separating pawls A are contacted non-uniformly to the transfer material carrying sheet, the transfer material carrying sheet is deformed at plural positions, by which the space formed between the transfer material carrying sheet and the transfer material to allow the separation pawls A to enter becomes small, with the result of unstable separation. If the separation pawls are strongly pressed to the transfer material carrying sheet in an attempt to assure the separation by the separating pawls, they can damage the transfer material carrying sheet with the result of deteriorated quality of the transferred image. This requires frequent exchanges of the transfer material carrying sheet. Therefore, an image forming apparatus wherein the transfer material can be satisfactorily separated from the transfer material carrying member has been desired. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide an image forming apparatus wherein a recording material carried on a recording material carrying member can be separated efficiently without damage to the recording material carrying member, whereby images having good quality can be provided. This and other objects, features and advantages of the present invention as defined in claim 1 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. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a transfer device usable with an image forming apparatus according to an embodiment of the present invention. Figure 2 is a perspective view of a transfer drum frame of the transfer device of Figure 1. Figures 3A and 3B illustrate the transfer materials wrapped around the transfer drum. Figure 4 is a side view of separating means used in the transfer device of Figure 1. Figure 5 is a perspective view illustrating the relationship between a connecting portion and the transfer sheet in the transfer device of Figure 1. Figure 6 illustrates a separating action by the separating means. Figure 7 is a sectional view of an image forming apparatus according to an embodiment of the present invention. Figure 8 is a perspective view of a transfer device according to another embodiment of the present invention. Figure 9 is a top plan view of separating means of the transfer device according to another embodiment of the present invention. Figure 10 is a sectional view of a color electrophotographic copying apparatus according to another embodiment of the present invention. Figure 11 is a perspective view of a conventional transfer device. Figure 12 illustrates means for detecting sizes of the transfer materials. Figures 13A and 13B illustrate the separating action by the separating means. Figure 14 is a perspective view illustrating the relationship between the connecting portion and the transfer sheet in the transfer device. Figures 15 and 16 are side views illustrating the relationship between a separating pawl and an auxiliary separating pawl. Figures 17A and 17B show the relationship between the raising amount of the transfer material and the separating pawl and the auxiliary separating pawl. Figure 18 is a perspective view of separating means equipped with an auxiliary separating member according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 7, there is shown a color electrophotographic apparatus as an exemplary image forming apparatus according to an embodiment of the present invention. In this embodiment, an image bearing member, that is, an electrophotographic photosensitive drum 1 which is rotatably supported for rotation in the direction indicated by an arrow is uniformly charged by a primary charger 2, and then is exposed to light image 3 corresponding to image information by exposure means including a laser beam exposure means or the like, so that an electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image is visualized as a toner image on the photosensitive drum 1 by a movable type developing device 4, for example. In this embodiment, the photosensitive drum 1 is an OPC (organic photoconductor) drum 1 having a negative charging property and is negatively charged by the primary charger 2, and the portion which is to receive the toner is exposed to light by the exposure means so that the potential of that portion is attenuated, whereby the latent image is formed. The latent image is reverse-developed by negatively charged toner particles in the developing device 4. The movable developing device 4 comprises, in this embodiment, four developing devices 4M, 4C, 4Y and 4B containing magenta color developer, cyan color developer, yellow color developer and black color developer, respectively, and a guide (not shown) for guiding the four developing units for movement in a horizontal plane. In the movable developing device 4, a desired developing unit is brought to a developing position where it is faced to the outer periphery of the photosensitive drum 1, to develop the electrostatic 16 latent image on the photosensitive drum 1. The recording material (transfer material) P is carried by the transfer device 30 which will be described in detail hereinafter and is conveyed by the developing device 30 in the direction indicated by an arrow in the Figure, and is brought into contact to the photosensitive drum 1 and receives the toner image which