An image forming apparatus

Abstract: An image forming apparatus includes an image bearing member for carrying a toner image; a recording material conveyer belt for electrostatically carrying and feeding a recording material; a roller, provided on a side of the recording material conveyer belt not carrying the recording material in a separation position where the recording material is separated from the recording material conveyer belt, for supporting the recording material conveyer belt; transfer charging means for electrostatically transferring the toner image from the image bearing member onto the recording material at a transfer position; discharging means, provided across the recording material conveyer belt from the roller at the separation position, for discharging the recording material upon separation thereof from the recording material conveyer belt; wherein the roller is electrically grounded through an element in which a voltage is generated when the current is supplied thereto.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus wherein an image is transferred onto a recording material carried on a recording material carrying member. Heretofore, various image forming apparatuses having a plurality of image forming stations have been proposed in which different color toner images are formed by the image forming stations, and the images are transferred superimposedly onto the same recording material (recording paper), thus forming a color image. In one of such apparatuses, a color copying machine of a multi-color electrophotographic type using an endless recording material carrying member is known as a high-speed image forming apparatus. Referring first to Figure 2, an example of color electrophotographic image forming apparatus will be described. In the apparatus, there are provided first, second, third and fourth image forming stations Pa, Pb, Pc and Pd, by which different color toner images are formed through latent image formation, development and image transfer processes. Each of said image forming stations is provided with an image bearing member 3a, 3b, 3c or 3d, and each color image is formed on the image bearing member which is in the form of an electrophotographic photosensitive drum. Adjacent to each of the photosensitive drum, a recording material carrying member in the form of a transfer belt 130 of dielectric material is disposed, and the toner image formed on the photosensitive drum is transferred onto the recording material p carried on the transfer belt 130. The recording material p now having the transferred image is subjected to operation of a separation charger 32 (corona charger) by which the attraction force to the transfer belt 130 is reduced, and it is separated from the transfer belt 130. Thereafter, the recording material p is fed to a fixing station 9, where the toner image is fixed by heat and pressure on the recording material, which is then discharged onto an outside tray 63 as a copy or print. However, this structure involves a problem, when, for example, a recording material having a low resistance resulting by keeping the recording material in a high humidity condition. More particularly, when the recording material may function as a short-circuit between the transfer position and the grounded electroconductive driving roller (separating means) 13 functioning as an opposing electrode, the toner image is not transferred in good order onto the recording material, or the toner image once transferred onto the recording material is transferred back onto the drum at a downstream portion of image forming station (retransfer), with the result of remarkably improper image transfer or formation. Generally, the volume resistivity of the recording material ranges between approximately 10 7 to 10 11 Ohm.cm depending on the material of the recording material or the water content thereof. This phenomenon will be described in more detail, referring to Figure 5. During image formation, the recording material short-circuits between the driving roller 13 and the photosensitive drum in the fourth image forming station. Then, back side positive charge of a part of charge couple at a portion of the transfer belt 130 which are in contact both with the recording material and the driving roller 13 (the hatched portion E in Figure 5) flows to the driving roller 13, and surface negative charge thereof flows into the transfer charging portion through the recording material. As a result, electric discharge occurs. At this time, the transfer current which is to flow from the transfer charger 24d to the photosensitive drum 3d (opposite electrode), indicated by the arrows A and B Figure 5, flows to the transfer belt E, as indicated by arrows A and D, with the result that transfer current is not enough. When the recording material short-circuits between the photosensitive drum 3d of the fourth image forming station and the driving roller 13, the portion E of the transfer belt 130 tends to supply the positive charge from the photosensitive drum 3d as well as taking all of the transfer current, the resistance of recording material further reduces. As a result, the current flows in the directions A and D and C and D. Moreover, when the separation discharger 32 is operated in order to separate the recording material and the transfer belt 130 simultaneously with image transfer, the negative charge is supplied from the separation discharger 32 to the photosensitive drum 3d through the recording material with the result of improper image transfer. The direction C is opposite from the direction B which is the direction of proper image formation. By the opposite current, the toner is not properly transferred, or the toner retransfers to the photosensitive drum 3d. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide an image forming apparatus capable of preventing improper image transfer from an image bearing member to the recording material carried on a recording material carrying belt. The object is achieved by an image forming apparatus as claimed in any of claims 1, 5 and 9. 