has been formed on the photosensitive drum 1 as the visualized image. The transfer material P is supplied to the transfer device 30 in synchronism with the image on the photosensitive drum 1 by the registration roller 6A. The residual toner particles remaining on the photosensitive drum 1 surface is removed by the cleaning device 30, so that the photosensitive drum 1 is prepared for the repeated color image forming process operation. Referring now to Figures 1 and 2, an example of the transfer device 30 used with the color electrophotographic apparatus according to an embodiment of the present invention will be described. In this embodiment, the transfer device 30 comprises cylindrical rings 31 and 32 at opposite longitudinal ends and a connecting portion 33 for connecting the rings 31 and 32. The rings 31 and 32 and the connecting portion 33 constitute a transfer drum frame for supporting a transfer material carrying sheet 34 which is a transfer material supporting member, including dielectric material film. In this embodiment, the drum frame wrapped with the transfer material carrying sheet constitutes an image transfer drum 30D. The transfer drum 30D has an outer diameter of 160 mm, in this embodiment. As shown in Figures 3A and 3B, the circumferential length of the transfer drum 30D has a circumferential length enough to carry one A3 size sheet or one B4 size sheet with the length of the sheet being codirectional with the circumferential direction of the transfer drum, and also enough to carry two A4 size sheets or two B5 size sheets with the length of the sheets being codirectional with the longitudinal direction of the transfer drum. In Figure 3B, there is a space between the two transfer materials P carried on the transfer material carrying sheet 34, but this space is not inevitable, the space between the transfer materials P may be removed. In this embodiment, it is possible that the size of the transfer material used is detected by detecting means, and also the number of sheets to be subjected to the image forming operation is detected by detecting means, and in response to those detections, a CPU can decide whether one or two transfer materials are wrapped on the transfer drum. As shown in Figure 12, the transfer material size detecting means may comprise projections 71 provided on the cassette 70 for accommodating the transfer materials and microswitches SW1 - SW4 on the image forming apparatus which are selectively depressed by the projections 71 when the cassette 70 is set in the apparatus. The configuration of the projection 71 is different depending on the size of the transfer material contained in the cassette, and therefore, the microswitch or microswitches depressed by the projection 71 are different. In response to the selective actuation of the microswitch, the signal is introduced into the CPU 72, and the number of the copies to be taken is set in the CPU 72 by an unshown input button. On the basis of those settings, the CPU 72 decides how many transfer materials P are attracted on the transfer drum, and it controls the attracting operation. In this embodiment, the transfer device 30 further includes separating means 40 which comprises a separation pawl supporting member 41 arranged along an axis of the transfer drum 30D and plural, three in this embodiment, separation pawls 42 fixed on the supporting member 41. The separating pawl 42 is provided at its end integrally with a roller 42a which is pressed to the outside of the transfer drum 30D to perform the function which will be described hereinafter. As will be understood from Figures 1 - 4, at the opposite ends of the supporting member 41, pressing rollers 45 and 46 are disposed through supporting plates 43 and 44. The pressing rollers 45 and 46 are contacted to the rings 31 and 32 of the transfer drum 30D when a half turn clutch 74 for operating the separation pawl operates. The rollers 45 and 46 are guided by the guiding grooves 35 and 36 formed in the rings 31 and 32 to rotate the separation pawl 42 to move its end downwardly, that is, in the direction substantially perpendicular to the surface of the transfer drum 30D. Referring to Figures 13A and 13B, the description will be made as to the operation of the separation pawl 42. As shown in Figure 13A, the separation pawl 42 is urged rightwardly by a spring 76, but is normally maintained at its non-operative position wherein it does not perform the separating operation. The supporting member 41 for supporting the separation pawl 42 is rotatable about a shaft 77. The supporting member 41 is driven by an actuating plate 73 having a generally a part-circular (half-circular) disk supported on an output shaft 78 of a half turn clutch 74 controlled by the CPU 72. When the half turn clutch 74 takes its engaging position, the driving force is transmitted to the output shaft 78 through driving means 75 in the form of a belt or chain. When the separating pawl 42 performs its separating action, a signal is produced from the CPU 72, and the shaft 78 is rotated through one half turn by the half turn clutch 74 taking its engaging position, from the state shown in Figure 13A. Therefore, the actuating plate 73 rotates the supporting member 41 about the shaft 77 in the