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. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of an image forming apparatus according to Embodiment 1 and Embodiment 2. Figure 2 is a schematic view of an image forming apparatus of Embodiments 1 to 7. Figure 3 is a schematic view of an image forming apparatus of Embodiments 3 and 4. Figure 4 is a schematic view of an image forming apparatus of Embodiments 5 to 7. Figure 5 is a schematic view of a conventional image forming apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS The image forming apparatuses according to the embodiments of the present invention will be described in conjunction with the accompanying drawings. In the following description, the image forming apparatus is in the form of a full color image forming apparatus shown in Figure 2 having been described in the foregoing. Around the photosensitive drums 3a, 3b, 3c and 3d, there are provided exposure lamps 111a, 111b, 111c and 111d, drum chargers 2a, 2b, 2c and 2d, potential sensors 113a, 113b, 113c and 113d, developing devices 1a, 1b, 1c and 1d, transfer chargers 24a, 24b, 24c and 24d, and cleaners 4a, 4b, 4c and 4d, respectively. In an upper portion of the apparatus, there are provided unshown light sources and polygonal mirrors. A laser beam emitted by the light source is scanningly deflected by a polygonal mirror 117, and the beam is deflected by a reflection mirror and is directed through a f-theta lens to the photosensitive drum 3a to 3d to scan it in the direction of the generating line of the drum, so that latent image is formed on the photosensitive drum 3a to 3d in accordance with image signals. The developing means 1a to 1d contain predetermined amount of cyan, magenta, yellow and black toner particles having negative charging property, respectively, supplied by unshown supplying means. The developing means 1a to 1d develop the latent images on the photosensitive drums 3a to 3d to visualize them into cyan toner image, magenta toner image, yellow toner image and black toner image through reverse development. The recording material p is contained in a recording material cassette 10, and is supplied to the transfer belt 130 by a plurality of feeding rollers 11 and registration rollers 12, and the recording materials are sequentially fed to the transfer station where the recording material is faced to the photosensitive drum 3a. The transfer belt 130 is of dielectric material sheet such as polyethylene terephthalate resin sheet (PET), polyvinylidenefluoride resin sheet polyurethane resin sheet. The opposite ends thereof are overlaid and bonded together with each other into a form of an endless film, or it may be a seamless and endless film of such a dielectric material. The electroconductive driving roller 13 and supporting rollers 14 and 15 rotate the transfer belt 130, and when it is detected that transfer belt 130 is at a predetermined position, the recording material p is fed to the transfer belt 130 from the registration rollers 12, and is carried to the transfer station of the first image forming station Pa. Simultaneously therewith, the image writing signal is turned on, and the image forming operation on the photosensitive drum 3a is started at a predetermined timing on the basis of the writing signal in the first image forming station Pa. Attraction chargers 5 and 6 are provided between the supporting roller 14 and the transfer charger 24a of the first image forming station Pa such that transfer belt 130 is interposed between the attraction chargers. The recording material thus fed is attracted on the transfer belt before the transfer operation. The transfer charger 24a applies the electric field or charged at the transfer position (nip formed between the transfer belt 130 and the photosensitive drum 3a, by which the toner image of the first color is transferred onto the recording material p from the photosensitive drum 3a. The attraction chargers 5 and 6 may be omitted, and the recording material p may be electrostatically attracted on the transfer belt 130 firmly. The recording material p is fed then to the second image forming station Pb and subsequent image forming stations. In other words, the recording material p may be electrostatically attracted on the transfer belt 130 simultaneously with the image transfer operation. In this example, the attraction chargers 5 and 6 are in the form of rollers, but they may be non-contact type chargers such as corona chargers, or may be contact type chargers using charging members such as blade or brush. In this example, the transfer chargers 24a to 24d are in the form of a transfer blade, but they may be non-contact type chargers such as corona chargers, or may be contact type chargers using charging members such as a blade or brush. The contact type charger is advantageous in that the amount of ozone production is much less and in that influence of the humidity and temperature of the ambience is small. This embodiment uses the contact charger for each of the attraction charger and the transfer charger. For the purpose of stability in the image transfer, discharging needles 7a, 7b, 7c and 7d may be provided downstream of the transfer chargers 24a, 24b, 24c and 24d with respect to movement direction of the transfer belt 130. The discharging needles 7a to 7d are out of contact with the transfer belt 130, but are effective to discharge a part of transfer current. With this structure, separation discharge which may occur at the transfer position upon separation of the recording material from the photosensitive drum can be prevented particularly when the humidity is low, for example. The image forming and transfer operations at the second, third and fourth image forming stations Pb, Pc and Pd are the same as in the first image forming station. The recording material p now having the 4-color toner image is electrically discharged at a separation position downstream of to transfer position by the separation charger 32 as a discharging means so that electrostatic attraction force to the transfer belt 130 is reduced, by which the recording material is separated from the transfer belt 130. The separation charger 32 acts on the recording material p to charge or discharge the recording material p while the toner image is unfixed, and therefore, a non-contact type charger (corona charger) is used. During the separating operation, the separation charger is supplied with an AC voltage having a peak-to-peak voltage 10kVpp and a frequency 500 Hz. The recording material p separated from the transfer belt 130 is fed to a fixing device 9 by feeding means 62 along a guiding member. The fixing device 9 comprises a fixing roller 51, pressing roller 52, heat resistive cleaning members 54 and 55 for cleaning the fixing and pressing rollers, roller heating heaters 56 and 57 disposed in the rollers 51 and 52, respectively, an oil application roller for applying parting oil such as dimethylsilicone oil, an oil container 53 for containing the oil, a thermister 58 for controlling the fixing temperature on the basis of a detected temperature of the surface of the pressing roller. The recording material p having a 4-color toner image is subjected to the image fixing operation so that the toner images are mixed and fixed on the recording material p, by which a full-color toner image is produced, and the recording material p is discharged onto the discharge tray 63. The photosensitive drums 3a to 3d after the image transfer operation, is cleaned by cleaners 4a - 4d, so that residual toner is removed therefrom to be prepared for the next latent image forming operation and the like. The toner and foreign matter remaining on the transfer belt 130 is wiped by a cleaning web (unwoven textile) 19. The contact of the cleaning web 19 to the transfer belt 130 is controlled by a supply roller 17, a take-up roller 18, a tension roller 22 and a backup roller 21. In addition, a predetermined current is applied between rollers 21 and 22 to electrically discharge the transfer belt 130. The transfer belt used in such an image forming apparatus is a dielectric member sheet such as PET sheet, polyvinylidene fluoride sheet or polyurethane sheet having a volume resistivity of 10 13 -10 18 Ohm.cm. The image is stabilized if the current contributable to the image transfer action of the transfer charging means is controlled to be at a proper constant level (constant current control). In this embodiment, therefore, a constant-current-control is carried out so as to provide a constant current even if the volume resistivity varies due to the kind (thickness, material or the like) of the recording material or due to the wetting condition of the paper or the like. In such a control, the transfer voltage applied to the transfer charger 24a-24d sequentially increases in accordance with charge-up of the transfer belt 130, for example, 1kV at the first image formation station, 2kV at the second image formation station, 3kV at the third image formation station, and 4kV at the fourth image formation station. The transfer belt 130 and the recording material p are separated from each other at the separation portion after a predetermined amount of charge is given thereto through the constant-current-control in the transfer process, and the transfer belt 130 is discharged by a transfer belt discharging station where the couple of rollers 21 and 22 are provided, and the recording material is discharged by recording material discharging station (not shown) after the fixing process. Embodiment 1 Referring to Figure 1, an image forming apparatus according to Embodiment 1 according to the present invention will be described. Examples of the material of the dielectric sheet of the transfer belt 130 include PET, polyacetal, polyamide, polyvinylalccohol, polyetherketone, polystyrene, polybutyleneterephthalate, polymethylpentene, polypropylene, polyethylene, polyphenylenesulfide, polyurethane, silicon resin material, polyamide-imide, polybarbonate, polyphenyleneoxide, polyethersulfon, polysulfone, aromatic polyester, polyetherimide, aromatic polyimide, or the like; engineering plastic resin material film or the like. In this embodiment, the use is made with polyimide resin material in view of the mechanical property, electrical property and incombustibility. It is a seamless type, and the volume resistivity thereof is 10 16 Ohm.cm, and the thickness thereof is 10µm. The process speed in the image forming apparatus of the embodiment (rotational speed of the transfer belt and the photosensitive drum) is 100mm/s. The transfer chargers 24a, 24b, 24c and 24d are of plate-like electroconductive rubber having a rectangular shape extending in a direction(thrust direction) perpendicular to a recording material feeding direction. The plate-like electroconductive rubbers are urged toward the associated photosensitive drums 3a, 3b, 3c, 3d through the transfer belt 130. The back side of the recording material p fed to the transfer portion is charged with the polarity (positive polarity) opposite from that of the toner by the transfer chargers 24a-24d, so that toner image is electrostatically transferred from the photosensitive drums 3a-3d onto the surface of the recording material p. In this embodiment, the constant-current-control is carried out, wherein the transferring current is 6µA. As shown in Figure 5 which has been described hereinbefore, the driving roller 13 is electrically grounded to the main assembly ground, and the distance between the transfer charger 24d and the driving roller 13 dl=50mm. The electric current flowing into the driving roller 13 during the image forming operation was 3µA under the high temperature and high humidity ambience (absolute water content (wt.(g) of watervapor in 1kg air) was approx. 