counterclockwise direction to move the separating pawl 42 to its operating position wherein it performs the separating function, as shown in Figure 13B. After the separating operation, the shaft 78 rotates through one half turn by the half turn clutch 74, and the restoring force of the spring 76 rotates the supporting member 41 and the separating pawls 42 in the clockwise direction to return them to the non-operative position shown in Figure 13A. Although not shown in Figures 13A and 13B, the rollers 42a, 45 and 46 and the separating plates 43 and 44 make the same motion as the separating pawls 42 and the supporting member 41. The connecting portion 33 is provided with cut-away portions 37 to assist or facilitate insertion of the separating pawls 42 between the transfer material carrying sheet 34 and the transfer material P attracted and carried on the transfer material carrying sheet 34. As shown in Figures 1 and 5, the edge of the transfer material carrying sheet 34 is provided with cuts 34a along the cut-away portions 37 of the connecting portion 33 within a non-image-forming region of the transfer material in which the toner image is not transferred onto the transfer material. Therefore, the transfer material carrying sheet 34 indicated by hatching line in Figure 5 is bonded to the connecting portion 33 so as to provide local large curvature portions. The transfer device 30 includes transfer material attracting means 50 for attracting and retaining on the transfer material carrying sheet 34 the transfer material P supplied to the transfer device. As shown in Figure 8, the transfer material attracting means 50 includes an attracting corona charger 51 which is disposed within the transfer drum 30 and which applies to the back side of the transfer material carrying sheet electric charge having the polarity opposite to that of the toner image on the photosensitive drum 1, that is, the positive charge in this embodiment, and a conductive roller 52 disposed outside of the transfer drum 30D. The conductive roller 52 is grounded and functions as an opposite electrode of the attracting corona charger 31 to inject the electric charge into the transfer material P and to electrostatically attract the transfer material P to the transfer material carrying sheet 34. Preferably, the transfer material P is supplied such that the non-image-forming region at its leading edge is overlapped with the cuts 34a of the transfer material carrying sheet 34 and is not overlapped with the image forming region. As for the continuous image formation, when the length of the transfer material measured along the direction of the transfer material transportation is less than a half of the circumferential length of the transfer drum capable of supporting the transfer material, two transfer materials are retained on the transfer material. More particularly, when the length of the sheet used is less than one half the length which is the circumferential length of the transfer drum subtracted by the width of the connecting portion, the second transfer material is supplied subsequently to the first sheet, so that the second transfer material is attracted on the transfer material carrying sheet 34 in the position diametrically opposite to the first transfer sheet, as shown in Figure 3B. Thus, two transfer materials P are simultaneously retained on the transfer material carrying sheet. The transfer material P attracted by the transfer device 30 is conveyed to the image transferring operation position where the transfer charger 15 is disposed. In the transferring position, the image transfer corona charger 15 applies to the back side of the transfer material carrying sheet 34 the electric charge having the polarity opposite to that of the toner, thus transferring onto the first transfer material P the toner image formed with the first color developer, the magenta toner, for example. Subsequently, the same latent image is formed on the photosensitive drum, which is developed with the same, that is, the first color toner, which is in turn transferred onto the second transfer material P retained on the transfer material carrying sheet 34. Prior to the first transfer material reaching for the second time to the conductive roller 52, the conductive roller 52 is released so that it is moved away from the transfer material carrying sheet 34, through, for example, not less than 2 mm, to a position where it does not disturb the toner image transferred onto the transfer material P. A second toner image is formed on the photosensitive drum in synchronism with the first transfer material having received the first color image and is transferred onto the first transfer material P by the transfer corona charger 15. Similarly, the second transfer material having received the first color toner image receives the second color toner image. In the similar manner, the two transfer materials P receive four color toner images, respectively. The transfer device 30 is provided with a pair of AC corona dischargers 16 at opposite sides of the transfer material carrying sheet 34 so as to weaken the attracting force between the transfer material and the transfer material carrying sheet after completion of the image transferring operation. It