22g/kg , the temp. and relative humidity were 30°C and 80%). The image formed at this time was unsatisfactory due to transfer defect. However, only when the recording material p is between the fourth photosensitive drum 3d and the driving roller 13 or between the third photosensitive drum 3c and the driving roller 13, the flow of the transferring current into the driving roller 13 (A-D direction, C-D direction) and the resultant transfer defect occur. The recording material p had a length larger than the distance between a position where the transfer belt 130 is contacted to the third transfer charger 24c to a position where it is contacted to the driving roller 13, and it was "GINKAN" (tradename) available from Nippon Seishi KABUSHIKI KAISHA, Japan having a basis weight of 157g/m 2 , which was kept under 5 the ambient condition for sufficient period. Here, it is considered that transferring current escapes from the fourth transfer charger 24d or the third transfer charger 24c to the driving roller 13, or the current flows into the driving roller 13 when the recording material p is discharged by the separation charger 32 (the negative charge flows toward the photosensitive drums 3c, 3d). In this embodiment, the driving roller 13 is not directly connected to the main assembly ground, but, as shown in Figure 1, the driving roller 13 is connected to the main assembly ground through a constant current source 70, and constant current source 70 effects the constant-current-control to prevent the current between the driving roller 13 and the transfer charger 24d or between the driving roller 13 and the photosensitive drum 3d. The sequential control is such that constant current source 70 is on only when the recording material short-circuits between the fourth photosensitive drum 3d and the driving roller 13 or when the recording material short-circuits between the third photosensitive drum 3c and the driving roller 13, and otherwise it is off even during the image formation. By doing so, the current through the driving roller 13 is controlled to be not more than a predetermined value level. In this embodiment, the current through the driving roller is controlled to be 0µA, so that transferring current is prevented from escaping to the main assembly ground through the driving roller and so that current is prevented from flowing into the driving roller 13 when the recording material p is electrically discharged by the separation charger 32, by which the above-described image defects attributable to the transfer defect can be avoided. In view of separation discharge which may occur at the separation portion in low humidity condition, the voltage source 70 is on-off-controlled by CPU74 as control means in accordance with the ambience humidity in this embodiment. More particularly, in the low humidity ambience, the separation charger 32 is on, and the constant current source 70 is off; in the high humidity ambience, the separation charger 32 is off and the constant current source 70 is on. The separation charger 32 is disposed above the most downstream portion of the transfer belt 130, namely, above the driving roller 13 of the transfer belt 130, and is provided with a discharge wire. The discharge wire is stretched in the thrust direction, and the tension thereof is kept by the provision of the spring at one end of the discharge wire. The electric energy supply to the discharge wire is effected through a connector provided in the main assembly and through an unshown electric energy supply contact, an electric energy supply pin and a spring. The driving roller 13 is connected to the main assembly ground through the constant current source 70, and functions also as an opposite electrode for the discharge wire. In this embodiment, the distance between the transfer charger 24d and the separation charge portion (the position where the recording material p is separated from the transfer belt 130) d2=50mm, and the separation charger 32 is supplied with an AC voltage having 10kVpp, 500Hz. As described hereinbefore, under the low humidity ambience, the electrostatic attraction force between the recording material and the transfer belt 130 is larger, and therefore, the effect of weakening the electrostatic attraction force by the separation charger 32 is significant. Under the low humidity ambience, the image defect attributable to the separation charge tends to occur upon separation between the transfer belt 130 and the recording material, and therefore, the separation charger 32 is effective as a countermeasure thereagainst. The transfer defect does not easily occur under the low humidity ambience, and therefore, the zero Ampere control (constant current control) for the driving roller 13 disclosed in Embodiment 1 is not necessary. So, it is preferable to render on the separation charger 32 upon separation, and the constant-current-control is off (non-operated), under the low humidity ambience. On the other hand, under the high humidity ambience, the electrostatic attraction force between the transfer belt 130 and the recording material is small as compared with the case of the low humidity ambience, and therefore, the effect of the separation charger 32 is relatively small. However, the transfer defect tends to occur, and therefore, the constant-current-control for the driving roller 13 is preferably carried out. Accordingly, under the high humidity ambience, the separation charger is off, and the constant-current-control is on. In this embodiment, on-off of the separation charger 32 and the constant-current-control is carried out in accordance with the humidity ambience (absolute water content) as follows: Water Content Separation Charger Constant Current Control Ambience A no less than 10g/kg off on Ambience B no more than 10g/kg on off The volume resistivity of the recording material p under the ambiences A and B are approx. 10 7 and 10 11 Ohm.cm, respectively, and the currents into the driving roller 13 is approx. 