is effective to electrically discharge the transfer material P and the transfer material carrying sheet 34. In order to separate the first transfer material P from the transfer material carrying sheet 34, the pressing rollers 45 and 46 are moved together with the supporting member 41 for the separating means 40 by the half turn clutch 74 for actuating the separating pawls, as will be understood from Figures 1, 4, 13A and 13B, so that they are contacted to the rings 31 and 32 of the transfer drum 30D. They are guided by the guiding grooves 35 and 36 of the rings 31 and 32. By this, the edges of the separating pawls 42 are rotated downwardly, that is, in the direction perpendicular to the transfer drum 30D surface toward the transfer material carrying sheet 34, whereby the pressing roller 42a moved integrally with the separating pawl supporting member 41 is pressed to the transfer material carrying sheet 34. The pressing roller 42a moves along the cut-away portion 37 of the connecting portion 33, and the separating pawl 42 wedges between the edge of the first transfer material and the transfer material carrying sheet 34 at the position where the curvature of the transfer material carrying sheet 34 is locally diffrent, so that the transfer material P is separated from the transfer material carrying sheet 34. In this manner, the leading edge of the transfer material is separated from the transfer material carrying sheet 34 by deformation of the transfer material carrying sheet 34, and therefore, the separating pawls 42 do not directly damage the transfer material carrying sheet 34, so that good transferring operation is assured. At this time, the inside separation roller 53 which is to be pressed to the inside of the transfer material carrying sheet is not driven. Referring back to Figures 4 and 6, when the second transfer material P is to be separated from the transfer material carrying sheet 34, the inside separating roller 53 disposed within the transfer drum 30D is pressed to the inside of the transfer material carrying sheet 34 by energization of the solenoid 79 controlled by the CPU 72. As shown in Figure 4, the solenoid 79 is coupled by a coupling member 81 to the supporting member 83 for supporting the inside separating roll 53. The coupling member 81 is rotatable about a shaft 80 and is rotatably joined with the supporting member 83 and with the solenoid 79. The supporting member 83 is urged downwardly by the spring 82. Therefore, as described hereinbefore, when the solenoid 79 is energized in the state shown in Figure 4, the coupling member 81 rotates clockwisely about the shaft 80 to raise the roller 53 supported on the supporting member 83 to provide the state shown in Figure 6. When the solenoid 79 is deenergized, the state of Figure 4 is restored. The outside separating pawl 42a is also pressed to the outside of the transfer material carrying sheet 34 by the pressing rollers 45 and 46 of the separating means 40 being guided along the grooves 35a and 36a formed at the opposite side of the connecting portion 33 for connecting the rings 31 and 32. As will be understood, the curvature of the transfer material carrying sheet is locally changed by the roller 42a, so that the edge of the transfer material is separated by the curvature change, and the separation is completed by inserting the separating pawl 42 between the transfer material P and the transfer material carrying sheet 34. In this embodiment, when the transfer material carrying sheet is deformed, both of the outside separating roller 42a and the inside separating roller 53 are operated, but only one of them is actuated to deform the sheet. It is preferable that a corona discharger 54 is used to perform an AC corona discharging operation so as to prevent disturbance to the image due to separation discharge which is produced when the transfer material P and the transfer material carrying sheet 34 are separated. When the length of the transfer material measured in the transfer material conveying direction is longer than the above-described (Figure 3A), the leading edge of the transfer material P is attracted on the transfer material carrying sheet 34 at the same position as the first transfer material described above, and the image transferring and the transfer sheet separating operations are performed in the same manner as the first transfer material described hereinbefore. At this time, the inside separating pawl 53 is not driven. After the image transferring and the transfer material separating operations are completed, the transfer material P is conveyed to the fixing device 18, where the toners are mixed and fixed by application of heat thereto. Then, the transfer material is discharged so that the image forming operation is completed. Figure 14 illustrates a transfer device 30 according to another embodiment of the present invention. In this embodiment, a raising member 57 for raising the leading edge of the first transfer material toward outside of the transfer material to separate it, is additionally provided in the cut-away portion 37 formed in the connecting portion 33 of the transfer drum frame. The first transfer material P is supplied in such a manner that it is attracted with its non-image-forming region (adjacent its leading edge) above the raising member 57. After the four color toner images are transferred from the photosensitive drum 1 onto the transfer material P supported on the transfer material carrying sheet 34, the raising member 57 is driven in the direction of an arrow at the position of the separating pawl 42 so as to separate the leading edge of the transfer material from the transfer material carrying sheet and so as to introduce the separating pawls 42 between the transfer material P and the transfer material carrying sheet 34, thus effecting the transfer material separating function. The separation of the second transfer material when two transfer materials are attracted on the transfer material carrying sheet 34, is the same as described with Figures 4 and 6, that is, by deformation of the transfer material carrying sheet 34. Here, a gripper or grippers for gripping the transfer material may be employed in addition to the raising member. The separation by deformation of the transfer material carrying member according to the present invention is applicable to a transfer device wherein the transfer material is sucked on a transfer drum surface. Referring to Figure 8, there is shown a further embodiment of the transfer device 30. In the foregoing embodiment shown in Figure 2, the transfer drum frame is constituted by rings 31 and 32 and connecting portion 33 therebetween, and the transfer material carrying sheet 34 is wrapped around the frame, and therefore, the transfer material attracting position on the transfer material carrying sheet 34 is determined. In the present embodiment, the transfer drum D is constituted by rings 31 and 32 which are fixed to a central supporting shaft 62 by proper supporting members 61 to constitute a transfer drum frame. On the frame, a seamless transfer material carrying sheet 34 is wrapped. In this embodiment, the transfer material carrying sheet 34 is in the form of a seamless cylinder, and therefore, the transfer material can be supported thereon at any position. The transfer material P can be supplied without strict control relative to the rotational position of the transfer drum. The transfer material P is attracted on the transfer material carrying sheet 34 by the attracting corona charger 51 and the conductive roller 52. As shown in Figure 7, when such a transfer device 30 is used, a transfer material detecting sensor 63 is disposed downstream of the attracting means 50 to detect the position of the transfer material. The transfer material detecting sensor 63 is disposed upstream of the transfer position by a distance on the circumference of the transfer drum 30, which is longer than a distance L from the image exposure position to the image transfer position measured along the circumference of the photosensitive drum 1. As for the continuous copying operation, when the length of the transfer material measured along its conveyance, is less than one half of the circumferential length of the transfer drum, the second transfer material is supplied sequentially. It is detected by the transfer material detecting sensor 63, and thereafter, the latent image is formed on the photosensitive drum 1 in synchronism with the detected transfer material, and the image is developed and transferred onto the transfer material. In this embodiment, the four color toner images on the photosensitive drum 1 are sequentially transferred, the first and second transfer materials P on the transfer drum are separated. During the separating operation, as shown in Figure 6 the inside separating roller 53 and the outside separating roller 42a movable together with the separating pawls 42 are pressed to the transfer material carrying sheet 34 to locally change the curvature of the transfer material carrying sheet 34, thus separating the leading edge of the transfer material by the curvature, and the separating is completed by inserting the separating pawls 42 between the transfer material and the transfer sheet. Referring to Figure 10, there is shown a transfer device according to another embodiment of the present invention. In this embodiment, the transfer device 30 includes, in place of the transfer material carrying dielectric film 34 described in the foregoing embodiments, a transfer material carrying sheet 34 having an insulating layer 34a contactable to the transfer material and a backing conductive layer 34b. The conductive layer 34b of the transfer material carrying sheet 34 is electrically isolated from the rings 31 and 32 constituting the transfer drum frame, and a DC bias voltage of a predetermined level is applicable to the conductive layer 34b, although it is not shown in the Figure. When the transfer material P is conveyed to the transfer drum 30D, the conductive layer 34b is supplied with a bias voltage having the polarity opposite to that of the toner image on the photosensitive drum 1, and simultaneously, the attracting corona charger 72 is energized so that it is weakly discharges toward the same polarity as the toner image. For example, when the polarity of the toner is negative, the discharge current of the attracting corona discharger is -5 micro-ampere, and the bias voltage is +1.5 KV, by which the transfer material P can be