3µA and 0µA respectively. In this embodiment, the current including the current which may occur by the separation charger 32 as the current generating source, is prevented from flowing. By doing so, an image forming apparatus is provided wherein the separation property between the transfer belt 130 and the recording material p and the image quality are maintained high even under the low humidity ambience, and the transfer defect is prevented from occurring even under the high humidity ambience. The ambience temperature/humidity is detected automatically by a temperature/humidity detection sensor provided in the main assembly of the image forming apparatus. Embodiment 2 The second embodiment of the present invention will be described. In this embodiment, the on-off of the separation charger 32 and the constant-current-control is controlled in accordance with the types of the recording material p by a CPU74 as the control means. The basis weight of the recording material used with the image forming apparatus ranges between approx. 50g/m 2 -200g/m 2 , and.the resistance of the recording materials are different. The recording material having a large basis weight has a relatively large thickness, and therefore, the resistance between its front side and the back side is large. Therefore, a high transfer voltage is required, and the transferring current tends to escape more to the driving roller 13. The larger thickness of the recording material means larger cross-sectional area through which the current flows, and results in the tendency of the transferring current escaping to the driving roller 13. Accordingly, an image forming apparatus is provided wherein when an image is formed on a recording material having a large thickness, the zero Ampere control(constant current control) between the transfer charge portion and the driving roller is carried out so that escape of the transferring current is prevented, and therefore, the transfer defect does not occur. The type of the recording material is automatically discriminated by a mechanical sensor or an optical sensor, but an operator may set on a control panel. Embodiment 1 and Embodiment 2 may be properly combined to control on-off switchings of the separation charger 32 and the constant current source 70 in accordance with the detection results of the humidity (absolute water content) and the type of the recording material p. Embodiment 3 In this embodiment, as shown in Figure 3, the driving roller 13 is supplied with a predetermined positive voltage (the voltage of the same polarity as the polarity of the voltage applied to the transfer charger 24a-24d or the voltage of the opposite polarity from the polarity of the toner image on the photosensitive drum) by a voltage source 72, so that potential of the driving roller 13 per se is made high, thus preventing the flow of the negative charge to the transfer portion. As a result, the negative charge does not flow to the transfer portion, and a remarkable transfer defect and image defect can be prevented. In this embodiment, the voltage source 72 is a constant voltage source. In view of separation discharge which may occur in the separation portion under a low humidity condition, the positive voltage bias applied to the driving roller 13 by the voltage source 72 (the voltage of the same polarity as the polarity of the voltage applied to the transfer charger 24a-24d) is changed by a CPU74 as a control means in accordance with the ambience humidity in this embodiment. The volume resistivities of the recording material p sufficiently kept under a low humidity ambience and the recording material p sufficiently kept under the high humidity ambience are different by about 4 digits. Therefore, it is considered that amount of flow of the negative charge varies significantly depending on the ambience. In this embodiment, the applied voltage is low under the low humidity ambience, and it is high under the high humidity ambience so that flow amount of the negative charge to the transfer portion is controlled. Under the low humidity ambience, the volume resistivity of the recording material p is high as compared with the case of high humidity ambience(for example, 10 12 Ohm.cm or higher), and therefore, the amount of the flow of the negative charge to the transfer portion is low. Therefore, it is preferable that positive voltage applied to the driving roller 13 from the voltage source 72 is small. On the other hand, under the high humidity ambience, the volume resistivity of the recording material p is low (for example, 10 10 Ohm.cm or lower), and therefore, the amount of flow of the negative charge to the transfer portion is large. Therefore, it is preferable that positive voltage applied to the driving roller 13 is high. In this embodiment, the applied voltage is controlled in accordance with the humidity (absolute water content) as follows: Ambience C (Absolute Water Content is not less than 20g/kg): 7kV Ambience D (Absolute Water Content is not less than 1.5g/kg and less than 20g/kg): 4kV Ambience E: (Absolute Water Content is less than 1.5g/kg): 1kV Under the ambiences C, D and E, the volume resistivity of the recording material p are approx. 10 10 , 10 11 and 10 15 Ohm.cm. With the above-described structure, an image forming apparatus is provided which is not influenced by the ambience, and the transfer defect does not occur. The ambience temperature/humidity may be detected automatically by a temperature/humidity detection sensor provided in the main assembly of the image forming apparatus, or the operator or a serviceman may set on a control panel. Embodiment 4 In this embodiment, the positive voltage bias applied to the driving roller 13 (the voltage of the same polarity as the voltage applied to the transfer charger 24a-24d) is changed by a CPU74 as a control means in accordance with types of the recording material. The basis weight of the recording material used with the image forming apparatus widely ranges between 50g/m 2 -200g/m 2 , and therefore, the resistances of the recording materials p are widely different, and the amount of the negative charge flowing to the transfer portion during the image formation is influenced