property attracted and carried on the transfer material carrying sheet 34, and also the toner image can be transferred onto the transfer material P from the photosensitive drum 1. If the apparatus is operated in the full-color production mode, the second and subsequent toner images are properly superposedly transferred if the bias voltage is incremented successively with 500 V for example. In this embodiment, the separating means 40 includes separation pawls 42 and a transfer material separating electrode 70 disposed faced to the separation pawls 42, wherein the electrode 70 is electrically connected to a DC source 71. As for the transfer material separating operation, when the leading edge of the transfer material P comes to the neighborhood of the separating pawl 42, the electrode 70 which is disposed away from the transfer material carrying sheet 34 by approximately 7 mm for example is supplied with a voltage having the polarity opposite to that of the voltage applied to the conductive layer 34b of the transfer material carrying sheet 34, for example, a voltage providing a potential difference of approximately 4 KV from the transfer material carrying sheet, for example. The portion of the transfer material carrying sheet 34 faced to the electrode 70 is electrostatically attracted to the electrode 70, by which the transfer material carrying sheet 34 is locally deformed to provide a larger curvature thereof. Therefore, the transfer material is separated from the transfer material carrying sheet at the deformed portion, and the separating pawl are introduced into between the separated transfer material and the transfer material carrying sheet to complete the separation therebetween. The separated transfer material is conveyed to a fixing device 18 where the images are mixed and fixed. Thereafter, the transfer material is discharged. Referring to Figure 9, there is shown another embodiment of the separating means 40′ usable with the transfer device according to the present invention. The separating means 40′ has the same structure as the separating means described in conjunction with Figure 1. However, as shown in Figure 9, the arrangements of the separating pawls 42 are different. More particularly, the central separating pawl 42 is closest to the transfer drum D, and auxiliary separating pawls 42b at both sides thereof are shorter toward the longitudinal ends of the transfer drum D. Further, an outer separating roller 42a is integrally mounted only to the central separating pawl 42. With the separating means 40′ thus contracted, the deforming means constituted by the outer separating roller 42a associated with the central processing pawl 42 and the inner separating roller 53, is first pressed to the transfer material carrying sheet, during the separating operation, by which the transfer material carrying sheet 34 is locally deformed to provide a different curvature thereof, by which the leading edge of the transfer material is separated from the transfer material carrying sheet 34. Subsequently, the central separating pawl 42 which is closest to the transfer material is first introduced into between the transfer material P and the transfer material carrying sheet 34. With advancement of the transfer material P, the transfer material P is gradually raised to expand the separated region so that the auxiliary separating pawls 42b are inserted into between the transfer material P and the transfer material carrying sheet 34, thus further expanding the separated region to complete the separation between the transfer material and the transfer material carrying sheet. In this embodiment, since the transfer material carrying sheet 34 is deformed at one position, it is easy to locally deformed the transfer material carrying sheet. Here, it is not inevitable that the inner separating roller 53 is employed. In the foregoing embodiments, the description has been made as to the case where the transfer material carrying sheet of the transfer drum is deformed to separate the transfer material from the transfer drum, but the present invention is not limited to this case. Rather, as the means for carrying the transfer material, it may be a photosensitive belt which functions as the image bearing member, too, wherein the photosensitive belt may be deformed when a transfer material is to be separated from the photosensitive belt. Various experiments and investigations made by the inventors have revealed that if the number of separation pawl which are to be inserted between the transfer material carrying sheet and the transfer material at least at the time of the start of the separation therebetween is one, the transfer material can be more effectively separated from the transfer material carrying sheet. Even if the transfer material is electrostatically attracted to the transfer material carrying member such as the transfer material carrying sheet, the transfer material can be very effectively separated from the transfer material carrying member by a single separation pawl. From the standpoint of statistics of separation, if there is n separation pawls each having a probability N of erroneous separation, the probability of at least one separating pawl failing, is where n is positive integer not less than 2. Therefore, N < N′, and therefore, the separation by a single separating pawl is efficient. Referring to Figures 15 and 16, there are shown side views of the separating pawl 42 and the auxiliary separating pawl 42b. As shown in Figure 15, the positions of the edges of the auxiliary separating pawls 42b may be changed in a single plane so as to be spaced more from the transfer drum 30D toward the longitudinal ends, or the positions thereof may be changed three-dimensionally, as shown in Figure 16. The latter case is particularly effective when the transfer material is soft so that the rigidity of the sheet can not be expected as assisting the separation, or when further stabilized and assured separation is required. With the separating means 40 having the above structure, the central separation pawl 42 closest to the transfer material first enter the space between the transfer material P and the transfer material carrying sheet 34. With the advancement of the transfer material P, it is raised gradually to expand the separated region. Then, the auxiliary separating pawls 42b at both sides of the central separating pawl are introduced into the space between the transfer material P and the transfer material carrying sheet 34 to further expand the separated region to complete the separation therebetween. The auxiliary separating member 42b may be, in place of the auxiliary separating pawls 42b described above, in the form of quarter circular thin member disposed at predetermined angles as shown in Figure 18. In this embodiment, the separation pawl 42 is made of a stainless steel plate having a thickness of 0.8 mm coated with tetrafluoroethylene resin, and the auxiliary separating pawls 42b are made of stainless steel plates having a thickness of 0.5 mm. Particularly, the auxiliary separating members 42b shown in Figure 18 are preferably made of metal plates or flexible films (PET, for example). If there is burr at the curved edge 42c, the conveyance of the transfer material is affected, and therefore, it is preferable that the edge 42c is abraded. In this embodiment, the main separating function is provided by the central separating pawl 42 and only one separating pawl is inserted between the transfer material and the transfer drum at the time of the start of the separation therebetween. The auxiliary separating members, for example, the auxiliary separating pawls 42b, function to prevent the downward bending of the transfer material until the transfer material is conveyed from the transfer drum 5 to a conveying member. Accordingly, when the auxiliary separating pawl or pawls are used, the auxiliary separating pawl or pawls can go above the transfer material with the result of erroneous separation, depending on the position of the auxiliary separating pawl 42b. The inventors have determined the relative proper positional relationship between the auxiliary separating pawl 42b and the separating pawl 42. Figure 17A illustrates the behavior of the transfer material when the transfer material P is being separated, wherein when the transfer material P is raised by an amount of d, it is separated by a length of 2 l . Figure 17B shows the state wherein the separating pawl 42 and the auxiliary separating pawl 42b are arranged three dimensionally, wherein the angle ϑ is formed between the separating pawl 42 and a tangent of the transfer drum 30D at the separating point, and the edge of the separating pawl 42 is at the point of origin. The edge of the separating pawl 42b is placed at a point Q (x1, y1) on an x-y plane. The point Q is on a one-dimensional function expressed by y = ax (a: constant). The amount of rise of the transfer material P is ax₁ tanϑ. Since d and 2 l are proportionally changed with advancement of the sheet, x₁: l = ax₁ tanϑ:d That is, a = d/ l tanϑ The values d and l are empirically obtained as follows: Since d/ l = 1/6, a = 1/6 x 1/tanϑ Therefore, erroneous separation does not occur, if the following is satisfied: y = 1/6 · 1/tanϑ · x which defines the position of the edge of the auxiliary separating pawl 42b relative to the separating pawl 42. With proper margin, y ≧ 1/6 · 1/tanϑ · x In the foregoing embodiments, the description has been made with respect to a transfer device having a transfer drum wrapped with a dielectric sheet or a transfer drum in the form of a bias roller, but the present invention is applicable to a transfer member in the form of a belt in place of the drum. In some of the above-described embodiments, the separating pawl 42 is shown as being located substantially at the longitudinal center of the transfer drum. This is shown as being usable for the case of central reference supply of the transfer material, and in the case of a lateral edge reference system, the separating pawl is placed at another position, more particularly, to the reference side lateral edge. In the foregoing, the present invention has been described in conjunction with a color image forming apparatus, but the present invention is not limited to the apparatus wherein the images are superposedly transferred onto the transfer material, but is applicable to another image forming apparatus such as a monochromatic image forming apparatus provided with the transfer device. The transfer device is not limited to the cylindrical configuration. As described in the foregoing, according to the present invention, there is provided an image forming apparatus wherein the recording material retained on the recording material carrying member is very efficiently separated from the recording material carrying member without damage to the recording material carrying member, thus providing a good quality images. 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 scope of the following claims.