by the property of the recording material p. A recording material p having a large basis weight has a large thickness, and therefore, the resistance between the front side and the back side thereof is large with the result of large amount of the negative charge flowing to the transfer portion and therefore the tendency of occurrence of said transfer defect. On the other hand, the recording material p having a small basis weight, has a small thickness, and the results are the opposite. Therefore, when the image formation is carried out on a recording material having a large thickness, the positive voltage applied to the driving roller 13 is made high by which the amount of the negative charge to the transfer portion is decreased, thus preventing the transfer defect. The types of the recording material may be automatically detected by a mechanical or optical sensor, or an operator may set the type of the recording material on a control panel. Embodiment 5 The inventors used the apparatus shown in Figure 2, and carried out image formations under a high temperature and high humidity ambience (room temperature of 30°C and relative humidity of 80%), during which the resistance between the photosensitive drum 3d and the transfer charger 24d was measured; and it was 100 MOhm. At this time, between the transfer charger 24d and the photosensitive drum 3d, a transfer belt 130 and the recording material p were interposed, the recording material p being paper having a basis weight of 157g/m 2 (print paper Ginkan 157g/m 2 , available from Nippon Seishi KABUSHIKI KAISHA). Simultaneously with the measurement, the resistance between the transfer charger 24d and the grounding portion of the driving roller 13 as the opposite electrode for the separation charger 32 was determined, and it was 10 MOhm. It was determined by applying a predetermined transfer voltage to the transfer charger 24d and measuring the current flowing through the photosensitive drum 3d and the driving roller 13. Accordingly, in this embodiment, the driving roller 13 is not directly connected to the main assembly ground GND, but as shown in Figure 4, the driving roller 13 is connected to the main assembly ground GND through the resistor R of 1000 MOhm, and the resistance between the transfer charger 24d and the grounding portion of the driving roller 13 is increased from 10 MOhm to 10+1000=1010 MOhm, that is, the resistance sufficiently higher than the resistance 100 MOhm between the photosensitive drum 3d and the transfer charger 24d. Therefore, in this embodiment, the transferring current by the transfer charger 24d does not escape to the main assembly ground GND through the driving roller 13, and therefore, sufficient transferring current is supplied to the photosensitive drum 3d, and the toner image is properly transferred onto the transfer material p from the photosensitive drum 3d, thus providing a high quality image without transfer defect. Furthermore, the occurrence of the transfer defect when the recording material is discharged by the separation charger 32 upon separation, can be prevented. In the foregoing, in order to prevent escape of the transferring current to the main assembly ground GND through the driving roller 13, a resistor R is connected between the driving roller 13 and the main assembly ground GND to increase the resistance (impedance) between the driving roller 13 and the main assembly ground GND, but a varister may be used in place of the resistor R. Alternatively, a high resistance member may be provided on the surface of the driving roller 13 to increase the resistance between the driving roller 13 and the transfer charger 24d so that transferring current is prevented from escaping to the main assembly ground GND through the driving roller 13. Embodiment 6 This embodiment is similar to Embodiment 5 shown in Figure 4, but a resistor R provided between the driving roller 13 and the main assembly ground GND is in the form of a variable resistor, and the resistance is changed by a CPU74 as a control means in accordance with the ambience humidity. As described in the foregoing, under a low humidity ambience, the electrostatic attraction force between the transfer belt 130 and the recording material is large as compared with the case of high humidity ambience, and therefore, the effect of weakening the electrostatic attraction force by the separation charger 32 is significant. However, the transfer defect which is a problem does not easily occur under the low humidity ambience, and therefore, it is not necessary to increase the resistance between the driving roller 13 and the main assembly ground GND at the cost of deteriorating the separation property for the recording material. Thus, under the low humidity ambience, it is desirable to lower the resistance of the variable resistor R to enhance the function of the driving roller 13 functioning as the opposite electrode for the separation charger 32. On the other hand, under the high humidity ambience, the electrostatic attraction force between the transfer belt 130 and the recording material is small as compared with the case of low humidity ambience, and therefore, the electrostatic attraction force reducing effect due to the separation charger 32 is not so significant. In addition, the transfer defect tends to occur, and therefore, the resistance between the driving roller 13 and the main assembly ground GND is high. Accordingly, under the high humidity ambience, the resistance of the variable resistor R is preferably large. In this embodiment, under the low humidity ambience the variable resistor R provides a low resistance, and under the high humidity ambience it provides a high resistance. Examples of the resistance of the variable resistor R in this embodiment are given below: Ambience F (Absolute water content is no less than 15g/kg): 1000 MOhm Ambience G (Absolute water content is 5-15g/kg): 100 MOhm Ambience H (Absolute water content is less than f 5g/kg): 0 Ohm Under the ambiences F, G and H, the volume resistivities of the recording material are approx. 