Claims: An image forming apparatus, comprising: an image bearing member (1); image forming means for forming an image on said image bearing member (1); flexible carrying means (34) for carrying a recording material (P); transfer means (30) for transferring the image from said image bearing member (1) onto the recording material (P) carried on said carrying means (34); separating means (40) for separating the recording material (P) from said carrying means (34); characterized in that said separating means (40) includes deforming means for deforming said flexible carrying means (34) to separate the recording material (P) therefrom. An apparatus according to claim 1, wherein said flexible carrying means is in the form of a sheet. An apparatus according to claim 1 or 2, wherein said deforming means is movable between an operative position for separating the recording material and an inoperative position in which it does not separate the recording material from said carrying means. An apparatus according to any one of the preceding claims, wherein said deforming means includes a pressing member for pressing against said carrying means at its side opposite from its side carrying the recording material. An apparatus according to any one of the preceding claims, wherein said deforming means includes a pressing member for pressing against said carrying means at its side carrying the recording material. An apparatus according to any one of the preceding claims, wherein said separating means includes a separating member insertable between the recording material and said carrying means to separate the recording material from said carrying means. An apparatus according to claim 3, wherein said separating means is movable with said deforming means between said operative position and said inoperative position. An apparatus according to claim 4 or 5, wherein said pressing member is integral with said separating member. An apparatus according to claim 6, 7 or 8, wherein said separating member is a separating pawl. An apparatus according to any one of the preceding claims, further comprising attracting means, disposed upstream of said transfer means with respect to a movement direction of said carrying means, for electrostatically attracting the recording material to said carrying means. An apparatus according to claim 4, 5 or 8, wherein said pressing member is disposed at a reference position for the recording material in a direction crossing the movement direction of said carrying means. An apparatus according to claim 4, 5, 8 or 11, wherein said separating means (40) includes a first separating member (42) for being inserted between said carrying means (34) and the recording material (P), said first separating member being disposed adjacent to said pressing member, and a second separating member (42b) for being inserted therebetween after insertion of said first separating member (42). An apparatus according to claim 12, wherein, when a separation by said separating means starts, only said first separating member is inserted between said carrying means and the recording material. An apparatus according to claim 12 or 13, wherein each of said first and second separating members is a separating pawl. An apparatus according to claim 12, 13 or 14, wherein a plurality of said second separating members are provided, which are disposed more downstream of said first separating member away from said first separating member, with respect to the movement direction of said carrying means. An apparatus according to claim 15, wherein said second separating members are disposed more downstream with respect to the movement direction of said carrying means away from said first separating member. An apparatus according to any one of the preceding claims, wherein said carrying means is capable of carrying simultaneously a plurality of the recording materials. An apparatus according to any one of the preceding claims, wherein said carrying means is in the form of a drum and includes ring members at longitudinal ends, a connecting portion for connecting the ring portions and a sheet member covering an opening defined by said ring portions and said connecting portion. An apparatus according to any one of the preceding claims, wherein toner images of different color toners can be formed on said image bearing member, and the toner images are superposedly transferred onto the recording material carried on said carrying means. An apparatus according to claim 11, wherein said appratus is capable of forming a full-color toner image on the recording material.

Inventors: HASEGAWA TAKASHI, TAKEDA ATSUSHI, HASEGAWA, TAKASHI, TAKEDA, ATSUSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G03G15
IPC codes: G03G15