10 10 Ohm.cm, 10 11 Ohm.cm and 10 15 Ohm.cm, and the measured resistances between the driving roller 13 and the transfer charger 24d are approx. 10 MOhm, 1000 MOhm and 1000 MOhm. As described in the foregoing, in this embodiment, the resistance between the driving roller 13 and the main assembly ground GND is changed in accordance with the ambience humidity to maintain high separation property between the transfer belt 130 and the recording material under the low humidity ambience while maintaining high image quality under the high humidity ambience. When the sufficient separation property is provided between the recording material and the transfer belt 130 under the high humidity ambience (the absolute water content is 15g/kg or higher), the driving roller 13 may be isolated from the main assembly ground GND (float) in place of increasing the resistance between the driving roller 13 and the main assembly ground GND. The temperature/humidity may be detected automatically by a temperature/humidity detection sensor provided in the main assembly of the image forming apparatus, and the resistance change may be made automatic in accordance with the detected humidity. Or, the temperature humidity may be detected by a temperature meter and a humidity meter, and the operator or a serviceman may manually input the temperature and humidity to manually change the resistance. Embodiment 7 This embodiment is similar to Embodiment 5 or 6 shown in Figure 4, but the impedance between the driving roller 13 and the transfer charger 24d is controlled by the CPU74 as the control means in accordance with the types of the recording material p. The basis weight of the recording material used with the image forming apparatus ranges approx. between 50-200g/m 2 . The resistance of the recording material significantly changes in accordance with the basis weight. The recording material having a large basis weight has a large thickness, and therefore, the resistance between the front side and the back side is large. Therefore, the required transfer voltage is large, and the transferring current further tends to escape to the driving roller 13 through the recording material p. When the image formation is effected on a recording material having a large thickness, the resistance between the driving roller 13 and the transfer charger 24d is increased to prevent the escape of the transferring current. By doing so, high quality images without transfer defect can be provided. The types of the recording material can be automatically detected by a mechanical or optical sensor, but may be manually set on a control panel. In the foregoing Embodiments 1-7, the transfer charger may be a corona charger, an electroconductive elastic roller, a brush or the like, and the same advantageous effects can be provided. The image bearing member is not limited to an electrophotographic photosensitive member, but may be a dielectric member in an electrostatic recording. The developing means 1a-1d for developing the electrostatic latent images on the image bearing members 3a-3d, will be briefly described. Generally, in the case of non-magnetic toner, it is applied on the sleeve using a blade or the like, and in the case of magnetic toner, it is applied on the sleeve using magnetic force. The toner is carried on the sleeve to a developing zone. There are an one-component non-contact development method wherein the sleeve is not contacted to the image bearing member, a one-component contact developing method wherein the sleeve is contacted to the image bearing member. In other types, the use is made with a developer containing toner particles and magnetic carrier particles mixed therewith, and the developer is carried by magnetic force. There are a two-component contact developing method wherein the developer is contacted to the image bearing member, and a two component non-contact development method wherein the developer is not contacted to the image bearing member. Such four types of the development is generally used. In this embodiment, two-component contact type developing system is used from the standpoint of the high quality and high stability of the image. However, the present invention is usable with any other types of development. 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 (3) for carrying a toner image; a recording material conveyor belt (130) for electrostatically carrying and feeding a recording material (P); a roller (13), provided on a side of said recording material conveyor belt not carrying the recording material in a separation position where the recording material is separated from said recording material conveyor belt, for supporting said recording material conveyor belt; transfer charging means (24) for electrostatically transferring the toner image from said image bearing member onto the recording material at a transfer position; discharging means (32), provided across said recording material conveyor belt from said roller (13) at said separation position for discharging the recording material upon separation thereof from said recording material conveyor belt; wherein said roller (13) is electrically grounded through a variable resistor (R). An apparatus according to claim 1, further comprising control means (74) for controlling a resistance value of said variable resistor (R). An apparatus according to claim 2, wherein the variable resistor is a varistor. An apparatus according to claim 2, further comprising detecting means for detecting a temperature and a humidity, wherein said control means (74) changes the resistance value in accordance with an absolute water content corresponding to the temperature and the humidity detected by said detecting means. An image forming apparatus comprising: an image bearing member (3) for carrying a toner image; a recording material conveyor belt (130) for electrostatically carrying and feeding a recording material (P); a roller (13), provided on a side of said recording material conveyor belt not carrying the recording material in a separation position where the recording material is separated from said recording material conveyor belt, for supporting said recording material conveyor belt; transfer charging means (24) for electrostatically transferring the toner image from said image bearing member (3) onto the recording material (P) at a transfer position; discharging means (32), provided across said recording material conveyor belt (130) from said roller (13) at said separation position, for discharging the recording material upon separation thereof from said recording material conveyor belt; means for detecting a temperature and a humidity; control means (74) for selectively effecting, in accordance with an absolute water content corresponding to the temperature and the humidity detected by said detecting means, a control of a current to said roller (13) to be a predetermined value upon separation of the recording material (P) from said recording material conveyor belt (130). An apparatus according to any of claims 2, 4 or 5, further comprising recording material detecting means for detecting a type of the recording material, and said control means (74) selectively effects the control in accordance with a detection result of said recording material detecting means. An apparatus according to claim 4 or claim 5, wherein said control means (74) selectively effects control of the discharging means (32) in accordance with the absolute water content. An apparatus according to claim 5 or 6 wherein said predetermined value of current is substantially zero. An image forming apparatus comprising: an image bearing member (3) for carrying a toner image; a recording material conveyor belt (130) for electrostatically carrying and feeding a recording material (P); a roller (13), provided on a side of said recording material conveyor belt (130) not carrying the recording material (P) in a separation position where the recording material is separated from said recording material conveyor belt, for supporting said recording material conveyor belt; transfer charging means (24) for electrostatically transferring the toner image from said image bearing member (3) onto the recording material (P) at a transfer position; discharging means (32), provided across said recording material conveyor belt (130) from said roller (13) at said separation position, for discharging the recording material (P) upon separation thereof from said recording material conveyor belt; voltage application means (72) for applying to said roller a voltage of a polarity opposite from a regular polarity of the toner image on said image bearing member; means for detecting a temperature and a humidity; control means (74) for controlling the voltage in accordance with an absolute water content corresponding to the temperature and the humidity detected by said detecting means upon separation of the recording material (P) from said recording material conveyor belt (130). An apparatus according to claim 9, further comprising recording material detecting means for detecting a type of the recording material, wherein said control means (74) changes the voltage in accordance with an output of said recording material detecting means. An apparatus according to claim 6 or claim 10, wherein said recording material detection means detects a thickness of the recording material. An apparatus according to claim 9, wherein said control means (74) controls said voltage so that current to said roller (13) is substantially zero when the absolute water content is higher than a predetermined level. An image forming apparatus according to claim 4, wherein said control means is arranged to control the resistance of the variable resistor such that the resistance of the variable resistor (R) is at a first value when an absolute water content corresponding to the temperature and humidity detected by said detecting means is equal to or higher than a predetermined level, and at a second value lower than said first value when the absolute water content is lower than the predetermined level. An apparatus according to claim 13, wherein when the absolute water content is not less than the predetermined level, the roller (13) is electrically floated. An apparatus according to claim 13 or 14, wherein when the absolute water content is less than the predetermined level, said roller (13) is electrically grounded. An apparatus according to any preceding claim, wherein said roller comprises an electroconductive member. An apparatus according to any preceding claim, wherein said discharging means (32) includes a corona charger. An apparatus according to any preceding claim, wherein said transfer charging means (24) is contacted to a side of said recording material conveyor belt (130) which does not carry the recording material (P) at the transfer position. An apparatus according to any preceding claim, wherein said recording material conveyor belt (130) comprises a dielectric member. An apparatus according to any preceding claim, wherein a plurality-of image bearing members (3a-d) are provided to carry toner images of different colors, and the toner images are sequentially transferred onto the recording material (P) carried on said recording material conveyor belt (130). An apparatus according to claim 20, wherein a plurality of transfer charging means (24a-d) are provided to effect the sequential image transfer. The combination of an apparatus according to any preceding claim and a sheet of recording material, wherein a length of the sheet of recording material (P) measured in a recording material feeding direction of said recording material conveyor belt (130) is larger than a distance (d 2 ) between said transfer position and said separation position.

Inventors: BESSHO YUJI, INOUE MASAHIRO, ITOU YOSHIKUNI, KIMURA YOICHI, BESSHO, YUJI, INOUE, MASAHIRO, ITOU, YOSHIKUNI, KIMURA, YOICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: G03G15
IPC codes: G03G15